MONITORING AND OPERATION OF 220kV/132kV

SUBSTATIONA Project Report submitted in partial fulfillment of the requirements

For the award of the degree of

BACHELOR OF TECHNOLOGY IN

ELECTRICAL AND ELECTRONICS ENGINEERING

by

P.NAGARJUNA REDDY (07281A0241)

MOMD.ZUBAIR (07281A0211)

S.TILAK (07281A0258)

Under the esteemed guidance of Mr.S VIGNESHWAR

Asst.Professor

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE (Affiliated to J.N.T.U, Hyderabad) Singapur, Karimnagar-505468

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KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE

(Recognized by A.I.C.T.E)

(Affiliated to J.N.T.U, Hyderabad)Singapur, Huzurabad, Karimnagar-505468

CERTIFICATE

This is to certify that this project work entitled “MONITORING AND

OPERATION OF 220kV/132kV SUBSTATION” Warangal, Mulugu road ” is

the bonafide work carried out by P.NAGARJUNA REDDY(07281A0241),

MOMD.ZUBAIR(07281A0211), S.TILAK(07281A0258) in partial fulfillment of

the requirements for the award of the degree of Bachelor of technology in

Electrical & Electronics Engineering students of final year B.Tech., EEE

Engineering (2007-2011) in partial fulfillment of the requirements for the award

of the Degree of B.Tech. Of the JNTU, HYDERABAD

PROJECT GUIDE HEAD OF DEPARTMENT

S.VIGNESHWAR Y.Y PUNDLIK

(Asst. Professor) (Assoc. Professor)

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ACKNOWLEDGEMENT

We wish to express our sincere thanks to our Project Guide Mr. S. VIGNESHWAR for having

been a source of constant inspiration, valuable guidance and generous assistance throughout the

period of our project work under his guidance.

We express our sincere gratitude to our, Sri YOGESH.Y.PUNDLIK, Head of dept of

ELECTRICAL & ELECTRONICS ENGINEERING, for having been a source of constant

inspiration, valuable guidance and generous assistance throughout the period of our project work.

We deem it as a privilege to have worked under his able guidance.

We wish to express our sincere thanks to our Principal, Dr. K. SHANKAR, for providing the

college facilities for completion of the project.

We wish to express our sincere thanks to Sri T.RAJESHWAR RAO(A.D,MRT), who granted

permission to perform this project under the guidance of P.SANDEEP(A.E,MRT) and also

Mrs. SAJINI(A.E,TL&SS), who took active part in completion of this project.

We express our gratitude for the encouraging remarks and valuable guide lines to all the

members of the Project Evaluation Committee.

Finally, we would like to thank all the faculty members, supporting staff of the

Department of EEE and friends for their cooperation and valuable help for completing this

project.

Project associates

P.NAGARJUNA REDDY (07281A0241)

MOMD.ZUBAIR (07281A0211)

S.TILAK (07281A0258)

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CONTENTS

Page No

ABSTRACT 1

CHAPTER-1 INTRODUCTION 2

1.1 What is substation 2

1.2 Classification of substations

1.3 220kV/132kV/33kV substation in Warangal 3

1.4 Single line diagram of 220kV/132kV/33kV substation 5

CHAPTER-2 MAJOR EQUIPMENTS USED IN SUBSTATION 6

2.1 Transformer 6

2.1.1 Introduction 6

2.1.2 Principle and constructional features of transformer

7 2.1.3 Name details of transformer

10 2.1.4 Transformer accessories

13 2.1.5 Dissolved gas analysis (DGA)

16 2.1.6 Norms of protection for

transformer 17 2.1.7 Instrument

transformer introduction 18 2.1.8 current

transformer and its specifications 18 2.1.9 current

voltage transformer 22 2.2 Energy meter

23 2.2.1

Introduction 23

2.2.2 Procedure for energy meter testing 24

2.2.3energy meter testing results 27

2.3 Auxiliary AC and DC power supply 28

2.4 Battery room 29

2.4.1 battery charger

29 2.4.2 Theorey of operation of float charger

4

32 2.4.3 Theorey operation boost charger

33

CHAPTER-3 TYPES OF BUSBAR CONNECTIONS 36

3.1 Introduction 36

3.2 Single bus 36

3.3 Double bus with double breaker 37

3.4 Inspection bus 38

3.5 Double bus with single breaker

39 3.6 Ring bus

40 3.7 Breaker and a half bus

41

CHAPTER-4 PROTECTION EQUIPMENT USED IN SUBSTATION 42

4.1 Circuit breakers 43

4.1.1 Introduction 43

4.1.2 Operation of circuit breaker

44 4.1.3 Different techniques used to extinguishes the arc

45 4.1.4 Arc interruption of circuit breaker

46 4.1.5 Air-blast circuit breakers

47 4.1.6 SF6 circuit breakers

47 4.1.7 Vacuum circuit

breakers 49 4.1.8 Advantages of

circuit SF6 circuit breaker 49 4.2 Lightening arrester

50 4.3 Wave trap

52 4.4 Protective relays

52 4.4.1

Introduction 52

4.4.2 The basic requirement s for the relay 53

4.4.3 Distance relay principle 54

4.4.4 Types of distance relays 54

5

4.4.5 Applications of distances relays

58 4.5 Substation earthing

60

CHAPTER-5 SUMMARY AND CONCLUSSIONS 62

5.1 Summary 62

5.2 Conclusions

REFERENCES 63

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ABSTRACT

Now a day’s everything is depending up on the power. So give the reliable

supply to the consumers. In distribution systems one of the major parts is

"SUBSTATIONS".

An electrical substation is a subsidiary station of an electricity, Generation,

Transmission and distribution systems where the voltage is transformed from high to

low or reverse using the transformers .Electric power may flow through several

substations between generating plant and consumer and may be changed in different

voltage levels .the equipment used in substation are Transformer, Lightening

arresters, isolator, bus bar, protective devices, Battery charger, earth switches, earth

rods. So for of supply the regular maintenance and checking is necessary from that

we conclude weather it is suitable or not for the desired operation.

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CHAPTER-1

INTRODUCTION

1.1 What is substation?

An electrical substation is a subsidiary station of an electricity generation,

transmission and distribution system where voltage is transformed high to low or the

reverse using transformers. Electric power may flow through several substations between

generation plant and consumer, and may be changed in voltage in several steps.

The main equipment used in substation are transformers lighting arresters, Circuit

breakers PLCC, isolators, bus bars, protective relays, Battery charger, earth switches, earth rods.

1.2 Classification of substations

Classification of substations based on

(1) Service requirements

(2) Constructional features

1. According to service requirements: According to service requirements substations are

classified into:

i. Transformer Substations

ii. Switching Substations

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iii. Power factor correction substations

iv. Frequency changer substations

v. Converting substations

vi. Industrial substations

2. According to construction features: According to constructional features substations

are classified as;

i. Indoor substations

ii. Outdoor substations

iii. Underground substations

iv. Pole mounted substations

1.3 220/132/33 kv sub-station warangal

There are four incoming feeders named as B.pad-2, RSS-1 (Nagaram), RSS-2.There are

six 132 KV incoming feeders connected to 132 KV bus. There are 33 KV out going feeders

connected 33KV bus. There are three 220/132 KV power transformers and two 132/33 power

transformer

Lighting arrestors is placed at the starting point of every feeder to every phase of the line.

Capacitive voltage transformer (CVT) follows the lightning arresters .Wave trap is placed next to

the CVT for collecting communication signals at higher frequencies

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Earth switch is there to transfer the induced voltage on the line when the isolator is open.

Then there is an isolator to open the line under maintenance. There is a current transformer to

step down the currents to the lower values for the purpose of protection and metering .Next to

CT circuit breaker is placed which functions under fault counditions.Then there is a another

isolator.

There is 220 KV bus with large size conductors. The bus is very important in the

substation. The incoming feeders are connected to it through the control devices. There is an

isolator followed by a high voltage 220 KV circuit breaker. Then 220 KV feeders are connected

to primary side of power transformer through an isolatar.There are three power transformers of

each 100 MVA, 220/132 KV Size.

The output is taken from the LV side of the transformer and connected to 132 KV bus bar

through the protecting devices. From every protecting control device cables are connevted to the

control center where the continuous monitoring and controlling is done through and relay panel.

