A

MINI PROJECT REPORT

ON

OPERATION AND MAINTENANCE OF 220/132KV

SUBSTATION

Submitted in partial fulfillment for the award of the Degree of

Bachelor of Technology in Electrical and Electronics Engineering

Submitted By

V.RAVALIKA (08281A0212)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE

(Affiliated to J.N.T.U, Hyderabad)

SINGAPUR, KARIMNAGAR -505468

(2008-2012)

ABSTRACT

A Substation receives electrical power from generating station via incoming

transmission line and delivers electrical power through feeders and this is used for

controlling the power on different routes. Substations are integral part of a power

system and form important part of transmission and distribution network of electrical

power system.

Their main functions are to receive energy transmitted at high voltage from

the generating stations, reduce the voltage to a value appropriate for local distribution

and provide facilities for switching some sub-station are simply switching stations

different connections between various transmission lines are made, others are

converting sub-stations which either convert AC into DC or vice-versa or convert

frequency from higher to lower or vice-versa.

The various circuits are joined together through these components to a bus-bar

at substation. Basically, Sub-station consists of power transformers, circuit breakers,

relays, isolators, earthing switches, current transformers, voltage transformers,

synchronous condensers/ Capacitor banks etc.

This mini project covers the important equipments & their function in a Sub-

Station. And also an attempt is made to cover the general maintenance of Substation

and Checks the observations to be made by Shift Engineer. As a part of case study we

are going to visit a 220/132Kv TRANSCO substation in Warangal.

i

CONTENTS

Chapter No TITLE Page no.

List of Abbreviations iii

List of Symbols iv

List of Figures v

List of Tables vi

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Construction of a substation 1

2 CLASSIFICATION OF SUBSTATIONS 3

2.1 According to the requirement 3

2.2 According to the constructional features 4

3 SINGLE LINE DIAGRAM 6

3.1 Feeder Circuit 6

3.2 Transformer Circuit 6

3.3 Auxiliary supply 7

4 BRIEF DISCRIPTION OF INSTRUMENTS IN THE

SUBSTATION 8

4.1 Lightening Arrestors 8

4.2 Earthing 12

4.3 Capacitor Voltage Transformer 13

4.4 Wave Trap 15

4.5 Isolator with ES (Earth Switches) 16

4.6 Instrument Transformers 17

4.7 Circuit Breakers 26

ii

Chapter No TITLE Page no.

4

4.8 Bus 31

4.9 Transformers 31

4.10 Capacitor Bank attached to the bus 35

5 TYPES OF CONTROL 37

5.1 Capacitors 38

5.2 Bus bar systems 38

5.3 Station battery 38

5.4 Insulators 40

6 PROTECTION FOR VARIOUS EQUIPMENTS 43

6.1 Transformer 43

6.2 Feeder 43

7 220/132KV SUBSTATION AT WARANGAL 44

7.1 Substation at Warangal 45

7.2 Salient Features of 220/132KV Substation 46

7.3 Important points to be kept in view while 48

laying out the substation

8 CONCLUSION 50

REFERENCES 51

iii

LIST OF ABBREVIATIONS

EHV–Extra high voltage

SLD– Single line diagram

PT – Potential transformer

CT – Current transformer

HVCT- High voltage CT

LVCT – Low voltage CT

CVT – Capacitor voltage transformer

LA – Lightening arrestors

ES - Earth switches

CB – Circuit breaker

HV side – High voltage side

LV side – Low voltage side

PLCC - Power Line Carrier Communication

OLTC – On load tap changer

HG Fuse - -Horn gap fuse

OTI – Oil temperature indicator

WTI – Winding temperature indicator

IDMT Characteristics – Inverse definite minimum time characteristics

iv

LIST OF SYMBOLS

X0 – Zero sequence reactance

X1- Positive sequence reactance

R0- Zero sequence resistance

Ip – Primary current

Np – Primary Winding Turns

Is – Secondary Current

Ns – Secondary Winding Turns

Vp – Primary voltage

Vs – Secondary voltage

Zs – Impedance attached at the secondary side coil

v

LIST OF FIGURES

Fig No Title Page No.

2.1 Construction of the substation 5

3.1 Single line dig of a 220/132kv substation 7

4.1(i) Surge diverter 9

4.1(ii) Characteristics of Non linear resistor 9

4.2 Lightening arrestors 9

4.3.1 Circuit diagram of CVT 14

4.3.2 Capacitor voltage transformer 14

4.4.1 Wave trap 15

4.5.1 Isolator with earth switch 17

4.6.1 Line diagram of CT 19

4.6.2.1 Line diagram of VT 24

4.6.2.2 Potential transformer 25

4.7.1 SF6 Circuit breaker 29

4.9.1.1 Electrical transformer 31

4.9.1.2 Ideal transformer 32

4.9.1.3 Mutual induction 33

4.9.3 Three phase 100MVA Auto transformer 34

4.10.1 Capacitor bank in the Distribution system 35

4.10.2 Reactive Losses 36

5.1 Types of control 37

5.3 Station Batteries 41

5.4 Ball and socket type Disc insulator 43

vi

LIST OF TABLES

Table No. Title Page No

4.1.3 LA voltage rating 11

4.1.4 The limits of LA and Transformers 11

4.6.1.5 The specifications of HVCT 22

4.6.1.6 The specifications of HVCT 23

5.4 Insulators 42

1

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION :

The present-day electrical power system is A.C. i.e. electric power is

generated, transmitted and distributed in the form of alternating current. The electric

power is produced at the power stations which are located at favourable places,

generally quite away from the consumers. It is delivered to the consumers through a

large network of transmission and distribution. At many places in the line of the

power system, it may be desirable and necessary to change some characteristic (e.g.

voltage, A.C. to D.C., frequency, Power factor etc.) of electric supply.

This is accomplished by suitable apparatus called sub-station. For example,

generation voltage (11KV or 6.6KV) at the power station is stepped up to high

voltage (say 220KV or 132KV) for transmission of electric power. The assembly of

apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly,

near the consumer’s localities, the voltage may have to be stepped down to utilization

level. This job is again accomplished by a suitable apparatus called ‘substation.

1.2 CONSTRUCTION OF A SUBSTATION

At the time of constructing a substation, we have to consider some factors

which affect the substation efficiency like selection of site.

2

1.2.1 SELECTION OF SITE:

Main points to be considered while selecting the site for EHV Sub-Station are

as follows:

i) The site chosen should be as near to the load centre as possible.

ii) It should be easily approachable by road or rail for transportation of equipments.

iii) Land should be fairly levelled to minimize development cost.

iv) The source of water should be as near to the site as possible. This is because water

is required for various construction activities;

(Especially civil works,), earthing and for drinking purposes etc.

v) The sub-station site should be as near to the town / city but should be clear of

public places, aerodromes, and Military / police installations.

vi) The land should be have sufficient ground area to accommodate substation

equipments, buildings, staff quarters, space for storage of material, such as store yards

and store sheds etc. with roads and space for future expansion.

vii) Set back distances from various roads such as National Highways, State

Highways should be observed as per the regulations in force.

viii) While selecting the land for the substation preference to be given to the Govt.

land over Private land.

ix) The land should not have water logging problem.

x) The site should permit easy and safe approach to outlets for EHV lines.

3

CHAPTER 2

CLASSIFICATION OF SUBSTATIONS

There are several ways of classifying sub-stations. However, the two most

important ways of classifying them are according to (1) service requirement and (2)

constructional features.

