Summer Training

AREPORT

ON SUMMER TRAINING

AT220 KV GSS SALAWAS ROAD

JODHPUR (RAJ.)

Submitted in partial fulfillment Of the

Requirement of the degree OF

BACHELOR OF TECHCONOLOGYIN

ELECTRICAL & ELECTRONICS ENGINEERING

SUBMITTED BY:- ANUPRIYA PANDEY

DEPT. OF ELETRICAL & ELECTRONICS ENGINEERING

RAJASTHAN INSTITUTE OF ENGINEERING & TECH.

AKNOWLEDGEMENT

I wish to express my sincere thanks to Mr. Sanwal who arranged my vocational training at 220 KV GSS, Jodhpur . I gratefully acknowledge the valuable cooperation rendered by Mr. S.D. Panwar (Executive Engineer) 220 KV. G.S.S. Jodhpur. I am also thankful to Mr. Prashant Lodha (Astt. Engineer.) for being helpful & providing me valuable instructions & study material . And Special thanks to staff & workers of 220 KV G.S.S, Jodhpur.

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PREFACE

I under went summer training at 220KV G.S.S., SALAWAS ROAD, Jodhpur. This is a part of total 30 days training program incorporated in the curriculum of RTU.As I am a student of electrical & electronics engg. So the training at G.S.S. has been particularly beneficial for me. I saw the various procedure process & equipment used in TRANSMISSION & DISTRIBUTION of electricity by equipments which were studied in the books & this has helped me in better understanding of transmission & distribution concepts of electrical power.G.S.S. is a very large concept & it is very difficult to acquire complete knowledge about it in a short span. I have tried to get acquainted with overall plant functioning & main concept involved there in.

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Contents

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INTRODUCTION

Energy is the basic necessity for the economic development of a country.

Energy exists in different forms in nature but the most important form is the electrical

energy.

The conversion of energy available in different forms in nature into electrical

energy is known as generation of electrical energy.

Various sources of energy available are:-

i) The Solar Energy

ii) The wind Energy

iii) Water

iv) Fuels

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v) Nuclear energy

More than 70 % of the Electricity in India is generated through fuels.

Electricity is generated at 6.6 KV & 11 KV levels which are further stepped up by

step up transformer to 220KV and further to 400 KV level. This Electric supply is

transmitted at 400 KV/220 KV/132 KV voltage level to interconnected Grid

Substations. From these Grid Substations Electric supply is further step down and

distributed to 33KV/11KV substations and finally this supply is made available at

consumer end at 415/220 Volt.

The whole electrical system is classified as

(a) Generation

(b) Transmission

(c) Distribution

(d) Utilization

(e) Switch Gear and Protection

Electricity is generated and is then distributed to various sub-stations, where

the voltage is reduced to 220 K.V. with the help of step down transformer. From these

sub-stations the energy is distributed to the consumer after reducing it to 33 K.V.

The 220kv G.S.S in Jodhpur is situated on Basni Phase-II, Salawas Road, and

9-10 KM away from Jodhpur Railway Station. The names of the Feeders connected to

this GSS are as under:-

1. 220 KV Pali

2. 220 KV Surpura Ist

3. 220 KV Surpura IInd

4. 220 KV Balotra

5. 132 KV Baori

6. 132 KV Banar

7. 132 KV Bilara

8. 132 KV Stainless Steel Factory

9. 132 KV Pali

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10. 132 KV CHB

11. 132 KV P.S.8

12.132 Jodhpur I

13. 132 KV Jodhpur II

14. 33 KV No. 1

15. 33 KV No. 2 (I.O.C)

16. 33 KV No. 3

17. 11 KV No. 1( Industrial Area)

18. 11 KV No. 2 (Industrial Area)

19. 11 KV No. 3 (Lift Canal )

20. 11 KV No. 5 (Industrial Area )

21. 11 KV No. 6 (Domestic Load )

22. 11 KV No. 7 (Domestic Area )

23. 11 KV No. 8 (Industrial Area )

24. 11 KV No. 9 (Industrial Area )

The various components of a substation are described below:-

Incoming Lines

Outgoing Lines

Transformers, main power transformers, auxiliary transformers.

HV & LV switch gear, circuit breaker, isolators etc.

Relay and metering panels, CTs, PTs, control panel

Shunt reactors, shunt capacitors.

Drop out fuses

Power Cables, control cables.

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Station service equipment such a lighting, Auxiliary battery supply,

transformer, Oil purification set, compressed air system, battery overhead

earth wires.

Station earthing system.

Mesh earthing system.

Galvanized steel structures

Communication equipment

SCADA system

Interphase metering system

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POWER TRANSFORMER

Transformer is a static electrical machine, which works on the principle of

electromagnetic induction. It transfers electrics power from one electric circuit to

another electric circuit with the help of magnetic path (flux) on constant frequency but

equal or different voltage and current. For this purpose two set of insulated windings

are wounded on a close terminated silicon steel core. Winding which connected to the

supply is called primary winding and that winding which connected to the load is

called secondary winding.

