LIST OF CONTENTS

1) INTRODUCTION

1.1 Location of Cheshmashahi Grid Station

1.2 Description of single line diagram
1.3 Substations
1.3.1 Design of substations
1.3.2 Location of Substations

2) EQUIPMENTS & SWITCH GEAR

INSTALLATIONS IN GLADNI GRIDSTATIONS

2.1 Incoming lines

2.2 Bus bars
2.2.1 Single bus bar
2.2.2 Double bus bar
2.3 Insulator
2.3.1 Pin type insulator
2.3.2 Suspension type insulator
2.3.3 Post type insulator
2.4 Isolating switches
2.5 Circuit breaker
2.5.1 Classification of circuit breaker
2.5.1.1 MOCB
2.5.1.2 Air blast circuit breaker
2.5.1.3 SF6
2.5.2 Applications
2.6 Lightening arrester

3) RELAYS

3.1 Protective relay

3.2 Types of relay
3.2.1 IDMTL
3.2.2 Buchol’z relay
3.3 Earthing
3.3.1 Neutral grounding
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 4) TRANSFORMERS

4.1 Introduction
4.2 Main circuits in transformers
4.2.1 Magnetic Circuit
4.2.2 Electric Circuit
4.2.3 Dielectric Circuit
4.2.4 Thermal Circuit
4.2.5 Structural Circuits
4.3 Instrument transformer
4.3.1 Current transformer
4.3.2 Potential transformer
4.4 Metering and indicating instruments
4.5 Power line carrier communication system

5) STRUCTURAL STRUCTURES

5.1 Conservator
5.2 Breather
5.3 Pressure relief device/expansion vent
5.4 Temperature indicators
5.5 Buchol’z relay
5.6 Bushings
5.7 Tap Changer
5.7.1 Off-Circuit tap changer
5.7.2 On Load Tap Changer
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 LIST OF ABBERVIATIONS

S.No. ABBERVIATIONS FULL NAME

1. ac Alternating current
2. dc Direct current
3. kV Kilo Volts
4. PT Potential transformer
5. CT Current transformer
6. HT High terminal
7. MOCB Minimum Oil Circuit Breaker
8. IDMTL Inverse Minimum Time Over
Current Relay
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 INTRODUCTION
In the beginning of the electrical age, electricity was generated,
transmitted and distributed in the form of direct current and voltages
were low. With the development of transfer, ac system has become
predominant. The present day large power systems have been possible
only due to adoption of ac system.

The present day electrical power generator system is ac. The

electrical power is produced at power stations which are located at
favorable places quite away from consumers. It delivers to consumers
through large network of transmission and distribution. For the further
distribution of electrical supply grid station plays an important role.

Sometimes it becomes necessary to use dc, at the time; ac can be

converted to dc by the use of rectifiers. In the short ac is of utmost
importance.
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 1.1 CHESHMASHAHI GRID STATION (105 MVA, 132/33 KV)

Location:- Cheshmashahi near Tulip Garden (J&K)

Introduction:-

In Cheshmashahi grid station there is a provision for 4 outgoing/incoming

transmission lines:-
i) 132kV Pampore-Cheshmashahi Ckt-1st
ii) 132kV Pampore-Cheshmashahi Ckt-2nd
iii) 132kV Habak-Cheshmashahi Ckt-1st
iv) 132kV Habak-Cheshmashahi Ckt-2nd
There are four (4) transformer banks 132/33kV with capacity
{(2x20)+(1x15)+(1x50)} MVA=105 MVA

Total 105 MVA

There is also provision for following 33 kV lines
1) 33 kV Auxilary,It is used by power station itself
2) 33 kV Shalimar (Feeds to Shalimar and Harwan Receiving Stations)
3) 33 kV TRC Feeder (Feeds to TRC, RajBagh & Neduse Receiving
Stations)
4) 33 kV Centaur Feeder (Feeds to Centaur, Bichwara & B.B. Cantt.
Receiving Stations)
5) 33 kV B.B.Cantt.( Feeds to B.B.Cantt. Receiving Stations through under
ground cable)
6) 33 kV Brane (Feeds to Brane and Nishat Receiving Station)
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 1.2 DESCRIPTION ABOUT SNGLE LINE DIAGRAM :-

