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Chapter 12
Electrical System
12
...
Power generation from the power plant is
difficult to understand
...
The
purpose of this chapter is to introduce the students to the electrical equipments used in power plant
...
2 GENERATORS AND MOTORS
In a generator, an e
...
f
...
The current
produced by the e
...
f
...
The electrical power ei is produced
therefore from the mechanical power supplied
...
If current is supplied to
the coil; a mechanical force is mani-fested and due to this force the coil will move
...
m
...
is induced, in opposition
to the current
...
m
...
Thus the
motor requires electrical power to produce a
Fig
...
1
corresponding amount of mechanical power
...

12
...
The coil is free to move about the axis O
...
The direc-tion of the
e
...
f
...
The action is reversible: i
...
the same arrangement may act either as generator or motor
...
At 50 c/s
...
p
...
and the latter at 1500
...

12
...
1 ROTORS
Rotors are most generally made from solid forgings of alloy steel
...
Test pieces are cut from the circumference and the ends to provide information
about the mechanical qualities and the microstructure of the material
...
One of the most important examinations is the ultrasonic test, which will
discover internal faults such as cracks and fissures
...

The rotor forging is planed and milled to form the teeth
...

(a) Windings
...
The heat developed in the
conductor's causes them to expand, while the centrifugal force presses them heavily against the slot
wedges, imposing a strong frictional resistance to expansion
...
As a result, when the machine is stopped and the copper coals, it
contracts to a shorter length than originally
...
With new machines the use of silver bearing copper, having a
much higher yield paint, mitigates the trouble
...
g
...
The slots are radial and the coils formed of flat strip with separators between turns
...
The insulation is usually micanite, but bonded asbestos and glass fabric have
both been used
...
The allowable current depends on cling and expansion
...

(b) Cooling
...
It is usual to
have a large gap (e
...
45 mm
...

For larger machines provision must be made for cooling the bottom of the slot, the con-ventional method
being that shown in Fig
...
2
...
For the
largest ratings elaborate ventilating arrangements are necessary, and for machines of 100 MVA
...
12
...
Conventionally Cooled Rotor
Slot and Ducts
...
12
...
Arrangement of Conductors Fop
...


methods are being developed for direct contact between the rotor coils and the gas coolant
...
12
...
The essential feature is the use of straight rectangular tribes for the slot
conductors, ventilated on a cooling circuit separate from that of the stator, the hydrogen gas coolant
being circulated through the tubes by a centrifugal impeller mounted on the outboard end of the rotor
...

The conductors are hard-drawn electrolytic silver-bearing copper, with synthetic-resin bonded glass
cloth laminate insulation
...
The appearance and arrangement of the overhang can be seen in Fig
...
4, which
also shows the spigot on which the retaining ring is
centred
...
Non-magnetic, magnesium and magnesiumnickel steels are used for the retaining rings, the nonmagnetic property being useful for avoiding excessive
magnetic leakage and stray load loss
...
The rotor may carry centrifugal or
Fig
...
4
...

2-Pole Rotor (English Electric)
...
They may be coupled to a
shaft extension or be separately driven
...
Slip rings are required for conveying the exciting current to and from the rotor winding
...
Fig
...
5 shows a typical
completed two-pole rotor
...
12
...
Completed Rotor (Parsons)
...
2
...
Fig
...
6 shows a
simple stator housing requiring two end plates and four intermediate plates, held apart by tie bars
...
The frame is then covered with sheet steel
...
12
...
Method of Fabrication Turbo-Generator Stator Housing
...
The active part of the stator consists of segmental lamina-tions of low-loss alloy steel
...
The
stamp-ings are rather complicated on account of the number of holes and slots that have to be produced
...
Here the flux direction is mainly circumferential, and by cutting the core- plate sectors in such a way that the
pre-ferred flux direction is at right-angles to their central radial axis,
Flux
Fig
...
7, a sub-stantial reduction in core-loss can be secured
...
The core plates are assembled between end
Grain
plates with fingers projecting between the slots to support the flanks
orientation
of the teeth
...
12
...
Use of Grain-Oriented
material, for this greatly reduces stray load loss
...

core plates may be stepped to a larger bore for the same reason
...
The windings of two-pole machines are comparatively straightforward
...
Single-layer concentric or twolayer short-pitched windings may be used
...
Chording is not possible so that
flux harmonics have full effect
...
m
...
wave
...