2.1 ACCORDING TO THE REQUIREMENT:

A sub-station may be called upon to change voltage level or improve power

factor or convert A.C. power into D.C. power etc. According to the service

requirement, sub-stations may be classified into:

(i) Transformer sub-stations: Those sub-stations which change the voltage

level of electric supply are called transformer sub-stations. These sub-stations receive

power at some voltage and deliver it at some other voltage. Obviously, transformer

will be the main component in such sub-stations. Most of the sub-stations in the

power system are of this type.

(ii) Switching sub-stations: These sub-stations do not change the voltage level

i.e. incoming and outgoing lines have the same voltage. However, they simply

perform the switching operations of power lines.

(iii) Power factor correction sub-stations: Those sub-stations which improve

the power factor of the system are called power factor correction sub-stations. Such

sub-stations are generally located at the receiving end of transmission lines. These

sub-stations generally use synchronous condensers as the power factor improvement

equipment.

4

(iv) Frequency changer sub-stations: Those sub-stations which change the

supply frequency are known as frequency changer sub-stations. Such a frequency

change may be required for industrial utilization.

(v) Converting sub-stations: Those sub-stations which change A.C. power into

D.C. power are called converting sub-stations. These sub-stations receive A.C. power

and convert it into D.C. power with suitable apparatus (e.g. ignitron) to supply for

such purposes as traction, electroplating, electric welding etc.

(vi) Industrial sub-stations:- Those sub-stations which supply power to individual

industrial concerns are known as industrial sub-stations.

2.2 ACCORDING TO THE CONSTRUCTIONAL FEATURES:

A sub-station has many components (e.g. circuit breakers, switches, fuses,

instruments etc.) which must be housed properly to ensure continuous and reliable

service. According to constructional features, the sub-stations are classified as:

Indoor sub-station

Outdoor sub-station

Underground sub-station

Pole-mounted sub-station

(i) Indoor sub-stations:- For voltages up to 11KV, the equipment of the sub-

station is installed indoor because of economic considerations. However, when the

atmosphere is contaminated with impurities, these sub-stations can be erected for

voltages up to 66 KV.

5

(ii) Outdoor sub-stations:- For voltages beyond 66KV, equipment is invariably

installed out-door.

It is because for such voltages, the clearances between conductors and the

space required for switches, circuit breakers and other equipment becomes so great

that it is not economical to install the equipment indoor.

(iii) Underground sub-stations:- In thickly populated areas, the space available

for equipment and building is limited and the cost of land is high. Under such

situations, the sub-station is created underground.

(iv) Pole-Mounted sub-stations:- This is an outdoor sub-station with equipment

installed over-head on H-pole or 4-pole structure. It is the cheapest form of sub-

station for voltages not exceeding 11KV (or 33 KV in some cases). Electric power is

almost distributed in localities through such sub-station.

Fig: 2.1 CONSTRUCTION OF THE SUBSTATION.

6

CHAPTER 3

SINGLE LINE DIAGRAM (SLD)

A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of

the concerned Electrical System which includes all the required electrical equipment

connection sequence wise from the point of entrance of Power up to the end of the

scope of the mentioned Work. As in the case of 132KV Substation, the SLD shall

show Lightening Arrestor, C.T/P.T Unit, Isolators, Protection and Metering P.T &

C.T. Circuit Breakers, again Isolators and circuit Breakers, Main Power Transformer,

all protective devices/relays and other special equipment like CVT, GUARD RINGS,

etc as per design criteria. And the symbols are shown below. There are several

feeders enter into the substation and carrying out the power. As these feeders enter

the station they are to pass through various instruments.

3.1 FEEDER CERCUIT:

1. Lightening arrestors; 2. CVT; 3. Wave trap; 4. Isolators with earth switch

5. Current transformer; 6. Circuit breaker; 7. Feeder Bus isolator

8. BUS; 9. Potential transformer in the bus with a bus isolator

3.2 TRANSFORMER CIRCUIT:

i) HV side:

1. Transformer bus Isolator 3. Current transformer

2. Circuit breaker 4. Lightning Arrestors

5. Auto Transformer 100MVA (220/132KV)

ii) LV side:

1. Lightening arrestors 5. Bus

2. Current transformer 6. Potential transformer with a bus isolator

3. Circuit breaker 7. A capacitor bank attached to the bus

4. Bus Isolator.

7

3.3 AUXILIARY SUPPLY:

220V.Battery system: To control and protect the substation equipment the

220 volts DC battery system is necessary. It is provided in the main control room. It

will be discussed below.

Fig: 3.1 SINGLE LINE DIAGRAM OF A 220/132KV SUBSTATION

WARANGAL.

8

CHAPTER 4

BRIEF DISCRIPTION OF INSTRUMENTS IN THE

SUBSTATION

4.1 LIGHTENING ARRESTORS:

4.1.1 Lightening Arrestors: Lightening arrestors are the instruments that are used in the incoming feeders

so that to prevent the high voltage entering the main station. This high voltage is very

dangerous to the instruments used in the substation. Even the instruments are very

costly, so to prevent any damage lightening arrestors are used. The lightening

arrestors do not let the lightening to fall on the station. If some lightening occurs the

arrestors pull the lightening and ground it to the earth. In any substation the main

important is of protection which is firstly done by these lightening arrestors. The

lightening arrestors are grounded to the earth so that it can pull the lightening to the

ground.

These are located at the entrance of the transmission line in to the substation

and as near as possible to the transformer terminals.

LA will be provided on the support insulators to facilitate leakage current

measurement and to count the no of surges discharged through the LA.

LA bottom flange will be earthed via leakage ammeter and surge counter.

Leakage current is to be recorded periodically. If the leakage current enters into the

red range from the green range, the LA is prone for failure. Hence, it is to be

replaced.

There should be independent earth pit for LA in each phase so as to facilitate

fast discharging and to raise the earth potential.

9

The lightning arresters or surge diverters provide protection against such

surges. A lightning arrester or a surge diverter is a protective device, which conducts

the high voltage surges on the power system to the ground.

Fig.4.1 (i) Surge diverter

(ii)Characteristics of the non linear resister

Fig 4(i) shows the basic form of a surge diverter. It consists of a spark gap in series

with a non-linear resistor. One end of the diverter is connected to the terminal of the

equipment to be protected and the other end is effectively grounded. The length of the

gap is so set that normal voltage is not enough to cause an arc but a dangerously high

voltage will break down the air insulation and form an arc. The property of the non-

linear resistance is that its resistance increases as the voltage (or current) increases

and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in

Fig 4 (ii).

Fig: 4.2 LIGHTENING ARRESTORS.

10

4.1.2. The action of the Lightning Arrester or surge diverter is as

under:

(i) Under normal operation, the lightning arrester is off the line i.e. it conducts

no current to earth or the gap is non-conducting.

(ii) On the occurrence of over voltage, the air insulation across the gap breaks

down and an arc is formed providing a low resistance path for the surge to the

ground. In this way, the excess charge on the line due to the surge is harmlessly

conducted through the arrester to the ground instead of being sent back over the line.

(iii) It is worthwhile to mention the function of non-linear resistor in the

operation of arrester. As the gap sparks over due to over voltage, the arc would be a

short circuit on the power system and may cause power-follow current in the arrester.

Since the characteristic of the resistor is to offer low resistance to high voltage (or

current), it gives the effect of short circuit. After the surge is over, the resistor offers

high resistance to make the gap non conducting.

4.1.3. Guide for selection of LA:

(i) Before selecting the LA it should be ascertained whether the system is

effectively earthed, non-effectively earthed or having isolated neutral.

(ii) The system neutrals are considered to be effectively earthed when the co-

efficient of earthing does not exceed 80%.

