Abc

A
PRACTICAL TRAINING REPORT
On
“220 K.V. GRID SUBSTATION”
SIROHI, RAJASTHAN
Submitted in Partial Fulfillment
of Requirement for the award of
Degree of
BACHELOR OF TECHNOLOGY
IN
“ELECTRICAL ENGINEERING”
x
Submitted To: Submitted By:
Mr. Ankur Malik Dashrath Singh
(Professor) 12EUSEE002
Department Of Electrical Engineering
Shri U.S.B. College Of Engg. & Manag, Aburoad(Sirohi)
Rajasthan Technical University, Kota(Raj.)
[2015-16]
1

ACKNOWLEDEMENT
I would like to express my gratitude for valuable cooperate rendered by
Mr.A.S.BAIRWA(AEN), Mr.D.K.JAIN(XEN), who provided me various facilities
and lot of knowledge about GSS and its different parts during my training session and
a very special thanks to Mr. D.K.JAIN , who encouraged me in training session.
I would like to express my thanks to staff members, technicians of GSS
Sirohi, those co-operated me for setting the knowledge of various equipment and their
operations and for their encouragement during training session.
During my training period of two months, were taught about the working
operation of different types and capacity of transformer, circuit breakers, CT’s, PT’s,
CVT’s and other connected equipment installed at the site.
Date : (Dashrath Singh)
Place : M-8963849025
3

ABSTRACT
A substation is an assembly of apparatus, which transform the characteristics
of electrical energy from one form to another say from one voltage level to another
level. Hence a substation is an intermediate link between the generating station and
the load units. The incoming feeders are connected to bus-bar through lighting
arrestors, capacitive voltage transformer, line isolator, circuit breakers current-
transformers; line isolator etc.
In the 220KV GSS the income 220KV supply is stepped down to 132Kv with
the help of transformers which is furthers supplied to different sub-station according
to the load. In addition to these transformers, there are many other equipment’s are
also installed in220 KV yards.
At" GSS SIROHI", there is a separate control room. There are meters for
reading purpose. Control panel s provided for remote protection of 220KV switch
yards transformer incoming feeder, outing feeders. Bus bar has their own control plant
in their control rooms. The control panel carry the appropriate relays. Necessary
meters indicating lamp control.
The training at grid substation was very helpful. It has improved my
theoretical concepts of electrical power transmission and distribution. Protection of
various apparatus was a great thing. Maintenance of transformer, circuit breaker,
isolator, insulator, bus bar etc. was observable. I had a chance to see the remote
control of the equipment’s from control room itself, which was very interesting.
4

CONTENTS
Certificate.................................................................................................ii
Acknowledgement...................................................................................iii
Abstract....................................................................................................iv
Contents....................................................................................................v
List of Figures........................................................................................viii
List of Tables...........................................................................................ix
Chapter 1: Introduction.........................................................................10
1.1 Overview of R.S.E.B.
Chapter 2: Single Line Diagram...........................................................11
2.1 Electrical Component
2.2 Single Line Diagram of 220kv GSS Sirohi
2.3 Lines Connected With 220kv GSS Sirohi
2.4 Aluminium Cored Steel Rainforced Overhead
Transmission Conductors
Chapter 3: Lightening Arrestor............................................................16
Chapter 4: Capacitor Voltage Transformer........................................18
Chapter 5: Potential Transformer........................................................19
Chapter 6: Isolator.................................................................................20
Chapter 7: Wave Trap ..........................................................................22
7.1 Transmitter
7.2 Receiver’s
Chapter 8: Circuit Breaker...................................................................24
8.1 Air Blast Circuit Breaker
8.2 Oil Circuit Breaker
8.3 SF6 Circuit Breaker
8.4 Vaccum Circuit Breaker
Chapter 9: Current Transformer.........................................................29
Chapter 10: Bus Bar...............................................................................30
5

Chapter 11: Transformer......................................................................32
11.1 Core
11.2 Winding
11.3 Transformer Oil
11.4 Tapping and Tap Changer
Chapter 12: Relay...................................................................................35
12.1 Over Voltage Relay
12.2 Over Current Relay
12.3 Earth Fault Relay
12.4 Directional Relay
12.5 Differential Relay
12.6 Invers Time Characteristic Relay
12.7 Buchholz’s Relay
Chapter 13: Insulator.............................................................................37
13.1 Pin Type
13.2 Suspension Type
Chapter 14: Earthing.............................................................................39
14.1 Neutral Earthing
14.2 Electrical Earthing
Chapter 15: PLCC..................................................................................41
15.1 Basic Principle of PLCC
15.2 Line Traps or Wave Traps
15.3 Coupling Capacitor
15.4 Drainage Coils
Chapter 16: Control Room....................................................................44
16.1 Synchronizing
16.2 Energy Meter
16.3 Watt Meter
6

16.4 Frequency Power
16.5 LT Meter
16.6 Ammeter
16.7 MVAR Meter
16.8 Maximum Indicator Demand
Chapter 17: Battery Room.....................................................................46
17.1 Charging of Batteries
17.2 Discharging of Batteries
Chapter 18: Capacitor Bank.................................................................47
Chapter 19: Conclusion.........................................................................48
Chapter 20: References..........................................................................49
7

LIST OF FIGURES
Fig 2.1: Single Line Diagram of 220kv GSS Sirohi..............................................13
Fig 3.1: Lighting Arrestor......................................................................................16
Fig 4.1: Capacitive Voltage Transformer...............................................................18
Fig 5.1: Potential Transformer...............................................................................19
Fig 6.1: Insolator....................................................................................................20
Fig 7.1: Wave Trap................................................................................................22
Fig 8.1: Axial Blast Air Circuit Breaker................................................................27
Fig 8.2: Oil Circuit Breaker...................................................................................28
Fig 9.1: Current Transformer.................................................................................29
Fig 10.1: Bus Bar...................................................................................................31
Fig 11.1: Transformer............................................................................................34
Fig 12.1: Buchhol’z Relay.....................................................................................36
Fig 13.1: Insulator..................................................................................................38
Fig 14.1: Earthing .................................................................................................40
Fig 15.1: PLCC Circuit..........................................................................................43
Fig 16.1: Control Room.........................................................................................45
Fig 17.1: Battery Room........................................................................................46
Fig 18.1: Capacitor Bank.......................................................................................47
8

