Q.1. a) Discuss the following terms : i) Diversity Factor ii) Coincidence Factor iii) Utilization
Factor iv) Contribution Factor
Ans:-
Q.1. b) Discuss in detail the classification of loads with their characteristics.
Q.2. a) Derive the relation between load factor & loss factor.
Q.2. b) A generating station has a maximum demand of 80 MW and a connected load of 150 MW.
If MWhr generated in a year are 40*10 3 . Calculate :
i) Load Factor
ii) Demand Factor
Ans:- i) Load factor=(average load over a period)/(maximum demand during the period)
LF =150/80
LF= 1.875
Demand factor= maximum Demand/Connected Load
DF= 80/150
DF= 0.533
Q.3. a) Draw a line diagram of a radial type primary feeder. Mention the factors that influence the
selection of primary feeders.
Ans:-
Q.3. b) Explain the voltage choice & feeder design for secondary distribution system.
Ans:-
Low voltage (LV) For a phase-to-phase voltage between 100 V and 1000 V. The standard ratings
are: 400 V - 690 V - 1000 V (at 50 Hz)
Medium voltage (MV) For a phase-to-phase voltage between 1000 V and 35 kV. The standard
ratings are: 3.3 kV - 6.6 kV - 11 kV - 22 kV - 33 kV
High voltage (HV) For a phase-to-phase voltage between 35 kV and 230 kV. The standard ratings
are: 45 kV - 66 kV - 110 kV - 132 kV - 150 kV – 220
Generating Station: The place where electric power produced by parallel connected three phase
alternators/generators is called Generating Station. The Ordinary generating voltage may be 11kV,
11.5 kV 12kV or 13kV. But economically, it is good to step up the produced voltage to 132kV,
220kV or 500kV or greater by Step up transformer (power Transformer).
Primary Transmission: The electric supply (in 132kV, 220 kV, 500kV or greater) is transmit to
load center by overhead transmission system.
Secondary transmission: Area far from city which have connected with receiving station by line is
called Secondary transmission. At receiving station, the level of voltage reduced by step-down
transformers up to 132kV, 66 or 33 kV, and Electric power is transmit by three phase three wire
overhead system to different sub stations.
Primary Distribution: At a sub station, the level of secondary transmission voltage (132kV, 66 or
33 kV) reduced to 11kV by step down transforms. Generally, electric supply is given to those
heavy consumer whose demand is 11 kV, from these lines which caries 11 kV and a separate sub
station exists to control and utilize this power.
For heavier consumer (at large scale) their demand is about 132 kV or 33 kV, they take electric
supply from secondary transmission or primary distribution (in 132 kV, 66kV or 33kV) and then
step down the level of voltage by step-down transformers in their own sub station for utilization (
i.e. for electric traction etc).
Secondary Distribution: Electric power is given by (from Primary distribution line i.e.11kV) to
distribution sub station. This sub station is located near by consumers areas where the level of
voltage reduced by step down transformers 440V by Step down transformers. These transformers
called Distribution transformers, three phase four wire system). So there is 400 Volts (Three Phase
Supply System) between any two phases and 230 Volts (Single Phase Supply) between a neutral
and phase (live) wires. Residential load (i.e. Fans, Lights, and TV etc) may be connected between
any one phase and neutral wires, while three phase load may be connected directly to the three
phase lines
Q.4. a) Explain the following :
i) Radial type feeders with tie & sectionalizing switches.
ii) Radial feeder with express feeder & back feed.
iii) Loop type primary feeder.
Ans:-
i) Radial type feeders with tie & sectionalizing switches.
Higher service reliability
Fast restoration of service by switching un-faulted sections of the feeder to adjacent primary
feeders
ii) Radial feeder with express feeder & back feed.
iii) Loop type primary feeder
Q.4. b) An industrial area near a city was found to have a load density of 0.5MVA/ km2 . The total
area was to be located between a rectangular screen of 8k*4km. Determine the suitable
number of 33KV/11KVsubstation, their capacity & feeder length. The loads are to be
served by 11 KV feeders.
Ans:-
Q.5.a) Derive the expression for power loss & voltage drop for the feeder line with uniformly
decreasing load.
Ans:-
Q.5.b) A 400KV 3 phase, 4wire system has balanced loads and is fed from a 11KV/ 400V, 3phase
100KVA transformer. Determine voltage drop, output KVA, KW & pf. of a transformer.
The impedances are :
Section oa = (0.06+ j0.04) ohm
Section ab = (0.1+ j0.05) ohm
Ans:-
Q.6.a) What are the different distribution systems for ac & dc? Give comparison.