In this case, the reactance ratio X0/ X1 (zero sequence reactance/positive

sequence reactance) is positive and less than 3 and at the same time the resistance

ratio RO/X1 (zero sequence resistance/positive sequence reactance) is less than 1 at

any point on the system. For this system the arrestor rating will be 80% of the highest

phase to phase system voltage.

11

(iii)The LA voltage rating corresponding to the system voltages normal are

indicated below :

Rated system

Voltage (KV)

Highest system

Voltage (KV)

Arrester rating in KV

Effectively earthed systems

11 12 9

33 36 30

66 72.5 60

132 145 120/132 (latex)

220 245 198/216 (latex)

400 420 336

Table: 4.1.3 LA voltage rating

4.1.4 LOCATION OF LIGHTING ARRESTORS:

The LAs employed for protecting transformers should be installed as close as

possible to the transformer. The electrical circuit length between LA and the

transformer bushing terminal should not exceed the limits given below:

Rated system

Voltage

KV

BIL

KV

Peak

Max. distance between L.A and

Transformer bushing terminal

(inclusive of lead length) (in metres)

Effectively earthed

11 75 12.0

33 200 18.0

66 325 24.0

132 550 35.0

650 43.0

220 900 Closes to

Transformer 1050

400 1425

1550

Table: 4.1.4 The limits of LA and Transformers

12

4.2 EARTHING:

The earthing practice adopted at generating stations, sub-stations and lines

should be in such a manner as to provide:

a) Safety to personnel

b) Minimum damage to equipment as a result of flow of heavy fault currents

c) Improve reliability of power supply

4.2.1 The primary requirements are:

The impedance to ground (Resistance of the earthing system) should be as low as possible

and should not exceed,

Large sub-stations -1 ohm

Small sub-stations -2 ohms

Power stations -0.5 ohms

Distribution transformer stations- 5 ohms

4.2.1.1 All exposed steel earthing conductors should be protected with bituminous

paint.

4.2.1.2 PLATE EARTHING:

i) EHT Substation - 1.3 M x 13 M.Ms cast iron plates 25mm thick Plates are

to be buried vertically in pits and surrounded by finely divided coke, crushed coal or

char coal at least 155 mm all round the plates. Plates should not be less than 15 m

apart and should be buried to sufficient depth to ensure that they are always

surrounded by moist earth.

4.2.1.3 PIPE EARTHING:

a) EHT substations Cast iron pipes 125 mm in diameter 2.75 m long and not less than

9.5 mm thick pipes 50.8mm in dia and 3.05m long. Pipes are to be placed vertically at

intervals of not less than 12.2 m in large stations surrounded by finely broken coke

crushed coal and charcoal at least 150 mm around the pipe on the extra depth.

a) Peripheral or main earth mat- 100 x 16 m MS flat

b) Internal earth mat- 50 x 8m MS flat to be placed at 5m apart

c) Branch connections- Cross section not less than 64.5 square meters

13

Joints are to be kept down to the minimum number. All joints and

connections in earth grid are to be brazed, riveted, sweated, bolted or welded. For rust

protection the welds should be treated with barium chromate. Welded surfaces should

be painted with red lead and aluminium paint in turn and afterwards coated with

bitumen. Joints in the earthing conductor between the switch gear units and the cable

sheaths, which may require to subsequently broken should be bolted and the joint

faces tinned. All joints in steel earthing system should be made by welding except the

points for separating the earthing mat for testing purposes which should be bolted.

These points should be accessible and frequently supervised.

4.2.1.4 In all sub-stations there shall be provision for earthing the following:

a) The neutral point of earth separate system should have an independent

earth, which in turn should be interconnected with the station grounding mat

b) Equipment frame work and other non-current carrying parts (two

connections)

c) All extraneous metallic frame work not associated with equipment (two

connections)

d) Lightning arrestors should have independent earths which should in turn be

connected to the station grounding grid.

e) Over head lightning screen shall also be connected to the main ground mat.

4.2.1.5 The earth conductor of the mat could be buried under earth to economical

depth of burial of the mat 0.5 meters.

4.3 CAPACITOR VOLTAGE TRANSFORMER (CVT):

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.

14

These are high pass Filters (carrier frequency 50KHZ to 500 KHZ) pass

carrier frequency to carrier panels and power frequency parameters to switch yard. 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 and a transformer

used to isolate and further step-down the voltage.

Fig: 4.3.1 CIRCUIT DIAGRAM OF CVT.

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 KV 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.

Fig: 4.3.2 CAPACITOR VOLTAGE TRANSFORMER.

15

4.3.1 Specifications of CVT:

CVT type : CVEB/245/1050

Weight : 665 kg

Total output simultaneous : 250 VA

Output maximum : 750 VA at 50O C

Rated voltage : A-N, 220/√3

Highest system voltage : A-N, 245/√3

Insulation level : 460/1050 KV

Rated frequency : 50Hz

Nominal intermediate voltage : A1-N, 20/√3 KV

Voltage factor : 1.2Cont. 1.5/30 sec

‘HF’ capacitance : 4400pF +10% -5%

Primary capacitance C1 : 4840pF +10% -5%

Secondary capacitance C2 : 48400 pF +10%-5%

Voltage ratio : 220000/√3/ 110/√3/110-110/√3

Voltage : 110/√3 110-110/√3

Burden : 150 100

Class : 0.5 3P

4.4 WAVE TRAP:

Wave trap is an instrument using for trapping of the wave. The function of

this wave trap is that it traps the unwanted waves. 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 in the

substation. Generally it is used to exclude unwanted frequency

components, such as noise or other interference, of a wave.

Note: Traps are usually unable to permit selection of unwanted

or interfering signals. Fig: 4.4.1 WAVE TRAP.

16

Line trap also is known as Wave trap. What it does is trapping the high

frequency communication signals sent on the line from the remote substation and

diverting them to the telecom/tele protection panel in the substation control room

through coupling capacitor.

This is relevant in Power Line Carrier Communication (PLCC) systems for

communication among various substations without dependence on the telecom

company network. The signals are primarily tele protection signals and in addition,

voice and data communication signals. The Line trap offers high impedance to the

high frequency communication signals thus obstructs the flow of these signals in to

the substation bus bars. If these are not present in the substation, then signal loss is

more and communication will be ineffective/probably impossible.

4.5. ISOLATOR WITH EARTH SWITCHES (ES):

Isolators are the no load switches and used to isolate the equipment. (Either

line equipment, power transformer equipment or power transformer). With the

isolators, we are able to see the isolation of the equipment with our naked eye. The

line isolators are used to isolate the high voltage from flow through the line into the

bus. This isolator prevents the instruments to get damaged. It also allows the only

needed voltage and rest is earthed by itself.

Isolator is a type of switching device. It has non control devices. Isolator are

operated after the circuit breaker is opened. While closing the circuit, first close the

isolator and after the circuit breaker is closed. Strictly speaking Isolators are operated

under no current condition. In the following cases it is permissible to use isolator for

making and breaking of the circuits.

17

Fig: 4.5.1 ISOLATOR WITH EARTH SWITCH.

Air break isolators or disconnecting switches are not intended to break load

though these are meant for transfer of load from one bus to another and also to isolate

equipment for maintenance. These are available mainly in two types vertical break

type and horizontal break type. The later type requires larger width. However the

space requirement can be reduced in the horizontal break isolators by having double

break with a centre rotating pillar.

Pantograph and semi-pantograph disconnects involve vertical movements of

contact arm and therefore require less separation between phases and thereby require

less separation between phases and thereby help in reducing the sub-station area to a

larger extent. The isolators could be operated mechanically or hydraulically or

pneumatically or by electric motor. Earthing facility shall be provided wherever

required.