LIST OF TABLES
TABLES PAGE NO.
Table-2.1 – Electrical Components with Symbols.....................................................12
Table-2.2 – List of Lines Connected with 220kv GSS Sirohi....................................14
Table-2.3 – Conductor Area, Current Rating, Voltage Rating of Conductor Wire....15
9

CHAPTER-1
INTRODUCTION
1.1 Over View of R.S.E.B.
“Rajasthan State Electricity Board” started working form 1 July, 1957. When
India became independent its overall installed capacity was nearly 1900 MW. During
first year plan (1951-1956) this capacity was only 2300 MW. The contribution of
Rajasthan state was negligible during 1&2 year plans the emphasis was on
industrialization for that end it was considered to make the system of the country
reliable. Therefore Rajasthan state electricity board came into existence in July 1957.
The first survey for energy capacity in Rajasthan is held in 1989 at that time the total
electric energy capacity of Rajasthan is 20116 MW. At that time the main aim of
RSEB is to supply electricity to entire Rajasthan in the most economical way and the
RSEB comes under northern zone. 220 kv grid sub-station sirohi has been augmented
from 132kv to 220kv n 1984. This grid sub-station is located on highway no.27.
The aim of RSEB is to supply electricity to entire Rajasthan state in the most
economical way. Government of Rajasthan on 19th July 2000 issued a gazette
notification unbundling Rajasthan State Electricity Board into Rajasthan.
i. Rajya Vidyut Utpadan Nigam Ltd (RRVUNL), the generation
Company.
ii. Rajasthan RajyaVidyutPrasaran Nigam Ltd, (RRVPNL), the
transmission Company.
iii. Regional distribution companies namely
a) JaipurVidyutVitran Nigam Ltd, (JVVNL)
b) AjmerVidyutVitran Nigam Ltd (AVVNL)
c) Jodhpur VidyutVitran Nigam Ltd (JVVNL)
10

CHAPTER-2:
SINGLE LINE DIAGRAM
2.1 Electrical Component
Any complex power system even though they are three phase circuits, can be
represented by a single line diagram, showing various electrical components of power
system and their interconnection. In single line representation of substation the
electrical components such as power transformers, incoming and outgoing lines, bus-
bars, switching and protecting equipment’s, are represented by standard symbols and
their interconnections
Between them are shown by lines. Single line diagrams are useful in planning
a substation layout. Some of the standard Symbols used to represent substation
components are given in Table below
Table: 2.1 Electrical Components with Symbols
S.NO. ELECTRICAL COMPONENTS SYMBOLS
1 AC GENERATOR
2 BUS BAR
3 POWER TRANSFORMER
TWO WINDING
TRANSFORMER
4 THREE WINDING
TRANSFORMER
5 CURRENT TRANSFORMER
11

6 VOLTAGE TRANSFORMER
7 CIRCUIT BREAKER
8 CIRCUIT BREAKER WITH
ISOLATOR
9 ISOLATOR
10 LIGHTENING ARRESTOR
11 EARTH SWITCH
12 WAVE TRAP
13 COUPLING CAPACITOR
12

2.2 Single Line Diagram of 220 KV GSS Sirohi
Fig.2.1 Single line Diagram of 220KV GSS Sirohi
13

2.3 Lines Connected With 220 KV GSS Sirohi:
Table-2.2 List of Lines Connected with 220 KV GSS Sirohi
NAME OF
LINES
VOLTAGE
(KV)
TYPE OF
FEEDER
USING
CONDUCTOR
Bali I 220 I/C ZEBRA
Bali II 220 I/C ZEBRA
Jalore 220 I/C ZEBRA
PGCIL 220 I/C ZEBRA
Pindwara I 132 O/G PANTHER
Pindwara II 132 O/G PANTHER
Reodar 132 O/G PANTHER
Bagra 132 O/G PANTHER
Sumerpur 132 O/G PANTHER
Ramseen 132 O/G PANTHER
Sirohi City 33 O/G
Krishnganj 33 O/G
Mount Abu 33 O/G
Veervara 33 O/G
Velangri 33 O/G
Paldi 33 O/G
Jawal 33 O/G
Goyali 33 O/G
14

2.4 Aluminium Cored Steel Reinforced Overhead Transmission
Conductors
Table-2.3 Conductor Area, Current Rating, Voltage Rating of Conductor Wire
CODE NAME CALCULATED
AREA
(mm2
)
CURRENT
RATING
(AMP)
VOLTAGE
RATING
(KV)
GOPHER 30.62 77
WEASEL 36.88 84 11
FERRET 49.48 98
RABBIT 61.70 112
HORSE 116.2 148
DOG 118.5 153 33
WOLF 194.9 162
DINGO 167.5 179
LYNX 226.2 178
CARACAL 194.5 205
PANTHER 261.5 191 132
BISON 431.3 208
JAGAUR 222.3 197
ZEBRA 484.5 202 220
MOOSE 400
15