It is the invariable practice with two-layer windings to make the coils as half turns and to joint the
ends
...
The conductors are

390

POWER PLANT ENGINEERING

insulated in many cases with bitumen-bonded micanite, wrapped on as tape, vacuum dried, then impregnated with bitumen under pressure and compressed to size
...
12
...
Each
copper bar A forming part of a conductor is insulated with mica tape, B and C
...
The conductor is insulated with layers of mica, tape, E; then the
conductors are assembled to form a
...
Fig
...
9 shows
typical conductors
...

Within the slots, the outer surface of the conductor insulation is at earth potential: in the overhang it will approach more nearly to the potential of the enclosed copper
...
g
...

Setting the winding deeply into the slots increases the slot inductance
...


A

B

C

D

E

F

G

Fig
...
8
...


Fig
...
9
...


12
...
3 VENTILATION
Forced ventilation and total enclosure are necessary to deal with the large-scale losses and high
rating per unit volume
...

(a) Air-Cooling
...
12
...
The water coolers are normally in two
sections, so that one can be cleaned while the machine is operating
...


391

ELECTRICAL SYSTEM

Shaft
Air
side

Air
side

H2 side
Spring

Axial
seal

H2 side
Spring

Oil Feed

Radial
seal

Fig
...
10
...


With integral fans mounted on the rotor, the air is fed to the space surrounding the stator overhang, and pipes and channels convey a proportion towards the centre of the stator core
...
With separate fans,
however, air can be fed directly to the middle as well as to the ends, as shown in Fig
...
10
...
Compared with
Gas cooler
H manifold
air, hydrogen has 1/14 of the density, reducing
Seal
windage loss and noise; 14 times the specific
heat; 1
...
As a result,
Oil
From couplingto
end seals
hydrogen cooling at 1, 2 and 3 atmospheres
Oil from
shaft
H side
seal
absolute can raise the rating of a machine by
Vent to
Gas
dryer
atmosphere
15, 30 and 40 per cent respectively
...
Oil- film gas-seals at the rotor
side
Vacuum
CO
H
pump
shaft ends are necessary
...
12
...

Liquid
detector
Oil is fed to the shaft and the flow is split, part
towards the interior (gas) side and part to the
Fig
...
11
...
The latter mingles with the bearing oil,
System
...

Fans mounted on the rotor circulate hydrogen through the ventilating ducts and internally mounted
gas-coolers
...
When filling or emptying the casing of the
machine, an explosive hydrogen-air mixture must be avoided, so that air is first displaced by carbon
dioxide gas before hydrogen is admitted: the process is reversed for emptying
...
The hydrogen purity is monitored by measurement
of its thermal conductivity
...
03 m3 per mW of rating per day
...
1 m3 for hydrogen
pressure of 2 atm
...
The gas consumption of synchronous capacitors, which do not need shaft seals,
is very much less
...
5–1
...
Fig
...
11
gives a diagram of the auxiliary equipment required for a hydrogen -cooled machine
...
Direct cooling of stator windings is applied at ratings rather higher than that
which makes the method necessary for rotors
...
12
...
A similar design serves for
water-cooling a stator
...
Insulating tubes convey the liquid to and from
the water “headers,” and the water itself must have adequate resistivity to limit conduction loss
...

12
...
4 HIGH-VOLTAGE GENERATORS
Although it is usual to combine a generator with a transformer to develop an output at high
voltage, machines have been built for feeding a 33 kV network direct
...
The insulation between conductors, and between the outer conductors and slot walls, is flexible micanite
...
The
innermost (or “bull”) conductor in every slot forms that third of the winding connected to the line
terminals, while the “outer” conductors are at the star-point end
...
The slot
reactance tends to be high
...
12
...
Three completely separate windings insulated respectively for 11, 22 and 33 kV, (line) are used, the 11 kV, section being at the
bottom of the slots
...
The thicker insulation of the higher- voltage
windings requires the copper to be deeper and more extensively laminated
...