Figure attached shows the key diagram of a typical132/33 kV Cheshmashahi grid

Station. The key diagram of this grid station is explained as under:-
i) There are four 132 kV incoming lines connected to the bus bars. Each
incoming lines is capable of supplying the substation load. All these lines
can be loaded simultaneously to share the grid-station load or one line
can be called upon to meet the entire load. The three line arrangement
increases the reliability of the system. In case there is a breakdown of one
incoming line, the continuity of supply can be maintained by the other
line.

ii) The grid station has double bus-bar system, one main bus-bar and the
other reserve bus-bar. The incoming lines can be connected to either
bus-bar with the help of an arrangement of ckt. Breaker & isolators. The
advantage of a double bus-bar system is that if t repair is to be carried on
one bus-bar or isolators, clamps or insulators of one bas bar, the supply
need not to be interrupted as the entire load can be transferred to the other
bus.

iii) There is an arrangement in Cheshmashahi grid station to step down the

incoming 132kV supply to 33kV by four transformer banks with capacity

2x20 MVA= 40MVA

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1x15MVA = 15MVA
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1x50 MVA = 50 MVA

 Total = 105MVA
iv) All the four incoming &out going lines are connected through ckt.
Breakers having isolators on there either ends. Whenever repair is to be
carried over the line or tower, the Circuit breaker of the said line is first
opened from both the end, then isolators of the line opened from both the
end & the line is earthed from both end.

v) The instrumental transformers (P.T.) and (C.T.) are used for metering &
protections of the system. The secondary’s of the C.T and P.T are
connected to the energy meter and relays through cables via cable trench.
of relay ckt. The P.T is connected right on the point where the line is
terminated. The CTs are connected at the terminals of each ckt. Breakers.

vi) The lighting arresters are connected near the transformers terminals (on
H.T. side) to protect them from lighting strokes.

vii) Again there is provision for further step down 132 kv supply to 33kv by
two transformers-2x50MVA-100MVA

viii) There are other is auxiliary in the grid station such as wave trapper,
capacitor bank for power factor improvement, earth connections, local
supply connections, dc supply connections.

1.3 SUBSTATIONS
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 Substations serve as a source of energy supply for the local areas of
distribution in which these are located. Their main functions are to receive energy
transmitted at high voltage from the generating stations, reduce the voltage to a
value appropriate for local distribution and provide facilities for switching. Some
substations are simply switching stations where different connections between
various transmission lines are made; others are converting substations which
convert AC into DC or vice versa or convert frequency from higher to lower or
vice versa. Substations have some additional functions. They provide points where
safety devices may be installed to disconnect equipment or circuit in the event of
fault. Voltage on the outgoing feeder can be regulated at a substation. A substation
is convenient place for purpose of improving power factor and make measurement
to check the operations of the various parts of the power system.
The substations may be defined as the assembly of apparatus, which transforms the
characteristics of electrical energy from one form to another: e.g- from AC to DC
or the voltage in operation from 66kv, 110kv or 220kv. However, 500kv will be
used for the national grid system in the near future. The consumer do not use such
high voltage and so they must be transformed to low voltage levels by means of
substations, thus a substation may be called as link between the generating stations
and the consumer.

1.3.1 DESIGN OF SUBSTATIONS :-

When a substation is to designed, the following procedure should be adapted :
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 1) Prepare a single line diagram of main electrical connections showing bus
bar arrangements, circuit breakers and transformers.
2) Decide the layout of switch gear keeping view capacity of substations,
method of control, and number of feeders, reliability, safety, flexibility,
space needed and construction.
3) The layout should be such that it should be possible to isolate any section
during fault, without affecting the service of the healthy section.
4) It should be possible to have an easy and safe access for maintenance and
inspection for different equipments.
5) An arrangement should be made to extinguish fire.
6) The earth conductor should of sufficient cross sectional area to carry the
fault current in severe conditions.
7) A proper and efficient automotive electrical protective gear should be
used. Power cables should be separate from the control cables.
8) Allow reasonable amount of expansion for substations.