4.6. INSTRUMENT TRANSFORMERS:

“Instrument Transformers are defined as the instruments in which the

secondary current or voltage is substantially proportional to the primary current or

voltage and differs in phase from it by an angle which is approximately zero for an

appropriate direction of connection”.

Basic Function of Instrument Transformers:

18

Direct measurement of current or voltage in high voltage system is not

possible because of high values and insulation problems of measuring instruments

they cannot be directly used for protection purposes.

Therefore an instrument transformer serves the purpose and performs the

following function:

Converts the higher line voltages or line currents into proportionally reduced

values by means of electromagnetic circuit and thus these values can be

measured easily.

Protects measuring instruments and distribution systems by sensing the

abnormalities and signals to the protective relay to isolate the faulty system.

Types of Instrument Transformers:

Instrument transformers are of two types:

Current Transformers

Voltage Transformers

4.6.1 Current transformers:

Current transformer is a current measuring device used to measure

the currents in high voltage lines directly by stepping down the currents to

measurable values by means of electromagnetic circuit.

4.6.1.1 Basic Design Principle of Current Transformers:

The basic principle induced in designing of current transformers is

Primary ampere turns = Secondary ampere turns

Ip Np = Is Ns

Where, Ip - Primary current

Np - Primary Winding Turns

Is - Secondary Current; Ns - Secondary Winding Turns

19

Ampere turns plays very important role in designing current transformers.

Current transformers must be connected in series only.

Current transformer has less no of turns in primary and more no of turns

in secondary.

The secondary current is directly proportional to primary current.

The standards applicable to CT's are IEC-60044-1 and IS – 2705.

4.6.1.2 Simple Line Diagram of Current Transformer:

The line diagram of a current transformer contains different components:

Fig: 4.6.1 LINE DIAGRAM OF CT.

Primary Winding: It is the winding which is connected in series with the

circuit, the current of which is to be transformed.

These are of two types:

1. Single turn primary winding 2. Multi-turn primary winding

Magnetic Core: Performance of any current transformer depends on its

accuracy of transformation and characteristics of the core material used.

Design of a current transformer depends on the frequency of excitation.

Secondary Winding: The winding which supplies the current to the measuring

instruments, meters, relays, etc.

Burden: The relay, instrument or other device connected to the secondary

winding is termed as 'burden' of a current transformer.

Ex. Burden for Metering is CT – 20 VA, 15 VA.

SECONDARY WINDING

PRIMARY WINDING

20

4.6.1.3 Tests generally to be conducted on CT:

Insulation resistance values (IR values): Primary to earth, primary to

secondary core1, primary to secondary core2, core1 to earth, core2 to earth and core1

to core2. Primary to earth and primary to secondary cores are to be checked with 5KV

motor operated insulation tester (megger) and secondary to earth values are to be

checked with 1000V insulation tester or preferably with 500V insulation tester.

Ratio test: Primary injection test is to be conducted for this purpose

TAN-DELTA test: on 132KV CTs and above

Polarity test at the time of commissioning (at least on the CTs connected to

revenue meters)

Excitation (saturation) characteristic check

Secondary and lead resistance check

Secondary injection check

Primary injection check

4.6.1.4 The accuracy of a CT is directly related to a number of factors including:

* Burden

* Burden class/saturation class

* Rating factor

* Load

* External electromagnetic fields

* Temperature and

* Physical configuration.

* The selected tap, for multi-ratio CTs

Number of secondary cores in the current transformer is based on its usage.

CTs used for 11KV and 33KV feeder will have 2 secondary cores. Core 1 is generally

for Over current and earth fault protection. Core 2 is for metering. Usage of core is

decided by the accuracy class of the CT .Core material decides the accuracy class

Core with accuracy class 1.0, 0.5 and latest is 2.0 is used for metering.

Allowable errors are +/-1.0% in case of 1.0 accuracy class CTs.

21

CT secondary current is proportionate upto120% of the rated primary current

with +/-1% error in case of 1.0 accuracy class CTs. This indicates that 0.2 accuracy

class CTs are expensive than 0.5 and 1.0 accuracy class CTs. Beyond 120% of the

rated primary current, the metering core get saturated.

Core with accuracy class 5P10, 5P15 and 5P20 is used for o/c & e/f

protection. In 5p10, the 5 denotes allowable errors i.e. +/-5%, P denotes protection

and 10 denotes accuracy limit factor. CT secondary current is proportionate upto10

times the rated primary current with +/-5% errors in case of 5P10 accuracy class CTs.

This indicates that 5P20 accuracy class CTs are expensive than 5P15 and 5P10

accuracy class CTs. CT with 2cores (protection core and metering) is used for

11KV& 33KV feeders and capacitor bank protection. CT with 3cores (protection,

special protection and metering) is used for 132/11, 132/33KV ptrs&132KV feeders

protection 220/132KV PTR LV CT is also having 3 cores.

CT with 4 cores (protection, special protection, special protection and

metering) is used for 220KV Bus couplers for the twin bus substations. CT with 5

cores (4 cores for special protection, and metering) is used for 220KV feeder

protection, In all the above cases, protection means O/L &E/L protection, special

protection means differential protection and REF protection in case of power

transformers, bus bar protection (bus differential protection) in case of bus, and

distance protection in case of feeders. At the rate of 220KV level we should use 1:5

cores Current transformer.

4.6.1.5 Specifications of HVCT:

Type : IT-245

Frequency : 50 Hz

H.S.V : 245 KV

BIL : 460/1050KV

Oil weight : 360kgs

Total weight : 1250kgs

Lth : 40/1 KA/sec

22

RATIO

800-600-400/1-1-1-1-1

CORE NUMBER 1 2 3 4 5

RATED PRIMARY

CURRENT (A) 800

RATED

SECONDARY

CURRENT(A)

1 1 1 1 1

OUTPUT(VA) -------- ---------- --------

----- ------- - 30

ACCURACY CLASS PS PS PS PS 0.5

I.S.F/A.L.F ---- --- --- --- <=5

TURN RATIO 2/1600 1200 800

RCT at 75 C AT 800/1

(ohms) 6 6 6 ---

Table: 4.6.1.5 Specifications of HVCT.

At the rate of LV (132KV) side we can use 1:3 core CT. The specifications of LVCT

are given below:

4.6.1.6 Specifications of LVCT:

Type : IT-145

Frequency : 50 Hz

HSV/NSV : 145/132 KV

BIL : 650/275 KV

Oil weight : 75Kg

Total weight : 550Kg

Lth : 31.5/1 kA/sec.

Ldyn : 78.75kAp

23

RATIO 500/1-1, 0.66-1

CORE NUMBER 1 2 3

RATED PRIMARY

CURRENT (A) 500

PRIMARY &

SECONDARY

CONNECTION

500/1 500/1 500/0.66 500/1

1s1-1s2 2s1-2s2 2s1-2s3 3s1-3s2

RATED SECONDARY

CURRENT(A) 1 1 0.66 1

OUTPUT(VA) 20 ------- ------------ 20

ACCURACY CLASS 5p PS 0.2

I.S.F/A.L.F 20 -------- --------- <=5

Rct at 75o C (Ohms) -------- <=5 --------- -------------

Table: 4.6.1.6 Specifications of LVCT

Important:

a) CT secondary circuit and PT primary should never be open circuited. It is

vulnerable to the CT/PT

b) CT primary circuit and PT secondary should never be short circuited.

NOTE:-Loose connections should not be allowed in the electrical circuit. It increases

the contact resistance which in turn the rises the temperature in that area due to load

current. It damages the oil seals in CTs and transformers bushings causing oil leak

and in term entry of moisture in to the equipment causing falling of IR values and

damages ‘O’ rings in circuit breaker causing SF6 gas leakage. Entry of moisture in to

the VCB insulator chamber cause vacuum interrupter failure and pull rod failure due

to electrical break down. Hence loose connections should not be allowed.

