CHAPTER- 1

OVERVIEW

1.1 Introduction

The economical generation of electrical energy and huge demands for power

requires creation of bigger and bigger powerhouses, may these be hydro electrical, steam

or atomic. The powerhouse may be far away from the load centers as in the case of

hydropower houses or they may be midst. Populated areas as in case to transformation

and switching stations house to be created. These are generally known as substation.

1.2 Type of Sub Station

Depending on the purpose of substation may be classified as

1. Step up sub station

2. Primary grid sub station

3. Secondary sub station

4. Distribution sub station

5. Bulk supply sub station

The step of substation is associated with the generation station. The generation

voltage is limiting to a low value and needs to step up to the primary transmission voltage

so that huge blocks of power can be carried over long distance to the centers

economically.

The primary grid sub stations are created at suitable load centers along that

primary transmission voltage is stepped down to a number of suitable secondary voltage.

From here secondary transmission lines are carried over to the load points.

Along this secondary transmission lines secondary substation are created to actual

load points where the voltage is further stepped down sub transmission and primary

distribution voltage.

The distribution substation are created where the sub transmission voltage/primary

is step down to supply voltage, these substation feed the extra consumer through a

network to distribution of and service lines.

The bulk supply and industrial sub stations are generally distributed substation

with the difference that the substation is created separately for a consumer and subsequent

distribution of the electric power or left to the particular consumer of large or medium

1

supply group. As may be designed by the standard laid down by the electrical supply by

undertaking.

A substation is immediate link between the generating stations. The generation of

electricity at low voltage link 6.6-KV or 11-KV is done although high voltage up to 33

KV is also possible but it is not expected due to economical condition. For economical

transmission of electric power, we transmit power at a very high voltage. As we know

voltage drop along the length of transmission line. We have to maintain the voltage at

such a high level that it may not be lost during transmission. Thus for the sake of efficient

transmission and considering the economical aspect all electrical authorities operating

high voltage at transmission level.

While at distribution level electrical service provides require a voltage level that

suits consumer electrical electrical equipment for this purpose we have to provide smoke

kind of stations for following reasons.

1. To maintain the voltage in original high transmission voltage.

2. To step down the voltage up to distribution load.

3. To cut the supply of desired related area, in case of any kind of damage.

All GSS are linked to each other to form network as power grid. In India, we have

two power grids; they are northern and southern power grid.

Generally we operate ring scheme of power grid.

2

CHAPTER- 2

INTRODUCTION TO 132-KV G.S.S., R.R.V.P.N.L.

MANSAROVER, JAIPUR

2.1 Introduction

R.R.V.P.N.L.. was constituted by government of Rajasthan power development

department with the general duty of promoting & coordinating development of

generation, supply & distribution of electricity within the state in the most efficient &

economic manner.

To make power system more reliable & efficient inter-state Grid system was

regional electricity Board to promote integrated operations of the constituent power

system:

1. Northern Zone

2. Western Zone

3. Eastern Zone

4. North-Eastern Zone

Regional load dispatch centers have been established to co-ordinate the activities

of state load dispatch centers so as to ensure optimum utilization of generation &

transmission facilities in the regions.

.

This is substation in Rajasthan. The various feeders are 132-KV single circuit line from

SANGANER & CHAMBEL.

2.1.1 Incoming Feeders:

1. Sanganer.(132-KV)

2. Chambel(132-KV)

2.1.2 Outgoing Feeders:

1. SMS stadium (132-KV)

2. Bishalpur-I (33--KV)

3. Kavari Path (33-KV)

4.Triveni nagar (33-KV)

5.Kiran path (33-KV)

3

6. Nirman nagar (33-KV)

In the layout so constructed the bus bar is double (main bus 1) & (main bus -2)

with a provision of transfer bus by which will be available the 132-kV switchyard layout

has been developed for total no. of 6 days each measuring 27 meters. Single line diagram

of 132-KV G.S.S., Mansarover, Jaipur.

2.1.3 Electrical Layout of 132-KV G.S.S.

1. The physical condition for each equipment.

2.Distance between various equipment.

2.2 Place of Practical Training:

For third year vocational training of 30-days, we undergo 30-days training at 132

kV G.S.S. Jaipur.

This 132-kV Grid substation is owned by R.R.V.P.N.L. is situated about 15 km

away from Jaipur on Mansarover.

2.3 Constructional Feature of 132-KV G.S.S. Mansarover:

In this G.S.S. breaker and half scheme is adopted between buses main-1 and main-

2. There are total 9 feeders in which 2 are incoming feeders from Sanganer & Chambal

which are incoming from and 7 outgoing feeders to two transformers in which one are of

50 MVA capacity & other is of 25 MVA capacity.

Hence total capacity of 132-kV G.S.S. is 25 + 50 = 75 MVA

These transformers are step down transformer 132-kV to 33-kV and fed three bus-

bar arrangements.

This bus bar arrangement provides option of utilization ant of two buses at a time

and leaving third one out of for maintenance or repair.

2.4Bus – Bar Arrangement in Switchyards:

There are several ways in which the switching equipments can be connected in the

electrical layout of generating station, receiving station or a switchyard in a distribution

system. Following aspect in general affect the station of the scheme.

1. Degree of flexibility of operation desired.

2. Importance of load conditions freedom from total shut down and its period desired.

3. Economic consideration, availability and cost.

4. Technical consideration.

5. Maintenance, safety of personal.

6. Simplicity

4

7. Provision of extension.

8. Protective zones.

With these basic requirements there are several bus bar arrangement are as

follows:-

1. Single bus bar arrangement.

2. Duplicate bus bar arrangement

3. Sectionalization of bus bar.

4. Ring bus.

5. One and a half breaker arrangement.

6. Mesh arrangement.

In 132-kV G.S.S. one and a half breaker arrangement scheme is adopted

between main-1 and main-2 busses.

2.5 One and A Half Breaker Scheme:

Breaker and a half scheme or one and a half breaker scheme is sometimes called the

three-switch scheme, as there are three breakers in series between the main buses. As two

circuits are connected between three breakers, hence the term breaker and the half breaker

are used for each circuit. Under normal operating conditions all breaker are closed and

both buses are energized. Opening the associated two circuit breakers trips the circuit.

This is scheme-having flexibility to attend any element in this switchyard without

losing the circuit. It occupies single bay space for two feeders oriented in diagonally

opposite directions. All the incoming feeders from generating units are in one direction

while out going feeders are oriented in opposite direction.

The breaker and half arrangement are more expensive than other scheme, however

breaker and a half scheme are superior in flexibility reliability and safety.

5

CHAPTER- 3

VARIOUS EQUIPMENT

3.1 Different Equipment of Switchyard Used In G.S.S.:

Some equipment are used in the G.S.S. for successful operation of breaker and a

half scheme between two buses, they are

1. Lightning arrester

2. Capacitor voltage transformer

3. Wave trap

4. Isolator

5. Circuit breaker

6. Current transformer

3.2 Lightning Arrester

3.2.1 Introduction:

Every instrument must be protected from the damage of lightning strokes. The three

protections in sub stations are essential:

1. Protection for transmission line from direct strokes.

2. Protection of power station or substation from direct strokes.

3. Protection of electrical apparatus against traveling waves.

Effective protection of equipment against direct strokes requires a shield to prevent

lightning from striking the electrical conductor together with adequate drainage facilities

over insulated structure.

