AN INDUSTRIAL TRAINING

REPORT ON
33/11KV SUBSTATION PERAVALI
Submitted in partial fulfillment of the requirement for the award of

DIPLOMA
In
ELECTRICAL AND ELECTRONICS
ENGINEERING

SUBMITTED BY
B V SURYA NARAYANA-21243-EE-005
Under the esteemed guidance of
Mr.D.DHANA PRASAD
Assistant professor

SRI VASAVI ENGINEERING COLLEGE

(Recognized by SBTET, A.P.Vijayawada, Approved by AICTE, New Delhi) Pedatadepalli,
Tadepalligudem-534101, W.G.District, A.P, INDIA
 STATE BOARD OF TECHNICAL EDUCATION AND TRAINING
VIJAYAWADA
DEPARTMENT OF
ELECTRICAL & ELECTRONICS ENGINEERING

SRI VASAVI ENGINEERING COLLEGE

(Recognized by SBTET, A.P.Vijayawada, Approved by AICTE, New Delhi) Pedatadepalli, Tadepalligudem-534101,
W.G.District, A.P, INDIA

This is to certify that the project report entitled “33/11kv substation peravali is a bonafide
work done by B V SURYA NARAYANA (21243-EE-005)submitted in partial fulfillment of
the requirements for the award of the DIPLOMA in Electrical & Electronics Engineering
during the academic year 2021-2024.The results of investigation closed in this report have
been verified and found satisfactory.

Mr. D.DHANA PRASAD;MTECH Dr.D.SUDHARANI;ME;PH.D

(PROJECT GUIDE) (HEAD OF THE DEPARTMENT)

Mr.G.RAMPRADSAD,M.Tech
(INCHARGE OF POLYTECHNICE COLLEGE) EXTERNAL EXAMINER
 ACKNOWLEDGEMENT

I would also like to thank Dr. G.V.N.S.R. Ratnakara Rao, B.E, M.E, Ph.D
Principal of our college for his constant support through out the Industrial Training.

I would like to thank Dr.SUDHARANIDONEPUDI,M.E,Ph.D Head of the

hedepartment of Electrical & Electronics Engineering for supporting us to do our
Industrial Training.

I would also like to thank Mr. G. Ramaprasad, M.Tech, Incharge of

Polytechnic Courses for permitting me to do my Industrial Training.

I would also like to thank our guide MR.DHANA,PRASAD M.Tech Asst.

Prof EEE department of our college for his constant support through out the
Industrial Training

I would like thank my family for always being there for me. Their love,
constant support and encouragement to achieve my goals.

Finally, we would also like to thank all those who directly or indirectly helped
me during this Industrial Training

B.V.SURYA NARAYANA
(21243-EE-005)
 ABSRACT

During my six months of in - plant training period. I have gathered more knowledge about

not only industrial electronics but also electrical installations, applications of electronic in

electrical field and mechanical field. In Loadstar, I gained theoretical knowledge about the

electronics in industrial usage. In my training period, I learnt that how to apply those

theoretical knowledge practical activities. I basically understood there is a big difference

between theoretical knowledge & practical knowledge. In here, present my practical

experiences which I gained within last six months. In this report, I present that back ground

of the training organization, its structure, staff levels, etc. And also, I express my training

experiences in Loadstar (pvt) Ltd. Further, under this, I describe industrial electronic devices,

CNC machines, Computer embedded systems… etc On the other hand, I mention that

problems which were encountered during my training period

 INDEX
CHAPTERS CONTENTS PAGE NUMBERS

1 INTRODUCTION 1-4

1.0. Substation 1
1.1 Operation of substation 1
1.2 Classification of substation 2
1.2.a. According to service requirement 2
1.2.b. According to constructional features 2
1.3. Single line diagram 3
1.3.a. 33/11kv ss peravali 4

2 TRANSFORMERS 5-13
2.0 About transformer 5
2.1 Working principle of transformer 6
2.2 Emf equation of transformer 7-8
2.3 Classification of transformer 9
2.4 Parts of transformer & their functions 9
2.4.a. Laminated core 9
2.4.b Windings 10
2.4.c Insulating material 10
2.4.d Transformer oil 10
2.4.e Buchhloz relay 10
2.4.f Tap changer 11
2.4.g Oil conservator 11
2.4.h Breather 11
2.4.i Explosion vent 11
2.4.j Radiator & fans 11
2.5. Specification of ptr 1 12
2.6. Specification of ptr 2 13

3 EQUPIMENT OF 33/11 KV SUB STATION 14-29

3.0 Bus bar 14

3.1 Single bus bar arrangement 14
3.1.a Single bus bar arrangement with bus sectionalization 15
3.1.b Main & transformer bus arrangement 16
3.2 Circuit breaker 17
3.3 Isolator’s 18
3.4 Types of isolator’s 18
3.5 Lightning arrester 19
3.5.a Working of lightning arrester 19
3.6 Types of lightning arrester 19
3.6.a Tyrite lightning arrester 20
3.6.b Metal oxide surge arrester 21
3.7 Wave traps 22
3.8 Capacitive voltage transformer 22
3.9 Insulators 23
3.10 Pin insulator 23
 3.11 Suspenision insulator 24
3.12 Strain insulator 25
3.13 Stay insulator 25
3.14 Shackle insulator 25
3.15 Types of conductors 26
3.16 Instrument transformer 26
3.16.a Current transformer 26
3.16.b Potential transformer 27
3.17 Capacitor bank 28-29

4 PROTECTIVE EQUIPMENT 30-50

4.0 Circuit breaker 30

4.1 Working principle of circiut breaker 30
4.2 Arc phenomenon 30
4.3 Different types of circuit breaker’s 31
4.3.a Based on voltage 31
4.3.b By installation of location 31
4.3.c Based on external design 31
4.3.d By interrupting mechanism 32
4.4 Relay family 33
4.5 Definition of relay 34
 Definition of protective relay
 Pick up level of activating signal
 Reset level
 Reset time of relay
 Research of relay
 Operating type of relay
4.6 Relays for transmission & distribution line protection 35
4.7 Transformer protection & different type of relays 35
4.7.a Differential relays 36
4.8Differential protection scheme in a power transformer 36
4.8.a Principle of protection 36
4.8.b Percentage of differential relay or based differential protection 37
4.8.c Working function of percentage different production 38
4.8.d Characteristics of percentage relay 39
4.8.e Philosophy of protective relaying 39
4.9 Earthing 40
4.9.a Pipe earthing 40
4.9.b Plate earthing 42
4.10Maintenance of schedules 43
4.10.a Transformers & reactors 43
4.10.b Circuit breakers 46
4.10.c Current transformer 47
4.10.d Potential transformers /capacitance voltage
transformer/coupling capacitor 47
4.10.eProtection system 48

5 Conclusion 51
 LIST OF FIGURES

CHAPTERS CONTENTS PAGE NUMBERS

1 Fig: 1.1 Single line diagram of 33/11 kv sub station 3

2 Fig:2.0 Single phase transformer 5

Fig :2.1 Diagram of transformer core 6
Fig :2.2 Sinusoidal wave form 7
Fig :2.3 Power transformer 1 12
Fig :2.4 Power transformer 2 13