Ans:- The transmission line is a closed system through which the power is transfer from generating
station to the consumers. The transmission lined are categorized into three categories.
Short Transmission Line – The length of the short transmission line is up to 80Km.Medium
Transmission Line – The length of the medium transmission line lies between 80km to 200km.
Long transmission Line – The length of long transmission line is greater than 150km.
The supports conductor, conductor, insulator, cross arms and clamp, fuses and isolating switches,
phases plates etc. are the main component of the transmission lines.
Key Differences Between AC and DC Transmission Line
1. The AC transmission line transmits the alternating current over a long distance. Whereas, the DC
transmission line is used for transmitting the DC over the long distance.
2. The AC transmission line uses three conductors for long power transmission. And the DC
transmission line uses two conductors for power transmission.
3. The AC transmission line has inductance and surges whereas the DC transmission line is free
from inductance and surges. The inductance and the surges are nothing but the wave of the high
voltage which occurs for short duration.
4. The high voltage drop occurs across the AC terminal lines when their end terminals voltage are
equal. The DC transmission line is free from inductance, and hence no voltage drop occurs across
the line.
5. The phenomenon of the skin effect occurs only in the AC transmission line. The skin effect
causes the losses, and this can be reduced by decreasing the cross-section area of the conductor.
The phenomenon of skin effect is completely absent in the DC transmission line.
6. At same voltage, the DC transmission line has less stress as compared to the AC transmission
line. Hence, DC requires the less insulation as compared to AC.
7. The communication line interference is more in the AC transmission line as compared to the DC
transmission line.
8. The corona effect is the phenomenon through which the ionization occurs across the conductor.
And this ionisation causes the losses in the conductor. The phenomenon of corona effect occurs
only in the ac transmission line and not in the dc transmission line.
9. The dielectric loss occurs in the ac transmission line and not in the DC transmission line.
10. The AC transmission line has the difficulties of synchronisation and stability whereas the DC
transmission line is free from stability and synchronisation.
11. The AC transmission line is less expensive as compared to the DC transmission line.
12. The small conductor is used for AC power transmission as compared to the DC transmission.
13. The AC transmission line requires the transformer for step-up and step-down the voltage.
Whereas in DC transmission line the booster and chopper are used for step-up and step-down the
voltage.
Q.7. a) Briefly explain the line drop compensation & voltage control.
Ans:- An electric power system having line drop compensation includes a controllable electric
power source having an output for supplying voltage to a power bus, a local voltage regulator for
monitoring the output voltage of the power source and for producing a control signal
representative of a desired nominal output voltage of the power source, and a remote voltage
regulator for sensing voltage on the power bus at a point of regulation located away from the
power source. The remote voltage regulator produces a pulse width modulated signal having a duty
cycle representative of the voltage at the point of regulation. A pulse width to trim bias converter
receives the pulse width modulated signal and produces the trim signal having a magnitude
representative of the duty cycle of the pulse width modulated signal. This trim signal is combined
with the control signal of the local voltage regulator to produce a modified control signal for
controlling the output voltage of the power source to produce a predetermined voltage at the point
of regulation.
Q.7. b) A 33 KV feeder has (0.1+ j0.25) ohm impedance per phase per km and is supplying a
load of 6 MVA over a distance of 80 km at 0.75 pf. What will be the receiving end voltage and
voltage drop of line if compensated to 50% by series capacitance compensation
Ans:-
Q.8. a) Explain the location of shunt & series capacitors in pf correction.
Ans:-
1. Shunt capacitors are used in parallel to load or supply points, either on individual basis
or as group correction in APFC panels/ manual operation. These compensate for lagging
power factor at various points from generation to consumer level to maintain high power
factor.
2. Series reactors (inductors) are used in series with shunt capacitors to limit surge currents
at switching, thereby protecting the capacitors and switchgear.
3. Shunt reactors (inductors) are used in rather complex FACTS system of power factor
control in case of net load turning leading power factor load.
4. Shunt reactors are also used as detuning reactors. Here they save the system from
resonance conditions.
5. Series capacitors are rather typical in the sense they are placed directly in series with
transmission lines at suitable locations and carry entire line current. They help keep good
power factor and reduce line drop by being a part of line impedance. They compensate
for losses or voltage drops due to line impedance in all load conditions
Q.9.a) What is SCADA? How does it help for distribution automation?
