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Title: basic electrical
Description: In the notes use to basic electrical law, electrical engineering questions
Description: In the notes use to basic electrical law, electrical engineering questions
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Question and answers
Electrical Maintenance Unit
1
...
If we join a charged conductor to another
conductor, electricity flows from one to the other
...
Static electricity is no use
...
2
...
Its symbol is ‘I’ and
measuring unit is Ampere measured by ammeter or ampere meter
...
What is electro-motive force (emf) or voltage?
It is the pressure that moves the electrons to flow in any conductor
...
Its symbol is ‘E’ or ‘V’ and measuring unit is volt
measured by voltmeter
...
What is potential difference (P
...
P
...
5
...
It’s symbol is VT
...
D
6
...
Its measuring unit is ohm and measured by ohmmeter, multi meter,
wheat stone bridge, and post office box
...
7
...
001118 gm/sec
...
What is volt?
The international volt is defined as 1/1
...
It
is that difference of potential which, when applied to a conductor whose resistance is
1 (one) international ohm, will cause a current of 1 (one) international ampere to
flow
...
What is matter?
The matter is defined as anything, which possesses weight and occupies space and
can be in any of three forms solid, liquid or gaseous
...
10
...
11
...
Human body is conductor and
when touched to the live conductor it completes its shortest root though the body and
the body gets electric shock in which its nervous system, the heart, respiratory
system may cease to function
...
What is fuse and what materials used for fuse wire?
Fuse is a weakest point in an electrical circuit, which melts when the excess current
flows through it in the electrical circuit
...
13
...
Fusing factor = minimum fusing current / current rating of fusing element
...
14
...
Soldering at high temperature using brass as solder is called brazing or hard
soldering
...
Its melting
point is 190°C and is widely used
...
What are the sources of electricity?
a
...
Generator (magnetism)
c
...
Light (photo electric or solar cell)
e
...
Friction (static electricity)
-2-
Question and answers
Electrical Maintenance Unit
16
...
Physical effect
...
Chemical effect
...
Magnetic effect
...
Heating effect
...
X-ray effect
...
What is fire?
Destructive burning of any material is called the fire
...
If any one among three is separated the fire will come to end
...
On what factor resistance of the substance depends (Laws of resistance)?
a
...
b
...
c
...
That is specific resistance of the material
...
The resistance of the conductor depends on its temperature
...
R=ρL Ω
A
Where ρ is the constant for the material called its specific resistance or resistivity
...
What is specific resistance or resistivity of the material?
Specific resistance of the material is the resistance of a piece of unit length and unit
cross-section (unit cube of that material)
...
Or the specific resistance of any material is the resistance offered by the opposite
face of that material
...
-3-
Question and answers
Electrical Maintenance Unit
20
...
The temperature co-efficient of
resistance of material is the ratio of increase in resistance of 1°C rise in temperature
to the original resistance of the material (strictly at 0°C)
...
R0 Resistance at 0°C
...
t Temperature rise
...
What are the effects of temperature on resistance?
The effects of temperature on resistance are
a
...
the resistance
increases with increasing temperature at fairly regular manner
...
b
...
the change in resistance
due to increasing temperature is irregular and negligible for a considerable range
of temperature
...
In case of certain materials belongs to insulators, electrolytes such as paper,
rubber, glass, mica, carbon, acids, alkalies etc
...
Such materials posses negative
co-efficient of resistance
...
What are the classifications of voltages?
a
...
That is 0 – 250V
...
Medium voltage: Voltage above 250V upto 650V comes under medium voltage
...
High voltage: Voltage above 650V upto 33 kV comes under high voltage
...
Extra high voltage: Above 33 kV voltages are extra high voltages
...
What is coulomb?
It is the unit of charge
...
The letter Q denotes it
...
24
...
A condenser has a
capacitance of one (1) farad, if it is capable to maintain a charge of one coulomb
under a potential difference of one volt between its plates
...
= Q/V
...
What is henry?
It is the unit of inductance and the letter H denotes it
...
1 henry = 1 volt sec / ampere
...
What is the least count of out-side micrometer?
The least count of out-side micrometer is 0
...
-5-
Question and answers
Electrical Maintenance Unit
27
...
Sl
...
p
...
State Greek alphabets and what for they are used?
Sl
...
What is conductance?
Conductance is the property of the conductor, which allows the flow of electric
current through it
...
The unit of conductance is mho
...
30
...
The conductance and conductivity is zero in insulators
...
Insulators
will not allow the leakage of current, short-circuiting current, shock to the operator
and isolates the electric current safely with out any diversion to any other place
...
b
...
d
...
f
...
It should be flexible
It should have good mechanical strength
It should easily moulded into any shape
It should not be effected by acid
It should be non-inflammable
It should have very high specific resistance to prevent leakage current
It should be withstand high temperature
...
That is resistance decreases with increasing temperature
h
...
What is electrode?
A conducting element used for converging (centering) current to and from a medium
is called electrode
...
A positive and other is negative
...
What is force?
Force is that which charge or tends to change a body state of rest or uniform motion
through a straight line
...
33
...
34
...
Gravitational
unit of force in M
...
S system is kilogram weight or 9
...
Weight is the force with which 1-kilogram mass is attracted by the earth towards its
center
...
What is bayer?
Bayer is the C
...
S unit of pressure and is equal to 1-dyne/cm2
...
What is conductor?
Substances such as metals, which have large number of free electrons are said to
offer a low resistance to the flow of electrons under the influence of emf and hence
are called conductors
...
Conductors are classified into three main groups
...
Good conductors
...
Semi conductors
...
Fair conductors
...
What are the properties of good conductor?
Properties of good conductor
a
...
c
...
e
...
g
...
i
...
k
...
It posses very low resistance or specific resistance
...
It is a good conductor of heat
...
It must be malleable and ductile
...
It posses better tensile strength
...
It can be drawn in very fine wires
...
It must be very low in cost
...
- 10 -
Question and answers
Electrical Maintenance Unit
37
...
b
...
d
...
f
...
h
...
j
...
l
...
n
...
p
...
r
...
Silver
Silver copper alloy
Copper (Hard down and Annealed)
Gold
Zinc
Platinum
Tin
Aluminum
Iron
Brass
Phosphorous bronze
Nickel
Lead
Germanium silver
Antimony
Platinoid
Mercury
Bismuth
Platinum iridium
38
...
That
is semiconductors gives opposition (resistance) to the flow of free electrons than that
of good conductor
...
Dilute acid
b
...
See water
d
...
Silicone
f
...
What is fair conductor?
Fair conductors are the materials, which have less conductivity than that of
semiconductor
...
Examples for fair conductors are
a
...
Coke
c
...
Plumbago
40
...
This property is well utilized
to convert electrical energy into heat energy
...
Examples are eureka, carbon, nichrome, tungsten, manganin,
germanium, and tentalum
...
- 12 -
Question and answers
Electrical Maintenance Unit
41
...
Rheostat: a variable resistance by sliding contacts on it the current can be varied is
called rheostat
...
42
...
In some cases certain % of bismuth and cadmium is also added to decrease the
melting point of the solder
...
Bismuth has a special quality in comparing to most of other metals
...
This property helps to shrink the solder and there by it allows
the joint become firm
...
To get stronger joint
add more tin in the solder
...
43
...
44
...
This effect of electricity flowing through the peripheral surface of the
conductor is known as skin effect
...
What are the advantages of stranded cables?
a
...
c
...
It gives flexibility
...
Increases current carrying capacity
...
e
...
- 13 -
Question and answers
Electrical Maintenance Unit
46
...
In a closed electrical circuit, at a constant temperature, the ratio between the resulting
unvarying current or direct current and applied voltage is a constant
...
OR
In simple manner Ohm’s law says that, in a closed electrical circuit the current is
directly proportional to the voltage and inversely proportional to the resistance of the
circuit
...
Ohm's Law / Power Formulas
P = watts
I = amps
R = ohms
E = Volts
47
...
Characteristics of series circuit
a
...
b
...
c
...
d
...
e
...
f
...
g
...
- 14 -
Question and answers
Electrical Maintenance Unit
48
...
Characteristics of parallel circuit
a
...
b
...
c
...
d
...
e
...
The reciprocal of total resistance is equal to the sum of the reciprocal of
individual parallel resistances (1/R = 1/ r1 + 1/ r2 +1/ rn)
...
Individual conductance is inversely proportional to the individual resistance
...
If two same value resistors are connected in parallel circuit the total resistance is
the resistance of one resistor
...
49
...
Capacity of a capacitor depends on following factors
a
...
b
...
That is if the
distance is more the capacity decreases or if the distance is less the capacity more
...
It depends on the nature of dielectric constant
...
On what factor voltage rating of the capacitor depend?
The voltage rating of the capacitor depends on the distance between the plates of the
capacitor
...
51
...
Paper capacitor
...
Rolled plastic cover or polyester type capacitor
...
Mica capacitor
...
Silver mica capacitor
...
Ceramic capacitor
...
Electrolytic capacitor
...
What is the resultant capacitance in series and parallel circuit?
In series circuit the resultant capacitance 1/CT = 1/c1+1/c2 + 1cn farad
...
- 15 -
Question and answers
Electrical Maintenance Unit
53
...
- 16 -
Question and answers
Electrical Maintenance Unit
54
...
Work
done is equal to force * distance
...
K
...
55
...
Power = Work / time
...
One mechanical horsepower is equal to 746 watts
(British) and 735
...
5 joules/sec
...
34
horsepower (British) and 1
...
56
...
The unit of energy is joule or watt-second or
watt-hour or kilo watt-hour
...
Define Joule’s law
...
H = I2 R t/J calories
...
2 Joules
...
24 I2 R t calories
...
What is electrolysis?
When current passes through an acid or a salt, it de-composes and the two decomposed portions tend to move in opposite direction
...
Or the process of decomposing a liquid by the passage of electric current (DC)
through it is called the electrolysis or electric analysis
...
What are the Faradays laws of electrolysis?
First law
The mass ions liberated at an electrode are directly proportional to the quantity of
electricity (coulomb Q) which has passed through the electrolyte
...
And M = Z I t
...
Second law
If the same quantity of electricity passes through several electrolyte the masses of the
ions liberated are proportional to their respective chemical equivalent
...
What is electro plating?
The process of depositing a metal on the surface of another metal by electrolysis is
known as electro plating
...
- 18 -
Question and answers
Electrical Maintenance Unit
61
...
Electro plating
...
Purification of copper and extraction of number of metals and number of
commercial compounds like sodium, hydrogen, hydroxide, oxygen etc
...
Electro typing
...
Determination of DC polarity
...
Electro refining of metals
...
State the laws of magnetism
...
Magnet imparts its magnetic properties to other metals
...
When a magnet is suspended freely horizontally, it stands at geographical north
and south
...
Every magnet has a north and its associated separable South Pole
...
If a magnet broken in any number of pieces, each piece will act as a separate
magnet having north and south poles
...
Like poles repulse and unlike poles attracts
...
The amount of attraction or repulsion is directly proportional to the pole
strength and inversely proportional to the square of the distance between them
...
63
...
The letter ‘B’ denotes it and it is measured in Weber/cm2
...
64
...
65
...
66
...
67
...
68
...
- 20 -
Question and answers
Electrical Maintenance Unit
69
...
Tough method
b
...
Induction method
70
...
71
...
Electro magnets can be magnetised very easily by sending DC through it
...
Changing the direction of the current through the coil can change the polarity of
the poles
...
The strength of the magnet can be controlled by the electric current
...
Electro magnets can be made in any shape depending upon the need
...
The magnetic strength remains constant as long as the current is constant
...
State ‘Cork screw rule’ and ‘Right hand thumb rule’
...
‘Cork screw rule’ says that, the direction of magnetic lines of force around a straight
current carrying conductor is the same as that in which the cork screw must be
rotated to cause to an advance in the direction of the current in conductor
...
Then the folded fingers or encircling
fingers must be in the direction of magnetic lines of force around the conductor
...
Who discovered electro magnetism?
‘Orsted’ a denish scientist discovered that whenever an electric current passes
through a conductor, a magnetic field will be produced around that conductor in
concentric circle
...
74
...
In 1831 Faraday discovered the production of electric current in electric conductor
by converting magnetism
...
First law
Whenever a conductor causes to cut the magnetic lines of force an emf will be
induced in that conductor
...
- 22 -
Question and answers
Electrical Maintenance Unit
75
...
Lenz’s law
The lenz’s law states that, electro magnetically induced current always flows in such
a way or direction that the action of magnetic field set up by induced current tends to
opposes the root cause which produces it
...
What is eddy current?
Eddy currents are those which are produced or induced in the mass of metal
whenever the metal are moved in magnetic field of the magnetic field is moved
across the mass metal so as to link it
...
Eddy current can be calculated by following equation
We = k Bmax2 f2 t2 v watt
...
Development of eddy current is made use in energy meters to provide controlling
torque and also in form of automatic starters in moving coil measuring instruments
...
What is magnetic Hysteresis?
Lagging of magnetization or induction flux density ‘B’ behind the magnetising force
‘H’ is known as magnetic hysteresis
...
What are the types of induced electro motive force?
a
...
b
...
Statically induced emf can be further divided into two groups
...
Mutually induced emf
...
Self induced emf
...
What are the use of mutual induction and self-induction?
Use of mutual induction
a
...
b
...
c
...
Use of self-induction
a
...
b
...
c
...
80
...
a
...
b
...
c
...
d
...
e
...
f
...
g
...
81
...
A generator works on under the principle of faraday’s laws of electro magnetic
induction
...
82
...
What are the types of generators?
There are two types of generator
...
Permanent magnet generator
...
Electro magnet generator
...
a
...
b
...
- 24 -
Question and answers
Electrical Maintenance Unit
84
...
a
...
b
...
c
...
There are different types of compound generator
...
Short shunt commulative compound generator
...
Short shunt differential compound generator
...
Long shunt commulative compound generator
...
Long shunt differential compound generator
...
a
...
b
...
c
...
85
...
Z = Total number of armature conductors
...
A = Number of parallel paths in armature
...
emf = emf generated in one parallel path and it is the emf generated of that generator
...
In
such cases A=2 and in lap wave wound armature parallel paths is equal to the
number of poles in the armature winding
...
What are the losses in DC generator?
There are two main losses
...
Copper losses or electrical losses
...
Stray losses or rotational losses or constant losses
...
Armature copper losses (Ia2 ra)
...
Field copper losses (Ise2 rse) or (Ish2 rsh)
...
Losses in brush
...
Magnetic losses (Iron loss or core loss)
...
Mechanical losses
...
What is efficiency of generator?
Efficiency = Out put / input
= Out put / out put + losses
- 26 -
Question and answers
Electrical Maintenance Unit
88
...
The effects of armature reaction are follows
...
Armature reaction destroys (cross magnetizes) and weakens the main field flux
produced by the main pole
...
It causes to reduce the induced emf in the armature
...
It causes to reduce the efficiency of machine
...
It causes to produce sparking at the brushes due to the shifting of M
...
A
(magnetic neutral axis)
...
At short-circuited loads or at very heavy loads, in case of self-excited generators
de-magnetising of pole cores (wiping of residual magnetism) may takes place
...
What are the remedies for armature reaction?
a
...
N
...
b
...
c
...
d
...
e
...
90
...
Whenever a brush contacts two or more commutator segments, the connected to
those segments are short-circuited
...
The change that takes place in the
coil after the period of short-circuiting of that coil is called commutation
...
If the commutation is not smooth, the spark may be more and that will damage the
commutator surface, commutator segments and so the winding
...
91
...
No load saturation characteristics or OCC or magnetic characteristics (E0/If)
...
Internal or total characteristics (E/Ia)
...
External characteristics (V/I)
...
What is motor? How DC motor works?
A motor is a machine, which takes electrical energy and converts that electrical
energy into mechanical energy
...
The direction of the force or rotation can be determine by “Fleming’s left hand rule”
93
...
This rotation is due to the turning or twisting force acted on
that conductor
...
T = force * radius Newton-meter
...
∴ Work done per revolution = force * 2πr
...
p
...
5 hp
...
What are the classifications of DC motor?
a
...
b
...
c
...
There are two types of DC compound motor
...
Differential compound motor
...
Commulative compound motor
...
What are the losses in DC motor?
The losses in DC motor are same as that of DC generator
...
96
...
a
...
It is also known as electrical
characteristics
...
Speed and armature current characteristics
...
Speed and torque characteristics
...
What is the necessity of DC motor starter?
Eb = V – Ia ra
...
At the time of starting from the rest there is no any back emf (Eb) in the armature
...
This very large current
blow out the fuses and before to that it will damage the commutator, commutator
brushes and winding
...
This starting resistance is gradually cut out as the motor gains speed and the
develops back emf (Eb) which regulates it’s speed and armature current
...
98
...
DC two point starter for series motor
...
DC three point starter for shunt motor
...
DC four point starter for compound motor
...
How speed control of DC motor can be achieved?
Induced emf in the armature E = P * φ * Z * N / A * 60 volts
...
N ∝ Eb / φ
N ∝ V – Ia ra / φ
...
If it so then N ∝ V/ φ
...
100
...
A cell
essentially requires two electrodes, electrolyte and container
...
What is battery?
The combination of two or more cells is called the battery
...
What are the classifications of cell?
a
...
b
...
- 29 -
Question and answers
Electrical Maintenance Unit
103
...
The common primary cells in use are,
a
...
b
...
c
...
d
...
e
...
Secondary cells are those cells, which can be re-charged and use again once they
discharged or used for the work for number of times with out re-newing it’s
materials
...
Lead acid cell
...
Nickel iron alkaline cell
...
Nickel cadmium alkaline cell
...
What is polarization? What is local action?
Polarization
The hydrogen bubbles which are clinging over the surface of copper electrode
(anode) becomes a thin film of hydrogen over the copper electrode
...
This effect is known as polarization
...
This is
due to the impurities (copper, iron, tin, and lead) in the commercial zinc
...
These
local cells consume always zinc and the emf developed by those local cells are
always opposite to the main emf
...
105
...
It gives high current capacity
...
Its internal resistance is very low
...
It gives a constant current
...
It posses very high efficiency
...
It posses fairly constant emf
...
It posses good mechanical strength
...
It posses large storage capacity
...
It can be renewed by charging after it is discharge
...
It is durable
...
What is Plante plate and Faure plate?
There are two types of positive plate preparation
...
Plante plate
As per plante process positive plate PbO2 are prepared by a process of repeated
charging and discharging of pure lead
...
This process of positive plate preparation
required very long time for it’s manufacturing and so it is very costly
...
107
...
The negative
plates are also of rectangular lead grid and the active material Pb in the form of
paste is held firmly in this lead grid
...
Why negative plates are one more than positive plates?
Negative plates are one more than positive plates so as to get negative plates on
both the sides of positive plates
...
The other reason is that both the
sides of positive plates will become active and the efficiency of the positive plate
and the cell will increase
...
What is electrolyte?
Electrolyte is the medium through which the current produces chemical changes
...
85 specific gravity (concentrated
sulphuric acid) diluted with distilled water in the ratio of 1:3 approximately, so the
specific gravity of the dilute sulphuric acid is 1
...
110
...
a
...
b
...
c
...
- 32 -
Question and answers
Electrical Maintenance Unit
111
...
a
...
Where n – total number of cells in
series and E – emf of one cell
...
The internal resistance ‘r’ also increases and equal to ‘nr’
...
Where R – external load
resistance
...
Total current is equal to one cell current
...
If
the internal resistance is negligible or less then current will be maximum
...
a
...
b
...
c
...
d
...
So we can understand that parallel useful when the external resistance is small as
compared to internal resistance of the parallel group
...
112
...
Mainly there are three types of charging of battery
...
Constant current charging system
...
Charging current = V – Eb / R + r amps
...
Constant voltage or potential charging
...
c
...
The continuous charging of a battery at a very low rate for keeping the battery
ready in good working condition is called the trickle charging
...
The value of the trickle charging current is
approximately 2% of the full charging current of the battery
...
What are the factors on which the capacity of the battery depends?
The capacity of the battery is measured in ampere-hour
...
a
...
b
...
A cell should not be discharged below 1
...
If it is
discharged below 1
...
c
...
Capacity decreases with increase rate of discharge
...
Specific gravity of electrolyte
...
When the specific gravity increases the capacity of the battery
increases
...
Quantity of electrolyte
...
f
...
The design of the separator should be thin
...
Temperature
...
114
...
Kirchhoff’s laws are used in complex network circuits to determine the equivalent
total resistance and the current flowing in various conductors of that circuit
...
a
...
b
...
Point law or current law
...
In other words the sum of the currents
flowing towards the junction or node or any point of network is equal to the total
current flowing away from that junction
...
The mesh law states that, in any closed electrical circuit the algebraic sum of the
potential drops is equal to the sum of the impressed emf’s acting in that close
circuit
...
115
...
a
...
b
...
116
...
a
...
b
...
- 34 -
Question and answers
Electrical Maintenance Unit
c
...
d
...
T
...
R
...
V
...
e
...
- 35 -
Question and answers
Electrical Maintenance Unit
117
...
a
...
b
...
c
...
d
...
e
...
118
...
For the same capacity alternators are cheaper than DC generators, because
alternator is not having commutator arrangement and there by small in size
...
Alternating current produces pulsating magnetic field and there by it posses the
property of inductance and capacitance
...
Alternating current can be step-up or step-down by static transformer
...
AC can be transmitted with very less cost in comparing to DC transmission
...
Alternating line losses are very less comparing to DC line losses
...
An alternators and AC motor requires very less maintenance
...
Charge per unit for AC is less than DC
...
Define AC
...
120
...
121
...
Its symbol is ‘T’
...
So the periodic time T = 1/50
seconds
...
122
...
123
...
124
...
- 36 -
Question and answers
Electrical Maintenance Unit
125
...
Average value = 0
...
What is root mean square value (R
...
S)?
The R
...
S value is also known as effective value or virtual value
...
Then divide to get the average with the number of instantaneous values
and find the square root of this average to calculate the R
...
S value of the emf or
current
...
M
...
R
...
S value = maximum value / √2 = 1/√2 = 0
...
∴ R
...
S value or effective value = 0
...
What is form factor?
The ratio of the R
...
S value to the average value is called the form factor
...
707 * Emax or Imax : 0
...
707 * Emax or Imax / 0
...
11
So that R
...
S value = average value * 1
...
M
...
11
128
...
M
...
So the
crest factor = maximum value / R
...
S value
...
414
129
...
Examples are force, emf, current etc
...
Examples are
temperature, mass, volume etc
...
What is phase?
- 37 -
Question and answers
Electrical Maintenance Unit
The development of an AC quantity through different stages is known as Phase
...
131
...
Here between those quantities there is no angle
...
What is out of phase?
When two alternating quantities voltage and current do not reaches their maximum
and minimum values simultaneously, then they are called out of phase
...
What is phase angle?
Phase angle is an angular displacement between two alternating quantities
...
134
...
135
...
136
...
137
...
138
...
I = V/R amps
...
(Where cosϕ is zero because the voltage and current
are in-phase
...
What is the relation between voltage and current in AC circuit containing only
inductance?
Current (I) is lags behind the voltage by 90°
...
XL = 2πfL ohms
...
(Where cosϕ is 90 because current lags behind voltage by
90°
...
- 39 -
Question and answers
Electrical Maintenance Unit
140
...
I = V/XC amps
...
P = I * V * cosϕ watts
...
So cosϕ 90° = 0)
∴ P = I * V * 0 = 0 watts
...
What is inductance and inductive reactance?
Inductance
A coil carrying alternating current produces an alternating flux, which causes to
link with same coil and produces an emf in the coil, which opposes the applied
emf
...
The unit for measurement is henry
...
The unit for
measurement is ohm
...
What is capacitance and capacitive reactance?
Capacitance
The property of a capacitor to store electrical energy in it, when it is connected to
an electric supply is called capacitance
...
Capacitor
store an electric energy in the unit of charge and the unit of charge is coulomb
...
The unit for measurement is
ohm
...
What is impedance?
The total opposition offered by an AC circuit for the flow of current through it is
called Impedance
...
∴
Z = √ R2 + (XL ∼ XC) 2
Z = √ R2 + (X) 2
Where ∼ indicates the difference of XL and XC and denoted in the letter X (net
reactance of the AC circuit)
...
What is ohm’s law for AC circuit?
I = V/Z amps
...
V = I * Z volts
...
What is the current and power in an AC circuit?
Current
AC circuit contains resistance ‘R’ and reactance ‘X’
...
Because resistance current (IR) is in-phase with
voltage (ER)
...
Because reactance current will lead or lag the
voltage (ER) by 90°
...
So that circuit
current I = √ (I cosϕ)2 + (I sinϕ)2 amps
...
Because I sinϕ is not taking any energy from the circuit
...
a
...
This true power is some times known as energy component or active
component or watt-full component
...
or this true power is the power
consumption of all source of electric circuit
...
Reactive power = E * I * sinϕ watts
...
c
...
The terminal voltage multiplied by the actual resultant current (I) is called the
apparent power or volt-ampere or VA
...
What is power factor?
So from the above power explanation,
Cosϕ = true power / apparent power = E * I * cosϕ / E * I
...
Cosine of angle of lead and lag of the resultant current with the applied voltage
...
The ratio of R/Z
...
The ratio of true power to the apparent power
...
What is resonance in series circuit?
If in an AC circuit inductive reactance XL and capacitive reactance XC is equal the
voltage across both will be equal and are 180° out of phase
...
If we are in a position to alter the frequency of supply voltage at a particular
frequency named as ‘resonant frequency’, AC series circuit’s XL = XC and the net
reactance will be zero
...
Because the controlling component of the circuit is resistance only and the current
is maximum and equal to V/R amps
...
148
...
Q-factor
= 1 √L/C
R
149
...
It is denoted by the letter
‘Y’ and the unit of measurement is mho
...
Equation used in admittance
a
...
b
...
c
...
d
...
150
...
That is 2πfL = 1/2πfC
...
If ‘R’ is negligible, then (FR) = 1/2π√LC cycles per second
...
What is poly phase?
A system with two or more the two phases is known as poly phase system
...
What is phase sequence?
- 43 -
Question and answers
Electrical Maintenance Unit
The sequence of attaining the maximum value of the induced emf in each set of
winding among those three sets is known as phase sequence
...
153
...
- 44 -
Question and answers
Electrical Maintenance Unit
154
...
155
...
156
...
157
...
Unbalance load
If the three-phases have different power factors and the phase current, then the load
is called the unbalance load
...
What is phase power and total power?
Phase power
The power measured between a phase and neutral is known as phase power
...
159
...
a
...
b
...
160
...
IL = IPh
...
VL = √3 VPh
...
Note: in star connection we are getting neutral point and we can able to measure
the phase as well as line voltage
...
VL = VPh
...
IL = √3 IPh
...
- 45 -
Question and answers
Electrical Maintenance Unit
- 46 -
Question and answers
Electrical Maintenance Unit
161
...
In 3-ϕ system power ‘P’ = 3 * VPh * IPh * cosϕ watts
...
Substituting the value of IPh and VPh
in the above 3-ϕ power equation,
P = 3 * VPh * IPh * cosϕ watts
...
P = √3 * VL * IL * cosϕ watts
...
Substituting the value of IPh and VPh
in the above 3-ϕ power equation,
P = 3 * VPh * IPh * cosϕ watts
...
P = √3 * VL * IL * cosϕ watts
...
So cosϕ = P/ √3 * VL * IL
...
What are the advantages of rotating field system?
a
...
b
...
This intern
reduces the size of the machine
...
Out put current can be taken directly from the fixed terminals on the stator
...
d
...
163
...
a
...
b
...
c
...
Depending on rotation there are two types
...
Armature rotating
b
...
Depending on number of phases there are two types
...
Single phase
b
...
With respect to excitation there are two types
...
Self excited
b
...
- 47 -
Question and answers
Electrical Maintenance Unit
- 48 -
Question and answers
Electrical Maintenance Unit
164
...
What is voltage regulation?
166
...
167
...
a
...
Field copper losses (Ife2 rf)
...
b
...
Mechanical losses includes bearing friction, slip ring friction and friction due to
windage
...
When the efficiency of the alternator is maximum or on what factor the efficiency
of the alternator depends?
Efficiency of an alternator depends on its load power factor for a given load
...
The efficiency for given load is maximum only when the
power factor is unity and it decreases as the power factor fall
...
What are the methods of synchronizing?
a
...
Dark lamp method and bright lamp method
...
Synchroscope method
...
What is synchroscope?
Synchroscope is an instrument, which shows the phase relationship of emf of the
incoming alternator and at the same time it also indicates whether it is running slow
or fast
...
It
consists of a small motor with rotor and stator
...
A
potential transformer connected to two of the main bus-bar give supply to the stator
‘A’ winding and another potential transformer of same type connected to the
corresponding terminals on the incoming machine supply to the stator ‘B’ winding
...
That means the
both the voltages in winding ‘A’ and ‘B’ are same and there is no resultant flux to
rotate the rotor
...
- 50 -
Question and answers
Electrical Maintenance Unit
171
...
172
...
Electrical energy may be transmitted economically over long distance by
stepping up of voltages to reduce the line losses
...
To distribute the low voltages at consumer side by stepping down the voltages
...
Transformer is a static machine and losses are very less
...
b
...
173
...
It says that, when ever a changing
flux links with a coil an emf is induced in it and this induced emf is proportional to
the rate of change of flux and the number of turns in the coils linking the flux
...
What are the types of transformer core?
a
...
b
...
c
...
d
...
175
...
k = <1 in step down transformer, where secondary turns are less than primary and
low voltage for consumer use
...
What is the use of conservator in the transformer?
It is a drum type cylinder mounted on the top of the transformer through a small
pipe
...
To indicate the level of oil in the
- 51 -
Question and answers
Electrical Maintenance Unit
transformer an indicator is fixed
...
That is it limits the air with oil due to its less
surface area
...
What is the use of breather in the transformer?