Five outgoing feeders are taken out from the 132 KV bus and transmitted the power at

lower voltage to the 132/33 KV substations which are located nearby load center.

10

11

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CHAPTER-2

MAJOR EQUIPMENTS USED IN SUBSTATON

2.1 TRANSFORMERS

2.1.1 Transformer introduction

Transformers are static piece of apparatus by means of which electric power in one

circuit is transformed to electric power of the same frequency in another circuit. It can raise or

lower the voltage in a circuit but with a corresponding decrease or increase in current.

Transformers employed in a substation are

1. Power transformers

2. Instrument transformers

a) Current transformers

b) Potential transformers

3. Station transformers

Power transformer

Power transformers convert power level voltages from one level or phase configuration to

another. They can include features for electric isolation, power distribution, and control and

instrumentation applications. Transformers typically relay on the principle of magnetic induction

between coils to convert voltage and /or current level.

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2.1.2 Principle and constructional features of transformer

Fig 2.1.2.1 Basic principle of transformer

In its simplest form, it consists of two inductive coils, which are electrically separeted but

magnetically linked through a path of low reluctance. The two coils possess high mural

inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up

in the laminated core and it produces mutually induced emf.If the second coil circuit is closed,

current flows in it and so electric energy is transferred from the first coil to the second coil. The

first coil in which electric energy is fed from AC supply mains, is called primary winding, while

the second coils is known as secondary winding.

The necessity of the transformer arises when voltages are required to b changed. For

example, the generated voltage of the alternators will b around 15KV.It is not economical to

have transmission and distribution systems at this voltage as, for the same power transmitted, the

current will b more when compare to high voltage transmission i.e.as the transmission voltage

increases, the current is reduced and their by the conductor diameter can b reduced resulting in

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saving of conductor material and weight of structures (supporting)and power losses during the

conductor(I^2R losses).

The efficiency of the transformer is very high as there are no moving parts and so there

are no mechanical losses. The only losses in transformers are

(a) Copper losses in primary in secondary winding

(b) Iron losses due to eddy current and hysteresis losses in the magnetic core

Constructional features

The simple elements of a transformer consist of two coils and a laminated steel core.

The two coils are insulated from each other and from the steel coil. Other parts are

A) Suitable container for the assembled core and windings

B) Suitable medium for insulating the core in windings from the container and

C) Suitable bushings for insulating and bring out the terminals of the winding from the tank.

The transformers are of two general types: distinguished from each other by the manner

in which the primary and secondary coils are placed around the laminator steel core. They are

a) Shell type b) core type

Steel surrounding the coil is shell type of construction:

Coil surrounding the steel is core form type of construction

In the simplified diagram for the core type transformer the primary and the secondary winding

interleaved to reduced leakage fluxes. This is half the primary and half the secondary winding

will place side by side or concentrically on each limb. Not primary on one limb and the

secondary on other limb.

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Core type transformer

Fig 2.1.2.2 construction core type transformer

The windings of transformers are made of copper wire or strip heavy current capacity

requires conductors of large cross section. To reduce the eddy current losses with in the

conductors, several small wire of parallel strips are preferable to one large strip. The coils used

are from wound and cylindrical type. The general form of this coils May be circular are

rectangular. But for large size core type transformers, because of their mechanical strength. Such

cylindrical coils are wound in helical layers, with the different layers insulated from each other

by paper, cloth etc.

Figure shows the general arrangement of these coils with the core and from each other.

Since the low voltage windings are easiest to insulate it placed nearest to the core

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Shell type Transformers

In this case, the coils are wound but are multilayer disc type. The different layers of such

multi layer discs are insulated from each other by paper. The complete winding consists of

stacked discs with insulation spaces between the coils with the spaces forming horizontal cooling

and insulation ducts. A shell type transformer may have a simple rectangular form as shown in

figure

2.1.2.3 Construction of shell type transformer

2.1.3 Name plate details of the transformer

Make : Bharat bijlee limited

Connection : YNyno

K V A Rating : 25/5o kva

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Cooling : ONAN/OFAF

No load : 132/33 kv

Phases HV/LV : 3/3

Frequency : 50 Hz

%impedance volts at75

℃

At tap minimum : 13.6%

At tap normal : 13.183%

At tap maximum : 12.7%

Untanking mass : 34450 kg

Mass/oil volume : 17545/20400kg

Total mass : 70500kg

Transformation mass : 58000kg

Maximum temperature rise over an ambient of 50℃

Oil : 50℃

Winding : 55℃

Insulation levels P>F/Impulse

HV : 230kv/550kvp

Hv(N) : 38kv/95kvp

LV : 70KV/170KVP

Short circuit duration current : 2 sec

% Impedance volts: IT is the percentage of HV volts required to create full load amperes in a

shorted LV winding .It is also indicative of percentage of LV volts to b applied so that full load

currents flow in a shorted HV winding.

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Max. Ambient temperature: 50degree C. This means the transformer is designed to work in good

condition when the surrounding temperature of the transformers increase up to a maximum of 50

℃

Winding temperature rise 50℃ : this means that the winding temperature can go up to 55℃ over

and above the atmospheric temperature

Vector group Y No stands for :

Y: primary winding star connected

N: availability of primary bush ring at top plate.

Y: small letter representing secondary star connected

n: small letter representing availability of secondary neutral bush ring at top plate

O: this letter is zero indicating that the angle of voltages between primary and secondary is zero

Insulation levels: for the safety point of view insulation will b provided for more than the

operating voltages of the transformer.

Total mass: it is the total weight of the transformer including the active part, oil, tank,

bushing, all the spares etc. This is for the purpose of cranes capacity and transport facility

required in case the full transformer is to be lifted and or transported from place to place.

Transport mass: normally transportation is done when active part is mounted in the tank

but without oil, bushings, all the spares etc. This is for the purpose of cranes capacity and

tan sport facility required in case the full transformer is to be lifted and or transported

from place to place.

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Untanking mass: weight of part plus top plate

2.1.4 Transformer accessories

Conservator tank

Conservator with the variation of temperature there is corresponding variation in the

oil volume. To account for this, an expansion vessel called conservator is added to the

transformer with a connecting pipe to the main tank. In small transformers this vessel is open

to atmosphere through dehydrating breather (to keep the air dry). In large transformers, an air

bag is mounted inside the conservator with the inside of bag open to atmosphere through the

breathers and the outside surface of the bag in contact with the oil surface.

Breather

Both the transformer oil and cellulosic paper are highly hygroscopic. Paper being more than

the mineral oil the moisture, if not excluded from the oil surface in conservator, thus will find its

way finally into the paper insulation and causes reduction insulation strength of transformer. To

minimize this, the conservator is allowed to breathe only through silica gel column, which

absorbs the moisture in air before it enters the conservator air surface.

Pressure relief device/expansion vent

Transformers tank is a pressure vessel as the inside pressure can group steeply whenever

there is a fault in the windings and the surroundings oil is suddenly vaporized. Tanks as such are

tested for a pressure with stand capacity of 0.35kg-cm^2.to preventing bursting of the tank and

thus catastrophe, these tanks are in addition provided with expansion vents with a thin diaphragm

made of Bakelite /copper/glass at the end. In present day transformers, pressure relief devices are

replacing the expansion vents. These are similar to safety valves on boilers.

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Buchholz’s relay

Whenever a fault in a transformer develops slowly, heat is produced locally, which begins to

decompose solid of liquid insulating materials and thus to produce inflammable gas and oil flow

this relay is applicable only to conservator type transformers. Buchholz relay is connected in the

pipe leading to the conservator tank and detect the gas produced in transformer tank.

Temperature indicators

Most of the transformers are provided with indicators that display oil temperature and winding

temperature. Oil temperature is that of the top oil, where as the winding temperature

measurement is indirect. This is done by adding the temperature rise due to heat produced in a

heater coil. When a current proportional to that flowing in windings is passed in it to that or top

oil. For proper functioning or OTI & WTI it is essential to keep the thermometers pocket clean

and filled with oil.

OLTC or ON-LOAD/OFF LOAD TAP CHANGER

In a transformer

Vp/Vs=Np/Ns OR Vs=Vp/Np since Ns is constant

Vp=voltage in primary

Vs=voltage in secondary

Np= number of turns in primary

Ns= number of turns in secondary

As Ns is constant, Vs is proportional to Vp/Np. Whenever there is a decrease in Vp, Np

should also be increased in order to maintain Vs at constant levels. The action of

decrease/increase in Np whenever there is decrease/increase in Vp is done by the OLTC.