MAIN PARTS OF POWER TRANSFORMER

1. CORE :

It consist of laminated silicon steel in which quantity of silicon is upto 4%

Thickness of lamination is 0.35 to 0.50 mm. Normally the shape of the core is

rectangular and it has three legs.

2. WINDING :

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Winding of power transformer is an important part. It consist of super

enameled copper wires, the size of wire (diameter) depends on the capacity of

transformer.

3. TAP-CHANGER :

Tap changer is a switching device by which transformation ratio can be

changed by changing the position of tap changing switch.

TAP-CHANGING SYSTEM ON G.S.S. POWER TRANSFORMER

On Load Tap Changer (OLTC):

On load tap changer are employed to change turns ratio of transformer to

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

introduction of on load tap changing the operating efficiency of electrical system has

considerably improved.

Now a days, almost all the large power transformer are fitted with no load tap

changer. All forms of no load tap changer ckt. posses an impedance, which is

introduced to prevent short circuiting of tapping section during tap changer operation.

The impedance can be either a resist for a center tapped reactor.

4. TANK :

It is a metallic tank which is filled with insulating oil. The transformer core

and wdg. Assembly is surrounded by the oil in this tank. It protects the wdg. And core

from the external mechanical damages. Rectangular tanks are simpler in fabrication.

However for large rating power transformer, shaping of tanks becomes

necessary to confirm to transportable profile, shaping is provided by rounded corners

at the ends, truncations of low portions by wall from consideration of loading is well

wages girder and on the covers to reduce the height to minimize the tank oil, the tank

profile may closely follow the electrical clearness along the coils. As is evident

shaping gives saving in tank material and oil but increases complexity and fabrication

cost.

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Transformer tanks may be classified as:

Plain tanks

Shaped tanks

Belt shaped tanks

Corrugated tanks

Stub end type tanks

The transformer tanks are used in GSS. Power transformer is rectangular box (plain

tanks) type in shape.

5. TERMINAL BUSHING:

It is used to isolate the leads, which is coming from the transformer.

6. COOLING SYSTEM:

In power transformer, the oil serves a dual purpose, as an insulating medium

as well as a cooling medium. The heat generated in the transformer is removed by the

transformer oil surrounding the source and is transmitted either to atmospheric air or

water.

This transform of heat is essential to control the temperature with in

permissible limits for the class of insulation, there by ensuring longer life due to less

thermal degradation.

THE DETAILS OF THE VARIOUS TRANSFORMERS USED IN THIS G.S.S.

IS LISTED AS UNDER-

1.)

MAKE-Crompton Greaves ltd.

CAPACITY-50MVA/70MVA/100MVA

VOLTAGE RATIO-220/132

% IMPEDENCE-10.11% at tap 9

NO. OF TAPS- 1-21

QTY. OF OIL-30600Ltr/34.65 tonnes

BUSHING TYPE- Hermetically sealed

TYPE OF COOLING- ONAN/ONFF/OFAR

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2.) MAKE-NGEF,Banglore

CAPACITY-50MVA/70MVA/100MVA

VOLTAGE RATIO-220/132

% IMPEDENCE-9.58% at tap 9

NO. OF TAPS- 1-21

QTY. OF OIL-43466Ltr/34.65 tonnes

BUSHING TYPE- Hermetically sealed

TYPE OF COOLING- ONAN/ONAF/OFAF

3.) MAKE-Telk

CAPACITY-50MVA/70MVA/100MVA

VOLTAGE RATIO-220/132

% IMPEDENCE-12.71% at max voltage

NO. OF TAPS- 1-21

QTY. OF OIL-40850Ltr/36.7 tonnes

BUSHING TYPE- Hermetically sealed

TYPE OF COOLING- ONAN/ONAF/OFAF

4.) .) MAKE- EMCO

CAPACITY-20MVA/25MVA

VOLTAGE RATIO-132/33KV

% IMPEDENCE-10.012%

NO. OF TAPS- 1-17

TYPE OF COOLING- ONAF

TYPE OF COOLING USED IN G.S.S POWER TRANSFORMER

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a) ONAN type cooling:

The generated heat can be dissipated in many ways. In case of smaller rating

of transformer, its tank may be able to dissipate the heat directly to the atmospheric

air whilst bigger rating may require additional dissipating surface in the form of

tubes / radiators connected to tank.

In these cases, the heat dissipation is from transformer oil to atmospheric air

by natural means. This form of cooling is known as ONAN (Oil Natural, Air Natural)

type of cooling.

b) ONAF type cooling:

For further augmenting the rate of dissipating of heat, other means such as

fans blowing air as the cooling surface are employed. The forced air tanks blows

away the heat at a faster rate, there by giving better cooling rate than natural air. This

type of cooling is called ONAF (Oil Natural Air Forced) type of cooling.

In this cooling arrangement, additional rating under ONAN condition viz. either after

shutting off fans, is available which is of the order of 70-75%

c) OFAF type cooling

This method is used for transformers above 60 MVA. In this method the oil is

cooled in external heat exchanger using air blast produced by fans.

