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QUESTION BANK
w it h S O L V E D 2 M A R K Q s

POWER SYSTEM ANALYSIS
UNIT 1: INTRODUCTION

1
...

Planning the operation of a power system requires load studies, fault calculations,
the design of means for protecting the system against lightning and switching surges and
against short circuits, and studies of the stability of the system
...
Define steady state operating condition
...

3
...

The two types of disturbances in a power system are,
i) Large disturbance ii) Small disturbance
4
...

If the power system is operating in a steady state condition and it undergoes
change, which can be properly analyzed by linearized versions of its dynamic and
algebraic equations, a small disturbance is said to have occurred
...

5
...

A large disturbance is one for which the nonlinear equations describing the
dynamics of the power system cannot be validly linearized for the purpose of analysis
...

6
...

7
...

8
...


1

9
...

The equation for base current Ib is,

Ib =

kVA b
3 kVb

The equation for base impedance is,

Zb =
Where,
Ib
kVAb
kVb
Zb

kVb x1000
3 Ib

= Line value of base current
...


10
...


Load impedance per phase,

Z=

Load admittance per phase, Y =

VL

2

P − jQ
1 P − jQ
=
2
Z
VL

Where,
P = Three phase active power of star connected load in watts
...

VL = Line voltage of load
...
Give the equation for load impedance and load admittance per phase of a
balanced delta connected load
...

Q = Three phase reactive power of delta connected load in VARs
...


2

12
...

13
...

The base impedance is the impedance which will have a voltage drop across it
equal to the base voltage when the current flowing in the impedance is equal to the base
value of the current
...


kVAb = kVb x I b
14
...

The per unit value of any electrical quantity is defined as the ratio of the actual
value of the quantity to its base value expressed as a decimal
...
What are the quantities whose base values are required to represent the power
system by reactance diagram?
The base value of voltage, current, power and impedance are required to represent
the power system by reactance diagram
...
Usually the base values of voltage and
power are chosen in kilovolt and kVA or mVA respectively
...

16
...
It will be convenient for analysis of power system if the
voltage, power, current and impedance ratings of power system components are
expressed with reference to a common value called base value
...

17
...

2

Z p
...
u , old

 kVb , old   MVAb , new 
 x

x
 kV
  MVA

b , old 
 b , new  

18
...

(1) The per unit impedance referred to either side of a single phase transformer is the
same
...

(4) The manufacturers usually provide the impedance values in per unit
...

19
...

20
...

(1) Increase of system voltage, use of AVR
...

(3) Reduction in system transfer reactance
...

21
...

22
...
These faults are characterized by increase in current and fall
in voltage and frequency
...
What is a reactor?
Reactor is a coil, which has high inductive reactance as compared to its resistance
and is used to limit the short circuit current during fault conditions
...
Give the equation for transforming base kV on LV side to HV side of a
transformer and vice versa
...
Give the equation for base current and base impedance of a balanced three phase
circuit
...
Why the line value of voltage directly used for per unit calculation in three phase
systems?
The per unit value of a line-to-neutral(VLN) on the line-to-neutral voltage
base(Vb,LN) is equal to the per unit value of the line-to-line voltage(VLL) at the same point
on the line-to-line voltage base(Vb,LL) if the system is balanced
...
e
...
Why the three phase kVA directly used for per unit calculation in three phase
systems?
The per unit value of a 3-phase kVA on the 3-phase kVA base is identical to the
per unit value of kVA per phase on the kVA per phase base
...
e
...


Possible 16-mark questions and answers

1
...
(APR/MAY 2004)
2
...
u form of representation?
3
...
How will you change the base impedance from
one set of base values to another set?
4
...

5
...