3 Fig :3.0 Bus bar 14

Fig :3.1 Schematic diagram of bus bar arrangement 15
Fig :3.1.a Schematic diagram of single bus bar arrangement with bus 15
sectionalization
Fig :3.1.b Schematic diagram of main & transfer bus arrangement 16
Fig :3.2 Circuit breaker 17
Fig :3.3 Isolator 18
Fig :3.4 Classification of isolator 18
Fig :3.5 Characteristics of voltage 19
Fig :3.6 Characteristics of current 19
Fig :3.7 Valve type of lightning arrester 20
Fig :3.8 Thyrite of lightning arrester 21
Fig :3.9 Metal oxide surge lightning arrestors 21
Fig :3.10 Wave traps 22
Fig :3.11 Capacitive voltage transformer 22
Fig :3.12 Pin insulator 23
Fig :3.13 Suspension insulator 24
Fig :3.14 Strain insulator 25
Fig :3.15 Stay insulator 25
Fig :3.16 Shackle insulator 26
Fig :3.17 Current transformer 27
Fig :3.18 Potential transformer 28
Fig :3.19 Capacitor bank 29

4 Fig :4.0 Principle of differential protection 37

Fig :4.1 Percentage differential relay or based differential protection 38
Fig :4.2Characteristics of percentage relay 39
Fig :4.3 Pipe earthing 40
Fig :4.3.a Schematic diagram of pipe earthing 41
Fig : 4.4 Schematic diagram of plate earthing 42
 CHAPTER 1
INTRODUCTION

1.0 SUBSTATION:
A substation is the part if an electrical generation transmission and distribution
system .Substation transform voltage from high to low, or the reverse or perform any of
several other important functions.

The present day electrical power system is A.C .i.e, electrical power is generated
transmitted & distributed in the form of the alternating current. The electric power is
produced at power plant stations which are located at favorable places generally quite
away from the consumers. It is delivered to the consumers through a large network of
transmission 7 distribution. At many places in the power system, it may be desirable
and necessary to change some characteristics e.g. voltage, ac to dc, frequency, power
factor etc. of electric supply. This accomplished by suitable apparatus called substation.
For example; generation voltage (11kv or 33kv) at the power station is set 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 in the substation Similarly near
the consumer’s localities, the voltage may have to be step down to utilization level.
This job is accomplished by suitable apparatus called substation.

1.1 OPERATION OF SUBSATION

Substation may perform the following operations.

 Switching operation

To switch on and off the power line.

 Power converting operation

To change ac power into dc power and vice versa.

 Frequency changing operation

To change the supply frequency level i.e .from high level to low level or viceversa.

 Power factor correction operation

To improve the power factor of the system using static capacitors, synchronous
condensers or phase advancers.

1
1.2 CLASSIFICATION OF SUBSTATIONS
The substation may be classified into several ways .the two important ways of

classifying the substations areas follows

1.2.a According to service requirement

In accordance with the service performed, substations are classified as

 Switching substation

 Transformer substation

 Converting substation

 Industrial substation

1.2.b According to constructional features

In accordance with the constructional features substations are classified as

 Indoor substation

 Outdoor substation

 Pole mount substation

 Plinth mount substation

2
 1.3. SINGLE LINE DIAGRAM OF 33/11 KV SS KATAKOTESWARAM

33kv Incoming supply(NIDADAVOLE)

33kv AB Switch

33 kv Alternating supply(Tanuku)

33kv AB Switch

33kv breaker

33kv AB Switch 33kv Bus Bar

33kv AB Switch

33kv HG Fuse

8MVA 33/11 kv Step-down Transformers 5MVA

LV-I LV-II

11kv AB Switch PT AB Switch DTR 11kv AB Switch

11kv Bus Bar

11kv AB Switches 11kv AB Switches 11kv AB Switches

Circuit Circuit Circuit Circuit Circuit Circuit

Breaker Breaker Breaker Breaker Breaker Breaker

CT CT CT CT CT CT

\ 11kv AB Switches 11kv AB Switches 11kv AB Switches

KAMASALIPALAM SANKARAPURAM SURAPURAM UNAKARAMILLI KATAKOTESWARAM KATAKOTESWARAM

LOCAL AGL

3
 1.3.a. 33/11KV SS PERAVALI
The 33kv substation PERAVALI which was commissioned on18-08-1982

The main bus 33kv is connected to grid located at 132/33kv,” TANUKU” Now the

transmission line first parallel connected with lighting arrestor to divergesurge, followed

CVT connected parallel. CVT measures voltage and step down at 110v. A.C. for control

Panel, a current transformer is connected in series with line which measure current and

Step down current at ratio 200:1 for control panel. Transformer step downs voltage from

33KV to 11KV. A step down transformer of 11kv/440v is connected to control panel to

provide supply to the equipment of the substation. Capacitor bank is connected to main

Bus of 11kv.It is provided to improve power factor &voltage profile.

The main equipment in Substation are

 Power transformers (PTR)

 Circuit breakers
 Current transformers (CT’s)
 Potential transformers (PT’s)
 Relay circuits
 Isolators
 Lightening arresters
 Capacitor bank
 Capacitive voltage transformer
33/11kv SS PERAVALI SIX feeders

 MALLESWARAM
 MUKKAMALA
 TEEPARRU
 KAKARPARRU
 KHANDAVALLI
 PERAVALI TOWN

4
 CHAPTER-2

TRANSFORMERS
2.0. ABOUT TRANSFORMER

Fig: 2.0 Single phase transformer

A transformer is a static device .Which transfer’s electrical energy from one circuit to
the other circuit without changing frequency, while doing so the voltage may be increased or
decreased with a corresponding decrement or increment of current.

A transformer consists of two windings. The winding connected to the supply is called
primary winding and the winding connected to the load is called the secondary winding. In an
ideal transformer the induced voltage in the secondary winding (vS) is in proportional to the

Primary voltage (VP) and is given by the ratio of number of turns in the secondary (NS) to the
number of turns in the primary (NP) as follows.

Vs/Vp=Ns/Np

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current
(A.C) voltage to be stepped up by making ns greater than Np, or stepped down by making ns
less than Np.

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.

5
2.1. WORKING PRINCIPLE OF TRANSFORMER

Fig: 2.1: Diagram of transformer core

The basic principle behind working of a transformer is the phenomenon of mutual induction
between two windings linked by common magnetic flux. Basically the transformer consists of
two inductive coils; primary winding and secondary winding The coils are electrically separated
but magnetically linked to each other. When, primary winding is connected to a source of
alternating voltage alternating magnetic flux is produced around the winding. The core provides
magnetic path for the flux, to get linked with the secondary winding. Most of the flux gets linked
with the secondary winding which is called as 'useful flux' or main 'flux', and the flux which does
not get linked with secondary winding is called as 'leakage flux'. As the flux produced is
alternating (the direction of it is continuously changing), EMF gets induced in the secondary
winding according to Faraday's law of electromagnetic induction. This emf is called 'mutually
induced emf', and the frequency of mutually induced emf is same as that of supplied emf. If the
secondary winding is closed circuit, then mutually induced current flows through it and hence
the electrical energy is transferred from one circuit (primary) to another circuit (secondary).

6
2.2. EMF equation of a transformer
Transformation Ratio In a transformer, source of alternating current is applied to the primary
winding. Due to this, the current in the primary winding (called as magnetizing current) produces
alternating flux in the core of transformer. This alternating flux gets linked with the secondary
winding, and because of the phenomenon of mutual induction an emf gets induced in the
secondary winding. Magnitude of this induced emf can be found by using the following EMF
equation of the transformer. EMF equation of the Transformer

Let,

N1 = Number of turns in primary winding

N2 = Number of turns in secondary Winding

Φm = Maximum flux in the core (in Wb) = (Bm x A)

f = frequency of the AC supply (in Hz)

Fig : 2.2 Sinusoidal wave form

As, shown in the fig., the flux rises sinusoidally to its maximum value Φm from 0. It reaches to
the maximum value in one quarter of the cycle i.e in T/4 sec (where, T is time period of the sin
wave of the supply = 1/f).

