SUB-STATION EQUIPMENT'S & ITS FUNCTIONS

1. SUB-STATION EQUIPMENTS & ITS FUNCTIONS
Lightening Arrester
Lightening arrestors are the instrument that are used in the incoming feeders so that
to prevent the high voltage entering the main station. This high voltage is very dangerous
to the instruments used in the substation. Even the instruments are very costly, so to prevent
any damage lightening arrestors are used. The lightening arrestors do not let the lightening
to fall on the station. If some lightening occurs the arrestors pull the lightening and ground
it to the earth. In any substation, the main important is of protection which is firstly done by
these lightening arrestors. The lightening arrestors are grounded to the earth so that it can
pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45°
making a cone.
C V T
A capacitor voltage transformer (CVT) is a transformer used in power systems to
step-down extra high voltage signals and provide low voltage signals either for
measurement or to operate a protective relay. In its most basic form the device consists of
three parts: two capacitors across which the voltage signal is split, an inductive element
used to tune the device to the supply frequency and a transformer used to isolate and further
step-down the voltage for the instrumentation or protective relay. The device has at least
four terminals, a high-voltage terminal for connection to the high voltage signal, a ground
terminal and at least one set of secondaryterminals for connection to the instrumentation or
protective relay. CVTs are typically single-phase devices used for measuring voltages in
excess of one hundred kilovolts where the use of voltage transformers would be
uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors
connected in series. This results in a large voltage drop across the stack of capacitors that

2. replaced the first capacitor and a comparatively small voltage drop across the second
capacitor, C2, and hence the secondary terminals.
Wave Trap
Wave trap is an instrument using for tripping of the wave. The function of this trap
is that it traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum.
It is connected to the main incoming feeder so that it can trap the waves which may be
dangerous to the instruments here in the substation.
Instrument Transformer
Instrument transformers are used to step-down the current or voltage to measurable
values. They provide standardized, useable levels of current orvoltage in a variety of power
monitoring and measurement applications. Both current and voltage instrument
transformers are designed to have predictable characteristics on overloads. Properoperation
of over-current protection relays requires that current transformers provide a predictable
transformation ratio even during a short circuit.
These are further classified into two types which are discussed below.
a. Current Transformers
b. Potential Transformers
Current Transformer
Current transformers are basically used to take the readings of the currents entering
the substation. This transformer steps down the current from 800 amps to 1 amp. This is
done becausewe have no instrument for measuring ofsuch a large current. The main use of
this transformer is
a. Distance Protection
b. Backup Protection
c. Measurement

3. A current transformer is defined as an instrument transformer in which the secondary
current is substantially proportional to the primary current (under normal conditions of
operation) and differs in phase from it by an angle which is approximately zero for an
appropriate direction of the connections. This highlights the accuracy requirement of the
current transformer butalso important is the isolating function, which means no matter what
the system voltage the secondary circuit need to be insulated only for a low voltage.
The current transformer works on the principle of variable flux. In the ideal current
transformer, secondary current would be exactly equal (when multiplied by the turns ratio)
and opposite to the primary current. But, as in the voltage transformer, some of the primary
current or the primary ampere-turns are utilized for magnetizing the core, thus leaving less
than the actual primary ampere turns to be transformed into the secondary ampere-turns.
This naturally introduces an error in the transformation. The error is classified into current
ratio error and the phase error
Potential Transformer
There are two potential transformers used in the bus connected both side of the bus.
The potential transformer uses a bus isolator to protect itself. The main use of this
transformer is to measure the voltage through the bus. This is done so as to get the detail
information of the voltage passing through the bus to the instrument. There are two main
parts in it
a. Measurement
b. Protection
The standards define a voltage transformer as one in which the secondary voltage is
substantially proportional to the primary voltage and differs in phase from it by an angle
which is approximately equal to zero for an appropriate direction of the connections. This
in essence means that the voltage transformer has to be as close as possible to the ideal
transformer.