CHAPTER-3
LIGHTENING ARRESTOR
Lighting arrestor is a device, which protects the overhead lines and other
electrical apparatus via transformer from
overhead voltages and lighting.
Lightning, is a form of visible discharge
of electricity between rain clouds or
between a rain cloud and the earth The
electric discharge is seen in the form of a
brilliant arc, sometimes several
kilometres long, stretching between the
discharge points How thunderclouds
become charged is not fully understood,
but most thunderclouds are negatively
charged at the base and positively
charged at the top However formed, the
negative charge at the base of the cloud
induces a positive charge on the earth
beneath it, which acts as the second plate
of a huge capacitor .When the electrical
potential between two clouds or between
a cloud and the earth reaches a
sufficiently high value (about 10,000 V
per cm or about 25,000 V per in), the air
becomes ionized along a narrow path and
a lightning flash results. The typical
lightening arrestor has high voltage
terminal and a ground terminal. When a
lightening surge travels along the power
line to the arrestor, the current from the
surge is diverted through the arrestor, Fig.3.1 Lightening Arrestor
16

in most cases to earth.
The Thirties Alugard lightning arrester consists of a stack of one or more
units connected in series depending on the voltage and the operating condition of the
circuit three single pole arresters are required for 3-phase installation. The arresters
are single pole design and they are suitable for indoor and out-door service.
Each arrester unit consists essentially of permanently sealed Proclaim housing
equipped with pressure relief and containing a number of thirties value-element dishes
and exclusive lunate gaps shunted by Thirties resistors metal fitting cemented of the
housing provide means for bolting arrester units into a stack. Each arrester unit is
shipped assembled.
No charging or testing operation is required before placing them in service.
The thirties station-class arrester is designed to limit the surge voltages to a
safe value by discharging the surge current to ground; and to interrupt the small power
frequency follow current before the first current zero. The arrester rating is a define
limit of its ability to interrupt power follow current. It is important, therefore, to
assure that the system power frequency voltage from line to ground under any
Condition switching, fault, over voltage, never exceeds the arrester’s rating.
17

CHAPTER-4
CAPACITIVE VOLTAGE TRANSFORMER
CVTs are special kind of PTs using capacitors to step
down the voltage. A capacitor voltage transformer (CVT), or
capacitance coupled voltage transformer (CCVT) is a transformer
used in power systems to step down extra high voltage signals and
provide a low voltage signal, for measurement or to operate a
protective relay.
In its most basic form the device consists of three parts:
two capacitors across which the transmission line signal is split, an
inductive element to tune the device to the line frequency, and a
transformer to isolate and further step down the voltage for the
instrumentation or protective relay.
The device has at least four terminals: a terminal for
connection to the high voltage signal, a ground terminal, and two
secondary terminals which connect to the instrumentation or
protective relay. CVTs are typically single-phase devices used for
measuring voltages in excess of one hundred kilovolts where the
use of voltage transformers would be uneconomical.
The CVT is also useful in communication systems CVTs in
combination with wave traps are used for filtering high frequency
communication signals from power frequency. Fig.4.1 Capacitive
Voltage
Transformer
This forms a carrier communication network throughout the transmission network.
The capacitive voltage transformer comprises of a capacitor divider with its
associated Electro-magnetic unit. The divider provides an accurate proportioned
18

voltage, while the magnetic unit transform this voltage to levels suitable for
measuring, phase metering, protection etc. Capacitive voltage transformers are
available for system voltage of 33kv to 420kv.
19

CHAPTER-5
POTENTIAL TRANSFORMER
Potential transformers are instrument transformers. They have a large number
of primary turns and a few numbers of secondary turns. It is used to control the large
value of voltage.
Potential Transformer is designed for monitoring single-phase and three-phase
power line voltages in power metering applications. The primary terminals can be
connected either in line-to-line or in line-to-neutral configuration.
The lines in substations operate at high voltages.
The Measuring instruments are designed for low value of voltages. Potential
transformers are connected in lines to supply measuring instruments and protective
relays. These
Transformers make the low voltage instruments suitable for measurement of
high voltages. For example of 11kV/110V PT is connected to a power line and the
line voltage is 11kV then the secondary voltage will be 110V.
Fig.5.1 Potential Transformer
20

CHAPTER-6
ISOLATOR
Isolators are employed in substations to isolate a part of the system for
general maintenance. Isolator switches are operated only under no load condition.
They are provided on each side of every circuit breaker.
Either the switch isolates circuits that are continually powered or is a key
element which enables an electrical engineer to safely work on the protected circuit.
In some designs the isolator switch has the additional ability to earth the isolated
circuit thereby providing additional safety. Such an arrangement would apply to
circuits with inter-connect power distribution systems where both end of the circuit
need to be isolated. The major difference between an isolator and a circuit breaker is
that an isolator is an off-load device, whereas a circuit breaker is an on-load device.
They do not have any making or breaking capacity.
On fundamental basis the isolating switches can broadly divided into following
categories: -
1. Bus isolator
2. Line isolator cum earthing switch
3. Transformer isolating switch.
The operation of an isolator may be hand operated without using any supply
or may be power operated which uses externally supplied energy switch which is in
the form of electrical energy or energy stored in spring or counter weight.
In a horizontal break, centre rotating double break isolator, 3 strokes are
found. Poles are provided on each phase. The two strokes on side are fixed and centre
one is rotating. The centre position can rotate about its vertical axis at an angle of 90.
In closed position, the isolating stroke mounts on galvanized steel rolled frame. The
three poles corresponding to 3 phases are connected by means of steel shaft.
21

Isolators are of two types -
1. Single pole isolator
2. Three pole isolator
Fig.6.1 Isolator
Isolator for three-phase we provided in such a manner that for each phase,
one frame of isolator. These three isolator must be operated all together. In each
frame, line is connected to terminal stud. Terminal stud is coupled with contact.
Contact arm are supported by insulators. Contacts are made or broken by motor
operated mechanism. When contact is to be open then both arms are rotated in
opposite direction, so that contact is broken. Same time earthing pole moves upward
to make contact with a female contact situated adjoined to terminal stud. Hence that
terminal gets earthen. On these criteria isolator can be carried out manually but for
quick operation motor is used.
22