1.3.2 LOCATION OF SUBSTATIONS :-

The following points should be taken mainly into consideration in choosing the
location of substation.
1) Locate substation as close to the load centre as possible.
2) Locate substations at such points that all the perspective loads may be
conveniently reached without under voltage regulation.
3) Allow access to the incoming transmission line and outgoing distribution
line. Choose the site where municipal restriction of property laws should
permit the type of building necessary for substation.
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 4) Keep load on the substations within such limits that an undue large area
or number of consumers will not be affected in case the station shut down
occurs.

The location of substations could be indoor, outdoor and underground or pole

mounted. The last two are for small size of distribution substation and the rural
electrification may be considered only in large crowded cities, with space
limitations. The indoor type of substations would need building to house the
equipments. In case of outdoor type of substations which is common used, all the
equipments are arranged outdoor and should withstand weather conditions.

The advantage of outdoor type is that do not need any buildings. In case of indoor
type the cost of transformers and switch gear equipments are less than that of
outdoor type. Also the conditions for inspections and maintenance are better in
indoor type than in outdoor types.

In case of substations at generating stations, to step up generation voltage to

transmission voltage, they are located in the outstation yard. So in the transmission
sub stations also the control and protective equipments are located inside the
buildings near the station yard.
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 2 EQUIPMENTS AND SWITCH GEAR INSTALLATION IN THE
GLADNI GRID STATION

2.2 INCOMING LINES:-

These lines supply power to substations from source of generations or from
transmission line at high voltage.
2.2 BUS BARS :-
When a number of lines operate at the same voltage have to be directly connected
electrically, bus bars are used as the common electrical component. Bus bars are
copper or aluminum bars (generally rectangular x-section) and operates at the
constant voltage. The incoming and outgoing lines in a substation are connected to
the busbars. The outdoor busbars are either of the rigid type or of the strain type. In
rigid type pipes are used for busbars and also for making the connections among
the various equipments wherever required. The strain type bus bars are over head
system of wires string between the two supporting structures and supported by
strain type insulators.
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SALAL 1 BUS BAR

 There are numerous variations of bus bar arrangements. The choice of a particular
arrangement depends on various factors viz, system voltage, position if the
substation in the system flexibility, reliability of supply and cost:-

1) Simplicity is the keynote of a dependable system.

2) Maintenance should be possible without interruption of supply or danger
to the operation personal.
3) Alternative arrangement should be available in the event of an outage of
any of the apparatus.
4) The installation should be as economical as possible keeping in view the
requirements and continuity of supply.

The most commonly used bus bars arrangements in substations are:-

2.2.1 SINGLE BUSBARS :-

This arrangement is the simplest and cheapest. However it suffers from two major
defects:- Maintenances without interruption of supply is not possible and extension
of the substation without a shutdown is not possible.

2.2.2 DOUBLE BUSBARS :-

The scheme use two identical bus bars so that:-
a) Each load may be fed from either bus
b) The indeed and load circuit may be divided into two separate groups
needed from operational considerations.
c) Either busbar may be taken out for maintenance and cleaning of
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insulators.
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 This arrangement has been quite frequently adopted where the load and continuity
of supply justify additional cost. This type of bus bar is used in Gladni(Bus 1 and
Bus 2).

2.3 INSULATORS
The line conductors supported on the towers in such a way that currents from
conductors do not flow to earth through supports. This is achieved by securing the
conductors to support with the help of insulators. The insulators provide necessary
insulation between the conductor and supports, thus prevent any leakage current
from conductors to earth.

The insulators serve two purposes. They support the conductors; confine the
current to the conductors. The most common material for manufacture of the
insulators is porcelain. Toughened glass is also sometimes used for insulators but
its use is limited to about 33 KV. The design of the insulators is such that the stress
due to contraction and expansion porcelain to come in direct contact with hard
metal screw thread. Normally cement is used between metal and porcelain.