Ans:-
Supervisory Control and Data Acquisition or simply SCADA is one of the solutions
available for data acquisition, monitor and control systems covering large geographical areas.
It refers to the combination of data acquisition and telemetry.
SCADA systems are mainly used for the implementation of monitoring and control system of an
equipment or a plant in several industries like power plants, oil and gas refining, water and waste
control, telecommunications, etc.
In this system, measurements are made under field or process level in a plant by number of remote
terminal units and then data are transferred to the SCADA central host computer so that more
complete process or manufacturing information can be provided remotely.
This system displays the received data on number of operator screens and conveys back the
necessary control actions to the remote terminal units in process plant.
Q.9.b) Write a short note on substation automation.
Ans:- Power-system automation is the act of automatically controlling the power system via
instrumentation and control devices. Substation automation refers to using data from Intelligent
electronic devices (IED), control and automation capabilities within the substation, and control
commands from remote users to control power-system devices.Since full substation automation
relies on substation integration, the terms are often used interchangeably. Power-system
automation includes processes associated with generation and delivery of power. Monitoring and
control of power delivery systems in the substation and on the pole reduce the occurrence of
outages and shorten the duration of outages that do occur. The IEDs, communications protocols,
and communications methods, work together as a system to perform power-system automation.
The term “power system” describes the collection of devices that make up the physical systems
that generate, transmit, and distribute power. The term “instrumentation and control (I&C) system”
refers to the collection of devices that monitor, control, and protect the power system.
Power-system automation is composed of several tasks.
Data acquisition
Data acquisition refers to acquiring, or collecting, data. This data is collected in the form of
measured analog current or voltage values or the open or closed status of contact points. Acquired
data can be used locally within the device collecting it, sent to another device in a substation, or
sent from the substation to one or several databases for use by operators, engineers, planners, and
administration.
Supervision
Computer processes and personnel supervise, or monitor, the conditions and status of the power
system using this acquired data. Operators and engineers monitor the information remotely on
computer displays and graphical wall displays or locally, at the device, on front-panel displays and
laptop computers
Control refers to sending command messages to a device to operate the I&C and power-system
devices. Traditional supervisory control and data acquisition (SCADA) systems rely on operators
to supervise the system and initiate commands from an operator console on the master computer.
Field personnel can also control devices using front-panel push buttons or a laptop computer.
Power-system automation processes rely on data acquisition; power-system supervision and
power-system control all working together in a coordinated automatic fashion. The commands are
generated automatically and then transmitted in the same fashion as operator initiated commands.
Q.10.a) What is earthing? How is it effectively done? What are the methods of reducing earth
resistance?
Ans:- Definition: The process of transferring the immediate discharge of the electrical energy
directly to the earth by the help of the low resistance wire is known as the electrical earthing. The
electrical earthing is done by connecting the non-current carrying part of the equipment or neutral
of supply system to the ground.
Methods of Earthing
There are several methods of earthing like wire or strip earthing, rod earthing, pipe earthing, plate
earthing or earthing through water mains. Most commonly used methods of earthing are pipe
earthing and plate earthing. These methods are explained below in details.
Earthing Mat
Earthing mat is made by joining the number of rods through copper conductors. It reduced the
overall grounding resistance. Such type of system helps in limiting the ground potential. Earthing
mat is mostly used in a placed where the large fault current is to be experienced. While designing
an earth mat, the following step is taken into consideration.
• In a fault condition, the voltage between the ground and the ground surface should not be
dangerous to a person who may touch the noncurrent-carrying conducting surface of the electrical
system.
• The uninterrupted fault current that may flow into the earthing mat should be large enough to
operate the protective relay. The resistance of the ground is low to allow the fault current to flow
through it.The resistance of the mat should not be of such a magnitude as to permit the flow of
fatal current in the live body.
• The design of grounding mat should be such that the step voltage should be less than the
permissible value which would depend on the resistivity of the soil and fault required for isolating
the faulty plant from the live system.
• The earth resistance or resistivity is totally depends on the structure of the area earth.But somehow
we can use some material to enhance the value of the earth resistance for our earthing system.By
using salt and charcoal (these are commonly used) we improve the ionization of the surrounding
earth and keep the moisture level.There are chemicals available in market which can also be used
to improve the earth resistivity for our earth pits
Q.10.b) Write short note on data acquisition system.