COOLING ARRANGEMENTS:

Depending upon the type of cooling and rating of the transformer, the cooling

equipments can be arranged in various ways.

ARRANGEMENTS WITH RADIATOR:

Radiator are commonly use for ONAN type of cooling. Radiators consist of

elements joined to top to bottom headers. Elements in this are made by welding two

previously rolled and pressed thin steel sheets to form a number of channels of flutes

through which oil flows. These radiators can be both mounted directly on the

transformer tank or in the form of a bank and connected to the tank through the pipes.

The surface are available for dissipation of heat is multiplied manifolds by using

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various elements in parallel. As oil passes downwards either due to natural circulation

or force of a pump in the cooling circuit, heat is carried away by the surrounding

atmosphere air.

7. TEMPERATURE METERS:There are two temperature indicating meter in

power transformer which indicate the oil temperature and winding

temperature. Temperature measured is in ◦C.

8. CONSERVATOR AND AIR CELL:

As the temperature of oil increases or decreases during operation, there is a

corresponding rise or fall in volume. To account for this an expansion vessel

(conservator) is connected to the transformer tank.

The conservator has got a capacity between the minimum and maximum oil

level equal to 7.5 and of the oil in transformer.

The atmoseal type conservator, it is filled with oil to level appropriate to the

filling temperature and in the remaining portion is air cell, which is connected to

atmosphere through a breather. As the breathing is through air cell, no moisture

comes in contact with oil, this protect the oil from deterioration or contamination.

A. An efficient barrier between oil and air.

B. A protection against water vapour.

C. The suppression of any gas bubbles formation in the Oil.

Air cell is made from coated fabric with external coating resistance to transformer oil

and inner coating to ozone and weather.

9. BUCHHOLZ RELAY :

The transformer is fitted with double float buchholz relay. It is fitted in the

feed pipe from conservator to tank. Any internal fault in transformer is detected by

buchholz relay; the gas liberated in the transformer is diverted by buchholz relay,

without being trapped anywhere.

10. DEHYDRATING BREATHER:

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The conservator is connected to outside through breather filled with

dehydrating material like silica gel crystals impregnated with cobalt chloride to make

sure that the air in conservator is dry. The material is blue when dry and a whitish

pink when damp.

11. PRESSURE RELIEF VALVE:

In case of severe fault in the transformer, the internal pressure may built up to

a very light level which may result in an explosion of tank. To avoid such a

contingency a pressure relief valve is fitted on the transformer. It is spring loaded and

has contacts for tripping the transformer.

12. OIL TEMPERATURE INDICATOR:Oil temperature indicator operates on

the principle of liquid expansion. The OTI provided with a maximum pointer and two

mercury switches are adjustable to make contact between 50◦ and120◦ with the fixed

differential of 10◦.

13. WINDING TEMPERATURE INDICATOR :

The indicator is fitted with four mercury switches 1st is used for alarm, 2nd is

the for trip, 3rd is for fans on and 4th for pumps control. All switches are adjustable.

14. EARTHING:

Connecting leads from core and end frame are being terminated at the top of

the cover by connecting them to tank cover, and end frame being earthed. For tank

earthing two number studs have been providing on tank.

15. TERMINAL BUSHINGS:

It is used to isolate the leads that are coming from transformer. The size of the

bushing is justified according to the operating voltage of particular winding. The

active part of the bushing consists if an Oil Impregnated Paper (O.I.P.) condenser core

manufactured from superior grade craft paper wound on aluminum tube. This bushing

is voltage graded by suitably interposed aluminum foils forming condenser layers.

16. INSULATING OIL: The insulating oil has three functions;

• Provides additional insulation

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• Protects the paper from dirt and moisture

• Carries away the heat generated in the core and coils.

The insulating oil should have the following properties:

(i) High Dielectric Strength.

(ii) Free from inorganic acid, alkali and corrosive sulphur to prevent injury to the

conductor or insulation.

(iii) Low viscosity to provide good heat transfer.

(iv) Free from sludge under normal operating conditions.

(v) Good resistance to emulsion so that the oil may throw away any moisture that

enters the apparatus.

RATING OF 33/0.4 KV STATION TRANSFORMER (UTTAM BHARAT ELECTRICALS PVT. LTD.)

Tap range in % 8x1.25 to 12 x 1.25

N 20 steps

Step voltage/phase 1588

Rated Current 330 Amp.(max)

Diverter Switch 3.3 OHMS. P.a.

KVA 500

Volts at no load HV 33000

LV 400

Ampere HV 8.75

LV 721.6

Phases HV/LV 3/3

Type of cooling ONAN

Frequency 50 Hz

Imp. Voltage 4.5%Core and wdg. 1050 kg.

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Oil’s wt. 400 kgTotal wt. 2040 kg.Qty. of oil 455 Lt.Maxc. Temp. Rise in oil 40 C

11/0.4 KV STATION TRANSFORMERS(TECHNICAL ASSOCIATES (P) LTD)

KVA 500

Volts At no load HV 11000 V

LV 433 VAmpere HV 2624

LV 666.66Phases HV/LV 3/3

Year of Mfg. 1986

Type of cooling ONFrequency 50 Hz A.C.

