CHAPTER- 1

INTRODUCTION

1.1 Synopsis

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

exists in different form in nature but the most important for is electrical energy. The

conversion of energy available in different forms of nature into electrical energy is known

as generation of different forms of nature into electrical energy is known as generation of

electrical energy. Various sources of energy available in nature are-

1 Solar Energy

2 Wind Energy

3 Tidal Energy

4 Nuclear energy

A great demand of electrical energy is notable feature of modern civilization. The

abundance of electrical energy completely changes the direction of the tempo of

civilization, living standard, vast development of rural and urban areas. Electricity has

become an essential commodity. The feature of electrical energy not only paralyses

industries and agriculture but also upsets the lives.

The whole electrical system is classified as:

1 Generation

2 Transmission

3 Distribution

4 Utilization

5 Switchgear and protection

Turbines are moved with the help of different sources of energy. The generation is

coupled with turbine to generate 11KV which is further stepped up by step up

transformers to 400KV and is then distributed to various sub-stations where the voltage is

reduced to 220KV with the help of step down transformers. From these sub-stations the

energy is distributed to the consumers after reducing it to 33KV.

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1.2 Installation

The 400KV GSS Jodhpur is one of those stations, which distributes energy to the

consumers. The station was installed in 2003. Jodhpur is the second largest city of the

state Rajasthan.

1.2.1 The Incoming And Outgoing Feeder

The incoming supply is as follows:

1. 400 KV – Kankroli

2. 400 KV – Merta I

3. 400 KV – Merta II

4. 400 KV – Jaisalmer(Under Construction)

5. 400 KV – Rajwest (Under Construction)

The outgoing feeder is as follows:

1. 220 KV – Tinwari I

2. 220 KV – Tinwari II

3. 220 KV – Jodhpur I

4. 220 KV – Jodhpur II

5. 220 KV – Bilara

6. 132 KV – Banar

7. 132 KV – Mathania

8. 33 KV – Salawas

9. 33 KV – Daijar

10. 33 KV – JU Alloy

11. 33 KV – DRDO

1.2.2 Location And Coverage

The 400KV GSS is situated on near SURPURA village, about 14-15 KM from

Jodhpur Railway Station. It covers as area of 110-120 beegha of land.

2

Fig. 1.1

3

CHAPTER- 2

IMPORTANCE OF ELECTRICAL ENERGY

2.1 Introduction

Energy may be needed as heat, as light as motive power etc. the present day

advancement in science and technology has made it possible to convert electrical energy

into other desired form. This has given electrical energy place of pride in the modern

world. In fact the advancement of country is measured in terms of per capita consumption

of electrical energy. Electrical energy is superior to all other forms of energy due to the

following reasons

2.2 Conventional Forms And Easy Control

Electrical energy is a very convenient form of energy as it can easily converted into

any form of energy like heat, light mechanical etc.

2.3 Greater Flexibility

One important reason for preferring electrical energy is flexibility that it offers. It can

be easily transported from one place to another.

2.4 Cheapness

Electrical energy is cheaper than other forms of energy. Thus it is economical to use

this form of energy in domestic commercial and industrial purpose.

2.5 Cleanliness

Electrical energy is not associated with smoke, fumes or poisonous gases. Therefore

its use ensures cleanliness and health conditions.

2.6 High Transmission Efficiency

The consumers of electrical energy are generally situated quite away from the centers

of its production. The electrical energy can be transmitted conveniently and efficiently

from the center of generation to the consumers with the help of overhead conductors

known as Transmission lines.

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CHAPTER-3

FUNCTION AND SPECIFICATIONS OF VARIOUS

EQUIPMENTS AT 400KV GSS

3.1 Metering & Indicating Instrument

There are several metering & indicating instruments e.g. Ammeters, voltmeters,

energy meters etc installed in a sub-station to keep watch on circuit quantities. The

transformer is invariably used with them for satisfactorily operation.

3.2 Bus Bars

Bus bars are the important components in a substation. There are several bus bar

arrangements the can be used in a substation. The choice of a particular arrangement

depends upon various factors such as system voltage, position of substation, degree of

reliability; cost etc. the following are the important bus bar arrangement used in sub-

stations:

a) Single bus bar systems

b) Double bus bars systems

c) Duplicate bus bars systems

In the 400KV GSS Surpura, Double bus bar substation and duplicate bus bar

system has been installed.

