This document provides an overview of Vinay Vashisht's industrial training project report on the 33/11 kV substation in Uttarkashi, Uttarakhand Power Corporation Ltd. The report includes acknowledgments, contents, and sections covering an overview of UPCL, training at the Uttarkashi substation, transformers, substation components like earthing materials and bus bars, protection equipment, and protection against lightning. It provides technical details and specifications of the equipment at the substation.

1. INDUSTRIAL TRAINING PROJECT
REPORT
ON
UTTARAKHAND POWER
CORPORATION LTD.
33/11 KV SUBSTATION
UTTARKAASHI
Submitted To: Submitted by:
Vinay Vashisht
ECE 7th sem
11-ECE-1616

2. ACKNOWLEDGEMENT
With profound respect and gratitude, I take the opportunity to convey my
great thanks to complete the training here, since training has very important
role in exposing real life situation in an industry.
I am extremely grateful to all the technical staff of UTTARKAASHI
substation UPCL, UTTARKAASHI for their co-operation and guidance that
helped me a lot during the course of training. I have learnt a lot working
under them and I will always be indebted of them for this value addition in
me.
I would also like to thank the training in charge of Rohtak Institute Of Tech.
& Management , Rohtak and all the faculty member of Electronics And
Communications Engineering department for their effort of constant co-operation
which have been significant factor in the accomplishment of my
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industrial training.

3. CONTENTS
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S.R.
NO.
TOPIC PAG
E NO
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
UPCL-AN OVERVIEW
TRAINING AT UPCL(UTTARKAASHI)
TRANSFORMER
COMPONENTS OF TRANSFORMER
TYPES OF TRANSFORMER
SUBSTATION
EARTHING MATERIAL
BUS BAR
INSULATOR
MISCELLANEOUS EQUIPMENT
PROTECTION OF SUBSTATION
CIRCUIT BREAKER
PROTECTION AGAINST LIGHTENING
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4
6
10
12
14
17
20
21
22
23
25
28

4. An Overview
Uttarakhand, the 27th State of India was created on 9th November 2000 as the 10th
Himalayan State of the country blessed with the natural and mineral resources in
abundance and poised to be a 20000 MW HYDRO POWER HUB of India in the
future.
Uttarakhand Power Corporation Ltd (UPCL), formerly Uttaranchal Power Corporation
Ltd was incorporated under the Companies Act, 1956 on February 12, 2001
consequent upon the formation of the State of Uttaranchal. UPCL has been entrusted to
cater to the Transmission & Distribution Sectors inherited after the de merger from
UPPCL since 1st April 2001. The Electricity Act. 2003 mandated the separation of
Transmission functions under Power Sector Reforms. On 1st June 2004, the Power
Transmission Corporation Limited (PTCUL) was formed to maintain & operate 132
KV & above Transmission Lines & substations in the State. Today UPCL,
the State Power Distribution Utility of the Government of Uttaranchal (GOU) caters to
the Sub –Transmission & Distribution Secondary Substations & Distribution Lines
66 KV & below in the State .UPCL - the Frontline State Power Distribution Utility &
service provider of QUALITY & RELIABLE POWER SUPPLY to over 1.59 million
consumers of electricity spread over the 13 Districts of Uttarakhand.
These electrical consumers are categorized depending on their domestic, commercial,
agricultural and industrial loads. UPCL is also the first electrical utility in India to
initiate women empowerment by employing local women through Self Help Groups,
as franchisees, for meter reading, bill distribution and revenue collection.
UPCL looks forward to a committed participation from a Team of professionals always
striving for performance excellence with new innovative technologies to strengthen the
Power Distribution Infrastructure of the STATE in Seamless Integration with
Generation & Transmission Utilities for the Socio – economic development.. A
comprehensive POWER EVACUATION PLAN is underway with construction of new
33/11 KV Substations in the State.
3

5. Training at UPCL (UTTARKAASHI)
I was appointed to do training from this organization from 20th June to 19rd July 2013.
In this duration I was assigned under the supervision of Mr. Shakti Prasad for
understanding distribution section, UPCL.
Definition of sub-station: “The assembly of apparatus used to change some
characteristics of electric supply is called sub-station”.
Introduction: The present day electrical power system is a.c. electric power is
generated, transmitted, and distributed in the form of Alternating current. The electric
power is produce at the power station, which are located at favorable places, generally
quite away from the consumers. It is delivered to the consumer through a large network
of transmission and distribution. At many place in the line of power system, it may be
desirable and necessary to change some characteristic of electric supply. This is
accomplished by suitable apparatus called sub-station for example, generation voltage
(11KV or 6.6KV) at the power station is stepped up to high voltage (Say 220KV to
132KV) for transmission of electric Power. Similarly near the consumer’s localities,
the voltage may have to be stepped down to utilization level. Suitable apparatus called
sub-station again accomplishes this job.
About the substation: The substation in UTTARKAASHI, Uttarakhand is one of the
important power grids in the state of Uttarakhand. Cause it supplies the Barkot
Industrial area & locality. The most important of any substation is the grounding
(Earthing System) of the instruments, transformers etc. used in the substation for the
safety of the operation personnel as well as for proper system operation and
performance of the protective devices. An earthen system comprising of an earthing
mat buried at a suitable depth below ground and supplemented with ground rods at
suitable points is provided in the substations. These ground the extra high voltage to
the ground. As it is dangerous to us to go near the instrument without proper earth. If
the instruments are not ground properly, they may give a huge shock to anyone who
would stay near it and also it is dangerous for the costly Instrument as they may be
damaged by this high voltage
Site Selection of 33 KV Substation: 33KV Sub-Station forms an important link
between Transmission network and Distribution network. It has a vital Influence of
reliability of service. Apart from ensuring efficient transmission and Distribution of
4

