The document discusses the functions and types of equipment used in electrical distribution substations. It provides descriptions of common substation equipment such as power transformers, circuit breakers, disconnecting switches, bus bars, current and potential transformers, capacitors, reactors, protective relays and batteries. It also provides examples of how to calculate primary and secondary currents in transformers of different ratings and briefly discusses some common bus schemes used in Pakistan.

1. Khairpur Mir`s
Assignment of Power Distribution & Utilizations System
SubmittedTo: Sir Irfan Ahmed
SubmittedBy: Zulfiqar Mangrio (K-13EL-79)
Topic:Substation equipment’s and its Function
Department: Electrical Engineering
Date: 28-Jan-2016

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Substation equipment’s and its Function
Distribution System
The major function of distribution system is to provide power to individual consumer
requirement. Supply of electric power to different consumers is done at low voltage level.
Any Distribution of electric power is done by distribution networks. Distribution
Substations distribute the power. As many equipment`s are used in substation which can
lower the voltage level Control the whole system. Which are given below;
Substation equipment’s
1. Power Transformer
2. Circuit Breaker
3. Disconnecting Switches & Isolator
4. Bus Bar
5. Current Transformer
6. Potential Transformer
7. Current Limiting Reactor
8. Shunt Reactor
9. Series Capacitor
10. Capacitor Voltage Transformer (CVT)
11. Coupling Capacitor Voltage Transformer (CCVT)
12. Shunt Capacitor
13. Grounding System
14. Lightning Arrester
15. Line Trap
16. Protective Relays
17. Station Batteries
1. Power Transformer
A transformer is an electric static machine used to change the voltage and current
without changing the frequency and power. The power transformer is different

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from the distribution transformer because due to its high rating or it depends on the
ratings. In case of a 220 KV or above KV level auto transformers are used.
As in casethe of lower KV line such as less than 132KV line double
winding transformers are used
2. Circuit Breaker
The circuit breakers are used to control (Break) the circuit if any fault occurs in any
of the Substation system. These circuit breaker breaks for a fault which can damage other
equipment in the station. For any fault over the substation station we need to break the
circuit it means that line current. This is done by automatically Circuit breaker by the
help of relay.
Some of the type of Circuit Breaker
 Oil circuit breaker.
 Air circuit breaker.
 SF6 circuit breaker.
 Vacuum circuit breaker

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3. Disconnecting Switches & Isolator
Isolator is a mechanical switch which can separate the part from circuit in the system
when it is required. Electrical isolators separate a part of the system from rest for safe
maintenance works. This isolator is to protect the transformer and the other equipment in
the system. The isolator isolates or cut the circuit from the high voltage to the ground. It
is protecting device.
4. Bus Bar
A bus bars in electrical power distribution are the conductors which are made up of thick
strips of copper or aluminum that conduct electricitywithin a switchboard, distribution
board, substation, or other electrical apparatus. The bus is a line in which the incoming
feeders come into and get into the instruments for further step up or step down. 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.

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5. Current Transformer
A current transformer is defined as an instrument transformer in which the secondary
current is substantially proportional to the primary current.Current transformers are used when
the magnitude of AC currents exceeds the safe value of current of measuring instruments.
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
because we have no instrument for measuring of such a large current. The main use of this
transformer is
a. Distance Protection
b. Backup Protection
c. Measurement
6. Potential transformer
The potential transformer usages a bus isolator care for 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 apparatus. There are
two main parts in it
a. Measurement
b. Protection

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7. Current Limiting Reactor
The Current limiting reactor is an electrical inductance made up of a number of turns of
wire of low ohmic resistance and inserted in series with a line to limit the current that can
flow under short circuit.
8. Shunt Reactor
Shunt reactor is the absorption of reactive power in the case of Ferranti effect, and thus
assembling the energy efficiency of the system. It is the most compact device normally
used to compensate for reactive power transmission lines in high-voltage long and cable
systems. Shunt reactor can be connected directly to the power line or to higher power
transformer. It is used to improve and adjust the reactive power consumed reactor can
also be a variable rating.

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9. Series Capacitor
• Increase power
• Transmission capability
• Improve system stability
• Reduce system losses
• Improve voltage profile
• Optimize power flow between
Parallel lines
10. Shunt Capacitor
A capacitor bank is very essential equipment of an electrical power system. The energy
required to run all the electrical devices are useful load power is the active power. It is
expressed in active power KW or MW. The maximum load connected to the electric
power system is essentially inductive in nature such as electric transformers, induction
motors, synchronous motor, electric ovens, and fluorescent lighting are all inductive in
nature. The current causes the capacitor to lead the effort; the capacitive reactance can be
used to cancel the inductive reactance of the system.

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11. Capacitor Voltage Transformer
A capacitor voltage transformer (CVT) is a transformer used in power systems to
stepping down high-voltage signals and providing additional low voltage signals either to
measure or for the operation of the protective relay. In its simplest form, the device
consists of three parts: two capacitors across the divided voltage signal, which is used to
adjust the inductive device on the pace of supply and transformers, used to isolate and
further stepped down the voltage of the devices or relay protective element. The voltage
level of this transformer is measure voltage 200KV same as potential transformer.
12. Capacitance Coupled Voltage Transformer (CCVT)
A capacitance Coupled voltage transformer (CCVT) is a same like CVT , which is 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. Mostly it is use EXTRA high
voltage 500kv to sample the voltage for measurement purpose.