3.3 Control Cables

The control cable and the control system are required for officiating automatic

system. The cables employed for this purpose are multi-core cables having 10 or 37 or

61 conductors are run to the required points. The conductors used in the 400 KV GSS are

moose and conductors used in the 220 KV GSS are tarantula.

3.4 Power Transformers

A transformer consists essentially of two or more electric circuit in the form of

winding magnetically interlinked by a common magnetic circuit. An alternating voltage

applied to one of the winding produces, by electromagnetic induction, a correspondence

emf in the other windings & energy can be transferred from the ordinary circuit to the

other circuit by means of the common magnetic flux and the principle of mutual

induction. A transformer is basically a static device in which two or more stationary

electric circuits are coupled magnetically, the winding being linked by a common time

varying magnetic flux. Even though the static transformer is not an energy conversion

5

device & involves only the interchange of electrical energy between two or more

electrical systems, it is an extremely important component in many conversation systems.

Fig. 3.1

3.4.1 Classification Of Power Transformer:

1. According to usages

2. Step-up transformer

3. Step –down transformer

4. According to type of construction used:

5. Core type

6. Shell type

7. According to number of winding

8. Two winding transformer

9. Three winding transformer

10. Multi winding transformer

3.4.2 Terms Related To Transformer

1 Primary Winding:

The winding that is excited or energized by connecting it to an input source is usually

referred to as the primary winding.

2 Secondary Winding:

The winding to which the electrical load is connected and forms, which the output

energy is taken, are known as the secondary winding.

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3 HV Winding:

The winding which s operated at the high voltage level is known as the HV (high

voltage) winding.

4 LV Winding:

The winding which is operated at lower voltage level is known as the LV (low voltage)

winding.

4 Regulating Winding:

It is winding, which is used to regulate the voltage at different levels y connecting tap

changers across the winding. It consists of discrete numbers of small windings with 2 or 3

terms in each ad they being connected in series.

6 Tertiary Winding:

In addition to the tradition primary and secondary windings, a transformer can also have

tertiary winding.

3.4.3 Main Parts Of Transformer Are As Follows

1. Core:

It consist terminated silicon steel in which quantity of silicon 13up to 4% thickness of

lamination is 0.35 to 0.50m. Normally the shape of core in rectangular and it has three

legs.

2. Windings: Windings of power transformer are an important part. It consists of

super enameled copper wires. The size of wire (diameter) depends on the capacity of

transformer connection of winding is r/r.

3. Tap changer:

Tap changer is switching device by which the transformation ratio can be changed by

the changing the position of tap changing the switch. Tap changing system on GSS of

power transformer on-load tap changer (OLTC): On load tap changers are employed to

change turn 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 day, almost all the large

power transformers are fitted with on load tap changer. All forms of on load tap changing

circuit posse’s impedance. This is introduced to prevent short-circuiting of tapping

section during tap changer operation. The impedance can be either a resist of or a centre-

tapped reactor.

7

4. Tanks:

It is metallic tank, which is filled of insulating oil the transformer core and winding

assembly are surrounding by the oil in this tank. It protects that the winding and core

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

However for large rating power transformer, shaping of tanks becomes necessary to

conform to transportable profile shaping is provided by rounded corners at the ends,

truncation of law portion of walls from considerable, of loading in well wagon grider and

on the covers to reduce the height to minimize the tank oil, the tank profile may closely

follows the electrical clearances along the coils. As is evident, shaping gives saving in

tank material and oil but increases complexity and fabrication costs.

Transformer tank may be classified as

1 Plain tanks.

2 Shaped tanks.

3 Belt shaped tanks.

4 Corrugated tanks.

5 Stub and type tank

The transformer tank used in GSS power transformer is rectangular box (plain tank) type

in shape.

5. 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

transforms of heat is essential to control the temperature within permissible limits for the

class of insulation, thereby ensuring longer life due to less thermal degradation.

Types of cooling used in GSS power transformer:

1 ONAN type cooling: The generated heat can be dissipated in many ways. In case of

smaller rating of transformers, its tanks may be able to dissipate the heat directly to the

atmospheric dry while bigger ratings may require additional dissipating surface in form of

tubes/ radiators connected to tank or in the term of radiator tank. In these cases, the heat

dissipation is form transformer oil at atmospheric air by natural means. This form of

cooling is known as ONAN (Oil Natural, Air Natural) types of cooling.