6. power, the sub-station interruptions in power Supply. Sub-Station is constructed as
near as possible to the load center. The voltage level of power transmission is decided
on the quantum of power to be transmitted to the load center. Transmission is decided
on the quantum of power to be transmitted to the load center.
Selection of site: Main points to be considered while selecting the site for Grid Sub-
Station are as follows:
i) The site chosen should be as near to the load center as possible.
ii) It should be easily approachable by road or rail for transportation of equipment.
iii) Land should be fairly leveled to minimize development cost.
iv) Source of water should be as near to the site as possible. This is because water is
required for various construction activities (especially civil works), earthing and
for drinking purposes etc.
v) The sub-station site should be as near to the town / city but should be clear of
public places, aerodromes, and Military / police installations.
vi) The land should have sufficient ground area to accommodate substation
equipments, buildings, staff quarters, space for storage of material, such as store
yards and store sheds etc. with roads and space for future expansion.
vii) Set back distances from various roads such as National Highways, State
Highways should be observed as per the regulations in force.
viii) While selecting the land for the Substation preference to be given to the Govt.
5
land over private land.
ix) The land should not have water logging problem.
x) Far away from obstructions, to permit easy and safe approach /termination of
high voltage overhead transmission lines. configuration should be such that it
enables easy maintenance of equipment and minimum

7. 6
Transformer
Transformer is a static machine, which transforms the potential of alternating current at
same frequency. It means the transformer transforms the low voltage into high voltage
& high voltage to low voltage at same frequency. It works on the principle of static
induction principle. The transformer is an electromagnetic conversion device in which
electrical energy received by primary winding is first converted into magnetic energy
which is reconverted back into a useful electrical energy in other circuits (secondary
winding, tertiary winding, etc.). Thus, the primary and secondary windings are not
connected electrically, but coupled magnetically. A transformer is termed as either a
step-up or step-down transformer depending upon whether the secondary voltage is
higher or lower than the primary voltage, respectively. Transformers can be used to
either step-up or step-down voltage depending upon the need and application; hence
their windings are referred as high-voltage/low-voltage or high-tension/low-tension
windings in place of primary/secondary windings.
Transformers mainly are of three types:
1. Step up transformer
2. Isolators
3. Step down transformer
Figure: Transformer

8. Major Transformer in Power Plant
7
Generator Transformer:
In generator transformer the generator is connected to this transformer by the means of
isolated bus-ducts. This transformer is used to step up the generated voltage to the grid
voltage. It is generally provided with OFB cooling and oil immersed circuit taps on the
high voltage side are also present. The transformer has an elaborate cooling system
consisting of a number of oil pumps and cooling fan apart from various accessories
described later.
GT Specification:
Rating(MVA) 125
Type of cooling OFB
Temp. rise of oil 45°C
kV (No Load) HV 233Kv
LV 10.5kV
Phase HV 3
LV 3

9. Unit Auxiliary Transformer:
It draws input from main bus duct connecting the generating transformer. Total KVA
capacity unit auxiliary transformer required can be determined by using 0.85 p.f.& 0.9
efficiency for total auxiliary motor load. It is usually safe and desired to provide 20%
excess capacity then calculated to provide miscellaneous auxiliaries.
UAT Specification:
1. UAT Rating (MVA) 12.5
2. Volts at NO Load (KVA) HV- 10.5
8
LV- 6.6
3. Line Current HV- 687 A
LV- 1047 A
4. Phase HV- 3
LV- 3
5. Type of Cooling ON
6. Frequency 50 Hz
7. Vector Group Symbol Delta- delta
8. Insulation Level HV- 75 kV (peak)
LV- 60 kV (peak)
9. Temperature Rise of Oil 450 C
10. Temperature Rise of Winding 550 C