Breather is a bottle shaped steel tube, which is attached to one side of conservator
to allow the air to pass in and out of the tank or conservator through the calcium
chloride and silica gel, which is filled in it to absorb the moisture contained in the
air
...
178
...
This relay is placed
in between the pipe connecting the conservator and the tank
...
It consists of two operating floats and is operated by two mercury switches
separately provided for the alarm and trip
...
179
...
180
...
That is in … of second
...
Average rate of change of flux = Qm / … f
...
= 4 f Qm Weber / second
...
But the rms
...
∴ rms
...
11 * 4 f Qm N volts
...
44 f Qm N volts
...
What are the losses in transformer?
In transformer there are losses due to,
1
...
2
...
Losses due to leakage reactance (leakage flux)
...
At loaded condition copper losses and leakage flux losses will exist in cosiderable
manner
...
- 53 -
Question and answers
Electrical Maintenance Unit
- 54 -
Question and answers
Electrical Maintenance Unit
182
...
Natural cooling
...
Air natural cooling (Dry type)
...
Oil immersed natural cooling
...
Oil immersed, forced oil circulation with natural cooling
...
Artificial cooling
...
Oil immersed forced air circulation with air blast cooling
...
Oil immersed blast cooling
...
Air blast cooling
...
Artificial cooling (water)
...
Oil immersed water cooling
...
Oil immersed forced oil circulation with water cooling
...
Mixed cooling (water)
...
- 55 -
Question and answers
Electrical Maintenance Unit
183
...
1
...
a
...
b
...
c
...
d
...
2
...
a
...
b
...
c
...
d
...
3
...
a
...
b
...
c
...
4
...
a
...
b
...
c
...
5
...
a
...
b
...
c
...
d
...
e
...
f
...
6
...
a
...
b
...
7
...
a
...
b
...
c
...
184
...
The vibrations are produced due to the change in polarity of an
alternating current or voltage and by the loose of lamination of the core
...
- 56 -
Question and answers
Electrical Maintenance Unit
185
...
1
...
a
...
b
...
2
...
a
...
1
...
• Capacitor start capacitors run motor
...
• Split face motor
...
Three phase motors
...
• 3φ double squirrel cage motor
...
• Slipring induction motor
...
Commutator motors
...
Single phase commutator motors
...
• Repulsion start induction’s run motor
...
• Series motor or universal motor
...
3φ commutator motors
...
• Compensated motor
...
What is the working principle of 3φ induction motor?
When 3φ supply is given to stator, a rotating magnetic field of constant magnitude
is produced
...
At the
initial moment the frequency of induced emf is equal to the frequency of the stator
supply voltage, when the rotor is stationary as in the case of secondary of a
transformer
...
In this case the cause
producing the rotor current is the relative speed between the rotating magnetic field
if stator and the rotor and is maximum when the rotor is stationary
...
The rotation of this rotor is
developed due tog the torque developed in the rotor by interaction between the
rotating magnetic field of stator and the field produced by the rotor current
...
What is torque?
As said above torque is a turning or twisting moment of a force about an axis and it
is measured by the product of force * radius at which the force acts
...
a
...
b
...
The letter ‘T’ denotes torque in induction motor and torque is proportional to
Ir φ cosϕr
...
That is T ∝ Ir φ cosϕr
...
Cosϕr = rotor power factor
...
What is slip?
The difference in speed of stator magnetic speed ‘Ns’
(synchronous speed) and rotor speed ‘Nr’ is called slip
or absolute slip and it is denoted by the letter ‘S’
...
Slip has no unit
...
5 to
2
...
In other words slip ‘S’ = fr / f
...
189
...
Inner cage
winding is of high reactance and low resistance
...
So the impedance of inner
cage winding is very high
...
That is a very high ratio of current
is passing through the outer cage winding at the time of starting and there by
produces very high starting torque
...
So that in
the running condition the reactance of the inner cage decreases to the lowest value
and hence the Impedance (XL = 2πfL)
...
So now
- 59 -
Question and answers
Electrical Maintenance Unit
inner cage winding produces more torque than outer cage at the time of running
and the motor running torque is good enough
...
Why starter is necessary to start the AC motor?
a
...
Therefore if motor is directly
started the supply voltage may be disturb
...
By the help of starters starting and stopping of
motors can be made easily as we required
...
c
...
d
...
e
...
191
...
Direct on line starter (air break) mechanically
...
Direct on line starter (air break or oil immersed)
electrically
...
Star delta starter
...
Slipring motor starter
...
Auto transformer starter
...
What are the speed control methods of induction motor?
a
...
b
...
c
...
d
...
193
...
If we are starting
the motor at this condition the motor get hesitated to
start or run due to the attraction developed between
those rotor and stator teeth or slots
...
This type of magnetic locking in
squirrel cage induction motor can be avoided either by
skewing the rotor slot or by selecting the rotor slot,
such that there is no common factor between the rotor
slot and stator slots
...
What is skewing?
Skewing can be done by turning the rotor slots about 15°
from the parallel position of slots with the shaft
...
195
...
Stator losses (stator copper losses,
losses)
...
Rotor losses (rotor copper losses,
losses)
...
Windage and friction losses
...
What is synchronous motor?
An alternator, which is running as a motor can be
called as synchronous motor and it runs at synchronous
speed
while
it
converts
electrical
energy
into
mechanical energy
...
- 62 -
Question and answers
Electrical Maintenance Unit
197
...
It’s a constant speed motor and is equal to
synchronous speed from no load to full load
...
It has good efficiency higher than induction motor
...
It can be run as a motor and also as an alternator as
per the requirement
...
Dis-advantages
a
...
Because
its speed can not be varied
...
As a motor it is not self-starting type and it can
not be started on load
...
It requires both AC and DC supply
...
Hunting is also produced in this motor
...
What are the applications of synchronous motor?
a
...
b
...
c
...
199
...
So the damper winding acts the magnetic lines of force
and causes to create the opposite torque, which keeps
the rotor in the same position of the particular load
...
To reduce this hunting damper winding
is helpful
...
What is synchronous condenser or phase advancer?
An over excited synchronous motor takes leading current
just like a condenser and gives leading power factor
...
201
...
- 63 -
Question and answers
Electrical Maintenance Unit
Such and alternating field is equivalent to two fields
of equal magnitude and speed rotating in opposite
direction
...
So the rotor can not rotate in
any direction
...
So a single-phase motor is not
self-starting
...
What are the methods to self-starting of single-phase
motor?
a
...
b
...
c
...
d
...
e
...
203
...
There are following
few methods of speed control
...
By changing the number of poles of stator
...
By changing the applied voltage to the stator
...
Frequency control method
...
Rotor rheostat control
...
By operating two motors in concatenation or cascade
or tandem method
...
By injecting an emf in the rotor circuit
...
By changing slip
...
What are the classifications of electrical measuring
instruments?
a
...
These instruments give the
value of the quantity to be measure in terms of the
constant of the instrument and their deflection only
...
b
...
Secondary instruments are
those, which are calibrated in comparison with some
absolute instrument so as to indicate the electrical
quantity to be measured with the deflection of needle
or pointer of that meter over a calibrated scale
...
What are the operating
measuring instruments?
a
...
b
...
c
...
d
...
e
...
f
...
principles
of
electrical
206
...
Indicating instruments
...
Recording instruments
...
Integrating instruments
...
What are the essentials of indicating instrument?
a
...
b
...
c
...
- 66 -
Question and answers
Electrical Maintenance Unit
208
...
Phase error: Field flux in induction meter does not
lag 90° behind the supply voltage due to its
resistance
...
b
...
c
...
d
...
It may be caused
due to incorrect friction compensator, stray magnetic
field, and excess voltage
...
This causes sufficient distortion of
the field to prevent rotation, when one of the holes
comes under one of the pole of the shunt magnet
...
What is illumination?
The quantity of a light emitted by a lighting source is
known as illumination
...
When a solid
or vapour is heated it begins to radiate energy in the
surrounding media
...
Lux is the
illumination produced by a uniform source of candle
power on the inner surface of a sphere of radius one
(1) meter
...
What are the laws of illumination?
a
...
ie E ∝ I
...
Inverse square law: The illumination of the surface
is inversely proportional to the square of the
2
distance of the surface from the source
...
c
...
- 67 -
Question and answers
Electrical Maintenance Unit
211
...
Nature of work
...
Determine the foot-candle illumination required after
studying the nature of work
...
c
...
212
...
Direct lighting: light directly comes from the source
to the surface
...
Indirect lighting: light reflects from the wall,
reflector or ceilings etc
...
Semi direct lighting: light comes through the shade
...
What are the properties of good illumination?
a
...
b
...
c
...
d
...
e
...
f
...
g
...
h
...
214
...
Incandescent lamps
...
Carbon arc lamps
...
Gas discharge lamps
...
What are the materials used in Neon sign tube lamps
for different colors?
Following are the materials used in neon sign tubes for
different colors
...
Reddish orange – Neon gas + Argon gas
...
Golden – Neon gas + Helium gas
...
By depositing fluorescent powder on the inner surface
of the tube varying colors in intensity can be made
...
What is the material used in florescent tube?
The fluorescent tube is filled with argon gas at law
pressure and some mercury after evacuating the tube
...
Initially the mercury is in the form of globules on the
inside of the tube surface
...
217
...
For traction purpose a DC series motor is most
important
...
b
...
c
...
- 69 -
Question and answers
Electrical Maintenance Unit
d
...
- 70 -
relays,
timer,
Question and answers
Electrical Maintenance Unit
218
...
Rectifier is a device which converts AC supply into DC
...
a
...
b
...
c
...
d
...
e
...
219
...
Core
...
Insulation of cable
...
Metallic sheath
...
Bedding
...
Armouring
...
Serving
...
What are the factors considered for selecting a cable?
Following factors considered for the selection of the
cable
...
System voltage
...
Condition of installation
...
Continuous current to be carried
...
Maximum operating conductor temperature (70°)
...
Ambient air temperature (40°)
...
Thermal resistivity of soil
...
Depth of laying
...
Short circuit current
...
Where ‘K’ is
constant (K = 109 for copper cables), ‘a’ is area and
‘t’ is time duration of short circuit in seconds
...
What are the advantages of high voltage transmission?
a
...
b
...
c
...
d
...
e
...
222
...
Radial distribution system
...
Ring distribution system
...
Grid distribution system
...
What are types of distribution of supply?
a
...
b
...
- 72 -
Question and answers
Electrical Maintenance Unit
224
...
Conductor material
...
Pole
...
Cross arm
...
Insulators
...
Strain insulator
...
Post insulator
...
Stay wire
...
Support with insulator and stay lightner
...
Write types of lightning arrestor
...
Horn gap lightning arrestor
...
Oxide film lightning arrestor
...
Pellet lightning arrestor
...
Thyrite lightning arrestor
...
- 73 -
Question and answers
Electrical Maintenance Unit
Motor, Generator and Exciter
...
What are the main classifications of alternator?
a
...
b
...
2
...
m
...
11 then emf generated
= 1
...
22 kd kc φm f Z
kf form factor
f
Frequency
Flux maximum
φm
Z
Turns per phase
kd
Breadth factor or distribution factor
kc
Coil span
3
...
4
...
For a constant load if frequency decreases motor current will increase and at the start
if voltage is more motor current also increases
...
What is the minimum voltage required for starting of 6
...
6
...
7
...
8
...
9
...
5 times the total impedance
...
What is the slip of an induction motor during normal running?
More than zero
...
What is the effect of increased load on power factor of induction motor?
Power factor of an induction motor increases with loading
...
Explain the behavior of generator when operating alone and operating parallel with
grid
...
a) The power factor of generator depends on load power factor
...
c) Governor decides the frequency of generator
...
Generator parallel with grid
...
It will remain practically constant as same as grid frequency
...
b) Increase in the excitation will not increase the terminal voltage
...
This reactive power supplies for
the magnetizing current of motors, transformers and etc
...
d) If generator is under excited it will draw leading reactive current from the grid
...
Draw and explain following
...
1
...
2
...
3
...
Leading power factor
Unity power factor
Lagging power factor
Ter
...
Load current
- 76 -
Question and answers
Electrical Maintenance Unit
b) Torque Vs slip characteristics of induction motor
...
Pull out torque (R = XL)
I2 – rotor current
Cosφ2 – rotor power factor
...
5 times)
Torque
Full load current
1
Slip
0
When motor is started from rest the slip is 1 (one) at time of starting and starting
torque is 1
...
As the motor accelerates slip reduces and torque
increases
...
At that
time torque is maximum (pull out torque)
...
Torque is zero when slip is zero, because there will be no relative motion between
stator magnetic field and rotor
...
- 77 -
Question and answers
Electrical Maintenance Unit
- 78 -
Question and answers
Electrical Maintenance Unit
14
...
Rotor – Hydrogen
...
15
...
These currents will induce emf in rotor and heat up the rotor
bars and cause breakage in them
...
16
...
Medium motors
Roller bearing at DE and deep groove bearing at NDE
...
Vertical mounted – face to face angular contact ball bearing at NDE and roller or ball
bearing at DE
...
1
...
2
...
3
...
1 Inch
...
What is the life insulation if temperature increased by 10°C?
The life of the machine insulation decreases by half if the temperature of the
insulation increases by 10°C
...
What is the value of vacuum maintained by vacuum pump in oil filteration machine?
27 Hg
...
What is the DC HV test voltage range?
1
...
5* rated voltage
...
What do you mean by term insulating resistance? How it is measured?
Insulating resistance: insulating resistance is the opposition offered by an insulating
material to the flow of current (electrons) through it when an high potential is
applied across it
...
First the equipment whose resistance is to be measured is disconnected from supply
...
So we have to discharge it by connecting a wire between the terminals and ground
for 15 minutes
...
After this we should remove the wire and we have to connect megger terminals (live
& earth) to the motor terminal and earth
...
Then rotate the megger at rated speed of 160 rpm and take the readings
...
What you mean by dielectric absorption test?
Whenever we apply a potential from the megger to test the IR value, initially the
needle of the megger will go to low value of the resistance
...
Because in the insulating material there is
strain on the molecules when the potential is applied
...
The negative charges are attracted to positive
terminal and positive charges are attracted to negative terminal
...
This aligning may
take more time
...
I
(µ Amps)
(A)
(B)
Time
If the insulation is good the graph is as shown as B and if there is dirt, moisture the
graph will flatten early as shown in A
...
- 81 -
Question and answers
Electrical Maintenance Unit
9
...
At this point the heaters are
Switched off
...
Temp
...
But on temperature increases the IR value starts to decrease
because the moisture entrapped in the coils are released due to rise in temperature
and this causes the IR value to go down
...
Then comes the point where all the moisture is released and then will be no decrease
in IR value or rise in the temperature
...
At this point the heaters are switched off
...
This is the third stage
...
The insulation resistance of a DC motor is observed to be 15 MΩ at a temp
...
what is its value corrected to 40°C
...
0
...
Temperature correction factor – 8
...
Rm = 8 * 15
Rt – resistance measured at +°C
...
Rm – corrected value to 40°C
The IR of DC motor corrected to 40°C is 120 MΩ
...
The armature of a 600 kW, 0
...
of 30°C
...
Correction factor for 30°C is
0
...
Data given are
kV – 0
...
- 30°C
Correction factor – 0
...
24 + 1
= 1
...
5
So value corrected to 40°C = Rm = kt * Rt
= 0
...
It should be sent for IR re-conditioning
...
What are the advantages of motorised valve actuators?
Advantages
a
...
b
...
c
...
The
motor has high torque
...
What are the four basic parts of any valve actuators?
a
...
b
...
c
...
d
...
3
...
It engages on a worm wheel
...
This dog teeth engages or hits the dig teeth on moving or sliding
clutch
...
So
when the sliding clutch rotates the valve also rotates simultaneously
...
Do you require separate limit switch for closing and separate limit switch for
opening? Ans
...
5
...
6
...
7
...
a
...
When torque function is selected?
Limit function – when limit function is selected in rotork valve, the limit switches
will operate when the limit set points are reached
...
Torque function – when torque function is selected the torque switches will act when
set point is reached
...
- 84 -
Question and answers
Electrical Maintenance Unit
8
...
For indication normally open contacts (NO) of opposite
direction of valve should be used so that when valve fully closes open indication
contact should remain NC only and vice-versa
...
What indication will you get in control room when
a) valve is open – green
b) valve is closed – amber
c) valve is intermediate – both
d) valve is closing and torque switch operate – both
10
...
This is
use full when valve is stuck in fully open or closed condition
...
How will you proceed to operate the valve manually after an electrical operation?
After electrical operation to operate manually we must tilt the lever provided on the
actuator to hand (manual) position by which the sliding clutch gets engaged with
hand drive
...
Know the setting procedure for all valves
...
Know the control and power circuit diagram of electrical motorised valve
...
What is the colour of silica gel in dry stage and when saturated with moisture in a
breather?
Dry stage – Deep blue
...
2
...
3
...
4
...
5
...
6
...
a
...
b
...
Iron losses – Iron losses constitutes of two losses
...
Eddy current losses, these are due to the induced emf in the core, which
constitutes a current in the core
...
Eddy current losses can be minimised by using laminated core immersed in
varnish
...
b
...
Hysterisis losses can be minimised by selecting proper magnetic material, like
silicon steel
...
These losses are depends on load
...
r – resistance of winding
...
- 86 -
Question and answers
Electrical Maintenance Unit
7
...
Voltage regulation is the difference between no load voltage and full load voltage by
no load voltage
...
Voltage regulation is mentioned in % (percentage)
...
Efficiency of a transformer is the ratio of output in watts and input in watts
...
% Efficiency = Output in watts * 100 / Input in watts
...
8
...
Conservator: This allows for shrinkage and swelling of transformer oil
...
When cools down it goes back to
main tank
...
Breather: It provides dry sir to conservator when transformer breathes
...
The
moisture is absorbed in breather by silica gel
...
There is
heating up of oil, which breaks down and gases are released
...
Also if there is a short circuit, the buchholz relay will trip the transformer
...
Gases can be taken from the relay to identify nature of fault
...
Core: To provide low reluctance path for the magnetic lines of force
...
HV Winding: High voltage is given to HV winding and low voltage is taken from
the LV winding
...
Cooling tubes: These are provided to cool the transformer oil so that the heat of oil
will be given to the atmosphere
...
LT bushing: Carries the LV terminals
...
There
are two types of tap changer
...
- 88 -
Question and answers
Electrical Maintenance Unit
9
...
Properties of transformer oil are,
a) Colour – pale yellow
...
c) Acidity (KOH/gramme of oil) – <0
...
d) Dielectric strength – limit value is 45 kV and preferable value is 60 kV
...
f) Inter surface tension – 30 to 40 dynes / cm or 0
...
4 Newton
...
How explosion vent works?
Explosion vent is provided on the transformer tank to relieve pressure if the pressure
in the transformer exceeds 5 psi
...
One at top and another at bottom
...
Wire meshes are provided below the bottom diaphragm and above
the top diaphragm
...
There is an oil level indicator provided above the bottom diaphragm
...
A ruptured diaphragm must be immediately replaced
...
11
...
Silica gel breather is used in a transformer to provide dry atmospheric air to the
conservator when transformer breathes
...
The inner container contains silica gel, which absorbs moisture
...
Also it will trap dust and dirt entering the
breather
...
After it gets saturated with
moisture it will turn into white pink
...
When the silica gel is
saturated with moisture it must be replaced or regenerated or recharged
...
- 89 -
Question and answers
Electrical Maintenance Unit
12
...
Tap changer (ON LOAD type) can be used to increase or decrease transformer
output voltage without break in the voltage to the load
...
The odd taps are taken on one side and
even on another side as shown in figure
...
Transition resistors are provided to limit the current when the windings are shortcircuited by the diverter switch
...
the position of diverter
switch is shown
...
We want to increase voltage to 420 V at tap 5
...
The diverter switch
switching time is very high
...
The whole assembly is immersed in oil
...
But in HT side current to be handled is lesser
than LV side
...
Explain the procedure for finding out the vector group of a transformer
...
See from nameplate what group the
transformer belongs
...
Take the IR value between
a) HV and LV with body grounded
...
b) HV and body with LV grounded and
c) LV and body with HV grounded
...
Measure voltage between
a
...
b
...
c
...
d
...
Draw the diagram of Dy11 and check that the readings got are correct
...
C2
B2
C2 c2 = B2 b2 = B2 c2
...
That is C2 b2 >> C c2
...
A2 / a2
For Dy1 transformer
B2 c2 will be greater than B2 b2, C2 c2, C2 b2
...
c2
C2
b2 B2
For Yy0 transformer
B2 c2 = C2 b2
B b2 = C c2
B b2 and C c2 will be lesser than B2 b2 and C2 b2
...
Explain the principle of working of impulse steam turbine
...
There will be no drop in pressure in impulse
steam turbine
...
Mention the four processes involved in Renkine cycle
...
4
1
Saturated vapour line
...
2 – 3 Constant pressure heat rejection
...
3 – 4 and 4 – 1 Constant pressure heating
...
What are the methods of removing moisture from turbine?
a
...
b
...
c
...
4
...
Capacity factor can be defined as net power produced by the plant divided by perfect
net power that can be produced in the plant
...
5
...
It reduces the steam inlet when turbine over speeds
...
What are the benefits of feed water heating?
a
...
b
...
- 92 -
Question and answers
Electrical Maintenance Unit
7
...
a
...
b
...
c
...
d
...
e
...
f
...
g
...
h
...
i
...
j
...
8
...
9
...
a
...
b
...
The water passes through the tubes
and steam passes through shell
...
10
...
It improves
efficiency of the turbine, as the heat rejection is less
...
11
...
12
...
Heat rate = Heat supplied in Btu / Power output in kWh
...
What is the purpose of gland steam system?
Gland steam system is provided to arrest the steam leak from the turbine and to
protect the air ingress into the turbine
...
Explain the main difference between impulse and reaction turbine
...
In this type the heat is
added is very less
...
There is reduction in pressure with the increase in kinetic energy
...
What is meant by hydrodynamic film lubrication?
In high-speed turbines the lubricating oil will be at the sides and there is metal to
metal contact when turbine at rest
...
When turbine finally achieves its
speed the oil film breaks the barrier and it takes the load on itself
...
16
...
17
...
This is a contact type
feed water heater
...
Why non-return valves are provided in the steam extraction lines?
Non-return valves are provided because when the turbine trips there will be an
instant drop in pressure inside the turbine
...
These will rush in to the turbine and overspeed
will be there in turbine
...
19
...
20
...
When we add more heat to a
substance
...
This added heat
is measured as sensible heat
...
This is latent heat
...
- 94 -
Question and answers
Electrical Maintenance Unit
21
...
- 95 -
Question and answers
Electrical Maintenance Unit
22
...
Seal oil is at higher pressure than
hydrogen
...
What are the base load and peak load power stations?
During certain periods the load demands are very high
...
During these time certain power plants like thermal plants gives
this extra power required
...
But during the rest of period that is when there is no peak power demand there are
some power stations, which cater to the base load always runs giving power to the
grid
...
These stations can not be
easily stopped or restated
...
24
...
Throttle governing system: In this a valve (just like tap water controlling) which
reduces the steam pressure controls the steam flow
...
Nozzle governing: In this the steam floe is reduced but the pressure remains the
same
...
This is efficient way of governing
...
How turbine oil purification is achieved?
Oil purification is achieved by centrifugal operation
...
Why morpholine is used?
Morpholine is used for pH control
...
By which material condenser tubes are made?
Condenser tubes are made up of aluminium brass
...
How boiler level is controlled?
Feed water control valves controls boiler level
...
Where magnetic filter is used?
Magnetic filter is used in stator water lines to remove magnetic particles from the
DM water
...
What is the purpose of accelerator governer?
The purpose of accelerator governer is to cut of steam momentarily when large
electrical loads are taken to prevent turbine speeding up
...
What is the use of jacking oil pump?
Jacking oil pump is used to initial lifting of turbine rotor by hydrostatic lubrication
...
What is the use of supplementary oil tank?
Supplementary oil tank is used to collect the oil drains from the CIES valves
...
How dearator pressure is maintained?
Dearator pressure is normally maintained by extraction steam
...
How dearator pressure is maintained after turbine trip?
Pegging steam is used to maintain dearator pressure after turbine trip
...
Where trust bearing is provided?
Trust bearing is provided between HP turbine and LP turbine rotor
...
How dissolved oxygen control is achieved in feed water system?
Hydrazine is added to feed water system to control dissolved oxygen
...
How seal oil pressure is maintained?
Differential pressure regulator maintains the seal oil pressure at 0
...
- 97 -
Question and answers
Electrical Maintenance Unit
14
...
- 98 -
Question and answers
Electrical Maintenance Unit
15
...
16
...
17
...
18
...
19
...
20
...
21
...
- 99 -
Question and answers
Electrical Maintenance Unit
Electrical system
1
...
Main output system
...
Generated voltage
is stepped up to 220 kV from 16
...
b
...
This system supplies the load inside the power station
...
6 kV and 415 V from 16
...
2
...
Main generator, Generator transformer, PT, CT, CVT, lightning arrestor, wave trap,
main 220 kV bus, transfer bus, SF6 circuit breakers and isolators, line protection
scheme, GT and Generator protection scheme, bus bar protection scheme etc
...
Why earth switches are provided in 220 kV bays?
When bay CB trips, both end (station and grid) CB will trip
...
4
...
To indicate if line is charged or not
...
To synchronize grid with generator
...
For power line communication and carrier tripping
...
What are the protections provided for 220 kV lines and bus bars?
a
...
b
...
c
...
d
...
6
...
6 kV buses?
a
...
6 kV from the
generator
...
Start up transformer, which steps down the grid voltage to 6
...
7
...
6 kV buses
...
Auxiliary transformers
...
PHT motors
...
BFP motors
...
e
...
g
...
CCW motors
...
Pressuring pump motors
...
What type breakers are provided in 6
...
Make, SF6 gas, 1250A and 2000A capacity circuit breakers are provided in
6
...
9
...
6 kV and 220 kV
breakers?
When lock out signal comes to 6
...
Where as in
case of 220 kV breakers the breaker will not trip
...
If it is in closed condition it will be closed
...
For how long 220 V DC batteries can supply power UPS?
220 V DC batteries can supply Power UPS for 30 minutes
...
11
...
12
...
Through control UPS 240 V AC
...
Through control UPS 220 V DC backed by 220 V batteries
...
Through control UPS 48 V DC backed by 48 V batteries
...
What are the lighting systems adopted in KGS?
There are two systems
...
Normal lighting with class IV power supply
...
Emergency lighting with class II power supplies and in control room with class I
power supplies
...
What are the two main classifications of analog instruments?
The two main classifications of instruments are,
a
...
Example tangent galvanometer
...
Secondary instruments
...
Analog instruments are
classified according to their electrical quantity they measure
...
Principles they work are moving coil, induction
...
What are three types of secondary instruments?
The three types of secondary instruments are,
a
...
Example:
Ammeter, Voltmeter, Frequency meter etc
...
Integrating type: It integrates (sums up) the quantity being measured
...
c
...
Example: 3 pen graphical recorder
...
Give three operating forces acting on indicating instruments
...
Deflecting force
...
Controlling force
...
Damping force
...
What are the advantages of digital instruments over analog instruments?
a
...
b
...
5
...
a
...
b
...
2 micro Ω to 11 Ω
...
Megger – Insulation resistances more than 100 kΩ
6
...
Tan delta
is angle between current due to surface leakage or current due to capacitance and the
capacitive current
...
7
...
To find the ratio of a transformer
...
To find the phase angle deviation of primary and secondary voltage of
transformer
...
To find the magnitude of magnetizing currents
...
Draw connection diagram of ammeter, voltmeter, energy meter, and wattmeter
...
Illustrate how can you use a single-phase wattmeter to measure 3 phase reactive
power in a circuit?
We can measure reactive power of 3 phase circuit by dingle phase wattmeter by
connecting the current coil in series with a line or load and connecting the pressure
coil across the other two lines
...
10
...
Alternator
A/D converter
BCD counter
decoder & LCD display
Vx
Alternator: It reduces the unknown voltage to a small value
...
A/D converter: It converts the analog signal from alternator to digital signals
...
Decoder and display: It decodes the binary code to decimal form and gives a visual
display of it
...
Draw a neat sketch and explain the use of CT and PT for measurement of power in a
single-phase circuit?
If wattmeter of proper range is not available or if voltage and current ranges are high
we can usr CT and PT of suitable ratio
...
The reading of wattmeter can be multiplied by the ratio to get the actual power
...
Explain the construction and working of moving coil instrument
...
The coil is placed between poles of a permanent magnet
...
There are two phosphor bronze springs of very less resistance
...
It is also serves the
purpose of control force
...
There is
a knife edge pointer and a counter weight to avoid its sagging
...
Moving coil meters work on this principle
...
That is FαI
...
13
...
14
...
B
P
kG
Q
A
I1
I2
I1
G
I2
X
D
S
C
Wheat stone’s bridge works on kirchoff’s law
...
P and Q are fixed standard resistances
...
X is the unknown resistance
...
No current will flow through galvanometer if the potentials across
its terminals are equal
...
This condition
is called the balanced condition
...
At balanced condition VAB = VAC And VBD = VCD
I1*P = I2*X & I1*Q = I2*S
Dividing both I1*P = I2*X
I1*Q = I2*S
- 106 -
Question and answers
Electrical Maintenance Unit
= P/Q = X/S
Unknown resistance (X) = P/Q * S Ω
15
...
- 107 -
Question and answers
Electrical Maintenance Unit
Protective relays and application
1
...
2
...
3
...
Ratio error
...
Phase angle error
...
Where core balance CT is used?
Core balance CT is used in earth fault protection
...
Define knee point voltage of a CT
...
That voltage is the knee point voltage
...
kpv decides the opening range of the CT
...
kpv = RCT + RLEADS + RRELAY
6
...
5 – composite error (Phase angle error + ratio error) 5%
...
10 – Accuracy limit factor
...