2 ONAF type of cooling: For further augmenting the rate of dissipating of heat, other

means such as fans blowing air on the cooling surfaces are employed. The forced air takes

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

8

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

arrangement, additional raring under ONAN condition viz. after shutting off fans, is

available, which is of the order of 70-75%.

5.1 Cooling Arrangements

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

equipment can be arranged in various ways.

5.2 Arrangement with Radiators

Radiators are commonly used for ONAN and ONAF types cooling. Radiator

consists of element joined to and bottom headers, elements are made by welding two

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

through which oil flow. These radiators can be either mounted directly on the transformer

tank or in a form of a bank or connected to the tank through the piper. The surface area

available for dissipation of heat is a multiple manifold by using various elements in

parallel. As oil passes downwards either due to natural circulation or force of a pump in

the cooling circuits, the surrounding atmosphere air carries heat away.

5.3 Arrangements with Fans

These fans deliver large air volume at moderate speed with minimum sound and

low power consumption. Ring mounted fans are designed to give maximum volume

under free airflow condition and resistance up to approximately 6mm WC. These fans

generally conform to IS2312 and are used for radiator cooling. Fan consists of a totally

enclosed continuously rated specially designed motor with class B insulation and IP-55

class of protection to meet fan duty, impeller constructed with four broad faced. Steel

sheet blades assembly on robust aluminum hub, four arms, pressed sheet mounting ring

and four rubber cushions.

9

Fig. 3.2

6. Temperature Meters

There are two temperatures indicating metering power transformer, which indicate

the oil temperature and winding temperature. Temperature measured in degree Celsius. A

complete assembly of a transformer with details of core, wingding, tank connections and

major accessories.

7. Conservator and Air Cell

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

corresponding rise or falling volume to account for this an expansion vessel (conservator)

is connected to the transformer tank. The conservator has got a capacity between the

minimum to maximum oil level equal to 7.5 & of the oil in transformer. The atmoseal

types conservator, it is filled with oil to level appropriate to filling temperature and in

remaining portion is air cell, which is connected to atmosphere through a breather. As the

breather is through air cell no moisture come in contact with oil, this protect the oil from

deterioration or contamination.Air cell is a flexible separator filled inside the conservator.

Oil being out of the air cell, the separator is in direct contact with the atmosphere. The

advantage of air deterioration or contamination.

1. An efficient barrier between oil and air.

2. A protection against water vapors.

3. 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 to coating to ozone and weather.

10

Fig. 3.3

8. Buchholz’s Relay

The transformer is fitted with a bubble float buchholz rely. 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 divided to the buchholz relay without being

trapped anywhere.

9. Dehydration Breather

The conservator is connected outside through dehydration (Silicage filled)

breather to make sure that the air in conservation is dry.

11

Fig. 3.4

10. Oil Temperature Indicator

Oil temperature indicator operates on the principal of liquid expansion. The OTI

provided with a maximum pointer and two mercury switches are adjustable to make

contact between 500 to 1200 with the fixed differential of 100. the temperature for alarm

and trip contact setting shall be as under:- alarm 800 to 900.

11. Winding Temperature Indicator

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

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

12. Earthing

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

cover. By connecting them to tank cover, core and end frames being earthed. For Bank

earthing two number studs have been provided on tank.

13. Terminal Bushings

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

bushing is justified according to operation voltage of particular winding. The active part

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

from superior grade craft paper would on aluminum tube. This bushing is voltage graded

by suitably interposed aluminum foils forming condenser layers. Thus the electrical stress

are controlled throughout the thickness and along the surface avoiding any highly stress

concentrations. The bushing is supplied fully assembled in a wooden packing case with

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the busing supported at an angle of 10 degree to the horizontal. The bushing should never

be placed horizontally.

14. Insulating Oil

The insulating oil has three functions:

1 Provides additional insulation

2 Protects the paper from dirt and moisture

3 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) Free from sludge under normal operating conditions.

vi) Good resistance to emulsion so that the oil may throw away any moisture that enters

the apparatus.

3.5 Lighting Arresters

They are used to protect the sub-station & transmission lines arrests is

earthed . Gap is adjusted in such a way that 50% over voltage is operators. We

will use value type lighting arresters this types is called non-linear diverter. In

this spark – gap & resistance disc are used . when there is less change in line

voltage than is not flashover in gap but when there is over voltage & rapid

change in voltage then even grounding of voltage will not possible the value of

flash over voltage depends on surge currents. Operation will start when voltage

will increase 10% of rated voltage.