6
...
Explain the per phase generator model with required diagrams
...
With neat diagrams, explain the transformer model used for per phase analysis
...
Discuss in detail about the modeling of transmission lines
...
Clearly explain the basic components of a power system
...
Co
...
,
1994
...
The equation for base current Ib is,

Ib =

kVA b
3 kVb

2
...
The equation for base current Ib is,

Ib =

kVA b
3 kVb

4
...
The base impedance is

(kVb ) 2 x1000
Zb =
kVA b
6
...
The equation for converting the per unit impedance expressed in one base to
another
...
u ,new = Z p
...
The equation for transforming base kV on LV side to HV side of a transformer
and vice versa
...
The equation for base current and base impedance of a balanced three-phase
circuit
...
3-phase kVA base is identical to the per unit value of kVA per phase on the kVA
per phase base
...
e
...
The line-to-line voltage base (Vb,LL) if the system is balanced
...
e
...
The per unit impedance referred to either side of a single-phase transformer is
the same
...
The per unit impedance referred to either side of a three phase transformer is the
same regardless of the three phase connections whether they are Y-Y, ∆-∆ or ∆-Y
∆
14
...

15
...

16
...

17
...


7

UNIT 2:MODELLING OF VARIOUS COMPONENTS

Possible 2-mark questions and answers
1
...

Symmetrical steady state is the most important mode of operation of power
system
...
They are,
1) Load flow problem
2) Optimal load scheduling problem
3) Systems control problem
2
...
The variables associated with each bus or node are,
a
...
Phase angle of voltage δ
c
...
Reactive voltamperes, Q
3
...
The solution provides the voltages at various buses, power flowing in
Various lines and line losses
...
What are the information that are obtained from a load flow study?
The information obtained from a load flow study are magnitude and phase angles
of bus voltages, real and reactive power flowing in each line and line losses
...

5
...
It is also essential
for designing a new power system
...
What are the works involved in a load flow study? (NOV/DEC 2004)
The following has to be performed for a load flow study
...
Representation of the system by single line diagram
...
Formation of impedance diagram using the information in single line diagram
...
Formulation of network equations
d
...


8

7
...
Load bus or PQ bus (P and Q are specified)
b
...
Slack bus or swing bus or reference bus (|V| and δ are specified)
8
...

A bus is called voltage controlled bus if the magnitude of voltage |V| and real
power (P) are specified for it
...
Voltage controlled bus is also called as Generator bus and PV bus
...
What is PQ bus(load bus)? (APR/MAY 2005)
A bus is called PQ bus or load bus when real and reactive components of power
are specified for the bus
...

10
...
The swing bus is the reference bus for load flow solution and it is
required for accounting for the line losses
...

11
...
In a power
system, the total power generated will be equal to sum of power consumed by loads and
losses
...
The slack bus is assumed to generate the power required for losses
...
They are
estimated through the solution of line flow equations
...
List the quantities specified and the quantities to be determined from load flow
study for various types of buses
...

Bus type

Quantities
specified
P,Q

Quantities to
be obtained
|V|, δ

Generator
Bus

P, |V|

Q, δ

Slack Bus

|V|, δ

P, Q

Load Bus

9

13
...

The load flow equation of Gauss method is given by,

V K +1 =
p

1
Y pp


















Pp − jQ p

n
K
− ∑ Y pq Vq
*
q =1
V K 
 p 
q≠ p






















The load flow equation of Gauss-Seidel method is given by,

V K +1 =
p

1


















Pp − jQ p
*

p −1
n
K
K
− ∑ Y pq Vq − ∑ Y pqVq
q =1
q = p +1


















V K 
 p 


K+1
K
Vp and Vp = (K+1) and Kth iteration voltage of bus ‘p’ respectively
...

Y pp

14
...

The load flow equation of Newton Raphson method is given by,

n
P p = ∑  e p (eq G pq + f p B pq ) + f p ( f q G pq − e p B pq )



q = 1
n
Q p = ∑  f p (eqG pq + f p B pq ) − e p ( f qG pq − e p B pq )



q = 1
2
2
V p = e2 + f p
p
15
...

(APR/MAY 2004)
In load flow solution by iterative methods, the number of iterations can be
reduced if the correction voltage at each bus is multiplied by some constant
...
The multipliers that accomplish this improved
converged are called acceleration factors
...
6 is normally used
in load flow problems
...
How will you account for voltage controlled buses in the load flow algorithm?
The acceleration factor is a real quantity and it modifies the magnitude of bus
voltage alone
...