Therefore,

average rate of change of flux = Φ m /(T/4) = Φ m /(1/4f)

Therefore, average rate of change of flux = 4f Φm ....... (Wb/s).

Now, Induced emf per turn = rate of change of flux per turn

Therefore, average emf per turn = 4f Φm ..........(Volts).

Now, we know, Form factor = RMS value / average value

Therefore, RMS value of emf per turn = Form factor X average emf per turn.

7
As, the flux Φ varies sinusoidally, form factor of a sine wave is 1.11

Therefore, RMS value of emf per turn = 1.11 x 4f Φm = 4.44f Φm.

RMS value of induced emf in whole primary winding (E1) = RMS value of emf per turn X
Number of turns in primary winding

E1 = 4.44f N1 Φm ............................. eq 1

Similarly, RMS induced emf in secondary winding(E2)can be given as

E2 = 4.44f N2 Φm. ............................ eq 2

from the above equations 1 and 2,

This is called the emf equation of transformer, which shows, emf / number of turns is same for
both primary and secondary winding.

For an ideal transformer on no load, E1 = V1 and E2 = V2 .

where, V1 = supply voltage of primary winding

V2 = terminal voltage of secondary winding

Voltage Transformation Ratio (K)

As derived above,

Where, K = constant This constant K is known as voltage transformation ratio.

▪ If N2 > N1, i.e. K > 1, then the transformer is called step-up transformer.

▪ If N2 < N1, i.e. K < 1, then the transformer is called step-down transformer.

8
2.3. CLASSIFICATION OF TRANSFORMERS

Transformers may be classified into four types.they are

1. Power transformer

a. Step up transformer

b. Step down transformer

2. Distribution transformer

3. Instrument transformer

a. Current transformer

b. Potential transformer

4. Welding transformer

2.4. Parts of Transformer and their Functions

Following are the various parts of transformer:

1. Laminated core
2. Windings
3. Insulating material
4. Tank
5. Terminals and bushings
6. Transformer oil
7. Tap changer
8. Buchholz relay
9. Oil conservator
10. Explosion vent
11. Breather
12. Radiator and fans

2.4.a. Laminated Core

laminated core is the most important part of transformer, used to support the windings of the
transformer. It is made up of laminated soft iron material to reduce eddy current loss and
hysteresis loss. Nowadays in the core of the transformer, laminated sheets are used to minimize
eddy current losses, and CRGO steel material is used to minimize hysteresis losses. The
composition of the core material depends on the voltage, current, and frequency of supply to
the transformer.

9
2.4.b. Windings
In a transformer always two sets of windings are placed on a laminated core and these are
insulated from each other. Winding consists of several no of turns of copper conductors that are
bundled together and connected in series.The main function of windings is to carry current
produce working magnetic flux and induce mutual EMF for transformer action. Windings are
classified in two ways

 Based on the input and output of the supply

 Based on the voltage level of the supply

2.4.c. Insulating Material

Insulation is required between each turn of windings, between windings, winding, and core, and
all current-carrying parts and the tank of the transformer.The main function of insulating material
is to protect the transformer against short circuits by providing insulation to windings so that it
does not come in contact with the core and any other conducting material. The
insulating material of the transformer should have high dielectric Properties and also good
mechanical strength and temperature withstand capability. Synthetic material, papers, cotton
cloth, etc. are used as insulating material in transformers.

2.4.d. Transformer Oil

The function of transformer oil is to provide insulation between windings as well as cooling due
to its chemical properties and very good dielectric strength.It dissipates the heat generated by the
core and windings of a transformer to the external environment. When the windings of the
transformer get heated due to the flow of current and losses, the oil cools down the windings by
circulating inside the transformer and transferring heat to the external environment through its
cooling tubes. Hydro-carbon mineral oil is used as transformer oil and acts as a coolant. It is
composed of aromatics, paraffin, naphthenes, and olefins.

2.4.e. Buchholz Relay

Buchholz relay is the most important part of a power transformer rated at more than 500kVA.
It The function of the Buchholz relay is to protect the transformer from all internal faults such
as short circuit faults, inter-turn faults, etc. When a short circuit occurs in winding it generates
enough heat to decompose transformer oil into. These gases move toward the conservator tank
through a connecting pipe, and then due to these gases, the Buchholz relay gets activated. It
sends a signal to trip and alarm circuits and activates it. Then circuit breaker disconnects the
transformer from the supply.

10
2.4.f. Tap Changer

The main function of the tap changer is to regulate the output voltage of the transformer by
changing its turn ratio. There are two types of tap changers.

1. On-Load Tap Changer:- In an on-load tap changer, tapping can be changed without
isolating the transformer from the supply. Hence it is capable of operating without
interrupting the power supply.
2. Off-Load Tap Changer:- In the off-load tap changer, the transformer needs to isolate
from the supply to change its tapping (turns ratio).

2.4.g. Oil Conservator

The function of the oil conservator tank is to provide adequate space for the expansion and
contraction of transformer oil. It is a cylindrical drum-type structure installed on the top of the
main tank of the transformer. It is connected to the main tank through a pipe and a Buchholz
relay mounted on the pipe. A level indicator is also installed on the oil conservator to indicate the
quantity of oil inside the conservator tank. It is normally half-filled with transformer oil.

2.4.h. Breather
The breather is a cylindrical container filled with silica gel and directly connected to the
conservator tank of the transformer. The main function of the breather is to supply moisture-free
fresh air to the conservator tank during the expansion and contraction of transformer oil. In a
breather, when air passes through silica gel the moisture present in the air is absorbed by the
silica gel crystal and hence moisture-free dry air is supplied to the conservator tank. Thus we can
also say that the breather is acting as an air filter for the transformer.

2.4.i. Explosion Vent

An explosion vent is a metallic pipe with a diaphragm at one end and installed on the main tank
slightly above than conservator tank. The main function of the explosion vent is to protect the
power transformer against explosion during excessive pressure build-up in the main tank due to
severe internal faults. It acts as an emergency exit for oil and hot air gases inside the main tank of
the transformer.

2.4.j. Radiator and Fans

The main function of cooling tubes or radiators is to transfer heat generated by the core and
windings to the environment by circulating heated oil throughout the cooling tubes. In a large
power transformer, forced cooling is achieved with the help of cooling fans fitted on the radiator.