3.2.2 Lightning Arrester:

Lightning Arrestor is a device, which protects the overhead lines & other

electrical apparatus viz., transformer from over head voltages & lighting. When the

positively charge cloud produce negative charge on the overhead line by electrostatic

induction than the negative charge is however presented just under the cloud & portion of

the line away from the cloud becomes positively charged. This charge on the line does not

flow. Figure 3.1 (below) shows the lightning arrester.

The positive charge on the far end flows to the earth through insulators, thus

leaving the negative charge on the line directly under the cloud. Now assume due to the

6

direct discharge occurring between this cloud & passing by negative charge on the line is

no more bound charge & is free to travel on the both direction in the form of waves.

Figure-3.1 Individual Lightning Arrestor

These traveling waves will be of light magnitude & have steep wave front, which

can damage the unprotected equipment connected to the line. These waves are passed to

the earth through the lighting arrestors.

In this G.S.S., the lighting arrestors used are “THYRITE ALUGARD STATION –

class lighting arrestors”, Lighting Arrestors are provided with proper Earthing. Lighting

Arrestors are installed on a concrete block which project above ground by 305 mm.

Lighting Arrestors are always with a mounting name “Discharge Counter”.

It consist of a isolator in series & connected in such a way that long isolator is in

upward & short isolator is in downward so that initially large potential up to the earth is

decreased to zero.

3.2.3 Valve Type Lighting Arrestor:

Basic Lighting Arrestor has spark gap & resistance with non-linear characteristics.

The spark gap consists of series of electrodes whose spark gap is broken. In Heerapura

G.S.S. the units consist of number of main gap, a non-linear resistor, in parallel with non-

linear resistors. A lighting surge of short duration mainly passes through parallel resistors

while a switching surge & power frequency current passes through magnetic coils.

7

3.2.4 Protection Against Lightning:

All electrical equipments are protected from the severe damage of the lightning

strokes which is studied from the view of following protection:

1. Power station & sub stations from direct strokes.

2. Overhead transmission line from direct strokes.

3. Electrical apparatus from traveling waves.

3.2.5 Protection of Transmission Line From Direct Strokes:

Effective protection requires a shield to prevent lighting from striking to electrical

conductor together with adequate drainage facility over instruction so that discharge can

drain to ground.

Although ground wire runs over lines protects against lighting & reduces induced

Voltage but inadequate to protect ant traveling waves which reaches the terminal of

electrical this causes following damage:

1. High Voltage causes flash over in internal winding.

2. High Voltage causes internal flash over between the terminals of electrical equipment.

3. Step wave front of the surge may cause internal flashover between the turns of

transformer.

3.2.6 Lightning Arrestor Meter:

This meter indicates the sum of surface leakage & internal grading current of the

arrestor. It has three different indicators to indicate different conditions:

1. Green- Arrestor is healthy.

2. Red- Defective, Remove from service.

3. Counter- Maintains counts of operation undergone by the arrestor.

3.2.7 Location of Arrestors:

It is common practice to install the lightning arrestor straight on the terminals of

transformer. The lighting arrestor may be installed at a distance less than maximum

permissible from the transformer. So as to include the other substation equipment within

the protective zone in case the lighting arrestor is installed towards line side of circuit

breaker then it needs to be ensured that the lighting arrestor has adequate thermal capacity

to discharge the switching surges particularly on extra high voltage lines when circuit

breaker are not restrict free.

8

3.2.8 Caution:

Test on high voltage by common meggar do not provide correct assessment of its

health. Never leave the earth terminal if the arrestor underground by pass the monitor

with an earth shunt engaged in any work on its terminal.

3.2.9 Arresters Voltage:

The thyrite station class arrester is designed to limit the surge voltages to safe

value of discharging a surge current to ground, and to interrupt the maximum power

frequency follow current before the first current. It is important, therefore to assure that

the system power frequency voltage from line to ground under any condition of switching

fault over voltage never exceeds the arresters rating.

3.3 Capacitor Voltage Transformer (C.V.T.):

Capacitor voltage transformer (C.V.T.) can be effectively employed as potential

sources of measuring, metering, protection, carrier communication and other vital

function of an available for system voltages of 33-kV to 132-kV.

The performance of C.V.T. is inferior to that of electromagnetic voltage

transformer. Its performance is affected by the supply frequency switching transient bus

bar etc. the C.V.T. is more economical then an electromagnetic voltage transformer when

the nominal system voltage increases above 66-kV.

The carrier equipments can be connected via the capacitor of the C.V.T. Thereby,

there is no need separate coupling capacitor.

Capacitor type V.T. is used for voltage 66-kV and above. At such voltage cost of

electromagnetic type V.T. is tensed to be too high. The capacitor connected in series acts

like potential dividers provided the current taken by the burden or load is negligible

compared with the current passing through the series connected capacitors.

However, the burden current becomes relative larger and ratio- error and phase

error is introduced. Tuning carriers compensation. The factor connected in series with the

burden is adjusted to such a value that it supply frequency it resonate with the sum of two

capacitor. This eliminates the error. The construction of capacitor type V.T. depends on

the form of capacitor voltage divider. Generally HV capacitors are enclosed in porcelain

housing. A large metal sheet box at the base encloses the tuning coil intermediate

transformer.

9

In P.T. we use the coupling capacitor but C.V.T. it is eliminated and if we use the

PT in yard. It is very expensive due to the fact that the transformation ratio is high and the

size of the PT is high than a C.V.T. of same capacity.

So that C.V.T. are used in G.S.S. as shown below in figure- 3.2.

Figure-3.2 Capacitor Voltage Transformer (C.V.T.)

3.4 Isolator and Earthing Switching:

When to carry out inspection or repair in the substation installation a

disconnecting switch is used called isolator. It’s work is to disconnect the unit or section

from all other line parts on installation in order to insure the complete safety of the staff

working. There are isolators –

1. Bus isolator.

2. Line Isolator cum earthing switch.

3. Transformer isolating switches.

Isolator (disconnecting switch) operators under no load condition it does not have

any specified current breaking capacity of current making capacity. Isolator is not even

used for breaking load current.

Circuit breakers can make and break electric circuit breaker, under normal current

or short circuit condition. Isolators are used in addition to circuit breakers and are

provided on each side of every circuit breaker to provide isolation and unable

maintenance.

While opening a circuit, the breaker is open first then isolator while closing a

circuit the isolator is closed first, then circuit breaker. The isolators are necessary on

supply side of circuit breakers in order to ensure isolation (disconnection) of the circuit 10

breaker from line parts for the purpose of maintenance. Automatic switching of isolators

is preferred. Not open it in live current flowing because high are damage its connect.

3.4.1 Working of Isolator:

Isolator for a 3-phase system is provided in such a manner that of each phase we

provided one frame of isolator. These three isolators must be operated all together. In

each frame, line connected to terminal stud. Terminal stud is connected or coupled with

contact arm. An isolator is shown below in figure- 3.3.