10. Station Transformer:
This transformer is placed after generator transformer. It steps down 220 kV to 440 V
for plant electricity like lighting, drinking water purpose and for general purpose motor
(not LT or HT motors which run by UAT). It works for all the time and in case of plant
shutdown, these transformers are fed by the national grid and also fulfill the purpose of
UAT for the plant startup.
Different Components of a Transformer:
Magnetic circuit:
It consists of a high permeance core over which both primary and secondary coils are
wound. Electrical energy transfer between two circuits takes place through a
transformer without the use of moving parts; the transformer therefore has higher
efficiency and low maintenance cost as compared to rotating electrical machines.
Winding:
The rectangular paper-covered copper conductor is the most commonly used conductor
for the windings of medium and large power transformers. These conductors can be
individual strip conductors, bunched conductors or continuously transposed cable
(CTC) conductors. In low voltage side of a distribution transformer, where much fewer
turns are involved, the use of copper or aluminum foils may find preference. To
enhance the short circuit withstand capability, the work hardened copper is commonly
used instead of soft annealed copper, particularly for higher rating transformers. In the
case of a generator transformer having high current rating, the CTC conductor is
mostly used which gives better space factor and reduced eddy current losses in
windings.
Conservator Tank:
A small tank placed on the top of main tank. It is half filled with air and half filled with
oil. It maintains the level of oil in transformer. If oil level falls air comes in conservator
through the breather to fill the vacuum created.
9

11. Breather:
It performs the function of releasing and taking atmospheric air. Further it is filled with
silica gel to prevent the contamination of transformer oil in the conservator by the
moisture present in the air entering the conservator.
Cooling Mechanism:
Low power transformers are generally air cooled. For large power transformers, air
cooling is used. Oil performs the dual role of a coolant (heat exchanger) and an
insulating medium.
Explosive Vent:
In case of severe fault in the transformer, the internal pressure may be build up to a
very high level, where it may result in an explosion of tank. Therefore this vent is
provided to remove the excess pressure from transformer if any such situation arises.
Buchholz Relay:
This relay is used as a protective device sensitive to the effects of dielectric failure
inside the equipment. On a slow accumulation of gas, due perhaps to slight overload,
gas produced by decomposition of insulating oil accumulates in the top of the relay and
forces the oil level down. A float switch in the relay is used to initiate an alarm signal.
If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator.
This flow of oil operates a switch attached to a vane located in the path of the moving
oil. This switch normally will operate a circuit breaker to isolate the apparatus.
Mulsifire Mechanism:
This is provided for protection in case of a fire break-out in the transformer. A pipe
filled with pressurized air at 2-3 kg/cm2 is connected to a glass bulb. This pressure
stops a valve which operates the flow of water through nozzles provided over the entire
tank. In case of a fire, the glass bulb shatters due to the heat, releasing the pressurized
air. This fall in pressure causes the mulsifire valve to open, releasing water sprays from
the nozzle, thereby quenching the fire.
10

12. SWITCH YARD
Switch yard is a switching yard and is defined as the enclosed areas at the power
stations containing switching facilities and equipment for the purpose of connecting to
the transmission network. It consists of Isolators, circuit breakers, Current
Transformers, Potential Transformers, Capacitance Voltage Transformers, Wave Trap
and Lightning Arrestors.
Switchyard forms an integral part of any power station i.e., thermal Power Utilities,
Gas turbines based power plants or Hydel power plants. Switchyard will exist at a
generating station to coordinate the exchange of power the generators and the
transmission lines in the area.
11
Basic Structure of a switch yard
Current Transformer is used for measuring high current and Potential Transformer is
used for measuring high voltage.
Capacitance Voltage Transformers are used for maintaing the constant voltage in case
of voltage drop in transmission line.
Isolators is off-load switching device which disconnects the connection between
busbars in off condition. It id on both the sides of switch yard.
In BTPS, SF6 circuit breakers are used disconnect the connection between busbars in
both off-load and on-load condition.
Wave Trap is used for sending and receiving of wave through transmission lines. It is
basically used for detecting any fault in transmission line.
Lightning Arrestors are used to supress the high voltage formed due to lightning to
ground.
TYPES OF TRANSFORMER
1 Power transformer

13. 12
2 Instrument transformer
3 Auto transformer
4 On the basis of working
5 On the basis of structure
Power Transformer:
1.Single phase transformer
2.Three phase transformer
Instrument Transformer
1.Current transformer
2. Potential transformer
Auto Transformer
1. Single phase transformer
2. Three phase transformer
ON THE BASIS OF WORKING
Step down: Converts high voltage into low voltage.
Step up: Converts low voltage into high voltage.

14. 13
ON THE BASIS OF STRUCTURE
Figure: core type
Figure: Shell type

15. 14
SPECIFICATION OF C.T.
Figure : Current transformer
1 Standard: IS-2785
2 Highest System Voltage: 145 KV
3 Frequency: 50Hz
4 C.T. Current: 25 KA/1Sec.
5 Rated primary current: 800 Ampere
SUBSTATIONS
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 (11 KV or 33 KV) at the power station is set up to high voltage (say 220 KV or