TYPES OF INSULATORS:-

There are several types of insulators and there use in the substation will depend on
the service requirements:-

 Pin Type
 Suspension Type
 Post Insulator Type
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 2.3.1 PIN TYPE INSULATOR :-

As the name suggests, the pin type insulator is secured to the cross-arm to
the pole. There is a groove on the upper end of the insulator for housing the
conductor. The conductor passes through this groove and is bound by the annealed
wire of the same material as of conductor.
The pin type insulators are normally used up to 33KV .It is not desirable to
use them beyond 50 KV as a cost of such insulators then increase much faster than
the voltage.

2.3.2 SUSPENSION TYPE INSULATOR :-

For high voltage greater than 33kv these insulators are used they
consist of number of porcelain discs connected in series by metal links in form of a
string. The conductor is suspended at the bottom and the end of this string while
the other end of the string secured to the cross-arm of the tower. Each disc is
design for 11KV. The number of discs in series would depend upon the working
voltage.

2.3.3 POST INSULATOR TYPE :-

Post insulator is used for bus bars. A post insulator consists of porcelain body, cast
iron cap and flanged cast iron base. The hole in the cap is threaded so that the bus
can directly bolt to the cap.
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 2.4 ISOLATING SWITCHES :

In substation, it is often desired to disconnect part of the system for

general maintenance and repairs .This is accomplished by the isolating switch or
isolator. An isolator is essentially a knife switch and is designed to open a circuit
under no load. In other words, isolator switchesare operated only when the lines in
which they are connected carry low current. Isolator are not fitted with are control
devices and function only for instantaneous changes of switching circuits
arrangements and for providing a visible break in a ckt. So as to make certain
operation that can be performed on an isolated section without running a risk.
Isolator is handled manually as well as automatically.
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 2.5 CIRCUIT BREAKER :

A circuit breaker is equipment which can be open or closed under

normal as well as fault conditions. It is so desired that it can be operated manually
or by remote control under normal conditions and automatically under fault
conditions. For the latter operations a relay is used in the ckt breaker. The basic
construction of any circuit breaker requires the separation of contacts in any
insulating fluid which serves two functions: - it extinguishes the arc drawn
between the contacts when the circuits breaker opens and it provide adequate
insulation between the contacts and from each contact to earth.

2.5.1 CLASSIFICATION OF CIRCUIT BREAKER:-

1) MINIMUM OIL CIRCUIT BREAKER (MOCB)

One of the important developments in the design of oil circuit breaker has
been to reduce the amount of oil needed. The other advantages are reduction in
tank size, reduction in total weight and reduction in cost. It used minimum amount
of oil and is only used for arc extinguishing. The current conducting parts are
insulated by porcelain or organic insulated material. Low oil circuit breaker
employs solid materials for insulations purpose and uses smallqtty.of oil which is
just sufficient for arc extinguishing .By using suitable arc control devices, the arc
extinguishing can be further facilitated in low circuit breaker. There are two
different design of the arching chamber in terms of the venting provided; - Axial
venting; radial venting.
Axial venting generates high gas pressure and high dielectric strength; it
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is used mainly for the interruption of low current at high voltages. The radial
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 venting is used for interruption of relatively heavy current at low voltages as a gas
pressure developed is low and also the dielectric strength is low.

MINIMUM OIL CIRCUIT BREAKER

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 2) SF6
In such circuit breaker sulphur hexafluoride gas is used as arc
quenching medium. The SF6is electronegative gas and has a strong tendency to
absorb free electrons. The contacts of the breaker opened in a high pressure
flow of SF6 gas and an arc is struck between them. The conducting free
electrons in arc are rapidly captured by the gas to form relatively immobile
negative ions. This loss of conduction electrons in the arc quickly builds up
enough insulating strength.

SF6 CIRCUIT BREAKER TYPE FXT14F

RATED VOLTAGE - 245KV
NORMAL CURRENT - 2500A
FREQUENCY- 50HZ
DURATION OF SHORT CIRCUIT CURRENT – 3 SEC
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 The SF6 circuit breaker has been found to be very effect able for high power and
high voltage services.SF6 has excellent insulating strength because of its affinity
for electrons i.e. whenever a free electrons collides with the neutral gas molecules
to form negatives ions, the electrons is absorbed by the neutral gas molecules may
occur in two ways
SF6 + e -> SF6
SF6 + e -> SF5 + F

The negative ion formed are relatively heavier as compared to free electrons and
therefore under a given electric field the ions do not attain sufficient energy to lead
cumulative ionization in the gas.