Figure-3.3 Isolator

One contact arm has a male contract while other has female contact. Insulation

supports contact arms. Contacts are made or broken by motor operated mechanism. When

contact is to be opened then both arms are rotated in opposite direction so that contact is

broken. Same time Earthing pole moves upwards to make contact with the female contact

situated adjoin to the terminal stud. Hence the terminal gets earthed. On this criterion

isolators can be distinguished as single earth or double earth. Through all the operation

can be carried out manually but for quick operation motor is used.

3.4.2 Rating of Air Breaker Isolator:

1. Make S&S power switchgear ltd.

2. Motor operated mechanism.

3. Rated impulse voltage :1425kV

4. Rated Amperes :2000 Amps

5. Rated mechanical term load :160 kg

6. Auxiliary voltage :220V

7.Company :ELC

11

3.4.3 Earthing Switch:

Earthing switch is connected between the line conductor and earth. Normally it is

open. When the line is disconnected, the earth switch is close so as to discharge the

voltage trap on the line. Though the line is disconnected, there is some voltage on the line

to which the capacitance between line and earth is charged. This voltage is significant in

high voltage system. Before starting the maintenance work these voltages are discharged

to earth by closing the Earthing switch.

3.5 Circuit Breaker

As in 132-kV G.S.S. a very high amount of current flows in circuits and whenever

it is necessary to open a circuit due to any reason like to work a transformer, breaker,

isolator or line circuit breaker is used.

3.5.1 Basic Concept of Using Circuit Breaker:

In any circuit carrying a large amount of current, if a contact is open then

normally a spark is produce to fact that current transverses its path through the air gap.

Arcing is harmful as it can damage precious equipments as well as it can harm any

personal nearby. So as to prevent that arcing circuit breaker are used in which different

arc quenching media are provided between contacts.

3.5.2 Different Arc Quenching Media:

1. SF6 circuit breaker

2. Oil circuit breaker

3. Air blast circuit breaker

4. Vacuum circuit breaker

In 132-kV G.S.S. only SF6 gas circuit breaker are used as for generally in G.S.S.

SF6 type breakers are very efficient.

3.5.3 SF6 Circuit Breaker:

The outstanding physical and chemical property of SF6 gas makes it an ideal

dielectric media for use in power switchgear. These properties are included: -

1. High dielectric strength

2. Unique arc quenching ability

3. Excellent thermal stability

4. Good thermal conductivity

In addition, at normal temperature SF6 is chemically inflammable, no corrosive and

non condensable at low temperature. A SF6 circuit breaker is shown below in figure- 3.4.

12

Figure-3.4 SF6 Circuit Breaker

3.5.3.1 Oil Vs SF6 Gas:

SF6 is not flammable and toxic like oil. It is easier to handle, maintain and repair

equipment fill with SF6.

In case of breakdown of oil strong surges of pressure may occur due to sudden

development of gaseous products. In case of breakdown of SF6 gas, the only pressure rise

will result from the thermal expansion of gas.

3.5.3.2 SF6 Vs Air/Gases:

SF6 has about three times, the dielectric strength of N2 in addition SF6 has the

property that disassociated molecules recombine rapidly after the sources of arcing is

removed when superior arching occurs. Excellent performance in operation confirms the

high level of reliability and safety irrespective of environmental condition.

3.5.3.3 Complete Working of SF6 Circuit Breaker:

Interrupter unit has two fixed contacts that are connected through a moving

contact. Fixed contacts are of rod shape. These contacts are known as male contacts.

Rated power closed position. Fixed contacts are joined by a moving contact is

known as female contact. This female contact is of hollow cylindrical shape. Main parts

of female contacts are blast cylinder. Contact tube and guide tube. In closed position,

female contact overlaps male contacts. Contacts tube shorts two male contacts are current

completes its path from one male contact to another through contact tube.

When it is required to open the contact then piston is forced to move vertically

downward by hydraulic or pneumatic pressure. This piston pulls operating rod downward.

Operating rod pulls blast cylinder using bell and crank mechanism. Contact tube moves

away from contact counteracting piston moves towards contact compressing the SF6

present in blast cylinder. When contact between male and female contacts is just going to

break, then counteracting piston reaches its extreme position performing. Maximum 13

compression of SF6 gas when the arc is produced SF6 at very high-pressure quenches the

arc.

In open position, blast cylinder reaches the extremes position again. Contact tube

lies entirely on mole contact.

3.5.4 Air Blast Circuit Breaker:

In this type of circuit breaker compressed air is used as an arc extinguishes

medium. In these circuit breakers arc is subjected to air blast axially or the arc. The air

blast cools the arc and sweeps away the arcing products to the atmosphere. This rapidly

increases the dielectric strength of the medium between the contacts and prevents the re-

establishing the arc. Consequently, the arc is extinguished and flow of current is

interrupted. An air circuit breaker is shown below in figure 3.5.

Figure-3.5 Air Circuit Breaker

3.5.5 Oil Circuit Breaker:

In bulk oil circuit breaker transformer oil is used which serves 2 purposes:

1. Media for extinguish arc as well as for

2. Provide insulation between current carrying parts and metallic tank.

These are used up to 132kV.

An oil circuit breaker is shown below in figure-3.6.

Figure-3.6 Oil Circuit Breaker

14

3.6 Rating of Circuit Breaker:

Specification : SF6 gas

Make : BHEL, Hyderabad

Type : 3AT3

Rated Voltage : 420 kV

Rated Impulse with stand voltage Lightening/switching = 1425/1050 kV

3.6.1 Rated Power Frequency:

Voltage : 520 kV peak

Rated Frequency : 50Hz

Rated Normal current : 200 Amp

Rated short time current : 40 K Amp

Rated Short Time Duration : 1 Sec

First Pole to clear Section : 1.3

Breaking Capacity Symmetrical : 40 K Amp

Equivalent 2900 MUA

Asymmetrical 49 K Amps

Rated making Current : 100 K Amp

Rated Operation Duty : 0.3 sec to 3 min

Rated pressure of Hydraulic : Operating Mechanism (Gauge)

250-350 bar

3.6.2 Rated Pressure of SF6 Gas:

20 dg. C (gauge) : 7.5 bar

Weight of complete breaker : 11700 kg

Weight of SF6 gas : 76.5 kg

Rated trip coil voltage : 220 V dc

Rated closed coil voltage : 220 V dc

3.7 Transformer

1. Transformer is an A.C. machine that

2. Transfers electrical energy from one electrical circuit to another

3. Does isolator without a change of frequency

4. Does isolator by the principle of electromagnetic induction and

5. Has an electric circuit that is linked by a common magnetic circuit.

15

When the transformer raises the voltage i.e. when the output voltage of a

transformer is higher than its input voltage, it is called the step-up transformer and when

it lowers the voltage it is called the step down transformer.

In our G.S.S. four step down transformers are used. There are changes the voltage

from 132-kV to 33-kV (r.m.s.) and all have the power of 25 MVA. One transformer is of

50 MVA and it also step down the voltage from 132-kV to 33-kV (r.m.s.). The voltage

transformation in a 3-phase network can be had by means of either a band of three single-

phase transformers or a single 3-phase transformer. A single unit 3-phase transformer

consists essentially of 3-phase transformer with them three cores united into a single core

assembly like single-phase transformer.