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In view of handling live currents, on load tap changers are provided in the H.V. side only

since current in H.V. side are much smaller in values of those of L.V. side. All off load tap

changers are provided on L.V. side considering economics of construction and operation since it

works at no load and no voltage conditions.

For the purpose as practiced at present the number of turns in H.V. is provided with

105% winding where 100% is the number of H.V turns derived for the required voltage class of

transformation.

Cooling of transformers

Heat is produced in the windings due to current flowing in the conductor (I-R) and in the

core on account of eddy current hysteresis losses. In oil immersed transformers heat is dissipated

by thermo-syphon system action. The oil serves as the medium for transferring the heat produced

inside the transformer to the outside atmosphere. Thermo-syphon action refers to the circulating

current set up in a liquid because of temperature difference between one part of the container and

other. When oil becomes hot it becomes lighter and therefore rises up, drawing in its wake colder

oil from below. This rising current of oil takes the heat away from the oil surfaces to the top of

the tank; from there it passes down the radiator tubes where the heat is radiated out into the

atmosphere. As the oil gets cooled it becomes heavier and sinks to the bottom.

As the size of the transformer becomes large, the rating of oil circulating by thermo-syphon

action becomes insufficient to dissipate all the heat produced and an artificial means of

increasing the circulation have to be adopted; namely forced oil circulation by electric pumps,

providing large radiators with forced air draft cooling by electric fans which are automatically

switched on and off depending upon the loading of transformer. In very large transformers

special coolers with water circulation may have to be employed.

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2.1.5 Dissolved gas analysis

Transformer undergoes electrical, chemical and thermal stresses during its service life

which may result in slow evolving incipient faults inside the transformer. The gases generated

under abnormal electrical or thermal stress are hydrogen, methane, ethane, ethylene, acetylene,

carbon monoxide, carbon dioxide, nitrogen and oxygen which get dissolved in oil. Collectively

these gases are known as FAULT gases, which are routinely detected and quantified at extremely

low level, typically in parts per million in dissolved gas analysis (DGA). Most commonly

method used to determine the content of these gases in oil is using a vacuum gas extraction

apparatus/head space sampler and gas chromatograph.

DGA is a powerful diagnostic technique for detection of slow evolving faults inside the

transformer by analyzing the gases generated during the fault which gets dissolved in the oil. For

DGA to be reliable, it is essential that sample taken for DGA should be representive of lot, no

dissolved gas to be lost during transportation and laboratory analysis be precise and accurate.

DGA can identify deterioration of insulation oil and hotspots, partial discharge, and

arcing. The health of oil is reflective of the health of the transformer itself. DGA analysis helps

the user to identity the reasons for gas formation and materials involved and indicate urgency of

corrective action to be taken.

.

Fault type Key gases

Arcing Acetylene, hydrogen

Corona Hydrogen

Over heated oil Ethylene, methane

Over heated cellulose Carbon monoxide and dioxide

Table 2.1.5.1 dissolved gas analysis

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2.1.6 Norms of protection for power transformer

Voltage ratio & capacity HV Side LV Side Common relays

i. 132/33/11KV upto 8

MVA

3 O/L relays + 1

E/L relay

2 O/L relays + 1

E/L relay

Buchholz, OLTC

Buchholz, OT, WT

ii. 132/33/11KV above

8 MVA and below

31.5 MVA

3 O/L relays + 1

dir. E/L relay

3 O/L relays + 1

E/L relay

Differential,

Buchholz, OLTC

Buchholz, OT, WT

iii. 132/33KV, 31.5 MVA

& above

3 O/L relays + 1

dir. E/L relay

3 O/L relays + 1

E/L relay

Differential, Overflux,

Buchholz, OLTC

PRV, OT, WT

iv. 220/33 KV, 31.5MVA

& 50MVA

220/132KV, 100

MVA

3 O/L relays + 1

dir. E/L relay

3 O/L relays + 1

dir. relay

Differential, Overflux,

Buchholz, OLTC

PRV, OT, WT

v. 400/220KV 315MVA 3 directional O/L

relays (with

dir.highset)

+1 directional E/L

relays. Restricted

E/F relay

+ 3 Directional

O/L relays for

3 directional O/L

relays (with

dir.highset)+1

directional E/L

relays. Restricted

E/F relay

Differential, Overflux,

Buchholz, OLTC

PRV, OT, WT and overload

(alarm) relay

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action

2.1.7 Instrument transformers introduction

Instruments transformer is a device used to the current and voltage in the primary system to

values suitable for the necessary instruments, meters, protective relays etc. they also serve the

purpose of isolating the primary system from the secondary system.

2.1.8 Current transformer and it’s specifications

A current transformer (CT) is a measurement device designed to provide a current in its

secondary coil proportional to the current flowing in its primary. Current transformer are

commonly used in metering and protective relaying in the electrical power industry where they

facilitate the safe measurement of large currents, often in the presence of high voltages. The

current transformer safely isolates measurements of large currents, often in the presence of high

voltages. The current transformer safely isolates measurement and control circuitry from the high

voltages typically present on the circuit being measured.

Rated primary current:

The value of current which is to be transformed to a lower value. In CT parlance, the load

of the CT refers to the primary current.

Rated secondary current:

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The current in the secondary circuit and on which the performance of the CT is based.

Typical values of secondary current are 1A or 5A. in the case of transformer differential

protection, secondary currents of 1/root 3A and 5/root 3 A are also specified.

Rated burden:

The apparent power of the secondary circuit in volt-amperes expressed at the rated

secondary current and at a specific power factor (0.8 for almost all standards)

Accuracy class:

In the case of metering CT’s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that

the errors have to be within the limits specified in the standards for that particular accuracy class.

The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and

100% of the rated burden at the specified power factor. In the case of protection CTs the CTs

should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as

well as composite error at the accuracy limit factor of the CT.

Fig 2.1.8.1 Current transformer

26

The rms value of the difference between the instantaneous primary current and the

instantaneous secondary current multiplied by the turns ratio, under steady state conditions.

Accuracy limit factor:

The value of primary current up to which the CT compiles with composite error

requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to

be within specified limits at 5, 10 or 15 times the rated primary current.

Short time rating:

The value of primary current that the CT should be able to withstand both thermally and

dynamically without damage to the windings, with the secondary circuit being short-circuited.

The specified time is usually 1 or 3 seconds.

Instrument security factor:

This typically takes a value of less than 5 or less than 10 though it could be much higher

if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error

of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This

means that heavy currents on the primary are not passed on to the secondary circuit and

instruments are therefore protected. In case of double ratio CT’s, FS is applicable for the lowest

ratio only.

Class PS/X CT

In balance systems of protection, CT’s with a high degree of similarity in their

characteristics are required. These requirements are met by Class PS (X) CT’s. Their

27

performance is defined in terms of a knee-point voltage, and the resistance of the CT secondary

winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio.

Knee point voltage:

That point on the magnetizing curve where an increase of 10% in the flux density

(voltage) causes an increase of 50% in the magnetizing force (current)

Name plate details

Company : W.S industry

Highest saturation

Voltage : 524kv

Basic insulation level : 460/1050kv

I th : 40/1kA/sec

I dyn : 10KAp

Oil weight : 360KG

Total weight : 1280Kg

Ratio : 800-600-400/1-1-1-1

Core number : 1 2 3 4 5

Rated primary current : 800A

Secondary current (A) : 1 1 1 1 1

O/p at 400/1(VA) : - - - - 30

Accuracy class : ps ps ps ps 0.5

ISF/ALF : - - - - ≤5

Turns ratio : 2/600-1200-800

Rct at 75 deg C at800/1 : 6 6 6 6 -

KPV at 800/1v : 2600 2600 720 720 -

28

LEXC at KPV at 800/1mA: 100 100 30 30

2.1.9 Current voltage transformer

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down

extra high voltage signals and provide low voltage signals either for measurement or to operate a

protective relay. In its most basic form the device consists of three parts: two capacitors across

which the voltage signal is split, an inductive element used to tune the device to the supply

Frequency and a transformer used to isolate and further step-down the voltage for the

Instrumentation or protective relay.