Vector Symbol DY-11Oil (Ltrs.) 400Weight of oil 340 kg.Wt. of oil & Wdg. 810 kg. Total Wt. 1585 kg.No load losses 840Load losses 5600 W.

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INSTRUMENT TRANSFORMER INSTRUMENT TRANSFORMER

The lines in a station operated at high voltage and carry current of a thousand

of amperes. The measuring instrument and protective device are designed for low

voltage generally 110V and current about 5A. Therefore, they will not work

satisfactorily if mounted directly on the power lines. This difficulty is overcome by

installing transformer 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.

CURRENT TRANSFORMER (C.T.)

Current transformer is an instrument and is used for protection and metering of

high values of current. Current transformers are used for reducing a.c. current from

higher value to lower value for measurement/protection/control.

There are two classes of instrument transformers:

A. Measuring current transformers

B. Protective transformers

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Measuring current transformers are used with ammeters, wattmeter, KVA

meters and KWH meters for reducing line current to 1 amp or 5 amps.

Protective current transformers are used over current protection, earth fault

protection, differential protection and impedance protection etc.

Definition of the different terms related with current transformer:

1. RATED PRIMARY CURRENT

The values of primary current on which the primary performance of the

current transformer is specified.

2. RATED SHORT TIME CURRENT

It is defined as R.M.S. value of a.c. component which the CT can carry

without damage.

3. RATED SECONDARY CURRENT

The value of secondary current marked on the rating plate.

4. RATED EXCITING CURRENT The R.M.S. value of current taken by the

secondary winding of a CT when sinusoidal voltage of rated frequency is applied to

secondary with primary winding open.

5. RATED BURDEN

The burden assigned by the manufacture at which the CT performs with

specified accuracy.

6. CURRENT ERROR & RATIO ERROR

The percentage error in the magnitude of secondary current is defined in terms

of current error.

BURDEN ON C.T.

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Rated burden of CT’s and VT’s refer to the maximum load in volt amperes

which may be applied across the secondary terminal, without the ratio and phase

angle errors exceeding the permissible limits. The burden depends upon the number

of relays and instruments connected and their individuals burden typical values.

VARIOUS TYPE OF CONSTRUCTION OF CT’S

A CT has following essential parts:

Magnetic core made up of continuous strip of nickel iron alloy of cooled

rolled grain oriented material.

Insulation over the core by tape.

Secondary winding having several turns would on the insulated core.

Bar primary passing through the window of the core and terminals.

Support porcelain or epoxy insulator.

Synthetic resin or oil insulation.

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VOLTAGE TRANSFORMERS (V.T.)

These are also instrument transformers and used for measurement and

protection. According they are either single phase or three phase. Voltage

transformers are necessary for voltage, directional and distance protection. The

primary of VT is connected directly to power circuit between phase and ground. The

volt-ampere rating of voltage transformer is a few VA to several hundred KVA.

TYPE OF CONSTRUCTION OF VT’S

Electromagnetic potential transformer in which primary and secondary are

wound on magnetic core like a usual transformer.

Capacitor potential transformer, in which the primary voltage is applied to a

capacitor group. The voltage across one capacitor is taken to auxiliary voltage

transformer. The secondary of auxiliary transformers is taken for protection or

measurement.

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CAPACITOR VOLTAGE TRANSFORMER (C.V.T.)

CVT are used for line voltmeters, synchroscope, protective relays, tariff meter etc.

The performance of CVT is affected by the supply frequency, switching transients,

magnitude of connected burden etc.

The CVT is more economical then an electromagnetic voltage transformer

when the nominal system voltage increases above 66KV. The carrier current

equipment can be connected via the capacitor voltage transformers. Thereby there is

no need of separate coupling capacitor. The CVT are used for voltage

above 66KV and above. The capacitor connected in series with the CVT acts like a

potential divider.

CIRCUIT BREAKER

A circuit breaker is a piece of equipment which can break a circuit

automatically under fault condition and make a circuit manually or by remote control

under fault condition.

OPERATING PRINCIPLE

A circuit breaker essentially consists of moving contacts, called electrodes.

Under normal operating conditions, these contacts remains closed and will not open

automatically until and unless the system close and will not open automatically until

and unless the system will becomes faulty. The contacts can be opened manually or

by remote control whenever desired. When a fault occurs on any part of the system,

the trip coils the breaker gets energized and the moving contacts are pulled apart by

some mechanism, thus opening the circuit. When the contacts of the circuit breaker

are separated under fault condition, an is struck between them. The current is able to

continue until the discharge ceases. The production of arc not only delays the current

interaction process but it also generates enormous heat, which may cause damage to

the system or breaker itself. Therefore the main problem in a circuit breaker is to

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extinguish the arc with in the shortest time so that the heat generated by it may not

reach a dangerous value.