24

4.6.2 Potential Transformers (PT):

An instrument transformer in which the secondary voltage, in normal

conditions of use, is substantially proportional to the primary voltage and

differs in phase from it by an angle which is approximately zero for an

appropriate direction of the connections.

Basic Functions of Voltage Transformers are:

To reduce the line voltage to a value which is suitable for standard measuring

instruments relays etc.

To isolate the measuring instruments, meters, relays etc. from high voltage

side an installation.

To sense abnormalities in voltage and give signals to protective relays to

isolate the defective system.

4.6.2.1 Simple Line Diagram of Voltage Transformer:

Fig: 4.6.2.1 LINE DIAGRAM OF VT.

Basic Design Principle Involved in Voltage Transformer’s:

The basic principle involved in the designing of Voltage Transformer is

Voltage Ratio = Turns Ratio

VP / VS = NP / NS

Thus NS VP = NP VS

25

As heavy primary voltages will be reduced to low secondary voltages, it will have

more turns in the primary & less turns in the secondary. It must always be connected

in parallel only. Even if we connect it directly from high voltage to earth, it is not

going to be a short circuit as its primary winding has very high resistance. Its core is a

set of assembled laminations. It operates at constant flux density. The standards are

IEC – 600044 – 2 and IS – 3156.

Fig: 4.6.2.2 POTENTIAL TRANSFORMER.

4.6.2.2 Tests generally to be conducted on the PTs:

Insulation resistance values (IR values): primary to earth, primary to

secondary core-1, primary to secondary core-2, core1 to earth, core 2 to earth and

core-1 to core-2. These values are to be checked with 1000V insulation tester

(megger) or preferably with 500V insulation tester.

Ratio Test: By applying single phase voltage across primary the voltage

induced in the secondary winding is to be measure. This is approximately equal to

voltage applied in the primary winding or voltage ratio of the PT.

Polarity test at the time of commissioning (at least on the PTs connected to

revenue meters)

PT secondary injection check

PT combined primary and secondary injection check

4.6.2.3 General checks for PT:

Mechanical alignment for PT power jaws

PT primary winding star earthing

26

Tightness of all connections

Primary/secondary fuse ratings

PT specifications

In PTs no of secondary cores is 1 or more than 1 based on the requirement.

Generally in 11KV or 33KV bus PTs, there is one secondary winding which is used

both for protection and metering and in 132KV and above, there are two secondary

cores. First core is of metering core with 1.0 or 0.5 or 0.2 accuracy classes. This will

be used metering, directional over current protection and distance protection.

The second core is protection core with 3P accuracy class. This will be used

for the directional earth fault protection (open delta voltage) of power transformers

and 132KV feeders.

Accuracy class 0.5 means +/- 0.5% errors are allowable and 3P means +/- 3%

errors are allowable and P denotes protection.

Permissible load to be connected on PT secondary winding is decided by the

burden of the PT secondary winding. It is expressed in volt-amperes (VA). If more

than rated burden is connected then error will be increased.

4.7. CIRCUIT BREAKER:

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.

These are load switches. It is able to make or break the normal load current as

well as the fault currents. The basic construction of any circuit breaker requires the

separation of contacts in an insulating fluid, which serves two functions. It

extinguishes the arc drawn between contacts when the CB opens and it provides

adequate insulation between the contacts and from each contact to earth. For

successful operation of the circuit breaker, two functions are to be performed.

a) Operating mechanism function, b) Arc quenching function.

There are

27

various operating mechanisms:

Spring charge mechanism, Pneumatic mechanism, Hydraulic Mechanism

Arc quenching medium:

Bulk oil (called bulk oil circuit breakers-BOCB)

Minimum oil (called minimum oil circuit breakers-MOCB)

Natural air (called air circuit breakers-ACB) (415v)

Forced air (called air blast circuit breaker-ABCB)

Vacuum (called vacuum circuit breaker-VCB)

SF6 gas (called Sulphur Hexafluoride-SF6 gas CB)

The present trend is up to 33KV, VCBs are preferred and beyond 33KV, SF6 gas

circuit breakers are preferred.

VCB is to be meggered periodically to know the healthiness of the vacuum

interrupter and the insulating pull rod. Vacuum integrity test is the correct test to

know the healthiness of the vacuum interrupter.

SF6 gas pressure is to be noted in log sheets at least twice in a day. If it is

reaching to SF6 gas pressure low alarm stage, it is to be brought to the notice of the

maintenance personnel.SF6 gas circuit breaker goes to lockout conditions after falling

to lockout pressure close and trip circuits will be blocked and circuit breaker

operation can’t be performed N<0 contacts of 63GLX were used in close and trip

circuits of the CB and 63GLX contactor is in picked up conditions when the gas

pressure is sufficient. Some of the SF6 gas circuit breaker automatically trips while

going to lockout stage N<C contacts of 63GLX contactor were used in close and trip

circuits and 63GLX is in drop off condition when the gas pressure is sufficient.

Oil condition in the air compressor is to be checked periodically. And it is

to be replaced based on condition of oil.

There are mainly two types of circuit breakers used for any substations. They are

(a) SF6 circuit breakers;

(b) Vacuum circuit breakers.

28

4.7.1 SF6 circuit breakers:

Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and

arc extinguishing properties. The dielectric strength of the gas increases with pressure

and is more than the dielectric strength of oil at 3 kg/cm2. SF6 is now being widely

used in electrical equipment like high voltage metal enclosed cables; high voltage

metal clad switchgear, capacitors, circuit breakers, current transformers, bushings,

etc. The gas is liquefied at certain low temperature, liquidification temperature

increases with the pressure.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur

in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each

bearing about 4 kg of sulphur. The steel box is made gas tight.

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

high input KV, 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 rings.

Some of the properties of SF6 are,

Very high dielectric strength

High thermal and chemical inertia

Superior arc extinguishing capability

Low decomposition by arcing

29

Fig: 4.7.1 SF6 CIRCUIT BREAKERS.

30

4.7.2 Vacuum circuit breakers:

Vacuum type of circuit breakers is used for small KV rated stations below

33KV. They are only used in low distribution side.

4.7.3 Control Circuit of Circuit Breakers:-

In closing circuit of the Circuit Breaker there are no. of series inter locks we can say

that it is an AND Gate and tripping circuit there are no.of parallel paths it is an OR

Gate.

For ‘closing’ the Circuit Breaker following conditions are to be met.

a) Local/Remote selector shall be in ‘Remote’ for closing the CB from

remote and it shall be in ‘Local’ for closing the CB from Local.

b) Spring is in charged condition in spring type CBs, Air pressure shall

be sufficient in kinematic CBs and Hydraulic Pressure is sufficient in

Aero shell fluid CBs.

c) SF6 Gas pressure is sufficient.

d) Master Trip is resettled.

For tripping the circuit breaker,

a) Local/Remote selector Switch shall be in ‘Remote’ for tripping the CB

from Remote and it shall be in ‘Local’ for tripping the CB from Local.

b) SF6 Gas pressure is sufficient.

c) Air Pressure is sufficient/Hydraulic Pressure is sufficient.

d) Protection trip bypasses the local/Remote selector switch.

Trip circuit healthiness is to be ensured immediately after closing the circuit breaker.

It is to be ensured at regular intervals at least once shift, as there is no trip circuit

supervision relay and annunciation relay for 33KV feeders and in case of old panels

of 132KV feeders If any deviation is found it is to be brought to the notice of

maintenance personnel.