Mention the important properties of relay contacts
...
Should be robust in construction
...
Self-cleaning (oxides easily breakdown)
...
Corrosion resistant
...
Bounces free and striction free (low contact resistance)
...
Able to carry rated continuous current and short time rated current
...
What is a composite error and write down the formula for composite error?
Basically composite error = Ratio error + Phase angle error
...
Composite error =100 * 1
T
oς
(kn * Is – Ip)2 dt
- 108 -
Question and answers
Electrical Maintenance Unit
T
Ip
- 109 -
Question and answers
Electrical Maintenance Unit
9
...
Pickup value: It is the smallest value of actuating quantity when its value is increased
from zero to pickup value, the relay will energise
...
10
...
0–5A
A
CT
V 0 – 300V Sec
Primary
240 V AC
Variac
kpv = RCT + RLEADS + RRELAY
V
Saturation
Knee point
Ankle point
A
Connect the circuit as shown
...
Increase it to 5 Volts and
take down the value of current from the ammeter
...
5 V) and take the current reading
...
5 V +0
...
05 V) and note down the current
...
At some value there will be 50% increase in
current for 10% increase in voltage
...
2 A
40
...
3 A (0
...
3 A)
...
From this point onwards a
little increase in voltage will lead to a large increase in current, because the core is
saturated fully
...
Protective relays operate between
ankle point and knee point
...
Measuring CT operate in the ankle region
...
Explain the procedure for finding out the polarity and ratio test in a CT with circuit
diagram
...
Now momentarily close the switch S and see the deflection in the
AVOmeter
...
If it is in opposite direction the polarity of CT is not correct
...
Ratio test: Connect circuit as shown in figure (2)
...
Take
down the readings of A1 and A2
...
Ratio = A1/A2
...
Explain the principle of operation of attracted armature relay with equation and
characteristics curve
...
When we place a
magnetic material inside the coil it is attracted
...
The spring force and magnetic force oppose each other
...
At verge (time) of pickup
Instantaneous select
2
k1 I = k2
f = k1 I2 = k2
Time
Time delay select
I = k 2 / k1
Where f – force
...
k2 – spring tension constant
...
- 111 -
Question and answers
Electrical Maintenance Unit
We can see the inverse characteristics from the above formulae
...
That is there is no intentional (fixed) time delay
...
- 112 -
Question and answers
Electrical Maintenance Unit
13
...
Commissioning checks on
CT
a) Terminal marking correctness
...
c) Insulation resistance between primary and secondary
...
e) Magnetization characteristic and knee point voltage test
...
PT
a) Terminal markings
...
c) Insulation resistance between primary and secondary
...
e) Ratio test
...
RELAYS
a) Pickup and dropout value check
...
c) Time delay (if relay is not instantaneous), operating time value check of relay
...
e) Continuity checks of contacts after energisation of relay
...
g) See if CT’s and PT’s are corrected in correct polarity
...
i) Primary injection test
...
- 113 -
Question and answers
Electrical Maintenance Unit
14
...
In core balance CT all the three phases go through the core and the resultant
magnetic flux is zero
...
So the
secondary output of CT is zero and the relay will not energise
...
Saturation is no problem because the core size is very big
...
RYB
During earth fault
...
Question and answers
Electrical Maintenance Unit
Logics and circuits
1
...
27C
Closing circuit supervisory relay
27T
Tripping circuit supervisory relay
3C
Interposing relay (closing coil)
3T
Tripping coil
52
AC circuit breaker
88
Auxillary motor (spring charging motor)
52Y
Anti-pumping relay
86
...
What is the operating voltage of 3C?
48V DC
...
DC relay coil or contactor coils must be connected to which side?
Negative side of the DC supply to avoid galvanic effect on the coil, which will corrode the coil
...
How special current limiting resistance is connected with the seal in contact?
Special current limiting resistance is connected in series with the seal in contact
...
How you will connect start and stop push button to control the motor from two different places?
Start push button should be connected in parallel and stop push button in series in the circuit
...
What are the basic principles of ED?
Basic principles of ED are,
a) All the contacts of corresponding relays and contactors are shown in de-energised condition
...
c) Power circuits are drawn in thick lines and control circuits are drawn in thin lines
...
e) Auxillary contacts acts with main device such as contactors and relay
...
What is anti pumping?
When a breaker is closed on fault condition there will be continuous tripping and closing of the
breaker because 3C is energized
...
- 115 -
Question and answers
Electrical Maintenance Unit
Switchgear
1
...
Breaker
a
...
b
...
c
...
Bus bars
a
...
b
...
c
...
2
...
a
...
b
...
c
...
3
...
If both close and trip signal is
present at same time (instant) the breaker will attempt to close and positively trip
...
4
...
Also it would not be very fast
...
It gives uniform timings irrespective of the operator and its action is fast
and closing and tripping time is very less
...
What are the built in protections provided in 415 V breaker?
a
...
This is placed under the lower
current terminal pole
...
It has a core that
rotates in the air gap
...
During protection the magnetic forces
developed overcome the spring tension and the core is attracted
...
This protection acts during the closing
of breaker if any fault exists
...
b
...
This protection acts when any fault comes during breaker in service
...
c
...
It is placed in front of the breaker
...
- 117 -
Question and answers
Electrical Maintenance Unit
MCC
1
...
Starters and contactors all will be a standard size
...
Spares inventory will be less
...
Cost of cables will be less because same size of cable is used for all capacity and
gland holes can be provided before hand
...
Maintenance and trouble shooting is easier in MCC
...
What type of motor starting adopted in MCC?
DOL (direct on line) starter
...
What is the purpose of grounding secondary of the control transformer?
To protect the operating personnel from high induced voltage
...
Based on what factors will you select rating of components for a starter cell?
Factors for selecting rating of components are,
a
...
b
...
c
...
d
...
e
...
5
...
Ensure that the load is tripped from control room and switch is in off position
...
b
...
c
...
d
...
e
...
f
...
g
...
h
...
i
...
j
...
k
...
l
...
m
...
- 118 -
Question and answers
Electrical Maintenance Unit
Maintenance on MCC panel
...
Ensure the permit and all isolations
...
Open the bus bar chamber and discharge the bus bar
...
Disconnect the cables connected to bus bar and take the IR value of bus bar and
cable individually
...
d
...
e
...
f
...
g
...
h
...
i
...
j
...
k
...
l
...
6
...
Auto reset relay closes its contacts when the bimetallic strip gets cooled
...
L & T type
OLR have only manual reset and siemens type has both manual and auto facility
...
Auto reset over load relay is reset by switching OFF the respective had switch
and again switching it ON
...
Manual over load relay is reseted by pushing the reset button provided on the
MCC cell
...
How you will prepare electrolyte for a lead acid battery?
While preparing electrolyte for lead acid battery sulphuric acid is added to distilled
water
...
How battery capacity is expressed?
Always battery capacity is expressed in Ampere – hour
...
What is the instrument used to measure the specific gravity?
The instrument used to measure the specific gravity is called Hydrometer
...
What you mean by SCR?
SCR is meant for silicon controlled rectifier
...
Define specific gravity and mention the specific gravity of a fully charged lead acid
battery?
Specific gravity of a substance is the comparison of density of the substance with the
density of pure water
...
= kg / cm2
kg / cm2
= (No unit)
Specific gravity is only number
...
Specific gravity of pure water is one
...
215
...
Corrected specific gravity is equal to indicated specific gravity ± (t - 27°C)*0
...
Indicated specific gravity = 1
...
6
...
Battery container
...
Battery cover
...
Positive plate (Pb o2)
...
Negative plate (Pb)
...
Cell connector
...
Grid
...
Cell separator (porous material)
...
Sediment chamber
...
Positive and negative terminals
...
Vent plugs
...
Dilutes sulphuric acid (electrolyte)
...
What are the indications of a fully charged cell?
a
...
This can be seen only if the
battery has transparent cover
...
Voltage per cell will be a 2
...
c
...
But the current is splitting up water to
H2 and O2
...
8
...
That is once
the cell is discharged it cannot be recharged
...
The cells can supply only low currents and have low efficiency
...
Their internal resistance is more
...
Secondary cell: These cells are chemically reversible
...
They can supply large currents because their internal resistance is less
...
These can supply constant
current
...
9
...
This Pb so4 is
chemically reversible by passing current
...
But under certain condition crystalline lead sulphate is formed (Example: under
charging after some time without trickle charging)
...
So if the sulphation occurs the battery life decreases
...
10
...
Equation for Nickel cadmium battery
...
)
Ni (OH2) + Cd OH2 + kOH Ni (OH4) + Cd +2 kOH (during discharge)
We see that there is no change in electrolyte
...
So there is no
need to change the electrolyte
...
Write down the theory of lead acid battery
...
The positive electrodes are Pbo2 (lead oxide) and the negative
electrodes are Pb (spongy lead)
...
On discharging the battery
Pbo2 + Pb +2 H2 so4 Pbso4 + Pbso4 + 2 H2o
Lead acid is converted into lead sulphate
...
Only water is remain
...
On charging the battery
Pbso4 + Pbso4 + 2 H2o Pbo2 + Pb +2 H2 so4
Here the products are converted to their original constituents and the acid is released
...
How much ever larger be the cell
the voltage of each cell will be approx
...
15 V when fully charged
...
All positive plates are made common and
connected to a grid
...
These are also grouped together
...
The electrolyte is prepared by adding sulphuric acid to distilled water drops by drop
and stirring it until the reasoned specific gravity is attained
...
The cells of a battery are
connected by a cell connector
...
12
...
Synchronizing transformer for phase sequence: This gives the synchronizing signal
to the firing card
...
The firing
card gives pulses to the SCR of R or Y or B depending upon which phase is positive
maximum
...
The firing card
controls the firing angle of SCR
...
Controller card: This card monitors the output and gives signal to firing card to
conduct at certain angle to maintain constant output voltage
...
- 123 -
Question and answers
Electrical Maintenance Unit
13
...
The supply for the charger is from MCC
...
LC filter is used for suppressing surge voltage
...
The
SCR conducts only when gate gets positive pulse
...
Firing card gives pulse to the
respective RYB SCR only when their phases are positive maximum
...
The filter is provided to smoothen the ripple output and the bleeder is used for
voltage regulation
...
Also it keeps the SCR in conducting state by drawing the minimum current which is
higher than the SCR holding current
...
DC CT is used for limiting output current
...
There is also provision for smooth rising of output voltage
...
What is purpose of freewheeling diode and DC filter circuit in the charger?
Freewheeling diode is used to protect the semiconductor components used in the
charger from the back emf, which is induced in the inductive coils of relays when the
supply to the relays is cut off
...
This emf is shunted by the freewheeling diode,
which is connected in reverse bios with the output
...
Ripple frequency is same
as system frequency for half wave rectifier and 2 times of system frequency for full
wave rectifier
...
Thus the ripple will not be allowed to come to zero
...
Why indoor switchyard is provided in MAPS?
The salt contamination in the switchyard is avoided by providing indoor switchyard
in MAPS
...
This salt will get deposited on the insulators and on the
conductors used in the switchyard
...
So to avoid unnecessary trip and to have
low cost of installation and spare parts of aluminium indoor switchyard is used
...
What do ABCB and ACB mean?
ABCB – Air blast circuit breaker
...
3
...
4
...
In star connection line current is equal to the phase current and line voltage is √3
times that of phase voltage
...
5
...
6
...
Separately excited generator: In a separately excited generator the field poles have
some residual magnetism
...
This is fed to the field winding and if the current direction is such that it adds the
residual magnetic flux to the field winding and the field strength is increased
...
- 125 -
Question and answers
Electrical Maintenance Unit
Power and control cables
...
Define conductor
...
Examle
wires, cables, bus bars etc
...
Define unprotected insulated wire
...
3
...
Cable: One or more insulated conductor enclosed in a protective mechanical
sheathing of either GI wire or GI strip or aluminium to protect the insulation from
mechanical damage
...
Define insulated wire
...
5
...
Armouring is required to protect the cores from mechanical damage
...
Explain briefly about grounding of cable trays
...
A grounding wire runs at the side of tray through a parallel groove clamp through out
the length of the cable tray
...
This saves extra ground wire
...
What are the differences between the power and control cable?
Power cable: It is used for supplying current to load
...
Single core
is available upto 1000 mm2
...
These cables
are graded for higher voltages and possess more cross section area
...
These are of lower current carrying capacity and voltage grading is
also less
...
8
...
It prevents
corrosion of aluminium conductor due to oxidation and due to saline atmosphere
...
Why aluminium armouring for single core 1000 mm2 is used?
- 126 -
Question and answers
Electrical Maintenance Unit
2
Aluminium armouring for single core 1000 mm is used so that heating will not take
place due to the flux around the conductor, as the aluminium is a non-magnetic
material
...
What is resistance of copper compare to the aluminium?
Copper is less resistive than aluminium
...
What is applied over the steel tape armour of PILC cable as serving?
Bituminous covered jute
...
What is used as insulation for PILC cable?
Impregnated paper
...
A small quantity of impurity reduces how much of conductivity of copper?
35% of conductivity will be reduced due a small impurity in the conductor
...
Why cast aluminium tri-foil clamp is used in single core cables laying?
When three conductors are clamped together the fluxes around the conductors are get
cancelled
15
...
Check nut
...
Nipple
...
Metal washer
...
Neoprene rubber
...
Metal washer
...
Compression nut
...
What are the advantages of PVC insulated cable?
a
...
Joints can be made easily
...
As PVC is light the injury caused to it while laying is less
...
It is corrosion resistant
...
It has high fire retarding property
...
It does not break down even if moisture enters
...
How many types of logic gates are there?
a
...
b
...
c
...
d
...
e
...
f
...
g
...
- 129 -
Question and answers
Electrical Maintenance Unit
Mechanical equipment fundamentals
1
...
2
...
Heat is transferred from the fluid, which has higher temperature
to the fluid, which has lower temperature
...
Conduction – Heat transfer in solids by momentum of molecules
...
Convection – Heat transfer in liquids by movement of molecules
...
Radiation – Heat transfer by energy waves
...
Parallel flow
...
Counter flow
...
Transverse flow
...
Why baffle plates are used in heat exchanger?
Baffle plates are used in heat exchanger so that the maximum heat can be transferred
and to avoid tube sagging
...
How pumps are classified?
A
...
b
...
d
...
f
...
Rotating pumps
a
...
Gear type
c
...
Reciprocating pumps
a
...
Plunger
c
...
What you mean by cavitation and NPSH?
Cavitation: Bubbles form in the liquids whenever there is pressure reduction inside
the pump
...
This is called the cavitation
...
It is the head available at the eye of the impeller
corrected to vapour pressure
...
Name the functions of the valve?
a
...
b
...
c
...
d
...
e
...
7
...
Less holdup
...
No support needed
...
Any actuator can be used
...
Quick opening and closing
...
Less space required
...
Used for low-pressure low temperature and large pipelines
...
What are the classifications of industrial instrumentation?
a
...
b
...
c
...
2
...
3
...
4
...
5
...
6
...
7
...
Manometer, Diaphragm gauges, Bellows, Strain gauges etc
...
State some elements of flow measurement
...
9
...
Copper – constantan, Iron – constantan
...
What is the purpose of instruments?
The purpose of instruments is to measure, safeguard the process for efficient plant
operation
...
This accuracy and speed is not possible
by human
...
In such cases instruments provide remote
operation
...
What is primary element and what should be its response?
Primary element is one, which senses the condition of process, and converts it to
some other form, which can be measured accurately
...
- 132 -
Question and answers
Electrical Maintenance Unit
The response of primary element is that it should convert the condition in to some
other form, which can be interpreted and measured easily
...
Name some process variables, which are appropriate for our plant with examples
...
Flow – flow of D2O in PHT system
...
Temperature – temperature of coolant in PHT system
...
Level – moderator level
...
Speed – speed of turbine
...
Voltage – voltage generated by main generator
...
Neutron flux – number of neutrons produced in reactor during operation
...
pH – pH of moderator
...
What is use of 2/3 logic in our plant?
All our protection instruments (system) are triplicated to have following uses
...
To increase system integrity
...
To decrease faulty trips
...
Maintenance can be done on one protective instrument without shutting down the
whole system
...
So we
have triplication (2/3 logics) in protection instruments
...
Of only one operates there will be no trip
...
14
...
This uses the property that the resistance of a metal changes (increases
of decreases) with temperature
...
These will be a wire, which
will senses the temperature and changes its resistance as the temperature changes
...
Different types of RTD’s are Platinum, copper, nickel
...
What is recorder and how it is useful to our plant?
Recorder is an instrument, which gives instantaneous values as well as records the
values
...
It also gives us past
information recorded in it
...
- 134 -
Question and answers
Electrical Maintenance Unit
Fire fighting
1
...
They are fuel, heat and
oxygen
...
Combustion can not survive without these
three
...
So wherever fuel,
oxygen and heat is there together combustion takes place
...
How many types of extinction media’s are used in fire fighting?
a
...
b
...
c
...
d
...
e
...
f
...
3
...
Class A – Ordinary fire like burning of paper, wood etc
...
Class B – Oil fire like burning of petrol, diesel, LPG etc
...
Class C – Gas and dust fire like burning of butane, acetone, natural gas etc
...
d
...
e
...
4
...
Soda acid type – suitable for Class A type of fires
...
Foam type – suitable for Class A and Class B type of fires
...
Carbon-di-oxide type – suitable for Class B, Class C and for Class E type of fires
...
Dry chemical powder – suitable for Class B, Class C, Class D and Class E fires
...
Halons BCF (bromo chloro difluoro methane) – suitable for Class A, Class B,
Class C and Class E types of fires
...
At what areas of risk the Co2 flooding system, mulsifyre systems are provided?
Co2 flooding system is provided in diesel generator and turbine oil tank area
...
6
...
Double female adapter (1 No)
...
Delivery hose pipe (50 feet – 2 Nos)
...
Branch pipe (1 No)
...
Valve wheel (1 No)
...
A hose box key (situated in a cabinet at side of hose box)
...
How water is used in a fire?
Water is used as a cooling effect in a fire
...
How foam is used in a fire?
Foam is used as a blanketing effect in a fire
...
Which extinguisher you use for electronic equipment fire?
Co2 or DCP type fire extinguisher can be used on fire involving electronic
equipments
...
What you mean by starvation method?
Starvation method means elimination of fuel from the fire
...
What is the name of powder used in Dry Chemical Powder extinguisher?
Sodium-bi-carbonate
...
What you mean by cooling method?
Cooling method means elimination of heat from the fire
...
What you mean by blanketing method?
Blanketing method means elimination of oxygen from the fire
...
Why Co2 is used on Class E fire?
Co2 is a non-conductor of electricity
...
What is the golden rule of first aid?
Do first thing first, artificial respiration, stop bleeding and treat shock
...
2
...
3
...
4
...
5
...
6
...
7
...
8
...
9
...
Give warm drinks and rest to the patient
and artificial breathing if necessary
...
What is the instrument name used for accurate measurement of IP?
Infra red spectro photometer
...
How D20 vapour is recovered?
Dryer recovers D20 vapour
...
What is the amount of D20 used in moderator?
140 tonnes
...
Name the heavy water plants in India
...
Nangal
...
Kota
...
Baroda
...
Tuticorn
...
Talcher
...
Thal (under construction)
...
Hazira (under construction)
...
Malugum (under construction)
...
Define reactor grade and down graded D20
...
7% then the D20 is reactor grade D20
...
7% then the
D20 is downgraded
...
What precautions should be taken while working in high tritium areas?
Use respirators, plastic suits, VP suits if concentration of tritium is very high
...
If by chance there is tritium intake in the body drink lots of fluids
...
Why spillage of D20 is to be avoided?
Cost consideration: D20 very costly and very valuable
...
Tritium hazard: D20 contains tritium, which when spilled becomes tritiated vapour
and finds access through human body
...
It is a beta
emitter
...
What is ice plugging?
If there is a need to repair a valve of D20 PHT system, there are no other valves to
shut of D20
...
Then liquid nitrogen
is poured inside the bag
...
This is called the ice
plugging
...
Name the methods by which D20 leak can be detected
...
By beetles
...
D20 losses through stack monitoring
...
Tritium monitoring
...
In heat exchangers the leakage can be found by taking samples of process water
from all heat exchanges and counting the tritium activity
...
Name the D20 recovery methods
...
Manual mopping and vacuum cleaning
...
Active drainage recovery
...
Vacuum mopping recovery
...
Dryers recovery
...
Vapour recovery
...
Name the features for reducing D20 leaks
...
Reduce valves and fittings in the pipelines
...
Use welded joints instead of flanged joint
...
State the law of conservation of mass and energy
...
When mass is lost there is a substantial gain in
energy and when energy is lost there is a increase in mass
...
Which of the following material is good moderator? Why?
a
...
D2
c
...
D2O
e
...
Graphite
...
It has to thermalise neutrons effectively
...
There should be only a minimum absorption of neutrons
...
It should not be toxic
...
It should not be inflammable
...
A neutron
cannot meet the nucleus in a definite manner
...
In case of H2O it is a good scatterer but is absorbs neutrons
...
So it cannot be used as a moderator
...
It has a moderating ratio of 21000
...
In case of Beryllium it is a toxic material
...
Graphite absorbs neutrons and is inflammable and therefore cannot be used
...
3
...
After how many half-lives will it come
down to 625 millicuries?
Activity of an Iodine – 131 = 10 curies
To find number of half-lives for coming it to 625 millicuries,
10 * 1st = 5
(1st half-life)
5* = 2
...
5 * = 1
...
25 * = 0
...
- 142 -
Question and answers
Electrical Maintenance Unit
- 143 -
Question and answers
Electrical Maintenance Unit
4
...
Example – Hydrogen has one proton and its atomic number is one (1)
...
Mixture: It is a substance formed by different elements and these elements can be
separated by physical methods
...
5
...
00759 amu (atomic mass unit)
6
...
025 eV (energy volt)
7
...
- 144 -
Question and answers
Electrical Maintenance Unit
Nuclear system
1
...
To thermalise neutrons to maintain criticality
...
Emergency core cooling when PHT fails
...
Structural cooling
...
What is the cooling water used in moderator heat exchanger during normal
operation? And during class IV failure?
During normal operation process LP water is used in moderator heat exchangers,
which transfers heat to seawater
...
3
...
4
...
The
circulation is maintained by flywheel, which increases the de-acceleration and
maintains the flow for two minutes
...
5
...
Pressure tubes – zircalloy-2
Fuel cladding – zircalloy-2
End shield – stainless steel
...
6
...
Impeller – To reduce the water flow
...
Self-injection Hx – Cools the water in case of leakage
...
Gland supply – Cools the seals
...
Primary and secondary seals – sealing the pump
...
Thrust bearing – To take the axial thrust
...
What are the purposes of bleed condenser?
The purposes of bleed condenser are,
a
...
- 145 -
Question and answers
Electrical Maintenance Unit
b
...
- 146 -
Question and answers
Electrical Maintenance Unit
8
...
This is called the header level control and achieved by means of manual operation of
valves and shutdown cooling pump
...
In MAPS Unit # 1 end-shield cooling system has got heaters
...
After 30 years of operation this will rise to +32°C,
because of radiation exposure
...
MAPS 1
end-shield cooling system is operating at elevated temperature
...
- 147 -
Question and answers
Electrical Maintenance Unit
Reactor general
1
...
They are V1 and V2
...
V2 – Wet volume area
...
Rest of the areas in
the reactor building is V2 area
...
F/M vault is a V1 area and F/M maintenance bay is a V2 area
...
When the reactor is in
operation and if we open the F/M vault door, suppose of there is an accident in F/M
vault the pressure released directly goes to F/M maintenance area, which is a V2
area
...
2
...
That is stratified flow of D2O is obtained b inlet manifold
...
It is made of zircalloy
...
Why bi-directional flow is chosen for PHT system?
Bi-directional flow is chosen for PHT system because,
a
...
b
...
4
...
To permit access to F/M vault during shut down
...
To provide tight clamping for fuelling machines
...
To support the calandria tubes and also system
...
What is grounding?
It is an electrical connection with the general mass of earth through an earth
electrode
...
What is difference between earthing and grounding?
Both have same meaning
...
K
...
S
...
ground means earth
...
What are types of grounding?
There are two types
a
...
b
...
4
...
For instance
grounding of generator neutral, transformer neutral etc
...
What does mean by equipment grounding?
Grounding of non-current carrying metallic parts is called equipment grounding
...
Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc
...
Support structures, tower, poles etc
...
c
...
d
...
6
...
Earth fault protection is based on the method of neutral earthing
...
System voltage during earth fault depends on neutral earthing
...
It is a protection against arcing grounds, unbalanced voltages with respect to earth
and lighting
...
What is the important of equipment grounding?
Equipment earthing ensures safety
...
How safety could be ensured by equipment grounding?
In order to enumerate this, let us first find out the effects of current and voltage
developed during fault condition
...
What is the permissible body current limit?
The magnitude and duration of current conducted through a human body at 50 Hz
should be less than those did that cause ventricular fibrillation
...
These below given data are also applicable for current limits to human body
...
9 – 25 mA
Muscular contraction
25 – 60 mA
60 – 100 mA
Muscular contraction
Ventricular fibrillation
detect a slight tingling in his hand or finger
This is often termed as let go currents
...
These are threshold values, since 10
...
May be painful and can make it hard or
impossible to release energised objects grasped
by the hand
...
Ventricular fibrillation, stoppage of heart or
inhibition of respiration might occur and cause
injury or death if time is more than 1 sec
...
- 150 -
Question and answers
Electrical Maintenance Unit
10
...
For 50 kgs body weight: fibrillation current (IB) = 0
...
03 – 3
sec
...
Note = Above equation results = 116 mA for 1 sec
...
For 70 kgs body weight: fibrillation current (IB) = 0
...
and 496 mA for 100 sec
...
Above limit dictates that grounding should e such that current magnitude through
human body should not increase the specified values
...
1
...
At the depth of 600 mm in switchyard
...
b
...
c
...
Here depth is kept more because to care
for under grounding services
...
2
...
a
...
b
...
Why only 5 meters and 3 meters depths have been selected is that the
resistance is diminishes rapidly with the first few feet of driving, but less so at
depths greater than 2 to 3 meters in soil of uniform resistivity
...
3
...
Purpose of using
gravel is by doing steps 1,2 above tough and step potential are computed and
compared with tolerable potential and found as given below
...
a switchyard with crushed rock surface
230V
2165 V
Step 2
...
5 V
- 151 -
Question and answers
Electrical Maintenance Unit
11
...
12
...
Step and tough potential will increase more than maximum tolerable value
...
Under single line to ground fault the voltage to earth of the two healthy phases
rises from their normal phase to neutral voltage to full line voltage, which may
result in insulation break down
...
The capacitive current in two healthy phases increases ª3 times the normal value
...
The capacitive current in the faulty phase is 3 times its normal value
...
Experience shows that capacitive current in excess of 4 amps may be sufficient to
maintain an arc in the ionized path of the fault and this persistent arc phenomenon
is called ARCING GROUND, which ultimately cause high voltage build up
...
f
...
13
...
Equipment grounding is obtained by Grounding of non-current carrying metallic
parts equipment
...
Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc
...
Support structures, tower, poles etc
...
c
...
d
...
14
...
Grounding system: Comprises all interconnected grounding facilities in a specific
area
...
- 152 -
Question and answers
Electrical Maintenance Unit
15
...
These are
described below
...
Fall of potential method: This method is applicable for small grid or sub station
where induction effect of voltage is less
...
Measurement of earth resistance by 61
...
Alternate – 1 of fall of potential method: This method is influenced by induction
effect
...
Alternate – 2 of fall of potential method:
16
...
11Ω,
then following are recommended to reduce it
...
a
...
b
...
Sodium carbonate (Na2 CO3)
d
...
Soft choke and
f
...
17
...
Dry earth weights
about 1440 kg per cubic meter and thus 10% moisture content is equivalent to 144 kg
of water per cubic meter of dry soil
...
Below 20%, the resistivity increases very abruptly with the decrease in
moisture
...
What is the effect of salt content in moisture on resistivity?
The resistivity decreases and the salt content is expressed in percent by weight of the
contained moisture
...
19
...
Below 0°C the water in the soil begins to freeze
and introduces a tremendous increase in the temperature co-efficient, so that as the
temperature becomes lower the resistivity rises enormously
...
What does mean by neutral floating or neutral displacement?
- 153 -
Question and answers
Electrical Maintenance Unit
When a ground fault occurs, there is a tendency of neutral shift with consequent
change in voltage on the un-faulted phases
...
- 154 -
Question and answers
Electrical Maintenance Unit
21
...
The magnetic fluxes produced by the three phases in a multi core power cable
almost cancel put each other, since the vector sum of these currents at any instant
is zero and practically there is no residual magnetic flux around the cable
...
b
...
If the metallic sheath is at one
end the potential of the unearthed end could be much above the earth potential
...
c
...
d
...
Hence keeping above all points in mind metallic sheath and armour of all multi core
power cables shall be earthed at both end equipment and switchgear end
...
The
sheaths of shielded control cables should be grounded at both ends to eliminate
induced potentials
...
In 220 kV switchyard why lightning arrestor should be properly grounded?
a
...
b
...
This will eliminate voltage build up
due to earth resistance
...
Soil resistivity a should be
minimum and may be it is 3
...
23
...
For example,
a
...
A large percentage of fatal accidents from voltage gradients are in fact
associated with operating handles
...
Use closer mesh in the vicinity of operating handle area (150-mm approx
...
2
...
- 156 -
Question and answers
Electrical Maintenance Unit
24
...
In order to minimise the effect of step
potential and touch potential following two philosophies could be adopted
...
Inclusion of the fence within the ground grid area and
b
...
With this effective area is increased and reduces ground grid resistance substantially
and maximum ground – grid voltage rise as well
...
5 m – 1
...
In either case, the perimeter ground conductor and fence are
bonded electrically at frequent intervals
...