The unique arrangement of the windings and the core in a 3-phase transformer

makes it possible to save a great deal of iron by inter locking the magnetic circuits

isolator that the same iron is used by the three phase simultaneously.

Since the entire three phase assembly is reduced in size and is placed in a single

tank, additional saving results because less transformer oil is used, fewer transformer

bushings are required (All polyphone connections are made inside the tank) and the

complete transformer occupation the less floor space than a bank of three single phase

transformer.

A 3-phase transformer is cheaper (about 15%) than a bank of three single-phase

transformers. It is due to saving in cost of the iron core, of the tank and oil of the bushing

and of the aux, apparatus.

A 3-phase transformer is slightly more efficient than a bank of 3 single-phase

transformers. This is due to the fact that it has shorter magnetic path and consequently the

core volume and hence the core loss is smaller. A 3-phase transformer is shown below in

figures-3.7.

Figure-3.7 three phase transformer

16

3.7.1 Tertiary Winding:

Transformer may be built with a third winding called the tertiary, in addition to

the normal primary and secondary and such transformer are called the triple wound (3

way transformer) transformer. The tertiary winding may serve any of the following

properties or purposes.

1. To supply the sustain auxiliaries of a voltage different from that of the primary of

secondary winding.

2. To supply phase compensating device, such as condensers operated at a voltage, which

is different from both primary and secondary voltages.

3. To interconnect three supply systems operating different voltages.

4. To load large split winding generators.

5. To measure voltage of an H.V. testing transformers.

6. In star/delta-connected transformers, to allow sufficient earth fault current to flow for

operation of protective equipment, to suppress harmonic voltages, and to limit voltage,

unbalance. When the main load is unsymmetrical.

The tertiary winding is called the auxiliary winding when it is employed. For

supplying on additional small load at a different voltage On the other hand, it is called the

establishing winding when it is employed to limit the short circuit current.

Tertiary winding are normally delta-connected isolator that when faults and short

circuits occur on secondary side the considerable unbalance produced in phase voltage

may be compensated by the circulating currents flowing in a closed delta. Reactance of

the winding should be large enough to limit produced in phase voltage may be

compensated by the circulating currents flowing in a closed delta. Reactance of the

winding should be large enough to limit the circulating currents in order that there is no

overheating of the windings.

3.7.2 Auto Transformer:

In auto-transformer the primary and secondary winding are completely insulated

from each other but are magnetically linked by a common core. In the auto-transformer,

the two windings, primary and secondary are connected electrically as well as

magnetically; in fact, a part of the single continuous winding in common to primary and

secondary section of this one winding are not the same magnetic circuit.

Due to use to auto-transformer some advantage is present.

1. Only one winding is used for both winding isolator the spacing is less required.

17

2. Required of copper is less isolator the ohmic lass is reduced.

3. Less insulation is required.

4. Handling of KVA is easier than the other two winding transformer.

3.7.3 Tap Changing Transformer:

The voltage variation in power system is a normal phenomenon owing to the rapid

growth in induction and distribution network. Therefore it is very important to keep the

consumer’s terminal voltage within the limit. The transformer output voltage and isolator

the consumer’s terminal voltage may be controlled by providing tapping on the

transformer winding (either on the primary or secondary side) the principle of regulating

the secondary voltage is based on changing the number of turns on the primary or

secondary i.e. on changing the ratio of transformation. Decreases in primary turn cause

inverses in e.m.f. per turn, and in secondary output voltage secondary output voltage can

also be increased by increasing secondary turns and keeping primary turn fixed.

There is always a tapping on the H.V. side which when connected to the rated

voltage gives rated voltage on the l.v. side.

3.7.4 Specification:

25 MVA Transformer

Type : SAFLOCR< FOM-3 AMNYSCP

Making : Telk

Capacity : 25 MVA

Voltage : 400/200/33 kV

H.V. – 150/200/250

MVA I.V.–150/200/250

L.V. – 50/66.67/83.33

H.V. – 220,000

Volt on no Load I.V. – 132,000

L.V. – 33,000

H.V. – 217/289/361

Line Current I.V. – 394/525/656

(Amp) L.V. – 845/1156/1485

18

Phase : 3

Frequency : 50 Hz

Diagram No. : TK204139

H.V. – I.V. 12.76%

Impedance I.V. – L.V. 38.51%

(250 MVA Base Normal Tap) I.V. – 25.76%

Zero Sequence Impedance : H.V. - L.V. 12.63%

Vector Symbol : Y Na Odll

Mass of Core + Winding : 136000 kg.

Mass of Oil : 63100 kg.

Total Mass : 261900 kg.

Transportation Mass : 168000 kg.

Untaking Height : 1000 mm

Untaking Mass : 22000 kg

Transformer (Total) – 70120 Liters

Oil Cooling Plant – 8900 Liters

OLTC (Inside each 0.5) – 190 Liters

Conservator (Full Volt) – 7240 Liters

Type of Cooling : ON AN/ ON AF/ OF AF

Air Circulation : 2000 m3/mm

Oil Circulation : 3400 X 2 Liters/min

Guaranteed maximum

temperature rise in oil of

winding : 45O C / 50O C

50 MVA Transformer

H.V. – 189/252/315

MVA I.V. – 189/252/315

L.V. – 63/84/105

19

H.V. – 273/364/455

Line Current I.V. – 497/62/828

L.V. – 1104/1741/1880

Vector Symbol : Y Na Odll

Mass of Oil : 65150 kg

Total Mass : 261200 kg

Untaking Height : 7760 mm

Impedance Voltage H.V. – I.V. HTAP 11.39%

(315 MVA Base) H.V. – I.V. RTAP 11.82%

No. of Phase 3 I.V. – I.V. LTAP 12.65%

Frequency – 50 Hz H.V – L.V. RTAP 39.16%

I.V. – L.V. RTAP 26.66%

3.7.5 Transformer Accessories:

1) Bushing: Bushing are held on the top of the part of transformer and through it he

conductor goes of outside for connection from the transformer inner part and vice-versa.

In step down transformer the conductors goes in on H.V. side through

bushing and take out the conductor on I.V. side.

Figure-3.8 Transformer Bushing

Capacitor type bushing is used in the G.S.S. outer part of bushing is

making of porcelain. In inner part as side of bushing a number of layers of the paper core

built up around a center pipe with aluminum foils at external dielectric strength. The

20

active part is built under heat and pressure on microprocessor controlled broad-band

winding machine. Bushing is shown above in figure-3.8.

2) Buchloz Relay: Gas operated relay, commonly known as Buchloz relay is used for

protection of oil immersed transformers, reactors etc.

All type of faults occurring with oil filled transformer is accompanied by

more or less violent generation of gases, which the heat liberate, from the oil. This

phenomenon has been fully utilized to provide complete internal protection of

transformation. The falling oil level, which may eventually lead to a dangerous situation,

is also detected by this relay in time.

Construction:

This is a hydraulic device, arranged in the pipe line between the mash transformer

tank and the conservator tank, it comprises, a cast iron having housing which contains

essentially two floats upper and lower. Each float carriers a switch, the leads from which

are taken to a terminal box.

The necessary pet-cucks for gas release site testing, and a drain plug on the body

of the housing are provided. Inspection windows are filled on both side of the relay

housing to see the oil level and to ascertain the volume of gas collected on a calibrated

scale ion cubic cm.