Ans:- Data acquisition is the process of sampling signals that measure real world physical
conditions and converting the resulting samples into digital numeric values that can be
manipulated by a computer. Data acquisition systems, abbreviated by the acronyms DAS or DAQ,
typically convert analog waveforms into digital values for processing. The components of data
acquisition systems include:Sensors, to convert physical parameters to electrical signals.Signal
conditioning circuitry, to convert sensor signals into a form that can be converted to digital
values.Analog-to-digital converters, to convert conditioned sensor signals to digital values.
Data acquisition applications are usually controlled by software programs developed usingvarious
generalpurpose programminglanguages suchas Assembly, BASIC, C, C++, C#, Fortran, Java, Lab
VIEW, Lisp, Pascal, etc. Stand-alone data acquisition systems are often called data loggers.There
are also open-source software packages providing all the necessary tools to acquire data from
different hardware equipment. These tools come from the scientific community where complex
experiment requires fast, flexible and adaptable software. Those packages are usually custom fit
but more general DAQ packages like the Maximum Integrated Data Acquisition System can be
easily tailored and is used in several physics experiments worldwide.
Q.11.a) Explain the various factors to be considered to decide the ideal location of substation.
Ans:- Meaning of Substations:
Substations serve as sources of energy supply for the local areas of distribution in which these are
located. Their main functions are to receive energy transmitted at high voltage from the generating
stations, reduce the voltage to a value appropriate for local distribution and provide facilities for
switching.
Some substations are simply switching stations where different connections between various
transmission lines are made, others are converting substations which either convert ac into dc or
vice versa or convert frequency from higher to lower or vice versa. Substations have some
additional functions. They provide points where safety devices may be installed to disconnect
equipment or circuit in the event of fault. Voltage on the outgoing distribution feeders can be
regulated at a substation.
A substation is convenient place for installing synchronous condensers at the end of the
transmission line for the purpose of improving power factor and make measurements to check the
operation of the various parts of the power system. Street lighting equipment as well as switching
controls for street lights can be installed in a substation.
Classification of Substations:
The substations may be classified in numerous ways such as on the basis of:
A. Nature of duties B. Service rendered C. Operating voltage D. Importance, and E. Design.
A. Classification of Substations on the Basis of Nature of Duties:
The substations, on the basis of nature of duties, may be classified into the following three
categories:
1. Step-Up or Primary Substations:
Such substations are usually associated with generating stations. The generated voltage, which is
usually low (3.3, 6.6, 11 or 33 kV), is stepped up to primary transmission voltage so that huge
blocks of power can be transmitted over long distances to the load centres economically.
2. Primary Grid Substations:
Such substations are located at suitable load centres along the primary transmission lines. In these
substations, the primary transmission voltage is stepped down to different suitable secondary
voltages. The secondary transmission lines are carried over to the secondary substations situated at
the load centres where the voltage is further stepped down to sub- transmission or primary
distribution voltages.
3. Step-Down or Distribution Substations:
Such substations are located at the load centres, where the sub- transmission/primary distribution
voltage is stepped down to secondary distribution voltage (415/240 V). These are the substations
which feed the consumers through distribution network and service lines.
Q.11.b) Discuss bus arrangement and switching systems in substations
Ans:-
The electrical substation is a junction point where two or more transmission lines terminate. In
actuality, most EHV and HV substations can be the point where more than half a dozen of lines
terminate. In many large transmission substations, the total numbers of lines
terminating exceeds one or two dozen.
A substation bus scheme is the arrangement of overhead bus bar and associated switching
equipment (circuit breakers and isolators) in a substation. The
operational flexibility and reliability of the substation greatly depends upon the bus scheme.
The first requirement of any substation design is to avoid a total shutdown of the substation for the
purpose of maintenance, or due to fault somewhere out on the line. A total shutdown of the
substation means complete shutdown of all the lines connected to the substation.
Clearly, a EHV or UHV transmission substation where a large number of critical lines terminate is
extremely important, and the substation should be designed to avoid total failure and interruption
of minimum numbers of circuits.
As the name implies, this substation configuration consists of all circuits connected to a single bus.
A fault on the bus or between the bus and circuit breaker will result in an outage of the entire
bus or substation. Failure of a single circuit breaker will also result in an outage of the entire bus.
Maintenance of any circuit breaker requires shutdown of the corresponding circuit/lineand
maintenance of the bus requires a complete shutdown of the bus. A bypass switchacross the
breaker should be used for maintenance of the corresponding breaker. Circuit protection
is disabled in this case.
The single bus substation configuration is the simplest and least expensive of all configurations.
This configuration requires less installation area and it can be easily expanded. Single bus
configurations are not considered reliable systems and they should only be implemented in
substations where high reliability is not required, such as large transmission
yards. Reliability and availability of this system can be improved by expanding and sectionalizing
the bus.