SF6 (SULPHUR HEXAFLUORIDE) CIRCUIT BREAKER

In such breaker, SF6 gas is used as the arc quenching medium. The SF6 is an

electronegative gas and has a strongly tendency to absorb free electrons. The contacts

of the breaker are opened in a high pressure flow of SF6 gas and an arc struck

between them. The conducting free electrons in the arc are rapidly captured by the gas

to form relatively immobile negative ions. This loss of conducting electrons in the arc

quickly builds up enough insulation strength to extinguish the arc.

1. CONSTRUCTION

It consists of fixed and moving contacts enclosed in a chamber (called arc

interrupted chamber) containing SF6 gas. This chamber is connected to SF6 gas

reservoir. When the contacts of the circuit breaker are opened the valve mechanism

permits a high pressure SF6 gas from the reservoir to flow towards the arc

interruption chamber. The fixed contacts are the hollow cylinder current carrying

contacts fitted with an arc horn. The moving contacts are also a hollow cylinder with

rectangular holes in the sides to permit the SF6 gas to let out. The tip of the fixed

contacts, moving contacts and arcing horn are coated with copper-tungsten arc

resistance material. Since SF6 gas is costly, it is reconditioned and reclaimed by

suitable auxiliary system after each operation of the breaker.

2. WORKING

In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a

pressure of about 2.8 kg/cm2. When the breaker operates, the moving contacts is

pulled apart and an arc is struck between the contacts. The movement of the moving

contact is synchronized with the opening of a valve which permits SF6 gas at 14

kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure

flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile

negative ions which are ineffective as charge carries. The result is that the medium

between the contacts quickly builds up high dielectric strength and causes the

extinction of the arc. After the breaker operation the valve is closed by the action of a

set of spring.

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RATINGS AND SPECIFICAT1ONS OF DIFFERENT CIRCUIT BREAKERS

MOUNTED IN GSS ARE GIVEN AS UNDER-

• breaker serial No. : 403784

• year of manufacture : 2002-03

• Type : 3AT-2

• Rated Voltage : 420 KV

• Rated power frequency : 1050/1425 KVP

withstand voltage

• Rated power frequency : 520/610KV

with stand and voltage

• Rated frequency : 50 Hz

• Rated Normal current : 3150 A

• Rated short time current : 40 kA

• Rated short circuit duration : 1 sec

• First pole to clear factor : 1.3

• Symmetrical : 40 kV

• breaking capacity equivalent : 29000 MVA

• Asymmetrical : 52.5 kV

• rated making current : 100 kA

• Rated pressure of SF6 gas at 20° C gauge : 7.5

• weight of compete breaker : 9280

• weight of SF6 Gas : 7.5 kg

• Rated tri coil voltage : 220±l0VDC

TOTAL NO. OF BREAKERS IN-

220KV SIDE- 9 Nos.

132KV SIDE-16 Nos.

33KV SIDE-10 Nos.

11KV SIDE-12Nos.

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PROTECTION SYSTEM

In order to generate electric power and transmit to customers, millions of

rupees must be spent on power system equipment. This equipment is designed to

work under specified normal conditions.

However a fault may occur causing the system to collapse. This fault occurs

because of

Over voltage due to switching.

Over voltage due to direct and indirect lightning strokes.

Bridging of conductors by birds.

Breakdown of insulation due to decrease of its dielectric strength.

Mechanical damage to equipment.

These short circuits may cause heavy damage to equipment and would also

cause intolerable interruption of service to customers.

Relays are the devices that detect abnormal conditions in electrical circuits by

constantly measuring electrical quantities, which are different under normal and fault

conditions. The basic electrical quantities, which may change under fault conditions,

are voltage, current, phase angle and frequency. Having detected the fault the relay

operates to competent the trip circuit which results in opening of the circuits breaker

and therefore in the disconnection of the faulty circuit.

BASIC REQUIREMENTS OF PROTECTIVE RELAYING

A well designed and protective relaying should have:

1) Speed

2) Selectivity

3) Sensitivity

4) Reliability

5) Simplicity

6) Economy

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TYPES OF PROTECTION

There are two types of protection known as primary and back up.

The Primary Protection is the first line of defense and primary relays clear faults in

the protected system as fast as possible. The reliability, not only of the protected

scheme but also of the associated C.T.’s, P.T.’s and the C.B.s cannot be guaranteed.

Therefore some sort of Back Up Protection must be provided. The back up relay

operates if the primary relay fails and covers not only the local section but the next

one also and have a time delay long enough before the primary relays to operate.

PROTECTIVE RELAYS are classified depending upon their construction and

principle of operation such as :

Ordinary electromagnetic relays consisting of moving plunger, moving iron,

attracted armature hinged and balance Static relays employing thermiontic

valves, transistors or magnetic

amplifiers to obtain the operating characteristics.

Electro-dynamic relays operate on the same principle as moving coil instrument.