The device has at least four terminals, a high-voltage terminal for connection to the high voltage

signal, a ground terminal and at least one set of secondary terminals for connection to the

instrumentation or protective relay. CVTs are typically Single-phase devices used for measuring

voltages in excess of one hundred kilovolts where the use of voltage transformers would be

uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors

connected in series. This results in a large voltage drop across the stack of capacitors that

replaced the first capacitor and a comparatively small voltage drop across the second capacitor,

C2, and hence the secondary terminals.

Name plate details

Company :W.S industry

Manufacturaing : 1996

Weight : 665 kg

Total o/p simulteanious : 2560VA

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Max.output : 750VA at 50℃

Rated voltage A-N : 220/√ 3 kv

Highest saturation voltage : 245/√ 3 kv

Insulation level : 460/1050

Frequency : 50 hz

Nominal intermediate voltage A-N : 20/√ 3

Voltage factor : 1.2 cont. 1.5/30 sec

HF capacitance : 440 pf 10% - 5 %

Primary capacitance : 4840 pf 10% -5%

Secondary capacitance : 48400 pf 10% - 5%

Voltage ratio : 220/√3/¿110 /√ 3—110/√ 3

Winding : 1a-1n 2a1-2a2 -2n

Voltage : 100/√ 3 kV 110-100/√3kv

Burden : 150VA 100VA

Class : 0.5 3p

2.2 Energy meter

2.2.1 Introduction

30

Electricity meters operate by continuously measuring the instantaneous voltage (volts) and

current (amperes) and finding the product of these to give instantaneous electrical power (watts)

which is then integrated against time to give energy used (joules, kilowatt-hours etc.). Meters for

smaller services ( such as small residential customers) can be connected directly in-line between

source and customer. For larger loads, more than about 200 amps of load, current transformers

are used, so that the meter can be located other than in line with the service conductors. The

meters fall into two basic categories, electromechanical and electronic.

2.2.2 Procedure for energy meter testing

1 All the equipment is set up on a meter test bench capable of handling 10 No. meters at one

time.

2 Change over switches for power factor are provided in each phase.

3 For upf phase sequence in RYB. For 0.5 pf phase sequence is BRY.

4 The current coils of both R.B.S. meter and MUT, i.e. meter under test are connected in

series, and the potential coil in parallel.

5 Performance of meter should be checked up at full load upf, full load 0.5 pt lag, 1/10 full

load upf.

6 For long range meters, say 5.20 Amps, Ib i.e., basic current is 5A. and Imax i.e.

maximum current is 20 Amps.

7 The meter should start at 0.375% rated current for cyclometer type register and 0.25% of

rated Current for dial and pointer type registers.

8 For full load upf test, pressure coils are connected across rated supply voltage and rated

full load current at upto passed through current coils.

9 The position of brake magnet is adjusted (for full load-upf test) to bring the meter speed

within the required limit.

31

10 The permissible errors are +2%.

11 The required calculation for evaluating the error is as follows :

For a 3 Phase 10 Amp

Meter constant is 240 rev/KWh.

RSS constant is 100 rev

MUT constant/RSS = 240/100 =

i.e. for every revolutions of MUT disc, RSS should record 2.5 revolutions.

If this is so, then MUT has no error.

12 Revolutions in RSS can be read upto second decimal place.

13 There is a red mark on MUT’s disc over a small length while counting the revolutions

one has to select one and means where the beginning of the marking crosses the centre

point or tie end of the marking crosses the centre point. This is to be followed for

“CLICK ON” as well as “CLICK OFF”.

14 If suppose, corresponding to 6 revolutions of MUT (for 3 Phase, 10 Amps meter

mentioned in (11) RSS makes 2.4 revolutions.

Then % error = 2.5-2.4/2.5x100=4%

This means MUT is faster.

But RSS too has an error.

Let RSS have an error of + 0.5%

= Net error = +4+5 = +4.5%

32

15 If suppose, corresponding to 6 revolutions of above said MUT, RSS makes 2.65

revolutions.

Then % error = 2.5-2.65/2.5x100 = -6%

This means MUT is slower.

RSS error = +0.5%

= Net error = -6+0.5 = -5.5%

16 For full load, 0.5 pt lag, pressure coils are connected across rated supply voltage and

rated full load current is passed through current coils at 0.5 pt lag.

17 Full load, 0.5 pt lag test, adjustment is made by means of loop on potential core or

Resistance phase loop on current electromagnet with a clamp.

18 1/10 full load upf; rated supply voltage is applied across pressure coils and a very low

current (1/10 of full load current) is passed through the meter at upto.

19 Creep test: All meters are to be tested for non-registration with the voltage ckts alone

energized at 10% over voltage with current ckts open.

20 While conducting creep test, if the meter is found to be creeping then meter is to be

adjusted. After the adjustments are done, errors at 1/10 full load are to be rechecked.

21 The two adjustments generally provided to prevent creeping of meters are (i) an iron

wireon the shaft, near to the end of the potential core (ii) two diametrically opposite holes

inthe disc.

22 Dial test is performed after full load-upf, full load 0.5pt lag, 1/10 full load upf, creep test

are performed and meter adjusted to run within permissible limits of error.

23 During dial test, the load is to be light load at upf.

33

24 The initial reading of the meter during, dial test is usually 99997 or 99999 and the test is

Continued till 00000 is obtained.

25 By the dial test it is ensured that apart from the meter revolutions being accurate for

different loads and power factors, the drive is communicated to the cyclometer and that

the final registration by the meter with reference to load connected is within limits of

accuracy.

26 After conducting the tests discussed, potential links which were kept removed, should be

properly fixed.

27 Before proceeding with testing, it should be ensured that the disc rotates on all three

phases.

28 Sometimes it so happens that the disc does not rotate on all three phases then the lower

bearing is to be adjusted.

29 Sometimes it so happens that the disc rotates in two phases and does not rotate on the

third phase. Then the terminal connections of that particular phase and connected coil

should be checked.

30 If the disc stops rotating during the course of testing then lower

bearing should be adjusted and all the tests should be repeated.

2.2.3 Energy meter Testing: (132kv feeder , Jammikunta)

34

Make : secure company

E.M s. no : Gec05782

Type of E.M : E3M021

Class of accuracy : 0.2

CT ratio adopted : 1200/1

PT ratio adopted : 33kv/110v

E.M CT ratio adopted : 800/1

E.M PT ratio adopted : 33kv/110v

Error : 0.15 %

Evaluation of test results:

From above results the error is less than the permissible limit

(“+ 2%), thus energy meter is in good working condition

2.3 Auxiliary AC and DC power supply

Auxiliary of AC power supply

35

AC supply in substations in generally made available through station auxiliary transformer or

through tertiary windings, if available in substation .However tertiary winding should be used

only to meet substation load requirement .substation load requirement in case of emergency.

Auxiliary DC power supply

Dc supply in substations is required mainly for operation of protection system and circuit

breaker operation .These virtual functions in a substation and hence reliable DC system should

be provided .for 132 kv substation there is requirement of one battery set with charger of

adequate capacity should be provided as correct operation is to be ensured taking into

Consideration grid security .trip coils of circuit breakers of rating 220kv and above should be

necessarily wired to different sources to maintain reliable operation and faults clearance .line

protections also need to be wired from two different sources for redundancy

2.4 Battery room

2.4.1 Battery charger

Battery charger is intended to:

1) Keep the lead acid battery on trickle or boost charge as required.

2) Supply DC power to the plant load.

The battery chargers consists of one float charger, one boost charger and a DC

distribution board the float charger converts the AC mains to the required DC and supplies load

current and float charging of the battery .The float charger out put is connected to the DC

distribution board through output isolator switch. During normal operation the float charger

supplies load current through DC distribution board and simultaneously trickle charges the

36

battery through interlocking contactors .In case of power failure the battery bank starts supplying

the load automatically through interlocking contactors

The boost charger converts the 3phase AC supply to DC output and capable of boost

charging the battery up to max cell voltage of 2.3v/2.75v/cell.The charger output is connected to

the battery through output isolators switch .when boost charger is made on ,the float charger

output is automatically isolated from the battery .