In this arrangement one or more busses is added to the single bus substation scheme. One or more
circuit breakers may be used in this arrangement to make connections between the main and
transfer bus. When no Tie CB is present, for maintenance of a circuit breaker, the transfer bus is
energized by closing the isolator switches to the transfer bus. Then the circuit breaker to be
maintained is opened and isolated on both sides. Circuit protection will be disabled in this
maintenance arrangement.
When a tie circuit breaker is present, circuit breaker maintenance is achieved by closing the tie
breaker. The transfer bus is energized and the isolator nearest the transfer bus of the breaker to
be maintained is closed. The circuit breaker to be maintained is now opened, isolated and removed
for maintenance. The circuit under maintenance is transferred to the transfer bus.
In the main and transfer bus configuration, the protective relay scheme is quite complex due to the
requirement of the tie breaker to handle each situation for maintenance of any other circuit breaker.
This bus scheme is more costly than the single bus configuration but is more reliable and can
be easily expanded.
The switching procedure is complicated for maintenance of any circuit breaker. Failure of a
breaker or fault on the bus results in an outage of the whole substation
This configuration utilizes two buses and two breakers per circuit. Both buses are normally
energized and any circuit can be removed for maintenance without an outageon the corresponding
circuit. Failure of one of the two buses will not interrupt a circuit because all of the circuits can be
fed from the remaining bus and isolating the failed bus.
Substations with the double bus double breaker arrangement require twice the equipment as
the single bus scheme but are highly reliable. Load balancing between buses can be achieved
by shifting circuits from one bus to the other. This scheme is typically found in EHV transmission
substations or generating stations
Substations utilizing this configuration are supplied with two busses. Each circuit is equipped with
a single breaker and is connected to both buses using isolators. A tie breaker connects both main
buses and is normally closed, allowing for more flexibility in operation. A fault on one bus requires
isolation of the bus while the circuits are fed from the opposite bus.
The double bus single breaker scheme is more expensive and requires more installation space than
the single bus configuration. It is common to find this scheme with an additional transfer bus in
EHV transmission substations.
In the ring bus configuration, as the name implies, the circuit breakers are connected to form a
ring, with isolators on both sides of each breaker. Circuits terminate between the breakers and each
circuit is fed from both sides. Any of the circuit breakers can be opened and isolated for
maintenance without interruption of service.
This scheme has good operational flexibility and high reliability. If a fault occurs in this
configuration, it is isolated by tripping a breaker on both sides of the circuit. By tripping two
breakers, only the faulted circuit is isolated while all the other circuits remain in service.
The main disadvantage of the ring bus system is that if a fault was to occur, the ring is split which
could result into two isolated sections. Each of these two sections may nothave the proper
combination of source and load circuits. This can be somewhat avoided by connecting the source
and load circuits side by side.
Ring bus schemes can be expanded to accommodate additional circuits, but its generally not
suited for more than six. Careful planning should be used with this scheme to avoid difficulties
with future expansion.
When expansion of the substation is required to accommodate more circuits, the ring bus
scheme can easily be expanded to the One and Half breaker configuration. This configuration
uses two main buses, both of which are normally energized with three
breakers connected between the buses.
In this bus configuration, three breakers are required for every two circuits - hence the "one and
half" name. Think of it as, to control one circuit requires one full and a half breaker. The middle
breaker is shared by both circuits, similar to a ring bus schemewhere each circuit is fed from both
sides.
Any circuit breaker can be isolated and removed for maintenance purposes without interrupting
supply to any of the other circuits. Additionally, one of the two main bussescan be removed for
maintenance without interruption of service to any of the other circuits.
If a middle circuit breaker fails, the adjacent breakers are also tripped to interrupt both circuits. If a
breaker adjacent to the bus fails, tripping of the middle breaker will notinterrupt service to the
circuit associated with the remaining breaker in the chain. Only the circuit associated with
the failed breaker is removed from service.
The breaker and half configuration is very flexible, highly reliable, and more economicalin
comparison to the Double Bus Double Breaker scheme. Protective relay schemes in this
configuration are highly complicated as the middle breaker is associated with two circuits. It also
requires more space in comparison to other schemes in order to accommodate the large number of
components.
Q.12.a) What are the different types of faults that can occur on distribution network? Explain them
with line diagram.
Ans:- Types of Faults
Electrical fault is the deviation of voltages and currents from nominal values or states. Under
normal operating conditions, power system equipment or lines carry normal voltages and currents
which results in a safer operation of the system.