The various types of Relays installed at 400 KV GSS are: -

(a) Over current relay

(b) Distance relay

(c) Differential relay

(d) Earth fault relay

(a) OVER CURRENT RELAY AND EARTH FAULT RELAY

Directional type over current relay works on the induction

principles and initiates corrective measures when current in the circuit exceeds the

predetermined value. The actuating source is a current in the circuit supplied to the

relay liom a current transformer. These relays are used on a.c. circuits only and can

operate for fault flow in either direction. But these relay are unsuitable for use as a

directional protective relay under short circuit conditions. When a short circuit occurs,

the system value falls to a low value and there may be insufficient torque developed

in the relay to cause its operation, this difficulty is over come in the directional over

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current relay, which is designed to be almost independent of system voltage and

power factor.

Operation:

Under normal operating conditions, powers flows in the normal direction in

the circuit protected by the relay. Therefore, directional power relay (upper element)

does not operate, thereby keeping the over current element (lower clement)

unenergised. However when a short circuit occurs, there is a tendency for the current

or power to flow in the reverse direction. Should this happen, the disc of the upper

element rotates to bridge the fixed contact I and 2. This completes the circuit for over

current element. The disc ol this element rotates and the moving contact attached to it

closes the trip circ u i t

This operates the circuit breaker which isolates final tripping of the current by

them is not made till the following conditions are satisfied:

(a) Current flows in a direction such as to operate the directional element

(b) Current in the reverse direction exceeds the pre-set value.

(c) Excessive current persists for a period corresponding to the time setting over

current element.

(b) DISTANCE RELAY: Distance protection is the name given to the protection,

whose action depends upon the distance of the feeding point to the fault. The time of

operation of such a protection is a function of the ratio of voltage and current, i.e.

impedance. This impedance between the relay and the fault is dependent upon the

electrical distance between them.

An impedance relay has an operating force proportional to the fault current

and restraining force proportional to the line voltage at the relay. As soon as the ratio

of this voltage to the fault current change i.e. falls below a certain value, the relay

operates. This value is dependent upon the distance of the fault, which is

predetermined. Hence for this reason the relay is discriminative and it does not

operate for any fault occurring outside this distance.

As it is very important to localize the fault, a relay of the above type is given a

controlled time lag, so that the relay nearest to the fault operates first. This time lag is

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made proportional to the distance of the fault by so designing the relay that it has time

lag characteristics, which is dependent upon the line voltage at the relay directly.

Again, the time lag characteristic is inversely proportional to the fault current that is

passing through the relay.

In case of a fault, there is a steady fall of voltage along the line from the .feeding point

to the fault. This voltage gradient can be utilized for grading of the time lags of the

relays, which controls a number of switches in a feeder.. These relays automatically

adjust their time of operation depending upon their distance from fault.

(c) DIFFERENTIAL RELAYS: A differential relay is one that operates when

the difference of two or more electrical quantities exceeds a predetermined value.

Almost any type of relay connected, in a cei-tain way, can be made to operate as

differential relay. There are two fundamental system of differential protection viz.

(a) Current Balance Protection (current differential relay)

(b) Voltage Balance Protection (voltage differential relay)

A current Balance Differential Relay is one that compares the current entering

a section of the system with the current leaving the- section. Under normal operating

conditions, the two currents are equal but as soon as a fault occurs, this condition no

longer applies. The difference between the incoming and outgoing currents in

arranged to flow through the operating coil of the relay. If this differential current is

equal to or greater than the pickup value, the relay will operate and open the circuit

breaker to isolate the faulty section. In Voltage Differential Relay, two similar current

transformers are connected at either end of the element to be protected by means of

pilot wires. The secondary of circuit breakers are connected in series with a relay in

such a way that under normal conditions their induced emfs are in opposition. Under

healthy condition, equal current flows in both primary windings Therefore, the

secondary voltages of the two transformers are balanced against each other and no

current will through the relay operating coil. When a fault occurs in the protected

zone the current in the two primaries will differ from one another and the secondary

voltage will no longer be in balance. This voltage difference will cause a current to

flow through the operating coil of the relay, which closes the trip circuit.

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AUXILLARY EQUIPMENT

LIGHTING ARRESTERS

Lighting arrestors is used to protect the sub-station and transmission lines

arrester is earthed. Gap is, adjusted in such a way that 50% over voltage it operators.

We will use valve type lighting arrestor this types of lighting arrestor is also

called non liner diverter. In this spark-gap, stuck and resistance discs are used. These

are isolated by mica foils.

The resistance of resistance discs with the increases of current value in light

arrestor spark gap and resister remains in series and these are protected from moisture

and atmospheric and these are protected from moisture and atmospheric changes by

keeping them in porcelain pot.

When there is less change in line voltage than there is not flashover in gap but

when there is over voltage and rapid changes in voltage then even grounding of

voltage will not possible the value of flash over voltage depends on surge

current.Operation will start when voltage will increase 10% of rated voltage.

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CAPACITOR BANK

The power factor can be improved by connecting capacitors in parallel with

the equipment operating at lagging power factor; the capacitor draws a leading current

and partly or completely neutralized the lagging reactive component of load current.