11
2.5. SPECIFICATIONS OF PTR-1(8MVA)

Fig : 2.3: Power transformer 1

Make - Ece Total mass - 20000kg

Rating - 8 MVA Year of manufacture - 29/09/23

Rated voltage Hv - 33KV

Lv - 11KV

Rated load current Hv - 139.96A

Lv - 419.89A

Frequency - 50 HZ

Impedance voltage - 8.35%

Mass of core & oil - 9500kg

Mass of tank &fitting - 6000kg

Mass of oil - 4500kg

12
2.6. SPECIFICATIONS OF PTR- II (5MVA)

Fig: 2.4 :Power transformer 2

Make - Apex Total mass - 15220kg

Rating - 5 MVA Year of manufacture - 2000

Rated voltage Hv - 33KV

Lv - 11KV

Rated load current Hv - 87.5A

Lv - 262.4A

Frequency - 50 HZ

Impedance voltage - 7.535%

Mass of core - 4130kg

Mass of tank &fitting - 5300kg

Mass of oil - 2620kg

13
 CHAPTER-3

EQUIPMENTS OF 33/11KV SUBSTATION

3.0. BUS BAR

An electrical bus bar is defined as a conductor or a group of conductor used for collecting
electric power from the incoming feeders and distributes them to the outgoing feeders. In other
words, it is a type of electrical junction in which all the incoming and outgoing electrical current
meets. Thus, the electrical bus bar collects the electric power at one location.

Fig : 3.0 : Bus bar

The most common of the bus-bars are 40×4mm (160 mm2); 40×5 mm (200 mm2) ; 50×6
mm (300mm2) ; 60×8 mm (480 mm2) ; 80×8 (640 mm2) and 100×10 mm (1000 mm2).

The various types of busbar arrangement are used in the power system. The selection of the bus
bar is depended on the different factor likes reliability, flexibility, cost etc.

 The bus bar arrangement is simple and easy in maintenance.

 The maintenance of the system did not affect their continuity.
 The installation of the bus bar is cheap.

3.1. Single Bus-Bar Arrangement

The arrangement of such type of system is very simple and easy. The system has only one bus
bar along with the switch. All the substation equipment like the transformer, generator, the
feeder is connected to this bus bar only. The advantages of single bus bar arrangements are

 It has low initial cost.

14
  It requires less maintenance
 It is simple in operation

Fig: 3.0: scehemetic diagram of bus bar arrengement

Drawbacks of Single Bus-Bars Arrangement

 The only disadvantage of such type of arrangement is that the complete supply is disturbed
on the occurrence of the fault.
 The arrangement provides the less flexibility and hence used in the small substation where
continuity of supply is not essential.
3.1.a. Single Bus-Bar Arrangement with Bus Sectionalized
In this type of busbar arrangement, the circuit breaker and isolating switches are used. The
isolator disconnects the faulty section of the bus bar, hence protects the system from complete
shutdown. This type of arrangement uses one addition circuit breaker which does not much
increase the cost of the system

Fig : 3.1.a. Schematic diagram of Single Bus-Bar Arrangement with Bus Sectionalized

15
Advantage of single Bus-bar Arrangement with Bus Sectionalization

The following are the advantages of sectionalized bus bar.

 The faulty section is removed without affecting the continuity of the supply.
 The maintenance of the individual section can be done without disturbing the system
supply.
 The system has a current limiting reactor which decreases the occurrence of the fault.
Disadvantages of Single Bus-Bar Arrangement with Sectionalization

 The system uses the additional circuit breaker and isolator which increases the cost of the
system
3.1.b. Main and Transfer Bus Arrangement
Such type of arrangement uses two type of busbar namely, main busbar and the auxiliary bus bar.
The busbar arrangement uses bus coupler which connects the isolating switches and circuit
breaker to the busbar. The bus coupler is also used for transferring the load from one bus to
another in case of overloading. The following are the steps of transferring the load from one bus
to another.

Fig :3.1.b schemetic diagram of Main and Transfer Bus Arrangement

 The potential of both the bus bar kept same by closing the bus coupler.
 The bus bar on which the load is transferred is kept close.
 Open the main bus bar.
Thus, the load is transferred from the main bus to reserve bus.

16
Advantages of Main and Transfer Bus Arrangement

 The continuity of the supply remains same even in the fault. When the fault occurs on any
of the buses the entire load is shifted to the another bus.
 The repair and maintenance can easily be done on the busbar without disturbing their
continuity.
 The maintenance cost of the arrangement is less.
 The potential of the bus is used for the operation of the relay.
 The load can easily be shifted on any of the buses.
Disadvantages of Main and Transfer Bus Arrangement

 In such type of arrangements, two bus bars are used which increases the cost of the system.
 The fault on any of the bus would cause the complete shutdown on the whole substation.

3.2. CIRCUIT BREAKER

Circuit breakers are used to open and close circuits. They can be operated manually to perform
maintenance or will automatically trip if a short circuit occurs. This function in the power system
is similar to that of the fuses or breakers in a household distribution panel.

Fig : 3.2 :circuit breaker

17
3.3. ISOLATORS

Isolator is a manually operated mechanical switch that isolates the faulty section of substation. It
is used to separate faulty section for repair from a healthy section in order to avoid the
occurrance of severe faults. It is also called disconnector or disconnecting switch.

Fig : 3.3 :Isolator

3.4. TYPES OF ISOLATORS

 Based on the no poles.(single &3pole)

 Based on their service (indoor &outdoor)
 Isolators for earthing
 Single break& double break isolators.

Fig: 3.4 :Classification of isolator

18
Applications of Isolator

The applications of isolator include the following.

 High Voltage Devices: Isolators are used in High Voltage devices.
 Isolator in Substation: When a fault occurs in a substation, then isolator cuts
out a portion of a substation.

 Signal Isolation: Isolators can be used for isolation of signal.

3.5. Lightning Arrester

The device which is used for the protection of the equipment at the substations against travelling
waves, such type of device is called lightning arrester or surge diverter. In other words, lightning
arrester diverts the abnormals high voltage to the ground without affecting the continuity of
supply. It is connected between the line and earth, i.e., in parallel with the equipment to be
protected at the substation

3.5.a. Working of Lightning Arrester

When a travelling wave reaches the arrestor, its sparks over at a certain prefixed voltage as
shown in the figure below. The arrestor provides a conducting path to the waves of relatively low
impedance between the line and the ground. The surge impedance of the line restricts the
amplitude of current flowing to ground.

Fig :3.5.Characteristics of voltage Fig :3.6. Characteristics of current

The lightning arrester provides a path of low impedance only when the travelling surge reaches
the surge diverter, neither before it nor after it. The insulation of the equipment can be protected
if the shape of the voltage and current at the diverter terminal is similar to the shape shown
below.

19
3.6. TYPES OF LIGHTNING ARRESTORS

Valve Type Lightning Arrester

The valve type arrester consists of a multiple spark gap assembly in series with the resistor of
nonlinear element. The each spark gap has two elements. For non-uniform distribution between
the gap, the non-linear resistors are connected in parallel across the gap.

Fig :3.7: valve type lightning arrester

3.6.a.THYRITE LIGHTNING ARRESTER

A lightning arrester may be a spark gap or may have a block of a

semiconducting material such as silicon carbide or zinc oxide Thyrite arrester is most
common and is mostly used for the protection against high dangerous voltages .It
consists the thyrite which is an inorganic compound of ceramic material. The resistance
of such material decreases rapidly from high value to low value and for current from a
low value to high value. When the lightning takes place, the voltage is raised, and
breakdowns of the gaps occur, the resistance falls to a very low value, and the wave is
discharged to earth. After the surge has passed the thyrite again come back to its
original position.

20
 Fig: 3.8 : .THYRITE LIGHTNING ARRESTER
3.6.b. Metal Oxide Surge Arrester
The arrester which uses zinc oxide semiconductor as a resistor material, such type of arrester is
known as a metal oxide surge arrester or ZnO Diverter. This arrester provides protection against
all types of AC and DC over voltages. It is mainly used for overvoltage protection at all voltage
levels in a power system.