CHAPTER-7
WAVE TRAP
To communicate between two G.S.S. we use power line itself. Power line
carrying 50Hz power supply also carries communication signals at high frequency.
Wave Trap is a device used for this purpose. It traps the frequency of desired level for
communication and sends it to P.L.C.C. room through CVT.
Rejection filters are known as the line traps consisting of a parallel resonant
circuit ( L and C in parallel) tuned to the carrier frequency are connected in series at
each end of the protected line such a circuit offer high impedance to the flow of
carrier frequency current thus preventing the dissipation. The carrier current used for
PLC Communication have to be prevented from entering the power equipment’s such
as attenuation or even complete loss of communication signals. For this purpose wave
trap or line trap are used between transmission line and power station equipment to
avoid carrier power dissipation in the power plant and reduce cross talks with other
PLC Circuits connected to the same power station.
For matching the transmitter and receiver unit to coupling capacitor and
power line matching filters are provided. These filters normally have air corral
transformers with capacitor assumed. The matching transformer is insulated for 7-10
KV between the two windings and performs two functions. Firstly, it isolates the
communication equipment from the power line. Secondly, it serves to match.
7.1 Transmitter:
The transmitter consists of an oscillator and an amplifier. The oscillator
generates a frequency signal within 50 to 500 HZ frequency bands the transmitter is
provided so that it modulates the carrier with protective signal. The modulation
process usually involves taking one half cycle of 50 HZ signal and using this to create
block to carrier.
23

Fig.7.1 Wave Trap
7.2 Receivers:
The receivers usually consist of an alternate matching transformer band pass
filter and amplifier detector.
The amplifier detector converts a small incoming signal in to a signal capable
of operating a relatively intensive carrier receiver relay. The transmitter and receiver
at the two ends of protected each corresponds to local as far as transmitting.
24

CHAPTER-8
CIRCUIT BREAKER
A circuit breaker is an automatically operated electrical switch designed to
protect an electrical circuit from damage caused by overload or short circuit.
Its basic function is to detect a fault condition and, by interrupting continuity,
to immediately discontinue electrical flow Unlike a fuse, which operates once and
then has to be replaced, a circuit breaker can be reset (either manually or
automatically) to resume normal operation. Circuit breakers are made in varying
sizes, from small devices that protect an individual household appliance up to large
switchgear designed to protect high voltage circuits feeding an entire city
Every system is susceptible to fault or damages which can be caused due to
overloading, short-circuiting, earth fault etc. Thus to protect the system and isolate the
faulty section C B8.2 are required. Apart from breaking and making contacts, a C B
should be capable of doing continuously carry the maximum current at point of
installation.
In any circuit, carrying a large amount of current, if a contact is opened then
normally a spark is produced due to fact that current traverses its path through air gap
Arcing is harmful as it can damage precious equipment. Media are provided between
contacts. There are different arc quenching media:-
1) Air blast
2) Oil
3) SF6 gas
4) Vacuum
In 220 kV GSS, SF6 gas circuit breaker are used, as for greater capacity GSS ,
SF6 type breakers are very efficient
25

8.1 Air Blast Circuit Breaker:-
Air blast circuit breakers are normally only used at low voltage levels but are
available with high current ratings up to 6000 A and short circuit ratings up to 100 kA
at 500V. The air blast circuit breakers according to type of flow of blast of
compressed air around the contacts are three namely
(i) Axial (ii) Radial (iii) cross flow of blast air type.
They use ambient air as the arc quenching medium.
As the circuit breaker contacts open the arc is formed and encouraged by
strong thermal convection effects and electromagnetic forces to stretch across splitter
plates. The elongation assists cooling and deionization of the air/contact metallic
vapour mixture. The long arc resistance also improves the arc power factor and
therefore aids arc extinction at current zero as current and circuit breaker voltage are
more in phase. Transient recovery voltage oscillations are also damped thus reducing
overvoltages. Arc products must be carefully vented away from the main contact area
and out of the switchgear enclosure.
8.1.1 Advantages:-
 There is no risk of explosion and fire hazard.
 Due to less arc energy in it as compared to that in O.C.B. burning of contacts
is less
 It requires less maintenance
 It provides facility of high speed reclosed
8.1.2 Disadvantages:-
 Compressor plant compressed air is required.
 Air leaks at the fitting of the pipe line.
 It is very sensitive to restriking voltage and Current Chopping.
26

8.2 Oil Circuit Breaker:-
Mineral oil has good dielectric strength and thermal conductive properties. Its
insulation level is, however, dependent upon the level of impurities. Therefore regular
checks on oil quality are necessary in order to ensure satisfactory circuit breaker or
oil-immersed switch performance. Insulation strength is particularly dependent upon
oil moisture content. The oil should be carefully dried and filtered before use. Oil has
a coefficient of expansion of about 0.0008per°C and care must be taken to ensure
correct equipment oil levels.
The oil can be moved into arc zone after the current reaches zero. The oil
circuit breakers may be classified as:
(i)Plain break oil circuit breakers.
(ii) Self blast or self-generated or arc control oil circuit oil circuit breakers.
(iii) Externally generated pressure oil circuit breakers
.
8.2.1 Advantage:
a) Arc energy is absorbed in decomposing of oil
b) The gas formed, which is mainly hydrogen have a high diffusion rate
c) The oil used such as transformer oil is a very good insulator and allows
smaller cleaner between live conductors and earth components
d) The oil has ability to flow into the arc space after current is zero.
8.2.2 Disadvantage:
a) There is a risk of formation of explosive mixture with air.
b) Oil is easily in flammable and may causes fire hazards.
8.3 SF6 Circuit Breaker
The outstanding physical and chemical property of SF6 gas makes it an ideal
dielectric media for use in power switchgear. These properties are included:
27

1) High dielectric strength
2) Unique arc quenching ability
3) Excellent thermal stability
4) Good thermal conductivity
In addition, at normal temperature SF6 is chemically inert, inflammable, noncorrosive
and non-condensable at low temperatures.
8.3.1 SF6 versus oil:
SF6 is not flammable and toxic like oil. It is easier to handle, maintain and
repair equipment filled with SF6.
In case of breakdown of oil, strong surges of pressure may occur due to sudden
development of gaseous products. In case of breakdown of SF6,the only pressure rise
will result from the thermal expansion of gas.
Rating of SF6 breaker:
Type: hydraulic operated: (1975)
 Rated voltage 245kv
 Rated impulse withstand voltage
 Lightening switching 1050KV
 Rated power frequency voltage 520KVp
 Rated frequency 50 Hz
 Rated normal current 2000 A
 Rated short time current 40KA
 Rated short circuit duration 1 sec
 Breaking capacity symmetrical 40 KA
 Weight of complete breaker 11700Kg
 Weight of SF6 76.5Kg
Rating of SF6 breaker:
Type: pneumatic operated
 Make: ABB
 Rated Voltage 245KV
 Rated normal current 2000KA
28