Fireproof conductor insulation
Bakelite
and bakelized asbestos wrap
strip

Micanite cell

Slip-plane here for
thermal expansion

Fireproof separator Grading shields

Bakelite wedge

Fig
...
12
...


The machines discussed are generators designed for water wheel and internal-combustion engine or gas-turbine drives, and salient-pole synchronous motors
...


12
...
Consequently
there is no concern with mechanical power, and only transformer e
...
f
...


393

ELECTRICAL SYSTEM

12
...
1
...
The chief elements of the construction are :
(1) Magnetic Circuits, comprising limbs, yokes, and clamping structures
...

(3) Terminals, tappings and tapping switches, terminal insulators and leads
...

Improvements are continually being made in construction, and the practice of different
manufacturers depends considerably on the size of unit made, the organization of the factory, and the
indivi-duality of the designers
...

The practice in Great Britain and Europe is to concentrate mainly on the single and three-phase
core types
...
A few
special constructions are sometimes employed
...

12
...
2
...
Induction densities up to 1
...
55 Wb/m2 are possible, the limit far 50 c/s being the loss and the magnetizing current
...
Paper, Japan, varnish china
clay or phosphate may be used
...

Burring of the edges of the plates may cause a considerable increase in core loss by providing
paths for eddy currents should the sharp edges cut through the insulation and establish contact between
adjacent plates
...
Silicon alloy steels are hard, and cause wears
of the punching tools, so that the removal of burrs needs special attention
...
Sheets are therefore cut as far as possible along the grain,
which is the direction in which the material has a higher permeability
...
In building the core, considerable pressures are used to minimize air gaps between the
plates, which would constitute avoidable losses of area and might contribute to noisy operation
...
of
Circum
...
0 0
...
85

0
...
9

0
...
16 0
...
16 0
...
14

0
...
12
0
...
12
...
Sections of Core-type Transformer Limbs
...
09
0
...
of packets
Ai = kd2
k=

(a) Square

(b) Cruciform

(c) Three-stepped (d) Four-stepped

64
58

79
71

84
75

87
78

1

3

5

7

0
...
56

0
...
62

The reduction of core sectional area due to the presence of paper, surface oxide, etc
...

1
...
As has been seen, iron losses make it imperative to laminate transformer cores
...

With core-type transformers in small sizes, the simple rectangular limb can be used with either
circular or rectangular coils
...
For this purpose the limbs can be square, as in
Fig
...
13(a); where the circle represents the inner circumference of the tubular former carrying the
coils
...
A very common
improvement is to employ cruciform limb sections, as in Fig
...
13(b)
...
With large transformers further core stepping may be introduced to reduce the length of mean
turn and the consequent I2R Loss
...
A typical core section with three steps is shown in Fig
...
13(c)
...
The three-stepped limb is commonly employed and even more steps may be used
for very large transformers
...

Cores for shell-type transformers are usually of simple rectangular cross-section, the Coils being
also rectangular
...
Assemply
...
In small sizes (e
...
below 50 kVA) string or cotton webbing may be employed to bind the
plates
...
Cores may also be clamped between
iron frames (after the fashion of miniature transformers), but the most usual way is to bolt the plates
together
...

The bolts must be insulated from the core both along their length and at their ends to avoid shortcircuiting the laminations, thereby providing eddy current paths
...
The slight tendency of the plates to “fan” out at their edges; due to central
clamping, increases the space between adjacent laminations and provides a safeguard against electrical
contact at the sharp edges
...
Stiffener plates are insulated from the cores
and are discontinuous at joints to obviate any tendency for the flux
Punched
to use them as a conducting path in parallel with the laminations
...

Very large cores, on the other hand, have a relatively small
surface/volume ratio, so that additional ducts must in some way augFig
...
14
...

ment the cooling surface
...

The first is easy, the second requires special punching
...
Heat flows twenty times more readily along the laminations to the
edges than from plate to plate across the intervening insula-tion, which has naturally low heat conductivity
...
12
...

In any but the smaller sizes it is impracticable to cut complete plates, i
...
complete magnetic
circuits
...
It therefore becomes necessary to make the coils separately, and to place hem on the
cores or to build up the plates through them
...