37

38

The DC distribution board consists of isolator switches and fuses and connects the DC output to

various loads .The float charger ,boost charger and DC distribution board are mounted on a

single Indoor floor mounting free standings type sheet steel cubicle.

Technical specifications

220 V battery chargers

1) AC input 415 volts, 3phase, 50 hz

2) DC output

Float charger 220 v to 250v, 8 amps. Voltage can be varied by a

Potentiometer

Boost charger 198 to 297v, 16 amps .voltage can be varied by

Coarse &fine rotary switch

3) Regulation from no load to full load ±1.0%

4) Max error around the set point ±1%

5) Ripple 2% peak to peak

6) Efficiency 75%at full load

7) Metering moving coil voltmeter and ammeter for DC output

Side

8) Cooling natural air cooling .louvers provide for ventilation

9) Dimensions 1800 mm (H) X 600 MM(D) X 900 MM(W)

10) Weight 250 kg

11) Cable entry from the bottom cubicle

12) Temperature rise above ambient wound components--70℃ Diode ---80℃

SCR --80℃

39

2.4.2 Theory of operation of float charger

The float charger converts the AC mains to regulated DC and supplies the load current and float

charging requirement of the battery .The float charger output is connected to DC distribution

Board through output isolators switches (sw1).

It is a thyristor controlled power supply transformer in double wound 3 phase, natural air cooled.

Rectifier bridge is 3 phase ,full wave half controlled .Each device is mounted on extruded heat

sink for efficient cooling .Each device is protected from voltage surge suppression net work and

from over load with fast acting HRC/semiconductor fuses.

Float charger is basically designed for constant voltage opertion .Voltage can be set

between 220 to 250 V for 220V charger .Float chargers are provided with current limit .during

over load the charger output drop below set level, thereby allowing to take current surges .

Float charger is provided with input MCB, contactor with overload relay, LED indication

lamps ; AC voltmeter, DC voltmeter and DC Ammeter and auto manual switch .All

disconnecting switches are normally closed and are for isolation purposes only.

40

2.4.3 Theory of operation boost charger

The boost charger converts the input three phase AC supply to DC output and capable of

boost charging the battery up to a maximum cell voltage of 2.3/2.7v cell .the boost charger

output is connected to the battery through output isolator switch (SW3).when the boost charger is

made ON ,the float charger output and load is automatically isolated from the battery and

102nd/84th cell of the battery is connected to the distribution board .this provide a means to

supply continuous power upon a power failure during boost charging

The three phase AC input is applied through the MCB2 to the boost input contactor CON-2.

The boost charger transformer TX6 have primary taps which can be used as per the

requirement .The secondary has multiples with the four way rotary switches RS6& RS7 designed

as coarse and fine control for boost charging output .

The AC voltage is applied after CON -2 is routed to the primary of TX6 through ballast

chokes CH2 to ch4 ,PL4 to PL6 indicates the healthiness of AC supply to boost charger .TX6

steps down the voltage down to suitable level.

41

42

The secondary voltage of TX6 as selected by the coarse, and fine control switches is applied to

the three phase full wave bridge rectifier consisting of six numbers Diodes D5 to D10,each diode

is protected by a HRC fuse (F10 to F15) and R-C surge suppressors RC8 to RC13.the dc output

is protected by HRC fuse F16 and F17.ammeter A2 with external shunt SH2 measures the boost

charger output current and BR2 is the bleeder resistor .

Boost charger is also provided with blocking diode and DC contactor CON3 for tapped cell

connection during operation of float boost charger .

DC contactor is inter locked with the input contactor, such that, when boost charger is switch on

DC contactor CON3 is demagnetized and battery will b connected to load circuit through diode

D12 maintain required voltage across load during boost charging.

Only the boost charger is provided with a contactor CON3 which is inter locked electrically with

its input contactor CON2.the DC contactor gets energized only when the boost charger is OFF

i.e.CON1 is OFF in which case full battery cell are floated across the float charger. When the

boost charger is switched ON, voltage from the tap cell gets connected to the float charger

through diode D12 tap cell diode D12 is protected through fuse F19.

43

CHAPTER-3

BUSBAR CONNECTIONS

3.1 Introduction

Various factors affect the reliability of a substation or switchyard, one of which is the

arrangement of the buses and switching devices. In addition to reliability, arrangement of the

buses/switching devices will impact maintenance, protection, initial substation development, and

cost. There are six types of substation bus/switching arrangements commonly used in air

insulated substations:

1. Single bus

2. Double bus, double breaker

3. Main and transfer (inspection) bus

4. Double bus, single breaker

5. Ring bus

6. Breaker and a half

3.2 Single Bus

This arrangement involves one main bus with all circuits connected directly to the bus.

The reliability of this type of an arrangement is very low. When properly protected by relaying, a

single failure to the main bus or any circuit section between its circuit breaker and the main bus

will cause an outage of the entire system. In addition, maintenance of devices on this system

requires the de-energizing of the line connected to the device. Maintenance of the bus would

require the outage of the total system, use of standby generation, or switching to adjacent station,

if available. Since the single bus arrangement is low in reliability, it is not recommended for

heavily loaded substations or substations having a high availability requirement. Reliability of

this arrangement can be proved by the addition of a bus tiebreaker to minimize the effect of a

main bus failure.

44

Fig 3.1 single bus connection

3.3 Double Bus, Double Breaker

This scheme provides a very high level of reliability by having two separate breakers available to

each circuit. In addition, with two separate buses, failure of a single bus will not impact either

line. Maintenance of a bus or a circuit breaker in this arrangement can be accomplished without

interrupting either of the circuits. This arrangement allows various operating options as

additional lines are added to the arrangement; loading on the system can be shifted by connecting

lines to only one bus. A area for the substation to accommodate the additional equipment. This is

especially double bus; double breaker scheme is a high-cost arrangement, since each line has two

breakers and requires a larger true in a low profile configuration. The protection scheme is also

more involved than a single bus scheme.

Fig 3.2 double bus and double breaker connection

45

3.4 Main and Transfer Bus

This scheme is arranged with all circuits connected between a main (operating) bus and a

transfer bus (also referred to as an inspection bus). Some arrangements include a bus tie breaker

that is connected between both buses with no circuits connected to it. Since all circuits are

connected to the single, main bus, reliability of this system is not very high. However, with the

transfer bus available during maintenance, de-energizing of the circuit can be avoided. Some

systems are operated with the transfer bus normally de-energized.

When maintenance work is necessary, the transfer bus is energized by either closing the

tie breaker, or when a tie breaker is not installed, closing the switches connected to the transfer

bus. With these switches closed, the breaker to be maintained can be opened along with its

isolation switches. Then the breaker is taken out of service. The circuit breaker remaining in

service will now be connected to both circuits through the transfer bus. This way, both circuits

remain energized during maintenance. Since each circuit may have a different circuit

configuration, special relay settings may be used when operating in this abnormal arrangement.

When a bus tie breaker is present, the bus tie breaker is the breaker used to replace the breaker

being maintained, and the other breaker is not connected to the transfer bus. A shortcoming of

this scheme is that if the main bus is taken out of service, even though the circuits can remain

energized through the transfer bus and its associated switches, there would be no relay protection

for the circuits. Depending on the system arrangement, this concern can be minimized through

the use of circuit protection devices (reclosure or fuses) on the lines outside the substation. This

arrangement is slightly more expensive than the single bus arrangement, but does provide more

flexibility during maintenance. Protection of this scheme is similar to that of the single bus

arrangement. The area required for a low profile substation with a main and transfer bus scheme

is also greater than that of the single bus, due to the additional switches and bus.

46

Fig 3.3 main and transfer bus connection

3.5 Double Bus, Single Breaker

This scheme has two main buses connected to each line circuit breaker and a bus tie breaker.

Utilizing the bus tie breaker in the closed position allows the transfer of line circuits from bus to

bus by means of the switches. This arrangement allows the operation of the circuits from either

bus. In this arrangement, a failure on one bus will not affect the other bus. However, a bus tie

breaker failure will cause the outage of the entire system. Operating the bus tie breaker in the

normally open position defeats the advantages of the two main buses. It arranges the system into

two single bus systems, which as described previously, has very low reliability. Relay protection

for this scheme can be complex, depending on the system requirements, flexibility, and needs.

With two buses and a bus tie available, there is some ease in doing maintenance, but

maintenance on line breakers and switches would still require outside the substation switching to

avoid outages

.