16. 132 KV) 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 again
accomplished by suitable apparatus called substation.
The assembly of apparatus to change some characteristic of electric power supply is
called substation.
TYPES OF SUBSTATION
15
According to the service requirement:
Transformer substation
Switch substation
Power factor correction substation
Frequency change substation
Converting substation
Industrial substation
According to the constructional features:
Indoor substation
Outdoor substation
Underground substation
Pole mounted substation
TRANSFORMER SUBSTATION
They are known as transformer substations as because transformer is the main
component employed to change the voltage level.
depending upon the purposed served transformer substations may be classified into:
STEP UP SUBSTATION
The generation voltage is steeped up to high voltage to affect economy in transmission
of electric power. These are generally located in the power houses and are of outdoor
type.
PRIMARY GRID SUBSTATION
Here, electric power is received by primary substation which reduces the voltage level
to 66KV for secondary transmission. The primary grid substation is generally

17. 16
SECONDARY SUBSTATIONS
At a secondary substation, the voltage is further steeped down to 11KV. The 11KV
lines runs along the important road of the city. The secondary substations are also of
outdoor type.
DISTRIBUTION SUBSTATION
These substations are located near the consumer’s localities and step down to 400V, 3-
phase, 4-wire for supplying to the consumers. The voltage between any two phases is
400V & between any phase and neutral it is 230V.
SUBSTATION CHARACTERISTICS:
 Each circuit is protected by its own circuit breaker and hence plant outage does
not necessarily result in loss of supply.
 A fault on the feeder or transformer circuit breaker causes loss of the transformer
and feeder circuit, one of which may be restored after isolating the faulty circuit
breaker.
 A fault on the bus section circuit breaker causes complete shutdown of the
substation. All circuits may be restored after isolating the faulty circuit breaker.
 Maintenance of a feeder or transformer circuit breaker involves loss of the
circuit.
 Introduction of bypass isolators between bus bar and circuit isolator allows
circuit breaker maintenance facilities without loss of that circuit.
STEPS IN DESIGNING SUBSTATION:
The First Step in designing a Substation is to design an Earthing and Bonding System.
Earthing and Bonding:
The function of an earthing and bonding system is to provide an earthing system
connection to which transformer neutrals or earthing impedances may be connected in
order to pass the maximum fault current. The earthing system also ensures that no
thermal or mechanical damage occurs on the equipment within the substation, thereby
resulting in safety to operation and maintenance personnel. The earthing system also

18. guarantees equipotent bonding such that there are no dangerous potential gradients
developed in the substation.
In designing the substation, three voltage have to be considered these are:
17
Touch Voltage:
This is the difference in potential between the surface potential and the potential at
earthed equipment whilst a man is standing and touching the earthed structure.
Step Voltage:
This is the potential difference developed when a man bridges a distance of 1m with
his feet while not touching any other earthed equipment.
Mesh Voltage:
This is the maximum touch voltage that is developed in the mesh of the earthing
grid.To determine the effective substation earthing resistance, from which the earthing
voltage is calculated.In practice, it is normal to take the highest fault level for
substation earth grid calculation purposes. Additionally, it is necessary to ensure a
sufficient margin such that expansion of the system is catered for.To determine the
earth resistivity, probe tests are carried out on the site. These tests are best performed
in dry weather such that conservative resistivity readings are obtained.
Earthing Materials
Conductors:
Bare copper conductor is usually used for the substation earthing grid. The copper bars
themselves usually have a cross-sectional area of 95 square millimeters, and they are
laid at a shallow depth of 0.25-0.5m, in 3-7m squares. In addition to the buried
potential earth grid, a separate above ground earthing ring is usually provided, to which
all metallic substation plant is bonded.
Connections:

19. Connections to the grid and other earthing joints should not be soldered because the
heat generated during fault conditions could cause a soldered joint to fail. Joints are
usually bolted, and in this case, the face of the joints should be tinned.
18
Earthing Rods:
The earthing grid must be supplemented by earthing rods to assist in the dissipation of
earth fault currents and further reduce the overall substation earthing resistance. These
rods are usually made of solid copper, or copper clad steel.
Switchyard FenceEarthing:
The switchyard fence earthing practices are possible and are used by different utilities.
These are:
 Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter.
The fence is then bonded to the grid at regular intervals.
 Place the fence beyond the perimeter of the switchyard earthing grid and
bond the fence to its own earthing rod system. This earthing rod system is
not coupled to the main substation earthing grid.
CONDUCTORS USED IN SUBSTATION DESIGN:
An ideal conductor should fulfills the following requirements:
 Should be capable of carrying the specified load currents and short time
currents.
 Should be able to withstand forces on it due to its situation. These forces
comprise self weight, and weight of other conductors and equipment, short
circuit forces and atmospheric forces such as wind and ice loading.
 Should be corona free at rated voltage.
 Should have the minimum number of joints.
 Should need the minimum number of supporting insulators.
 Should be economical.