Fig:3.9: Metal oxide surge arrester

21
BENEFITS OF LIGHTENING ARRESTER TESTING

 Lightning protection testing would make sure that all structures, key electrical
and electronic installations are safe from the effect of lightening strike.
 The financial benefits are determined as follows: how does the total annual cost
for a lightning protection system compare to the costs of potential damage
without a protection system? The cost evaluation is based on the expenditures
for the planning, assembly, and maintenance of the lightning protection system
3.7. WAVE TRAPS

High frequency waves above 50 Hz are captured using a wave trap or line trap. A wave trap
is used to produce a high impedance to prevent high-frequency carrier waves from entering
undesirable locations, usually substations. All communication in carrier wave technology is sent
at a frequency between 150 kHz and 800 kHz.

Fig: 3.10: wave traps

3.8. CAPACITIVE VOLTAGE TRANSFORMER

The capacitive voltage transformer step-down the high voltage input signals and provide the low
voltage signals which can easily measure through the measuring instrument. The Capacitive
voltage transformer (CVT) is also called capacitive potential transformer

Fig: 3.11: capacitive voltage transformer

22
3.9. INSULATORS

Insulators will generally be used in substations to separate and support electrical conductors
while not letting electrical currents flow through themselves. When electrical materials including
cables are wrapped in insulate material, this is referred to as insulating them

Properties of Insulators

 It has large resistance and specific resistance.

 Large di-electric strength.

 High mechanical strength.
 Resisting high temperature.
 May not get change in nature due to temperature.
 It should not absorb water.
 Can be made to any shape.
 Cannot get fire simply

There are 5 types of insulators used in transmission lines as overhead insulation:

 Pin insulator
 Suspension insulator
 Strain insulator
 Stay insulator
 Shackle insulator
Pin, suspension, and strain insulators are used in medium to high voltage systems.
While stayand shackle insulators are mainly used in low voltage applications.

3.10. PIN INSULATOR

A pin insulator is a device that isolates a wire from a physical support such as a pin (a wooden or
metal dowel of about 3 cm diameter with screw threads) on a telegraph or utility pole. It is a
formed, single layer shape that is made out of a non-conducting material, usually porcelain glass.

Fig : 3.12 : Insulator

23
Advantages of Pin Insulator

 It has high mechanical strength.

 The pin type insulator has good creep age distance.
 It is used on the high voltage distribution line.
 The construction of the pin type insulator is simple and requires less maintenance.
 It can be used vertically as well as horizontally.

Disadvantages of Pin Insulator

 It should be used with the spindle.

 It is only used in the distribution line.
 The voltage rating is limited, i.e., up to36kV
 The pin of the insulator damaged the insulator thread.

3.11. SUSPENSION TYPE INSULATOR

A suspension-type insulator protects an over headed transmission line like a conductor.

Generally, it is made up of porcelain material that includes a single or a string of insulating discs
hung over a tower. It operates at above 33KV and overcomes the limitation of pin-type
insulators. In higher voltage, beyond 33kv, it becomes uneconomical to use pin insulator because
size, weight of the insulator become more. Handling and replacing bigger size single unit
insulator is quite difficult task. For overcoming these difficulties, suspension insulator was
developed.

Fig: 3.13: SUSPENSION TYPE INSULATOR

24
3.12. STRAIN INSULATOR

When suspension string is used to sustain extraordinary tensile load of conductor it is referred as
string insulator. When there is a dead end or there is a sharp corner in transmission line, the line
has to sustain a great tensile load of conductor or strain. A strain insulator must have
considerable mechanical strength as well as the necessary electrical insulating properties

Fig :3.14: STRAIN INSULATOR

3.13. STAY INSULATOR

A stay insulator is a type of low voltage insulator designed to fasten and counterweight the dead-
end pole by connecting with a stay wire or guy grip, it is also called a stay type insulator or egg
insulator.

Fig:3.15 : STAY INSULATOR

3.14. SHACKLE INSULATOR

The shackle insulator (also known as a spool insulator) is usually used in low voltage distribution
network. It can be used in both the horizontal or vertical positions. The use of such insulator has
decreased recently after increasing the using of underground cable for distribution purpose. The
tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility

25
of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with
the help of soft binding wire

Fig :3.16 : SHACKLE INSULATOR

3.15. TYPES OF CONDUCTORS

AAC : ALL ALUMINIUM CONDUCTOR

AAAC : ALL ALUMINIUM ALLOY CONDUCTOR

ACSR : ALLUMINIUM CODUCTORS STEEL- REINFORCED

ACAR : ALLUMINIUM CONDUCTORS ALLOY- REINFORCED

IACS : INERNATIONAL ANNEALED COPPER STAND

The panther conductor is of (30+7)/3.00mm at 33/11 KV SS PERAVALI

The 33kv bus of single panther conductor at 33/11 KV SS PERAVALI

The following sizes have now been standardized by CEA for transmission lines of different
voltages

 For 132kv lines : panther ACSR having 7- strands of steel of dia 3.00mm and 30-strands
of aluminium of dia3.00mm.
 For 220kv lines : “zebra” ACSR having 7-strands of steel of dia 3.18 mm and 54-strands
of aluminium of dia3.18mm.
 For 400kv lines : twin “moose” ACSR having 7-strands of steel of dia 3.53 mm and 54-
strands of aluminium of dia 3.53mm

3.16. INSTRUMENT TRANSFORMER

TYPES OF INSTRUMENRT TRANSFORMER

3.16.a. CURRENT TRANSFORMER (CT)

26
 A current transformer is designed to maintain an accurate ratio between the currents in its
primary and secondary circuits over a defined range. The alternating current in the primary
produces an alternating magnetic field in the core, which then induces an alternating current in
the secondary.

Fig :3.17 : Current transformer

.The ratio of the primary current and the secondary current is known as a current transformer
ratio of the circuit. The current ratio of the transformer is usually high. The secondary current
ratings are of the order of 5A, 1A and 0.1A. The current primary ratings vary from 10A to
3000A or more. The symbolic representation of the current transformer is shown in the figure
Above

3.16.b. POTENTIAL TRANSFORMER

A potential transformer (PT), or a Voltage Transformer (VT) is an Instrument Transformer used

to measure voltage. It is specially designed to maintain the correct voltage phase angle reference
between the Instrument Transformer's source and output and excellent voltage regulation to
obtain precise voltage measurements. Potential transformers are step-down transformers, i.e.,
they have many turns in the primary winding while the secondary has few turns. The figure
shows a typical potential transformer for the measurement of high alternating voltage. From the
figure, it is clear that a P.T. is a well designed step down transformer.

27
 Fig :3.18 : potential transformer

The stepped down voltage by the Potential transformer can be measure using a low range AC
voltmeter. The potential transformer has shell type construction of its magnetic core for better
accuracy. One end of the secondary winding of the potential transformer is grounded to provide
the proper protection to the operator.

The primary winding of the potential transformer is connected across the high voltage power line
whose voltage is to be measured and a low-range AC voltmeter (usually 0-110V) is connected
across the secondary winding of the P.T.