31

4.8 BUS:

The bus is a line in which the incoming feeders come into and get into the

instruments for further step up or step down. The first bus is used for putting the

incoming feeders in la single line. There may be double line in the bus so that if any

fault occurs in the one the other can still have the current and the supply will not stop.

The two lines in the bus are separated by a little distance by a conductor having a

connector between them. This is so that one can work at a time and the other works

only if the first is having any fault.

4.9 TRANSFORMERS:

Transformers come in a range of sizes from a thumbnail-sized coupling

transformer hidden inside a stage microphone to huge units weighing hundreds of

tons used to interconnect portions of national power grids. All operate with the same

basic principles, although the range of designsis wide. While new technologies have

eliminated the need for transformers in some electronic circuits, transformers are still

found in nearly all electronic devices designed for household ("mains") voltage.

Transformers are essential for high voltage power transmission, which makes long

distance transmission economically practical.

Fig: 4.9.1.1 ELECTRICAL TRANSFORMER.

32

4.9.1 Basic Principle:

The transformer is based on two principles: firstly, that an electric current can

produce a magnetic field (electromagnetism) and secondly that a changing magnetic

field within a coil of wire induces a voltage across the ends of the coil

(electromagnetic induction).

Changing the current in the primary coil changes the magnetic flux that is

developed. The changing magnetic flux induces a voltage in the secondary coil.

Fig: 4.9.1.2 IDEAL TRANSFORMER.

An ideal transformer is shown in the adjacent figure; Current passing through

the primary coil creates a magnetic field. The primary and secondary coils are

wrapped around a core of very high magnetic permeability, such as iron, so that most

of the magnetic flux passes through both primary and secondary coils.

4.9.1.1 Induction law:

The voltage induced across the secondary coil may be calculated from

Faraday's law of induction, which states that, where VS is the instantaneous voltage,

NS is the number of turns in the secondary coil and Φ equals the magnetic flux

through one turn of the coil.

33

If the turns of the coil are oriented perpendicular to the magnetic field lines, the

flux is the product of the magnetic field strength and the area A through which it cuts.

The area is constant, being equal to the cross-sectional area of the transformer core,

whereas the magnetic field varies with time according to the excitation of the

primary.

Fig: 4.9.1.3 MUTUAL INDUCTION.

Since the same magnetic flux passes through both the primary and secondary

coils in an ideal transformer, the instantaneous voltage across the primary winding

equals Taking the ratio of the two equations for VS and VP gives the basic equation

for stepping up or stepping down the voltage Ideal power equation The ideal

transformer as a circuit element.

If the secondary coil is attached to a load that allows current to flow, electrical

power is transmitted from the primary circuit to the secondary circuit. Ideally, the

transformer is perfectly efficient; all the incoming energy is transformed from the

primary circuit to the magnetic field and into the secondary circuit. If this condition is

met, the incoming electric power must equal the outgoing power.

Giving the ideal transformer equation Transformers are efficient so this

formula is a reasonable approximation. If the voltage is increased, then the current is

decreased by the same factor. If an impedance ZS is attached across the terminals of

the secondary coil, it appears to the primary circuit to have an impedance of ZS =

(VS/IS).

34

4.9.2 Detailed operation:

The simplified description above neglects several practical factors, in

particular the primary current required to establish a magnetic field in the core, and

the contribution to the field due to current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible

reluctance with two windings of zero resistance. When voltage is applied to the

primary winding, small current flows, driving flux around the magnetic circuit of the

core. The current required to create the flux is termed the magnetizing current; since

the ideal core has been assumed to have near-zero reluctance, the magnetizing current

is negligible, although still required to create the magnetic field.

The changing magnetic field induces an electromotive force (EMF) across

each winding. Since the ideal windings have no impedance, they have no associated

voltage drop, and so the voltages VP and VS measured at the terminals of the

transformer, are equal to the corresponding EMFs. The primary EMF, acting as it

does in opposition to the primary voltage, is sometimes termed the "back EMF". This

is due to Lenz's law which states that the induction of EMF would always be such

that it will oppose development of any such change in magnetic field.

There are three transformers in the incoming feeders so that the three lines are

step down at the same time. In case of a 220KV or more KV line station auto

transformers are used. While in case of lower KV line such as less than 132KV line

double winding transformers are used.

Fig: 4.9.3 THREE PHASE 100MVA AUTO TRANSFORMER.

35

4.9.3 Specifications of 220/132KV Auto transformer:

Rated MVA: 100MVA

Frequency: 50HZ

No of phases: 3

Insulation level: HV LI 900 AC 395

HVN LI 95 AC 38

IV LI 550 AC 230

LV LI 170 AC 70

Type of cooling: ONAN DNAF

Rated MVA : 75 100

Rated KV at no load: HV 220KV --

IV 132KV --

LV 11KV --

Line Amperes : HV 196.8 262.4

IV 328.0 437.4

LV 1299.0 1732.1

Temperature Rise oC: Top oil - 50oC

Avg.WDG - 55OC

Impedance volts HV-IV 7.667 10.222

Normal Tap conditions) HV-LV 24.55 32.72

IV-LV 17.69 23.59

4.10 CAPACITOR BANK ATTACHED TO THE BUS:

The capacitor banks are used across the bus so that the voltage does not get

down till at the require place. A capacitor bank is used in the outgoing bus so that it

can maintain the voltage level same in the outgoing feeder.

Fig: 4.10.1 CAPACITOR BANK IN THE DISTRIBUTION SYSTEM.

36

Capacitor Control is usually done to achieve the following goals:

Reduce losses due to reactive load current; Reduce KVA demand, decrease customer

energy consumption, Improve voltage profile, and increase revenue.

Indirectly capacitor control also results in longer equipment lifetimes because of

reduced equipment stresses.

Experience shows that switched feeder capacitors produce some of the fastest

returns on equipment investment Sources of Energy Loss. Energy losses in

transmission lines and transformers are of two kinds: resistive and reactive. The

former are caused by resistive component of the load and cannot be avoided. The

latter, coming from reactive component of the load, can be avoided. Reactive losses

come from circuit In the case of concentrated industrial loads, there should be a bank,

sized to almost equal the reactive load current, located as close to each load as

possible (Fig. 5.10).

Fig: 4.10.2 REACTIVE LOSSES.

37

CHAPTER 5

TYPES OF CONTROL

VAR control is the natural means to control capacitors because the latter adds

a fixed amount of leading VARs to the line regard less of other conditions, and loss

reduction depends only on reactive current. Since reactive current at any point along a

feeder is affected by downstream capacitor banks, this kind of control is susceptible

to interaction with downstream banks. Consequently, in multiple capacitor feeders,

the furthest downstream banks should go on-line first and off-line last. VAR controls

require current sensors.

Current control is not as efficient as VAR control because it responds to total line

current, and assumptions must be made about the load power factor. Current controls

require current sensors. Voltage control is used to regulate voltage profiles; however

it may actually increase losses and cause instability from highly leading currents.

Voltage control requires no current sensors.

Fig: 5.1 TYPES OF CONTROL

38

Temperature control is based on assumptions about load characteristics.

Control effectiveness depends on how well load characteristics are known. Not useful

in cases where those characteristics change often. Temperature control does not

require any current sensors. Time control is based on assumptions about load

characteristics. Control effectiveness depends on how well load characteristics are

known. Not useful in cases where those characteristics change often. Time control

does not require any current sensors.

Power factor control is not the best way to control capacitor banks because

power factor by itself is not a measure of reactive current. Current sensors are needed.

Combination control using various above methods is usually the best choice.

If enough current, and/or other sensors are available, a centrally managed

computerized capacitor control system taking into account the variety of available

input parameters can be most effective, though expensive to implement.