20. The most suitable material for the conductor system is copper or aluminums. Steel may
be used but has limitations of poor conductivity and high susceptibility to corrosion.
In an effort to make the conductor ideal, three different types have been utilized, and
these include: Flat surfaced Conductors, Stranded Conductors, and Tubular Conductors
19
Overhead Line Terminations
Two methods are used to terminate overhead lines at a substation.
 Tensioning conductors to substation structures or buildings
 Tensioning conductors to ground winches.
The choice is influenced by the height of towers and the proximity to the substation.
The following clearances should be observed:
VOLTAGE LEVEL MINIMUM GROUND CLEARANCE
less than 66kV 6.1m
66kV - 110kV 6.4m
110kV - 165kV 6.7m
greater than 165kV 7.0m
POWER LINE CARRIER COMMUNICATION
Introduction:
Reliable & fast communication is necessary for safe efficient & economical power
supply. To reduce the power failure in extent & time, to maintain
the0020interconnected grid system in optimum working condition; to coordinate the
operation of various generating unit communication network is indispensable for state
electricity board.
In state electricity boards, the generating & distribution stations are generally located at
a far distance from cities. Where P & T communication provided through long
overhead lines in neither reliable nor quick. As we have available very reliable

21. physical paths viz. the power lines, which interconnected, hence power line carrier
communication is found to be most economical and reliable for electricity boards.
20
APPLICATIONS:
The PLCC can be used for the following facilities:
 Telephony
 Teleportation
 Remote control or indication
 Telemetry
 Teleprinting
BUSBARS
When numbers of generators or feeders operating at the same voltage have to be
directly connected electrically, bus bar is used as the common electrical component.
Bus bars are made up of copper rods operate at constant voltage. In large stations it is
important that break downs and maintenance should interfere as little as possible with
continuity of supply to achieve this, duplicate bus bar system is used. Such a system
consists of two bus bars, a main bus bar and a spare bus bar with the help of bus
coupler, which consist of the circuit breaker and isolator.
In substations, it is often desired to disconnect a part of the system for general
maintenance and repairs. An isolating switch or isolator accomplishes this. Isolator
operates under no load condition. It does not have any specified current breaking
capacity or current making capacity. In some cases isolators are used to breaking
charging currents or transmission lines. While opening a circuit, the circuit breaker is
opened first then isolator while closing a circuit the isolator is closed first, then circuit
breakers. Isolators are necessary on supply side of circuit breakers, in order to ensure
isolation of the circuit breaker from live parts for the purpose of maintenance. A
transfer isolator is used to transfer main supply from main bus to transfer bus by using
bus coupler (combination of a circuit breaker with two isolators), if repairing or
maintenance of any section is required.

22. INSULATORS
The insulator serves two purposes. They support the conductors (bus bar) and confine
the current to the conductors. The most common used material for the manufacture of
insulator is porcelain. There are several types of insulators (e.g. pin type, suspension
type, post insulator etc.) and their use in substation will depend upon the service
requirement. For example, post insulator is used for bus bars. A post insulator consists
of a porcelain body, cast iron cap and flanged cast iron base. The hole in the cap is
threaded so that bus bars can be directly bolted to the cap.
With the advantage of power system, the lines and other equipment operate at very
high voltage and carry high current. The arrangements of switching along with
switches cannot serve the desired function of switchgear in such high capacity circuits.
This necessitates employing a more dependable means of control such as is obtain by
the use of the circuit breakers. A circuit breaker can make or break a circuit either
manually or automatically under all condition as no load, full load and short circuit
condition.
A circuit breaker essentially consists of fixed and moving contacts. These contacts can
be opened manually or by remote control whenever desired. When a fault occurs on
any part of the system, the trip coils of breaker get energized and the moving contacts
are pulled apart by some mechanism, thus opening the circuit.
When contacts of a circuit breaker are separated, an arc is struck; the current is thus
able to continue. The production of arcs are not only delays the current interruption,
but is also generates the heat. Therefore, the main problem is to distinguish the arc
within the shortest possible time so that it may not reach a dangerous value.
The general way of classification is on the basis of the medium used for arc extinction.
21
MISCELLANEOUS EQUIPMENT:
Capacitor Bank: The load on the power system is varying being high during

23. morning and evening which increases the magnetization current. This result in
the decreased power factor. The low power factor is mainly due to the fact most
of the power loads are inductive and therefore take lagging currents. The low
power factor is highly undesirable as it causes increases in current, resulting in
additional losses. So in order to ensure most favorable conditions for a supply
system from engineering and economical stand point it is important to have
power factor as close to unity as possible. In order to improve the power factor
come device taking leading power should be connected in parallel with the load.
One of the such device can be capacitor bank. The capacitor draws a leading
current and partly or completely neutralize the lagging reactive component of
load current.
Figure: Capacitor bank
Capacitor bank accomplishes following operations:
 Supply reactive power
 Increases terminal voltage
 Improve power factor
A fuse is a short piece of wire or thin strip which melts when excessive current through
it for sufficient time. It is inserted in series with the circuit under normal operating
conditions; the fuse element is at a nature below its melting point. Therefore it carries
the normal load current overheating. It is worthwhile to note that a fuse performs both
detection and interruption functions.
22