1. Rod gap arresters

2. Horn gap arresters

3. Multigap arresters

4. Expulsion type arresters

5. Value type arresters

3.6 Circuit Breakers

Classification of circuit breakers

1) Are quenching (Medium Wise)

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a) Air Blast CB

b) Oil CB

c) Air Blast CB

d) Vacuum CB

e) SF₆ CB

2) Application wise :

a) Generator CB

b) Transformer Line CB

c) Industrial CB

d) Distribution CB

3) Voltage Level Wise:

a) HV/EHV CB

b) MV CB

c) LV CB

4) Base on Construction:

a) Dead Tank Breaker

b) Live Tank Breaker

G.S.S has SF₆ Circuit Breaker’s which have the following mechanism.

6.1 SF₆ Circuit Breaker: In this CB, the SF₆ gas is used as an quenching agent. The

process of extinction by the gas is shown the below block diagram

At the time of fault:

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Contracts of CB open

The Valve mechanism permits high pressure SF₆ gas from the reservoir to flow towards the are interruption chamber from the

Trip Valve, which is now NO-normally open.

The high pressure flow of the SF₆ gas rapidly absorbs the free electrons in the are path to form immobile negative ions, which

are ineffective are charge carriers.

As a result, the medium between the contacts quickly builds up high Dielectric Strength & causes the extinction of the arc.

Fig. 3.5

3.6.1.1 Electrical properties of SF₆

Electron affinity: The excellent insulation properties of sulphur hexafluoride are

attributable to the strong electron affinity of the SF ₆ molecule. This is based mainly on

two mechanisms, resonance capture and dissociative attachment of electrons in

accordance with the equations:

1) SF₆ +e→ SF₆

2) SF₆ + e → SF₅ + F

The process represented by equations (1) applies to electron energies of 0.1 eV

with an energy range of 0.05eV and that represented by equations (2) applies to an energy

range of 0.1 eV.

1. Are-quenching capacity: On account of its thermal properties and low

ionization temperature, sulphur hexafluoride exhibits outstanding characteristics for the

extinguishing of electric arcs. The quenching time using SF₆ is about 100 times less than

that using air.

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2. Dielectric Strength: The strong interaction of hi-energy electrons with the

polyatomic SF₆ reaches that of transformer oil at pressure of only 3 bars. The breakdown

strength of SF₆ is independent of frequency. It is inert gas. Chemical inertness of this gas

is advantageous in switchgear. The components do not get oxidized or deteriorated. The

life of metallic part, Contacts is longer in SF₆ gas. Hence the maintenance requirements

are reduced.

3.6.1.2 Operating Principals Of Sf₆ Circuits Breaker:

The SF6 breaker operates on what is usually referred as the puffer principle. In puffer

type SF6 circuit breaker , the entire breaker is filled with SF6 gas at single pressure of 5

Kg/cm2 or about 7 bar. The breaker is a sealed unit . during the opening stroke the SF6 gas

is compressed released through the nozzle of insulating material. The compressed gas

flow through the nozzle at a high velocity and takes away the heat produced by the arc the

arc is quenched at a current zero. The high dielectric strength of gas is useful in giving

good with stand voltage SF6 circuit breaker are explosion free , can quench capacitive

currents , short circuit current etc. Early and are used for high voltage circuit breakers for

voltage above 3.3 KV During manufacture of the breaker pole it is dried internally

through pumping, the breaker pole is then to pressurized and also tested against leaks

inside the pile there is a absorption medium for the decomposition products of the gas.

The breakers pole should only be opened by trained person at the manufacturing factory.

3.7 Isolators

Then 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 in the voltage across the terminal e.g. each pole of the isolator

will result from the operation.

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 connections

and very short length of cable.

16

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 .

3.8 Instrument Transformer

Is defined as a transformer intended to feed the measuring instruments, meters,

relays etc. Generally protective system are relays are connected to the secondary

of a current transformer as they cannot withstand high currents. These IT’s help

in reducing these voltages & currents to acceptable level for operation of

voltmeters & ammeters.

3.8.1 Current Transformer

A CT is an instrument transformer in which the secondary current is

substantially reduced proportional to the primary current & differs from it by the

angle which is approx. direction of current . These transformers are different from

general power transformers.