10

17
...
The solution of nonlinear equations can be obtained only
by iterative numerical techniques
...
What do you mean by a flat voltage start?
In iterative methods of load flow solution, the initial voltage of all buses except
slack bus are assumed as 1+j0 p
...
This is referred to as flat voltage start
...
When the generator bus is treated as load bus? What will be the reactive power
and bus voltage when the generator bus is treated as load bus?
If the reactive power of a generator bus violates the specified limits, then the
generator bus is treated as load bus
...

20
...
Calculations are simple and so the programming task is less
b
...
Useful for small systems
...
What are the disadvantages of Gauss-Seidel method?
The disadvantages of Gauss-Seidel method are,
a
...

b
...

c
...

22
...

23
...

The elements of Jacobian matrix will change in every iteration
...

24
...
This load flow method is faster, more reliable and he results are accurate
...
Requires less number of iterations for convergence
...
The number of iterations are independent of the size of the system
...
Suitable for large system
...
What are the disadvantages of Newton-Raphson method?
The disadvantages of Newton-Raphson method are,
a
...

b
...

c
...

26
...
The N-R method has quadratic convergence characteristics and so converges
faster than G-S method
...
The number of iterations for convergence is independent of the system in N-R
method
...
In N-R method, the convergence is not affected by the choice of slack bus
...
Compare G-S method and N-R methods of load flow solutions
...
Variables are expressed in polar co1
...

rectangular co-ordinates
...
Computation time per iteration is more
2
...

3
...
3
...

4
...
The number of iterations required for
of the size of the system
...

5
...

5
...


28
...
In FDLF method, the weak coupling between P-δ and Q-V
are decoupled and then the equations are further simplified using the knowledge of
practical operating conditions of a power system
...
How the disadvantages of N-R method are overcome?
The disadvantage of large memory requirement can be overcome by decoupling
the weak coupling between P-δ and Q-V (i
...
, using decoupled load flow algorithm)
...
The simplifications are based on the practical operating
conditions of a power system
...
Write the equation for power flow in the transmission line
...
(Ypq’/2)
Sqp = Pqp-jQqp
= Eq* iqp
= Eq*[Eq-Ep]Ypq + Eq*Eq
...
Define primitive network
...
The performance equations of
primitive network are given below
...

J and E are source vectors
...

32
...
The
bus is a conductor made of copper (or) aluminium having negligible resistance
...

33
...

For the specific system, we can obtain the following relation (relation between
element voltage and bus voltage)
...
Its
elements are found as per the following rules
...

= -1, if ith element is incident to but oriented towards the kth node
...

34
...
It is given by the admittance matrix Y in the
node basis matrix equation of a power system and it is denoted as Ybus
...

35
...

The equation for bus admittance matrix is,
YbusV = I
where
Ybus = Bus admittance matrix of order (n x n )
V = Bus voltage matrix of order (n x1)
I = Current source matrix of order (n x1)
n = Number of independent buses in the system

13

36
...
Y1n  V1 
Y
  
 21 Y22 Y23
...
Y3n  V3 

...


...
=

  

...


...


...


...


  
Yn1 Yn 2 Yn 3
...

 

...

 
 I nn 

37
...

The equation to find the kth bus voltage is,

Vk =

1
[∆ 1k I 11+ ∆ 2 k I 22+ ∆ 3k I 33+
...

Ijj = Sum of the currents injecting current to node j
...

38
...

i) Data preparation is simple
...

iii) Since the bus admittance matrix is sparse matrix(i
...
, most of its elements are zero),
the computer memory requirements are less
...
With the help of a neat flow chart, explain the Newton-Raphson method of load flow
solution when the system contains voltage controlled busses in addition to swing bus and
load bus
...
Compare Gauss-Seidel method and Newton-Raphson method of load flow studies
(NOV/DEC 2004)
3
...

(NOV/DEC 2004)
4
...

(APR/MAY 2005)
5
...


14

(APR/MAY 2005)
6
...

(APR/MAY 2005)
7
...
Derive the basic equations for the load flow study using Gauss-Seidel method
...
Acceleration factor
...
Handling of PV buses
...
Draw the representation schemes for
a
...
Tap changing transformer
10
...