24. PROTECTION OF SUBSTATION
Transformer protection:
Transformers are totally enclosed static devices and generally oil immersed. Therefore
chances of fault occurring on them are very easy rare, however the consequences of
even a rare fault may be very serious unless the transformer is quickly disconnected
from the system. This provides adequate automatic protection for transformers against
possible faults.
23
Conservator and Breather:
When the oil expands or contacts by the change in the temperature, the oil level goes
either up or down in main tank. A conservator is used to maintain the oil level up to
predetermined value in the transformer main tank by placing it above the level of the
top of the tank.
Breather is connected to conservator tank for the purpose of extracting moisture as it
spoils the insulating properties of the oil. During the contraction and expansion of oil
air is drawn in or out through breather silica gel crystals impregnated with cobalt
chloride. Silica gel is checked regularly and dried and replaced when necessary.
Marshalling box:
It has two meter which indicate the temperature of the oil and winding of main tank. If
temperature of oil or winding exceeds than specified value, relay operates to sound an
alarm. If there is further increase in temperature then relay completes the trip circuit to
open the circuit breaker controlling the transformer.

25. 24
Transformer cooling:
When the transformer is in operation heat is generated due to iron losses the removal of
heat is called cooling.
There are several types of cooling methods, they are as follows:
 Air natural cooling:
In a dry type of self cooled transformers, the natural circulation of surrounding
air is used for its cooling. This type of cooling is satisfactory for low voltage
small transformers.
 Air blast cooling:
It is similar to that of dry type self cooled transformers with to addition that
continuous blast of filtered cool air is forced through the core and winding for
better cooling. A fan produces the blast.
 Oil natural cooling:
Medium and large rating have their winding and core immersed in oil, which act
both as a cooling medium and an insulating medium. The heat produce in the
cores and winding is passed to the oil becomes lighter and rises to the top and
place is taken by cool oil from the bottom of the cooling tank.
 Oil blast cooling:
In this type of cooling, forced air is directed over cooling elements of
transformers immersed in oil.
 Forced oil and forced air flow (OFB) cooling:
Oil is circulated from the top of the transformers tank to a cooling tank to a
cooling plant. Oil is then returned to the bottom of the tank.
Bus-bar :- When a no. of lines operating at the same voltage have to be directly
connected electrically, bus-bar are used, it is made up of copper or aluminum bars
(generally of rectangular X-Section) and operate at constant voltage.
The bus is a line in which the incoming feeders come into and get into the instruments
for further step up or step down. The first bus is used for putting the incoming feeders
in LA single line. There may be double line in the bus so that if any fault occurs in the
one, the other can still have the current and the supply will not stop. The two lines in
the bus are separated by a little distance by a Conductor having a connector between
them. This is so that one can work at a time and the other works only if the first is
having any fault.

26. Circuit breaker :
A circuit breaker is an equipment, which can open or close a circuit under normal as
well as fault condition. These circuit breaker breaks for a fault which can damage other
instrument in the station. It is so designed that it can be operated manually (or by
remote control) under normal conditions and automatically under fault condition. A
circuit breaker consists of fixed & moving contacts, which are touching each other
under normal condition i.e. when breaker is closed. Whenever a fault occurs trip coil
gets energized, the moving contacts are pulled by some mechanism & therefore the
circuit is opened or circuit breaks. When circuit breaks an arc is stack between
contacts, the production of arc not only interrupts the current but generates enormous
amount of heat which may cause damage to the system or the breaker itself. Therefore
the main problem in a circuit breaker is to extinguish the arc within the shortest
possible time so that the heat generated by it may not reach a dangerous value. The
medium used for arc extinction is usually Oil, Air, Sulfur Hexafluoride (SF6) or
vacuum.
Circuit breakers can be classified on the basis of medium used for arc
extinction:
A. Oil Circuit Breakers:-
These are the oldest type of circuit breakers & have the virtues of reliability,
simplicity of construction & relative cheapness. These are mainly of two types:
a. Bulk Oil Circuit Breakers using large quantity of oil are also called the dead
tank type because the tank is held at earth potential. Such circuit breakers may
further be classified as:-
i. Plain Break Oil Circuit Breakers are very simple in construction & widely
used in low voltage d.c & a.c circuits. For use on higher voltages, they become
unduly large in size & need huge of transformer oil. In addition, such breakers
are not suitable for high-speed interruption; therefore, these cannot be used in
auto-closing.
ii. Self-Generated Pressure Oil Circuit Breakers are of three types viz. Plain
explosion pot having limited breaking capacity, cross jet explosion pot suitable
25