4. In an ideal transformer, the secondary voltage vector is exactly oppositeand equal
to the primary voltage vector when multiplied by the turn’s ratio.
In a practical transformer, errors are introduced because some current is drawn for
the magnetization of the core and because of drops in the primary and secondary windings
due to leakage reactance and winding resistance. One can thus talk of a voltage error which
is the amount by which the voltage is less than the applied primary voltage and the phase
error which is the phase angle by which the reversed secondaryvoltage vector is displaced
from the primary voltage vector.
Bus Bar
The bus is a line in which the incoming feeders comeinto 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.
A bus bar in electrical power distribution refers to thick strips of copper or aluminum
that conduct electricity within a switchboard, distribution board, substation, or other
electrical apparatus. The size of the bus bar is important in determining the maximum
amount ofcurrent that canbe safely carried. Bus bars are typically either flat strips orhollow
tubes as these shapes allow heat to dissipate more efficiently due to their high surface area
to cross sectional area ratio. The skin effect makes 50-60 Hz AC bus bars more than about
8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current
applications. A hollow section has higher stiffness than a solid rod of equivalent current
carrying capacity, which allows a greater span between bus bar supports in outdoor
switchyards. A bus bar may either be supported on insulators or else insulation may

5. completely surround it. Bus bars are protected from accidental contact either by a metal
enclosure or by elevation out of normal reach.
Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly
onto any metal chassis of their enclosure. Bus bars may be enclosed in a metal housing, in
the form of bus duct or bus way, segregated-phase bus, or isolated-phase bus.
Circuit Breaker
The circuit breakers are used to break the circuit if any fault occurs in any of the
instrument. These circuit breaker breaks for a fault which can damage other instrument in
the station. For any unwanted fault over the station we need to break the line current. This
is only done automatically by the circuit breaker. There are mainly two types of circuit
breakers used for any substations. They are
a. SF6 circuit breakers
b. Spring circuit breakers.
The use of SF6 circuit breaker is mainly in the substations which are having high
input kv input, say above 220kv and more. The gas is put inside the circuit breaker by force
i.e. under high pressure. When if the gas gets decreases there is a motor connected to the
circuit breaker. The motor starts operating if the gas went lower than 20.8 bar. There is a
meter connected to the breaker so that it can be manually seen if the gas goes low. The
circuit breaker uses the SF6 gas to reduce the torque produce in it due to any fault in the
line. The circuit breaker has a direct link with the instruments in the station, when any fault
occur alarm bell rings.
The spring type of circuit breakers is used for small kv stations. The spring here
reduces the torque produced so that the breaker can function again. The spring type is used
for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in
low distribution side.

6. Transformer
There are three transformers in the incoming feeders so that the three lines are step
down at the same time. In case of a 220KV or more KV line station auto transformers are
used. While in case of lower KV line such as less than 132KV line double winding
transformers are used.
The transformer is transported on trailor to substation site and as far as possible
directly unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally oil
filled, and those of higher capacity are transported with N2 gas filled in them +ve pressure
of N2 is maintained in transformer tank to avoid the ingress of moisture. This pressure
should be maintained during storage, if necessary by filling N2 Bushings - generally
transported in wooden cases in horizontal position and should be stored in that position.
There being more of fragile material, care should be taken while handling them. Radiators
– These should be stored with ends duly blanked with gaskets and end plates to avoid in
gross ofmoisture, dust, and any foreign materials inside. The care should be taken to protect
the fins of radiators while unloading and storage to avoid further oil leakages. The radiators
should be stored on raised ground keeping the fins intact.
Oil Piping. The Oil piping should also be blanked at the ends with gasket and blanking
plates to avoid in gross of moisture, dust, and foreign All other accessories like temperature
meters, oil flow indicators, PRVs, buchholz relay; oil surge relays; gasket ‘ O ‘ rings etc.
should be properly packed and stored indoor in store shed. Oil is received in sealed oil
barrels. The oil barrels should be stored in horizontal position with the lids on either side in
horizontal position to maintain oil pressure on them from inside and subsequently avoiding
moisture and water ingress into oil. The transformers are received on site with loose
accessories hence the materials should be checked as per bills of materials.