3.17. CAPACITOR BANK

Capacitor banks are usually installed at specific points in the system, such as substations or
feeders, where they can provide the optimal amount of reactive power compensation for the load
or network conditions. Capacitor banks are generally used for improving power factor of
electricity consumed by bulk consumers e.g.industry. Usually the electrical power supply
companies impose a penalty, if the average power factor over a stipulated period falls below a
certain value. A large part of load of bulk consumers are inductive in nature because of use of
induction motors. This causes low power factor (lagging because of predominately inductive
loads). Capacitors, on the other hand, constitute leading power factor load; thus compensating a

28
major part of the inductive loads and result in power factor close to unity but still lagging in
nature. This improvement of power factor fulfills the requirements of the supply company. Loads
with low power factor draw more current from supply than same active load with high power
factor and hence cause increased power loss in supply line. This increase of power loss is a waste
for the supply company. Further supply companies need to install higher size transmission /
distribution equipment to supply the additional load current because of the low power factor. To
prevent these, the penalty for low power factor is imposed

Fig: 3.19 : Capacitor bank

29
 CHAPTER-4
PROTECTIVE EQUIPMENTS

4.0. CIRCUIT BREAKER (C.B)

A.C.B is a piece of equipment, which can

1. Make or break an electrical circuit either manually or by remote control under normal
conditions.

2. Break a circuit automatically under fault conditions.

3. Make a circuit either, manually or by remote control under fault conditions

4.1. WORKING PRINCIPLE OF CIRCUIT BREAKER

Circuit breaker essentially consists of fixed and moving contacts. These contacts are touching
each other and carrying the current under normal conditions when the circuit is closed. When the
circuit breaker is closed, the current carrying contacts, called the electrodes, engaged each other
under the pressure of a spring.

During the normal operating condition, the arms of the circuit breaker can be opened or closed
for a switching and maintenance of the system. To open the circuit breaker, only a pressure is
required to be applied to a trigger.

Whenever a fault occurs on any part of the system, the trip coil of the breaker gets energized and
the moving contacts are getting apart from each other by some mechanism, thus opening the
circuit

4.2. Arc Phenomenon

When the contacts of a circuit breaker are separated, there is a luminous electric discharge
between these two contacts known as ‘Arc’. This arc may continue until the discharge ceases.
The production of arc may delay the current interruption process and generate enormous heat
which may cause serious damage to system or to circuit breaker itself. Therefore, the main
problem in a circuit breaker is to extinguish the arc within the shortest possible time.

30
4.3. DIFFERENT TYPES OF CIRCUIT BREAKERS

4.3.a. Based on Voltage

 High voltage circuit breakers: These breakers are rated for use at voltages greater than
2 kV. High voltage circuit breakers are further subdivided into transmission class
breakers

 Low voltage circuit breakers: These breakers are rated for use at low voltages up to 2
kV and are mainly used in small-scale industries

Those which are rated 123 kV and above

Medium voltage class (lesser than 72 kV) circuit breakers

4.3.b. By Installation Location

 Indoor circuit breakers: These are designed to use inside the buildings or in weather
resistant enclosures. They are typically operated at a medium voltage with a metal clad
switchgear enclosure

 Outdoor Circuit breakers: You can use these breakers outdoors without any roof due to
their design. Their external enclosure arrangement is strong compared to the indoor
breakers and can withstand wear and tear.

4.3.c. Based on External Design

 Dead tank circuit breakers: The breakers whose enclosed tank is at ground potential are
known as dead tank circuit breakers

 .Their tank encloses all the insulating and interrupting medium. In other words, the tank
is shorted to ground or it is at dead potential.

 Live tank circuit breakers: These breakers have a tank housing interrupter that is at a
potential above the ground. It is above the ground with some insulation medium in
between

31
4.3.d. By Interrupting Mechanism
Air circuit breaker-This breaker uses air as an insulating and interrupting medium.
The breaker is sub-classified into two types

1. Low voltage circuit breaker whosevalueliesbelow1000V

2. High voltage circuit breaker whose value is1000Vandabove.It is further classified into oil
circuit breakers and the oil-less circuit breaker
 Oil circuit breaker-It uses oil as an interrupting and insulating medium. These
breakers are divided into two types based on the pressure and amount of oil
used.

 Vacuum circuit breakers-These breakers use vacuum as the interrupting

medium due to its high dielectric and diffusive properties.

 MCB (Miniature Circuit Breaker)-The current ratings for this breaker are less
than 100A and has only one over-current protection built within it. The trip
settings are not adjustable in this circuit.

 MCCB ( Moulded Case Circuit Breakers)-Current ratings for these breakers

are higher than 1000A. They have earth fault protection along with current
protection. The trip settings of the Molded Case Circuit Breaker can be adjusted
easily.

 Single pole circuit breaker-This breaker has one hot wire and one neutral wire
that operate at 120 V. When there is a fault, it will interrupt just the hotwire.

 Double pole circuit breaker-This is used for 220 V. There are two hotwires
and both the poles need to be interrupted.

 GFI or GFCI circuit breaker (Ground fault circuit interrupter)-These are

safety switches that trip on ground fault current. The GFCI breaker interrupts
the electrical circuit when it detects the slightest variance between phase and
neutral wires.

 Arc Fault circuit interrupter (AFCI) -The AFCI breaker interrupts the circuit
during excessive arc conditions and prevents fire. Under the normal arcing
condition, this breaker will be idle and won’t interrupt the circuit.
.

32
 4.4 RELAY FAMILY

ELECTRO MAGNETIC STATIC NUMERICAL MECHANICAL

Based on Time character

1. Thermal
Based on Actuations a) OT Trip
1. Define time relays
b) WT trip
1. Current relays 2. Inverse time relays with c) Lamp trip etc.
define minimum time 2. Float Type
2. Voltage relays (IDMT) a) Buchholz
3. Frequency relays b) OSR
3. Inverse with d.mt and
instantaneous element c) PVR
d) Water level
controls

Auxiliary relays Inclusive of logic

1. Annunciate relays 1. Differential

2. Inter trip relays 2. Unbalance

3. Multi contactor 3. Neutral displacement

D.C operated relays 4. Directional

4. Time delay relays
5. Restricted earth fault
5. Fuse failure relays
6. Over fluxing
6. Trip circuit
supervision relays,etc 7. Distance schemes

8. Bus bar protection

33
4.5. Definition of Relay:

A relays is a sensing device which is used to detect the faults in an electrical circuit and trip
circuit breaker. Over load

Definition of protective relay :

A relay is automatic device which senses an abnormal condition of electrical circuit and closes
its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence
make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from
rest of the healthy circuit .Now let’s have a discussion on some terms related to protective relay.

Pickup level of actuating signal:

The value of actuating quantity (voltage or current) which is on threshold above which the relay
initiates to be operated .If the value of actuating quantity is increased, the electromagnetic effect
of the relay coil is increased, and above a certain level of actuating quantity, the moving
mechanism of the relay just starts to move.

Reset Level:

The value of current or voltage below which a relay opens its contacts and comes in original
position.

Reset time of relay:

The time which elapses between the instant when the actuating quantity becomes less than the
reset value to the instant when the relay contacts return to its normal position.

Reach of relay:

A distance relay operates whenever the distance seen by the relay is less than the pre specified
impedance. The actuating impedance in the relay is the function of distance in a distance
protection relay. This impedance or corresponding distance is called the reach of relay. Power
system protection relays can be categorized into different types of relays.

Operating time of relay:

Just after exceeding pickup level of actuating quantity the moving mechanism (for example
rotating disc) of relay starts moving and it ultimately closes the relay contacts at the end of its
journey. The time which elapses between the instant when actuating quantity exceeds the pickup
value to the instant when the relay contacts close. Now let’s have a look on which different
protective relays are used in different power system equipment protection schemes.