Although joints introduce gaps in the continuity of the circuit, the Mates call be suitably arranged to
reduce the effect of the gaps on the magnetic conductivity of the joints
...
For example singlephase shell-type transformer of small output may have a core composed of T- and U-shaped stampings
...

In assembling such a transformer, the coils are made and finished, and the core plates inserted on
each side until the complete core is built up, after which it is clamped
...
After assembly, the pins are withdrawn,
leaving the holes ready for receiving the insulated core-clamping bolts
...
12
...
Core CONa Truct 7ON of Core-Type Transforator
...
12
...
For core-types the clamps and flitch
plates are also arranged in a jig with upright pins, on
which the core plates are threaded
...

12
...

It is usual to build with laminations in threes
or fours to shorten the building time and reduce the
chance of buckling the plates
...
Sheets of
pressboard may be inserted at intervals in a thick core,
(c)
...
For grain-oriented steel intricate mitred joints have been developed
...

After the core has been built, the second set of
clamps and flitch plates is added and the core tightened, The yoke is removed to admit the coils after the
whole core has been stood upright it is then replaced
...
12
...
Single Phase Shell Type transformer in
vided in a similar way to the limb, and the relative
Course of Assembly
...

Sometimes (in core-type transformers) the yokes are made, as a whole, of about 20 per cent greater area
than the limbs, thereby reducing the iron loss in parts, which do not involve an increase in the length of
copper
...
It becomes necessary to reduce the
overall height
...
12
...
The core losses are, however, generally larger by 5-10 per cent
...
g
...
distribution) transformers employs cores made from lengths of cold
...

Then either (i) the core is cut across to make a pair of
“C” cores, threaded into coils and clamped; or (ii) it is left uncut
and the coils are wound on it by a special winding machine
...
Fig
...
18 illustrates
the method
...
12
...
Five-Limbed Core
...


397

ELECTRICAL SYSTEM

Core
sections

Joint

Fig
...
18
...


3
...
Considerable use is made of channel and angle section rolled
steel in the framework of core- type transformers
...
The bottom pair of channels has cross channels
as feet
...

12
...
3 WINDINGS
In addition to the classification as circular or rect
...
The terms are almost self-explanatory
...
12
...
12
...
The latter are used almost invariably with
shell-type transformers
...
12
...


H
...


L
...
H
...


( a)

L
...


L
H
L
H
L
( b)

Fig
...
19
...


On account of the easier insula-tion facilities, the low-voltage winding is placed nearer to the
core in the case of core-typo and on the outside positions in the case of shell-type transformers
...

1
...
Cylindrical concentric helix windings, commonly employed
for core-type transformers, can often be built up (generally with axial spacing strips to improve oil
circulation between the coil and the tube) on bakelite tubes, which facilitate erection, and form a strong
foundation for winding the coils
...
Usually the voltage of the low-voltage side is sufficiently small to permit of this, and frequently a
helical winding in one or two layers can be used for the high-voltage winding
...
In this way it becomes
unnecessary to put insulation between successive layers over and above that on the wires themselves
...
per coil, unsection-alized
...
Care has to be taken to wind
the coils tightly and to keep them perfectly circular
...
They can be stressed up to about 20 kV per cm
...

2
...
Cross-over coils are suitable for currents not exceeding about 20A
...
v
...
The coil is wound on a former with several layers of several turns per layer,
tape being interleaved axially to give greater rigidity to the coil
...

Disc coils are made up of a number of flat sections, comprising layers wound spirally from inside
outwards as shown in Fig
...
20
...
Sectional or continuous disc coils are commonly used
...

Helix Coils
Cross-over Coils

Dise Coils

Fig
...
20
...


3
...
Sandwich windings commonly employed for shell type transformers,
allow of easy control over the reactance
...
If it were possible to accommodate the two coils in the same space, the whole flux would link both windings, and there would be no
leakage flux
...
Each high-voltage section
lies between two low-voltage sections
...
In order to balance the magneto- motive forces of adjacent sections, each normal
section, whether high or low voltage, carries the same number of ampere-turns
...

4
...
Conductors of large cross- section are not employed, as
being too stiff to handle, and leading to excessive I2R loss
...
m
...
’s which produce circulating currents and additional
losses (often referred to as stray losses)
...
A 7
...
square conductor might be approaching the upper limit of size
for a 50 c/s transformer
...