47

Fig 3.4 double bus with single breaker connection

3.6 Ring Bus

In this scheme, as indicated by the name, all breakers are arranged in a ring with circuits

tapped between breakers. For a failure on a circuit, the two adjacent breakers will trip without

affecting the rest of the system. Similarly, a single bus failure will only affect the adjacent

breakers and allow the rest of the system to remain energized. However, a breaker failure or

breakers that fail to trip will require adjacent breakers to be tripped to isolate the fault.

Maintenance on a circuit breaker in this scheme can be accomplished without interrupting any

circuit, including the two circuits adjacent to the breaker being maintained. The breaker to be

maintained is taken out of service by tripping the breaker, then opening its isolation switches.

Since the other breakers adjacent to the breaker being maintained are in service, they will

continue to supply the circuits. In order to gain the highest reliability with a ring bus scheme,

load and source circuits should be alternated when connecting to the scheme. Arranging the

scheme in this manner will minimize the potential for the loss of the supply to the ring bus due to

a breaker failure. Relaying is more complex in this scheme than some previously identified.

Since there is only one bus in this scheme, the area required to develop this scheme is less than

some of the previously discussed schemes. However, expansion of a ring bus is limited, due to

the practical arrangement of circuits.

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Fig 3.5 ring bus

3.7 Breaker-and-a-Half

The breaker-and-a-half scheme can be developed from a ring bus arrangement as the number of

circuit’s increases. In this scheme, each circuit is between two circuit breakers, and there are two

main buses. The failure of a circuit will trip the two adjacent breakers and not interrupt any other

circuit. With the three breaker arrangement for each bay, a center breaker failure will cause the

loss of the two adjacent circuits. However, a breaker failure of the breaker adjacent to the bus

will only interrupt one circuit. Maintenance of a breaker on this scheme can be performed

without an outage to any circuit. Furthermore, either bus can be taken out of service with no

interruption to the service. This is one of the most reliable arrangements, and it can continue to

be expanded as required. Relaying is more involved than some schemes previously discussed.

This scheme will require more area and is costly due to the additional components.

49

Fig 3.6 breaker-and- a half bus connection

50

CHAPTER-4

PROTECTION EQUIPMENT USED IN SUBSTATION

4.1 circuit breakers

4.1.1 Introduction

Circuit breakers are mechanical devices designed to close or open contact members, thus

closing or opening of an electrical circuit under normal abnormal conditions.

The circuit breakers are used to break the circuit if any fault occurs in any of the

instrument these circuit breaker breaks for a fault which can damage other instrument in the

station. For any unwanted fault over the station we need to break the line current. This is only

done

Automatically by the circuit breaker. There are mainly two types of circuit breakers used

for anySubstations. They are (a) SF6 circuit breakers; (b) spring circuit breakers. There

are six types of circuit breakers depending up on the arc quenching media

1. water circuit breakers

2. oil circuit breakers

3. air blast circuit breakers

4. SF6 circuit breakers

5. Air circuit breakers

6. Vacuum circuit breakers

The use of SF6 circuit breaker is mainly in the substations which are having high input kv

input, say above 220kv and more. The gas is put inside the circuit breaker by force i.e. under

high pressure. When if the gas gets decreases there is a motor connected to the circuit breaker.

The motor starts operating if the gas went lower than 20.8 bar. There is a meter connected to the

breaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gas

to reduce the torque produce in it due to any fault in the line. The circuit breaker has a direct link

with the instruments in the station, when any fault occur alarm bell ring.

51

The spring type of circuit breakers is used for small kv stations. The spring here reduces the

torque produced so that the breaker can function again. The spring type is used for step down

side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution

side.

4.1.2 Operation of the circuit breaker

All circuit breakers have common features in their operation, although details vary

substantially depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is

usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are

usually arranged with pilot devices to sense a fault current and to operate the trip opening

mechanism. The trip solenoid that releases the latch is usually energized by a separate battery,

although some high-voltage circuit breakers are self-contained with current transformers,

protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the

circuit; some mechanically-stored energy (using something such as springs or compressed air)

contained within the breaker is used to separate the contacts, although some of the energy

required may be obtained from the fault current itself. Small circuit breakers may be manually

operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy

to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and

must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made

of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is

limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are

52

usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit

breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled,

and extinguished in a controlled way, so that the gap between the contacts can again withstand

the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the

medium in which the arc forms.

4.1.3 Different techniques are used to extinguish the arc

Lengthening of the arc

Intensive cooling (in jet chambers)

Division into partial arcs

Zero point quenching (Contacts open at the zero current time crossing of the AC waveform,

effectively breaking no load current at the time of opening. The zero crossing occures at

twice the line frequency i.e. 100 times per second for 50Hz ac and 120 times per second for

60Hz ac )

Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore

power to the interrupted circuit.

4.1.4 Arc interruption of the circuit breaker

53

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings

will have metal plates or non-metallic arc chutes to divide and cool the arc magnetic

blowout coils deflect the arc into the arc chute. In larger ratings, oil circuit breakers rely upon

vaporization of some of the oil to blast a jet of oil through the arc.

Gas (usually SF6) circuit breakers sometimes stretch the arc using a magnetic field, and

then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact

material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum

circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the

contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus

blowing out the arc. Circuit breakers are usually able to terminate all current very quickly:

typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been

tripped, depending upon age and construction of the device.

4.1.5 Air-blast circuit breakers

54

Air is used as insulator in outdoor-type substations and for high-voltage transmission

lines. Air can also be used as extinguishing medium for current interruption. At atmospheric

pressure, the interrupting capability, however, is limited to low voltage and medium voltage

only. For medium voltage applications up to 50 kV, the breakers are mainly of the magnetic air-

blast type in which the arc is blown into a segmented compartment by the magnetic field

generated by the fault current. In this way, the arc length, the arc voltage, and the surface of the

arc column are increased. The arc voltage decreases the fault current, and the larger arc column

surface improves the cooling of the arc channel. At higher pressure, air has much more cooling

power, and air-blast breakers operating with compressed air can interrupt higher currents at

considerable higher-voltage levels.

Air-blast breakers using compressed air can be of the axial-blast or the cross-blast type.

The cross-blast type air blast breaker operates similar to the magnetic-type breaker: compressed

air blows the arc into a segmented arc-chute compartment. Because the arc voltage increases

with the arc length, this is also called high-resistance interruption; it has the disadvantage that the

energy dissipated during the interruption process is rather high. In the axial-blast design, the arc

is cooled in axial direction by the airflow. The current is interrupted when the ionization level is

brought down around current zero. Because the arc voltage hardly increases this is called low-

resistance interruption. When operating, air-blast breakers make a lot of noise, especially when

the arc is cooled in the free air, as is the case with AEG’s free-jet breaker (Freistrahlschalter)

design.

4.1.6 SF6 circuit breakers

The superior dielectric properties of SF6 were discovered as early as 1920. It lasted until

the 1940s before the first development of SF6 circuit breakers began, but it took till 1959 before

the first SF6 circuit breaker came to the market. These early designs were descendants of the

axial-blast and SF6 circuit breakers. Air-blast circuit breakers, the contacts were mounted inside

a tank filled with SF6 gas, and during the current interruption process, the arc was cooled by

55

compressed SF6 gas from a separate reservoir. The liquefying temperature of SF6 gas depends

on the pressure but lies in the range of the ambient temperature of the breaker. This means that

the SF6 reservoir should be equipped with a heating element that introduces an extra failure

possibility for the circuit breaker; when the heating element does not work, the breaker cannot

operate. Therefore the puffer circuit breaker was developed and the so-called double pressure

breaker disappeared from the market. In the puffer circuit breaker the opening stroke made by

the moving contacts moves a piston, compressing the gas in the puffer chamber and thus causing

an axial gas flow along the arc channel. The nozzle must be able to withstand the high

temperatures without deterioration and is made from Teflon. Presently, the SF6 puffer circuit

breaker is the breaker type used for the interruption of the highest short-circuits powers, up to

550 kV–63 kA per Interrupter made by Toshiba. Puffer circuit breakers require a rather strong

operating mechanism because the SF6 gas has to be compressed. When interrupting large

currents, for instance, in the case of a three-phase fault, the opening speed of the circuit breaker

has a tendency to slow down by the thermally generated pressure, and the mechanism (often

hydraulic or spring mechanisms) should have enough energy to keep the contacts moving apart.

