Substations

Instructor
Engr. Muhammad Yaseen
Reviewed By
Engr Muhammad Imran

Differentiate between substation and grid
station
Substation:
 An electrical power substation is a conversion point between
transmission level voltages (such as 132 KV) and distribution level
voltages (such as 11 KV).
 A substation has one or more step-down transformers and serves a
regional area such as part of a city or neighborhood.
 Substations are connected to each other by the transmission ring
circuit.

Differentiate between substation and grid
station
Grid Station:
 An electrical grid station is an interconnection point between two
transmission ring circuits, often between two geographic regions.
 They might have a transformer, depending on the possibly different
voltages, so that the voltage levels can be adjusted as needed.
 The interconnected network of grid stations is called the grid, and
may ultimately represent an entire multi-state region.
 In this configuration, loss of a small section, such as loss of a
power station, does not impact the grid as a whole, nor does it
impact the more localized neighborhoods, as the grid simply shifts
its power flow to compensate, giving the power station operator the
opportunity to effect repairs without having a blackout.

Distribution sub-station
Introduction:
 It is link between sub-transmission and distribution system. The main
function of sub-station is to transform higher voltage (132 kV) to
distribution voltage level (11 kV).
 Power is delivered to high voltage bus through circuit breaker and
isolators.
 High voltage bus bars are then connected to the distribution transformers
which further step down the voltage up to the level of 11 kV.
 The power is transformed to low voltage bus bar which is made of large
cross-sectional area.
 From low voltage bus bars energy is fed to the feeders through circuit
breakers.
 The three phase primary feeders distributes energy from low voltages bus
of sub-station to its load centers.
 Normally 100-500 kVA transformer are connected to each primary feeder
or sub-feeders.

Classification of Substation
 There are several ways of classifying substations. However the two most
important ways of classifying them are according to (1) Service
requirement and (2) constructional features.
(1) According to service requirement
 A substation may be called upon to change voltage level or improve
power factor or convert a.c. power in to d.c. power etc. According to the
service requirement, substation may be classified into:
(a) Transformer Sub-stations.
 Those substations which change the voltage level of electric supply are
called transformer substations.
 These substations receive power at some voltage and deliver it at some
other voltage.
 Obviously, transformer will be the main component in such substations.
Most of the substations in the power system are of this type.

(b) Switching sub-stations.
 These substation do not change the voltage level i.e. incoming and
outgoing lines have the same voltage. However, they simply perform the
switching operations of the power lines.
(c) Power factor correction sub-stations.
 Those substations which improve the power factor of the system are
called power factor correction sub-stations.
 Such sub-stations are generally located at the receiving end of the
transmission lines.
 These sub-stations generally use synchronous condensers at the power
factor improvement equipment.
(d) Frequency changer sub-stations.
 Those sub-stations which change the supply frequency are known as
frequency changer sub-stations.
 Such a frequency change may be required for industrial utilization.

(e) Converting sub-stations.
 Those sub-stations which change a.c. power in to d.c. power are called
converting sub-stations.
 These sub-stations receive a.c. power and convert it in to d.c. power with
suitable apparatus (e.g. ignitron) to supply for such purposes as traction,
electroplating, electric welding etc.
(f) Industrial sub-stations.
 Those sub-stations which supply power to individual industrial concerns
are known as industrial sub-stations.
(2) According to constructional features
 A substation has many components (e.g. circuit breakers, switches, fuses,
instruments etc.) which must be cover properly to ensure continuously
and reliable service. According to constructional features, the substations
are classified as:

(a) Indoor substations.
 For voltages up to 11 kV, the equipment of the substation is installed
indoor because of economic consideration.
 However, when the atmosphere is polluted with impurities, these sub-
stations can be erected (setup) for voltages up to 66 kV.
(b) Outdoor substations.
 For voltages beyond 66 kV, equipment is invariably (constant) installed
outdoor. It is because of such voltages, the clearances between
conductors and the space required for switches, circuit breakers and other
equipment becomes so great that it is not economical to install the
equipment indoor.
(c) Underground substations.
 In thickly populated areas, the space available for equipment and
building is limited and the cost of land is high.
 Under such situations, the sub-station is created underground.

(d) Pole-mounted substations.
 This is an outdoor sub-station with equipment installed over-head on H-
pole or 4-pole structure.
 It is the cheapest form of the sub-station for voltages not exceeding 11
kV.
 Electric power is almost distributed in localities through such sub-
stations.
 For more discussion on pole mounted sub-station please refer the
principle of power system by v.k. Mehta.

Substation equipments and its functions
(1) Lightening Arrester.
 Lightening arrestors are the instrument that are used in the incoming
lines 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.

(2) Capacitor Voltage Transformer (CVT):
 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 secondary terminals 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.

220 kV Capacitor Voltage
Transformer
Wave Trap

(3) Wave trap:
 Wave traps are used at sub-stations using Power line Carrier Communication
(PLCC). PLCC is used to transmit communication and control information at a
high frequency over the power lines.
 This reduces need for a separate infra for communication between sub-stations.
(4) Instrument Transformers:
 The line in sub-stations operate at high voltages and carry current of thousand
amperes.
 The measuring instruments and protective devices are designed for low voltages
(generally 110 V) and current (about 5 A). Therefore they will not work
satisfactorily if mounted directly on the power lines.
 This difficulty is overcome by installing instrument transformer on the power
lines.
 The function of these instrument transformer is to transfer voltages or currents in
the power lines to values which are convenient for the operation of measuring
instruments and relays.