Fig. 3.6

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Shown below are the major differences that are noticeable:

Conventional Transformer Current Transformer

Driving Function: voltage. Driving Function : Current

Secondary load impedance determines the

secondary current.

CT primary current is the determining

and predominated factor.

Corresponding to the secondary current the

primary current flows.

Secondary current follows the primary

current.

3.8.1.2 Function / Application of a CT

1. For Metering function : It transforms the high value Primary Current substantially

low value secondary current which can be fed directly to measuring instruments for

measuring the current & power in the main circuit.

2. For Protection Purpose: The secondary current can also feed Protective Relays which

operate the protective system in the main circuit in case of any abnormality in the system.

Definition of the different terms related with current transformers :

a) Rated Primary Current: The value of primary current on which the primary

performance of the current transformers is specified.

b) Rated Short Time Current: Its defined as r.m.s value of a.c component which the

CT can carry without damage.

c) Rated Secondary Current: The value of secondary current marked on the rating

plate.

d) Rated Exiting Current: The RMS value of current taken by the secondary winding

of a. C.T. When sinusoidal voltage of rated frequency is applied to secondary with

primary winding open.

e) Rated Burden: The burden assigned by the manufacturer at which C.T performs with

specified accuracy.

f) Current Error Ratio Error: The percentage error in the magnitude of secondary

current is defined in the terms of current error.

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3.8.1.3 Burden on C.T.

Rated burden of CTS and VT’s referring to the maximum load in volt-amperes 9

VAO which may be applied across the secondary terminals without the ratio and phase

angle error-exceeding the permissible limits. The burden depends upon the number of

relays and instruments connected and their individuals burden typical values.

3.8.1.4 Various Types of Construction of CTs

A CT has following essential parts

1. Insulation over the core by taps

2. Secondary winding having several turns would on the insulated core.

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

4. Support porcelain or epoxy insulator.

5. Synthetic resin or oil insulation.

6. 3.8.1.5 CT’s For High Voltage Installations

Separately mounted post type CT’s are suitable for outdoor service. The primary

conductor is at high voltage with respect to the earth. Hence it is insulated by means of

insulation column filed with dielectric oil. In high voltage CT’s the primary and

secondary windings are situated at the upper end of the unit. The primary wdg. Normally

being of bar type. The top – fabricated housing is at line potential and is supported on the

porcelain insulator.

3.8.1.6 Specification Of 400 KV Current Transformers

1. Type – Dead tank, single phase out door, oil immersed & Hermetically sealed .

2. Manufacturer’s Designation – 420 kV CT.

3. Rated voltage ( KV ) – 420

4. Short time thermal rating for One second ( KA rms ). – 40

5. Rated dynamic current of primary 100 ( Kapeak).

6. Flux density at knee point voltage, 14.5 Wb/cm2

7. NO. of primary turns – single

8. No. of secondary turns – 200-1000-500

9. Core area , cum2 – 65.55

10. Core length, (Average magnetic path) cms. – 104.46

11. Type of primary winding – Hair pin type.

12. 1 Temperature rise (degree C) at rated continuous thermal current over max. ambient

temperature at site for –

i) Winding 40

19

ii) Oil at top 40

iii) Exposed current carrying parts 55

12.2 Temperate rise at normal rating over max. Ambient temperature site for

i) Winding 40

ii) Oil at top 40

iii) Exposed current carrying parts.

13. Total creep age distance, mm 10500

14. Protection creep age distance 5250

15. One minute power frequency with stand test voltage ( kv rms ) 630

16. 250/2500 micro seconds switching impulse withstand voltage (kv peak) 1050

17. 1.2/50 micro second impulse withstand test voltage (kv peak) 1425

18. Power frequency withstand test 4 kv for core IV & V and Voltage on secondary

(kv rms) 3 kv rms for core III

19. Weight of oil per C.T. kegs. 510

20. Governing standard for oil IS 335

21. Whether pressure relief device Yes Provided.

22. Total weight per CT kgs. 2150

23. Overall dimension .mm 1150 x 750 x 5200

24. Current density in primary winding at

i) Normal rating A/cm2 208

ii) Thermal rating for 1 sec., A.cms 249

iii) Dynamic rating, A/cm2 10405

25. Visual corona extinction voltage 320 (k Vrms)

3.8.2 Potential Transformers

Potential transformers (PT) are mainly instrument transformers that are basically used

for the following purposes :

1. For stepping down the voltage for measurement

2. As line voltmeters

3. Protective relays

4. Tariff meters

3.8.2.1 Construction

The PT is mostly step down and shell type. The secondary voltage is generally 110 V

potential transformers are of two types :

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a. Magnetic type

The magnetic type PT work on the principles of power transformers. The design is

particularly for the system voltage of 132 K.V. and above where it becomes increasingly

more economical. Main parts of the PT are stated below :-

1. Core: The core may be shell type in its construction. Shell type

core is suitable for low voltage transformers.