3) VACCUM CIRCUIT BREAKER;

In such breakers (degree of vacuum being from 10-7 to 10-5tore) is used as arc
quenching medium. Since vacuum offers the high insulating strength, it has
superior quenching properties than any other medium e.g. when contacts of the
breaker are opened in vacuum , the interruption occurred at first current zero with
dielectric strength between the contacts building at a rate of 1000 th of times higher
than that obtained with other circuit breaker . Thus a vacuum arc is different from
the general class of low & high pressure arc. In the vacuum arc the neutral atoms,
ions and electrons do not come from the medium in which the arc is drawn but
they are obtained from the electrodes themselves by evaporating its surface
material, because of the large mean free path for the electrons, the dielectric
strength of the vacuum is a 1000 times more than when the gas is used as the
interrupting medium. In this range of vacuum the break down strength is
independent of the gas density and depends only on the gap length and upon the
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condition of electrodes surface.

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 INSIDE BOTTLE OF VACUUM CIRCUIT BREAKER

2.5.2 APPLICATIONS :
For a country like India where distance are quite large & accessibility
to remote areas difficult the insulation of such outdoor ,maintenance free circuit
breaker should prove a definite advantage . Vacuum circuit breaker are being
employed for outdoor application ranging from 22KV to 66KV with limiting of
say 62 to 100 MVA ; they are suitable for a majority of applications in rural
areas.
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 2.6 LIGHTENING ARRESTERS :

It is protective device which conducts the high voltage surges on the power
systems to the ground.

Surge voltages are abnormal voltage that may cause break down of insulation of
electrical equipment. These voltages may result from switching disturbance in the
electrical installation circuit or from lightning stroke.

In Gladni grid station valve type arrester is used. It consists of two assemblies:
Series spark gaps and non-linear resistor discs. These both are connected in
series under normal conditions; the normal system voltage is insufficient to
cause the breakdown of air gap assembly. On the occurrence over voltage, the
breakdown of series spark gap take place and the surge current is conducted to
earth via the non-linear resistor. They provide effective protection.
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 4.3 INSTRUMENT TRANSFORMERS :
The lines I substation operate at high voltage and carry current of
1000 of amperes. The measuring instruments and protective devices are
designed for low voltages (for generally 110 volts) and currents (about 5 A).
Therefore, they don’t work satisfactory if mounted directly on the lines this
difficulty is overcomes by installing instrument transformers on the power
lines. The function of these instrument transformers is to transfer voltage or
current in the power lines to values which are convenient for the operation of
measuring instrument and relays. There are two types of instrument
transformers viz

4.3.1 CURRENT TRANSFORMERS:

C.T is essentially a transformer which steps down current to a known

ratio. The primary winding of this transformer consists of one or more turns
of thick wire connected in series with the line. The secondary consists of
large number of turns of fine wire and provides for the measuring
instruments & relays a current which is a constant fraction of the current in
the line. Suppose current transformer rated at 100/5 A is connected in the
line to measure current in primary the current in primary the current in the
line 100 A, then secondary of C.T will be 5 A .Similarly, if current in the
line is 50 A, then secondary of C.T will have current of 2.3 A .Thus C.T
under consideration will step down the line current by a factor of 20.
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Current Transformer
 4.3.2 POTENTIAL TRANSFORMER:

It is essentially a step down transformer & step downs the voltage to

known ratio. The primary of this transformer consists of large number of
turns of fine wire connected across the line instrument relays a voltage a
known fraction of the line voltage. Suppose a potential transformer rated at
66kv/ 110v is connected to a power line .If line voltage is 66kv, then the
voltage across secondary will be 110V.

Potential Transformer
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 4.4 METERING & INDICATING INSTRUMENTS :

There are several metering & indicating (e.g. ammeter, voltmeter,

energy meter etc) installed in a substation to maintain watch over the ckt
quantities .The instrument transformer are invariably used with them for
satisfactory operation.