LOAD INTERRUPTER SWITCHES

The switches are designed and used to close and open high voltage circuits

under normal working conditions. The are extinguishing device of the load interrupter

is made in the form of a split a molded plastic chute fitted with organic glans, inserts

this chute surrounds the moving knife of the arc extinguishing system. The stationery

acting contact is located in the lower part of the chute.

INSULATORS

The insulators serve two purposes. They support the conductors and confine

the current to the conductors. The most commonly used material for the manufacture

of insulator is porcelain. There are several types of insulators and there use in the

substation will depend upon the service requirement. For example, post insulator is

used for the bus bars. A post insulator consists of a porcelain body, cost iron cap and

flanged cost iron base. The hole in the cap is threaded so that bus bars are directly

bolted to the cap.

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SHUNT REACTOR

Shunt reactor are provided at sending end and receiving end of long EHV &

UHV transmission line. They are, switched in when the line is to be charged or when

the line is on low load.When the line is on no-load or low load, shunt capacitance

predominate and received and voltage is higher then the sending end voltage.

The receiving end voltage of 400KV, 100KM long line may be as high as 800KV.

The shunt capacitance of such line neutralized by switching in the shunt reactor,

during high loads, the series inductive reactance of the line produces IX drop and the

receiving end voltage drops, the shunt reactors are switched off.

Shunt reactors may be connected to the low voltage territory winding of transformer

via a suitable circuit breaker, EHV shunt reactor may be connected to transmission

line without any EHV circuit breaker. Usually oil immersed magnetically shielded

reactor with gapped core are used.

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ISOLATORS

While carrying out inspection or repair in a substation installation, it is

essential to disconnect reliably the unit or the section, on which the work is to be

done, from all other live parts of the installation in order to ensure complete safety of

the working staff. To guard against mistakes it is desirable that an apparatus, which

makes a visible break in the circuit, should do this apparatus is the isolating switch. It

may be defined as a device used to pen (or close) a circuit either when negligible

current is interrupted or whn no significant change in the voltage across the terminal

e.g. each pole of the isolator will result from the operation P.A.

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ISOLATORS ARE CLASSIFIED AS-

1. OFF LOAD ISOLATOR

It is an isolator which is operated when the isolator is already disconnected

from all sources of supply or when the isolator is already disconnected from the

supply and current may be due to capacitance current of bushings bus bar connected

and very short length of cable.

2. ON LOAD ISOLATOR

It is isolator, which is operated in a circuit where there is a parallel path of

low impedance so that no significant change in the voltage across the terminals of

each pole occurs when it is operated.

When the switch is opened, the working contacts between which are is drawn

separate. Acted upon by the high temp of arc the wall of the organic material inserts,

generated gases (mainly hydrogen), which create a loadinterputer switches.

CONTROL CABLES

The control cable and conduit system is required for affecting automatic

controls. The control system generally operates at 110V or 220V and the cables

employed for this purpose are multi core cable having 10 or 37 or 61 conductors

according to requirement, for laying these cables generally ducts is run from control

room basement to centrally located juriction box from where the conduits are run the

required points.

METERING AND INDICATING INSTRUMENTS

There are several metering and indicating instruments e.g. ammeters,

voltmeters, energy meter etc. installed in a sub-station to maintain watch over the

circuit satisfactory operation.

BATTERY CHARGER

D.C. supply (110V) is the heart of the GSS. It energises all

protection .Generally for the normally working the float charger unit supplied the

permanent load at 110V (+ - 1%) and also supplies the trickle charging current to the

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battery on float condition. In battery Room 5 battery Set are installed and

interconnected providing D.C. Current for controlling panels.

FLOAT CHARGING

It is intended to supply float charging current of the battery and

simultaneously the permanent load of station.

BOOST CHARGING

Boost charging is used to charge the batteries after power resumption.

POWER LINE CARRIER COMMUNICATION (PLCC)

1. INTRODUCTION

For exchange of dates and transfer of message between grid substation

voices communication is necessary. For this purpose high frequency carrier current

(30 to 50 kcls) is transmitted on same transmission line on high power is also

transmitted, hence such communication is a “power line carrier communication” or

shortly “PLCC. High frequency carrying current audio singles is generated,

transmitted and received with the help of identical carrier current equipment provided

on each end. Carrier current equipment comprises of following:

(1) Coupling capacitor

(2) Wave trap unit

(3) Transmitter/receiver

1. Coupling Capacitor:

It acts like a filter, it blocks power frequency (50 Hz) while offer low

reactance to carrier frequencies (30-500KC! s) as allow them to pats through because:

For examples A 2000 Pf capacitor offers 1.5 mega ohms to 50 Hz while it just

offer 150 ohms to 500 KHZ. Thus coupling capacitor allows carrier frequency signal

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to enter the carrier equipment but does not allow 50 Hz power frequency current to

enter the carrier equipments.

2. Wave trap unit:

It is parallel turned circuit comprising of I & C. It has low impedance (less

then 0.1 ohms) to 50 Hz and high impedance to carrier frequencies. Thus power

frequencies get passed through wave trap & carrier frequencies passes through

coupling capacitor & reaches carrier current. Wave traps are mounted in outdoor

switchyard.