8.4 Vacuum Circuit Breaker:-
Vacuum interrupter tubes or ‘bottles’ with ceramic and metal casings are
evacuated to pressures of some 10-6 to 10-9 bar to achieve high dielectric strength.
The contact separation required at such low pressures is only some 0 to 20mm and
low energy mechanisms may be used to operate the contacts through expandable
bellows.
8.4.1 Advantages:-
a) Low maintenance
b) Vacuum ‘bottles
c) Easy to replace
d) Complete isolation of the interrupter from atmosphere and Contaminants
e) Absence of oil minimizes fire risk
8.4.2 Disadvantages:-
a) Limited availability
b) May be found for open terminal
c) Designs up to 72.5kV were used in conjunction with SF6 insulation system
d) Spare vacuum ‘bottle’ holding require
Fig. 8.1 Axial Blast Air Circuit Breaker
29

Fig.8.2 Oil Circuit Breaker
30

CHAPTER-9
CURRENT TRANSFORMER
The lines in substations carry currents in the order of thousands of amperes.
The measuring instruments are designed for low value of currents. Current
transformers are connected in lines to supply measuring instruments and protective
relays. For example a 100/1A CT is connected in a line carrying 100A, and
Then the secondary current of CT is 1A.
When current in a circuit is too high to directly apply to measuring
instruments, a current transformer produces a reduced current accurately proportional
to the current in the circuit, which can be conveniently connected to measuring and
recording instruments A current transformer also isolates the measuring instruments
from what may be very high
Current transformer is a instrument transformer which is mainly used for
measuring currents where very high currents are flowing. According to the
construction of the current transformer the primary winding of transformer is in series
with high current carrying line & measuring instrument is connected to the secondary.
Fig.9.1 Current Transformer
31

CHAPTER-10
BUS BAR
When number of lines operating at the same voltage levels needs to be
connected electrically, bus-bars are used. Bus-bars are conductors made of copper or
aluminium, with very low impedance and high current carrying capacity.
This bus bar arrangement is very useful for working purpose as every GSS. It
is a conductor to which a number of cut .Are connected in 220 KV GSS there are two
bus running parallel to the each other, one is main and another is auxiliary bus is only
for standby, in case of failure of one we can keep the supply continues.
If more loads are coming at the GSS then we can disconnect any feeder
through circuit breaker which is connected to the bus bar. This remaining all the
feeders will be in running position .if we want to work with any human damage. In
this case all the feeders will be on conditions. According to bus voltage the material is
used .Al is used because of the property & features and it is cheap.
PROPERTIES COPPER ALUMINIUM
Electricity Resistively at 20 c 0.017241 0.00403
Temp coff. Of resistively 0.00411 0.00403
Softening tem. 200 180
Thermal 932 503
Conductivity Meting point 1083 57
Fig. 10.1 Bus Bar
32

CHAPTER-11
TRANSFORMER
A transformer is a device that transfers electrical energy from one circuit to
another through inductively coupled conductor -the transformer's coils.
A varying current in the first or primary winding creates a varying magnetic
flux in the transformer's core and thus a varying magnetic field through the secondary
winding. This varying magnetic field induces a varying electro-motive force, or
voltage in the secondary winding. This effect is called mutual induction. If a load is
connected to the secondary, an electric current will flow in the secondary winding and
electrical energy will be transferred from the primary circuit through the transformer
to the load.
In an ideal transformer, the induced voltage in the secondary winding (Vs) is
in proportion to the primary voltage (Vp), and is given by the ratio of the number of
turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:
The three major parts of transformer:
11.1 Core
Core is the main part of the transformer. It is subjected to magnetic flux. For
efficient operation, it is essential that the core of transformer must be constructed
from laminated magnetic material of low hysteresis loss and high permeability.
The core is constructed by stacking layers of thin steel laminations. Each
lamination is insulated from its neighbours by a thin non-conducting layer of
insulation. The effect of laminations is to confine eddy currents to highly elliptical
paths that enclose little flux, and so reduce their magnitude. Thinner laminations
reduce losses, but are more laborious and expensive to construct.
34

11.2 Winding
Core type transformers use concentric type of winding. Each limb is wound
with a group of coil consisting of both primary and secondary winding, which are
concentric to each other. Low voltage winding is placed near to the core (which is at
earth potential) and high voltage winding is placed outside, however L T and H T
windings are inter-leaved to reduce the leakage reactance.
The type and arrangement used for winding in core type transformers depend
upon many factors. Some of the factors are given below:
1. Current rating
2. Shot circuit strength
3. Temperature rise
4. Impedance
5. Surge voltage
6. Transportation facilities
The layered winding may have conductors wound in one, two or more layers
and is therefore accordingly called one, two or multi-layer winding.
11.3 Transformer Oil
Oil in transformers construction, serves the double purpose of cooling and
insulating. For use in transformer tank, oil has to fulfil certain specifications and must
be carefully selected.
In the choice of oil for transformer Use, following characteristics have to be
concern:
a) Viscosity: It determines the rate of cooling and varies with the
temperature. A high viscosity is an obvious disadvantage because of
the sluggish flow through small aperture, which it entails.
b) Insulating properties: It is usually unnecessary to trouble about the
insulating properties of oil. Since it is always sufficiently good. A
35

more important matter is however, the reduction of the dielectric
strength due to the presence of moisture, which must be avoided.
c) Flash point: The temperature vapour above an oil surface ignites
spontaneously is termed as the flash point. Flash point of oil, used in
transformer, is not to be less than about 160 degree for the reasons of
safety.
d) Sledging: Sledging means, the slow formation of semi-solid
hydrocarbons, sometime of an acidic nature, which are deposited on
winding and tank walls. The formation of sludge is due to heat and
oxidation. In its turns. It makes the whole transformer hotter thus
aggravating the trouble, which may proceed until the cooling ducts are
blocked. The chief remedy available is to use oil, which remains
without sludge formation even it is heated in the presence of oxygen,
and to employ expansion chambers to restrict the contact of hot oil
with the surrounding air. Among the products of oxidation of
transformer oil are volatile, water soluble, organic acids and water.
This in combination can attack and corrode iron and other metals. The
breathers prevent the moisture produced by oxidation of the oil.
11.3 Tapping and Tap Changer
The transformer has an on load tap changer to cater for a variation of +5% to -
15% in the HV voltage. The tap-changer may be either manually operated or motor
driven.
Tap changer is used to change the HV voltage. We use tap changer in HV side
only because in HV side current is less hence it is easy to handle lower amount of
current.
36