399

ELECTRICAL SYSTEM

5
...
The insulation between the h
...
and l
...
windings, and between l
...
winding and
core, comprises bakelite-paper cylinders or elephantide wrap
...
The paper is wrapped
round the conductor in a suitable machine, preferably without overlap of adjacent turns
...
The wrapped conductor is lashed with
cotton strands wound openly, to give some mechanical protection
...
12
...
Core and Windings of three-phase 200 HVA, 50 cycle
6000/440 V Core-type Transformer (Bruce Peebles)
...

The high-voltage winding is separated from the low-voltage winding by a series of, ducts and
Bakelite cylinders or barrels
...
It
will be seen that the end turns, i
...
those turns in coils 3, 4, and 6, are more heavily insulated than Nos
...
The reason for this is connected with phenomena occurring during switching operations,
or with line disturbances
...

For large h
...
Transformers the end-turn reinforcement is a matter of careful design
...

The winding insulation has to be coordinated with the means adopted for controlling the distribution of
impulse electric stresses, Fig
...
22(a)
...
V
...
V
...
v
...
12
...
Transformer Insulation Details
...

Small thin-wire coils may be varnished for adhesion
...
This predisposes
to movement of the coils and breakdown on sudden short-circuit
...
They may then be assembled and tightened up
without danger of undue further shrinkage
...
The electric
stress is therefore twice as great on the oil in the annular ducts as on the bakelite cylinders
...
g
...
The radial width of the oil ducts is
determined by the electric stress and has to be made much wider than would be necessary from considerations of cooling
...
The paper, oil-saturated, has a high dielectric strength, so
that the h
...
—l
...
spacing can be reduced
...
See Fig
...
22(b)
...
v
...
12
...
The innermost layer is wound over a neutral shield
...
The outermost layer, enclosed by a static shield, is connected to the line terminal, and the
successive layers joined in series to give the electric stress distribution effect of a capacitor bushing to
surge voltages applied to the line terminal
...
Tappings are usually
required on modern transformers
...
Leads and Terminals
...
The shape and size of the
conductors are of importance in very high voltage systems, not on account of the current-carrying capacity, but because of dielectric stresses, corona, etc
...

7
...
Up to voltages of about 33 kV, ordinary porcelain insulators can be used, which do
not require special comment
...
Of course, any conductor can
be effectively insulated by au provided that it is at a sufficient distance from other con-ducting bodies
and sufficiently proportioned to prevent corona phenomena
...

The oil filled bushing consists of a hollow porcelain cylinder of special shape with a conductor
(usually a hollow tube) through its centre
...
The dielectric field strength is greatest
at the surface of the conductor, and this breaks down at a much lower voltage in air than in oil
...

Under the influence of the electric field, foreign substances in the form of dust, moisture or
metallic particles, have a tendency to arrange themselves in radial lines, giving rise to paths of low
dielectric strength, with consequent danger of breakdown
...
The effect is to break
up radial chains of semi-conducting particles
...
The capacitor-type hushing is constructed of thick layers of bakelized
paper alternating with thin graded layers of tin-foil
...
Their length and the radial separation of their tin-foil plates control the
capacitance of the capacitors
...
12
...
If the thickness of bakelized paper separating
successive tin-foil layers is kept constant, and the capacitances of the capacitors are kept constant by
successively reducing the length of the tin-fail layers proceeding outward, then the voltage across each
capacitor will be the same, giving a practically uniform dielectric stress throughout the radial depth of
the insulator
...
In Fig
...
23 the short stepped end is oil-immersed beneath the tank cover, the smooth long end projecting outwards
...
The rain-sheds are corrugated circumf6rentially to accord with the estimated
electric field distribution and to provide a long leakage path
...
12
...
v
...


12
...
Its core and conductor areas are k2 times greater and its rating (with the same flux and current
densities) increases k4 times, The lasses increase by the factor k3 but the surface area is multiplied only
by the factor V
...
Large transformers are
therefore more difficult to coal than small ones, and require more elaborate methods
...
Luckily the losses are comparatively small,
and the problem of cooling (which is essentially a problem of preserving the insulation-solid and liquidfrom deterioration) can in most cases be solved by reliance on natural self-ventilation
...