11
...

12
...


Reference books:
R1 – Hadi saadat, “Power System Analysis”, Tata McGraw Hill
R2 – I J Nagarath , D P Kothari ’Modern Power system Analysis’, TMH Pub
...
Ltd
...

R3 – Nagoor Kani, “Power System Analysis”

15

UNIT 3:POWER FLOW ANALYSIS

Possible 2 mark questions:
1
...
The protective scheme consists of current and
voltage sensing devices, protective relays and circuit breakers
...

2
...
Most of
the components of the power system have inductive property which opposes any sudden
change in currents, so the faults are associated with transients
...
What is meant by a fault?
A fault in a circuit is any failure which interrupts with the normal flow of current
...
The faults may cause damage to the equipments, if it is allowed to persist
for a long time
...
Why faults occur in a power system?
Faults occur in a power system due to insulation failure of equipments, flashover
of lines initiated by a lightening stroke, permanent damage to conductors and towers or
accidental faulty operations
...
How are the faults classified?
In one method, the faults are classified as,
1
...
Series faults - due to open conductors
...
Symmetrical faults - fault currents are equal in all the phases and can be
analyzed on per phase basis
2
...

6
...

Various types of shunt faults are
1
...
Line-to-line fault

16

3
...
Three phase fault
Various types of series faults are,
1
...
Two open conductor fault
7
...

This fault conditions are analyzed on per phase basis using Thevenin’s theorem or using
bus impedance matrix
...

8
...

Load flow studies
1
...

2
...

3
...


the power system for load flow and short
Fault analysis
Resistances are neglected
...

The voltages can be safely assumed as 1
p
...
and the prefault current can be
neglected
...
For a fault at a given location, rank the various faults in the order of severity
...
The various faults in the order of decreasing severity are,
1) 3 phase fault
2) Double line-to-ground fault
3) Line-to-line fault
4) Single line-to-ground fault
5) Open conductor fault
10
...
The currents in the various parts of the system and in the fault
locations are different in these periods
...

11
...

2) Each generator is represented by an emf behind either the subtransient or transient
reactance depending upon whether the short circuit current is to be found immediately
after the short circuit or after about 3 – 4 cycles
...

4) All network impedances are purely reactive
...


17

5) Shunt capacitances and shunt branches of transformers are neglected
...

12
...
e
...
It
is the sum of leakage reactance and the reactance representing armature reaction
...

13
...
(APR/MAY 2004)
The subtransient reactance is the ratio of induced emf on no-load and the
subtransient symmetrical rms current, (i
...
, it is the reactance of a synchronous machine
under subtransient condition)
...

14
...

The transient reactance is the ratio of induced emf on no-load and the transient
symmetrical rms current
...
e
...
It is given by,

Eg
1
'
X d = ' = Xl +
Transient reactance,
1
1
I
+
Xa X f
Where
Xl = Leakage reactan Xa = Armature reaction reactance

Xf = Field winding reactance

15
...
The maximum momentary short circuit
current rating of the circuit breaker used for protection or fault clearing should be less
than this initial fault current
...
Most of
the circuit breakers open their contacts only during this period
...

16
...

The equation is,

I =

Eg
Xd

,

I' =

Eg
Xd

'

where

I

= Steady state symmetrical fault current

I ' = Transient symmetrical fault current

Xd = Direct axis synchronous reactance
Xd’ = Direct axis transient reactance
E g = RMS voltage from one terminal to neutral at no load
...
Write the equation for subtransient and transient internal voltage of the
generator
...
Write the equation for subtransient and transient internal voltage of the motor
...
How symmetrical faults are analyzed?
The symmetrical faults are analyzed using per unit reactance diagram of the power
system
...
The currents and
voltages at various parts of the system can be estimated by any of the following
methods
...

20
...

Doubling effect:
If a symmetrical fault occurs when the voltage wave is going through zero then
the maximum momentary short circuit current will be double the value of maximum
symmetrical short circuit current
...

DC off-set current:
The unidirectional transient component of short circuit current is called DC offset current
...
Differentiate between subtransient and transient reactance
...