2. Winding: The primary and the secondary winding are coaxial to

reduce leakage to minimum. The primary winding may be single coil but must be

subdivided.

3. Insulation: Cotton type and varnished cambric are used or soil

construction. Hard fiber separators are used between coils. At low voltages, the

transformers are usually filled without above 700 volts been developed for use up to 45

K.V.

b. Capacitor voltage transformers (CVT)

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

The supply frequency-switching transients, magnitude of connected burden etc, affect the

performance of CVT. The CVT is more economical then an electromagnetic voltage

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

equipment can be connected via the capacitor voltage transformers, thereby there is no

need of separated coupling capacitors. The CVT are used for voltage above 66 KV and

above. At such voltage the costs of electromagnetic voltage transformer is too high. The

capacitor connected in series with the CVT acts like a potential divider. The burden

provided by the capacitor is negligible. The construction of CTV depends on the form if

the capacitors voltage divider. Generally HV capacitors are enclosed in porcelain

housing. Schedule of guaranteed data and technical particulars for 4400-pf-400KV

capacitor voltage transformers.

3.9 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. It is stronger mechanically than glass gives less

trouble from leakage & is less affected by change of temperature.

Type of insulators

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i) Pin type Insulator

ii) Suspension type insulator

iii) Strain Insulator

iv) Shackle Insulators

v) Post Insulators.

Their use in the substation will depend upon the service requirement.

3.9.1 Pin type Insulator

Pin type of insulator is not economical beyond 33 KV. For high voltage (>33 kV), it is

a usual practice to use suspension type insulators. They consist of a number of porcelain

discs connected in series by metal links in the form of a starting. The conductor is

suspended at the bottom end of the string while the other end of the string is secured to

the cross-arm of the tower. Each unit or disc is designed for low voltage. If working

voltage is 66kv, and then six discs in series will be provided in the string.

3.9.2 Strain Insulators

When there is a dead end of the line or there is corner or sharp curve the line is

subjected to greater tension. In order the line of excessive tension are used. For high

voltage transmission lines, strain insulators consists of an assembly of suspension the

tension in lines is exceedingly high, 2 or more strings are used in parallel.

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CHAPTER- 4

POWER LINE CARRIER COMMUNICATION (PLCC)

4.1 Introduction

For exchange of dates & transfer of message between GSS voice communication is

necessary. High frequency carrying currents audio signals is generated, transmitted &

received with the help of identical carrier current equipment provided on each end.

Carrier current equipment comprises of following:

1. Coupling Capacitor

It acts like a filter. It blocks power frequency (50hz) while offer low reactance to

carrier frequencies as allows them to pats through because. For examples A 2000 pf

capacitors offer 1.5-mega ohm to 50hz while if just offer 150 ohms to 500 kHz. Thus

coupling capacitor allows carrier frequency signal to enter the carrier equipment bus

does not allow 50hz power frequency current to enter the carrier equipments.

2. Wave Trap Unit

It is parallel turned comprising of c & I. It has low impedance to 50hz & high

impedance to carrier frequencies get passed through wave trap & carrier frequencies

passes through coupling capacitor & reaches carrier current Wave traps are mounted in

outdoor switchyard. Wave trap mounted at GSS is “under hung”.

3. Transmitter & Receiver Unit

Carrier current unit acts like both transmitter receiver carrier frequencies are

generated in master oscillator can be tuned to a particular frequency selected for the

application output voltage of oscillator is held constant by voltage stabilizers. Output of

oscillators is fed to amplifiers, which increases the strength of signal to be transmitted to

overcome the transmission losses. Line losses vary with length of line frequency type of

line losses in overhead lines. Receiving unit comprises of an alternator. Band pass filter

restricts the acceptance of uncounted signal & matching transformer or matching element

matches the impedance of line & receiving unit block diagram of receiving of receiving

unit.