34
4.6. Relays for transmission & distribution lines protection

Sl.NO Lines to be protected Relays to be used

1 400 KV transmission line Main-I: non switched or numerical distance scheme

main-ii: non switched or numerical distance scheme
2 220 KV transmission line Main-I : non switched distance scheme (fed from bus pts)
main-ii: switched distance scheme (fed from line CVTS)
with a changeover facility from bus pt to line CVT and vice
versa.
3 132 KV transmission line Main protection: switched distance scheme (fed from bus
pt). Backup protection: 3 nos. Directional IDMT o/l relays
and 1 no. Directional IDMT e/l relay
4 33 KV lines Non-directional IDMT 3 o/l and 1 e/l relays.

5 11 KV lines Non-directional IDMT 2 o/l and 1 e/l relays.

4.7. Transformer protection –different types of relays

Buchholz relay
Earth fault relay
Over current relay
Differential relay
Digital relay

35
4.7.a. Differential relay

Generally differential protection is provided in the electrical power transformer rated more than
5mva.The differential protection of transformer has many advantages over other schemes of
protection.
The faults occur in the transformer inside the insulating oil can be detected by buchholz relay.
But if any fault occurs in the transformer but not in oil then it cannot be detected by buchholz
relay. Any flash over at the bushings are not adequately covered by buchholz relay. Differential
relays can detect such type of faults. Moreover buchholz relay is provided in transformer for
detecting any internal fault in the transformer but differential protection scheme detects the same
in faster way.
The differential relays normally response to those faults which occur inside the differential
protection zone of transformer.

4.8. Differential protection scheme in a power transformer

4.8.a Principle of differential protection

Principle of differential protection scheme is one simple conceptual technique. The differential
relay actually compares between primary current and secondary current of power transformer, if
any unbalance found in between primary and secondary currents the relay will actuate and inter
trip both the primary and secondary circuit breaker of the transformer. Suppose you have one
transformer which has primary rated current ip and secondary current is. If you install ct of ratio
ip/1a at the primary side and similarly, ct of ratio is/1a at the secondary side of the transformer.
The secondaries of these both CT's are connected together in such a manner that secondary
currents of both CT’s will oppose each other.
In other words, the secondaries of both CT’s should be connected to the same current coil of a
differential relay in such an opposite manner that there will be no resultant current in that coil in
a normal working condition of the transformer. But if any major fault occurs inside the
transformer due to which the normal ratio of the transformer disturbed then the secondary current
of both transformers will not remain the same and one resultant current will flow through the
current coil of the differential relay, which will actuate the relay and inter trip both the primary
and secondary circuit breakers. To correct phase shift of current because of star-delta connection
of transformer

36
 Fig :4.0 Principle of differential protection

winding in the case of three-phase transformer, the current transformer secondaries should be
connected in delta and star as shown here.
At maximum through fault current, the spill output produced by the small percentage unbalance
may be substantial. Therefore, differential protection of transformer should be provided with a
proportional bias of an amount which exceeds in effect the maximum ratio deviation.



4.8.b. Percentage differential relay or biased differential protection:

Generally differential protection relay means the relay operates when the phasor difference
between the two or more electrical quantities exceed the pre-set value. The electrical quantity
may be voltage or current. But mostly voltage based relays are not preferred. We use to prefer
current based differential protection, but it has some limitations such as both ct should be
identical ct ratio, identical burden, extension cable resistance nuisance trip etc. In order to avoid
these, percentage differential protection can be used.

The percentage differential relay is designed to operate the differential current in terms of its
fractional relation with actual current flowing through the circuit. It is used to protect the system
under current transformer saturation, unequal ct ratios, nuisance trip etc. It increases the stability
of the differential protection relays.

37
 Fig :4.1 : Percentage differential relay or biased differential protection

4.8.c. Working function of percentage differential protection:

Two coils are there in the relay. One is operating coil and another one is restraining coil. Here
restraining coil produce force or torque which will oppose the operating coil of the relay. Let’s
take n is the number of turns in the operating coil and nr is the number of turns in the restraining
coil. The connection is made as shown in the figure. In this two coils are placed and the
operating coil k carries the differential current which means i1-i2 and another one coil is
restraining coil r carries the current proportional to (i1+i2)/2 because of the coil k is connected in
midpoint the restraining coil. Normally current i1 flows in the restraining coil in nr/2 parts, the i2
current flows another nr/2 parts... Hence the effective ampere turns are...

That’s why we have taken the total current through the restraining coil as i1+i2)/2

Under normal condition, the force produced by the restraining coils is greater than the force
produced by the operating coils. Therefore, relay does not operate.

Hence

38
During fault condition…the operating force become higher than the restraining force, due to
this the operating coil trips the mechanism.

Thus the ratio of differential current to average restraining current is always a fixed percentage.
Therefore it is called as percentage differential relay.

4.8.d. Characteristics of percentage relay:

Fig :4.2 . Characteristics of percentage relay

4.8.e. Philosophy of protective relaying:

Function of protective relaying is to cause prompt removal from service of any element of a
power system when it suffers a short circuit or when it starts to separate in any abnormal manner
that might cause damage or otherwise interfere with the effective operation of the rest of the
system.

39
4.9. EARTHING

In an electrical installation, an earthing system or grounding system connects specific parts of

that installation with the earth's conductive surface for safety and functional purposes. The point
of reference is the earth's conductive surface. The choice of earthing system can affect the safety
and electromagnetic compatibility of the installation. Regulations for earthing systems vary
considerably among countries, though most follow the recommendations of the international
electro technical commission. Regulations may identify special cases for earthing in mines, in
patient care areas, or in hazardous areas of industrial plants.
In addition to electric power systems, other systems may require grounding for safety or
function. Tall structures may have lightning rods as part of a system to protect them from
lightning strikes. Telegraph lines may use the earth as one conductor of a circuit, saving the cost
of installation of a return wire over a long circuit. Radio antennas may require particular
grounding for operation, as well as to control static electricity and provide lightning protection.
1. Pipe earthing
2. Plate earthing

4.9.a. Pipe Earthing:

A galvanized steel and a perforated pipe of approved length and diameter is placed vertically in a
wet soil in this kind of system of earthing. It is the most common system of earthing. The size of
pipe to use depends on the magnitude of current and the type of soil. The dimension of the pipe
is usually 40mm (1.5in) in diameter and 2.75m (9ft) in length for ordinary soil or greater for dry
and rocky soil. The moisture of the soil will determine the length of the pipe to be buried but
usually it should be 4.75m(15.5ft).