What are the specifications for procurement of grounding conductor and grounding
rods?
Grounding conductor, pad, rods etc
...
Copper
: 91
...
9%
b
...
0 to 3
...
Tin
: 0
...
5%
d
...
0 to 2
...
Iron
: 0
...
0%
Impurities must be limited to the percentage specified below:
a
...
3% maximum
...
Antimony : 0
...
c
...
04% maximum
...
Phosphorous : 0
...
26
...
Copper is cathodic with
respect to other metals that are likely to be burried in the vicinity
...
Grid of copper forms a galvanic cell with burried steel structures, pipes and any
of the lead based alloys that might be present in cable sheaths, it is likely be
hasten the corrosion of the latter
...
Use of tinned copper conductor accelerates and concentrates the natural corrosion
of metal in small area however cell potential with respect to steel and zinc
reduces by about 50% and practically eliminates this potential with respect to
lead
...
What should be the frequency of measurement of earth resistivity?
As per IS: 3043, 1987, measurement of earth resistivity should be carried out
annually or biannually and value should be recorded
...
What should the statutory provision of earthing?
a
...
b
...
In the case of high and extra high voltages, the neutral
points shall be earthed by not less than two separate and distinct connections with
earth, each having its own electrodes at the generating station or substation and
may be earthed at any other point provided no interference is caused by such
earthing
...
c
...
d
...
If they are poorly made or inadequate for
the purpose for which they are intended, loss of life or serious personal injury
may result
...
Each earth system shall be so devised that the testing of individual earth electrode
is possible
...
f
...
g
...
are capable of carrying the new value of earth fault current
which may be obtained by this addition
...
No cut-out link or switch other than a linked switch arranged to operate
simultaneously on the earthed or earthed neutral conductor and the live
conductors, shall be inserted on any supply system
...
i
...
used in earthing shall conform to Indian standard
specifications, wherever these exist
...
What maintenance of earth electrodes should be done?
The neighbouring soil to the earth electrode shall be kept moist where necessary, by
periodically pouring water through a pipe where fitted along with it or by pouring
water in the immediate vicinity of the earth electrode
...
- 158 -
Question and answers
Electrical Maintenance Unit
30
...
Earthing conductors in outdoor areas shall be burried 500 mm below finished
grade level unless stated otherwise
...
Minimum 6000 mm spacing between rod pipe electrode shall be provided unless
stipulated otherwise
...
Earthing conductor around the building shall be burried in earth at a minimum
distance of 1500 mm from the outer boundary of building
...
Earthing conductors embedded in the concrete floor of the building shall have
approximately 100-mm concrete cover
...
Earthing conductors along their run on columns, beams, walls etc
...
f
...
g
...
it shall be
laid 800 mm below them
...
Earthing conductor shall be burried 1000 mm outside the switchyard fence
...
i
...
31
...
: 500 to 3000 Ω
...
What is the effect of voltage frequency and current on resistance of the human body?
a
...
even for
one person
...
For higher touch voltages in order of approximately 50V to 100V the skin
impedance decreases considerably and becomes negligible when the skin breaks
down
...
Wet hand contact resistance becomes very low at any voltage
...
With increase in frequency, impedance of skin decreases
...
What are the paths of current through the body?
A value of 1000 Ω is selected for the calculations that follows as representing the
resistance of a human body from hand to both feet and also from hand to hand or
from one foot to other foot
...
a
...
However deaths have occurred during foot to foot current flow
...
34
...
Next, a single fast automatic re-closure could in a second shock initiated within less
than 500 ms from the start of first
...
With manual re-closure the possibility of exposure to a second shock is
reduced since the time interval may be substantially greater
...
State DC/AC equivalent factor (K)
...
K = I DC fibrillation / I AC fibrillation (rms)
...
Only the making and breaking of
current leads to painful and cramp like contractions of muscles
...
Below approximately 300 mA a
sensation of warmth is felt in the extremities during the flow of current
...
36
...
To produce the same
excitatory effects the magnitude of direct current flow of constant strength in 2 to 4
times greater than that of alternating current
...
What are the characteristics of transformer oil?
Characteristics
Requirement
Method of testing
Appearance
The oil shall be clear A representative sample
and transparent and of the oil shall be
free from suspended examined in a 100-mm
matter of sediments
thick layer at 27°C
...
89 gm / cm
IS-1448(P:16):1977
Density at 29
...
Kinematic viscosity
IS-1448(P:25):1976
max
...
27°C
under consideration
b
...
04 N/m
IS- 6104:1971
27°C minimum
IS-1448(P:21):1970
Flash point pensky- 140°C
marten (closed)
minimum
Pour point max
...
Total acidity max 0
...
Inorganic
nil
IS-1448(P:2):1967
acidity/alkalinity
Corrosive sulphur
Electric strength
(Breakdown voltage)
a
...
b
...
02 N
should be
in place of
0
...
Dielectric dissipation 0
...
- 161 -
See note 2
See note 2
Question and answers
Specific resistance
(resistivity)
a
...
b
...
Oxidation stability
a
...
b
...
Ageing
characteristics after
accelerated ageing
(open beaker method
with copper catalyst)
a
...
b
...
c
...
Total sludge max
...
SK value
Electrical Maintenance Unit
IS-6103:1971
See note 2
35 * 1012 Ω-cm
1500* 1012 Ω-cm
0
...
1% by weight
...
5 * 10 Ω-cm
0
...
20
0
...
05% by weight
...
Density of the oil may be measured at ambient temperature and converted to 29
...
29
...
5)}
Where t = Ambient temperature (in °C)
ρt = Density measured at temperature t
X = Correction factor (Equal to 65 * 10-5)
...
As a consequence of the tendency for water absorption to occur due to breathing on
storage even when drums are sealed the oil shall be filtered to remove moisture and
particulate contaminates present in the original sample before the test as follows
...
A sufficient quantity of oil is heated to 90 ± 2°C, then filtered hot under vacuum
corresponding to an absolute pressure of about 2
...
- 162 -
Question and answers
Electrical Maintenance Unit
b
...
The remaining hot
filtrate is immediately used for measuring dielectric dissipation factor at 90°C and
specific resistance at 90°C
...
For both phenol and amine types of indicators, qualitative methods shall be adopted
...
Value of 0
...
) shall be treated as absence of DBPCPhenolic type inhibitor (quantitative method for amine is under consideration)
...
What are the permissible limits for the transformer oil?
Test
Equipment
Permissible Importance
required
voltage
limits
Electric
Above 145 kV 50 kV
The electric strength does not give a
strength
145 – 72
...
An oil which is
voltage)
significantly oxidised under high
min
...
The
presence of oil deterioration particles,
water and foreign contaminants results
in general overall reduction in the
efficiency of the equipment
...
It is therefore not
advisable to rely solely on the electric
strength of the oil by periodic tests
without
verifying
its
other
characteristics
...
)
resistivity
...
Dielectric
Above 145 kV 0
...
This characteristic is very sensitive to
dissipation Below 145 kV 1
...
the presence in the oil of soluble
factor (Tan
contaminants and ageing products
...
If tan
δ delta) at
delta increases resistivity decreases
...
This is highly influenced by
temperature, voltage, and frequency of
the equipment
...
1 * 10 The specific resistance is another
important test for the quality of oil
...
High resistivity reflect low contents of
free ions and ion forming particles and
normally indicates low concentration of
conducting
contaminants
...
Neutralizati All voltages
0
...
)
KOH/g
Sediment
and
precipitable
sludge
All voltages
Flash point
All voltages
Interfacial
All voltages
tension at
27°C (min
...
) PPM
oxidation of the oil activity encourage
deterioration of insulating paper and
pressboard
...
The test
is required to be performed more
frequently if value exceeds 0
...
The presence of those particle normally
No
sediments reduces the electric strength of the oil
of sludge and in addition deposits hinder heat
should be exchange, thus encouraging from the
detectable deterioration of the insulating material
...
this test should be made more
frequently if the oil has been subjected
to high temperature or shows any sign
of unusual odour
...
018 N/m The interfacial value of oil against
water provided a very sensitive means
of determining the degree of oil
contamination
...
A low interfacial value
indicates that the oil is damaged
...
Large amount of these gases
is an indication of an incipient due to
overheating, sparking, hotspot, arcing,
selector
breaking
current,
solid
insulation deterioration etc
...
To obtain such information and rectify
the faults at regular intervals (using
- 165 -
Question and answers
Electrical Maintenance Unit
dissolved gas analysis method) ensures
trouble free operation and safety of
equipment
...
1
...
1
= 245 kV
2
...
3
...
= 220 * 6
...
)
b
...
0
3
= 380 kV (Approx
...
Cable charging breaking current requirement:The CB for opening high voltage cable or cable networks should be capable of
interrupting the charging currents of cables successfully with the over voltage within
specified limits
...
5
...
Impulse withstand kV
One
minute
power
Nominal Highest
Rated voltage
crest
frequency test voltage kV
system
(L-L) kV
(rms)
voltage
(L-L) kV (rms)
Full
Reduced
Full
Reduced
(rms)
insulation
insulation insulation
insulation
(kV)
(kV)
(kV)
(kV)
220
245
1050
900
460
395
Note: Reduced insulation value – applies where internal insulation is mire important
...
6
...
8 cms
...
8 cms
...
7
...
- 168 -
Question and answers
Electrical Maintenance Unit
8
...
Closing time - 45 to 54 m sec
...
9
...
10
...
mV drop test for the main contact – 11 mV for 1250 A isolators
...
5 mV for 2000 A isolators
...
Interrupting capacity of magnetising current – 0
...
15 PF (lag)
c
...
11
...
- 169 -
Question and answers
Electrical Maintenance Unit
Electrical or electronic equipment design factors
1
...
a
...
b
...
c
...
The factors leading to explosion also depends upon the inherent properties of gas and
its concentration in the atmosphere
...
For design and selection of an equipment for hazardous area, it is very much
essential to know the parameters or characteristics of the atmosphere
...
Those hazardous locations are classified in to zones and
areas as per NEC and IEC classifications
...
Areas where explosive atmospheres can occur despite the explosion protection
measures employed, only explosion protected electrical equipment may be used
...
Protection
type
Flame –
proof
enclosure d
Increased
safety e
Pressurised
apparatus p
Basic principal
Principal application
Part which can ignite an explosive
atmosphere are placed in an enclosure
which, if there is an ignition of an
explosive mixture internally, will
withstand the pressure and prevent the
explosion being transmitted to the
atmosphere around the enclosure
...
The formation of an explosive
atmosphere inside an enclosure is
prevented by using a protective gas to
maintain an internal overpressure
Switchgear and switching
installations, control and
display
units,
control
boards,
motors,
transformers,
heating
devices, light fittings
...
Switching and control
cabinets, analysis devices,
large motors
...
Intrinsic
The equipment placed in the hazardous
safety i
are contains only intrinsically safe
circuits
...
Oil
Electrical equipment or parts of
immersion
electrical equipment are immersed in a
(o)
protective liquid in such a way that an
explosive atmosphere above the surface
or outside the enclosure cannot be
ignited
...
Moulding m Parts, which can ignite an explosive
atmosphere, are embedded in a casing
compound so that the explosive
atmosphere cannot be ignited
...
Transformers,
resistors
...
Question and answers
Electrical Maintenance Unit
2
...
Protection against solids
Protection against liquids Mechanical protection
IP Principal
IP Principal
IP Principal
0 No protection
...
0 No protection
1 Protected against solid 1 Protected
against 1 Impact energy 0
...
mm (eg:- accidental
drops
of
water
contact with the hand)
...
2 Protected against solid 2 Protected
against 2 Impact energy 0
...
mm (eg:- finger of the
falling at upto 15°
hand)
...
3 Protected against solid 3 Protected
against 3 Impact energy 0
...
5
drops of rain water
joule
...
at upto 60° from the
vertical
...
00
bodies larger than 1
projections of water
joule
...
small wires)
...
00
(no harmful deposit)
...
directions
...
00
against dust
...
force to heavy seas
...
Protected
against
prolonged
effects
of
immersion under pressure
...
a
...
b
...
c
...
d
...
e
...
But in general the silicon controlled rectifier is referred to as thyristor
...
In our station thyristors are used in the following equipments
...
Main generator static excitation system
...
Power UPS
...
Control UPS
...
Diesel generator excitation system and etc
...
The biasing at the three
junctions J1, J2, J3 determine the state of the thyristor
...
This is shown in the figure below
...
The condition for the conduction of a diode is that the anode must be positive
with respect to the cathode
...
By controlling the instant
of the pulse release the conduction of the thyristor can be controlled
...
They are as
follows
...
Forward blocking region
...
Forward conduction region (Useful region of operation)
...
Reverse blocking region
...
Reverse conduction or breakdown region
...
Forward blocking region: When an external voltage is applied to the thyristor
making anode positive with respect to the cathode, the thyristor is said to be ‘forward
biased’
...
Junctions J1 & J3 are forward biased
...
Junction J2 is reverse biased
...
A small forward leakage current flows which increases with the applied voltage
...
This is represented by region OA in the characteristic graph
...
Forward conduction region: As the forward voltage is increased, a point is reached
where the junction J2 gets forward biased and allows a large current to flow through
the device
...
Above this
point the voltage across the device falls to a low value and the current is limited only
by the external load resistance
...
As seen above thyristors can conduct even in the absence of gate pulses provided the
forward voltage across them is more than the break over voltage
...
This characteristics of the thyristor
makes it possible to control its conducting period in each cycle of the applied voltage
by the release of gate pulses at the desired instant
...
This is shown in the diagram given below
...
(Firing ckt)
- 175 -
Pulse
amplifier
Question and answers
Electrical Maintenance Unit
- 176 -
Question and answers
Electrical Maintenance Unit
c
...
Under this condition,
1
...
2
...
Only a small leakage current flows through the device, which increases with the
applied voltage
...
d
...
The voltage at this point is known as the ‘reverse
breakdown voltage’
...
Current
C
Forward conduction region
IL
IH
O
D
Reverse blocking
region
Reverse conduction
region
IL – latching current
...
VBO – break over voltage
...
- 177 -
A
B
IG1
IG=0
VBO
Voltage
Forward blocking region
Question and answers
Electrical Maintenance Unit
Some important technical terms
a
...
In control and power
ACVR’s this has been achieved by bleeder or dummy load resistances connected
across the output terminals (DC side)
...
Holding current: It is the value of anode current below, which the thyristor in
conduction (ON state) turns OFF
...
c
...
For rectifier mode of operation the firing angle will be between 0° to 90°
...
This is
illustrated in the figure given below
...
Triggering: The process of switching the thyristor ON by the application of the gate
pulse is known as triggering
...
Ripple: The AC components in the DC output of any rectifier are called the ripple
...
This AC voltage indicates the conduction of all the
thyristors in the bridge
...
f
...
Inductors and capacitors are used as
filters in the output of rectifiers
...
a
...
b
...
Short circuit / over current
...
This rating specified for a
particular thyristor should not be exceeded because it would lead to spurious triggering
(switching ON) of the thyristor
...
R
C
Thyristor
High di/dt: This rating of the thyristor indicates the maximum rate of rise of ON-state
current
...
Small area of conduction then spreads from these points to the whole crystal
...
Connecting an inductor in series with the thyristor shown below
provides protection against high di/dt
...
The semi conductor fuses operate very fast with prearcing time less than 0
...
Hence the fault
current will be interrupted by these fuses before it reaches its maximum value
...
C
...
Resistance checks: The anode – cathode resistance and gate – cathode resistance of
the thyristor should be as follows
...
In the reverse direction – about 1 MΩ
...
In case of fused thyristor these resistances will be zero Ω
...
Current deflection test: The thyristor should be connected to a power supply as
shown below
...
If there is no deflection in the meter it shows that the thyristor is faulty
...
The above testing can also be done
with the help of a motwane analog multimeter
...
A
K
G
(+)
Ammeter
Switch
(–)
Motwane ammeter selected in resistance range
...
Switch closed zero resistance
...
What are precautions to be taken while doing maintenance or repair work on power
cables?
A research organised by EPRI (electric power research institute) on medium voltage
XLPE cables found that DC high potential at 80% of the factory value
a
...
It did not identify significantly weakened cable
...
Maintenance and repair:
Before attempting for any corrective maintenance on power cables like replacement
of lugs or jointing of cables following should be taken care
a
...
b
...
Before jointing is commenced it is advisable that IR of both sections of
cable to be jointed be checked
...
Before jointing a paper insulating cable (for PVC cables this step is not required),
the paper insulation should be tested for the presence of moisture by immersion in
hot compound for paraffin wax at a temperature between 120°C and 140°C
...
Use only single strip of the paper
...
In case dia of die to be used for crimping is slightly more than dia of cables, then
use some loose strand before doing crimping of lug
...
2
...
During DC high voltage testing flow of following currents will take place
...
Capacitance charging current
...
Dielectric absorption current
...
Surface leakage current
...
Partial discharge current (corona)
...
Volumetric leakage current
...
Capacitance charging current
...
E – voltage in kilovolts
...
c – capacitance in micro farads
...
The charging current is a function of time and will decrease as the time of the
application of voltage increases
...
Test readings should be
taken until this current has decreased to a sufficiently low value
...
Dielectric absorption current
...
This current can be calculated by
the formula
-n
ia =VCDT
Where ia – dielectric absorption current
...
C – capacitance in micro farads
...
T – time in seconds
...
Again time should be allowed before recording test readings so that this current has
decreased sufficiently
...
Surface leakage
...
d
...
The partial discharge current, also known as corona current is caused b ionization of
air due to high-test voltage
...
Semi-conducting tape is used to separate the conductor from the insulation to prevent
possible damage of the insulation from the corona and ionization
...
The application of semi-conducting smoothes
the voltage stress and keeps such voltage stress constant and to a minimum
...
Volumetric leakage current
...
This is the
current that is used to evaluate the condition of the insulation under test
...
- 185 -
Question and answers
Electrical Maintenance Unit
Predictive (preventive) maintenance on Induction Motor
...
What are the reasons for high current in motor?
a
...
b
...
8 Hz current will be 102% of the normal current
...
High voltage
...
Under voltage
...
Mechanical over loading
...
Process requirement
...
What are the reasons for unbalanced current in motor?
a
...
b
...
c
...
3
...
1
...
a
...
b
...
c
...
d
...
e
...
f
...
g
...
h
...
i
...
j
...
- 186 -
Question and answers
Electrical Maintenance Unit
2
...
a
...
b
...
c
...
d
...
e
...
f
...
g
...
If the vibration is because of electrical fault, de-energise the machine and
watch the vibration as it runs down
...
Twice the power supply frequency – it indicates that the vibration is developed
by unbalanced power supply voltages, unbalanced air gap, unbalance in rotor
winding, slackened stator core etc
...
Multiple of power frequency – the stator and rotor slots co-ordinate to develop
radial lines of force to deform and pulsate the cores
...
Twice the slip frequency – magnetic unbalance due to unbalance air gaps,
slackened rotor core, interturn short in the rotor-winding etc
...
d
...
4
...
In case temperature is more, then the following could be the possible
reasons
...
Electrical overloads
...
Over and under voltage
...
Over and under frequency
...
Voltage unbalance
...
(% Voltage unbalance) = 100 * maximum deviation from average voltage
average voltage
...
45%
...
45)*(6
...
d
...
- 187 -
Question and answers
e
...
g
...
i
...
Unbalance current
...
Long acceleration cycle
...
- 188 -
Question and answers
Electrical Maintenance Unit
2
...
a
...
b
...
c
...
d
...
3
...
a
...
b
...
c
...
d
...
4
...
a
...
5
...
In case temperature is higher
than the 90°C the following could be the possible reasons
...
Inadequate lubricants inside the bearing
...
Faulty bearing
...
Bearing is jammed
...
Over greasing
...
Improper grade of lubricant
...
What are the reasons for abnormal sound or noise?
Motors in general should run very quietly and no abnormal noise is desired
...
a
...
b
...
c
...
d
...
For permissible limits of noise levels for rotating electrical machines IS: 12065:1987
is being reffered
...
What are the reasons for harmonics in the motor?
Generally even harmonics are not expected to be present in three phase motors
...
are also not expected
...
Presence of strong 2nd harmonics indicates unbalance voltage, unbalance winding
impedance, rotor defects, magnetic imbalance, faulty rotor skewing etc
...
Overloading causes overheating, resulting in non-linear magnetization
- 189 -
Question and answers
Electrical Maintenance Unit
rd
which gives high 3 harmonic winding faults, short circuits
...
- 190 -
Question and answers
Electrical Maintenance Unit
8
...
a
...
b
...
c
...
d
...
9
...
a
...
b
...
c
...
d
...
10
...
While proceeding for point as above said, following factors are to be kept in mind
which affect the insulation resistance measurement
...
Surface condition
...
Moisture
...
Temperature
...
Magnitude of test voltage
...
Duration of application of test voltage
...
Residual charge in the winding
...
Ageing of the insulation
...
Mechanical stresses
...
What are the minimum recommended PI values for AC and DC rotating machines?
Following minimum recommended PI values criteria is to be followed
...
1
...
b
...
0 for class – B insulation
...
2
...
- 191 -
Question and answers
Electrical Maintenance Unit
12
...
3 (minimum recommended value)
IR value for 15 seconds
Tips:
a
...
However for
machines in good condition substantially the same IR is obtained for any test
voltage up to the peak value of the rated operating voltage
...
If the IR value decreases significantly with an increase in applied voltage it is an
indication of imperfections or fractures of the insulation aggravated by the
presence of dirt or moisture or may be due to the effects of dirt or moisture alone,
or may result from numerous other phenomena not necessarily associated with
any defect or weakness
...
IR value for good dry winding continue to increase for hours with constant test
voltage continuously applied, however a fairly steady value is usually reached in
10 to 15 minutes
...
d
...
IR = kV + 1
Where IR = recommended minimum IR in mega ohms at 40°C of the entire
machine winding and kV = rated machine voltage in kilo volts
...
e
...
Therefore when the three phases
are tested separately, the observed insulation resistance of each phase should be
divided by two to obtain a value which after correction for temperature, may be
compared with the recommended minimum value of IR
...
What is use of Tan – Delta test? And what are the recommended values?
The very purpose of this test is to detect moisture content, voids, cracks and
deterioration in the insulation and same is to be conducted on HT motors
...
S
...
127 may 1980) the contamination level of motor
winding is to be assessed from the given Tan – Delta value
...
Low void content
...
Clean
...
Some dirt
...
Dirt and moisture
...
Gross contamination
...
Above 20%
...
Severe oil and carbon contamination
...
What are important guidelines for conducting HV test?
Based on the recommendations given in IS: 4029:1977 decided DC test voltage
= (2E+1kV) 1
...
2
...
M = derator factor which is a function to be decided on the basis of the age and
condition of equipment
...
A plot
leakage current Vs test voltage is to be plotted as the test progress
...
a
...
If however a second test to be made at 80% of the voltage given by the
empirical formula given above
...
Test voltage for completely rewound motor = full test voltage for new motor
...
Partially rewound or overhauled motor = 75% * full test voltage for a new motor
...
Before the test for the old parts of the winding shall be carefully cleaned and
dried
...
Before attempting of HV DC test a minimum PI value of motor should be
obtained
...
What are the uses of high voltage surge test?
This test gives distinct wave forms giving indications of various defects such as,
a
...
b
...
c
...
d
...
e
...
f
...
g
...
h
...
i
...
- 194 -
Question and answers
Electrical Maintenance Unit
16
...
Type of enclosure No
...
Thumb rule for calculating positive sequence and negative sequence current in
motors
...
Positive sequence current: Average of all three phases currents
...
Negative sequence current: Maximum deviation of any of the phase currents from
the average
...
How you evaluate the insulation condition based on PI value?
Evaluation of insulation condition based on PI value
PI value
Insulation condition
Recommendation
1
...
5
Bad
Drying is mandatory
1
...
0
Doubtful
Drying is recommended
2
...
0
Adequate
No drying is needed
3
...
0
Good
No drying is needed
> 4
...
What are the conditions monitoring for the motor bearings?
Bearing oil analysis is a useful tool in determining bearing performance and possible
deterioration
...
Oil analysis tests
Symptoms
Possible cause
Test
Cost
Viscosity
Water or high Water content
Low
temperature
ASTM 445 viscosity
Low
ASTM 974 neutralization number
Viscosity change Oxidation
Low
ASTM 664 neutralization number
colour change
Moderate
ASTM 2296 neutralization number
Moderate
Particles
Bearing
Spectroscopy
Low
deterioration
or Particle count
Moderate
Direct reading ferrography Moderate
foreign matter
Analytical ferrography High
- 195 -
Question and answers
Electrical Maintenance Unit
- 196 -
Question and answers
Electrical Maintenance Unit
Important test on electrical equipments
1
...
Test
Purpose
IR value
Detects serious flaws,
And
moisture absorption and
PI value
...
insulation
Tan delta or Indicates
deterioration,
dielectric
contamination
and
loss or
power factor physical damage
...
Excitation
Indicates defects in the
current at
magnetic core structure,
high voltage
...
Turns ratio
Indicates short circuited
turns
and
internal
connections
Winding
Detects
poor
resistance
connections
and
conductor shorts
Core IR and Indicates deterioration
inadvertent
of
core
insulation
grounds
system
Water
Indicates moisture level
contents
in oil
Total acidity, Measures organic and
neutralization inorganic acids
number
Dissolved
Indicates specific gases
gas analysis generated
Furanite
Indicates
cellulose
compounds degradation
- 197 -
Item
Winding
...
Winding, Winding has to be isolated, oil
oil and sample should be collected
...
Winding
Winding has to be isolated
...
Test on Circuit breakers
Test
Purpose
IR value
Detects serious flaws,
moisture absorption and
cleanliness
...
Tests on power cables
Test
Purpose
IR value
Detects serious flaws,
moisture absorption
and cleanliness
Dielectric
Shows
insulation
loss or tan δ deterioration,
contamination
and
physical damage
DC
step Determines condition
voltage test
of insulation
Resistance
Detects
of
bolted connections
connection
Component Required condition of machine
Overall
Cable has to be isolated
insulation
system
Overall
Cable has to be isolated
insulation
system
Overall
insulation
system
poor Bolted
connection
- 199 -
CB has to be isolated
Cable has to be isolated
Cable has to be isolated
Question and answers
Electrical Maintenance Unit
- 200 -
Question and answers
Electrical Maintenance Unit
4
...
discharge sites
ELCID test
Determines healthiness of
(optional)
stator
core
inter
lamination insulation
Online motor
To
determine
the
current
healthiness of the motor
signature
by giving all the electrical
analysis
parameters,
harmonic
analysis, rotor bar heath
and bearing problems
Wear debris
Bearing
condition
- 201 -
Arrestor has to be isolated
Item
5
...
Tests on HV generator
Test
Purpose
IR and PI
Tan delta or
power factor
test
DC winding
resistance
DC
step
voltage
or
leakage
current test
Partial
discharge or
corona
or
TVA probe
ELCID test
Wedge
tightness
check
AC
impedance
test
Recurrent
surge
oscillograph
Item
Detects serious flaws,
moisture absorption and
cleanliness of winding
Evaluation of stress
grading,
dielectric
losses and homogeneity
of
the
winding
insulation
Detects
poor
connections
and
conductor shorts
Detects
insulation
weakness
and
possibility of warning of
breakdown of incipient
fault
Evaluation of stress
grading system and
location
of
Partial
Discharge sites
Required condition of machine
Stator and Bus bar and neutral connection
field
has to be isolated
winding
Stator
Bus bar and neutral connection
winding
has to be isolated
Stator and Bus bar and neutral connection
field
has to be isolated
winding
Stator
Bus bar and neutral connection
winding
has to be isolated
Stator
winding
Bus bar and neutral connection
has to be isolated
...
Determines
wedge Stator
Rotor has to be threaded out
tightness
wedge
Detects the presence of Field
short circuit turns
winding
Rotor winding should be isolated
from the excitation system
Detects interturn and Field
earth faults in winding
winding
Rotor winding should be isolated
from excitation system
...
C
...
To evacuate generated electrical power
...
To provide required power to SUT, UT, DG, UPS, and CUPS
...
To provide required emergency power from onsite DG, UPS & CUPS
...
To provide Fast transfer in event of Class IV failure
...
e
...
f
...
g
...
h
...
i
...
j
...
k
...
l
...
m
...
• What are the design guidelines for electrical system?
a
...
As per studies seismic condition is not there within 5 kms and
nearest zone is away from 20 kms
...
Safety related equipments are separated from suitable fire barriers of 3 hrs rating
by horizontal and vertical clearances and from turbine building which are houses
high energy rotating equipments and where potential for fire is exist
...
Separate switchyard control is provided in case of non-availability of main
control room with line and bus coupler protection and bus bar protections
...
d
...
e
...
f
...
g
...
h
...
- 205 -
Question and answers
Electrical Maintenance Unit
MAIN GENERATOR AND IPBD
• How output of the generator is depends?
Output of the generator is the function of volume, length, dia, airgap, and speed
...
While sucking the gas, gas comes through the stator parts
also and removes the heat from the stator parts
...
Thus the stator and
rotor of the generator is cooled
...
• When the hydrogen explosion will take place in main generator?
When hydrogen concentration in air is more than 4% and less than 74% causes the
explosion
...
High thermal capacity
b
...
Low viscosity
d
...
Simple heat exchanger i
...
it can be circulate easily and cooled by heat exchanger
• How rotor windings are held in position against centrifugal force?
Rotor windings are held by duraluminium wedges and by non-magnetic steel
retaining rings in the overhang portion
...
• How rotor cooled?
Hydrogen picked up from stator core backspace, passes through ventilation canals on
rotor and comes out through adjacent canals
...
Heat
from the hydrogen removed by 4 nos
...
(NAPW)
• What is rotor E/F relay setting?
1
...
- 207 -
Question and answers
Electrical Maintenance Unit
• How generator is protected from switching surges and lightning surges?
Surge capacitor and lightning arrestor
...
• What is the difference between PT and normal transformer?