7. Isolator
The use of this isolator is to protect the transformer and the other instrument in the
line. The isolator isolates the extra voltage to the ground and thus any extra voltage cannot
enter the line. Thus an isolator is used after the bus also for protection.
Control and RelayPanel
The controland relay panel is ofcubical construction suitable for floor mounting. All
protective, indicating and control elements are mounted on the front panel for ease of
operation and control. The hinged rear door will provide access to all the internal
components to facilitate easy inspection and maintenance. Provision is made forterminating
incoming cables at the bottom of the panels by providing separate line-up terminal blocks.
For cable entry provision is made both from top and bottom. The control and relay panel
accepts CT, PT aux 230 AC and 220V/10V DC connections at respective designated
terminal points. 220V/10V DC supply is used for control supply of all internal relays and
timers and also for energizing closing and tripping coils of the breakers. 230V AC station
auxiliary supply is used for internal illumination lamp of the panel and the space heater.
Protective HRC fuse are provided with in the panel for P.T secondary. Aux AC and battery
supplies. Each Capacitor Bank is controlled by breaker and provided with a line ammeter
with selector switch for 3 phase system & over current relay (2 phases and 1 Earth fault for
3 phsystem). Under voltage and over voltage relays. Neutral Current Unbalance Relays are
for both Alarm and Trip facilities breaker control switch with local/remote selector switch,
master trip relay and trip alarms acknowledge and reset facilities.
Protective Relaying

8. Protective relays are used to detect defective lines or apparatus and to initiate the
operation of circuit interrupting devices to isolate the defective equipment. Relays are also
used to detect abnormal or undesirable operating conditions other than those caused by
defective equipment and either operate an alarm or initiate operation of circuit interrupting
devices. Protective relays protect the electrical system by causing the defective apparatus
or lines to be disconnected to minimize damage and maintain service continuity to the rest
of the system. There are different types of relays.
i. Over current relay
ii. Distance relay
iii. Differential relay
iv. Directional over current relay
i. Over Current Relay
The over current relay responds to a magnitude of current above a specified value.
There are four basic types of construction: They are plunger, rotating disc, static, and
microprocessor type. In the plunger type, a plunger is moved by magnetic attraction when
the current exceeds a specified value. In the rotating induction-disc type, which is a motor,
the disc rotates by electromagnetic induction when the current exceeds a specified value.
Static types convert the current to a proportional D.C mill volt signal and apply it to
a level detectorwith voltage or contact output. Such relays can be designed to have various
current-versus-time operating characteristics. In a special type of rotating induction-disc
relay, called the voltage restrained over current relay. The magnitude of voltage restrains
the operation of the disc until the magnitude of the voltage drops below a threshold value.
Static over current relays are equipped with multiple curve characteristics and can duplicate
almost any shape of electromechanical relay curve. Microprocessor relays convert the
current to a digital signal. The digital signal can then becompared to the setting values input
into the relay. With the microprocessorrelay, various curves or multiple time-delay settings

9. can be input to set the relay operation. Some relays allow the user to define the curve with
points or calculations to determine the output characteristics.
ii. Distance Relay
The distance relay responds to acombination ofboth voltage and current. Thevoltage
restrains operation, and the fault current causes operation that has the overall effect of
measuring impedance. The relay operates instantaneously (within a few cycles) on a 60-
cycle basis for values of impedance below the set value. When time delay is required, the
relays energizes a separate time-delay relay or function with the contacts or output of this
time-delay relay or function performing the desired output functions. The relay operates on
the magnitude of impedance measured by the combination of restraint voltage and the
operating current passing through it according to the settings applied to the relay. When the
impedance is such that the impedance point is within the impedance characteristic circle,
the relay will trip. The relay is inherently directional. The line impedance typically
corresponds to the diameter of the circle with the reach of the relay being the diameter of
the circle.
iii. Differential Relay
The differential relay is a current-operated relay that responds to the difference
between two or more device currents above a set value. The relay works on the basis of the
differential principle that what goes into the device has to come out .If the current does not
add to zero, the error current flows to cause the relay to operate and trip the circuit.
The differential relay is used to provide internal fault protection to equipment such
as transformers, generators, and buses. Relays are designed to permit differences in the input
currents as a result of current transformer mismatch and applications where the input
currents come from different system voltages, such as transformers. A current differential
relay provides restraint coils on the incoming current circuits. The restraint coils in
combination with the operating coil provide an operation curve, above which the relay will
operate. Differential relays are often used with a lockout relay to trip all power sources to