Strong and reliable operating mechanisms are costly and form a substantial part of the price of a

breaker. For the lower-voltage range, self-blast circuit breakers are now on the market. Self-blast

breakers use the thermal energy released by the arc to heat the gas and to increase its pressure.

After the circuit breaker moving contacts are out of the arcing chamber, the heated gas is

released along the arc to cool it down. The interruption of small currents can be critical because

the developed arcing energy is in that case modest, and sometimes a small puffer is added to

assist in the interrupting process. In other designs, a coil carrying the current to be interrupted

creates magnetic field, which provides a driving force that rotates the arc around the contacts and

thus provides additional cooling. This design is called the rotating-arc circuit breaker. Both self-

blast breakers and rotating arc breakers can be designed with less powerful (and therefore

cheaper) mechanisms and are of a more compact design than puffer breakers.

56

4.1.7 Vacuum circuit breakers

Between the contacts of a vacuum circuit breaker a vacuum arc takes care of the

interruption process. As already discussed in the introduction to this chapter about the switching

arc, the vacuum arc differs from the high-pressure arc because the arc burns in vacuum in the

absence of an extinguishing medium. The behavior of the physical processes in the arc column of

a vacuum arc is to be understood as a metal surface phenomenon rather than a phenomenon in an

insulating medium. The first experiments with vacuum interrupters took place already in 1926,

but it lasted until the 1960s when metallurgical developments made it possible to manufacture

gas-free electrodes and when the fire.

4.1.8 Advantages SF6 circuit breaker

Due to the superior arc quenching properties of SF6 gas, it has many advantages over the

oil circuit breakers

1) circuit breakers have very short arcing time

2) Since dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can

interrupt much larger currents.

3) There are no carbon deposits so that tracking and insulation problems are eliminated.

4) There is no risk of fire in such breakers because SF6 gas is inflammable.

5) The SF6 breakers have low maintenance cost, light foundation requirements and

minimum auxiliary equipments.

6) The closed gas enclosure keeps the interior dry so that there is no moisture problem.

7) The SF6 circuit breakers give noiseless operation due to its closed gas circuit and no

exhaust to atmosphere unlike the air blast circuit breaker.

57

4.2 Lightening arrester

These lightening arrestors can resist or ground the lightening if falls on the incoming

feeders. The lightening arrestors can work in a angle of 30°around them. They are mostly used

for protection of the instruments used in the substation. As the cost of the instrument in the

station are very high to protect them from high voltage from lightening these lightening arrestors

are used.

.

It is a device used on electrical power systems to protect the insulation on the system from the

Damaging effect of lightning. Metal oxide varistors (MOVs) have been used for power system

Protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a

High voltage terminal and a ground terminal. When a lightning surge or switching surge travels

Down the power system to the arrester, the current from the surge is diverted around the

protected Insulation in most cases to earth.

Landscape suited for purpose of explanation: (1) Represents Lord Kelvin's "reduced" area of

The region, (2) Surface concentric with the Earth such that the quantities stored over it and under

It are equal; (3) Building on a site of excessive electrostatic charge density; (4) Building on a site

of low electrostatic charge density.

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In telegraphy and telephony, a lightning arrester is placed where wires enter a structure,

Preventing damage to electronic instruments within and ensuring the safety of individuals near

Them. Lightning arresters, also called surge protectors, are devices that are connected between

Each electrical conductor in a power and communications systems and the Earth. These provide

Short circuit to the ground that is interrupted by a non-conductor, over which lightning jumps. Its

Purpose is to limit the rise in voltage when a communications or power line is struck by

Lightning.

The non-conducting material may consist of a semi-conducting material such as silicon carbide

Or zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simply open to the air,

But more modern varieties are filled with dry gas and have a small amount of radioactive

Material to encourage the gas to ionize when the voltage across the gap reaches a specified level.

Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow

Lamp) connected between the protected conductor and ground, or myriad voltage-activated solid

state switches called varistors or MOVs. Lightning arresters built for substation use are

Impressive devices, consisting of porcelain tube several feet long and several inches in

Diameter, filled with disks of zinc oxide. A safety port on the side of the device vents the

Occasional internal explosion without shattering the porcelain cylinder

Name plate details

Company : W.S industries

Rated voltage : 20kv (rms)

Long duration discharge class : 3

Frequency : 50Hz

Surge monitor type : CRM-SMX

Style : SMX

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Type : zodiver

MCOV : 120 kV

Normal discharge current : 10 kA

Pressure relief current : 40 kA

Y.O.M : 1990

4.3 Wave trap

A device used to exclude unwanted frequency components, such as noise or other

interference, of a wave. A device used to exclude unwanted frequency components, such as noise

or other interference, of a wave. Wave trap is an instrument using for tripping of the wave. The

function of this trap is that it traps the unwanted waves. Its function is of trapping wave. Its shape

is like a drum. It is connected to the main incoming feeder so that it can trap the waves which

may be dangerous to the Instruments here in the substation

4.4 Protective relays

4.4.1 Introduction

It is to cause a prompt removal from service of any element of a power system when it

suffers a short circuit or when it starts to operate in any abnormal manner that might cause

damage or otherwise interfere with the effective operation of the rest of the system. The relaying

equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty

element when they are called upon to do by the relaying equipment.

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Fig 4.4.1 basic connection of a protective relay

4.4.2 The functional requirement of the relay

i) Reliability: The most important requisite of protective relay is reliability since they supervise

the circuit for a long time before a fault occurs; if a fault then occurs, the relays

must respond instantly and correctly.

ii) Selectivity: The relay must be able to discriminate (select) between those conditions for which

prompt operation is required and those for which no operation, or time delayed

operation is required.

iii) Sensitivity: The relaying equipment must be sufficiently sensitive so that it operates reliably

when required under the actual conditions that produces least operating tendency.

iv) Speed: The relay must operate at the required speed. It should neither be too slow which

may result in damage to the equipment nor should it be too fast which may result

in undesired operation.

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The general arrangement of protective zones

Fig 4.4.2 different types of protective zones

4.4.3 Distance relaying principle

A distance relay compares the currents and voltages at the relaying point with Current

providing the operating torque and the voltage provides the restraining torque. In other words an

impedance relay is a voltage restrained over current relay.

The equation at the balance point in a simple impedance relay is K1V2 = K2I2 or V/I = K3

where K1, K2 and K3 are constants. In other words, the relay is on the verge of operation at a

constant value of V/I ratio, which may be expressed as an impedance.

4.4.4 Types of distance relays

(1) Impedance relay

(2) Reactance relay

(3) Mho relay

(4) Modified impedance relay

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(1) Impedance relay:

Characteristics of an impedance relay on R-X diagram is shown in fig

Fig 4.4.3 impedance relay

The numerical value of the ratio of V to I is shown as the length of the radius vector, such as Z and

the phase angle between V and I determines the position of the vector, as shown.

Operation of the impedance relay is practical or actually independent of the phase angle

between V and I. The operating characteristic is a circle with its center at the origin, and hence the

relay is non-directional.

Characteristic of Directional Impedance Relay:

Characteristic of a directional impedance relay in the complex R-X phase is shown in fig.

Fig 4.4.4 directional impedance relay

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Along the line impedance locus line, the positive sequence impedance of the protected line as

seen by the relay between its location and different points along the protected line can be plotted.

The directional unit of the relay causes separation of the regions of the relay characteristic shown

in the figure by a line drawn perpendicular to the line impedance locus. The net result is that

tripping will occur only for points that are both within the circles and above the directional unit

characteristic.

(2) The Reactance-type Distance Relay

Reactance relay measures V/I Sin Ø (i.e. Z sin Ø). Whenever the reactance measured by the

relay is less than the set value, the relay operates. The operating characteristic on R-X diagram

is indicated below:

Fig 4.4.5 reactance relay

The resistance component of impedance has no effect on the operation of reactance relay, the

relay responds solely to reactance component of impedance. This relay is inherently non-

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directional. The relay is most suitable to detect earth faults where the effect of arc resistance may

render other types of relays to detect faults with difficulty.