4.5 POWER LINE CARRIER COMMUNICATION SYSTEM :

Whenever some breakdown occurs in the transmission network at any

place, the message to rectify the fault must be sent to all substations.
Quickness is the first priority .The ordinary telephone network cannot be
used as it already overloads and remains engaged. Therefore, the same
power lines, which carry energy, are used for this purpose. The signal is
modulated by HF carrier and is send as radio waves through the power lines.
This system is known as power line carrier communication system.
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A WAVE TRAP
 3) PROTECTIVE RELAY AND EARTHING

3.1 PROTECTIVE RELAYS :-

It is a device that detects the faults and initiates the operation of the
ckt breaker to isolate the defective element from the rest of the system .The
relay ensures the safety of the ckt equipment from any damage which might
be otherwise caused by the fault.

3.2 THE TYPES OF RELAYS INSTALLED AT GLADNI

 IDMLT type relay (Directional & non directional)

 Primary relays such as buchol’z Restricted earth fault etc.

3.2.1 IDMTL (INVERSE DEFINITE MINIMUM TIME OVER CURRENT

RELAY

IDMTL is one in which the operating time approx inversely

proportional to the fault current near pick up value and becomes substantially
constant slightly above the pickup value of the relay. This is achieved by using a
core of the electromagnet which gets saturated for currents slightly greater than
pick up current. It has two types:-

( a ) Non – directional relay

( b ) Directional relay

( a ) Non directional ( over current or earth leakage ) relay :-

This types of relay will not be able to discriminate when used on transmission
lines, whether the fault has been taken place in the section where the relay is
located or it has taken place in the adjoining section. Under the condition of high
power factor or leading power factor, the impedance seen by relay is a very low or
even negative.
( b ) Directional (Over current or earth fault ) relay :- The non
directional relay can operate for fault flow in either direction. In order to achieve
operation for the fault flowing in a specific direction, it is necessary to add a
directional element to the non directional element. Such a relay which responds to
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fault flow in a particular directional is called a directional relay.

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 CONTROL PANEL SHOWING RELAYS

3.2.2 BUCHOL’Z RELAY :-

Buchol’z relay is a gas actuated relay used for protecting oil immersed
transformer against all types of internal fault and makes use of the fact that fault
decompose oil thus generating gases. The device relies on the fact that an electrical
fault inside the transformer tank is accompanied by generation of gas, and if the
fault current is high enough by a surge of oil from the tank to the conservator.
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 3.3 EARTHING:-

Connecting of an electrical equipment or apparatus to the earth with

the help of a connecting wire of negligible resistance is known as “earthing” or
“grounding”.

In an electric installation, if a metallic part of an electric appliance

comes in direct contact with a bare or livewire, the neutral being a good conductor
of electricity is charged static charge on it will accumulate. Now if any person
comes in contact with this charged metal part, he will get a severe shock. But if the
metallic parts of the equipment or apparatus are earthed, the charge will be given to
earth immediately as the metallic part comes in direct contact with a bare or live
wire or breakdown occurs. And as the discharge takes place to the earth, the
impedance of the path of the current is low, heavy current flows to earth, the
instant the current exceeds the limiting value the protective device (usually a fuse)
operates and isolates the appliance or equipment form the supply. Thus to provide
safety to the users as well as for the operating and maintenance personals it is
essential to provide earthing.

(1 ) SYSTEM EARTHING :-

It is required to provide low fault impedance to the ground fault currents for proper
operation of the protective relays and for meeting the system requirement by
effectively earthed system.

( II ) SAFETY EARTHING :-

It is required to provide protection to the operating staff working in the yard and
substation from any injury during fault condition by keeping the voltage gradient
with in safe limits. The above two parts have common earth mat from which flat
iron risers are taken out to connect all the non-current carrying metal parts of the
equipment. At the same time the earth mat conductor rise to voltage, which is
equal to the resistance of the earth mat multiplied by ground fault current. This
difference of potential results in voltage gradients.
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 3.3.1 NEUTRAL GROUNDING:-

In practice, the neutral of a three- phase system is earthed at substation. This is

known as neutral earthing or grounding .An earthed neutral system has the
following advantages :-

A) It provides a better protection against earth faults.