2) Flux created by induced currents in the
damper winding is included
...

4) This cannot be extrapolated
...

2) There is no damper winding and hence
no flux is created
...

4) This can be extrapolated backwards in
time

22
...
Positive sequence components
Negative sequence components
Zero sequence components
...
Write the symmetrical components of three phase system
...
They are,
1) Positive sequence components
2) Negative sequence components
3) Zero sequence components
...
Define negative sequence and zero sequence components
...
Va2, Vb2 and Vc2 are the negative sequence components of Va,
Vb and vc
...
Vao, Vbo and Vco are the zero sequence
components of Va, Vb and Vc
...
Express the unbalanced voltages Va, Vb and Vc in terms of symmetrical
components Va1, Va2 and Va0
...
Express the symmetrical components Va1, Va2 and Va0 in terms of unbalanced
vectors Va, Vb and Vc
...
Define the operator ‘a’ and express the value of ‘a’ and ‘a2’ in both polar and
rectangular form
...
The value of ‘a’ is 1∠120o
...
5 + j0
...
5 - j0
...
what are sequence impedances and sequence networks?
The sequence impedances are the impedances offered by the devices or
components for the like sequence component of the current
...

29
...
The reverse is the case for negative
sequence quantities wherein HV quantities lag the corresponding LV quantities by 30o
...
What is an unsymmetrical fault? List the various unsymmetrical faults
...
The unsymmetrical faults in a power system are,
1) Single line-to-ground fault
...

3) Double line-to-ground fault
4) Open conductor fault
...
Define positive sequence and negative sequence impedances
...

The negative sequence impedance of an equipment is the impedance offered by
the equipment to the flow of negative sequence current
...
Explain the need for short circuit studies
...
Draw the relationship between the phase components and the sequence
components
...
The phase ‘b’ of a three phase circuit is open
...
Determine the positive, negative and zero sequence
components of the current in phase ‘a’
...
With the help of a detailed flow chart, explain how a symmetrical fault can be
analysed using ZBus
...
What are the various types of faults? Discuss their frequency of occurrence and
severity? Find the fault current when an L-L-G fault occurs at the terminals of an
unloaded generator
...
Derive an expression for the positive sequence current Ia1 of an unloaded
generator when it is subjected to a double line to ground fault
...

7
...

8
...

9
...

10
...
Illustrate the answer by considering a symmetrical fault
...
Co
...
,
1994
...
What is meant by a fault?
A fault in a circuit is any failure which interrupts with the normal flow of current
...
The faults may cause damage to the equipments, if it is allowed to persist
for a long time
...
Give the reason for faults in power system?
Faults occur in a power system due to insulation failure of equipments, flashover
of lines initiated by a lightening stroke, permanent damage to conductors and towers or
accidental faulty operations
...
List the various types of symmetrical and unsymmetrical faults
...
Three phase fault
Unsymmetrical faults:
6
...
Line-to-line fault
8
...
For a fault at a given location, rank the various faults in the order of severity
...
The various faults in the order of decreasing severity are,
6) 3 phase fault
7) Double line-to-ground fault
8) Line-to-line fault
9) Single line-to-ground fault
10) Open conductor fault
5
...
The currents in the various parts of the system and in the fault
locations are different in these periods
...


24

6
...
e
...
It
is the sum of leakage reactance and the reactance representing armature reaction
...

7
...
(APR/MAY 2004)
The subtransient reactance is the ratio of induced emf on no-load and the
subtransient symmetrical rms current, (i
...
, it is the reactance of a synchronous machine
under subtransient condition)
...

8
...

The transient reactance is the ratio of induced emf on no-load and the transient
symmetrical rms current
...
e
...
It is given by,

Eg
1
'
X d = ' = Xl +
Transient reactance,
1
1
I
+
Xa X f
Xa = Armature reaction reactance
Where Xl = Leakage reactance
Xf = Field winding reactance
9
...
The maximum momentary short circuit
current rating of the circuit breaker used for protection or fault clearing should be less
than this initial fault current
...
Most of
the circuit breakers open their contacts only during this period
...

10
...