Tap changers are of two types:
a) No Load Tap changer: In this type tap changer, we have to cut off load
before changing the taps. These kinds of tap changer are used in small
transformers only.
b) On Load Tap changer: In this type tap changer load remains connected to
transformer while changing the taps. This kind of tap changer requires special
construction. Tapping winding is placed over HV winding.
Fig.11.1 Transformer
37

CHAPTER-12
RELAY
A relay is an electrically operated switch. It is device to sense the fault in
system. Current flowing through the coil of the relay creates a magnetic field which
attracts a lever and changes the switch contacts. The coil current can be on or off so
relays have two switch positions and they are double throw (changeover) switches.
Relays allow one circuit to switch a second circuit which can be completely separate
from the first.
12.1 Types of Relays
These are called normally opened, normally closed in GSS control room there is
panel in which the relays are set and there are many types of relays:
a) Over voltage relays
b) Over current relays
c) I D M T fault relay
d) Earth fault relay
e) Buchhloz’s relay
f) Differential relay
12.1.1 Over Voltage Relay: This protection is required to avoid damage of
system in case line becomes open circuited at one end .These fault would trip the local
circuit breaker thus block the local and remote ends This relay is operated i. e. ,
energized by CVT connected to lines
12.1.2 Over Current Relay: This relay has the upper electromagnet of non-
directional relay connected in series with lower non-directional electromagnet. When
the fault current flow through relay current coil which produces flux in lower magnet
of directional element. Thus the directional relay has the winding over the
38

electromagnets of non-directional element and produces a flux in lower magnet and
thus over current operates.
12.1.3 Earth Fault Relay: when a conductor breaks due to some reason
and it is earthen then earth fault occurs. The fault current is very high thus, there is
need to of over current relay this relay has minimum operating time
12.1.4 Directional Relay: It allows flowing the current only in one
direction then only this relay operates It has a winding connected through the voltage
coil of relay to lower magnet winding called current coil Which is energized by CT if
fault occurs This relay operates when v/I is less than theoretical value The v/I is
normally constant .
12.1.5 Differential Relay: This relay operates when phase difference of two
electrical quantities exceeds the predetermined value. It has always two electrical
quantities; hence in 400kv GSS for transformer differential relay is used.
12.1.6 Invers Time Characteristics Relay: The relay using here having the
inverse time characteristics having the time delays dependent upon current value. This
characteristic is being available in relay of special design.
These are:
a) Electromagnetic Induction type
b) Permanent magnetic moving coil type
c) Static type
12.1.7 Buchholz’s Relay: It is the protective device of the transformer. When
any fault occurs in the transformer then it indicates about fault and we disconnect the
transformer from the circuit. It is used in the power transformer. It is connected
between the tank and conservator. It has two floats on which two mercury switch are
attached. One float is used for the bell indication and other float is used for the
tripping. In the normal position the relay is filled with the oil and contacts of the
mercury switch are opened. When the earth fault occurs in the transformer then it
39

increases the temperature of oil and oil flows into the conservator through relay.
On the way it makes the contacts of the tripping circuit short. So in the we can say
that this relay works as circuit breaker.
Fig.12.1 Buchholz Relay
40

CHAPTER-13
INSULATOR
In order to avoid current leakage to the Earth, through the supporting structure
provide to the conductor of overhead transmission lines, insulators are used. The
conductors are secured to the supporting structures by means of insulating feature,
which do not allow current to flow through these support and hence finally to the
earth . Bus support insulators are porcelain or fibreglass insulators that serve to the
bus bar switches and other support structures and to prevent leakage current from
flowing through the structure or to ground. These insulators are similar in function to
other insulator used in substations and transmission poles and towers.
An Insulator should have following characteristic:
a) High Insulation resistance, High mechanical strength, No internal impurity or
crack Disc.
b) Generally Porcelain or glass is used as material for insulators. Porcelain
because of its low cost is more common.
Insulators can be classified in following ways:
13.1 Pin Type: These are designed to be mounted on a pin, which in turn is
installed on the cross arm of a pole.
13.2 Suspension Type: These insulators hang from the cross arm, there by
forming a string. The centre post carries the moving contact assembled at the
extremities the moving contact engages the fixed contacts are generally in the form of
spring loaded finger contact.
The insulator consist of following parts
a.)Contacts : The contacts are rated for line current and designed to withstand
electromagnetic strains and prevent charging at rated shortly time current the contact
are made of electrolytic fixed in housing.
41

b.) Switching blade: The blade is made of electrolytic copper.
c.) Tandom pipe: All three phases are opened or closed simultaneously with a
tandem pipe this is dipped galvanized and provided with on or off insulators and pad
locking.
d.) Motor operated: This is meant rotary motion of the linear operating pipe for
either of opening or closing for remote level local operation. Hand operation is also
provides with detectable handle that can be fitted and square.
Fig. 13.1 Insulator
42