B) It ensures nearly constant voltage of healthy phases because neutral point is

not shifted.

C) This system provides a better reliability of service .

D) It is safer for personal and equipment.

E) It requires lesser maintenance expense as compared to the unearthed neutral

(isolated system).

F) In the system, transient voltages produced are very small.

G) Ground fault relaying is simple . The earth may be utilized to operate

protective relays to isolate the fault.

H) Persistent arcing ground can be eliminated by employing protective gears.

In addition to above, there are following equipments in a substation:-

A) Fuses

B) Carrier-current Equipment

C) Substation AuxiliarySupplies.
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 FUSES :

Fuse is a essentially a short piece of metal ( or a fusible material )

inserted in a circuit which melts when a predetermined value of current flows
through it and thus breaks the circuits .The protective element of the fuse is a fuse-
link inserted in series with the circuit being protected . The most generally material
used for fuse element is a low melting point material such as tin, lead or zinc
.Fuses may be low voltage type or high voltage type : low voltage can be further
divided into two classes namely semi- enclosed rewire able fuse and the cartridge
type fuse.

SUBSTATION AUXILIARY SUPPLY :

In small –unattended substations only small amount of power for

electric lightening during regular periods of inspection, maintenance and repair is
required. In regional substation the electric power is required for the auxiliaries-
the lightening circuits, air blast fans of power transformers, battery changing sets,
compressor units in case of air blast circuit breakers, ventilating fans of the
substation buildings etc. In large substation it is wide practice to connect two
transformers to the main bars supply of the auxiliaries.
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 4. TRANSFORMERS

4.1 INTRODUCTION:-

As in well known transformers is a piece of apparatus without

continuously moving parts, which by electromagnetic induction transforms
alternating voltage and current in one or more windings usually from one value to
different values of voltage and current and at same frequency.

Transformers is heart of power system and is most important and

costliest in power system .It is estimated that one MW of additional power earning
capacity requires about 7 to 8 MVA transformers’ capacity .The demand for
transformers are expected to grow at a rapid pace, since the generating stations
conventionally Hydroelectric, thermal are situated geographically far apart from
load centers and since these are linked by transformer.

4.2 MAIN CIRCUITS IN TRANSFORMERS:-

4.2.1 MAGNETIC CIRCUITS :

Cold rolled grain oriented silicon (CRGOS) Steel (with lower specific
losses to the flow of magnetic flux along the direction of grain
orientations is universally utilized.
With the advent of computer controlled machines for cutting the
laminations precisely, step lap lamination joints are economically used to
achieve low no- load losses.
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 4.2.1 ELECTRIC CIRCUIT:-

It consists of primary and secondary winding which are costliest and which have to
meet stringent requirement of Dielectric, thermal and mechanical stresses expected
during testing as well as service conditions. ANNEALED rectangular shaped
copper conductors most commonly used and for Distribution transformer and high
voltage testing transformers and conductor are also used. In order to minimize the
stray losses in the winding, continuously transposed conductor (CTC) are used
type of winding normally include Helical, continuous Disc, Multilayer and inter
leaved Disc winding.

4.2.3 DIELECTRIC CIRCUIT:-

It comprises of insulation:

- Minor insulation between lure to lure on within the layers of same winding.

- Major insulation between winding and between winding and earth.

- Oil serves as an insulation as well as coolant paper oil combination has been

established as effective insulation system even up to HV transformers.

4.2.4 THERMAL CIRCUIT:-

The losses produced in transformer should be dissipated in order to limit

the temperature rise of oil and winding working at 98mof winding
temperature, over weighted ambient of 32, transformers have continuous
healthy life during its entire service span and every 6 rise ova 98
reduces the life to half .By providing additional cooling through
radiators, fans .etc heat can be dissipated and capacity of transformer can
be updated

4.2.5 STRUCTURAL CIRCUIT:-

This is essential to house the transformer oil, flitting and accessories such as
terminal arrangements, conservator, Breather, safety device etc. The structural
32

design should also withstand seismic forces.