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13. Grounding System
To provide an earth mat for connecting neutral points, equipment body, support structures
to the ground. For the safety of staff and to enable ground fault protection to provide a
path to empty the land of neutral currents, errors, surge arresters, overhead shielding wire
etc. with a safe step and touch potential possibilities.
14. Lightning 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 unsafe from
the equipment`s used in the substation. Even equipment are very expensive, so as to
prevent any damage using lightning arresters. The lightning arresters do not let the
lightning to fall on the substation. In the case of some lightning and lightning arresters

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drag and ground it to the ground.
15. Line Trap
A line trap also called wave trap free parallel resonant circuit, connected series with high
voltage transmission power lines to provide high frequency (40 kHz to 1000 kHz) carrier
signals of power line communication purpose.
16. Protective Relays
Protectionrelays used to detect defective or device lines and to isolate the defective
equipment. Relays are also used to detect abnormal operating conditions or undesirable
from those caused by damaged equipment and either a warning or isolate the equipment.
Protectionrelays are used in electrical system protection by causing defective device or
lines to be separate or disconnect the equipment and to maintain continuity of service to
the rest of the system.

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18. Station Batteries
Batteries are supply the direct current to the relays for control purpose, as
well as store the energy so use for the when there is no power in the substation it
means that all the system is open or any fault condition. These are connected in
series to increase the voltage level.
Another Apparatus
 Protection system
To provide alarm or automatic tripping of faulty part from healthy part and also to
minimize damage to faulty equipment and associated system
 Control cable
It provide supply to various auxiliary equipment’s and machines.
 Auxiliary standby power system
For supplying starting power, standby power for auxiliaries.
 Firefighting system

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To sense the occurrence of fire by sensors and to initiate water spray, to disconnect power
supply to affected region to pin point location of fire by indication in control room.
How to Determine the Primary current and secondary current in
transformer? As the input voltage is 11kv, output phase voltage is 220v
and rated as;
1. TransformerRated At 50 KVA
(a) Primary Current:
As we know that Power transformer’s Primary winding connected in Delta Connection,
So,
IL =√3 x Ip
VL = Vp
Now,
Apparent power is;
S = V.I => I = S/V
IPrimary =
5000
11000
IPrimary =4.545 A
(b) Secondary Current:

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The Secondaryof the Power transformer winding are connected in Delta or
Wye.
So,
IL = IP
VL =√3 x Vp
As Output Voltages are 220 V
S = V.I => I = S/V
ISec =
5000
220
ISec = 227.27 A
2. Transformerrated At 100KVA
(a) .Primary Current:
As we know that Power transformer’s Primary winding connected in Delta Connection,
So,
By Using Apparent power Formula, we get;
S = V.I => I = S/V
IPrimary =
100 ,000
11000
IPrimary =9.090 A
(b) Secondary Current:
So,
Using apparent power formula, we get;
IL = IP
VL =√3 x Vp
As Output Voltages are 220 V

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S = V.I => I = S/V
ISec =
100,000
220
ISec = 454.54 A
3. TransformerRated At 200 KVA
(a) Primary Current:
As we know that Power transformer’s Primary winding connected in Delta Connection,
So,
By Using Apparent power Formula, we get;
S = V.I => I = S/V
IPrimary =
200 ,000
11000
IPrimary = 18.181 A
(b) Secondary Current:
So,
Using apparent power formula, we get;
IL = IP
VL =√3 x Vp
As Output Voltages are 220 V
S = V.I=> I = S/V
ISec =
200,000
220
ISec = 909.09 A

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4. TransformerRated At 400 KVA
(a) Primary Current:
As we know that Power transformer’s Primary winding connected in Delta Connection,
So,
By Using Apparent power Formula, we get;
S = V.I => I = S/V
IPrimary =
400 ,000
11000
IPrimary = 36.36 A
(b) Secondary Current:
So,
Using apparent power formula, we get;
IL = IP
VL =√3 x Vp
As Output Voltages are 220 V
S = V.I => I = S/V
ISec =
400,000
220
ISec =1818.18 A
5. TransformerRated At 630 KVA
(a) Primary Current:

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As we know that Power transformer’s Primary winding connected in Delta Connection,
So,
By Using Apparent power Formula, we get;
S = V.I => I = S/V
IPrimary =
630 ,000
11000
IPrimary = 57.27 A
(b) Secondary Current:
So,
Using apparent power formula, we get;
IL = IP
VL =√3 x Vp
As Output Voltages are 220 V
S = V.I=> I = S/V
ISec =
630,000
220
ISec = 2863.63 A
Bus Scheme`s Used in Pakistan

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I) breaker and Half scheme:
II) Siemens GIS substation with Main & Transfer bus scheme
(2 BB)

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