Burden of PT is less and burden of normal transformer is high
...
• When TG works as induction generator?
When excitation alone lost
...
In DG motoring prevented, because of unburned fuel catches fire in DG
...
Emf = 4
...
• What are the protective parameters to changeover AVR auto to manual?
a
...
b
...
c
...
d
...
e
...
f
...
• Why AVR changes over to manual on AVR PT fuse failure?
Because loss of feedback to voltage corrector
...
- 208 -
Question and answers
Electrical Maintenance Unit
• Why generator differential provided?
For generator phase to phase faults and 3 phase faults
...
Leading VAR taken from grid leads to severe voltage dips in grid, if grid is weak
...
Stator overheating
...
• What are the limiters provided in excitation system?
a
...
Rotor angle limiter (75° lag)
c
...
d
...
Stator current limiter (lag 10000 A)
f
...
• What is the effect of unbalance currents in generator?
Double frequency currents are induced in metal parts of rotor and overheating of
retaining rings and non-magnetic wedges
...
So even if turbine trip, relay picks up, there is no trip actually
...
54%) used to detect the condition
...
• Why not high resistance for earth fault than using grounding transformer & resistor
0
...
Difficult to manufacture
...
Other methods are
a) Auto transformer
...
• Will rotor earth fault relay operate for earth fault in main exciter winding or RCU
Diode Bridge also?
Yes
...
• How moisture entry is prevented into bus duct?
Silicagel breathers at either end
...
Short circuit forces are less
...
Because the fault currents fed by the both
generator and GT adds up within UT bus duct in case there is a fault in UT bus duct
...
• Why cannot we have cables instead of bus duct in main generator?
Very large number of cables in parallel required problems of sealing the
terminations
...
• Which parameter indicates the rotor short-circuited turns (Not involving earthfault)?
Vibration increases
...
• What are the routines checks on slip rings?
a
...
b
...
c
...
d
...
e
...
• Can we trip GFB from control room during unit operation?
No only if generator breaker is off
...
• How hydrogen purity reduces?
Due to seal oil vapour mixing
...
This also covers
partially faults inside generator, time delayed to coordinate with 230 kV-distance
protection
...
• Why 50 Hz chosen?
Earlier 25 Hz generally used
...
• Why oil cannot be used instead of water in generator stator?
Oil has high thermal capacity than gas, but low thermal capacity than water
...
High viscosity of oil causes linear flow and poor surface heat transfer in small ducts
...
• What is the purpose of JOP?
Lifts the rotor by injection of oil at high pressure, when BGM is in service
...
• What is the purpose of lubrication oil?
Keep oil film in bearings, avoid metal to metal contact between bottom of journal
and bearings avoid damage to bearings by lubricating the bearings
...
• What are the purposes of barring gear?
Start rotor from rest
...
Avoid direct contact journals and bearings
...
- 214 -
Question and answers
Electrical Maintenance Unit
• How shaft voltage produced by turbine?
Due to un-symmetry in the flux path of core, non-uniform air gap, un-symmetry in
the rotor magnetic field during short circuit in the rotor winding, causes voltage to
develop across the ends of rotor shaft
...
• What is the necessity of shaft voltage measurement?
It is to observe insulation of bearing 6 & 7 and hydrogen seal assembly
...
If the leakage current >100mA, clean the insulation provided between bearing
pedestal and seal housing with earth
...
• What type of insulation is done for stator bars?
Bar insulation is done with epoxy mica thermosetting insulation
...
This insulation is more reliable
for higher voltages
...
After curing the insulation the epoxy resin (glue) fill all voids in the insulation
...
This eliminates the
formation of creepage sparks during operation and during HV test
...
• How cooling is done for slip ring and brush gear?
A centrifuge fan is mounted on the shaft in between two slip rings for ventilation of
the slip rings and brush gear
...
Now a day we are using LFC554 for economical reasons
...
- 216 -
Question and answers
Electrical Maintenance Unit
• What is the need of staggering of brushes and helical grooves?
The need of staggering is for uniform wear of brush and slip rings
...
The
forced ventilation fan removes carbon dust from the helical grooves
...
If shaft earthing is not done the leakage current will flow through
the bearings to ground and pitting of bearings will result
...
• What is the purpose of POLARIZATION INDEX (PI) value?
It is used to assess the degree of dryness of windings
...
Initially for a new insulator free ions are less and hence more
resistance will be more
...
So the PI value for new insulation
will be more and for old insulation it will be less
...
5 micro mho/cm
...
• What is the necessity of Seal oil system?
The annular gap between stator and rotor of the generator are to be sealed to prevent
hydrogen leak from the casing
...
5 kg/cm2
• What is the paint used in the surface and interior of enclosure and why it is?
Matt black paint, for efficient heat dissipation
...
• Why flexible expansion joints are used in IPBD?
To cater thermal expansion and contraction due to heating and to eliminate
mechanical vibrations to the equipment
...
This for a vital link between generator, GT, UAT, SPPT, SET and neutral
grounding transformer
...
The continuous enclosure operating at ground potential limits the leakage flux
outside the enclosure to a very low value thereby eliminating the problem of
inductive heating of magnetic materials in the vicinity of the busduct
...
Shielding effect of the enclosure reduced the electromagnetic forces under fault
conditions between bus to bus to a great
...
The IPBD consists of high purity aluminium alloy bus supported by high strength
porcelain insulator (24 kV class) within enclosure separates adjacent conductor
by air
...
e
...
f
...
Because the conductors are of circular type
having very little skin effect and has a very large cooling surface
...
Conductors are painted with epoxy Matt black paint results in heat dissipation and
the temperature rise is small and current carrying capacity is improved
...
High dielectric strength as conductors are supported on porcelain insulators
...
Air tight, watertight and dust free bus conductors
...
j
...
Hence changing parts makes it easy
...
• Why aluminium bus bars are silver-plated in IPBD?
Aluminium bus bars are silver-plated at flexible connection to prevent the galvanic
corrosion ant also for low contact resistance
...
• What are the precautions to be taken while working at SPPT?
PT trolley should be isolated very carefully so as to isolate secondary terminals first
and primary (HT) terminals next
...
When fuse is blown the temporary earth should be done at the HT side of the fuse to
replace the fuse
...
- 219 -
Question and answers
Electrical Maintenance Unit
• What is use of hot air blower in IPBD?
To remove moisture and to prevent moisture condensation inside the duct at
commissioning time or in long shutdown periods
...
7 cm
• Specification of NGT & NGR
...
5 kV / 250V, 50 kVA
...
5Ω, 250V, 288A (continuous) and
temperature rise allowed to 375 °C
...
Surge protector – non-inflammable, synthetic liquid impregnated and hermetically
sealed, 24KV, 0
...
PT – 16500/√3 /110/√3 volts
...
15A
...
Reactive power can
vary by the Generator main field voltage variation
...
Active power is produced by source and used effectively
...
• What is the protection for IPBD?
Generator – GT overall differential protection
...
• Why and where the magnetic filter is provided in stator water circuit?
Magnetic filter is provided to catch the metal particles in stator water circuit, which
are produced in the pipelines
...
• What are the isolations required for working on IPBD/ Generator?
a
...
b
...
- 221 -
Question and answers
c
...
e
...
Electrical Maintenance Unit
Barring gear motor stopped and tagged
...
CB 472 and CB 474 open and PT’s are isolated and tagged
...
- 222 -
Question and answers
Electrical Maintenance Unit
• What are the futures of turbine generator?
a
...
Moisture control (HP-0
...
)
c
...
70% steam dumping to the condenser to avoid reactor trip
...
Gland sealing
f
...
Generator stator and rotor cooling
h
...
Static excitation
• What are the intervals for generator overhauling?
a
...
2nd inspection after 8000 hrs of 1st inspection
c
...
4th inspection after 48000 hrs of 1st inspection
• Write nameplate details of the main generator
...
9 lag
...
• What are the torque settings used in IPBD connection?
M12 (Nut bolt)
4506 100 kg-cm or 55 NM
M16 (Nut bolt)
9006 250 kg-cm or 80 NM
M20 (Nut bolt)
18006300 kg-cm or 100 NM
• Write critical speeds of turbine generator?
Generator rotor
1st critical speed
1283 rpm
nd
2 critical speed
3600 rpm
Combined turbine generator
1st critical speed
1938 rpm
nd
2 critical speed
2120 rpm
- 223 -
Question and answers
rd
3 critical speed
4th critical speed
Electrical Maintenance Unit
2385 rpm
2837 rpm
- 224 -
Question and answers
Electrical Maintenance Unit
• What type of governing system used in turbine and what are the purposes of the
system?
Hydraulic governing system of centrifugal (speed) governer type is adopted in
turbine
...
1 kg/cm2 (max)
...
7
kg/cm2 relief valve is attached for on load testing
...
At speed of 2560-rpm governer system becomes effective and starts draining of
sensitive oil to 2
...
Once the speed takes over by governer, governer valves position comes to
closing side and then CIES valves are opening fully
...
HP speeder gear controls HP governer valves and LP speeder gear controls LP
governer valves and closes fully when 6% over speed which starts when 3% over
speeding
...
a
...
b
...
c
...
d
...
e
...
f
...
g
...
Once
synchronised the grid frequency and speeder gear controls the speed
...
• How patina formation is done
...
(Epoxy insulation in the stator winding absorbs no moisture)
...
b) Physical inspection of copper braided flexibles for discoloration
...
d) Torque tightness of flexibles
...
f) Tightness of supporting insulator, seal off bushing, inspection windows
g) Inspection of rubber bellows
h) Electrical checks on SPPT, NGT, CT, mVDT of copper flexible connection
...
j) Healthiness checks of lightning arrestor
...
• What are the works to be done in generator in major overhauling?
Works on stator
a) Hydro test (DM water at 5 kg/cm2 pressure, leak acceptable is 5% for 24 hrs)
...
d) Drying out of stator conductor
...
e) IR value check
...
g) Inspection of Teflon tubes and rubber grummets
...
i) Inspection of RTD’s
...
k) Hot air and hot water test of stator conductors to check whether flow through all
stator conductors is uniform
...
m) Capacitance and tan-delta measurements
...
o) Winding resistance measurement
...
q) ELCID (electromagnetic core imperfection detection) test
...
b) Inspection of rotor slots
...
d) DP test on slip-ring hub to detect micro crack
...
f) Measurement of IR and PI value
...
h) Recurrence surge oscillograph
...
j) Slip-ring groove cutting and machining
...
Then OCC test
...
By detecting the changes in these parameters failures can be
revealed
...
The very purpose of this test is to detect moisture content in the insulation
...
This indicates amount of ionization
...
The discharges are partial since there is some
insulation remaining to prevent a complete breakdown
...
This can be tested by electromagnetic probe, which is a detector that is sensitive to
the radio frequency signals produced by the partial discharges within the winding
...
- 227 -
Question and answers
Electrical Maintenance Unit
• What is the purpose of HV / leakage current test on stator?
This is to find out weakness in the stator winding
...
The record of voltage versus leakage current provides the condition of the winding
for present and future use and may permit prediction of breakdown voltage whether
it is within or slightly above the test voltage
...
5 * rated voltage for AC
And DC voltage = (2E + 1 kV) 1
...
1
...
Application of HV voltage also depends on the age factor or condition of the
machine
...
a) Capacitance charging current
...
c) Surface leakage current
...
e) Volumetric discharge current
...
• What is Recurrent Surge Oscillograph (RSO) Test?
RSO test is performed to detect faults in rotor windings
...
The faults from the winding to the
body and the faults between the parts of the winding (inter-turn faults)
...
• What is the purpose of rotor AC Impedance measurement?
Periodic measurement of rotor impedance using an AC power supply is another
means of detecting the presence of shorted turns in a rotor winding
...
This is because the induced backward current in a single shorted turn
opposes the MMF of the entire coil, thus greatly reducing the reactance
...
When the DC voltage applied three components flow,
a) A charging component flows into the capacitance of the winding
...
c) A leakage component over the surface between exposed conductor and ground
which is highly dependent on the state of dryness of the winding
...
The third component is
determined by the presence of moisture or ground fault and relatively constant with
time
...
If this leakage current is larger than the
first two components then the total charging current will not change significantly
with time
...
The ration of the 10 minutes reading over the oneminute reading is called the PI (Polarization Index)
...
• What is the generator IR value when generator is filled with stator water and
hydrogen?
Generator IR when filled with stator water and hydrogen is about 100 kΩ only
...
As we are
measuring insulation with 1 kV or 5 kV megger, the ionic current or leakage current
will be same and the IR value will be approximately same
...
Ionic current
Leakage
Current
Saturation region
Voltage (kV)
Water and hydrogen are self-restoring insulators
...
e
...
As the field voltage and stator voltage raises the heat
produced in the stator and rotor will increase the IR value of the machine in running
condition
...
- 229 -
Question and answers
Electrical Maintenance Unit
• What is the purpose of DC winding resistance test?
To detect the shorting of winding and loose or poor connection of the windings
...
This is
done for personnel safety of the operator and for the equipment safety by blowing
the fuse when earth fault current flows through the fuse
...
When fault occurs the system
voltage increases ♦3 times
...
Mainly there are three types of neutral earthing
a) Directly
b) Resistance
c) Reactance
• What is Arcing Grounds?
When earth faults occurs, arc with the ground and phase will occur
...
This is called Arcing
Ground
...
3 kV and below 22 kV resistance grounding is preferred
...
For below 3
...
Because normal earthing (plate earthing) gives 1
...
This limits current to
E (R∅)
Ω
230/1
...
For above 22 kV solid or direct grounding is used
...
to neutralize capacitive current by
adding reactive current
...
Generator 16
...
Only star point of the generator is grounded
...
Since the machine is grounded with reactance XL (transformer), a flow of 3rd
harmonic current is there in between ground and the machine neutral
...
Thus the 3rd harmonic voltage
(Vs) at machine line (VL3) and neutral end (VN3) should bear a constant ratio
...
In the extreme case if a fault occurring on the
machine neutral side, VN3 becomes zero and VL3 becomes Vs and vice versa
...
Blind zone
Neutral
Line
Earth
Fault
• How generator protections are classified in nuclear power station?
Classification of generator protection in nuclear power station
...
MAIN Protection
2
...
Differential
Over freq
...
EXCITATION Protection
Unbalance current
Excitation transformer over current
3
...
and Rotor o/v
Phase o/c during startup
48 V DC fail
E/F during startup
more than 3 Bridge fail (¾ logic)
Manual channel fails
Transformer over temperature (Class –B)
• State torque formula
...
S = stator flux
IR = Rotor current
cosθ = Rotor power factor
- 232 -
Question and answers
Electrical Maintenance Unit
• How main generator protection grouped?
The various protections associated with the generator, Generator transformer and Unit
transformer are connected to a trip unit through trip relays 86A, 86B and 86C
...
These are routed through
trip relays 86A
...
Certain protections such as loss of excitation, negative sequence protection, overfluxing etc
...
These protections are classified as
Class B
...
This relay initiates the
tripping of turbine (closure of stop valves) and also the LV side breaker of UT through
trip relay 86B1 and 86B2
...
These interlocks are wired in series with the Class B
trip relay 86B, which is wired to trip the generator breaker, generator field
...
Some protections
such as Bus bar differential, generator under frequency etc, requires tripping only of
the 220 kV side of the generator transformer to isolate the external fault
...
These protections are wired to trip relay 86C, which initiate
only the tripping of the generator transformer HV side breaker
...
- 233 -
Question and answers
Electrical Maintenance Unit
• What are the generator protection parameters?
Generator protection parameters are as follows
...
rotor earth fault (64F2)
100% stator earth fault (64A)
GT restricted earth fault (64 GT)
UT restricted earth fault LV A (64 UT A)
UT restricted earth fault LV B (64 UT B)
Gen
...
interturn (87 IT)
GT overall differential (87 GT)
UT differential (87 UT A/B/C)
Reverse power (37 G)
GT buchholz, OLTC oil surge, fire (30 A/G/D)
CLASS A2
Stator O/C during start (50S ABC)
Stator E/F during start (64 C)
Stator backup E/F (64 B)
GT backup O/C (50/51 GT)
GT backup E/F (51 N GT)
Gen
...
field fail with U/V (27/40G)
UT backup O/C (51 UT ABC)
UT backup E/F LV-A (51N LV-A)
UT backup E/F LV-B (51N LV-B)
LBB protection (50 Z)
UT buchholz, fire (30 A/D)
Excitation O/C stage – 2
Rotor + & - ve over voltage
Excitation 48 V DC fail
More than 3 bridge fails (3/4 logic)
CLASS B
Gen
...
negative phase sequence (46 G/GT)
Gen
...
backup impedance stage – 2(21G – 2)
Gen
...
under frequency (81 – 1 / 2)
GT backup earth fault (51N GT)
Question and answers
Electrical Maintenance Unit
• State class – B process side trip parameters
...
Reactor trip +
2
...
High 5
...
Exhaust hood steams temp
...
Lub
...
Relay oil pressure low
21 kg/cm2
6
...
Condenser vacuum low 696
...
Stator water cond
...
Stator water flow low
30 M3 / hr
10
...
75 kg/cm2
149°C
[0
...
38 kg/cm2
[ 3
...
114 kg/cm2
660 mm Hg
559 mm Hg
13
...
Action/2nd ann
...
high
75°C
80°C
>80°C manual trip
Bearing outlet oil temp
...
Manual trip
DM water outlet temp
Unload
85°C
>85°C Rundown trip
Stator winding temp high
Unload
75°C
>75°C Rundown trip
Hot gas temp high
Unload
75°C
>75°C Rundown trip
Stator core temp high
Unload
95°C
>105°C Rundown trip
Rotor winding temp high
Unload
110°C
>110°C Rundown trip
Temp of cold hydrogen gas
Unload
55°C
>55°C Rundown trip
Temp of inlet water to gas coolers
37-48°C Unload
>48°C Rundown trip
Temp of inlet water to stator winding 44-48°C Unload
>48°C Rundown trip
Generator seal oil outlet temp
65°C
>65°C manual trip
Purity of hydrogen in casing
<97%
<95%
<95% manual trip
*Unload – Decreasing load to a lower value manually
*Rundown – Reducing load to no-load condition (manually/automatic)
• Why boiler level high trip has been provided in turbine?
In condition of boiler level high moisture contents in the steam will rise and rise in
moisture content is harmful to turbine
...
The protective relay should have the following
characteristics:
a) Reliability
The protective relay should operate positively and isolate the faulty portion of the
power system as and when required
...
When a fault occurs the protection is required to select
and trip the only the nearest circuit breaker
...
d) Speed
The function of automatic protection is to isolate fault from the power system in a very
much shorter time than could be isolated manually, even with great deal of
supervision
...
A protective system is said to be sensitive, if the
primary operating current is low
...
However the residual magnetism in the field circuit may
provide small voltage build up even when the machine is run upto its rated speed without
excitation
...
Hence
separate protection (Phase & Ground) are provided with low current ranges
...
The
signal is compared with the internal reference
...
b) GROUND FAULT PROTECTION DURING START UP
The generator neutral current as measure in series with the resistance of the secondary of
the earthing transformer is fed to the relay through CT
...
This is compared with the internal resistance references
...
- 237 -
Question and answers
Electrical Maintenance Unit
OTHER PROTECTIONS
a) STATOR EARTH FAULT PROTECTION (64A, 64B, 64C)
The conventional unit type generator has the neutral earthed through a resistance loaded
distribution type transformer
...
Low magnitude of fundamental ground current may flow under normal conditions,
possibly due to generator winding imbalance or due to fault on HV side of generator
transformer or on the secondary of generator PT
...
To allow for these low magnitude earth fault
current, trip setting of the overvoltage ground relay are set to detect neutral displacement
voltage in excess of 5-10% of the phased neutral voltage
...
The second earth fault may result from
insulation deterioration caused by transient over voltage due to erratic, low current,
unstable arcing at first fault point
...
A 100% stator earth fault protection is designed to detect earth fault occurring in the
region of the machine windings close to the neutral end
...
It
works on the principle of monitoring the neutral side and the line side of the component
of third harmonic voltage produced by the generator in service
...
However no third harmonic voltage appear across the star/delta
connected generator, though there will be a certain magnitude of third harmonic voltage
between each phase and ground of the machine output
...
In fact under normal healthy operating condition the third
harmonic voltage generated in the machine is shared between the phase to ground
capacity impedance at the machine terminal and neutral to ground impedance at the
machine neutral
...
V3
= Generated third harmonic voltage
...
VN3 = Third harmonic voltage at machine neutral end
V3
VN3
VL3
Fig (1)
Whenever fault occurs at the point (Figure-2) say F on the machine winding, the voltage
distribution VN3 / VL3 undergoes a change from that during the running condition
...
Similarly when the fault occurs on the phase terminal, VN3 become equal to V3
...
N
Line
Fault
VN3
Faulty
V3
VL3 Healthy
VN3
Healthy
VL3 Faulty
- 239 -
Question and answers
Electrical Maintenance Unit
rd
Fig (2) 3 harmonic voltage distribution during healthy and faulty condition
...
The slopes of two lines are suitably set to ensure stability
...
The 3rd
relay 64C used for the protection of stator earth fault during start-up
...
This current if allowed to persist,
can cause serious over heating
...
In order to avoid unnecessary tripping of the machine, the
time characteristics of the relay should match the heating characteristics of the machine
...
phase sequence current creates magnetic flux wave in the air gap, which induces
current in the rotor body iron
...
Heating occurs in these areas due to
watt loss and quickly raises the temp
...
The input from the
CT which are connected in the each phase of the generator supply (Fig-2) are fed to a
negative sequence filter (Fig-3) which gives an a
...
output voltage proportional to the
negative sequence current
...
The output of the squaring circuit is proportional to the square of the input voltage and is
applied directly to the main timing circuit to give the required relation ship between I2t
and relay operating time (t)
...
This means that even when the negative current is less than the
relay setting, the timer circuit will partially charges and reduces the relay operating time
when the current exceeds the setting value
...
The other input comes from the 0
...
When the both inputs to the
AND gate are present the relay will operate and trip the generator from fault
...
A fourth auxiliary transformer is provided to get a phase
shift of 180o Ic – A in figure–3
...
It can be seen that the output produced
when negative sequence current is present, but zero when the current are of positive
sequence
...
On loss of field, the machine operates as
an induction generator excited by the reactive power drawn from the system to which it
connected
...
One parameter which changes significantly when the machine is subject to severe loss of
excitation is the impedance measured at the terminals and it move into the negative
reactance area
...
OPERATION
Figure-1 shows the fundamental block diagram of the relay vector V and I are voltage
and current input to relay terminal
...
The relay characteristic angle is
continuously variable from 45o to 75o lagging by means of a potentiometer (Q)
...
Output vector S2 proportional to the vector V ± I Z of the voltage mixing transformer (T2)
forms the second input signal of the phase angle comparator
...
The
output of the comparator is fed into a squaring amplifier whose output switches ON for a
positive input and OFF for a negative input
...
The squared output is averaged by an auxiliary element set to just to
operate for an equal mark/space ratio
...
The L/R
ratio of the auxiliary coil and pick up level are accurately set
...
Fig-3 shows the typical circuit connection for field failure protection
of generator
...
If the system stability is retained, the stationary stage will take over
...
If
the angular displacement of the rotor exceeds the stable limit, the rotor will slip a pole
pitch
...
This relay operates on the principle of measuring impedance course on R-X diagram and
operates to trip on pole slipping condition
...
When gen
...
Operation can cause winding stresses, pulsating torque and mechanical
resonance that have potential of damaging the Turbine Generator
...
The high impedance principle is used for thorough fault stability even
under current transformer saturation
...
A
non- linear resistance is connected across the relay to limit the over voltage during
internal fault
...
It does not cover phase to
ground fault as the ground fault current is limited to a very low value
...
- 243 -
Question and answers
Electrical Maintenance Unit
- 244 -
Question and answers
Electrical Maintenance Unit
f) GENERATOR INTER TURN DIFFERENTIAL PROTECTION: (87 GI)
This protection is by means of a differential current relay connected across crossconnected CT on the two parallel winding of each of the phase of the generator as shown
in figure-2
...
A
non-linear resistance is connected across the relay to limit the over voltage during the
internal fault
...
PRINCIPLE OF OPERATION (DIFFERENTIAL)
Fig-3 shows the simplified diagram of differential current protection of generator
winding, the CT’s of both end of the generator winding will sense the current which is
flowing through the stator winding
...
When the fault ‘F’ occurs on the stator winding, the differential current will be sensed by
the CT and these differential current passes through the operating coil of the relay which
gives trip signal to the circuit breaker of the generator
...
A
turbine blade is designed to have its natural frequencies sufficiently displaced from rated
speed and multiples of N (speed) to avoid a mechanical resonant condition that could
result in excessive mechanical Stresses in blades
This is a three stage under frequency protection, which consists of a time delay unit and 3
timer
...
The timer which
gives the cumulative operating time of turbine during under frequency which calls for
turbine inspection/maintenance as per the design formula
...
5-F) t < 3
...
From the above formula, it can be seen that the turbine can be operable at 48
...
The cumulative timer which gives alarm in Data acquisition
system then call for turbine inspection
...
The incoming frequency signal is connected to an input circuit, which then drive an
impulse generator to produce pulse at the beginning of each period of the input voltage
...
A comparator compares the two-time interval and this triggers an adjustable timer, which
then operate the output voltage
...
b) OVER FREQUENCY PROTECTION (81)
Generator over frequency protection is provided to limit the over speeding of turbine,
which leads to greater vibration due to resonance
...
This protection schemes also
similar to under frequency
...
c) GENERATOR OVER VOLTAGE ALARM (59)
This protection give time delayed alarm for continuous operation of the generator at more than
permissible voltage of AVR failure or during manual control of excitation
...
While generator is still in line
...
Since
the field excitation should remain same, The same reactive power would flow as before the motoring and
generator will operate as a synchronous motor driving the turbine
...
It is detected by low forward power relay
...
a) EXCITATION TRANSFORMER OVER CURRENT PROTECTION:
Time delayed over current protection with instantaneous high set unit is provided for the
short circuit protection of the excitation transformer, which trips the field breaker by
energizing class-B trip
...
On overvoltage the gap flasher over and the arrestor connects the resistor directly
across the field
...
Field forcing will happen only when PT
actual voltage value comes down due to the PT fuse drop or due to any other reason
...
That is actual voltage value is less and field forced
to increase the voltage
...
Means in some earth faults in the grid, the voltages may come down
to 110 kV and PT will sense this voltage as the generator is synchronised with the grid
...
If the fault
not cleared the generator will trip after some time delay
...
But in some grid disturbances or power swing conditions the stator and rotor voltage and
current changes
...
This protection is used to protect
machine from this type of over voltage
...
That force may be sufficient to spring the
shaft and make it eccentric
...
The method of detecting rotor first earth fault using the principle of negative biasing,
where by an earth fault anywhere in the field circuit can be detected
...
The rectified output of the supply provides a biasing potential of approximately 65V
...
When the fault occurs, the current flows
through the relay coil which intern operate the circuit breaker
...
- 248 -
Question and answers
d) ROTOR 2
ND
Electrical Maintenance Unit
EARTH FAULT (64F)
While the machine is continuous in service with one earth fault, appearance of 2nd earth
fault will severely affect the magnetic balance in the air gap and result in rotor distortion
and severe damage
...
However, to take care of the situation of 2nd
earth fault appearing immediately after 1st stator earth fault before the machine is taken
out, 2nd rotor earth fault protection is provided
...
The scheme consists of a bridge circuit which to be balanced manually with the 1st rotor
earth fault in the machine
...
It can be seen in the below diagram the protection of the field winding on either side of
the first earth fault and the balancing potentiometer forms a dc bridge with 64F2 (Relay)
connected across the pair of opposite modes
...
The differential transformer protection measuring
circuit is based on the well-known MERZ-PRICE circulating current principle
...
The output from each bias resistance
transformer T3 to T5 proportional to the appropriate primary line currents, are rectified
and summed to produce a bias resistance voltage
...
The output from T1 is rectified and combined
with the bias voltage to produce a signal, which is applied to the amplitude comparator
...
An unrestrained high set circuit, which monitors the differential current, will over ride the
amplitude comparator circuit and operate the relay output element when the difference
current is above the high set settings
...
The currents I1, I2,
& I3 are the output of generator CT, UT CT and GT CT respectively
...
b) OVER FLUXING PROTECTION (59V/F)
This is designed to protect the transformer from damages caused by the flux density in the
core exceeds the designed value
...
Over fluxing can be caused by regulator failure, load
reduction or excessive excitation with generator off-line it can also result from decreasing
speed while the regulator or the operator attempts to maintain rated stator voltage
...
The flux density in the core depends on the ratio of terminal voltage (V) divided by the
frequency (f)
...
Whenever the v/f ratio of the transformer exceeds the pre-set time, the relay will operate
and initiate
• Running down the AVR if the machine is off the bus bar
...
- 251 -
Question and answers
Electrical Maintenance Unit
- 252 -
Question and answers
Electrical Maintenance Unit
OPERATING PRINCIPLE:
The basic principle of the relay is to produce an alternating voltage, which is proportional
to the ratio of voltage & frequency, and to compare this with a fixed voltage
...
At
the end of the fixed timer cycle the second adjustable timer is initiated
...
This V/R is arranged to flow through a capacitor C to produce
an output voltage
V/2 π f RC
...
5 to 1
...
After the end of fixed time delay, the 2nd variable timer
initiates
...
c) GENERATOR TRANSFORMER RESTRICTED EARTH FAULT PROTECTION (64)
In addition to overall differential protection, a restricted earth fault protection covering
the transformer HV winding only is provided
...
The relay is high impedance type and high speed of operation
...
This protection energizes Class-A trip of the turbo-generator
...
The relay is 3-pole version of
very inverse time over current relay plus high set instantaneous over current relay
...
This may also serve limited back up protection function for fault
external to the transformer
...