The equation is,

I =

Eg
Xd

,

I' =

Eg
Xd

'

where

I

= Steady state symmetrical fault current

I ' = Transient symmetrical fault current

Xd = Direct axis synchronous reactance
Xd’ = Direct axis transient reactance
E g = RMS voltage from one terminal to neutral at no load
...
Write the equation for subtransient and transient internal voltage of the
generator
...
Write the equation for subtransient and transient internal voltage of the motor
...
Define doubling effect and DC off-set current
...
This effect is called doubling effect
...

14
...

Subtransient reactance

Transient reactance

1) This is the ratio of induced emf and
subtransient current
...

3) This is the smallest reactance among the
reactance values
...


1) This is the ratio of induced emf and
transient current
...

3) This is larger than the subtransient
reactance
...
What are symmetrical components?
An unbalanced system of N related vectors can be resolved into N systems of
balanced vectors called symmetrical components
...

16
...
(MAY/JUNE 2006)
In a 3-phase system, the three unbalanced vectors (either current or voltage
vectors) can be resolved into three balanced system of vectors
...

17
...

Negative sequence components consist of three phasors equal in magnitude,
displaced from each other by 120o in phase, and having the phase sequence opposite to
that of the original phasors
...

Zero sequence components consist of three phasors equal in magnitude and with
zero phase displacement from each other
...


27

18
...

The expression of unbalanced voltages in terms of symmetrical components are,
V =V +V +V
a ao a1 a 2
2
V = V + a V + aV
b
ao
a1
a2

2
V = V + aV + a V
c
ao
a1
a2

(Or)

Va  1 1
V  = 1 a 2
 b 
Vc  1 a
  

1
a

a2 


Va 0 
V 
 a1 
Va 2 
 

19
...

The expression of symmetrical components in terms of unbalanced vectors are,
V
V
V

ao

a1
a2

=
=
=

1
3

(V + V + V )
a
b
c

1

2
(V + aV + a V )
a
b
c
3
1

2
(V + a V + aV )
b
c
3 a

(Or)

Va 0 
1 1
V  = 1 1 a
 a1  3 
Va 2 
1 a 2

 

1
a2 

a


Va 
V 
 b
Vc 
 

20
...

An operator which causes a rotation of 120o in the anticlockwise direction is
known as operator ‘a’
...

The polar form and rectangular form of operator ‘a’ is given by,
--------polar form
a = 1∠120o
= -0
...
806 ----------rectangular form
The polar form and rectangular form of operator ‘a2’ is given by,
--------polar form
a2 = 1∠240o
= -0
...
806 ----------rectangular form

28

Possible 16 marks:
11
...


12
...
(MAY/JUNE 2006)
13
...

14
...
The phase ‘b’ of a three phase circuit is open
...
Determine the Fpositive, negative and zero sequence
components of the current in phase ‘a’
...
What are the various types of faults? Discuss their frequency of occurrence and
severity?
17
...
Derive an expression for the positive sequence current Ia1 of an
unloaded generator when it is subjected to a double line to ground
fault
...

18
...
What symmetrical components? Explain the symmetrical component
transformation
...
Write about the impedances in phase and sequence form
...
What is meant by sequence impedance? Explain the sequence network of an
unloaded generator
...
Explain the procedure for making short circuit studies of a large power system
using digital computer
...

(NOV/DEC 2004)

30

UNIT 5:STABILITY ANALYSIS

Possible 2 marks
1
...

The stability of a system is defined as the ability of power system to return to a
stable operation in which various synchronous machines of the system remain in
synchronism or ‘in step’ with each other, when it is subjected to a disturbance
...
Define steady state stability
...
e
...

3
...

The transient stability is defined as the ability of a power system to remain stable
i
...
, without loosing synchronism for large disturbances
...
Write any three assumptions upon transient stability
...
Rotor speed is assumed to be synchronous
...

b
...

c
...

5
...
In steady
state, the power transferred by synchronous machine of a power system is always less
than the steady state stability limit
...
What is transient stability limit?
When the load on the system is increased suddenly, maximum power that can be
transmitted without losing synchronism is termed as transient state stability limit
...