10. the device and prevent the device from being automatically or remotely reenergized. These
relays are very sensitive. The operation of the device usually means major problems with
the protected equipment and the likely failure in re-energizing the equipment.
iv. Directional Over current Relay
A directional over current relay operates only for excessive current flow in a given
direction. Directional over current relays are available in electromechanical, static, and
microprocessor constructions. An electromechanical overcorrect relay is made directional
by adding a directional unit that prevents the over current relay from operating until the
directional unit has operated. The directional unit responds to the productof the magnitude
of current, voltage, and the phase angle between them or to the productof two currents and
the phase angle between them. The value of this productnecessary to provide operation of
the directional unit is small, so that it will not limit the sensitivity of the relay (such as an
over current relay that it controls). In most cases, the directional element is mounted inside
the same case as the relay it controls. For example, an over current relay and a directional
element are mounted in the same case, and the combination is called a directional over
current relay. Microprocessorrelays often provide a choice as to the polarizing method that
can be used in providing the direction of fault, such as applying residual current or voltage
or negative sequence current or voltage polarizing functions to the relay.
DC Power Supply
I . DC Battery and Charger
All but the smallest substations include auxiliary power supplies. AC power is
required for substation building small power, lighting, heating and ventilation, some
communications equipment, switchgear operating mechanisms, anti-condensation heaters
and motors. DC power is used to feed essential services such as circuit breaker trip coils
and associated relays, supervisory control and data acquisition (SCADA) and
communications equipment. This describes how these auxiliary supplies are derived and

11. explains how to specify suchequipment. It has Single 100% battery and 100% charger, Low
capital cost, No standbyDC Systemoutage formaintenance. Need to isolate battery/charger
combination from load under boost charge conditions in order to prevent high boost
voltages.
I I . Battery and Charger configurations
Capital cost and reliability objectives must first be considered before defining the
battery and battery charger combination to be used for a specific installation. The
comparison given in Table 5.1 describes the advantages and disadvantages of three such
combinations.
Capital cost and reliability objectives must first be considered before defining the
battery/battery charger combination to be used for a specific installation. The comparison
given describes the advantages and disadvantages of three such combinations
III . 400V DC Battery
Make: Exide
Capacity: 300 AH at 27°
No. of Cells: 110 No.
Date of installation: 06/2001
Make: Universal,
Sr. No. : BC 1020/82
Date of manufacturing: 4/2000
Input Rating: Voltage: 415 V + 10 %
Output Rating : Float: 220 V, 10 Amp
Boost: 180 V, 30Amp
Functions of AssociatedSystemin Substation
Functions of Associated System in Substation is as shown below in table-4.1
Table-4.1 Functions of Associated System in Substation

12. Sr. System Function
1. Substation Earthing system
- Earth mat
- Earthing spikes
- Earthing risers
To provide an earth mat for connecting neutral points,
equipment body, support structures to earth. For safety of
personnel and for enabling earth fault protection. To
provide the path for discharging the earth currents from
neutrals, faults, Surge Arresters, overheads shielding wires
etc. with safe step-potential and touch potential.
2. Overhead earth wire shielding or
Lightning masts.
To protect the outdoorsubstation equipment from
lightning strokes.
3. Illumination system (lighting)
- for switchyard
- buildings
- roads etc.
To provide proper illumination to substation yard.
4. Protection system
- protection relay panels
- controlcables
- circuit breakers
- CTs, VTs etc.
To provide alarm or automatic tripping of faulty part from
healthy part and also to minimize damage to faulty
equipment and associated system.
5. Control cable For Protective circuits, control circuits, metering circuits,
communication circuits
6. Power cable To provide supply path to various auxiliary equipment and
machines.
7. PLCC system
power line carrier communication
system
For communication, telemetry, tele-control, power line
carrier protection etc.

13. 8. Telephone, telex, microwave,
OPF
For internal and external communication
9. Auxiliary standby power
system
Forsupplying starting power, standbypowerforauxiliaries.
10. Fire Fighting system
- Sensors, detection system
- water spray system
- fire port, panels, alarm
System.
- water tank and spray system
To sense the occurrence of fire by sensors and to initiate
water spray, to disconnect power supply to affected region
to pinpoint location of fire by indication in control room.