5.1 CAPACITORS:

a) Before commissioning a capacitor bank, capacitance of each capacitor shall be

measured with a capacitance meter. These shall be compared with the value

obtained by calculation using the formula,

C = KVAR x 109 Micro Farads

2 ∏f (V) 2

Where V is the rated voltage of capacitor and KVAR is the rated KVAR of capacitor.

As per IS the tolerance in the capacitance value for a capacitor unit is + 10% to – 5%.

b) In the event of failure of one capacitor unit (say in R-phase) it is observed that

balancing is done by removing one capacitor each from Y and B-phases.

c) It is therefore necessary that number of capacitor units connected in parallel in

each series group in all the three phases on one star bank shall be same.

39

5.2 BUS BAR SYSTEM:

5.2.1 Mesh (Ring) bus bar system:

Merits: 1. Busbars gave some operational flexibility

Demerits: 1. If fault occurs during bus maintenance, ring gets separated into two

sections.

2. Auto-reclosing and protection complex.

3. Requires VT’s on all circuits because there is no definite voltage reference point.

These VT’s may be required in all cases for synchronizing live line or voltage

indication.

4. Breaker failure: During fault on one circuit causes loss of additional circuit

because of breaker failure.

Remarks1.Most widely used for very large power stations having large no. of

incoming and outgoing lines and high power transfer.

5.2.2 Bus bar Isolator:

These can be used for the protection of the instruments in the substation by isolating

the buses at the required instant.

5.3 STATION BATTERY:

“Observe me every day” is a slogan mentioned on the batteries provided for

the vehicles. This holds good to battery at substations also. Battery is the heart of the

substation at control and protection side and this is the uninterrupted power source to

operate the switchgear and protection.

5.3.1 Periodical works on Batteries:

Pilot voltages and specific gravities are to be recorded by the shift personnel

in the morning shift by switching off the battery charger.

40

While switching off the battery charger, one should observe the battery for the

sparks if any due to loose connections. Once charger is switched off, entire DC load

off the station is to be met by the battery. Voltage of all the cells and their specific

gravities, are to be recorded once in a month by the maintenance personnel. If any

deviation is found either in cell voltages or specific gravities, the battery may be kept

in boost mode duly topping up the electrolyte levels with the distilled water and

keeping the cell caps in open position. Specific gravity of the healthy cell is 1200+/-

20 i.e. it ranges from 1180 to 1220 and the voltage is about 2.1v.

5.3.2 During Boost Charging:

a. Boost charger can be switched on duly keeping the coarse and fine selector

switches in position-1 to maintain the boost charger output voltage at minimum so

that boost charger current is minimum during starting. Later, coarse and fine selector

switches are to be adjusted as per the requirement,

b. Boost charging current should not exceed 1/10th of the battery Ampere Hour

capacity i.e. 8 Amperes for 80 AH battery. Cell temperature should not exceed 50

Deg.Cen. Boost charger voltage should not exceed 297V (i.e.2.7v/cell).

c. Float charger shall also be kept in service otherwise load will be connected

across first 84 cells and boost charger will be connected across 110 cells leading

under charging of first 84 cells or over charging of 85th cell to 110th cell causing

damage to the cells. Once float charger is switched on, load will automatically

connects across the float charger as float charger output voltage is generally more

than the first 84 nos. cells voltage.

d. At the end of the boost charging, all the cells shall be thoroughly cleaned, caps

shall be kept back and petroleum jelly is to be applied at the cell terminals to avoid

exposing of electrodes direct to atmosphere which will cause formation of sulphation

on the terminals due to oxidation. Cell terminals shall be tightened periodically duly

keeping brass bolts & nuts as spares to meet the requirement. Battery shall be

discharged yearly once. It increases the battery life period. Earth leakage is to be

avoided as far as possible to discharging of 50% of the cells.

41

Fig: 5.3 STATION BATTERIES.

5.3.3 Specifications of VRLA (Valve Regulated Led Acid) batteries:-

System details:

Make : AMARA RAJA

System voltage : 220v

Capacity at 27oc : 200AH

Cell type : 2V

No. of cells : 110

Charging Requirements:

Float voltage : 2.45-3V

Boost voltage : 2.53-3V

Maximum charging : 40A

Maximum allowable ripple: 2.1 rms

Current in each cell : 2A

5.4 INSULATORS:

Ball and socket type disc insulators are assembled to the 132 KV, 220 KV and

400 KV suspension and tension hardware, certain important design aspects and other

details are indicated below: in next page:

42

Sl.

No.

Description 132 KV lines 220 KV lines 400 KV

lines

1. Type of insulators Ball and socket

type disc insulator

Ball and socket

type disc insulator

Ball and

socket type

disc

insulator

2. Dimensions of insulators of

suspensions string

255mm x 145mm 280mm x 145mm 280mm x

145mm

3. Dimensions of insulators for

tension string

280mm x 145mm 280mm x 145mm 280mm x

170mm

4. Number of insulator disc per single

suspension string

9 nos. 13 nos. 23 nos.

5. Number of insulator disc per double

suspension string

2 x 9 nos. 2 x 13 nos. 2 x 23 nos.

6. Number of insulator discs per

single tension string

10 nos. 14 nos. 24 nos.

7. Number of insulator discs per each

double tension string

2 x 10 nos. 2 x 14 nos. 2 x 24 nos.

8. Electro Mechanical strength for

tension string insulator

7000 Kgs. 7000 Kgs. 11,500 Kgs.

9. Electro Mechanical strength for

suspension string insulator

11,500 Kgs. 11,500 Kgs. 16,500 Kgs.

10. Total creapage distance of each disc

insulator for suspension strings

280 mm 280 mm 315 mm

11. Total creapage distance of each disc

insulator for tension string

280 mm 280 mm 330 mm

12. Minimum impulse dry withstand

voltage (wave of 1 x 50 Micro

second) for each disc insulator

110 KV 110 KV 120 KV

43

(I.E.C standard)

13. One minute power frequency

withstand voltage for each disc

insulator

70 KV (dry)

40 KV (wet)

70 KV (dry)

40 KV (wet)

70 KV

(dry)

40 KV

(wet)

14. Power frequency puncture voltage

per each disc insulator

110 KV

(Suspension

strings)

140 KV

(Tension Strings)

110 KV

(Suspension

strings)

140 KV

(Tension Strings)

140 KV

(Suspension

strings)

140 KV

(Tension

Strings)

15. Size and designation of bal pin

shank for suspension string discs

16 mm 16 mm 20 mm

16. Size and designation of bal pin

shank for tension string discs

20 mm 20 mm 20 mm

17. Maximum Radio Influence Voltage

at 10 KV (RMS) for each disc

insulator

50 Micro Volts at

1 MHz

50 Micro Volts at

1 MHz

50 Micro

Volts at 1

MHz

18. Corona extinction voltage for

complete (RMS) string both

suspension and tension strings

- -

(RMS)

320 KV

Table: 5.4 Insulators

Fig: 5.4 BALL AND SOCKET TYPE DISC INSULATOR.

44

CHAPTER 6

PROTECTION FOR VARIOUS EQUIPMENTS

6.1 TRANSFORMER PROTECTION:

a) Station Transformer: HG Fuse protection on HV side and fuse protection on LV side

and Vent pipe.

b) Power transformers up to 7.5MVA:

HV side: O/L & Directional E/L protection with highest element in O/L relays.