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CHAPTER- 5

SUBSTATION

5.1 Introduction:

Substations are important part of power system. The assembly of apparatus used to

change some characteristics (e.g. voltage, arc. to o.k. frequency, p.f etc) of electrical

supply is called substation.

5.2 Classification of Substation

There are several ways of classifying substations. However, the two most important

ways of classifying them are according to:

1) Service requirements and

2) Constructional features

1. According to service requirements:

a) Transformer sub-stations

b) Switching substations

c) Power factor control substation

d) Frequency changer sub stations

e) Converting substations

f) Industrial sub stations

2. According to constructional features:

a) Indoor sub-stations

b) Outdoor sub-stations

c) Underground sub-stations

d) Pole-mounted sub-stations

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CHAPTER- 6

PROTECTIVE RELAY

6.1 Introduction

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:

1) Over voltage due to switching.

2) Over voltage due to direct and indirect lighting strokes.

3) Bridging of conductors by birds.

4) Breakdown of insulation due to decrease of its dielectric strength.

5) Mechanical damage of equipment.

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

intolerable interruption of service to customers.

6.2 Relays

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 faults the relays operates

to competent the trip circuit which result in opening of the circuits breaker and therefore

in the disconnection of the faulty circuits.

Basic requirements of protective relaying:

A well designed and protective relaying should have

i) Speed

ii) Selectivity

iii) Sensitivity

iv) Reliability

v) Simplicity

vi) Economy

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6.3 Types of Protection

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

protection is the first line to defense and primary relays clear faults in the protected

system as fast as possible. The reliability, not only if 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 backup relay operates if the primary relays fails

and covers not only the local primary relays to operate. Protective relays are classified

depending upon their construction and principles of operation such as:-Ordinary

electromagnetic relays consisting of moving plunger, moving iron, attracted armature

hinged and balanced beams types of relays are various examples, D.C. actuated such

replays. Electromagnetic induction or simply induction relays use the principles of

induction motors (whereby torque is developed by induction in rotor) in their operation.

Such relays are actuated by A.C. quantities only. Electro thermal relays (thermal overload

protection using bimetallic strip) Physic-electrical relays: Bucholy’s relays are examples

of this type. Static relays employing thermionic valves, transistors or magnetic amplifiers

to obtain the operating characteristics. Electro-dynamic relays operate on the same

principles as moving coil instrument.

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

1) Over current relays

2) Distance relays

3) Differential relays

4) Earth fault relays

1) Over Current Relays:

Directional type over current relays works on the induction principles and initiates

corrective measures when current in the circuit. Exceed the pre-determined value. The

actuating source is a current in the circuit supplied to the relay from a current transformer.

These relays are used on a.c. circuits and can operate for fault flow in either direction. But

their relays 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 in the relays to cause its operation. This

difficulty is over come in the directional over current relays, which is designed to be

almost independent of system voltage and power factor.

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Operation:

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

protected by the relays. Therefore, directional power relays (upper element) does not

operate, thereby keeping the over current element (lower element) energized. However

when a short circuit occurs, there is tendency for the current or power to flow in the

reverse direction. Should this happen, the disc of the upper elements rotates to bridge the

fixed contact 1 and 2. This completes the circuits for over current elements. The disc of

this element rotates and the moving contact attached to it closes and the trip circuit. This

operates this circuits 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 exceed the pre-set value.

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.

There are four main elements in any distance protection as follows: -

(i) Operating elements “O”: The element brings protection into action whenever a fault

occurs within the protected zone.

(ii) Directional elements “S”: This gives directional features to the operation of the

system and is useful in network having duplicate feeder. As soon as the fault current into

the bus bar from the line this element operates.

(iii) Distance element “Z”: This is sensitive to the ratio of the operating voltage to

the fault current i.e. V/ if or upon fictitious impedance when looking into the system from

the fault.

Zf=V/1f

The value of Zf is dependent upon the distance of the fault from the relays. The

principle of this element is more or less like ohmmeter.

(iv) Time delay element “T”: This element creates a time lag, the importance of which

has already been discussed above. This time lag depends upon the distance of the fault

point from the relay.

2) 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

27

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 made proportional

to the distance of the fault by so designing the relay that it has a time lag characterizes,

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 longer be in balance. This

voltage difference will cause a current to flow through the operating coil of relay, which

closes the trip circuit.