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 5. Structural Structures:-

5.1 CONSERVATOR:

Conservator with variation of temperature there is corresponding variation in oil

volume. To account for this, an expansion vessel called conservator is added to
hemp with a connecting pipe to main tank. In smaller transformers this vessel is
open to atmosphere through dehydrating breathers in large transformer; an air bag
is mounted inside the conservator with inside bag open
Atmosphere through breathers and outside surface of bag in contact with oil
33

surface.
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 5.2 BRAETHER:-

Both transformer oil and celluloses’ paper are highly hygroscopic .Paper being

more hygroscopic than mineral oil .The moisture, if not excluded from oil surface

in conservator, this will find its way finally into paper insulation and causes

reduction insulation strength of transformer to minimize this the conservator is

allowed to breath only through silica gel column, which absorb the moisture in air

before it enters the conservator air surface.

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 5.3 PRESSURE RELIEF DEVICE / EXPANSION VENT:-

Transformers tank is a pressure vessel as the inside pressure can group steeply
whenever there is a fault in the windings and the surrounding oil is suddenly
vaporized. TANKS as such are tested for a pressure with stand capacity of 0.35 kg
/cm. To prevent bursting of the tank, these tanks are in addition providing with
expansion vents with a thin diaphragm made of Bakelite /Copper / glass at the end.
In present day transformer, pressure relief device are replacing expansion vents.
These are similar to safety valves on boilers.

5.4 TEMPRATURE INDICATORS :-

Most of transformer (Small transformers have only OTI) are providing with
indicators that display oil temperature and winding temperature .There are
thermometers.
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 5.5 BUCHOL’Z RELAY:-

Buchol’z relay is a gas actuated relay used for protecting oil immersed
transformer against all types of internal fault and makes use of the fact that fault
decompose oil thus generating gases. The device relies on the fact that an electrical
fault inside the transformer tank is accompanied by generation of gas, and if the
fault current is high enough by a surge of oil from the tank to the conservator.

5.6 BUSHINGS:

Transformer are connected to HV lines and therefore care is to taken to prevent flash-over from

high voltage connections to earthed tank .Connections from cables are made in cable boxes , but

overhead connections are to be brought through bushing specially designed for different classes

of voltages.
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 BUSHINGS

5.7 TAP CHANGER:-

It is very essential to maintain system voltage within prescribed limits for better
health of electrical equipments voltage of system can be varied by changing the
true ratio of transformer .The device tap changer is used for adding a cutting out
turns of primary or secondary winding of transformer .It is of basically two types :

1. Off-Circuit tap changer

2. On-Load tap changer (OLTC)

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 5.7.1 OFF-CIRCUIT TAP CHANGER:-

The cheapest method of changing turn ratio of transformer is use of off-circuit tap

changer. It is essential to de-energize the transformer before changing tap.

5.7.2 ON – LOAD TAP CHANGER:-

On-load tap changers are employed to change the turn ratio of the transformer to

regulate system voltage while transformer is delivering normal load. With the

introduction of on-load tap changer, operating efficiency of electrical system has

considerably improved.
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 133.3 MVA TRANSFORMER (SINGLE PHASE)
39
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 160 MVA POWER Transformer (3 Phase)

TYPE OF OFAF ONAF ONAN

COOLING
Rating 160 128 96
HV&IV(MVA)
Rating LV (MVA) 53.33 42.67 32
No load HV(KV) 220 220 220
No load IV(IV) 132 132 132
No load LV(KV) 11 11 11
Line current 419.8 335.91 251.94
HV(A)
699.8 559.8 419.8
IV(A)
2799.27 2239.42 1679.5
40

LV(A)
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 50MVA EMCO Transformer

TYPE OF COOLING ONAN ONAM

Rated power HV KVA 40000 50000
LV 40000 50000
Rated voltage at no load 132 132
HV(KV)
33 33
IV

LV
41

Rated line current HV(A) 174.95 218.69

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LV 699.8 874.7
 50 MVA CGL Transformer
TYPE OF COOLING ONAN ONAF
No load voltage HV(KV) 132.8
LV 33
Current HV 218.7
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43

160 MVA Transformer Control panel

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 50 MVA EMCO Transformer Control Panel
44
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