- 253 -
Question and answers
Electrical Maintenance Unit
e) GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION (51N)
This is a simple inverse type over current relay connected to the neutral CT of
transformer
...
- 254 -
Question and answers
Electrical Maintenance Unit
f) OVER LOAD MONITORING (49)
Measuring oil temperature and winding temperature indirectly monitors the loading of the
generator transformer
...
The oil temperature /winding
temperature trips are routed through Class-C trip
...
The relay has two contacts one
closes on slow gas formation and initiate alarm
...
The relay consists of two float switches contained in a closed housing, which is located in
the pipe from transformer to conservator tank
...
In this two float relay the top float responds the
slow accumulation of gas due to mild and incipient fault, the lower float being deflected
by the oil surges caused by a major fault
...
- 255 -
Question and answers
Electrical Maintenance Unit
• What are the set values of generator protection?
TYPE OF PROTECTION AND ITS SET VALUES
No
...
Generator differential
0
...
2
...
5A(10%) 5000/5
Inst
...
5%
10000/5
5 sec
4
...
6
...
8
...
10
...
12
...
14
...
16
...
18
...
20
...
22
...
24
...
1
...
5 Hz
120%
1
...
1A
----PSM-1
...
– 8
...
2
TMS-0
...
4V
Fault
Stator Earth Fault During Starting 100 mA
Low Forward Power
0
...
77 Hz
GT Winding Temp
...
High
90O C
Class
Class-A1
Class-A1
Class-A1
(stage 2 Tx trip)
16
...
0 sec
Class-A1
--16
...
5 kV/110V
10000/5A
800/1A
-----800/1 A
--0
...
0 sec
Inst
...
Inst
...
TMS=0
...
0 sec
Class-A1
10000/5A
2
...
Class-A2
16
...
3
Class-A2
300/1A
10000/5A
----10000/5A
10000/5A
16
...
2
...
- 256 -
Question and answers
Electrical Maintenance Unit
Arc – the electrical discharge between two electrodes is called the arc
...
- 257 -
Question and answers
Electrical Maintenance Unit
• What precautions should be taken while meggering main generator?
All PT’s are racked out
...
Barring gear shall be kept off
...
Generator flexible lines shall be disconnected to isolate GT/UT
...
44 kc kd f ∅ T volts
...
11
∴Average emf induced / Cycle = ∅ N P/ 60
= ∅ P ∗120 f
60 * P
=2 f ∅ volt
If Z is the number of conductors = 2T (T = two sides of conductor)
emf induced = 2 f ∅ Z =2 f ∅ 2T = 4 f ∅ T
∴ RMS value of emf induced = form factor * emf
= 1
...
44 f ∅ T volts
...
That is for lap winding it is equal to Z and for wave
winding it equal to 2
...
This is negligible
...
c) Voltage drops due to armature reaction
...
In Alternator power factor contributes
more importance in Armature reaction
...
So
voltage variation will not be too much
...
That is armature flux is lagging 90ο by main field flux
...
Due to less field flux less voltage at the alternator
terminals and excitation required is more
...
The
result is additive and main field strength is more and excitation has to be reduced
...
• What is meant by voltage regulation?
If there is a change in load, there is a change in terminal voltage
...
The voltage regulation is
termed as the rise in voltage when full load is removed divided by rated terminal
voltage (Excitation and speed remains constant)
...
PF leading
Terminal
Voltage
PF unity
PF lagging
Load current
Generator voltage characteristics
• Why double squirrel cage motor used in barring gear? Why?
To have high starting torque
...
i
...
∴ T = k * φ (flux)* I2 * cos φ2
...
- 259 -
Question and answers
Electrical Maintenance Unit
In double squirrel cage motor inner cage is low resistive and high inductive
...
In case of inner cage Z (impedance) is
less (XL = 2 f L)
...
So torque
is less
...
At starting inductance is high and the Z is---------
- 260 -
Question and answers
Electrical Maintenance Unit
• What are the logics adopted to close the field breaker?
a) Turbine speed 2880 rpm
...
c) Auto/manual reference minimum
...
e) FB closing circuit healthy
...
The positive phase sequence is represented
below and the vectors arrive along X-axis in order 1, 2, 3 and conscript P has been
used to designate as positive sequence
...
That
is the order is reversed
...
The current or voltages in the 3-phase circuit vary simultaneously in
all the 3- phases
...
E1O
E2O
E3O
- 262 -
Question and answers
Electrical Maintenance Unit
• What is rotor and stator resistance values?
Rotor resistance = 98
...
1 /3
...
1 /3
...
1 /3
...
15 Amps
...
025 mm /1000 hrs
...
9 to 1 kg)
...
Windage losses are less
...
b
...
Hence output per volume is increased
...
No corona discharge, which makes insulation life long
...
Lesser denser and penetration and cooling more
...
No fire risk at purity 4% to 74%
...
Non toxic and low viscosity
...
High thermal conductivity
...
Low conductivity
...
Freedom from fire risk
...
External heat exchanger used
...
b) Relative humidity in % (30% is nominal)
...
5 is acidic and more than 7 is alkaline where oxygen
is not forming)
...
b) Conductivity
...
d) Dissolved oxygen (to trace corrosion and 1
...
- 263 -
Question and answers
Electrical Maintenance Unit
- 264 -
Question and answers
Electrical Maintenance Unit
• What are the logic’s adopted in barring gear motor?
For start permission
a) Local or remote start
...
c) Motor hand barring is permissive
...
e) Turbine speed is <100 rpm
...
35 kg/ cm2
...
d) No one-DG condition
...
b) Barring gear engage or motor speed reached to 1475 rpm
...
d) Bearing oil pressure is >0
...
e) No thermal over load of 42
...
• What is the equation for resistance measurement of PT 100 thermocouple?
°C = (R-100) / 0
...
b) Voltage should in-phase and angle should not be more than 10°
...
d) Frequency should be same and difference of 0
...
e
...
05 Hz is allowed
...
Without stator water recommended
insulation value for the generator is R60 = kV + 1 MΩ
R60 – minimum recommended IR in MΩ of entire winding at 40°C of 60 Sec
...
For the IR measurement 1 kV megger should be used
...
• State HP & LP turbine steam values
...
02 kg/cm
Wetness (I/L)
Wetness (O/L)
Flow
1333 t/h
Temp
250°C
0
...
058%
- 266 -
Pressure
5
...
9°C
Question and answers
Electrical Maintenance Unit
• State turbine governor setting
...
• What is requirement of speeder gear assembly?
To bring the turbine to synchronous speed and get tight lock with grid by grid
frequency
...
• What is the purpose of LLG?
To ensure that the turbine load never exceed the reactor output, to incorporate
turbine follow reactor feature governing system
...
On following occasions the over speed limiting gear acts
...
• What is the logic in lubrication oil pump system?
Normally main oil pump (MOP) will feed the required lub oil to turbine governor
and lubrication
...
3 kg/cm2 6
...
Oil Pump
will start
...
65 kg/cm2 Flushing Oil Pump will start
...
35 kg/cm2 Emergency Oil Pump will start
...
• What is the purpose of TOPP (turbine oil purification plant)?
The purpose of TOPP is to remove the water ingress in turbine oil system from the
gland leaks, cooler leakage, and solid metal particles, which are produced due to rust,
wear of bearings and to normalize the low quality oil
...
- 267 -
Question and answers
Electrical Maintenance Unit
RELAYS
• Write down the relay numbers and their designation
...
RESET)
TRIPPING (HAND RESET)
TRIPPING (SELF RESET)
CONTROL
CHECK ALARM
LIMITING
SEMAPHORE OR MOTOR
NEGATIVE SEQUENCE
OUT OF STEP
POTENTIAL FAILURE
ALARM
FOURTH LETTER
M – SPECIAL VARIATION
Sl
...
SECOND LETTER – MOVEMENT
A
ATTRACTED ARMATURE
B
BUCHHOLZ
C
INDUCTION CUP
D
INDUCTION DISC
G
GALVANOMETER (MOVING COIL)
T
TRANSISTOR
R
S
T
U
V
W
WA
WJ
X
Y
Z
ZS
E E Relay
RE CLOSING
SYNCHRONISING
TIMER OR TRANSFORMER
DEFINITE TIME
VOLTAGE TIME
PILOT WIRE
INTERPOSING
INTER TRIPPING
SUPERVISORY
FLASH BACK (BACK FIRE)
SPECIAL APPLICATION
ZERO SEQUENCE
Application
1
CTM
Motor protection
2
CTU
Locked rotor
...
D
...
T
...
- 269 -
Question and answers
Electrical Maintenance Unit
5
VAGM
Under voltage
6
WDG
Under /Over power for DG set
7
FTG
Under frequency
8
VAPM
Fuse failure
- 270 -
Question and answers
Electrical Maintenance Unit
• What is Knee point voltage?
EMF applied to secondary of current transformer (CT) which, when increased by
10% voltage causes the excitation current to increase by 50%
...
D
...
T?
Inverse time relay with definite minimum time is called IDMT
...
Their effect is to setup a field rotating in opposite direction to the main field
...
• Purpose of over current relay (Inverse); type- CDG
It is a self powered inverse time over current and earth fault relay, used for selective
phase and earth fault protection in time graded systems for A
...
machines,
transformers, feeders etc
...
The relay has a high torque movement combined with low burden and low
overshoot
...
• Purpose of Directional inverse Over current & earth fault relay; type- CDD
Directional phase or earth fault protection of ring-mains, parallel transformers,
transformer feeders, parallel feeders etc
...
; This
relay comprises an inductive disc over current unit with wound shading coils and a
directional high speed induction cup unit
...
The inductive disc unit is thus directionally controlled and it
operates only when the current flows in the tripping direction
...
its current coil is connected in series with the
operating coil of the induction disc unit
...
• Purpose of Over current & earth fault relay; type- CAG
This relays are designed for instantaneous phase or earth fault protection and
instantaneous high set over current protection
...
It consists an operating coil mounted on a
cylindrical iron core bolted to a frame at one end
...
The armature is 'L' shaped and pivoted at its corner so that one arm can be
attracted to the end of the core while the other arm to operate a set of contacts
...
There remains however the possibility of the circuit breaker itself
failing to operate and this hazard is traditionally covered by remote breaker back-up
...
A particular feature of the CTIG relay is a fast reset, which enables
the time delay to be set closer to the breaker trip-time
...
The scheme consists of a centre tapped resistor, a
measuring relay, plug setting bridge, auxiliary relay and rectifier bridge to provide
unidirectional supply to the measuring relay coil
...
Variable sensitivities are
provided by means of the tapped coil whose taps are connected to the plug setting
bridge
...
Under healthy condition no current flows through the measuring relay coil
and in any pole of the battery or wiring insulation failure, current flows through the
measuring relay coil and the relay operates
...
This is because, if allowed to run
with an E/F on the rotor, a subsequent second E/F can cause severe damage to the
machine
...
The start of
the second rotor earth fault detection scheme is a very sensitivity transductor
element
...
The Dc winding of the transductor on the other hand is connected
in series with the rotor E/F circuit
...
e
...
Hence the relay remains deenergised
...
Hence, the applied
voltage is fully available across the relay and it operates
...
In such cases a very sensitive current relay will be required for this
purpose
...
Also, its low burden enables it to be used with existing CT's/ Relays without
affecting the performance
...
The
signal is compared with an internal reference
...
• Purpose of Negative phase sequence current relay; Type- CTN/CTNM
Negative phase sequence current in the stator of a generator, caused due to
unbalanced loads or faults, it induces double frequency eddy current in the rotor
...
The inputs from the current transformers, which are connected in each phase of the
generator supply, are fed to a negative sequence filter which gives an AC output
voltage proportional to the negative sequence current
...
The output from the squaring
circuit is proportional to the square of the input voltage and is applied directly to the
main timing circuit to give the required relationship between I22t and the relay
operates time t
...
The definite time relay
offers a considerable advantage over inverse time relays in instances where there ia a
wide variation in line impedance
...
When the thermal overload relay does not provide protection
against stalling, separate definite time O/C relay type CTU can be used to provide
the same
...
CTU relay combines the advantage of complete static measurements with
characteristic of the robust, well proved attracted armature unit
...
Each phase
comprises a static overload detector and timer, which is accurate over a 10:1 time
setting range
...
Instantaneous high set unit when fitted uses alternate half cycle for measurement and
through a separate level detector drives a separate output relay
...
The overfluxing withstand time is generally found to be varying
inversely with the working flux density in the core, having higher withstand times
during extreme overfluxing conditions
...
The basic operating principle is to produce an ac voltage, which depends upon the
ratio between AC input voltage and the frequency
...
This circuit is a simple operational amplifier
integrator with the provision for V/F pickup adjustment
...
to dc converter
...
The frequency output is
given to a curve shaping circuit, which involves counter and comparators
...
• Purpose of Biased Differential Relay: Type-MBCH
This relay is suitable for protection of two or three winding power transformers, auto
transformers or generators transformer units
...
• Purpose of Digital frequency relay: Type-MFVUM
This relay is used to monitor the frequency of an electrical system
...
The operating principle of the relay is the comparison of the time interval of the
incoming frequency with that of a preset time derived from an accurate oscillator
within the relay
...
The preset time interval is obtained
from an OSCILLATOR and COUNTER and adjustment is achieved using
SELECTOR switches, which drive a DECODER circuit
...
• Purpose of Stator Earthfault Relay: Type-PVMM
- 276 -
Question and answers
Electrical Maintenance Unit
A 100% stator earthfault protection is designed to detect earthfault occurring in the
regions of machine winding close to the neutral end
...
It works on the principle involving monitoring of the
neutral side and line-side components of the third harmonic voltages produced by
AC generators in service
...
This relay is used with on load transformer tap changers and induction regulators to
provide close and accurate automatic voltage regulation on power systems of any
voltage
...
• Purpose of Directional power relay: Type-MWTU
...
25% to 18
...
This makes the relay
suitable for sensitive reverse power applications
...
It is also suitable for low forward power interlock and under
power protection
...
This relay is used to prevent interconnection of badly synchronised supplies
...
Phase measurement is achieved by
algebraically subtracting the 2 supply voltage waveforms and comparing the
resultant modulated beat waveform envelope with a Dc reference voltage
...
• Purpose of Static distance protection: Type-SHPM
...
3 zones of protection are included, each employing separate measuring
elements, one element each for 3 phase-to-phase and 3 phase-to-earth faults
...
Poly phase measuring elements are not used in QUADRAMHO as in some of the
contemporary schemes
...
• Purpose of Static offset MHO relay: Type- YTGM
...
With suitable current/voltage input selection, the relay
can be made to have the required characteristic in the R-X plane for various
applications such as Generator Field failure protection, Generator backup impedance
- 278 -
Question and answers
Electrical Maintenance Unit
protection and as offset MHO relay for use in conjunction with generator pole
slipping protection
...
This power relay is a sensitive Poly phase induction cup unit, providing under power,
reverse power and over power protection
...
The electrical quantities energize
windings on an eight pole laminated stator
...
Only a small arc of rotation is needed to cause contact
closer
...
• Purpose of pole slipping relay: Type-ZTO
...
The relay consists
basically of one directional relay and one blinder relay operating in conjunction with
a 40-80 milli seconds static timer
...
This relay consists of a
rectified AC voltage operated hinged armature unit
...
Each coil is connected across one of the voltage transformer secondary fuses
and under healthy conditions, this coil is short circuited by the fuse and cannot be
energized
...
- 280 -
Question and answers
Electrical Maintenance Unit
GENERATOR PROTECTION
Sl Des
1 87G
2 87GI
3 32A
2/32A
4 32B
2/32B
5 32C
27 A,B
6 37
2A/37
2B/37
7 21G1
2/21G
8 21G2
Description
Generator Differential
Protection
Generator Inter Turn fault
CT ratio
10000/5
Type
CAG-34
Remarks
Setting
P/S = 10% or 0
...
5 A SR = 86
...
5% Fixed
Generator Back-up
Impdence Stage-1
10000/5
VTT-11
WCD-13
VTT-11
WCD-12
VAGM-22
WCD-11
VTT-11
VTT-11
YTGM-15
2
...
5% Fixed
2
...
2 V
0
...
0, K2=1
...
0,
VTT-11
YTGM-15
K4=1
...
0, K10= 0
Zf=14
...
5 Sec
K1=1
...
0, K3=2
...
0, K5=5, K10= -1
Zf=3
...
0 Sec
K1=6
...
0, K3=4
...
0, K5=1
...
7
...
24
2
...
0 Sec
73
...
0 Sec
K1=4
...
0, K3=1
...
0, K5=5, K10= -1
Zf=4
...
0 Ohm
K1=0
...
67, K3=4
...
5A
Vs=5
...
Negative phase sequence 10000/5
VDG-14
CTIGM-15
CTNM-31
2/46G
17 50 ABC
18 49 G
19 50 S
ABC
20 64 F1
21 2/64F1
Timer
Instantenuous Over current
10000/5
Generator Over load protn
...
0 Sec
This is to be set during
commissioning by Alstom engineer
PSM=5
...
3
Setting = 0
...
7, K3= 1
Alarm=70%
120
...
4A, Thermal Ref
...
05 A
First rotor E/F protection
Timer
VAEM - 21
VTT-11
Setting = 1
...
0 Sec
13 64A
- 281 -
Question and answers
22 64F2
Second rotor E/F Protection
23 81 - 1 81-1RL1 Under Frequency Stage - 1
81-1RL2 Under Frequency Stage - 2
Electrical Maintenance Unit
CAEM-33
Setting = 1
...
96 Hz + 1
...
6 Hz
+2Sec,U#2=47
...
6 Hz +4
Sec,U#2=47
...
5Hz +0
...
5Hz +0
...
0 Hz + 1
...
5 Hz + 15
Sec,U#2=51
...
01+2
...
2Sec
24 81 - 2 81-2RL1 Under Frequency Stage - 3
25 81 - 3 81-3RL1 Over Frequency Stage - 1
81-3RL2 Over Frequency Stage - 2
26 81 – 4
Rate of rise of frequency (
df/dt)
Setting = 50
...
0Hz/Sec + 0
...
5 Hz + 0
...
5 Hz + 0
...
B
...
Protn
...
T
...
V
...
T
...
Timer
WTI Alarm
WTI Trip
OTI Alarm
OTI Trip
GT Over flux Stage-1
Timer
GT Over flux Stage-2
Type
MBCH-13
Setting
Settings = 20%
CDG-63
CTIG-39
VTT-11
CAG-14
CDG-11
VTT-11
Timer
VTT-11
PSM=1
...
4, Inst = 800%
Setting = 5%
Setting = 250 mSec
Setting = 0
...
2, TMS=0
...
0 Sec
WTI Set 90 degree C alarm
WTI Set 100 degree C trip
OTI Set 70 degree C Alarm
OTI Set 80 degree C Alarm
Setting K1=1
...
3
Setting = 10
...
15, 99G2A=1
...
0 Sec
GTTM-22
VTT-11
GTT-21
UNIT AUXILIARY TRANSFORMER PROTECTION
Sl
...
1 87 UAT
2 64UAT A /
64UAT B
3 50/51 UT
4 51SN1/51SN2
6
7
8
9
10 AVR
11 50 RYB
Description
Relay Type
Settings
UAT Differential Protection
UAT REF Protection
MBCH-13
FAC-14
Setting=20%
Setting=125 V
UAT B/U O/C Protection
B/U E/F UAT LV-A
CDG-63
CDG-11
PSM=1
...
32, Inst= 600%
PSM=0
...
44
...
Regulated voltage=110 V
2
...
5%
3
...
e
...
95A
WTI Trip
OTI Alarm
OTI Trip
Automatic Voltage Regulator VTJCM-13
OLTC O/C Protection
CAG-39
- 282 -
86C Operation
SUT breaker trip
86C Operation
Question and answers
Electrical Maintenance Unit
START-UP TRANSFORMER PROTECTION
Sl
...
Designation Description
1
2
3
4
5
6
7
8
87 SUT ABC
64 HV
64LVA/LVB
67 ABC
67 N
99SUT
51SNA/51SNB
50Z
2/50Z
9 WTI
Relay Type
Differential Protection
REF Protn
...
LV Sides
Directional O/C Protection
Directional E/F Protection
Over fluxing Protection
LV Side B/U E/F Protn
...
Timer
Winging Temperature
10 OTI
Oil Temperature
11 AVR
Automatic Voltage
Regulator
MBCH-13
FAC-14
FAC-14
CDD-41
CDD-41
GTTM-22
CDG-11
CTIG-39
VTT-11
12 81-5
EMCO EE301-M
SUT Over Frequency Protn
...
75, TMS=0
...
2, TMS=0
...
1, K2=1
...
2, TMS=0
...
Setting=0
...
25 Sec
95 degree C Alarm
105 degree C Trip
85 degree C Alarm
95 degree C Trip
1
...
Nominal Value = 110 V
3
...
75V (2
...
R Setting = 2
...
5%)
5
...
0 Hz + 20
...
• Why REF is now is used for HV side also in GT/SUT?
The E/F setting of differential is poor
...
Therefore very slow for internal faults/terminal faults/uncleared LV
faults
...
• Why do we use O/C & E/F protection on both sides of transformer?
Power in feed exists on both ends
...
Transformer prevents E/F currents from grid into DG neutral
...
Earlier felt that the Electro magnetic stress
due high external fault currents near 4% of neutral may not be high to cause E/F
here
...
• How 100% winding protection is given there?
a) Inject sub harmonic AC current into generator neutral
...
E/F
impedance reduces so current drawn increases and trips (Not used)
...
3rd harmonic voltage reduces to zero
...
• What is the basic purpose of class-B protection?
Class-B avoids load rejection
...
Therefore trip only for internal faults
...
- 284 -
Question and answers
Electrical Maintenance Unit
- 285 -
Question and answers
Electrical Maintenance Unit
• What are the effects of GT over fluxing?
a
...
b
...
Winding temp increases
d
...
e
...
• Why in transformer the LV CB also be tripped along with HV CB for a primary side
fault?
Auxiliary transformer 415v delta star transformer, if HV CB alone tripped then back
feeding from LV side (say DG runs parallel with transformer)—arcing voltage at the
fault on primary—fault fed for more time – more damage
...
Net spill current
will operate
low impedance CAG relay, so high impedance scheme with CAG
relay and stabilising resistor used
...
• What is the advantage of housing CT’s with in bushings?
Bushing acts as a primary insulator for the CT
...
5 kV rated in main generator
even though actual voltage during the E/F is root 3 times less?
The transformer should not saturate during E/F otherwise it will cause
ferroresonance with the GT winding capacitance
...
During loss of load or field forcing conditions, the transformer
voltage increases to cause saturation
...
• Where are the following relays used?
a) Very inverse b) extremely inverse relays c) definite time O/C Relay d)
instantaneous O/C Relay
...
- 286 -
Question and answers
Electrical Maintenance Unit
b) Extremely inverse – Used for feeders subjected to peak making currents
...
g
...
c) Definite time O/C Relay – Where neutral is resistance earthed- fixed ground current
...
Disadvantage – Under minimum generation it may not operate
...
Then for any earthfault on secondary
terminal, the primary current distribution is so for external fault, the differential is
likely to operate if sequence current from flowing into relay
...
• Show the CT characteristics
...
• When will you say that the CT is saturated?
When checking the CT with the secondary injection method a 10% increases in the
voltage causes a 50% increase in the current the CT is said to be saturated
...
In some other case when the currents in the two phases are
compared for relay operation the relay may operate due to unbalance
...
Pass dc current in the secondary and vary it from zero to maximum
...
• What precaution should be taken while removing a current operated relay when the
equipment is in service?
- 288 -
Question and answers
Electrical Maintenance Unit
Ensure that the CT is not getting opened by shorting the appropriate terminals
...
It senses the zero sequence current
...
Alf
...
Metering CT
- Error
...
Differential CT
- Class PS
...
Primary current
Rating of CT
- 1
...
• Why differential protection for PHT motors?
For more than 2500 kW motors it is required to provide differential protection
...
Insensitive to
starting current and stalling current
...
10p
...
At rated current?
5p
- 1 % Ratio error ± 60 min phase error
10p
- 3 % Ratio error ± 60 min phase error
15p
- 5 % Ratio error ± 60 min phase error
• What is the operating point in the Magnetising characteristic of protection CT &
measuring CT?
Protection CT – Operation at ankle point only
...
Then check secondary
for insulation
...
If we use only one secondary winding can we
keep the unused secondary winding short circuited?
No
...
The turns of primary
winding will be shared between 2 secondary windings
...
• But is it advisable to keep the secondary of CT in open conditions? Will not induce
very high voltage?
- 290 -
Question and answers
Electrical Maintenance Unit
If the CT has only one secondary winding, we should keep it always short cktd for
safety, but if the CT has multiple secondary, then if one secondary voltage is kept
limited by suitable loading, then the other secondary voltage is eventually limited
proportionately
...
• What is the 2 stage stalling protection for PHT motor?
Because locked rotors withstand time of motor is less than starting time of motor
under reduced voltage conditions
...
If generator is excited with internal fault the over current 50s trip
the generator to prevent major damage
...
The relays are
polarised dc armature type, sensitive to all frequencies, since the frequency need not
to be 50 Hz initially during start up
...
• What is the standard CT polarity?
Primary current enters at P1 and secondary current leaves at S2
...
It depends on the motor load and the motor winding (star or delta)
...
Current will increase √3 times
...
More heat in stator and rotor parts
...
Insulation failure and short circuit & Ground fault may occur
...
• Where CT secondary of 1A we are using?
For long distance current transmission, to reduce the IR drop
...
• Define knee point voltage
...
- 293 -
Question and answers
Electrical Maintenance Unit
• What is differential protection?
It is the current balance type protection, in which vector difference between current
entering the winding is used for relay operation
...
• What is Local Breaker Back up protection?
In case of local breaker fails to operate during fault due to mechanical failure this
protection will protect the system from sever damage
...
- 294 -
Question and answers
Electrical Maintenance Unit
TRANSFORMERS
• Give transformer nameplate details of GT, SUT, UAT, SET, 415 V Aux transformer
and Lighting transformer
...
L Trans
...
5/14/14 MVA 24
...
9/6
...
5/6
...
9
Volts
235/16
...
13 (14) %
9
...
82%
10 ±10% HV
22 ±10% LV
Vector
YNd11
Yn yno yno
(d1)
D yn1 yn1 Dyn
Oil
42000 Lt
...
19750 Lt
...
5 %
ϒ12% in 1
...
5% steps
Tap change
Tapchange%
3
50 Hz
157
...
Since the zero sequence components are by definition in time phase
with another their sum can not be zero at the junction point as per kirchoff’s law
...
• What is E/F current limit for SUT and UT?
400a limited by 10 ohms resistor
...
6 kV breaker end
...
It can be mounted indoor
...
- 296 -
Question and answers
Electrical Maintenance Unit
• Why 6
...
b) To reduce mechanical stresses in equipment
...
d) To reduce momentary line voltage dip due to ground fault
...
6 kV XLPE (cross-linked polyethylene)
...
• During unit operation can we have one UT feeding both unit 6
...
• During unit operation, can we parallel UT & SUT continuously?
No, due to switchgear limitation
...
6 kV aluminium bus bars?
a) Temperature rises not exceed 90 ºC
...
c) Take care of thermal expansion
...
6 kV side of transformers?
I – Trips 6
...
It gives primary protection for 6
...
I1 – Trips the both HT and LT breakers
...
• Why core balance CT is preferred over residual connected CT’s to sense earth fault
in 6
...
c) High pickup and TMS avoided in IDMT earth fault relay
...
b) Density
c) Coefficient of thermal expansion
d) Viscosity
e) Specific heat
f) Thermal conductivity
...
ONAF – Air forced radiators cooling – 10 to 100 MVA depending on availability of
area
...
- 298 -
Question and answers
Electrical Maintenance Unit
• Why off load tap changer was chosen for GT?
Because our plant works on base load always
...
• What is the type of lightning arrestor for GT?
Zn O (zinc oxide) types
...
Therefore there is no leak of water into oil
...
When transformer operates, due to pressure head between top and
bottom small quantity of oil flows through filters (absorbent material activated
alumina grade g-80 removes the moisture from oil)
...
Absorbent unit is reactivated at regular intervals
...
Efficient barrier between
oil and air
...
• Why main generator/UT is not provided with separate overfluxing protection?
Since GT is provided with overfluxing protection, it is adequate to protect main
generator / UT also
...
If a
generator CB is used, separate overfluxing protection is essential for main generator
...
However the Pressure relief device
closes back when the pressure drops
...
Fire hazard due to transformer oil does not exist after the
closure of Pressure relief device
...
b) But reactance changeover the tap range increases
...
Therefore leakage current from HV
to LV is not included
...
?
a) 3 wire to 4 wire conversion, since neutral is required for lighting load
...
• Why neutrals are solid grounded above 33 kV?
a) Less transient over voltage due to arcing grounds
...
(No neutral shifting)
c) Allows graded insulation of transformer (low cost)
d) Fast E/F protection
...
• Why all 415V transformers are chosen Dyn 11? What are the protections provided
for the 415V transformers?
a) To facilitate interchange
...
Protections
a) Door interlock to trip HT and LT breakers
...
c) Instantaneous O/C and inverse O/C (50 + 51)
...
e) IDMT E/F and restricted E/F (51N + 64)
...
• Why neutrals are solid grounded below 600v?
- 301 -
Question and answers
Electrical Maintenance Unit
Human safety
Permits enough E/F current because ground resistance is large in less than 415v,
hence fast fault clearance,
Equipment safety against over voltage
...
• Why resistance grounding preferred for less than 33 kV and more than 415 V?
a) To limit the earth fault current for equipment safety else, high short ckt forces
dislocate in windings/bus bars etc,
b) Over voltage due to arcing ground reduced
c) Permits earth fault protection (not possible in ungrounded system)
• What is meant by tan-delta measurement?
It is the tan of the angle between the capacitive current and the total current
...
Hence it shows a weak insulation
...
6 kV bus
...