7
...
Increase of system voltages
b
...

c
...
Use of high speed reclosing breakers
...
What is stability study?
The procedure of determining the stability of a system upon occurrence of a
disturbance followed by various switching off and switching on actions is called stability
study
...
How do you classify steady state stability limit
...

Depending on the nature of the disturbance, the steady state stability limit is
classified into,
a
...

b
...

10
...

1
...
Those having two finite inertia machines swinging with respect to each other
...
Give the expression for swing equation
...
(APR/MAY 2005)

Where H = Inertia constant in MJ/MVA
...

M = Inertia constant in p
...

Pm= Mechanical power input to the system (neglecting mechanical losses) in p
...

Pe = Electrical power output of the system (neglecting electrical losses) in p
...

12
...

2) Rotor speed changes are insignificant that had already been ignored in
formulating the swing equations
...

13
...
What is the use of swing curve?
The swing curve is the plot or graph between the power angle δ, and time, t
...
From the nature of variations of δ, the stability of a system for
any disturbance can be determined
...
Give the control schemes included in stability control techniques?
The control schemes included in the stability control techniques are:
a
...
Turbine valve control
c
...
Faster fault clearing times

32

15
...
Minimum transformer reactance
b
...
Additional transmission lines
...
What are coherent machines? (APR/MAY 2004)
Machines which swing together are called coherent machines
...
This is used in
stability studies involving many machines
...
State equal area criterion
...

There is a simple graphical method of determining whether the system comes to rest with
respect to each other
...
What are various faults that increase severity of equal area criterion?

The various faults that increases severity of equal area criterion are,
A Single line to ground fault
A Line to line fault
A Double line to ground fault
A Three phase fault
19
...
List the types of disturbances that may occur in a single machine infinite bus bar system of the equal area criterion stability

The types of disturbances that may occur are,
Sudden change in mechanical input
Effect of clearing time on stability
Sudden loss of one of parallel lines
Sudden short circuit on one of parallel lines
i)
Short circuit at one end of line
ii)
Short circuit away from line ends
iii)
Reclosure

33

21
...

22
...


The critical clearing time , tcc can be defined as the maximum time delay that can
be allowed to clear a fault without loss of synchronism
...

23
...
C offset currents and harmonic components are neglected
...

• The symmetrical components are used for the representation of
unbalanced faults
...

24
...
If it increases,
i
...
If every machine is stable, then the system having any
number of machine is stable
...
What is meant by an infinite bus?
The connection or disconnection of a single small machine on a large system would
not affect the magnitude and phase of the voltage and frequency
...

26
...

The assumptions made are,
• The mechanical power input to each machine remains constant during
the entire period of the swing curve computation
• Damping power is negligible
• Each machine may be represented by a constant transient reactance in
series with a constant transient voltage
...


27
...


34

The mechanical torque Tm and the electrical torque Te are considered positive for
synchronous generator
...
Under steady-state operation of the
generator Tm and Te are equal and the accelerating torque Ta is zero
...

Possible 16 marks
1
...
State
the assumptions if any and state the usefulness of this equation
...

R4-Pg
...
Discuss the various factors affecting the transient stability of the system
...
No 5
...
With the help of a neat flowchart, explain the modified Euler method of solving the
swing equations
...
No 5
...
State the bad effects of instability
...

R1-Pg
...
6
5
...

R1-Pg
...
45, R2 - 346
6
...
How do you apply equal area criterion to find
the maximum additional load
...
No5
...
Describe the equal area criterion for transient stability analysis of a system
...
No5
...
Mention the assumptions clearly and developing necessary equations, describe the step
by step solution of swing bus
...
No5
...
Derive the swing equation of a synchronous machine swinging against an infinite bus
...

R1-Pg
...
8 NOV/DEC 2004
10
...

R1-Pg
...
8 APR/MAY 2005
...
Explain critical clearing time and critical clearing angle, deriving the expressions
...
No5
...
No5
...
Explain the solution of swing equation by Runge Kutta Method
...
No5
...
Power System Analysis – K
...
Hemalatha, S
...
JayaChrista
...
Co
...
,
1994

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