Fig :4.3: . Pipe Earthing:

40
Fig : schematic diagram of pipe earthing

41
4.9.b. Plate earthing:

In plate earthing system, a plate made up of either copper with dimensions 60cm x 60cm x
3.18mm (i.e. 2ft x 2ft x 1/8 in) or galvanized iron (gi) of dimensions 60cm x 60cmx
6.35 mm (2ft x 2ft x ¼ in) is buried vertical in the earth (earth pit) which should not be less than
3m (10ft) from the ground level

Fig : schemetic diagram of plate earthing

42
 Max. value of earth resistance to be achieved

Equipment to be earthed Max. value of the earth resistance to be

achieved in ohm
1. Large power station 0.5

2. Major substation 1.0

3. Small substation 2.0

4. Factories substation 1.0

5. Lattice steel tower 3.0

6. Industrial machine and equipment 0.5

The earth resistance depends upon the moisture content in the soil

4.10.MAINTAINANCE OF SCHEDULES

4.10.a. Transformers and reactors:

without shutdown activities:

Sl. NO Checking of the equipment Maintenance

1 Checking of bushing oil level Monthly

2 Checking of oil level in conservator Monthly

3 Checking of oil level in OLTC conservator Monthly

4 Checking of the leaks Monthly

5 Manual actuation of cooler oil pumps and fans Monthly

6 Checking conditions of silica gel in breather Monthly

7 Checking of oil level in seal of breather Monthly

8 Testing of oil for DGA and other oil parameters 6 Monthly

43
9 Vibrations measurements (for shunt reactors only) 2 Monthly

Shut down activities:

Sl. NO Checking of the equipment Maintenance

1 BDV, PPM of OLTC diverter switch compartment oil (less Yearly

frequently if operations are not more)

2 External cleaning of radiators Yearly

3 Cleaning of all bushings Yearly

4 Checking of auto starting of cooler pumps and fans Yearly

5 Marshalling boxes of transformer/reactors Yearly

1.cleaning of marshalling boxes of transformers/reactors and Yearly

OLTC

2.lightening of terminations Yearly

3.checking of contractors ,space heaters, illuminations ,etc. Yearly

6 Maintenance of OLTC driving mechanism Yearly

7 Checking of all remote indication (wti and tap position Yearly

indicator) and top up oil in pockets, if required

8 Electrical checking /testing of pressure relief device Yearly

,buchholz relay, OLTC surge relay / checking of alarm/trip
and checking/replacement of gaskets of the terminal box

9 Checking/testing of buchholz relay by oil draining Yearly

10 Frequency response analyzing SOS(as and when

required)

11 Tan measurement of bushing Yearly

12 Recovery voltage measurement SOS

44
  Minimum oil circuit breakers:

Sl. NO Checking of the equipment Maintenance

1 Checking of oil leak from grading capacitors Monthly

2 Checking for oil leakage /oil level and n2 pressure Monthly

(if applicable)

3 Testing of oil for BDV Monthly

4 Maintenance of breather and changes of silica gel Monthly

 Spring operated mechanism:

Sl. NO Checking of the equipment Maintenance

1 Oil leakages from close and open dashpots, replace the Yearly
same if leakage observed

2 Greasing/lubrication of gears and various latches in the Yearly

operating mechanism

3 Maintenance of spring charging motors ,cleaning of Yearly

carbon brushes and contactors

4 Checking of play of gaps in catch gears Yearly

5 Replacement of oil in dashpot SOS

45
4.10.b. Circuit breakers:

SF6 circuit breakers:

Sl. NO Checking of the equipment Maintenance

1 Checking of oil from grading capacitors Monthly

2 Sf6 gas leakage test SOS

3 Dew point measurement of sf6 gas 3 years

4 Checking tightness of foundations bolts years

 Vacuum circuit breakers:

Sl. NO Checking of the equipment Maintenance

1 Cleaning of control cubicle and checking for loose Quarterly

connections

2 Checking of on/off indicator, spring indicator and Half yearly

checking manual and electrical operation

3 Checking vacuum of interrupter by application of high Yearly

voltage by disengaging with operating mechanism

4 Checking erosion of contacts by erosion mark on Yearly

operating rod or measurement of gap specified in
closed position of contacts(wherever provided)

5 Checking tightness of foundation bolts Yearly

6 Replacement of vacuum interrupter SOS

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4.10.c. Current transformers:
Sl. NO Checking of the equipment Maintenance

1 Checking of below expansion Monthly

2 Visual inspection of ct for oil leakage and crack Monthly

in insulators ,etc.

3 Thermo vision scanning of ct Yearly

4 Checking of oil leakage in terminal box Yearly

5 Checking of primary connection strips , if Yearly

provided externally

6 N2 pressure checking 2 Yearly

7 Measurement of tan and capacitance 2 Yearly

8 IR measurement (DAR) 2 Yearly

9 Checking of primary connection strips ,if SOS

provided internally

10 Measurement of ct secondary resistance SOS

11 Magnetization characteristics SOS

12 Ct ratio test SOS

13 DGA and testing of other parameters of oil SOS

14 Checking of burden on the secondary winding SOS

4.10.d. Potential transformers/capacitance voltage transformers/coupling

capacitors:
Sl. NO Checking of the equipment Maintenance

1 checking of oil leaks Monthly

47
 2 Measurement of voltage at control room panel Half yearly

3 Visual checking of earthing HF point (in case it Yearly

is not being set for PLCC)

4 Checking of any breakage or cracks in cementing Yearly

joint

5 Cleaning of CVT capacitors stacks and tightness Yearly

of terminal connections

6 Thermo vision scanning of capacitors stacks Yearly

7 Capacitors and tan measurement 3 Yearly

8 Testing of emu tank oil for BDV (if oil found SOS
discolored)

9 Checking for rust and painting SOS

*To be repeated before 1 year from commission and then as per schedule .this test is not possible
to be conducted at site if isolated of neutral of intermediate pt is not possible at site.

4.10.e. Protection systems:

Distance protection:
Sl. NO Checking of the equipment Maintenance

1 Reach check for all 4 zones Yearly

2 Time measurement Yearly

3 Power swing blocking check Yearly

4 Switch on the fault(soft) check Yearly

5 Level detectors of PPS Yearly

6 Fuse failure check Yearly

7 Polarization check Yearly

8 Negative phase sequence(NPS) detectors check Yearly

9 CVT fuse failure checking Yearly

48

Differential relays:

Sl. NO Checking of the equipment Maintenance

1 Pick up current at the fixed /selected setting Yearly

2 Operation of high set element/ instantaneous unit at Yearly

the fixed /selected setting

3 Operation of the relay at the selected restraint Yearly

features

4 Checking of 2nd harmonic current restraint feature Yearly

5 Operation of alarm and trip contacts Yearly

6 Through current stability checks on the existing Yearly

load

 Under voltage relay:

Sl. NO Checking of the equipment Maintenance

1 Starting and pick up of the relay as per plug setting Yearly

2 Relay operating time as per relay characteristics Yearly

3 Operation of alarm and rip contacts Yearly

4 Verification of input voltage on relay terminals Yearly


Over voltage relay

Sl. NO Checking of the equipment Maintenance

1 Staring and pick up of the relay Yearly

2 Relay operating time as per relay characteristics Yearly

49
 3 Operation of high set element/instantaneous unit at Yearly
voltage setting .if applicable

4 Operation of alarm and trip contacts Yearly

5 Verification of input voltage on relay terminals Yearly


Over current and earth fault relay:

Sl. NO Checking of the equipment Maintenance

1 Staring and pick up of the relay as plug setting Yearly

2 Time of operation as per relay characteristic Yearly

3 Operation of high set element/instantaneous unit at Yearly

current setting ,if applicable

4 Operation of alarm and trip contacts Yearly

5 Verification of input currents Yearly

6 Verification of directional features, if applicable Yearly

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 CHAPTER-5

CONCLUSION

Now from this report one can conclude that electricity plays an important role in
our life at the end of the training. I came to know about the various parts of
substation and how they are operated .also I learnt about how transmission is done
in various parts of East Godavari district. As evident from the report, a substation
plays a very important role in the transmission system. That’s why various
protective measures are taken to protect the substations from various faults and its
smooth functioning .APTRANSCO takes such steps so that a uniform and Supply
of electricity can reach in every part of this state.

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