27. for interrupting heavy current t high voltage (66kV) & self-compensated
explosion pot suitable for operation both at heavy currents as well as low
currents. Plain explosion pot cannot be used either for very low currents because
of increased arcing time or for very heavy currents because of risk of bursting of
pot due to high pressure.
iii. Impulse Type Oil circuit Breakers have the main advantage, over other
conventional design, of reduced requirement of oil (roughly one-fourth). The
possibility of current chopping can also be avoided by using resistance
switching.
b. Low oil or Minimum Oil Circuit Breakers are also called the live tank circuit
breakers because the oil tank is insulated from the ground. Such circuit breakers are
now available for all type of voltages (3.6, 7.2, 12, 36, 72.5,145,245 & 420 kV) &
for the highest breaking capacities. The MOCB with rated voltage of 12 kV has a
single interrupter per phase without extra support insulator.
B. Low Voltage Air Circuit Breakers:-
These breakers are designed for use on d.c circuits & low voltage a.c circuits for the
protection of general lighting & motor circuits. These breakers are usually provided
with an over current tripping mechanism which may be of instantaneous or time delay
type or combination of both. Trip devices may be set over a range from about 80 to
160 percent of rating. The breakers may also be provided with over tripping ranges &
arrangements such as low voltage trip, shunt trip connected to ever voltage, reverse
current or over current relays. Such breakers are of rating of to & including 6,000 A a.c
& 12,000 A d.c, voltage ratings are 250 to 600 V a.c & 250 to 750 V d.c. Special
breakers available up to 3,000 V for d.c services.
26
C. Air Blast Circuit Breakers:
The air blast circuit breakers employs compressed air (at a pressure of 20
kg/c.m2) for arc extinction & are finding their best application in systems
operating 132 kV & above (up to 400kV) with breaking capacity up to 7,500
MVA (during short circuit fault)& above, although such breakers have also been
designed to cover the voltage range of 6,600 Volts to 132,000 Volts. These
breakers have the advantages of less burning of contacts because of less arc
energy, little maintenance , facility of high speed reclosure, no risk of explosion
& fire hazard & suitability for duties requiring frequent operations. The
drawbacks of such breakers are additional need of compressor plant for
supplying compressed air, current chopping, sensitivity restriking voltage & air
leakage at the pipe line fittings.

28. 27
D. Vacuum Circuit Breakers:
The idea behind the vacuum circuit breakers is to eliminate the medium between
the contacts-vacuum. The dielectric strength of vacuum is 1000 times more than
that of any medium. In construction it is very simple circuit breaker in
comparison to an air or oil circuit breakers. These breakers are used for reactor
switching, transformer switching, capacitor bank switching where the voltages
are high & the current to be interrupted is low.
E. Sulphur Hex-fluoride Circuit Breakers:
SF6 gas has unique properties, such as very high dielectric strength, non-reactive to the
other components of circuit breakers, high time constant & fast recombination property
after removal of the source energizing the spark, which proves it superior to the other
mediums (such as oil or air) for use in circuit breakers.
SF6 circuit breakers have the advantages of very much reduced electrical clearances,
performance independent of ambient conditions, noise less operation, reduce moisture
problem, minimum current chopping, small arcing time, no reduction in dielectric
strength of SF6, low maintenance, reduced installation time & increased safety. Such as
circuit breakers are used for rated voltages in the ranges of 3.6 to 760 kV.
For the later operation a relay wt. is used with a C.B. generally bulk oil C.B. are used
for voltage up to 66 KV while for high voltage low oil & SF6 C.B. are used. For still
higher voltage, air blast vacuum or SF6 cut breaker are used. The use of SF6 circuit
breaker is mainly in the substations which are having high input kv input, say above
132kv and more. The gas is put inside the circuit breaker by force ie under high
pressure. When if the gas gets decreases there is a motor connected to the circuit
breaker. The motor starts operating if the gas went lower than 20.8 bar. There is a
meter connected to the breaker so that it can be manually seen if the gas goes low. The
circuit breaker uses the SF6 gas to reduce the torque produce in it due to any fault in
the line. The circuit breaker has a direct link with the instruments in the station, when
any fault occur alarm bell rings.
Protective relay
A protective relay is a device that detects the fault and initiates the operation of the
C.B. is to isolate the defective element from the rest of the system”. The relay detects
the abnormal condition in the electrical circuit by constantly measuring the electrical
quantities, which are different under normal and fault condition. The electrical
quantities which may change under fault condition are voltage, current, frequency and
phase angle. Having detect the fault, the relay operate to close the trip circuit of C.B.
There are two principle reason for this; Firstly,if the fault is not cleared quickly, it may
cause unnecessary interruption of service to the customer. Secondly, rapid