This is achieved by assigning different vector group to the
transformers
...
• Why water pressure is kept below the oil pressure? How it is maintained?
Incase of a heat exchanger tube failure the water should not go inside the
transformer
...
- 303 -
Question and answers
Electrical Maintenance Unit
• What is the requirement oil in a transformer?
Oil is used removal of heat produced in the transformer and also as insulating
medium
...
This will result in higher core loss and subsequent heating
of core and ultimate failure of transformer
...
• What type oil pumps are used?
Canned rotor pumps
...
The treated oil will be in par with the new oil
...
• Why there is no mixing of oil of tap changer and transformer?
When the tap changing takes place arc is struck between the contacts
...
If both oil get mixed up the
quality of transformer tank oil will come down
...
Hence both
oils are kept separately
...
In case HV wining the load current will be less
...
• What is the purpose of conservator?
To accommodate the change in volume of oil during increase in temperature
...
• Why REF is provided in the LV side of SUT and UT?
The LV sides of the two transformers are earthed through the resistance
...
Hence the differential protection
may not act for a LV earth fault
...
• Why twin secondary SUT?
As per is, the rating single secondary power transformer is limited to 25MVA (6
...
6 kV-bus supply was chosen?
11 kV was rejected in view of the high insulation cost with 11 kV motors
...
3 kV was rejected, since max motor size with 3
...
But
we are having the motors having rating more than 2 MW, which cannot suit to 3
...
6
...
- 306 -
Question and answers
Electrical Maintenance Unit
• Why oil transformers are out door?
Oil fire point = 170 ºC easy catching of fire
...
S used in transformers?
IS – 1866 FOR MAINTENANCE AND SUPERVISION OF OIL
IS – 10593 FOR GAS ANALYSIS
IS – 1886 FOR INSTALLATION AND MAINTENANCE
• When oil filteration is required?
On reweaving oil test results
...
• What are the types of oil used for in transformer for cooling?
Paraffin based and naphtha based (in INDIA)
• What are the types of bushing used in transformer?
Condenser type bushing
Porcelain type bushing
• What are the precautions to be taken while terminating the bushings?
Contact surfaces with intermediate plates,
Mating surfaces should be identical
...
• Why ICT are used?(INTERPOSING CT)
To correct the system primary CT errors in case of high current faults out side CT
zone (ICT’s primary CT is 800/1, but in fault current may go to thousands of amps
...
a) Matching the ratios
...
• How CT is connecting in ckt?
If the primary of CT is delta connected load the CT will be in star connection and
vice versa
...
- 307 -
Question and answers
Electrical Maintenance Unit
• What type of gasket and adhesive are used in transformer?
Gasket – Neoprene based rubberised cork type RC70-C
...
This is the replacement for the explosion vent
...
9kV where as bus voltage is 6
...
9kV voltage level adequately takes into account voltage
drop during loaded condition to cater station buses at 6
...
• Why our GT having off load tap changer?
Because our station is base load station
...
6kV buses
...
GT
HV solidly grounded
LV (delta)
UT
HV (delta)
LV cast stainless steel 9
...
SUT
HV solidly grounded
LV cast stainless steel 9
...
• What are the protections for GENERATOR TRANSFORMER?
a) Differential protection
b) Restricted earthfault protection
c) Backup earthfault protection
d) GT phase back up protection
e) Overfluxing protection
f) Oil surge (gas) protection
g) High winding temperature and oil temperature protection
...
LV back up over current and earth fault protection
Over fluxing protection
Buchholz and high oil, winding temperature protection
...
• What is the purpose of carona ring?
To minimize the arcing current during switching operations of disconnecting
switches
...
• Why input transformer of PUPS module 1 is delta-delta and module 2 is delta-star?
With the help of this arrangement, combined DC output from both chargers is
equivalent to that from a 12-pulse rectifier
...
Harmonics injected into system by rectifier
are low
...
- 311 -
Question and answers
Electrical Maintenance Unit
- 312 -
Question and answers
Electrical Maintenance Unit
MOTORS AND MCC
• What is Motor?
It is a device, which converts electrical energy into mechanical (rotating) energy
...
However motors may be rated as intermittent duty,
periodic duty or varying duty and must be turned off and allowed to cool after a
fixed operating time
...
3) Jogging: The starting and stopping of a motor at frequent intervals
...
• What is Motor speed?
The shaft speed of the three-phase squirrel cage motor is determined by the
frequency of the supply voltage and the number of poles in the motor
...
rpm = cycles per second x 60 - slip
Poles
(Where slip is the difference between the speed of the rotating magnetic field and the
speed of the rotor
...
the over current
protection device must be capable of carrying the starting current to exceed 400% of
the motor full load current
...
Overloads
on a motor may be mechanical or electrical
...
Damage to the driven machinery
can be result if plugging is applied improperly
...
- 314 -
Question and answers
Electrical Maintenance Unit
• What is Service factor?
The amount of overload that may be permitted without causing significant
deterioration of the insulation on a motor
...
15, the motor can be safely be subjected to an 11
...
• What is Starting current or Locked rotor current?
The current flow in the motor at the instant of starting
...
The most common locked rotor current is
about 6 times the full load current
...
• What is Torque?
The twisting force produced by the motor is called torque
...
), torque is related to horsepower by the following formula
...
a) MCC are made up of sheet steel enclosure, indoor floor mounting and free
standing, Dust and vermin proof, modular type and of double front and single
front (X1, Y1)
...
c) 0
...
8 * 2
...
d) 0
...
6 * 2
...
e) Parts are incoming panel, Cable entry, TB compartment, MCC module
compartment
...
g) Main buses are horizontally mounted and vertical buses are connected to MCC
cells
...
i) CT and PT are used for current and voltage measurements
...
k) Voltage meter and ammeter are provided
...
m) Control building and SRPH MCC are safety related and SB, TB, RAB, CWPH,
DM Plant MCC’s are non safety related
...
- 315 -
Question and answers
Electrical Maintenance Unit
• What are the types of MCC?
Type
Application
A
Receptacles, Cranes, Elevators, Local control panels
B
Locally controlled heaters
C
Remote controlled heaters
D
Remote controlled loads <29 kW (49 in control panel)
F
For valve motors
G
Locally controlled loads <29 kW
H
Locally controlled loads >29 kW (RTM installed)
I
DG MCC
J
Remote controlled loads <29 kW
K
Remote controlled loads >29 kW (RTM installed)
L
Barring gear motor MCC
SP
For F/M supply and PHT S/D cooling pumps
Protection
Fuse
Fuse, 49
Fuse, 49
Fuse
Fuse, 49
Fuse, 49
Fuse, 49
Fuse, 49
Fuse, 49
Fuse, 49
CT, PT Used
• What is the operating life of bearings?
a) Continuous 24 hrs operation – 40000 to 50000 hrs
...
c) Operating temperature
...
All MCC bus bars are made of aluminium
...
Type
Bus Rating
Class
Location
Cable Used
K1
K2
L1
L2
L3
M1
M2
N1
N2
W1
W2
P1
P2
Q1
Q2
X1
Y1
PMCC S
1000 A
1500 A
1000 A
1000 A
1500 A
1000 A
1500 A
1000 A
1000 A
1500 A
1000 A
1500 A
1500 A
1500 A
1500 A
600 A
600 A
630 A
IV
IV
IV
IV
IV
IV
IV
IV
IV
IV
IV
III
III
III
III
III
III
II
- 316 -
TB 111 Mts
...
RAB 108 Mts
...
5 Mts
...
SB 106
...
TB 111 Mts
...
RAB 108 Mts
...
DM plant 98 Mts
...
CB 106
...
CB 100 Mts
...
5 Mts
...
SRPH 100 Mts
...
5 Mts
...
5 Mts
...
3 kW
> 9
...
8 kW to 2
...
3 kW
9
...
6 to 45 kW
48 to 67
...
It works on principle of joule
law (I2Rt)
...
Current rating is depends on the type of material, cross section area, length and size
of terminal (large size terminal dissipates more heat)
...
These materials having the differential
coefficient of expansion
...
The range of the relay provided for the feeders are such that the full load rating of the
feeders is comfortably within the range of the relay (range will be at Centre) except
in very minimum loads ranging from 0
...
16 A
...
It splits the arc by this arc resistance increases
...
• What are IP (ingressive protection) and IC?
IP means ingressive protection to the motor against the dust and water entry
...
The second digit indicates protection against ingress of water, foreign bodies (liquid
particles)
...
1φ
...
3φ
...
Squirrel cage induction motor for fixed torque
...
Wound rotor motor for variable torque
...
LT motor - <200 kW
...
HT motor - >200 kW
...
Class – A (Normal torque and normal starting current
...
g
...
b
...
c
...
E
...
Double sq
...
Class – D (high starting torque and high starting current)
...
Drip proof (IP 54)
...
b
...
c
...
d
...
e
...
Principle is thermosymphony E
...
- CEP)
...
Explosion proof
...
5 to 2
...
Open drip proof
...
TEFC
...
Totally enclosed air over type
...
Open drip proof
...
Weather protected I
c
...
Totally enclosed water-cooled
...
Totally enclosed pipe ventilated
...
b
...
d
...
f
...
h
...
j
...
l
...
n
...
Excessive moisture (winding IR low and bearing lubrication loss)
...
Poor ventilation
...
Misalignment
...
Excessive starts and repetitive surges
...
Shaft currents (bearing pitting)
...
Manufacture defect or wrong design
...
Maintenance improper
...
Excessive starts may subject stator winding to high current for more time
...
Bearing also may damage
...
High stator current and over heat of stator winding
...
More harmonic currents in end parts
...
High vibration
...
Stator winding loose bracing
...
Rotor high vibration and bearing vibration
...
Frame vibration and more harmonics in side bands
...
- 322 -
Question and answers
Electrical Maintenance Unit
a
...
b
...
c
...
- 323 -
Question and answers
Electrical Maintenance Unit
• What are the effects of imbalance stator winding resistance?
If the stator winding resistances are imbalance phase to phase give pulsating fluxes
and pulsating torque on rotor and vibration may increase
...
• What reflects the change in speed?
a
...
b
...
• What is use of BORESCOPE inspection?
BORESCOPE inspection method makes it easy to observe the end winding condition
of the motor
...
• What you mean by CRAWLING and COGING?
Crawling
The motor fails to rotate at rated speed or motor rotates at … or 20% speed is called
motor crawling
...
Coging
Motor fail to start atoll is called motor coging
...
Because of air gap between stator and rotor
...
• State construction details of the motor
...
Rotor conductor
Heavy bars of copper or aluminium alloy
...
• What is the use of making rotor skewing?
1
...
2
...
• Why under voltage tripping of motor is incorporated in motor feeder breakers?
The under voltage can occur in case of bus fault
...
Due to the back feeding from
the motor the motor will slow down very fast
...
(In case pht motor will not rotate for the designed 3 minute period in
case of bus under voltage)
...
b) VAR load increases on generator leads to heating up of rotor
c) Stator current increases for same power export leads to stator over heating
...
• What is the difference between fixed trip and trip free?
Fixed trip: Breaker will trip only after closing even if trip impulses are existing
...
• What is the making current capacity of a 3-phase breaker as derived from its
symmetrical breaker capacity?
Making capacity = 2
...
• Why intermediate contacts in English electric breaker?
To prevent even slightest arcing on main contacts
...
• Why do we grease the bearings?
a) Grease lubricant gives good protection against ingress of moisture and dirt into
motor
...
c) Low friction torque at starting
...
• What FCN was implemented to avoid reactor trip on 220V-DC failure of PHT and
PPP breakers?
- 326 -
Question and answers
Electrical Maintenance Unit
The breaker close position supervision relay VAA 21 is changed by VAJC type,
contacts position do not change if 220V-DC is lost now
...
b) Oil lubrication recommended
...
d) When heat to be conducted away from bearing
...
f) High viscous oil for low speed machine, low viscous oil for high speed machine
...
• What are the causes of failure of bearings?
a) Faulty mounting
b) Faulty lubrication
c) Foreign matter in lubrication
d) Water in the bearing arrangement
e) Vibration
f) Inoccurrences of form of shaft or housing seating
...
h) Metal fatigue
...
6310
6
= single row deep groove ball bearings
...
5 times the motor rated
current instead of 6 times the rated current
...
Hence the
static starter is set to limit the starting current
...
• What is the speciality of the inverter output transformer? Why it is provided?
a) This eliminates all 3rd harmonics in the output voltage
...
c) The primary has to be star (not delta), since 3 separate inverters operates on
isolated primary winding
...
The inverters cannot withstand this
...
Then why copper contacts are silver-plated?
The silver plating avoids the oxidation of copper, especially in outdoors
...
But its oxide is a good electrical conductor but copper oxide
produce a film of insulation
...
is existing
...
• When the static bypass is blocked?
When the phase error is more than 20º
...
classIII bus
• What is the difference between a contactor and a breaker?
Contactor is not designed to open on short circuit condition (fuse will take care of
this situation)
...
• What is the difference between isolator and contactor?
Contactor is used for on load operation
...
They
posses arc chamber and arc chutes
...
Isolator is off load devices
...
The arc time is
more in slow acting devices and operated only in off load
...
- Short circuit protection
• What are the protections provided in PMCC circuit breakers?
1
...
2
...
- 330 -
Question and answers
Electrical Maintenance Unit
3
...
- 331 -
Question and answers
Electrical Maintenance Unit
• What is requirement of having DG’s?
To establish class III supply when class IV fails
...
6 kV supply
...
• What are the characteristics of CB, OLR and HRC Fuse?
CB characteristics (it is back up fuse)
Current
Margin to avoid fuse operation
During starting
Fuse characteristics
OLR characteristics
Minimum fusing current
Staring current
Running current
Time
CB Protection
Fuse Protection
OLR Protection
I
Time
When CB is used the CB characteristics should be below the fuse, because the CB
should operate first and then fuse
...
Because CB is the main protection
or main breaking device
...
In case of primary and secondary of the control transformer is
getting the main fuse will blow off
...
• What is interlocks provided for the valve MCC
- 332 -
Question and answers
Electrical Maintenance Unit
a) Mechanical interlock, which will not allow the other contactor to close if one
contactor, is closed
...
42-1 contact in 42-2 and 42-2
contact in 42-1
...
5 times kW rating
...
• What is the safety interlock provided in MCC cell?
The MCC cell door can't open if the cell is in on condition
...
5 times the full load current
...
(By filament lamps)
By providing internal heating by applying the low voltage
...
• Why 110V has been chosen in MCC cell?
To isolate control circuit from power circuit for Human safety at control circuit side
...
If it is not provided the stored energy will affect the PLC card
circuit
...
OLR for over load and single phasing protection
...
• What is the plugging of an induction motor?
It is an electrical braking of an induction motor by sudden reversal of phase
sequence
...
Thus these relays permit heavy starting conditions of
motors and offer dependable protection against overload
...
• How many earthing should be done for motors? Why?
- 334 -
Question and answers
Electrical Maintenance Unit
Two
...
• What is the significance of frame size of motor?
In order to make practical choice, interchangeability and large scale production
possible
...
b) It gives a quantitative information about the insulation with respect to moisture,
dirt and other contamination
...
0 indicates a need for immediate reconditioning
...
If insulation is weak it leads to more leakage current
due to high potential
...
S3 – Intermittent periodic operation (resting and loading e
...
cranes, lifts etc)
S4 – As for S3 but with starting
S5 – As for S3 with electric braking
S6 – Continuous cyclic operation
...
Insulation resistance of any electrical machine (motor or generator) should be above
0
...
• What are the classes of insulation?
Y – 90οC (max) cotton, silk, paper, wood without oil impregnation
A – 105οC Materials of class Y impregnated with natural resins,insulating oils
...
B – 130οC Mica, glass fibre, asbestos with suitable bonding substance
...
H – 180οC Glass fibre and asbestos, mica with silicon resins
...
• What are the checks on the motor during the preventive maintenance?
IR Value
Resistance and Inductance measurement
PI value (should > 1
...
- 336 -
Question and answers
Electrical Maintenance Unit
- 337 -
Question and answers
Electrical Maintenance Unit
• What are the causes of motor vibration?
a) Broken rotor
...
c) Slacked rotor core
...
• What are the causes for motor high current?
a) High frequency (51 Hz - 105% current)
b) Low frequency (48 Hz – 102% current)
c) High voltage
d) Low voltage
e) Mechanical over load
• What are the causes for motor unbalance current?
a) Loose connection
b) Voltage unbalance
c) Turns short circuit
• What are the sources of 240 V AC class I supply? What are the functions of each
part of UPS?
Six sources
...
Two 60 kVA UPS for non-safety related loads
...
These all UPS are back up by 220V DC batteries
...
Functions
1
...
2
...
3
...
Inverter
This converts DC to AC supply for loads
...
Static switch
...
Manual bypass
To take main UPS to maintenance by putting stand by UPS into service
...
In
this condition class II power UPS will feed the necessary loads for a 30 minutes of
duration
...
- 339 -
Question and answers
Electrical Maintenance Unit
220 kV SYSTEM
• What is meant by Dielectric strength?
The maximum electrical potential gradient that a material can withstand without
rupture usually specified in volts/millimeter of thickness
...
• Give switchyard specification
...
Type
: Out door
...
Scheme
: Double main bus bar with bypass switching scheme is provided
...
3
...
4
...
Impulse voltage : 1050 kV (peak)
6
...
)
7
...
8
...
9
...
Phase to phase – 2100 mm
...
Sectional clearance – 4300 mm
...
– (Protected) 2800 mm
...
Maximum temperature rise above ambient - 45°C
...
CB – SF6
12
...
13
...
• Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor
...
Make – ABB
2
...
Close Block – 17
...
b
...
7 bar
...
Auto reclose block – 19 bar
...
SF6 pressure block
a
...
2 bar
...
Rated – 6 bar
...
Limit – 5 to 6 bar
...
Open block – 5 bar
4
...
5
...
6
...
7
...
5 kg/cm2
...
Type : High-pressure pressure relieving isolator (HPPR) central pole double
break
...
CB and Isolator clearances
: Phase to Phase – 4500 mm
...
- 341 -
Question and answers
Electrical Maintenance Unit
Current transformer
1
...
2
...
Capacitor voltage transformer (CVT)
This is capacitive potential divider and inductive medium mineral oil sealed
...
make 245 kV/110 √3 V
...
Lightning arrestor
Type
: WS surge arrestor of ZODIVER type and SMX style
...
Multi unit construction for transport, storage and erection
...
Operating voltage
: 184 kV rms
...
• What is the purpose of the CVT?
To provide synchronising signal
To provide voltage indication
To facilitate the carrier communication
• What is the purpose of wave trap?
Carrier communication signals are sent through the lines
...
This signal should be prevented from entering the switchyard
...
Since it is connected in series with the line it
will effectively block the carrier signal entering into the switchyard
...
If
equipment’s
...
In
order to avoid the failure of insulation the LA is used
...
(It will divert the over voltages to earth and protect the substation)
• What is meant by restriking voltage?
The high voltage that will appear across the contact just after the quenching of the
arc is called restriking voltage
...
This voltage is known as switching surges
...
(Arc quenching is taken care by SF6 gas)
• Why switchyard is located indoors of coastal plants?
Saline atmosphere will deposit on the insulators causing its flashover
...
• Why disc insulators grooved at bottom?
To increase the creepage distance, reduce the chances of flash over
...
• What is the material of cap, pin, and insulator?
Cap = galvanised cast iron
Pin = forged steel pin
Insulator = porcelain
...
5 ohms
...
125A/1A – SUT
• Advantage of CVT over EMPT
...
• What are the main parts of 220 kV Circuit Breaker?
Pole column filled with SF6
Pneumatic drive system with compressed air circuit
Control cubicle unit
• What is the type of 220 kV circuit breaker?
220 kV, SF6 breaker, single pole, puffer type
...
2 bar
- alarm
5
...
- 345 -
Question and answers
Electrical Maintenance Unit
• What is the difference between circuit breaker and isolator?
Isolator is a disconnecting switch which is not having the making and breaking
capacity
...
(To discharge the residual potential)
• What is the type of Lightning Arrestor?
Station type, heavy duty, gap less zinc oxide
...
• What are the properties of SF6 gas?
Physical properties
1
...
Odorless
3
...
Pure SF6 gas is not harmful to the health
...
Non-inflammable
...
Density- more gas density, 5 times that of air at 20°C and at atmospheric
pressure
...
The temperature of
liquefaction depends on pressure
...
Hence this gas is not suitable for high pressures >15 kg f / cm2
6
...
5 times that of air at same pressure
...
Chemical properties
1
...
2
...
The life of the metallic parts, contacts is
longer in SF6 gas
...
Hence the
maintenance requirement is reduced
...
In the presence of the moisture, hydrogen fluoride is formed
during arcing which can attract the metallic and insulating parts in the circuit
breaker
...
Electro negative gas – Ability of an atom to attract means carrying a negative
electric charge
...
- 346 -
Question and answers
Electrical Maintenance Unit
• What are the protections are there for BUSBAR?
Instantaneous over current protection
Bus bar differential protection
Local breaker back up protection
- 347 -
Question and answers
Electrical Maintenance Unit
• What are the protections are there for lines?
Directional earthfault protection
Directional over current protection
Local breaker back up protection
Pole discrepancy
Main protection (distance protection)
Directional OverCurrent Relay For Line-1&2
MICOM-P127
(This relay is in addition to the existing electro-mechanical directional O/C relay)
Setting Details
CTR= 600/1A
VTR= 220kV/110V
Directional Over Current Setting
Description
IDMT Directional over
current (Stage#1)
Symbol in Relay
Set value
Secondary 1
...
1
LED Indication
(67ABC)
Directional
O/C(Stage#2)
Secondary 3
...
(67ABC)
Directional
O/C(Stage#3)
Secondary 5Amps
Primary (3000Amps)
Instantaneous
...
2Amps Primary
(120Amps) TMS = 0
...
Secondary 6Amps
Primary (3600Amps)
Instantaneous
...
0Amps Primary
(800Amps) TMS = 0
...
94 Amps
Primary
(750Amps)
Instantaneous
...
5Amps
Primary (2000Amps)
Instantaneous
...
(67N)
Directional Earth Fault
(Stage#3)
Secondary NOT USED
Primary
Instantaneous
...
(21)
QUAD
1
0
32
0
0
...
6
0
...
5
6
0
...
120
QUAD
2
0
32
1
0
...
04
1
1
2
0
...
9
1
1
0
...
5
N/A
Inst
...
8
N/A
4
0
...
02
1
0
...
02
1
1
2
0
...
5
1
2
0
...
25
1
400
800
LENT
4
0
...
3
0
...
3
0
...
5
4
0
...
7
0
...
)
CT RATIO
PSM
TMS
High set
3
PSM
TMS
High set
4
PSM
KNOB
2/50Z
5
2/47T
6
TIMER(29)
7
PSM
TIME
8
KNOB
2/50 ABC
LINE-02
LINE-03
LINE-04
REMARKS
600/1 A
600/1 A
800/1 A
800/1 A
1
...
1
5A
1
...
1
5A
1
...
2
5A
1
...
2
5A
L#2 Settings
changed on
18/6/02 & L#1 on
21/6/02
L#2 Settings
changed on
18/6/02 & L#1 on
21/6/02
0
...
1
2A
20%(0
...
225
400%(4A)
20%(0
...
225
400%(4A)
0
...
25 sec
0
...
25 sec
0
...
25 sec
0
...
1 sec
0
...
sec
94%
MIN
...
2A
0
...
2
1
0
...
1 sec
ISOLATOR
PARALLEL(29)
25 sec
POWER RELAY
(32)
N/A
N/A
INST, OVER
CURRENT
RELAY
(50A,,B,C
...
The state of charge of a battery not knowing by the specific gravity of a battery
...
• What are the problems of hydrogen concentration?
If the concentration of hydrogen more than 4% and less than 74% explosion
problems will be there
...
• Why lead acid battery requires so much large initial charging?
Initially for a new battery, negative plate will be PbO instead of Pb
...
• What are the protections adopted in UPS or PMCC supply?
LV (incomer) or UPS input
CTZM, Over current, Short circuit protections
PMCC S or T input or UPS output
Under voltage (27), 51RYB, 51N protections
...
PbO2 + H2 SO4 PbSO4 +2 H2 (during discharge)
Pb + SO4 PbSO4 (during charge)
• Is the chemical reaction of plante cells same as tubular lead acid cells?
No
...
During charging, H2O H2 + O2
O2 react with Pb to form PbO2 + (+ve plate)
During discharge, Pb + c PbSO4 (on -ve plate)
While PbO2 Pb + O2 (on +ve plate)
...
Therefore PbSO4 formed only on -ve plate
...
• Why battery room should be located separately in a power station?
Possibility of battery explosion
Corrosive atmosphere by acid spray
...
- 352 -
Question and answers
Electrical Maintenance Unit
• What are the disadvantages of nickel cadmium battery?
Status of charge not known
Number of cells are more
Cost very high
Environmental protection agency considers cadmium as a hazardous material,
difficult to discard at the end of life
...
Lower electrolytic temp - ah capacity reduces since chemical reaction rate reduces
...
• How H2 O2 generated in lead acid battery?
At end of charge, when most of the Pb is converted
...
O2
appears as gas at positive plate
...
e
...
• Why current reduced after gassing?
Excessive gassing shortens the life of battery by scouring the active materials at the
surface of the plates
...
Therefore more charging current require more water consumption
...
Shedding increases with overcharging, heavy discharge, batteries short ckt
...
• Why temperature correction required?
As temperature increases, specific gravity decreases
...
Therefore 10ºC raises,
7 points to be added,
corrected to get 27 ºC reading
...
If the cell is left under charged, lead
sulphate would form which will not reverse back into lead and lead peroxide during
charging
...
- 354 -
Question and answers
Electrical Maintenance Unit
• What you meant by shedding?
During the charging and discharging the active materials will undergo volumetric
changes
...
This loss of
active material is called shedding
...
• Acid should be poured to water
...
Hence there is chances of
splashing of the liquid
...
In the
other case splashing will be of water with concentration of acid, which will not
hazardous as the other one
...
Even if one ground has occurred then also these controls should be available
...
• Why battery capacity limits to 20 minute?
Battery cost is more
...
20 min, is enough to shutdown the unit safely
...
Container
b
...
Lead negative plates
d
...
Separators and retainers
f
...
Inter cell connector (lead plated copper)
• What are the different types of charging?
(Normally always) Float charging – 2
...
Equalizing charging (2
...
Boost charging
Boost charging is a quick charging process, which is generally required, if the battery
is drained to a large extent
...
• What are the effects of over charging?
Gassing
Heating
Loosening of plate active material
• What you mean by Drooping characteristics of charger?
When the charger is connected to excess load of charger rating the charger should
able to supply the load with out over loaded by maintaining the terminal voltage
within limit without over load trip
...
• What are the effects of under charging?
Sulphation
Buckling of plates
• What are the effects of high temperature?
Gassing of electrolyte and evaporation
Service life is halved for every 8 deg increase above 25 deg
...
• What type of thermometer is used for acid batteries?
Alcohol type thermometer
...
• What are the functions of charger?
1
...
2
...
3
...
4
...
5
...
- 357 -
Question and answers
Electrical Maintenance Unit
• What is the rating of switchyard battery charger and batteries?
Switchyard is having Float cum boost charger of 282 V DC maximum and 100
Amps rated charger of six-pulse full wave thyristerised controlled rectifier
...
+ve plate is made up of low antimory lead selenium
(Pb) and –ve plate is made up of paste plate type (O2)
...
1
...
2
...
3
...
8 V DC and total number of cells are 106 in battery bank
...
Float voltage 2
...
18 V per cell
...
Maximum system voltage is 106 * 2
...
Specific gravity 1110 ± 5 and specific gravity after 10 hrs discharge is 1150 ± 5
...
Over load (49)
...
Over voltage (59)
...
Short-circuit (3250 Amps)
...
Phase sequence and phase fail
...
di/dt and dv/dt protection
...
6 kV, and 415 V
...
6 kV (SF6 circuit Breaker)
415 V (Air Circuit Breaker)
Type
HPA12/1240C(Tr
...
/ PM)
610 (Tie/PM/MCC)
3037 (B/c, Incomer)
Standard
IEC 56
Rated voltage
12 kV (6
...
)
Making current
100 kA
105 kA (peak)
Short ckt withstand 40 kA/sec
50 kA/sec
Closing time
52 milli sec
60 sec (III/IV) & 30 sec (II)
Opening time
75 milli sec
35 sec (III/IV) & 40 sec (II)
SF6 pressure
2
...
8 bar (2
...
Breaker is adequately de-rated for use in dc circuits
...
DIT 5 will not work for dc
...
ACCB
The inherent current zero of sine wave helps arc quenching
...
• What are the indications used in 415V and 6
...
6 kV Breaker
Open
Green
Green
Close
Red
Red
Test
White
Service
Blue
Auto trip
Yellow
White
Spring charge
Blue
Gas pressure
Yellow
Voltage (RYB)
Red
- 359 -
Question and answers
Electrical Maintenance Unit
- 360 -
Question and answers
Electrical Maintenance Unit
• What are the advantages and disadvantages of 415V English Electric breaker?
Advantages
1
...
2
...
3
...
4
...
Switches contacts
...
Slow closing facility independent of closing spring
...
, Spring charging time 14 sec 4 sec at
timing, trip extends in test position also
...
• Type of closing spring is compression type
...
• Why auto transfer is blocked for back up protection?
Because Backup protection operates normally for bus faults
...
• What decides the control transformer VA rating?
Contactor coil VA rating
...
• Can we use ac coils in dc circuit?
Yes with economy resistor in series
...
Why not for dc
contactor?
Because the force developed is not steady in ac therefore contacts will chatter but if
shading ring is used force developed becomes steady due to splitting of phases of
flux, therefore contacts becomes bounce free and humming sound reduces
...
Poor surface property , large closing force
required
...