CHAPTER-14
EARTHING
Earthing is the provision of a surface under the substation, which has a uniform
potential as nearly as zero or equal to Absolute Earth potential. The provision of an
earthing system for an electric system is necessary by the following reason.
a) In the event of over voltage on the system due to lighting discharge or other
system fault. Those parts of equipment which are normally dead as for as
voltage, are concerned do not attain dangerously high potential.
b) In a three phase circuit, the neutral of the system is earthed in order to
stabilize the potential of circuit with respect to earth.
The resistance of earthing system is depending on shape and material of earth
electrode used. The earthing is of two principal types:-
14.1 Neutral Earthing
Neutral Earthing also known as System Neutral Earthing (or Grounding)
means connecting the neutral point i.e. the star point of generator, transformer etc. to
earth. In rotating machines, generator, transformer circuit etc., the neutral point is
always connected to earth either directly or through a reactance. The neutral point is
usually available at every voltage level from generator or transformer neutral.
14.2Electrical Earthing
Electrical Earthing is different from neutral earthing. During fault condition,
the metallic parts of an electrical installation which do not carry current under normal
conditions may attain high potential with respect to ground. As human body can
tolerate only I=0.165A/T current for a given time t so to ensure safety we connect
such metallic parts to earth by means of Earthing system ,which comprises of
43

electrical conductor to send fault current to earth. The conductor used is generally in
the form of rods, plates, pipes etc.
Fig.14.1 Earthing
44

CHAPTER-15
POWER LINE CARRIER COMMUNICATION
Power line communication or power line carrier (PLC), also known as Power
line Digital Subscriber Line (PDSL), mains communication, power line telecom
(PLT), or power line networking (PLN), is a system for carrying data on a conductor
also used for electric power transmission. Broadband over Power Lines (BPL) uses
PLC by sending and receiving information bearing signals over power lines.
All power line communications systems operate by impressing a modulated
carrier signal on the wiring system. Different types of power line communications use
different frequency bands, depending on the signal transmission characteristics of the
power wiring used. Since the power wiring system was originally intended for
transmission of AC power, in conventional use, the power wire circuits have only a
limited ability to carry higher frequencies.
The propagation problem is a limiting factor for each type of power line
communications. A new discovery called E-Line that allows a single power conductor
on an overhead power line to operate as a waveguide to provide low attenuation
propagation of RF through microwave energy lines while providing information rate
of multiple Gbps is an exception to this limitation.
Data rates over a power line communication system vary widely. Low-
frequency (about 100-200 kHz) carriers impressed on high-voltage transmission lines
may carry one or two analog voice circuits, or telemetry and control circuits with an
equivalent data rate of a few hundred bits per second; however, these circuits may be
many miles long.
The problem associated with the PLC channel is the requirement to put the
carrier signal onto the high voltage line without damaging the carrier equipment.
Once the signal is on the power line it must be directed in the proper direction in order
for it to be received at the remote line terminal.
45

15.1 Basic Principle of PLCC
In PLCC the higher mechanical strength and insulation level of high voltage
power lines result in increased reliability of communication and lower attenuation
over long distances.
Since telephone communication system cannot be directly connected to the
high voltage lines, suitably designed coupling devices have therefore to be employed.
These usually consist of high voltage capacitors or capacitor with potential devices
used in conjunction with suitable line matching units (LMU’s) for matching the
impedance of line to that of the coaxial cable connecting the unit to the PLC transmit-
receive equipment.
Also the carrier currents used for communication have to be prevented from
entering the power equipment used in G.S.S as this would result in high attenuation or
even complete loss of communication signals when earthed at isolator.. Wave traps
usually have one or more suitably designed capacitors connected in parallel with the
choke coils so as to resonate at carrier frequencies and thus offers even high
impedance to the flow of RF currents.
15.2 Line Traps or Wave Traps
The carrier energy on the transmission line must be directed toward the remote
line terminal and not toward the station bus, and it must be isolated from bus
impedance variations. This task is performed by the line trap. The line trap is usually
a form of a parallel resonant circuit which is tuned to the carrier energy frequency. A
parallel resonant circuit has high impedance at its tuned frequency, and it then causes
most of the carrier energy to flow toward the remote line terminal. The coil of the line
trap provides a low impedance path for the flow of the power frequency energy. Since
the power flow is rather large at times, the coil used in a line trap must be large in
terms of physical size. Once the carrier energy is on the power line, any control of the
signal has been given over to nature until it reaches the other end. During the process
46

of travelling to the other end the signal is attenuated, and also noise from the
environment is added to the signal. At the receiving terminal the signal is decoupled
from the power line in much the same way that it was coupled at the transmitting
terminal. The signal is then sent to the receivers in the control house via the coaxial
cable.
15.3 Coupling Capacitor
The coupling capacitor is used as part of the tuning circuit. The coupling
capacitor is the device which provides a low
Impedance path for the carrier energy to the high voltage line and at the same time, it
blocks the power frequency current by being a high impedance path at those
frequencies.
It is desirable to have the coupling capacitor value as large as possible in order
to lower the loss of carrier energy and keep the bandwidth of the coupling system as
wide as possible. However, due to the high voltage that must be handled and financial
budget limitations, the coupling capacitor values are not as high as one might desire.
15.4 Drainage Coils
The drainage coil has a pondered iron core that serves to ground the power
frequency charging to appear in the output of the unit. The coarse voltage arrester
consists of an air gap, which sparks over at about 2 KV and protects the matching unit
against line surges. The grounding switch is kept open during normal operation and is
closed if anything is to be done on the communication equipment without interruption
to power flow on the line. The matching transformer is isolated for 7 to 10 KV
between the two winding and former two functions. Firstly it isolates the
communication equipment for the power line. Secondly it serves to match the
characteristic impedance of the power line 400-600 ohms to that of the co-axial
vacuum arrester (which sparks) is over at about 250 V is provided for giving
additional protection to the communication equipment.
47