ELECTRICAL SYSTEM

403

12
...
1 SIMPLE COOLING
AN: Natural cooling by atmospheric circulation, without any special devices
...
This method is confined to very small units at a few kV at low
voltages
...

ON: The great majority of transformers are oil-immersed with natural cooling, i
...
the heat developed in the cores and coils is passed to the oil and thence to the tank walls, from which it is dissipated
...

OB: In this method the cooling of an ON-type transformer is improved by air blast over the
outside of the tank
...

OFB: For large transformers artificial cooling may be used
...

OW: An oil-immersed transformer of this type is cooled by the circulation of water in cooling
tubes situated at the top of the tank but below oil-level
...

12
...
2 MIXED COOLING
ON/OB: As ON, but with alternative additional air-blast cooling
...

A transformer may have two or three ratings when more than one method of cooling is provided
...
5; for ON/OB/OFB in the
ratio 1/1
...

12
...
3 NATURAL OIL COOLING
The diagram in Fig
...
24 is drawn to indicate on the left the thermal flow of oil in a transformer
tank
...
A continuous circulation of oil is completed by the heated oil flowing to the tank sides (where cooling to the ambient air occurs) and falling
again to the bottom of the tank
...

Fig
...
24 also shows on the right a curve typical of approximate temperature distribution, the
figures quoted being rises in degrees centigrade
...
The maximum oil temperature may be about 10° less
than the coil figure, and the mean oil temperature another 15° less
...
But to dissipate the loss in a large
transformer a plain tank would have an excessively large surface area and cubic capacity, and would
require a great quantity of oil
...


404

POWER PLANT ENGINEERING

Artificial means for increasing the surface
area without increasing the cubical contents have,
therefore, been developed
...


70

Ambient temp

These are illustrated in Fig
...
25
...

Method (c) is extremely common for a wide range
of sizes, while (d) is used when there is insufficient room to accommodate all the tubes required
by a large transformer
...
V
...
V
...
12
...
Oil-Circulation and Temperature
Distribution
...
The tubes may sometimes be "gilled," i
...

wound with a strip-on-edge metal helix, to increase cooling surface and the eddying airflow that more
effectively removes heat
...

Ribs or Fins

Corrugations

Oil
Oil
Tank wall
(a )

( b)
Main tank
Radiator
tank

Tubes

(c )

(d )

405

ELECTRICAL SYSTEM
Conservator

Radiator
For O
...


Oil pump
for O,F,B,

Fan
motor

(e) O
...
and O
...
B
...
F
...
type

Fig
...
25
...


For the largest sizes the radiator type of cooling is used, where separate radiator tanks with fins or
corrugations, connected at top and bottom to the main tank, dissipate the heat by oil circulation
...
12
...

The limit of output with oil-insulated, self-cooled transformers is reached when the tank becomes too large and costly
...
In carriage by road
the available routes with their grades, bridges, etc
...
For larger units, transport in parts must be resorted to, with erection on site
...

12
...
4 FORCED OIL COOLING
When forced cooling becomes necessary in large high-voltage oil-immersed transformers
...
Air-blast
cooling can be used, a hollow-wailed tank being provided for the transformer and oil, the cooling air
being blown through the hollow space
...

A cheap method of forced cooling where a natural head of water is obtainable is the use of a
cooling coil, consisting of tubes through which cold water is circulated, inserted in the top of the tank
...
Any leakage will, therefore, be from the water to the oil, so that
there is a risk of contaminating the oil and reducing its insulating value
...
The inlet and outlet pipes are
lagged to avoid water from the ambient air condensing on them and getting into the oil
...
When the cooling medium is water, this has the advantage that the oil can be arranged to work at a

406

POWER PLANT ENGINEERING

higher static head than the water, so that any leakage will be in the direction of oil to water
...

Fig
...
25(e) and (f) shows diagrammatically the usual methods of cooling employed where
separate radiators are necessary
...

Until recently all large units employed oil-circulating systems, but considerable advances have
been made towards increasing the size of self-cooled units by special radiators
...
The addition of an air-blast system to circulate cooling air over the radiators permits the increase of size to about 75000 kVA
...
A temperature device can be used
to bring the fan into action when the oil temperature reaches a desired limit; this improves the overall
efficiency at small loads
...
12
...