Principle of Operation:

When the transformer is healthy, the entire relay housing remains filled with oil

and the buoyancies of the respective floats flits the mercury which to the ‘open’ position.

When a slight or incipient fault occurs in the transformer, decomposition of gas

occurs & small bubbles of gas will be generated and these, attempting to pass from the

tank to the oil conservation, will be trapped in the upper portion of the relay housing. As

the gas accumulates, the oil level in the relay depresses, causing the upper float of built

thereby closing the mercury switch. The alarm circuit, which is usually connected to this

switch, gets emerged to ring an alarm bell, with a server internal fault; the gas generation

is rapid, causing the displaced oil to surge through the relay. This oil flow impinges on

the lower float. The lower float is thus deflected closing its mercury switch and

completing the trip circuit of the circuit breaker.

If the transformer suffers a loss of oil, causing the oil level to drop below the level

of relay, the buoyancy of both the floats will vanish on offer another, felting the

21

respective floats. First the alarm and then the surge elements will operate of close their

respective circuits.

3) Silica Gel Breather:

During the load cycle of a transformer the oil in which it is immersed expands and

contracts as its temperature rises and falls. This decreases and increases the air space

above the oil in the transformer and causes the transformer to breathe. During the cooling

position of the cycle air from the atmosphere is drawn into the transformer. The air may

be moist and, as the presence of moisture in transformer oil reduces the dielectric strength

of the oil and adversely affects the insulation of the transformer immersed in it, some

means of drying the incoming air is desirable.

In the dehydrating breather the air is desired by passing it through a tube

containing crystals of silica gel a substance with a great affinity for water. The crystals

are tirted blue when dry but saturated with moisture, and no longer effective then turn

pink. The gel, however, is unchanged chemically and the crystals can be restored to their

original dryness and blue color by heating in a shallow pan at a temperature of between

150o C to 200o C for two to three hours.

Figure-3.9 Silica Gel Breather

The breather is piped to the air space above the oil in either the transformer tank

or the conservation and forms the only entry and exit for the air this space. The crystals

are contained in a metal cartridge with a window to give a clear view of the color of the

gel the metal cartridge can be withdrawn by unscrewing the using nuts/hex nuts and then

removing the lower portion of breather.

At bottom of the breather, cup containing transformer oil is covered. This oil acts

as a seal, preventing the crystals from absorbing moisture except when breathing is taking

22

place. Oil level is marked on the maintained up to the level indicated before

commissioning the transformer.

If the color of the gel is pink, it is to be reactivated by heating. A silica gel

breather is shown above in figure-3.11.

4) Conservator: Oil is collected in the conservation. It is situated at the top of the

transformer. In this tank air is prohibited between the upper oil layer and the top inside of

conservation.

5) Pressure Relief Valve:

Pressure relief valve is designed to be used on power transform. When

pressure in the tank rises above predetermined safe limit this valve apparatus and

performs following function.

Allow the pressure to drop by instantaneously opening a part of about 150

mm diameter.

Give valve operation by raising a flag.

Operates a micro switch. This switch has four terminals hence switch can

be effectively used in conform circuit.

Construction and Working:

The Pressure relief valve has an integral flange with six holes for mounting. The

valve can be mounted vertically or horizontally on the tank. The Pressure relief valve has

got a part of about 150 mm diameter. A stain seals this part less steel. The diaphragm

rests on and ‘O’ ring and is kept pressed by two heavy-duty springs, thereby keeping the

port closed. The other side of diaphragm is exposed to tank. Pressure rises due to any

reason, the same pressures acts on diaphragm from inside. When pressure rises above

predetermined safe limit the diaphragm gets lifted from it seat. This lifting is

instantaneous and allows vapors, gases or liquid to come out of the tank depending upon

the position of valve on tank. The diaphragm restores its position as soon as pressure in

the tank below set limit. The lit of the diaphragm is utilized to operate flag and micro

switch with the help of red. The flag and switch remain operated until they are reset

manually.

6) Oil Level Indicator:

It is manufactures considering transformer application. It can also be used as

content gauge on other tanks where level of liquid inside the tank is required to be

indicated continuously on a dial.

23

Construction Working:

A float is used as sensor of liquid level inside the conservator. Swing of hinged

float due to the change in liquid level is utilized to indicate level on a calibrated dial and

to operated at which for external alarm unit.

User of magnetic coupling in the indicator achieves completes sealing of liquid

inside the conservation from surrounding atmosphere.

The result in avoiding any leakage of costly oil and avoiding contamination of

insulating oil due to leakage of surrounding air in the conservator.

The glass mercury switch is nylon encapsulated to avoid breakage and spilling of

mercury.

Switch is accessible for servicing while indicator is mounted on the conservator

without any necessity of draining the oil.

One mercury switch is provided for low oil level alarm. The normally open switch

close when oil drops to 10 mm above empty levels i.e. 75 mm room bottom of

conservator heads from mercury switch are brought into a terminal box positioned at the

bottom of indicator.

7) Winding temperature indicator:

As the thermometer pocket houses the temperature-sending device of the winding

temperature indicator. The WTI records the winding hot spot temperature. The working

and construction of the thermometer are described in the attached leaflet. Three sets of

adjustable mercury contacts are provided, one to actuate the fans when the load exceeds

the ‘ON’ rating of the transformer, and the other two contacts for actuating the alarm and

trip contacts on the main control panel. The instrument is housed in the box.

8) Marshalling Box:

The marshalling box is of sheet steel weather proof construction mounted on the

side of the transformer. It is provided with hinged door and pad lock, and houses the

following instruments and terminal blocks.

1. Winding Temperature indicator

2. Oil temperature indicator

3. Terminal block for alarm and tripping contacts of Buchloz Relay

4. Terminal blocks for oil level alarm contacts of magnetic oil level gauge.

5. Cooler supply switch

6. Heater supply switch

24

7. Fuses links and terminal block etc.

A heater in the marshalling box to avoid condensation of moisture inside the box.

9) Cooling equipment:

Transformer having mixed cooling ONAF and ANOF is provided with through

values.

3.8 Current Transformer

Instruction transformers used in conjunction with ammeter, overcorrect relays, etc.

current ratio is transformer step-down current from high value to a low value. Their

current ratio is substantially constant of given range of primary current tend phase angle

error is within specified limits. The VA rating of current transformer is small as compare

with that of a power transformer.

A current transformer is provided for the winding temperature indicating

equipment. The current transformer has a rectangular core, and the primary is formed by

phase ‘V’ of the transformer LV located inside the tank. The secondary of the C.T. is

connected to heater element located in the thermometer itself. A pocket on the tank cover

houses the temperature sensing device of the temperature recorded two terminals and

brought out from the current transformer secondary circuit to the tank cover.

An instruction plate giving the diameter of connection for testing the C.T. and for

connection to the heater oil has been provide inside the terminal box.

3.8.1 Rating of Current Transformer

Make : W.S. insulation of India Limited

Rated Frequency : 50 Hz

Hide Side Volt : 420 kV

Weight of Oil : 750 kg

BIL : 630/1425 kV

Current Ratio : 2000-1000/12000-1000-500

Turn Ratio : 2000-1000/2000-1000-500

Core No. : 1 and 2, 3, 4 and 5

Output (VA) : 40 Accuracy Class P.S.O.