Lower voltage drop)
- 362 -
Question and answers
Electrical Maintenance Unit
• What are the types of contactors?
Type
Contacts
Rating
3 TB 41
2 NO + 2 NC
12 A
3 TA 22
2 NO + 2 NC
30 A
3 TA 13
2 NO + 2 NC
38 A
3 TA 24
4 NO + 2 NC
70 A
3 TA 16
4 NO + 2 NC
105 A
3 TA 28
4 NO + 2 NC
170 A
3 TA 28
3 NO + 3 NC
170 A
3 TB 56
4 NO + 2 NC
400 A
3 TD 11
2 NO + 2 NC
12 A
3 T I 22
2 NO + 2 NC
30 A
Use
0 – 5
...
5 – 11 kW
15 kW
18 – 38 kW
40 – 48 kW
55 – 80 kW
55 – 80 kW
93 kW for F/M supply PM
...
5 kW for valve motors
...
6 – 7
...
• What material used for contactor?
Silver-nickel for <100A
Silver-cadmium oxide for large currents
...
Large force is required to overcome the high reluctance
initially
...
Hence drop voltage is reduced
...
• How the contact resistance can increase?
Humidity + salty air, dust, poor contact pressure
• Fusing current- the current at which the fuse element melts depends upon the
material, length and diameter
...
25 -1
...
• What is rated current and short circuit current?
Rated current = VA / √3 * V Amps
...
• What is the advantage of lower cut off current?
- 363 -
Question and answers
Electrical Maintenance Unit
Less electromagnetic forces on contactor / CB’s
• How two fuses in-series are discriminated?
Total I2t of minor fuse should be less than pre-arcing I2t of major fuse
...
5 times the minor fuse
...
• What is the safety reason to keep the contactor / MCC remote from motor?
Contactor = sparking equipment
...
• Where wound type CT used and where not used?
Used where low CT ratio req
...
Current exists
...
• Why fuses with fusing factor more than 1
...
Full loading of
PVC only possible if it has close excess current protection (i
...
) Fusing Factor = 1
...
Consistent and stable characteristics for accuracy of discrimination
...
It is inverse time characteristics, as the
current is high the time taken to break the circuit is less
...
Arc quenching is reliable
...
Quartz does not produce more gas after observing
heat as its sand powder observes more heat of the arc
...
Non deteriorating since it is sealed
...
Characteristics
1
...
This determines the energy that element can pass and to
determine the cut off characteristics
...
Inverse time characteristics, which is useful for selection of the fuse for motor
...
2 0
...
7 0
...
O/C current relay?
Inv
...
CMM: accounts for both +ve and –ve sequence currents i
...
Single phasing /
unbalance supply conditions and gives three times more weightage for the –ve phase
sequence current heating than + ve sequence current heating
...
e
...
Therefore CMM relay protection characteristic is closely matched to motor heating
characteristic
...
• What is the purpose of anti-pumping relay?
When closing signal is continuously existing even after the closing of the breaker the
anti-pumping relay will be picked up and it will not allow the breaker to close back
in case of tripping of the breaker
...
So if the air pressure is low effective arc quenching will not take
place which will result in damage of CB
...
• Why neutral breaker used in DG neutral grounding?
In case of high earth fault currents it is therefore normal practice to install a circuit
breaker in the neutral of the generator in order to reduce the total fault clearance
time
...
51 (inverse over current)
2
...
27 (under voltage)
4
...
64 (REF)
Class IV 415 V LV side
1
...
51N (earth fault)
- 368 -
Question and answers
Electrical Maintenance Unit
• What are the protections used in 415 V Class II side?
LV to UPS
1
...
50 (instantaneous over current)
3
...
51 RYB (inverse over current)
2
...
51N (earth fault)
From Class III to Class II tie
CTZM at Class III and 51, 51N at Class II
...
Insulation level – it is the combination of rated voltage, the corresponding impulse
withstand voltage, which together characterize the insulation of the equipment as
regards its ability to withstand the electrical stresses
...
Making current – it is the peak value of first loop of current of short circuit current
which the circuit breaker is capable of making at the rated voltage
...
5 times rated breaking current
...
Impulse withstand voltage – it is the amplitude of the standard voltage wave with the
insulation of equipment can withstand
...
• What is switchgear?
Equipment which is used for switching, controlling and protecting an electrical
circuit
...
6kV and 415 V
...
per phase, 1 no
...
6kV
Copper
Silver coated joints
Insulation level – 27kV
Fault level estimated – 27kA
Designed – 40kA
• What is the type of arc extinction in switchgears?
415V – resistance method (through arc splitter)
6
...
When the sum of two is
zeroing the arc is quenching
...
• What is the significance of SF6 gas pressure in 6
...
5bar)
Yellow – pressure for breaking system fault current (refilling should be done)
Red – SF6 pressure less than 2 bar, which indicates leak in the system
...
033 kg/cm2
...
07031 kg/cm2
...
6 kV?
Closing coil and Trip coil 2 supply from one source
...
Protections supply from one source
...
1 and 86
...
1 and under voltage protection is wired to 86
...
- 371 -
Question and answers
Electrical Maintenance Unit
• What is the difference between lockout of 6
...
Manual / auto reset
...
There is no
mechanical latch
...
6 kV it is of mechanical latch type relays
...
6 kV breakers
1250 A breaker contact resistance is <100 µς
...
If contact resistance is found more than this value should be sent to the manufacturer
for repair
...
6 kV switch gear?
6
...
Service lever cannot move while breaker closed
...
415 V
Breaker door cannot be opened when breaker is in service
...
Breaker closed cannot be rack in or out
...
6 kV)
Bus X to Bus Y
(CL III 415V)
Bus S to Bus T
(CL II 415 V)
• What are the properties of SF6 gas?
This is inert gas
...
The special property of this gas is
Electro negativity
...
- 372 -
Question and answers
Electrical Maintenance Unit
- 373 -
Question and answers
Electrical Maintenance Unit
CABLES
• What are the purposes of cable trays?
a
...
Give mechanical support
• What are the disadvantages of paper insulation?
a
...
b
...
• What are the advantages of XLPE?
a
...
Easy maintenance
c
...
No sheath (no fatigue)
e
...
High dielectric strength
g
...
More rated current, overload, short ckt capacity
i
...
Very light
k
...
• What is the specified cable life?
50 years
...
3 core cables -- grounded at both ends but not including core balance CT’s, since
even small induced current causes 50N operation
...
• How the required conductor size can be reduced by use of HRC fuses?
HRC fuse limits the peak amplitude of fault current
...
- 374 -
Question and answers
Electrical Maintenance Unit
• What are the types of cables used in 6
...
6 kV system AC (unearthed) grade having stranded aluminium conductors
cables are used
...
1
...
2
...
In RB copper conductor stranded cables are used
...
FS – fire survival insulation
...
HR PVC – heat resistant insulation
...
Their
insulations are HR PVC and FS type
...
1 kV grade cables used for 415V?
To take care of the both earthed / unearthed systems
...
Continuity of metallised paper ensures less voltage gradient, hence preventing
puncture of insulation
...
• Why bimetallic washers provided in aluminium copper transition joints?
To avoid galvanic corrosion failure
...
- 375 -
Question and answers
Electrical Maintenance Unit
- 376 -
Question and answers
Electrical Maintenance Unit
EMTR AND AUTO TRANSFER
• What is meant by EMTR?
In case of normal supply failure to CL-III & CL-II the loads will be fed by the
alternative/standby sources
...
• How EMTR is initiated?
EMTR is initiated on sensing the CL-III & CL-II bus under voltage
...
The DG’s will start
...
DG breaker will close on dead bus
...
CL-II bus under voltage
...
• What does load-shedding mean?
When there is only one source to feed the two buses, the total loads can not be fed by
this single source
...
• What does total load-shedding mean?
Even after the load shedding the is continue to deliver more than the rated power
sensed by overpower relay or running with under frequency sensed by the under
frequency relay the total load shedding will take place
...
• What does auto transfer mean?
If one of the sources is tripped on main protection its breaker will be tripped and the
tie-breaker will close
...
To restore the class IV whenever the UT or SUT is lost Auto
transfer is provided
...
Hence the auto
transfer is prevented
...
Fast transfer scheme to close the tie-breaker in less than 200 milli seconds
...
- 377 -
Question and answers
Electrical Maintenance Unit
2
...
- 378 -
Question and answers
Electrical Maintenance Unit
• What are the conditions for Auto transfer scheme?
Backup protections are not allowed to initiate the Auto transfer
...
Similarly protection in LV of the transformers are not
initiating the Auto transfer scheme
...
Conditions
1
...
2
...
3
...
4
...
5
...
• What are the uses of EMTR scheme?
1
...
2
...
3
...
4
...
• What are the sequences of motor load restoration in EMTR?
1
...
2
...
3
...
4
...
5
...
6
...
7
...
8
...
9
...
10
...
11
...
12
...
• Why synchronizing scheme has been adopted? What are interlocks provided?
To check running and incoming buses, which are going to be interconnected are in
synchronism with each other
...
Synch selector
...
Only one breaker can operate at a time
...
Master synch relay contact should available (bypass will bypass this synch
contact)
...
- 379 -
Question and answers
Electrical Maintenance Unit
• What are the settings provided in synchronizing scheme (SKE Relay)?
Voltage – V1- V2 = 10%
Frequency – t = 0
...
5 Hz)
% Slip – 0
...
- 380 -
Question and answers
Electrical Maintenance Unit
• What are the major losses in nuclear power generating system?
Condenser
- 500 MW
...
C/V and E/S system - 3
...
• Give the specification of DG and NGR of DG
...
KVA
2815
Volts
6600 V
Amps
246 A
Phase
3
Frequency
50 Hz
Insulation
Class-F
PF
0
...
2 A (Brush – less of permanent magnet, electronic
automatic voltage regulator)
Ambient temp 50°C
NGR specification
Resistance at 20°C
Voltage
Insulation class
Transient current
Continuous rating
95
...
6 kV/√3
7
...
10 Amps
...
Whenever class IV fails DG sets (2 + 1 standby) are started by EMTR and capable
of restoring class III loads within 30 Seconds
...
Event
Minutes Seconds
Milli seconds Difference
Class IV fail
00
00
280
00:00:280
EMTR initiate
00
01
303
00:01:023
DG start (1, 2, 3)
00
01
336
00:00:033
Voltage, Speed reached 00
07
257
00:05:921
CB 351, 361, 370 Closed 00
07
491
00:00:234
- 381 -
Question and answers
Electrical Maintenance Unit
Restoration will be done in 07 seconds and 491 milli seconds
...
Then
load restoration starts as per EMTR scheme
...
DG’s are capable of paralleling with 6
...
3
...
- 382 -
Question and answers
Electrical Maintenance Unit
• What are the design criteria’s of DG’s?
1
...
2
...
3
...
4
...
5
...
6
...
7
...
8
...
9
...
10
...
6
kV and 0
...
11
...
12
...
Because
in case of unit is not avail then for cooling water is available in other unit
...
Monorail of 3 Ton is provided
...
Provisions are made for filtered air and ventilation and combustion
...
Co2 fire fighting system is provided for smoothening effect in case of fire
...
Active process water from class III is provided
...
Starting air system
...
This system operates at high pressure and also provided with soft start of 8-bar
pressure in testing of system periodically
...
Lub oil system
...
This also
supplies oil to bearing lubrication, crankshaft, piston, and wiper
...
Pre- lubricating circuit is controlled by PLC, but at first commissioning
and overhauling pre-lub start immediately
...
Water cooling system
...
Fresh water chemistry is controlled to avoid organic growth and corrosion
...
This water is cooled
by active process water
...
Before transferring fresh water chemical addition should be done
...
Fuel oil system
...
Low-pressure fuel from fuel pump is supplied to individual injection
pump is injected to individual cylinders through injectors
...
Combustion air and exhaust gas system
...
Each bank cylinder is provided with a Turbo – charger
...
The exhaust is passes through silencer
...
Speed governing system
...
The governer is linked to the fuel racks
...
For full load 30°is sufficient and remaining for overloading
...
Speed droop setting
...
Oil sight glass
...
Load limiter
d
...
e
...
7
...
One starts at
respective DG breaker close and other at >45°C
...
8
...
For
DG – 1 auxiliaries MCC P1
...
9
...
10
...
• When DG’s start is not possible?
- 384 -
Question and answers
Electrical Maintenance Unit
1
...
2
...
3
...
• What are stages of DG starting?
1
...
2
...
3
...
4
...
5
...
- 385 -
Question and answers
Electrical Maintenance Unit
• What are the protections provided for DG?
1
...
2
...
3
...
4
...
5
...
6
...
6 kV switchgear protections
...
Excitation failure
...
Emergency stops push button
...
- 386 -
Question and answers
Electrical Maintenance Unit
Electronics
• What is Diode?
Diode is a two-layer semiconductor device, conducts only positive cycles when
applied to anode
...
It is also called Silicon Controlled Rectifiers (SCR)
...
• What is material used in making semiconductor?
Silicon & Germanium are the raw materials used for making semiconductor
...
• What is material used in making non-linear resistor & purpose of it in field discharge
resistor?
Silicon carbide materials used for making non-linear resistor
...
These resistors are
connected in parallel to the main field winding (Rotor)
...
Depending on the type of construction of the
machine and the slip this voltage can become un-permissibly high, for this purpose
over voltage protectors are provided in the field breaker cubicle
...
With a
higher than 90º the negative areas are greater so the total voltage becomes negative
...
• What is the purpose of RC network across thyristor?
RC network across each thyristor protects against Hole Storage Effect
...
They also effect a proper sharing of load
among thyristor bridges when connected in parallel
...
• What is purpose of Load angle limiter?
Load angle limiter, it either limits the angle between grid load center and the rotor
axis or generator terminal & rotor axis
...
• What is purpose of Rotor current limiter?
It limits overloading of rotor
...
• What is purpose of Slip stabilization?
It avoids oscillation of the AC machine (Rotor oscillations)
...
• What is the advantage of Static Excitation?
Fast response time, high reliability, interchangeability of parts during operation, less
wear & tear due to static devices & less maintenance
...
• When the negative sequence reactance arises?
Negative sequence arises whenever there is any unbalance present in the system
...
• When the Zero sequence reactance arises?
If a machine is operating with an earthed neutral, a system earth fault will give rise to
zero sequence current in the machine
...
• What is Voltage?
Voltage is the difference in potential (charge) between two points or voltage is the
amount of driving force or pressure applied to a circuit, it is measured in Volts (V)
...
A
resistor restricts or limits the amount of current flowing in a electrical circuit, it is
measured in Ohm (Ω)
...
The total resistance is equal to the sum of the individual resistors
...
I tot = I1 = I2 = I3
The applied voltage divides across each component in a circuit in proportion to the
resistance of the component
...
The total resistance of parallel resistors is equal to the reciprocal of the
sum of the reciprocal of the individual resistors
...
However, carbon composition resistors are too small for this
method of identification so a color coding system is used
...
Each color represents a numerical value as indicated below
...
1- Gold ±10% - Silver }
2- Red
5- Green
8- Grey
0
...
The resistance
value is determined as follows:
4 7 00 = 4700Ω
Yellow = 4
Violet = 7
Red
= 2 (two zeros)
Gold
= ±5%
4700+5% = 4935} The actual resistance should be between 4467 and 4935 ohms
...
001% per 1000 hours
Orange 0
...
1% per 1000 hours
Brown 1
...
I= V/R
...
" Σ V= 0
• What does Kirchhoff's current law states?
Kirchhoff's current law states that " the algebraic sum of current entering and leaving
a node is zero
...
)
• What is a Capacitor?
When two conductors are placed side by side, separated by a nonconductive
material, and connected across a battery, free electrons drift in the direction of the
- 391 -
Question and answers
Electrical Maintenance Unit
driving voltage is called capacitor
...
- 392 -
Question and answers
Electrical Maintenance Unit
• What is Inductor?
Inductors use the ability of electrical current to create a magnetic field
...
The more times the wire is coiled and the more current there is in the coil,
the greater the strength of the magnetic field
...
• What is Inductance?
The property of a coil that opposes a change in the current flow is called inductance
...
Inductance is proportional to the
square of the number of turns in the coil
...
The larger the diameter of the coil, the higher the
inductance
...
4) The length of the coil
...
• What is Power?
Power is a rate of doing work, or works done per unit in time
...
Power in watts is equal to the product of the applied voltage
and the current flowing
...
The components of an ac circuit causes a time period to be
introduced between current and voltage; that is, current and voltage are out of phase
...
2) Capacitive reactance (XC): measured in ohms, is the opposition to a change in
current flow provided by a capacitor
...
3) Cycle: The portion of a waveform contained in one period of time
...
7071 times the
maximum value)
...
6) Impedance (Z): The opposition to current flow in an ac circuit
...
7) Inductive Reactance (XL): Inductive reactance, measured in ohms, is the
opposition to a change in current flow produced by a coil of wire
...
8) Instantaneous value: The magnitude of a waveform at any instant of time
...
10) Periodic waveform: A waveform that continually repeats itself after the same
time interval
...
Resistance
in an ac circuit acts the same as resistance in dc circuit
...
It is always connected in parallel to the power
supply
...
It is always connected
in series with the load
...
It is primarily used to test the insulation of conductors
...
• What is Clamp-On Ammeter?
Clamp-on ammeter is used to check the current in a circuit, without being physically
connected in a circuit
...
• What is Infrared or Thermal scanner?
Infrared or thermal scanners are used to measure temperature without contact with
the equipment
...
• What is Phase sequence indicator?
Phase sequence indicator used to indicate the 3-phase direction- comes in two styles;
Lights and meters
...
• What is Rotation tester?
This device is used during the installation of a motor to determine the direction of the
motor once it is installed
...
- 395 -
Question and answers
Electrical Maintenance Unit
DC Machines
Conversion of one form of energy into another enables us to use natural power
sources as well as manufactured power sources to produce our electrical power
supply
...
• What is Electric Generator?
Electric generators are called a dynamo that converts mechanical energy into
electrical energy
...
• How electromotive force is created in a generator?
When a conductor cuts the magnetic lines of forces, an Electro motive force (emf) is
generated
...
• What is DC motor?
An electric motor converts electrical energy in to mechanical energy
...
DC series motor: series motor speed varies greatly as load changes
...
This
motor has the good starting torque characteristics provided by the series field, while
the shunt field provides for a relatively constant speed
...
The principles involved in control functions, components
and circuit analysis, along with the basic laws of electricity
...
Don't jump to
conclusions
...
Learn how the equipment in your
area is supposed to operate both electrically and mechanically
...
Electricity can be dangerous
...
Be careful when opening the circuit
...
•
No matter how complex or expensive an electrical control system is, the
components of the system begin to deteriorate as soon as they are installed and
failure of some components in the system will ultimately result
...
•
Troubleshooting can be generalized in 3 steps:
1) Determine the symptoms; that is, find out how it acts
...
)
2) Decide by logical reasoning what might be wrong
...
)
3) Determine what has to be done to correct the problem
...
•
The first step- determine the symptoms- can best be accomplished by working
with the machine operator and following the machine through its sequence to the
point of failure
...
Contacts that open and close a circuit and coils that operate the contacts,
keeping in mind the control voltage
...
Remember the operator knows the machine operation and can be an asset to you
in your troubleshooting
...
•
•
•
- 397 -
Question and answers
•
•
•
Electrical Maintenance Unit
Anyone attempting to troubleshoot without a drawing and a meter is wasting the
time
...
Finally take time to think
...
Annual Dose Limit (ADL) = 1 5 mSev or 1
...
Annual Dose Limit (ADL) = 1 mSev or 100 mRem for Public
...
01 mSev or 1 mRem
...
• What are the gases discharged to the stack?
Argon-41, Tritium, fission products, noble gases & Iodine particulates
...
The area around the Kaiga generating station is divided into the
following Zones up to 16 km radius
...
6 km around the
central plant zone of 0
...
This zone is
physically isolated from out side areas by plant fencing and is under the control of
Kaiga Generating Station
...
Natural growth is however allowed in this Zone
...
This Zone is defined to restrict the
population to an easily transportable number in case of an emergency
...
Secondary Zone: The secondary Zone extends up to 16 km from central plant Zone
protective measures like sheltering control on food stuff are required against
possible exposure from ingestion of radioactivity
...
The
effect of the emergency may be restricted either to a small area of the plant or a few
individuals or it may pose damage to the installation staff
...
Accordingly the emergencies are classified into:
1
...
Site emergency
3
...
The situation may result from
high radiation exposure or significant contamination or abnormal intake of
radioactivity by personal
...
Emergency Alert/Emergency Standby: This involves abnormal conditions, which
have a potential to proliferate in to a more serious situation but still provide time
for pre-cautionary and constructive steps to prevent an emergency situation or
migrate its consequences
...
Plant emergency This involves excessive release of radioactive materials or high
radiation fields in a section of the plant requiring operator action and/or automatic
operation of the safety system
...
The examples of plant emergency conditions or listed in
Annexure-III
...
The
protective measures such as incorporation of stable Iodine, sheltering and evacuation
of personal from plant areas other than control room to areas designated to be
habitable under the site emergency conditions and evacuation of non-essential
persons from the site may be considered
...
- 401 -
Question and answers
Electrical Maintenance Unit
Off-site Emergency
An Off site emergency situation results when the release of radioactive materials
from the plant is of a magnitude necessitating protective action to be taken for
members of the public in the neighborhood of the plant
...
Declaration of Emergency: Siren will be sounded as described below for declaring
emergency
...
Siren: Short intermittent siren 5 seconds on, 5 seconds off for a period of two
minutes
...
THE ASSEMBLY AREA IS ……………………………………
THE EMERGENCY CONTROL CENTRE IS……………
...
This announcement shall be repeated thrice in English, Hindi and Kannada
...
Termination of Emergency: A continuous Siren is sounded for 2 minutes
...
Notification Codes:
The messages for notification of start/termination of on site and off-site emergencies
are indicated as follows
...
The
codes for notification of commencement or termination of various types of
emergencies are:
a) External radiation exposure (mSv)
DAC-hr (HTO) DAC-hr(I-131)
DAC-hr (I-131) ------(≤ 1)
(For meeting iodine thyroid dose limit of 50 mSv)
The explanatory notes for these guidelines are given in Annexure-IX
...
1
...
Administration of Stable Iodine
3
...
4
...
5
...
6
...
Decontamination of Affected Areas and Buildings
...
1 mSv/hr
Domain 2 = 0
...
01 mSv/hr
Stochastic and Deterministic effects
...
Example: Cancer
...
Examples Cataract, permanent or temporary sterility
...
Intervention: Any human action intended to reduce or avert exposures to sources
which are not part of controlled practices or which are out of control as a
consequence of an accident is "Intervention"
...
- 404 -
Question and answers
Electrical Maintenance Unit
Principles of Radiation Protection:
Practices:
a
...
b
...
c
...
The reduction in dose should be sufficient to justify the harm and the costs to
the individual and the society due to the intervention
...
The benefit of the reduction in dose less the cost of intervention should be As
Large As Reasonably Achievable
...
Dose limits do not apply in case of intervention
...
Dose limits: Occupational Workers
a
...
b
...
c
...
The whole body exposure level should Remain less 10 mSv (1 Rem) in any month
and 15 mSv (1
...
In case of intakes of radioactive material into the body, the total amount of activity
taken into the body in a calendar year shall not exceed one ALI (Annual Limit on
Intake)
...
Whole body dose Ii -- + Σ -- < 1
0
...
- 405 -
Question and answers
Electrical Maintenance Unit
Planned Exposure: Situation may occur in-frequently during normal operations when
it may be necessary to permit a few workers to receive dose in excess of the annual
whole body dose limit
...
- 406 -
Question and answers
Electrical Maintenance Unit
External Exposure Control: Any external whole body exposure that exceeds 5 mSv
(0
...
Internal Exposure Control
a
...
b
...
KGS-Operating Manual on Radiation Protection Procedures
For assessment of internal exposure due to tritium, bioassay of urine will be taken as
the standard reference
...
For control of intake of tritium the following procedures
shall be l Exposure Control followed:
Permissible Contamination levels
Air borne Contamination the levels of air borne contamination in working areas at
the station should be maintained below the Derived Air Concentration (DAC) values
DAC (Bq/m3) = ALI Bq/2400 m3
Investigation of Doses
Investigation levels Whole body dose: Committee (SDIC) shall investigate these
exposures
Dose Reference Levels for Investigation
Tissue/Organ Investigation Levels mSv (Rem)
Monthly
Quarterly
Whole body
10 (1)
15 (1
...
To investigate fully the causes of the doses above the investigation levels and
to prepare a factual report
...
To suggest Remedial measures to prevent recurrence of such doses
...
To suggest further action in respect of work to be allocated to the exposed
person
...
To recommend Remedial measure and medical follow up wherever necessary
...
Chairman,
SARCOP shall constitute a special committee for investigation of such exposures
...
3 years, Biological = 7 days, tritium effective =
(TR *TB) / (TR +TB)
(12
...
3+365+7) = 7 Days
REVERSE SQUARE LAW: Dose at a rate form the point of source is inversely
proportional to the square of the distance
...
8 TBq / day, Tritium = 13TBq/day, Ar 41 = 2
...
295 TBq/day
RADIOACTIVE TRANSPORT INDEX: 1 meter from the source shield
...
Essential sequence of maintenance Jobs execution
...
2
...
These are established as convenient measures to evaluate
current performance levels against standard as well as an index to compare with
past performance
...
(Objective should
be zero)
e) Number of respective failures during reporting period
...
Maintenance Performance indicator based on work control
a
...
of DR received from control room per month
No
...
Maintenance performance indicator based on maintenance man hours
1) Man hours spent on breakdown maintenance
2) Man hours spent on PM including implementation of ECN/FCN’s etc
...
This includes dirt, debris, broken or missing parts, slag, tools rags,
liquids/chemicals, lapping compounds, grinding particles and any other item that
would affect the intended operation of a system or component
All personnel shall assume responsibility for preventing the introduction of foreign
material into systems
...
Such as
corrosion, fuel damage, component malfunction, or failure, changes in chemistry
...
Specific actions includes the following
Work packages will be planned using field walk downs to determine specific FME
recommendations
If temporary dams are installed which will not be readily visible upon system
closure, verification of removal shall be included in the checklist
...
Expressing concerns describing assignments, discussing
problems, are few aspects of maintenance of communication
...
The following additional communication practices will be followed
...
Repeat back is used to ensure accurate communication, especially when portable
radios, headsets, or telephones are being used
...
Upon completion of a task, technicians shall report job completions to their
supervisors and seek additional assignments
...
To confirm to the principle of solving problems at the lowest possible level,
potential grievance issues are to be discussed with the first line supervisor
...
Plant approved terminology, equipment identification and abbreviations are to be
used at all times
...
2-way communication is required at times!
f
...
Understand
h
...
- 411 -
Question and answers
Electrical Maintenance Unit
• Importance of self checking peer verification
a
...
A- Act Touch the component without actuating it
...
R- Review, verifies that the actual responses is the expected response
...
Self-checking is a self-verification step or action before it is performed
...
Consistently applied this
will minimize error by forming a barrier against complacency and over confidence
...
For examples relays, positioning
switches, breaker or valves, lifting/landing wires, connecting test equipment,
removing or installing fuses
...
Any deficiency found in the field like labels, nameplate missing/tampered
should be intimated to the supervisor
...
Do it right the first time
...
Peer verification is achieved through the use of inspection points, these include
dual verification, independent verification, supervisory verification and quality
verification
...
f
...
- 412 -
Question and answers
Electrical Maintenance Unit
The distinction between predictive and periodic maintenance is presented below
...
Use predictive maintenance results to trend and monitor equipment performance
so that needed corrective or preventive maintenance can be performed before
equipment failure
...
Predictive maintenance actions are determined by the data required to monitor
equipment condition
...
Infrared surveys (thermography) are performed on heat producing equipment
such as motors, circuit breakers, batteries, load centers, bus ducts, transformers
and insulated areas to monitor for high resistance or insulation breakdown
...
Motor operated valves are diagnostically tested and analysed
...
c
...
Periodic maintenance can also be
initiated because of the results of predictive maintenance, vendor
recommendation, or experience
...
Scheduled valve re-packing to avoid leakage based on previous experience
...
Replacement of bearings or pump realignment as indicated from vibration
analysis and/or lubricating oil analysis
c
...
d
...
Preventive Maintenance Programme Effectiveness
Continually review the preventive maintenance programme for effectiveness, and
change if necessary based on changes in plant design, operating conditions,
regulatory commitments and as found conditions
...
The primary objectives of the programme are to reduce future
component failures, optimize preventive maintenance tasks and use of resources,
identify programme scope and satisfy regulatory and utility concerns
...
Enhancement,
provide additional guidance on methods to determine preventive maintenance
effectiveness
...
1
...
manpower, tools,
tackles, required for maintenance is well planned and readily available or
reserved for planned job
2
...
3
...
4
...
5
...
6
...
Disadvantages of Unplanned maintenance
...
Unplanned job won’t have any expectations when to start, when to stop
...
Man power/tools tackles were available/not available at the right time is not
ensured
...
Job may have to do in hurry which can lead to mistaken or job can be done
leisurely (no sufficient work front for the available manpower
...
4
...
• What is pre-job briefing and post job briefing?
Pre-job briefing: Unit no, DR/WP/, USI/system/load, Job description, Eqpt history,
Scope of Job, Any special tool or equipment required, Safety/Alarm, Procedures,
expectation for the job, tech specifications requirements, communication, FME
requirements, environmental concepts, any abnormal conditions
...
- 415 -
Question and answers
Electrical Maintenance Unit
• What are the activities by which performance of the station will be judged?
Activities of station by which performance is judged by public
a
...
b
...
c
...
d
...
e
...
f
...
g
...
h
...
i
...
j
...
k
...
l
...
m
...
- 416 -
Title: basic electrical
Description: In the notes use to basic electrical law, electrical engineering questions
Description: In the notes use to basic electrical law, electrical engineering questions