Tank-less, air-insulated
...


Core and windginds of ON/OFB 22
...


Fig
...
26
...
5 MVA, 132/33 kV Transformer
...
4
...


407

ELECTRICAL SYSTEM

A coil design, which allows the copper heat to flow radially out-wards with little cross insulation
in the path of the flow, leads to economical rating in that a high current density can be employed for a
given temperature rise without sacrifice of efficiency
...
In some designs the flow of heat can be so much
improved that the transformer output entails a larger size of tank
...
Owing to the laminated nature of the cores, and the presence (on the surfaces of
the plates) of oxide films and paper, varnish, etc
...
On account of the rather greater exposure of iron to the oil in shell-type transformers, these are
better than the core-type as regards the cooling of the iron
...

1
...
Small tanks are constructed from welded sheet steel, and larger ones from plain boilerplate
...
The fittings include
thermometer pockets; drain cock, rollers or wheels for moving the transformer into position, eyebolts
for lifting, conservators and breathers
...

Conservators are require to take up, the expansion
and contraction of the oil with changes of temperature in
service without allowing the oil to come in contact with
the air, from which it is liable to take up moisture
...
The
tank is filled when cold, and the expansion is taken up in
the conservator
...
Some tank details are illustrated in Fig
...
27
...
12
...
2000 kVA, 20/6
...


2
...
Oil in transformer construction serves the double purpose of cooling and
insulating
...
All oils are good insulators, but animal oils are either too viscous or tend to form fatty acids,
which attack fibrous materials (e
...
cotton) and so are unsuitable for transformers
...

Of the mineral oils, which alone are suitable for electrical purposes, some have a bituminous and others
a paraffin base
...

In the choice of oil for transformer use the following char-acteristics have to be considered
...
This determines the rate of cooling, and varies with the temperature
...


408

POWER PLANT ENGINEERING

Insulating Property
...
A more important matter, however, is the reduction of the dielectric
strength due to the presence of moisture, which must be rigorously avoided
...

Flash Point
...
A flash point of not less than about 160°C is usually demanded for reasons of
safety
...
The temperature at which an oil will ignite and continue burning is about 25 percent
above the flash point, or about 200°C
...
The oil must not contain impurities such as sulphur and its compounds
...

Sludging
...
Sludging means the slow formation of semisolid hydrocarbons, sometimes of an acidic nature, which are deposited on windings and tank walls
...
In its turn it makes the whole transformer hotter, thus
aggravating the trouble, which may proceed until the cooling ducts are blocked and the transformer
becomes unusable owing to overheating
...
The chief remedy available is to use oil, which remains without
sludge formation after long periods of heating in the presence of oxygen, and to employ expansion
chambers to restrict the contact of hot oil with the surrounding air
...
Among the products of oxidation of transformer oil are CO, volatile water-soluble organic acids, and water
...
The provision of breathers not only prevents the ingress of damp air, but also helps on out-breathing to absorb
any moisture produced by oxida-tion of the oil
...

3
...
The deterioration of oil during its working life can be retarded by the use of
anti-oxidants, particularly oxidation “inhibitors
...
Inhibitors greatly prolong the phase in the service life of the oil, which precedes the onset of
deterioration, and during which the acid and sludge formations are substantially zero
...
Synthetic Transformer Oil
...
Chlorinated diphenyl, a synthetic oil suitable for transformers,
is chemically stable, non-oxidizing, rather volatile, and heavier than water
...
The permittivity is 4
...
5
...
The oil is a powerful solvent of most varnishes, gums, binders and paints, which must consequently be barred from transformers designed for synthetic oil cooling
...

5
...
The temperature rises permitted in the British Standard Specification for
power and lighting transformers
...
5 BUS-BAR
When a number of generators or feeders operating at the same voltage have to be directly connected electrically, bus-bars are used as the common electrical component
...
We shall discuss some important bus-bars arrangements used for power stations and sub-stations
...

12
...
1 SINGLE BUS-BAR SYSTEM
The single bus-bar system has the simplest design and is used for power stations
...