25

CHAPTER- 4

PROTECTION AND CONTROL

4.1 Protection and Control:

Protective will are used for protection. Protective relay are connected in the

secondary circuit of current transformer or potential transformer. The relay senses the

abnormal condition & closed the trip circuit of associated circuit breaker. The circuit

breaker open it’s contents one are between the contacts. As they separate the arc is

extinguished by suitable medium technique.

The relay distinguish between normal & abnormal condition whenever an

abnormal condition develops, the relay closed it’s contacts there by the trip circuit

breakers & circuit breakers opens & faulty point is disconnected flow the supply, various

types of relays used for power system.

4.2 The Various Type of Relay Installed at 400 KV G.S.S. are:

1. Buchloz Relay

2. Over Current Relay

3. Earth Fault Relay

4. Over Voltage Relay

5. Differential Relay

6. Bus Bar Relay

1) Buchloz Relay: Gas operated relay, commonly known as Buchloz relay is used for

protection of oil immersed transformers, reactors etc. It is as shown below in figure-4.1.

Figure-4.1 Buchloz Relay

All type of faults occurring with oil filled transformer are accompanied by

more or less violent generation of gases, which the heat liberate, from the oil. This

26

phenomenon has been fully utilized to provide complete internal protection of

transformation. The falling oil level, which may eventually lead to a dangerous situation,

is also detected by this relay in time.

Construction:

This is a hydraulic device, arranged in the pipe line between the mash transformer

tank and the conservator tank, it comprises, a cast iron having housing which contains

essentially two floats upper and lower. Each float carriers a switch, the leads from which

are taken to a terminal box.

The necessary pet-cucks for gas release site testing, and a drain plug on the body

of the housing are provided. Inspection windows are filled on both side of the relay

housing to see the oil level and to ascertain the volume of gas collected on a calibrated

scale ion cubic cm.

Principle of Operation:

When the transformer is healthy, the entire relay housing remains filled with oil

and the buoyancies of the respective floats flits the mercury which to the ‘open’ position.

When a slight or incipient fault occurs in the transformer, decomposition of gas

occurs & small bubbles of gas will be generated and these, attempting to pass from the

tank to the oil conservation, will be trapped in the upper portion of the relay housing. As

the gas accumulates, the oil level in the relay depresses, causing the upper float of built

thereby closing the mercury switch. The alarm circuit, which is usually connected to this

switch, gets emerged to ring an alarm bell, with a server internal fault; the gas generation

is rapid, causing the displaced oil to surge through the relay. This oil flow impinges on

the lower float. The lower float is thus deflected closing its mercury switch and

completing the trip circuit of the circuit breaker.

If the transformer suffers a loss of oil, causing the oil level to drop below the level

of relay, the buoyancy of both the floats will vanish on offer another, felting the

respective floats. First the alarm and then the surge elements will operate of close their

respective circuits.

2) Over Current Relay:

Over current protections in which the relay picks up when the magnitude of

current exceeds the set pick up level. Over current protection is the protection from

overload. Overloading of a machine or equipment/feeders means the machine/feeders are

27

taking more current than its rated current, with overloading there is an associated

temperature rise.

Several protection devices are used for over current protection, which ultimately

isolate the circuit. These include (1) Fuses (2) circuit breaker fitted with overload coils (3)

series connected trip coils (4) over current relays in conjunction with current transformer.

3) Earth Fault Projection:

When the fault current flows through earth return path the fault is called Earth

faults, which don’t involve earth, are called phase fault. Since earth faults are relatively

frequency, therefore relays are necessary in protective systems. It is noted that phase

relays also sense the earth fault current. Following are the method of earth fault

protection.

Residually Connected Earth Fault Relay:

In absence of earth fault

Irresidual = Ias + Ibs +Ics = 0

when earth fault occur and if residual current is above the pickup value of earth fault relay

then earth fault relay will be operated.

Combined Earth Fault & Phase Fault Protection:

The increase in current of phase cause corresponding increase in respective

secondary currents. The secondary flows through respective relay unit. Very often only-

phase relays are provided instead of three because in case of phase fault currents in any

two phases must increase. Hence two relays are enough. The earth fault relay is residually

connected.

For various fault the relay will also operated as follows:

1. Between R & Y phases R phases o/c relay will operate.

2. Between Y & B phases B phases o/c relay will operate.

3. Between R & B phases R & B phase o/c relay will operate.

4. Between R phase & earth o/c R & e/f relay will operate.

5. Between B phase & Earth o/c B & e/f will operate.

6. Between Y phase & Earth e/f relay will operate.

4) Over Voltage Protection:

There are many reasons for the Voltage in the power system mainly classified as

external & internal source. The most once are:

1. Lightning

28

2. Neutral displacement during line to ground fault.

3. Operating of switching and over current protective equipment.

4. Resource effects associated with inductance and capacitance in series.

The system can be protected against external over voltage by using a shielding method

which does not allow an arc path to form between the line conductor and ground, there by

inherent protection in the line designs. For protection against internal voltage normally

non-shielding methods are used which allow an arc path between the ground structure and

the line conductor but means are provided to quench the arc. The use of ground wire is a

shielding method where the use of spark gap and lighting arrestors are non-shielding

methods.

5) Differential Protection:

In this protection a different relay operates when the vector difference of two or

more similar electrical quantities exceeds a predetermined value. Most of the different

relays are current differential relays are current differential relay in which vector

difference between current entering the protected element and current leaving the

protected element is used for relay operation. For this purpose two current transformers

have the same ratio of transformation & their secondary are interconnected. Differential

protection is unit type protection.

6) Bus Bar Protection:

To protect bus bar of important G.S.S. and to clear Bus faults, bus bar protection

is used. This is based on the differential basis. All the secondary of current transformer on

the same bus are connected in such a manner that during normal condition the sum of

CT’s secondary current is zero. During the fault condition the current reaching to the bus

is more than the leaving the bus as some current is fed to this fault and the excess current

is fed to a relay which operates and tripping signal is given to all the breakers connected

to the bus thus isolated the faulty bus.

Synchronizing

Before coupling any incoming feeder to bus base it must be synchronized with bus

bar voltage.

The voltage can be checked by voltmeters. The function of synchornoscope is to

indicate the difference in phase and frequency and voltage of bus bar and incoming

feeder.

Energy meters are fitted on different panels to record import and export of power.

29

Frequency meters are fitted on different or each feeder of measure frequency.

There are analog of digital.

Voltmeter, ammeter and MUAR meter are provided on each panel for the

indication of voltage, line current and reactive power being supplied.

Power factor meters indication of p.f. of power max. demand indicator record the

max., this records the average power of over successive predetermined period.

4.3 Power Line Carrier Communication

This type of protection for transmission lines carrier current of frequency range

30-200 KHz in USA and 80-500 KHz in determined and transmitted and received through

the transmission lines provided with identical equipment consisting of transistor, receiver,

line tuning unit, master oscillator power amplifier etc.