(3) Mho relay

This is a directional impedance relay, also known as admittance relay. Its characteristic on R-X

diagram is a circle whose circumference passes through the origin as illustrated in figure

showing that the relay is inherently directional and it only operates for faults in the forward

direction.

Fig 4.4.6 mho relay

(4) Modified impedance relay

Also known as offset Mho relay whose characteristic encloses the origin on R-X diagram as

indicated below:

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Fig 4.4.7 offset mho relay

This offset mho relay has three main applications: -

i) Bus bar zone backup

ii) Carrier starting unit in distance/carrier blocking schemes.

iii) Power Swing blocking.

4.4.5 Application of distance relaying

Relay Setting:

Since the distance relays are fed from the secondaries of line CTs and bus PTs/line CVTs, the

line parameters are to be converted into secondary values to set the relay as per requirements.

Zsecy = Zpri/Impedance ratio

(where Impedance ratio = P.T.Ratio/C.T.Ratio)

It is to be noted that C.T Ratios (and P.T Ratios) and relay settings are inter-related. Hence any

changes in C.T .ratio have to be effected along with revision of relay settings only.

For the lines, the impedance in Ohms per KM is approximately as under:

KV Z1 (=Z2) Line Angle

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132 KV 0.4 60 to 70 Deg

220 KV 0.4 70 to 80 Deg

400 KV 0.3 80 to 85 Deg

A distance relay is either 3 zones or 4 zones to provide protection.

To ensure proper coordination between distance relays in power system, it is customary to

choose relay ohmic setting as follows: -

S.No. Zones Reactance Time

1. Zone-1 80% of ZL Instantaneous

(no intentional

time delay)

2. Zone-2 100% of ZL + 40-50% of ZSL 0.3 to 0.4 seconds

3. Zone-3 100% of ZL + 120% of ZSL 0.6 to 0.8 seconds

4. Zone-4 100% of ZL + 120% of ZLL 0.9 to 1.5 seconds

Where ZL = Positive sequence impedance of line to be protected.

ZSL = Positive sequence impedance of adjacent shortest line.

ZLL = Positive sequence impedance of adjacent longest line.

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Note:

i) Where a three zone relay only is available, the zone 3 will be set to cover the adjacent

longest line.

ii) The zonal timings will be carefully selected to properly grade with the relays on

adjoining sections

4.5 Substation earthing

The substation grounding system is an essential part of the overall electrical system. The proper

grounding of a substation is important for the following two reasons:

1. It provides a means of dissipating electric current into the earth without exceeding the

operating limits of the equipment.

2. It provides a safe environment to protect personnel in the vicinity of grounded facilities from

the dangers of electric shock under fault conditions

The grounding system includes all of the interconnected grounding facilities in the

substation area, including the ground grid, overhead ground wires, neutral conductors,

underground cables, foundations, deep well, etc. The ground grid consists of horizontal

interconnected bare conductors (mat) and ground rods. The design of the ground grid to control

voltage levels to safe values should consider the total grounding system to provide a safe system

at an economical cost.

Modern substation earthing system has buried horizontal mesh of steel rods and vertical

elctr5odes welded to the mesh further the vertical rises and the galvanizes steel grounding strips

or copper bars etc are connected between the grounding mesh and the points to be grounded.

The conventional criterion of “low earth resistance” and low current earth resistance

measurement continues tomb in practice for substation and power stations up to and including

220kV.

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The following information is mainly concerned with personnel safety. The information

regarding the grounding system resistance, grid current, and ground potential rise can also be

used to determine if the operating limits of the equipment will be exceeded.

Safe grounding requires the interaction of two grounding systems:

1. The intentional ground, consisting of grounding systems buried at some depth below the

earth’s surface

2. The accidental ground, temporarily established by a person exposed to a potential gradient in

the vicinity of a grounded facility

It is often assumed that any grounded object can be safely touched. A low substation

ground resistances not, in itself, a guarantee of safety. There is no simple relation between the

resistance of the grounding system as a whole and the maximum shock current to which a person

might be exposed. A substation with relatively low ground resistance might be dangerous, while

another substation with very high ground resistance might be safe or could be made safe by

careful design.

The substation earthing is provided for the fallowing reasons.

Safety of operational and maintenance staff

Discharge electrical charges to ground

Grounding of overhead shielding wires

Electro-magnetic interferences

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CHAPTER-5 SUMMARY AND CONCLUSSIONS

5.1 Summary

Transmission and distribution stations exist at various scales throughout a power system.

In general, they represent an interface between different levels or sections of the power system,

with the capability to switch or reconfigure the connections among various transmission and

distribution lines. On the largest scale, a transmission substation would be the meeting place for

different high-voltage transmission circuits. At the intermediate scale, a large distribution station

would receive high-voltage transmission on one side and provide power to a set of primary

distribution circuits. Depending on the territory, the number of circuits may vary from just a few

to a dozen or so.

The major stations include a control room from which operations are coordinated.

Smaller distribution substations follow the same principle of receiving power at higher voltage

on one side and sending out a number of distribution feeders at lower voltage on the other, but

they serve a more limited local area and are generally unstaffed. The central omponent of the

substation is the transformer, as it provides the effective in enface between the high- and low-

voltage parts of the system. Other crucial components are circuit breakers and switches. Breakers

serve as protective devices that open automatically in the event of a fault, that is, when a

protective relay indicates excessive current due to some abnormal condition. Switches are

control devices that can be opened or closed deliberately to establish or break a connection. An

important difference between circuit breakers and switches is that breakers are designed to

interrupt abnormally high currents (as they occur only in those very situations for which circuit

protection is needed), whereas regular switches are designed to be operable under normal

currents. Breakers are placed on both the high- and low-voltage side of transformers. Finally,

substations may also include capacitor banks to provide voltage support.

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5.2 conclusions

% Impedance volts: IT is the percentage of HV volts required to create full load

amperes in a shorted LV winding .It is also indicative of percentage of LV volts to

b applied so that full load currents flow in a shorted HV winding.

Max. Ambient temperature: 50degree C. This means the transformer is designed

to work in good condition when the surrounding temperature of the transformers

increase up to a maximum of 50℃

Winding temperature rise 50℃ : this means that the winding temperature can go

up to 55℃ over and above the atmospheric temperature

Insulation levels: for the safety point of view insulation will b provided for more

than the operating voltages of the transformer.

Major insulation: between primary and secondary phase to phase and inter coil to

core, this is achieved by Bakelite, wooden blocks, cellulosic paper cylinder.

Transformer oil: this is derivate of petroleum crude. This has a good dielectric

strength and improves the dielectric strength of transformer when filled under

vacuum by displacing air from all cavities .this is also a good cooling medium and

absorb heat from the windings in adequate movement of cooling medium.

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Thus mineral oil has a flash point of 140deg c and 160dec c five point. This also

can sustain the combusting with its own energy, one its catches fire. Thus this is

unsuitable for the transformer

The indoor transformers are filled with a synthetic liquid know as silicate liquid.

This is fire assistant and has flash point well above 300oc.as hauls

polychlorinated biphenyls, which were quite popular with indoor transformers

earlier have since been banned all over the world due to bioaccumulation

properties.

The insulation resistances values of the circuit breaker are in the order of gaga

ohms then it is suitable for the desired operation.

The lightening arrester discharges the high voltages to ground through nonlinear

resistance. How many times surges are diverted to the ground is displayed on the

screen. Lightening arrester having two regions, they are green and red. if needle

comes under the green it is working else damaged in red region.

In substations the energy meter testing is performed quart alley, the permissible

amount of error is ± 2%. if it is exceeds the limit ,instead of old one replaced by

new one.

Wave trap blocks the high frequency carrier waves are in the range of 24KHz to

50KHz, let power waves passes (50KHz -60KHz) through it.

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REFERENCES

Operation manual of MRT, APTRANSCO “relays principle, working and applications of distances

relays”

Hand book of MRT engineers, APTRANSCO “SF6 circuit breakers description”

Power system by J.B Gupta, S.K katria& sons publications, Tenth Edition “substation

earthing”

Electric power substations engineer by John D.Mcnonald “bus bar arrangements and

its related matter”

Principles of power system by V.K Mehta& Rohit Mehta, first multicolor edition,

“substation introduction and its classifications”

www.wikipedia.com “ circuit breakers, energy meter and some description about

transformers”

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