29. disconnection of faulty apparatus limits the amount of damage to it & a prevents the
effects from speeding into the system. A protective relay is a device that detects the
fault & initiates the operation of circuit breaker to isolate the defective element from
the rest of the system. Most of the relays operate on the principle of electromagnetic
attraction or electromagnetic induction. The following important types of relays are
generally used in electrical distribution & transmission line:
28
1. Induction Type Over Current Relay
2. Induction Type Over Voltage Relay
3. Distance Relay
4. Differential Relay
5. Earth Fault Relay
1. Induction Type Over Current Relay: This type of relay operates on the principle
of electromagnetic induction initiates corrective measures when current in the circuit
exceeds a predetermined value . The actuating source is a current in the circuit supplied
to the relay by a current transformer . These relays are used on ac circuits only and can
operate for fault flow in either direction.
Under normal condition the resulting torque is greater than the driving torque produced
by the relay coil current. Hence the Aluminum disc remains stationary, by during fault
current in the protective circuit exceeds the preset value. The driving torque becomes
greater than the starting torque & the disc starts to rotate, hence moving contact bridges
are fixed contact when the disc rotates to a preset value. Trip circuit operates the circuit
breaker, which isolates the faulty section.
2. Induction Type Over Voltage Relay: This type of relay operates on the principle
of electromagnetic induction & initiates corrective measures when current in the circuit
exceeds a predetermined value. Under normal condition the aluminum disc remains
stationary. However if the voltage increases at any cost the disc starts to rotate, hence
moving contact bridges to the fixed contact when the disc rotates through a preset
angle. Trip circuit operates the circuit breaker, which isolates the faulty section.
3. Distance Relay: Under normal operating condition, the pull is due to the voltage
element. Therefore the relay contacts remains open. However when a fault occurs in
the protected zone the applied voltage to the relay decreases where the current
increases. The ratio of voltage to current faults is below the predetermined value.
Therefore, the pull of the current element will exceed that due to voltage element &
this causes the beam to tilt in direction to close the trip circuit.

30. 4. Differential Relay: It compensates the phase difference between the power
transformer’s primary & secondary. The C.T.s on the two sides are connected by pilot
wires at both ends are same & no current flows through the relays. If a ground or
phase-to-phase fault occurs, the currents in the C.T.s no longer will be the same &
the differential current flowing through the relay circuit will clear the breaker on
both sides of transformers. The protected zone is limited to the C.T.s on the low
voltage side & C.T.s on the high voltage side of the transformer.
This scheme also provides protection for short circuits between turns of the same phase
winding. During a short circuit, the turn ratio of power transformer is altered & cause
unbalance in the system which cause the relay to operate. However, such sorts are
better taken care by Buchholz relay.
5. Earth Fault Relay: This scheme provides no protection against phase to phase
faults unless & until they develop into earth faults. A relay is connected across
transformer secondary. The protections against earth faults are limited to the region
between the neutral & line current transformer.
Under normal operating condition, no differential current flows through the relay.
When earth fault occurs in the protected zone, the differential current flows through the
operating coil of the relay. The relay then closes its contacts to disconnect the
equipment from the system.
29
Protection Against Lightening:
Transients or Surges on the power system may originate from switching or other
causes, but the most important & dangerous surges are those which caused by
lightning. The lightning surges may cause serious damage to the expensive equipments
or strokes on transmission lines that reach the equipments travelling as a wave. Thus it
is necessary to provide a protection against lightning surges. They are:-
1. Earth Screen.
2. Overhead Ground Wire.
3. Lightning Arrestor.
1. Earth Screen: The power stations & the substations are generally have much
expensive equipments. These stations can be protected from direct lightning strikes by
providing earthing screens. It consists of a network of Copper conductors mounted all

31. over the electrical equipments in the substation or Power station. The screen is
properly connected to earth on at least two points through low impedance. On the
occurrence of direct stroke on the station the screen provides a low resistance path by
which lightning surges are connected to the ground. In this way station equipments are
protected against lightning.
2. Overhead Ground Wires: The most effective method of providing protection
against direct lightning strokes is by the use of overhead ground wires. The ground
wires are placed over line conductors at such position that practically all lightning
strokes are intercepted by them. The ground wire is ground at each tower or pole
through as low resistance as possible. When the direct lightning strokes occur on the
transmission line will be taken you by the ground wire. The heavy current flows to the
ground through the ground wire, so it protects the line from harmful effects of
lightning.
3.Lightening Arrestors: Firstly, we can see lightning arrestors. These lightning
arrestors can resist or ground the lightning, if falls on the incoming feeders. The
lightning arrestors can work in an angle of 30 degrees around them. They are mostly
used for protection of the instruments used in the substation. As the cost of the
instruments in the substation are very high to protect them from high voltage lightning
these arrestors are used.
It is a device used in Electrical Power systems to protect the insulation o the system
from the damaging effect of lightning. Metal Oxide arrestors (MOVs) have been used
for power system protection the mid 70s.The typical lightning arrestor is also known
surge arrestor has a high voltage terminal and a ground terminal. When a lightning
surge or switching surge travels down the power system to the arrestor, the current
from the surge is diverted around the protected insulation in most cases to earth.
Lightning arrestors with earth switch are used after the current transformers to protect
it from lightning i.e. from high voltage entering into it. This lightning arrestor has an
earth switch that can directly earth the lightning. The arrestor works at 30o to 45o angle
of the lightning making a cone. The earth switch can be operated manually, by pulling
the switch towards the ground. This also helps in breaking the line entering the station.
By doing so maintenance repair of any instrument could be performed.
Types of lightning arrestors:- There are several types of lightning arrestors are in use,
differs only in their constructional detail but they are electrically identical & operate on
the same principle.
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They are-

32. 31
a. Rod gap arrestor
b. Horn gap arrestor
c. Valve type arrestor