Coupling Capacitor:

The carrier equipment is connected to the transmission lines through the

coupling capacitor, which is such value that it offers a low reactance to the carrier

frequency but high reactance to the power frequency. For example 2000 p.f. capacitor

offers 1.5 Mega ohms to 50 Hz and 150 ohm to 500 KHz. Thus coupling capacitor allows

equipment but doesn’t allow 50 Hz power frequency currents to enter the equipments. To

reduce the impedance further a low inductance is connected in series with the coupling

capacitor to form a resonance at carrier frequency.

30

CHAPTER- 5

OTHER PLANTS

5.1 Other Plants:

In 132-kV G.S.S. there is number of department. We will discuss them one

by one. The departments are:

1. Control Room

2. Power Line Carrier Communication Room

3. Fire Fighting Plant

4. Oil Testing Lab

5. Battery Room

6. Air Conditioning Plant

5.1.1 Control Room

The remote control of power switchgear requires the provision of suitable control

panels located at a suitable point remote from the immediate vicinity of CBS and other

equipment.

On each panel a control switch is provided for remote control operation of CBS

control switch for each isolator is provided. Indicator lamps are also provided to show

whether the isolator or CB is open or closed.

The color or signal lamps are as

Red: circuit breaker of isolator is closed position.

Green: Circuit breaker or isolator is open position.

In addition to visual indication an alarm is also providing for indication

abnormal conditions. When any tripping relay operates, its contents emerges an auxiliary

alarm relay, which on operation completes the alarm bell circuit.

Figure-5.1 Control Room

31

Synchronizing:

Before coupling any incoming feeder to bus base it must be synchronized with bus

bar voltage.

The voltage can be checked by voltmeters. The function of synchornoscope is to

indicate the difference in phase and frequency and voltage of bus bar and incoming

feeder.

Energy meters are fitted on different panels to record import and export of power.

Frequency meter are fitted on different or each feeder of measure frequency. There

are analog of digital.

Voltmeter, ammeter and MUAR meter are provided on each panel for the indication

of voltage, line current and reactive power being supplied.

Power factor meters indication of p.f. of power max. demand indicator record the

max., this records the average power of over successive predetermined period. A control

room is shown above in figure-5.1.

5.1.2 Power Line Carrier Communication

This type of protection for transmission lines carrier current of frequency range 30-

200 KHz in USA and 80-500 KHz is determined and transmitted and received through the

transmission lines provided with identical equipment consisting of transistor, receiver,

line tuning unit, master oscillator power amplifier etc.

1. Coupling Capacitor:

The carrier equipment is connected to the transmission lines through the coupling

capacitor, which is such value that it offers a low reactance to the carrier frequency but

high reactance to the power frequency. For example 2000 p.f. capacitor offers 1.5 Mega

ohms to 50 Hz and 150 ohm to 500 KHz. Thus coupling capacitor allows equipment but

doesn’t allow 50 Hz power frequency currents to enter the equipments. To reduce the

impedance further a low inductance is connected in series with the coupling capacitor to

form a resonance at carrier frequency.

2. Line Trap Unit:

The line trap unit is inserted between bus bars and connection of coupling

capacitor of the line. It is parallel tuned circuit comprising L & C. it has a low impedance

approximately 1 ohm to 50 Hz and high impedance to the frequency signals from entering

the neighboring line and the carrier currents flows only in the protected lines.

32

3. Protection and Earthing:

Once lightening and switching equipment and line cause voltages on

power lines trap unit. Non-linear resistor in series with a protective gap is connected

across the line trap unit and inductor of coupling unit. The gap is adjusted to the spark at a

set value of over voltages. Base of coupling unit is earthen by earth rod in the vicinity to

obtain a low earth resistance. Carrier panel is usually installed in the relay room and

connected to the station Earthing.

4. Electronic Equipment:

There are generally identical unit at each end consisting of:

1. Transmitting unit

2. Receiver unit

3. Relay unit

5. Drain Point:

It is a pure inductance of 100 mH, which direct the high frequency lighting

stroke to pass through the spark gap and not to reach at the generator.

6. Matching Circuit or Connecting Filter:

It consists of an inductance and a capacitance, which matches the

impedance of the transmission line to the carrier equipment’s.

7. Transmitters:

The high frequency signals arriving from the remote end are received by

the receiver and are then fed to receive unit which comprises of an attenuator, filter and

matching element which matches the impedances of the line & receiving unit.

We have got storage batteries being floated to the system so that in case of

failure of rectifier the communication is not interrupted.

Application of Carrier Current Relay:

Pilot channels such as carrier current or the power line provided

simultaneous tripping of circuit breakers at both the ends of the line one of the three

cycles. Thus high-speed fault clearing is obtained which improves the system stability.

Besides there are several other merit of carrier current relay:

These are

1. Fast, simultaneous operating of CB’s at both the ends

2. Auto reclosing, simultaneous reclosing signal is sent. Thereby simultaneous reclosing

of CB’s is obtained.

33

3. Fast clearance prevents shocks to the system.

4. Tripping due to synchronizing power surges does not occur yet during internal fault

clearing is obtained.

5. For simultaneous faults, carrier current protection provides easy description.

Other Uses:

1. Station to station communication: In power line stations, substation and receiving,

telephones are provided. These are connected to the carrier current equipment &

conversation can be carried out by means of carrier current communications.

2. Remote control of power stations equipment by carrier signals.

3. Telemetering.

5.1.3 Fire Fighting System

For only transformer: In G.S.S. costlier and heavier equipment is the transformer.

So first need is the fire fighting system for protection in transformer.

For transformer “mulsifire system” is used. Basic principle of this system is that

its block’s the incoming oxygen by cover or enclosed the entire transformer by the use of

water with pressure.

Red pips are covered the concerns of the transformer as a cube. And yellow pipes

are also in the form of cube. In water lines the nozzles are kept one entire pipe around the

pipe transformer.

On the air pipe line quarter values are present. In these values has a glass tube,

which is broken, when the temperature of surrounding are increased beyond the

predetermined limit.

“Dilues Value” is present on water pipelines. It is controlled by pneumatic values.

When the fire s present at transformer due to increase in temperature the glass tube

of quarter values is broken so the pressure is reduced due to decrease of pressure the

dilutes value also opened and water came out from pipe through the fountain and make a

water net around the transformer. Thus the fire is quenched.

If the pressure is decreased from a certain level of pressure due to a failure

of electrical supply it is stated automatically.

For the Yard:

Hydrant System: It gives only the availability of water at entire yard. Some points are

available in yard so that when fire is present manually those points are opened and water

34

throws through cloth pipes. For this system 75 HP motors are available. A DG set is also

available for option of the failure of electricity supply.

Halon System:

In this system the building is chosen as Halon zone such as control room. Load

dispatch. In the halogen zone region nozzle are provided so that the Halon are came out

from nozzles and quenched the arc for this purpose it is kept in an automatic system. For

this automatic system some detectors are present in the Halon Zones. Detectors are based

on the principle of smoke, photo electronics.

1. Detectors are connected to solenoid.

2. But the government bans Halon. So that this system is not working at this time.

Fire Alarm System:

The areas, which are not the Halon area the different type of detector, are used. By

which the indication and alarm of fire is present the control area. The panel shows the fix

area. By detecting the area fire fighting is done. Fire is quenched by different medium

such as:

1. Foam cylinder

2. CO2

3. Co + Power

5.1.4 Oil Testing Lab

In 132-kV G.S.S. the different types of test are done. Oil works as a coolant and dielectric

medium.