The LMU which consists of the matching transformer and tuning capacitors
indicated above is tailor-made to suit the individual requirements of the coupling
equipment and is generally tuned to a wide band of carrier frequencies-(100-450 KHz
typical).
a) Advantages of PLCC
i. No separate wires are needed for communication purposes as the power
lines themselves carry power as well as the Communication signals. Hence the
cost of constructing separate telephone lines is saved.
ii. When compared with ordinary lines the power lines have appreciably higher
mechanical strength. They would normally remain unaffected under the
condition which might seriously damage telephone lines.
iii. Power lines usually provide the shortest route between the power stations.
iv. Power lines have large cross-sectional area resulting in very low resistance per
unit length. Consequently the carrier signal suffers lesser attenuation than
when travel on usual telephone lines of equal lengths.
v. Power lines are well insulated to provide negligible leakage between
conductors and ground even in adverse weather conditions.
vi. Largest spacing between conductors reduces capacitance which results in
smaller attenuation at high frequencies. The large spacing also reduces the
cross talk to a considerable extent.
b) Disadvantages of PLCC
i. Proper care has to be taken to guard carrier equipment and persons using them
against high voltage and currents on the line.
ii. Reflections are produced on spur lines connected to high voltage lines. This
increases attenuation and create other problems.
iii. High voltage lines have transformer connections, which attenuate carrier
currents. Sub-station equipment’s adversely affect the carrier currents.
48

iv. Noise introduced by power lines is much more than in case of telephone lines.
This due to the noise generated by discharge across insulators, corona and
switching processes.
Fig.15.1 PLCC Circuit Diagram
49

CHAPTER-16
CONTROL ROOM
To remote control of power switch gear requires the provision of suitable
control plates located at a suitable point remote from immediate vicinity of CB’s and
other equipment’s.
At "GSS SIROHI" the separate control room provided for remote protection of
220KV switch yards transformer incoming feeder, outing feeders. Bus bar has their
own control plant in their control rooms. The control panel carrier the appropriate
relays. Necessary meters indicating lamp control switches and fuses. There are meters
for reading purpose.
On each panel a control switch is provided for remote operation of circuit breaker.
There are two indicators which show that weather circuit breaker is closed or open. A
control switch for each insulator is also provided. The position indicator of isolator is
also done with the help of single lamp and indicator. The colour of signal lamps are as
follows:-
c) RED - For circuit breaker or isolator is close option
d) GREEN - For circuit breaker is in open position.
e) AMBER - Indicates abnormal condition requiring action.
In addition to used indication an alarm is also providing for indicating abnormal
condition when any protective relay or tripping relay has operated. Its constants
energies on auxiliary alarm. Relay which on operation completes the alarm belt
circuit.
16.1 Synchronizing
There is a hinged Synchronizing panel mounted at the end of control panel.
Before coupling any incoming feeders to the bus bar. It just be synchronized with
switches. When the synchronous copy shows zero we close the circuit breaker.
Synchronoscope is used to determine the correct instant of closing the switch which
connects the new supply to bus bar. The correct instant of synchronizing when bus
50

bars incoming voltage: Are in phase, Are equal in magnitude, Are in some phase
sequence, having same frequency
The voltage can be checked by voltmeter the function of synchronoscope is to
indicate the difference in phase and frequency.
16.2 Energy Meter
These are fitted on different panel to record transmitted energy and recorded in
energy hours. For this purpose MWH meter have been provided.
16.3 Watt Meter
This is mounted on each feeder panel to record import or export power.
16.3 Frequency Power
Provided to each feeder to measure frequency which analog or digital.
16.4 LT Meter
Provided on each panel or the purpose of indication of voltage.
16.5 Ammeter
These are used to indication the line current.
16.6 MVAR Meter
Provided for indicating power factor of import and export.
16.7Maximum Indicator Demand
Chief requirement of these indicators to record the minimum power factor
taken by feeder during a particular period. This record the average power successive
predetermined period.
51

CHAPTER-17
BATTERY ROOM
There is a battery room which has batteries for 132KV section and for 220KV
section. Therefore D.C. power available is for functioning of the control panels. A
battery charger to charge the battery. Various parts of lead acid batteries:-
Plates, Separators, Electrolyte, Container, Terminal port, Vent plugs etc.
17.1 Charging of Batteries
Initial charging: It is the first charging given to batteries by which the
positive plates are converted to “lead peroxide”, whereas these plates will converted
to spongy lead. Also in a fully charged battery the electrolyte specific gravity will be
at its highest value or 1.2 and its terminal voltage will be 24 volts.
17.2 Discharging of Batteries
When a fully charged battery delivers its energy out by meeting a load the lead
peroxide of the +ve plates slowly gets converted to lead sulphate and the spongy lead
of the –ve plates also gets converted into lead sulphate during this time the specific
gravity of the electrolyte also decreases the value around 1.00 and the terminal
voltage also decreases from its initial to a lower value which may be around 1.85 or
1.8.
Fig. 17.1 Battery Room
53

CHAPTER-18
CAPACITOR BANK
Capacitor banks are used to improve the quality of the electrical supply and
the efficient operation of the power system. Studies show that a flat voltage profile on
the system can significantly reduce line losses. Capacitor banks are relatively
inexpensive and can be easily installed anywhere on the network.
The capacitor unit is made up of individual capacitor elements, arranged in
parallel/ series connected groups, within a steel enclosure. The internal discharge
device is a resistor that reduces the unit residual voltage to 50V or less in 5 min.
Capacitor units are available in a variety of voltage ratings (240 V to 24940V) and
sizes (2.5 kvar to about 1000 kvar).
Fig.18.1 Capacitor Bank
54

CONCLUSION
An Engineer needs to have not just theoretical but practical as well and so
every student is supposed to undergo a practical training session where I have
imbibed the knowledge about transmission, distribution, and maintenance with
economic issues related to it. During my 60 days training session I have acquainted
with the repairing of the transformers and also the testing of oil which is a major
component of transformer. At last I would like to say that practical training taken at
220KV GSS has broadened my knowledge and has widened my thinking as a
professional.
55

REFRENCES
[1] Principles of Power System-by V.K.MEHTA
[2] Electrical Power System-by C.L.WADHWA
WEBSITE:-
• www.rvpnl.com
• www.ieee.com
• www.electricalforu.com
• www.nptel.com
[3] REPORT BY- Dashrath Singh U.S.B. College, ABUROAD Dept. of Electrical
Engg.
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