3. Transmitter & Receiver Unit:

Carrier current unit shown in Figure acts like both transmitter receiver carrier

frequencies are generated in master oscillator can be tuned to a particular frequency

selected fro the application output voltage of oscillator is held constant by voltage

stabilizers. Output of oscillator is fed to amplifier, which increases the strength of

signal to be transmitted to over come the transmission losses.

Line losses very with length of line, frequency, weather conditions size & type of

line; losses in overhead line arc affected by weather. In fair weather the attenuation

(weakening of signal) is about 0.1 dB 1 Km at 80 KI IZ rising to 0.02 dB 1 Km. at

380 KHZ.

Receiving unit comprises of an alternator which reduces signal to

safer value. Band pass filter restricts the acceptance of uncounted signal & matching

transformer or matching element matches the impedance of line &

receiving unit.

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EARTHING

Connecting of an electrical equipment or apparatus to the earth with help of

connecting wire of negligible resistance is known as “earting” or “grounding”.

The provision of an earth electrode for an electrical system is necessitated by

following reasons:

1) All the parts of electrical equipment like casing of machine, switches and circuit

breaker, lead sheathing and armoring of cables, tanks of transformers etc., which have

to be at earth potential, must be connected to an earth electrode. The purpose of this is

to protect the various parts of the installation, as well as the person working against

damage in case the insulation of system fails at any point.

By connecting these parts to an earthed electrode, a continuous low resistance path is

available for leakage currents to flow to earth. This current operates the protective

devices and thus the faulty circuit is halted in case a fault occurs.

2) The electrode ensures that in the event of even voltage on the system due to

lighting discharge or other system faults, those parts equipment which are normally

“dead” as for as voltage are concerned do not attain dangerously high potential.

3) In a three phase circuit the neutral of the system is earthed in order to stabilize the

potential of the circuit with respect to earth. In electrical installations the following

component must be earthed:

(i) The frames, tanks and enclosures of electric machines, transformer, and apparatus,

lighting fitting and other items of equipment.

(ii) The operating mechanism of the switchgear.

(iii) The framework of the switchboards control boards individual panel hoards,

cubicles.

(iv) The structural steel work of substations, metal cable jointing boxes, the metal

sheaths of the cables the rigid metal conduit runs and similar metal work.

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There are two methods of earthing:

1 Pipe earthing.

2 Plate earthing

EARTHING ARRANGEMENT AT 220 KV GSS:

In an grid sub-station of any magnitude the various non current carrying

equipment to be earthed namely substation structures, shielding wires or masts,

equipments tanks and treadles etc are spread over large area and therefore it becomes

necessary to lay a grounding bus connect the various items to be earthed to be ground

bus through suitable connection to heave duplicate earthing is broken the sub-station

may remains safe under all condition. It generally, therefore, becomes desirable to

form a ring of the earthing hut, which can then be connected to the earthing

electrodes. In large sub station the earthing bus itself it said to a depth of 600 to 650

mm. saves as a grounding mat and no separate earthing mat or electrodes may be

required although use of some electrodes for making use of good earth conductivity at

depth unaffected by other condition is considered advisable particular))’ near lighting

arresters and transformers neutral earthing points where lighting surges are required to

be discharged in to earth.

Another way of looking into the sub station earthing problem is that a very

low earthing resistance value is required in a large area occupied by the substation and

obviously such can only be obtained by using a number of rod electrodes and joining

them in parallel. Further to be most effective the electrodes must be placed at distance

such that their areas of influence are not over lapped. If this is done the connections

between the various electrodes if laid at suitable depths may between 400 to 600 mm

will also take part in ground current dissipation to earth like horizontal counterpoises

and equipment to he earthed can also connected to them.

In substation the earthing system invariably takes the shape of grounding mat

with necessary or additional rounding rods accepts in the case of’ very small sub

stations.

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Common earth electrodes should he use for both system earths and equipment

earths. Here also it is recommended to have common earth bus for high voltage and

low voltage system. Where there are manual operating handle to the system earth

electrode. To remove any voltage gradient that may exist between the operating levers

and the ground level and shall be connected to the system earth electrodes.

PLATE EARTHING

In plate earthing plate either of copper of dimension 60cm x 6cm x 3.15mm or

of galvanized iron of dimension 60cm x 60cm x 6.30 is burled into the ground with its

face vertical at a depth of not less that 3 meters from wound levels. The earth plate is

embedded in alternate layers of coke and salt for a minimum thickness of 15 cm. The

earth wire GI wire for GI plate earthing and coppers wire for copper plate earthing is

securely bolted to an earth place with the help of a bolt nut and washer made of

material of that earth plate (made of copper in case ot copper plate earthing and of

galvanized iron in case of GI plate earthing).

A small masonry brick wall enclosure with a cast iron cover or top an

RCC pipe round the earth plate is provided to facilitate its identification and for

carrying out periodical inspection and tests.

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