3) 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

certain way, can be made to operate as differential relays. There are two fundamental

system of differential protection viz.

1) Current balance protection

2) Voltage balance protection

A current balance differential relay is one that compares the current entering a section

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

condition no longer applies. If this differential current is equal to or greater than the pick-

up value, the relay will operate & open the circuit breaker to isolate the faulty section.

Under healthy condition equal current flows in both primary windings. Therefore the

secondary voltages are balanced against each other & no current will through the relay-

operating coil.

4) Earth Fault Relays:

Directional type over current relays work on the induction principle and initiates the

char-active measures. When current in the circuit exceeds the predetermined values. The

actuating source is a current in the circuit supplied to the relay from a CT. these relays are

unsuitable for use as directional protective relays under short-circuit conditions. When a

28

short circuit occurs, the system values falls to a low value and there may be insufficient

torque developed in the relay to cause its operation. This difficult is overcome in the

directional over current relay, which is designed to be almost independent of system

voltage and power factor.

29

CHAPTER- 7

EARTHING

7.1 Introduction

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

connecting wire of negligible resistance is called as “Earthling or grounding”. The

provision of earth electrode for an electrical system is necessity by following reasons.

1. All the parts of an electrical equipments like casings of machines circuit breaker,

lead sheathing & armoring of cables, tanks of transformer etc, which have to be the at

earth potential, must be connected to an earth electrodes. This current operates the

proactive device & thus the faulty circuits is halted in case occur.

2. The electrode ensures that in the event of over voltage of an the system due to lighting

discharge or other system faults which are normally “dead” as for as voltage are

concerned do not attain dangerously high potentials.

3. In a 3-phase circuit the neutral of the system is earthed in order to stabilize the

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

components must be earthed: -

a) The flames, tanks & enclosed of electric machines transformers and apparatus,

lighting fitting.

b) The operating mechanism of the switchboards control boards individual panel boards,

cubicles.

c) The structural steel work of sub-stations, metal cable jointing boxes, the metal sheaths

of the cable s the rigid metal conduct runs & similar metal work.

There are 2 methods of earthing:

1- Pipe earthing.

2- Plate earthing.

7.2 Earthing Arrangements 400 KV GSS

In a GSS or any magnitude various non current carrier equipment to be

earthed namely substation structures , shielding g wires or masts ,equipments tanks

spread over large areas 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 conditions . It generally, therefore becomes desirables to form a ring

30

of the earthing electrodes. Another way of looking into the sub - station earthing

problem is that a very low

Earthing problem is that a very low earthing resistance value is required

resistance in a very large low earthing value is required in a large areas occupied

but the sub - station such can only be obtained by using a number of rod &

joining them in parallel .In a sub -station the earthing system invariably takes the

shape of grounding meet with necessary or additional rounding rods accepts in

the case of very small sub stations. Common earth electrodes should be use for both

system earths & equipments earth. Here also it is recommended to have common earth

bus for high voltage system. Where there are manual operating handle to the system .A

typical earthing arrangements for a GSS .

7.3 Plate Earthing

In plate earthing plate either of copper of dimensions 600cm * 60cm

*3.15mm or of galvanized iron of dimensions 60cm * 60cm * 6.30cm s burled into

the ground with its face vertical at a depth of not less that 3mt from ground

levels .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 & for

carrying out periodical inspection & tests. The earth wire GI wire of GI plate earthing

is securely. Bolted to an earth place with the help of a bolt nut & washer made of

material & of galvanized iron in case of GI plate earthing.

31

CHAPTER- 8

LABORATORY

8.1 Introduction:

The Laboratory at 400 KV GSS substation is equipped with various instruments to

test the transformer oil. It is very important to test the oil at regular intervals. It also used

to test the oil in failure conditions to find out the reason of failure.

8.2 Importance of Transformer Oil

1. The Oil serves dual purpose of insulating medium & coolant.

2. Heat generated inside a Transformer is dissipated to the Atmosphere through

Insulating Medium.

3. This ensures Longer Life & Less Thermal Degradation of Insulation.

4. Provide Arc Quenching Medium.

8.3 Deterioration of Transformer Oil

1. Accidental Leakage of Water

2. Chemical Decomposition

3. Contamination by Gases

4. Electrical Stresses

5. Thermal Stresses

6. Effect of Oxidation Products

7. Physical Contamination

32