Fig
...
28 shows the single bus-bar system for
a typical power station
...
Each
generator and feeder is controlled by a circuit breaker
...
The
chief advantages of this type of arrangement are low
initial cost, less maintenance and simple operation
...
B
...
B
...
B
...
B
...
B
...
12
...
A Single Bus-bar System
...

(2) If a fault occurs on the bus-bar itself, there is complete interruption of supply
...

12
...
2 SINGLE BUS-BAR SYSTEM WITH SECTIONALISATION
In large generating stations where several units are installed, it is a common practice to sectionalise
the bus so that fault on any section of the bus-bar will not cause complete shut down
...
B
...
B
...
B
...
B
...
B
...
B
...
B
...
B
...
12
...
B
...
12
...
Three principal advantages are claimed for this arrangement
...
Secondly, if a fault occurs on any feeder, the fault current is much lower than with
unsectionalized bus-bar
...
Thirdly,
repairs and maintenance of any section of the bus-bar can be carried out by de-energizing that section
only, eliminating the possibility of complete shutdown
...
Moreover, the circuit
breaker itself should be provided with isolators on both sides so that its maintenance can be done while
the bus-bars are alive
...
5
...

Main
Spare

C
...


C
...


C
...


C
...


C
...

Bus coupler

Fig
...
30

In order to achieve this objec-tive duplicate bus-bar system is used in important stations
...
12
...
Each generator
and feeder may be connected to either bus-bar with the help of bus coupler which consists of a circuit
breaker and isolators
...
12
...
However, if it were desired to switch a circuit from one to another without interruption of
service, there would have to be two circuit breakers per circuit
...

Advantages
(1) If repair and maintenance it to be carried on the main bus, the supply need not be interrupted
as the entire load can be transferred to the spare bus
...

(3) If a fault occurs on the bus-bar, the continuity of supply to the circuit can be maintained by
transferring it to the other bus-bar
...
6 BUSBAR PROTECTION
Busbars and lines are important elements of electric power system and require the immediate
atten-tion of protection engineers for safeguards against the possible faults occurring on them
...
The modifications are necessary to cope with the protection problems
arising out of greater length of lines and a large number of circuits connected to a busbar
...
Fortunately, less expensive methods are available which are reasonably effective in
providing protection for the busbars and lines
...

Busbars in the generating stations and sub-stations form important link between the incoming
and outgoing circuits
...

The busbar zone, for the purpose of protection, includes not only the busbars themselves but also the
isolating switches, circuit breakers and the associated connections
...

The standard of construction for busbars has been very high, with the result that bus faults are
extremely rare
...
Improved relaying methods have
been developed, reducing the possibility of incorrect operation
...

12
...
1 DIFFERENTIAL PROTECTION
The basic method for busbar protection is the differential scheme in which currents entering and
leaving the bus are totalised
...

When a fault occurs, the fault current upsets the balance and produces a differential current to operate a
relay
...
B
...
T
...
T
...
B
...
T
...
B
...
12
...
12
...
A
generator and supplies load to two lines feeds the busbar
...
The protective relay is connected
across this parallel connection
...
Under normal load conditions or external fault conditions, the sum of the
currents entering the bus is equal to those leaving it and no current flows through the relay
...

The difference of these currents will flow through the relay and cause the opening of the generator,
circuit breaker and each of the line circuit breakers
...
6
...
This can be
achieved by providing earthed metal barrier (known as fault bus, surrounding each conductor throughout its entire length in the bus structure
...
By directing the flow of earth-fault
current, it is possible to detect the faults and determine their location
...

Conductor

Insulators

Supporting structure
(fault bus)

C
...


Relay

To trip circuit

Fig
...
32

Fig
...
32 show the schematic arrangement of fault bus protection
...
A relay is connected across the secondary of
this CT
...
A fault involving a connection between a conductor and earthed supporting
structure will result in current flow to ground through the fault bus, causing the relay to operate
...


THEORETICAL QUESTIONS
1
...

2
...

3
...


ELECTRICAL SYSTEM

413

4
...

5
...
Describe the various generator-cooling methods
...
Give a list of various electrical protective equipment used in a power station
...
Describe the various methods of controlling the voltage at the consumer terminal used in
power supply system
...
Write short notes on the following:
(a) Over-head and underground power transmission system
...




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