Appearance:

At first the insulating oil is seeing by the engineers. Some judgment is taken, such

as which test is done at first etc. by appearance far good condition of insulating oil the

color may be lemon yellow type.

Break down voltage test:

Break down is the voltage at which the oil breaks down when subjected to an AC

electric field with a continuously increasing voltage contained in a specified apparatus.

The voltage is expressed in kV. It depends upon the degree of determine of liquid by

solid, especially hydroscope particles such fiber water contents of oil. Pressure of oil,

temperature of oil and dielectric strength are as follows-

35

Specification Breakdown Voltage Water contents

Oil 186-1883 30 kV (before titration) 25 ppm

50 kV (after titration)

Oil 1883-1888 30 kV (before titration) 15 ppm

60 kV (after titration)

Density:

For testing the density hydrometer is used. Standard value of density of good oil is

0.850 to 0.890 in g/cm3 at 27 degree C.

Tan delta test:

Oil immersed cylindrical capacitor. When the oil is at proper level. The graph

between voltage and current is perfectly capacitive.

But the testing oil due to the increasing the carbon in oil leakage current is present

so that the graph between voltage and current is filled by some degree from previous

graph. The tangent of the diverting angle is called tan (delta). It is also called loss angle.

At 90O C, tan (delta) is less than 20%. Tan (delta) increases than resistivity decreases. At

90O C, resistivity is (0.1 X 1012) ohm-cm.

Specific Resistance Test:

In the apparatus the special jar is present in which the testing oil is filled.

And put it in the apparatus. At room temperature at the time the apparatus should be

discharge. When sample is put down charge it for one minute and take the reading. Now

again discharge it and increase the temperature to 90O C to 87O C again it is charged for

one minute.

Water content Test:

This test is done to check and removal of the moisture from oil. The device used

for this purpose is called coulometer which is made by Meterohen Company. This

equipment is based on Micro-processor. In this the anodic solution is Hydranal coulonat

A % Cathodic solution is Hydranal coulonat C. the anode t=has 40 ml solution and

cathodic has 5 ml solution. In this equipment there are two electrodes, first is Generating

electrode in which silica gel is filled to absorb the moisture and second one is indicating

electrode in which iodine is filled.

For testing 1 mm oil is injected in the cathode. In the cathode solution a magnetic

capsule of sterur is placed to uniform the density which is rotated by magnetic fan.

36

25 ppm moisture permissible for above 145 kV.

35 ppm moisture permissible for below 145 kV.

Gas contents test:

Gas chromoto graph is used for this test. At first take the 50 ml of testing oil. Now

maintain the vacuums in special apparatus-inject the 50 ml testing oil and starting the

starrier.

In the gas chromate graph, after same time later automatic a graph is drawn which

shows the quantity of different gas in sample. For good oil Acetylene should be

negligible. Standard results of various gas content in the oil are as shown in table 5.1

below.

Table 5.1- Standard Results of Various Gas Content In The Oil

Gas Contents Standard Result for Oil

1 Hydrogen 100-150

2 Methane 50-70

3 Ethane 30-50

4 Ethylene 100-150

5 Acetylene 20-30

6 Carbon dioxide 3000-3500

7 Carbon mono oxide 200-300

Flash Point Test:

This is the minimum temperature of the oil at which vaporized oil ignites

momentarily. Flash point of transformer oil should be around 125 a machine also

performs 140O C. In this test oil is filled in a cooper container. Oil heated by a heating

coil immersed in oil. A sensor is provided to detect the flash in oil. The temperature at

which flash occurs is noted down. This process is entirely automatic.

Inter Facial tension:

It is tension between molecules of oil and water. We measure this in oil testing

LAB. Its unit is dyne per cm.

37

5.1.6 D.C. Supply System

The batteries used are lead acid type having solution of sulfuric and distilled. At

very high voltages G.S.S. independent source of supply is necessary for signaling, remote

position control alarm circuit etc. Thus D.C. supply is most suitable for this purpose. D.C.

can be obtained from 400 V 3-phase A.C. supply via rectifier & in event of A.C. failure

from the fixed batteries, which are kept charge in normal condition by rectifier supply.

Lead acid is more commonly used at power station & substation because of commonly

high cell voltage and lower cost.

Batteries System:

Water electrolyte. In charged state, it has a specific gravity of 1.2 at temperature

of 30. In the battery room, batteries are mounted on wooden stand. The cells are installed

from stand by porcelain.

The conductor connecting the cells are greased and coated with electrolyte

resisting varnish. The room is kept continuously maintainable so that room is not

accumulated with acid vapors to avoid risk of explosion, smoking, winding etc. any arc is

prohibited in this room.

Operation:

The station batteries are normally kept floating across the float rectifier which also

supply the current and boost charge is off. The controller maintains the DC voltage, with

allowable limit i.e. 2%. The batteries are kept in healthy state at 2.1 to 2.2 Volt/Cell.

In case of emergency due to main failure station batteries supply the load when

the supply is restarted the batteries held to be recharged to achieve this. Boost charger is

switch ‘On’ by closing switch open thereby demurring the coil of contractor and batteries

are boost charge and the constant load is provided by connecting the load to coil for

continuity of supply between battery and load.

Specification:

Ampere-hour capacity is that quantity of electricity at fully discharged battery- 1.1

at temperature 20.

1. Fixed charge current – 24 A

2. First charge ampere-hour input – 2300 A

3. Normal recharge current – 45 A

4. Finishing charge current – 2 A

38

5. Recommended heating voltage of batteries – 2.16 to 2.18

Charging:

The input of charging source should have a voltage equal to the number of cells in

the batteries multiplied by 2.7 and should be capable of delivering continuously a current

in amperes equal to the normal charging rate of the battery.

Specification:

1. No. of cells – 110

2. Manufacture’s cell design – 10 SRG

3. Capacity at 10-hour rate discharge – 450 Amp/hour

4. Quantity of acid required per cell for the first cell filling – 1.18 at temperature of 20

5. Recommended specific gravity of electrolyte in fully charged condition – 1.210

+/- .005 at 25 temperatures.

6. Charging equipment is required for

a) Initial charging

b) For quick charging after batteries discharge

5.1.7 Air Conditioning Plant

This plant is held for air conditioning the control room, class rooms, etc. an ideal

air conditioning system should maintain correct temperature, humidity, air party, air

movement and noise level.

In the 132-kV G.S.S. the load is about 40 tones. So that central station air

condition system is used. The area where all times there are no requirements of the

system FCU is used. When the switch is on the system is on for that place and vice-versa.

5.2 Annunciator

In a control room, the Annunciator is must. It consists of a sounding alarm and an

indicator, which indicates the nature of fault and the zone in which the fault occurs. The

alarm keeps ringing until the Annunciator is not accepted. The Annunciator is of two

sections. The first one is minor fault in which there is a buzzer sounding and indicates

come on the panel for the nature of fault. Fault such as the batteries charger is out of

order, low oil lines in transformer cooling system of transformer is out of order etc. are

the minor faults & are just warned the Annunciator to the authority concerned.

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