LV side: O/L & E/L protection Buchholz Relay OLTC Buchholz Relay OTI and

WTI.

c) Power transformers from 8.0MVA and above: HV side O/L & Directional E/L

protection with high set element in O/L relays. LV side O/L & E/L protection:

differential protection Buchholz Relay OLTC Buchholz Relay OTI, WTI and PRV.

d) Power transformers from 31.5MVA and above: Over flux protection & LV WTI in

addition to protection.

e) 220/132KV power transformers: Over flux protection on both HV & LV sides LBB

protection on HV side OLTC Buchholz phase wise in addition to protection.

6.2 FEEDER PROTECTION:

a) 33KV feeders: Non directional O/L & E/L protection with highest and IDMT

characteristics.

b) 132KV feeders: Main protection: Distance protection.

Backup protection: Directional O/L & E/L protection.

c) 220KV feeders: Main-1 protection: Distance protection

Main-2protection: Distance protection, LBB protection, pole discrepancy

Relay.

45

CHAPTER 7

220/132KV SUBSTATION AT WARANGAL

The present-day electrical power system is A.C. i.e. electric power is

generated, transmitted and distributed in the form of alternating current. The electric

power is produced at the power stations which are located at favourable places,

generally quite away from the consumers. It is delivered to the consumers through a

large network of transmission and distribution. At many places in the line of the

power system, it may be desirable and necessary to change some characteristic (e.g.

voltage, A.C. to D.C., frequency, P.f. etc.) of electric supply.

This is accomplished by suitable apparatus called sub-station. For example,

generation voltage (11KV or 6·6KV) at the power station is stepped up to high

voltage (say 220KV or 132KV) for transmission of electric power. The assembly of

apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly,

near the consumer’s localities, the voltage may have to be stepped down to utilization

level. This job is again accomplished by a suitable apparatus called ‘substation’.

7.1 SUBSTATION AT WARANGAL:

The substation in Mulugu cross road, Warangal (dist.), Andhra Pradesh was

completed by the year 1969, under APTRANSCO; it is one of the largest substations

in the state of Andhra Pradesh.

This substation has the carrying capacity of 300MW at different voltage levels

of 220KV and can step down to 132KV and again step down to 33KV, using four

input lines through the incoming feeders.

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7.2 SAILENT FEATURES OF 220/132/33KV SS WARANGAL

The 220/132/33KV Substation Warangal has the following equipment and

feeder bays

1) 220KV Feeders – 4 Nos.

2) 220/132KV 100MVA PTRs – 3 Nos.

3) 132KV Feeders – 6 Nos.

4) 132/33KV 50MVA PTR – 1 Nos.

5) 132/33KV 80MVA PTR – 1 Nos.

6) 33KV Feeders – 10 Nos.

7) 33KV Capacitor Bank – 2 Nos.

10MVAR

7.2MVAR

The 220KV supply is fed from either 220KV Nagaram –I&II or 220KV

Budidampadu ckt I&II.

7.2.1 220KV Features:

20KV Bus Twin Zebra Bus

3 No’s 220/132KV PTRs Namely

i) 100MVA PTR-I Make-TELK

ii) 100MVA PTR-II Make-GEC ALSTHOM

iii) 100MVA PTR-III Make-TELK

7.2.2 132KV Features:

132KV Bus Twin Zebra Bus

6 No’s 132KV Feeders Namely

i) 132KV Waddepally

ii) 132KV Jangaon

iii) 132KV Jammikunta

iv) 132KV RTS-B

v) 132KV Narsampet

vi) 132KV Nekkonda

47

7.2.3 132/33KV PTRs:

2 Nos 132/33KV PTRs

i) 80MVA PTR-I Make-BBL

ii) 50MVA PTR-II Make-BBL

7.2.4 33KV Features:

Tubular Copper Bus with Bus Coupler

i. 33KV Gorrekunta

ii. 33KV Machapur

iii. 33KV Atmakur

iv. 33KV Pothana

v. 33KV A.J.Mills

vi. 33KV Parkal

vii. 33KV Kamalapur

viii. 33KV Chintagattu

ix. 33KV KUC33KV

x. 33KVBalasamudram

xi. 10MVAR Capacitor Bank Make-NGEF

xii. 7.2MVAR Capacitor Bank Make-UNISTAR

7.2.5 DC SYSTEM

i.Battery Bank-A:

Make: STAR Batteries 220V DC, 200AH, Lead Batteries

Connected Battery Charger:

Make: HEE 220V Dc, 200AH

Float Current: 8 Amps, Boost Current: 16 Amps

ii.Battery Bank-B:

Make: AMARARAJA, 245V DC, 200AH, VLRA Batteries

Connected Battery Charger:

Make: HEE 220V DC, 200AH

Float Current: 8 Amps, Boost Current: 16 Amps

48

7.2.5.1 Chargers:

Main Input AC supply is fed from AC Distribution Board-I

33KV/415V, 250KVA Station Transformer on Bus-II side and it is connected to AC

Distribution Board-I.

Any Substation mainly consists of Transformers. These transformers are like

the heart of substation. These transformers are step down to the required voltage

levels. And the different types of equipments are used in the substation.

The assembly of apparatus used to measure and protect the require parameters

of the power transformer like (e.g. voltage, AC to DC, frequency, P.F. etc.) of electric

supply is called a substation.

EHV (Extra High Voltage) Sub-Station forms an important link between

Transmission network and Distribution network. It has a vital influence of reliability

of service. Apart from ensuring efficient transmission and Distribution of power, the

sub-station configuration should be such that it enables easy maintenance of

equipment and minimum interruptions in power supply.

Flexibility for future expansion in terms of number of circuits and

transformer MVA Capacity also needs to be considered while choosing the actual

configuration of the substation.

EHV Substation is constructed as near as possible to the load centre. The

voltage level of power transmission is decided on the quantum of power to be

transmitted to the load centre. Generally, the relation between EHV Voltage level and

the power to be transmitted is as follows:

49

7.2.4 Power to be transmitted Voltage level:

1) Up to 80MVA to 132KV.

2) From 100MVA to 300MVA 2KV.

3) 300 MVA to 1000 MVA 400 KV.

7.3 IMPORTANT POINTS TO BE KEPT IN VIEW WHILE LAYING OUT

THE SUBSTATION:

Substations are important part of power system. The continuity of supply

depends to a considerable extent upon the successful operation of sub-stations. It is,

therefore, essential to exercise utmost care while designing and building a substation.

The following are the important points which must be kept in view while

laying out a substation:

(i) It should be located at a proper site. As far as possible, it should be located at

the centre of gravity of load.

(ii) It should provide safe and reliable arrangement. For safety, consideration must

be given to the maintenance of regulation clearances, facilities for carrying out repairs

and maintenance, abnormal occurrences such as possibility of explosion or fire etc.

For reliability, consideration must be given for good design and construction, the

provision of suitable protective gear etc.

(iii) It should be easily operated and maintained.

(iv) It should involve minimum capital cost.

50

CHAPTER 8

CONCLUSION

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.

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 component 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.

51

REFERENCES

[1] Principles of Power Systems by V.K. Mehtha

[2] Electrical Power Systems by C.L. Wadhwa

[3] Power System Engineering by ML. Soni

[4]www.littelfuse.com/.../Littelfuse-Protection-Relay-Transformer- Protection

[5]www.osha.gov/SLTC/etools/electric_power/.../substation.html.

[6]http://www.scribd.com/doc/13595703/Substation-Construction-and-

Commissioning.

[7]http://www.authorstream.com/Presentation/marufdilse-881803-electrieal-

power-trasmission/

[8]http://skindustrialcorp.tradeindia.com/Exporters_Suppliers/Exporter17825.

277078/66-KV-Disc-Insulator-Ball-Socket-Type.html.

[9]http://en.wikipedia.org/wiki/Electrical_substation.