 Theses are two types of instrument transformer
(i) Current Transformer (CT) (ii) Potential Transformer (PT)
(i) Current Transformer (CT):
 A current transformer is essentially a step-up transformer which step down the
current to a known ratio.
 The primary of this transformer consists of one or more turns of thick wire
connected in series with power line.
 The secondary consists of large number of turns of fine wire and provides for
measuring instruments and relays a current which is a constant fraction of the
current in the line.
 Suppose a current transformer rated at 100/5 A is connected in the line to
measure the current .
 If the current in the line is 100 A . Then current in the secondary will be 5 A .
 Similarly, if current in the line is 50 A, then secondary of CT will have current
of 2.5 A.
 Thus the CT under consideration will step-down the line current by a factor 20.

(ii) Potential Transformer (PT)
 It is essentially a step-down transformer and step down the voltage to a known
ratio.
 The primary of this transformer consists of large number of turns of fine wire
connected across the line.
 The secondary winding consists of a few turns and provides for measuring
instruments and relays a voltage which is known fraction of the line voltage.
 Suppose a potential transformer rated at 66 kV/110 V is connected to power line.
If line voltage is 66 kV, then voltage across secondary will be 110 V.
(5) Bus-Bars:
 The bus is a line in which the incoming line come into and get into the
instruments for further step up or step down.
 The first bus is used for putting the incoming line in a 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.

 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 of current
that can be safely carried.
 Bus bars are typically either flat strips or hollow tubes as these shapes allow heat to
dissipate more efficiently due to their high surface area to cross sectional area ratio.
(6) 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 different types of circuit breakers used for any substations. They are
a. SF6 circuit breakers b. Oil circuit breaker
c. Spring circuit breakers. d. Vacuum circuit breaker

(7) Power Transformer:
 A power transformer is used in a sub-station to step-up or step-down the
voltage.
 The modern practice is to use 3-phase transformers in sub-stations; although
3 single phase bank of transformers can also be used.
 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.
(8) 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.
 Isolator is installed before and after the circuit breaker

(9) Protective Relaying:
 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.
 There are different types of relays.
i. Over current relay
ii. Distance relay
iii. Differential relay
iv. Directional over current relay

(10) D.C. Power Supply (D.C. 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.

Potential Transformer (PT) Current Transformer (CT)

Oil Circuit Breaker

SF6 Circuit Breaker Isolator

Substation Bus Schemes
 The selection of bus schemes is based upon safety, reliability, economy,
simplicity and other considerations.
 The most commonly sub-station schemes used at are given as under 5
schemes
1. Single Bus Single Breaker Scheme
2. Double Bus Double Breaker Scheme
3. Main Transfer Bus Scheme
4. Ring Main Arrangement Scheme
5. One and Half Breaker Scheme

1. Single Bus Single Breaker Scheme
Bus
IsolatorIsolator
C.BC.BC.BC.B
Line (OH/
UG)
C.B
Isolator Isolator
Isolator

Advantages
Disadvantages
i. Lowest cost
i. Failure of bus or any C.B results shutdown (power outage) in entire sub-
station.
ii. Difficult to do maintenance.
iii. Bus can not be extended without its de-energising.
iv. This can be used where loads have other supply arrangements if bus
looses supply.

2. Double Bus Double Breaker Scheme
Common point
Bus 1
Isolator
C.BC.BC.BC.B
C.BC.BC.BC.B
Bus 2
Line
Line
Line
Line
Isolator Isolator Isolator
Isolator

Advantages
Disadvantages
i. Each circuit has two breakers.
ii. It allows feeder circuits to be connected to be either bus.
iii. Any breaker can be taken out from service for maintenance.
iv. High reliability.
i. Most costly or expensive.
ii. Would loose half of the circuit for breaker failure. If circuits are not
connected to both buses.

3. Main and Transfer Bus Scheme
Out going 11 kV feeder
Main Bus
Incoming line 1 Incoming line 2
C.B
Here, “OR” logic is
used
Transfer
Bus
Isolator
Bus
Coupler

Advantages
Disadvantages
i. Low initial cost.
ii. Any breaker can be taken out from service for maintenance.
iii. Potential device may be used on the main bus for relaying.
i. It requires one extra breaker for bus tie.
ii. Switching is complicated when breaker maintenance.
iii. Failure of bus or any of the breaker results in shutdown of entire sub-
station.

4. Ring Main Arrangement Scheme
Out
going
Incoming
lineC.B
Isolator
Incoming
line
Incoming
line
Out
going
Out
going
Isolator Isolator
C.B C.B
C.BC.B
C.B C.BC.B C.B C.B
C.BC.B

Advantages
Disadvantages
i. Separation of ring if fault occurs.
ii. It requires potential devices on all circuits.
iii. Automatic reclosing.
i. Low initial cost.
ii. Flexible operation.
iii. Any of the breaker can be taken out of service.
iv. It requires one C.B per circuit.
v. Does not use main bus.
vi. Each circuit is fed from two circuit breakers.

5. One and half breaker Scheme
Out
going
Incoming
line 1
Isolator
C.B
Incoming
line 2
Ckt 1 Ckt 2
C.B
Main
Bus
Isolator
C.B
Transfer
Bus
IsolatorIsolator
Out
going

Advantages
Disadvantages
i. 1 and ½ breaker per circuit.
ii. Relaying and automatic reclosing are somewhat involved since the
middle breaker must be responsive to either of its associated circuits.
i. High flexibility operation.
ii. High reliability.
iii. Very simple in operation.
iv. Breaker failure of bus side breaker removes only one circuit from service
v. All switching is done by breakers.
vi. Bus failure does not remove any feeder circuits from service.

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