But when fault occurs, it causes excessively high currents to flow which causes the damage to
equipments and devices. Fault detection and analysis is necessary to select or design suitable
switchgear equipments, electromechanical relays, circuit breakers and other protection devices.
There are mainly two types of faults in the electrical power system. Those are symmetrical and
unsymmetrical faults.
1.Symmetrical faults
These are very severe faults and occur infrequently in the power systems. These are also called as
balanced faults and are of two types namely line to line to line to ground (L-L-L-G) and line to line
to line (L-L-L).
Symmetrical Fault
Only 2-5 percent of system faults are symmetrical faults. If these faults occur, system remains
balanced but results in severe damage to the electrical power system equipments.
Above figure shows two types of three phase symmetrical faults. Analysis of these fault is easy
and usually carried by per phase basis. Three phase fault analysis or information is required for
selecting set-phase relays, rupturing capacity of the circuit breakers and rating of the protective
switchgear.
2.Unsymmetrical faults
These are very common and less severe than symmetrical faults. There are mainly three types
namely line to ground (L-G), line to line (L-L) and double line to ground (LL-G) faults.
Unsymmetrical faults
Line to ground fault (L-G) is most common fault and 65-70 percent of faults are of this type.
It causes the conductor to make contact with earth or ground. 15 to 20 percent of faults are double
line to ground and causes the two conductors to make contact with ground. Line to line faults
occur when two conductors make contact with each other mainly while swinging of lines due to
winds and 5- 10 percent of the faults are of this type.
These are also called unbalanced faults since their occurrence causes unbalance in the system.
Unbalance of the system means that that impedance values are different in each phase causing
unbalance current to flow in the phases. These are more difficult to analyze and are carried by per
phase basis similar to three phase balanced faults.
Causes of Electrical Faults
• Weather conditions: It includes lighting strikes, heavy rains, heavy winds, salt deposition on
overhead lines and conductors, snow and ice accumulation on transmission lines, etc. These
environmental conditions interrupt the power supply and also damage electrical installations.
• Equipment failures: Various electrical equipments like generators, motors, transformers,
reactors, switching devices, etc causes short circuit faults due to malfunctioning, ageing, insulation
failure of cables and winding. These failures result in high current to flow through the devices or
equipment which further damages it.
• Human errors: Electrical faults are also caused due to human errors such as selecting improper
rating of equipment or devices, forgetting metallic or electrical conducting parts after servicing or
maintenance, switching the circuit while it is under servicing, etc.
Q.12.b) Give the arrangement of a single transformer 11KV/ 415Vsubstation & its layout.
Ans:-
Substation provides the energy supply for the local area in which the line is located. The main
function of the substation is to collect the energy transmitted at high voltage from the generating
station and then reduce the voltage to an appropriate value for local distribution and gives facilities
for switching. The substation is of two types one is the simple switching type where the different
connection between transmission line are made and the other is the converting stations which
convert AC to DC or vice versa or convert frequency from higher to lower or lower to higher.
The substation has an additional function like they provide points where safety devices may be
installed to disconnect equipment or circuit in the event of the fault. The synchronous condenser is
placed at the end of the transmission linefor improving the power factor and for measuring the
operation at the various part of the power system. Street lighting, as well as the switching control
for street lighting, can be installed in a substation.
The single line diagram of an 11 KV substation is shown in the figure below. The single line
diagram makes the system easy and it provides the facilitates reading of the electrical supply and
connection.
Single Line Diagram of 11kV/400V Substation
Main Components of 11kV Substation
The working of the electrical equipment used in the substation is explained below in details.
1. Isolator – The isolator connects or disconnects the incoming circuit when the supply is already
interrupted. It is also used for breaking the charging current of the transmission line. The isolator
is placed on the supply side of the circuit breaker so that the circuit breaker isolated from the live
parts of the maintenance.
2. Lightning Arrester – The lightning arrester is a protective device which protects the system
from lightning effects. It has two terminals one is high voltage and the other is the ground
voltage. The high voltage terminal is connected to the transmission line and the ground terminal
passes the high voltage surges to earth.
3. CT Metering – The metering CT measure and records the current when their secondary terminal
is connected to the metering equipment panel.
4. Step-down Transformer – The step-down transformer converts the high voltage current into the
low voltage current.
5. Capacitor Bank – The capacitor bank consists series or parallel connection of the capacitor. The
main function of the capacitor bank is to improve the power factor of the line.
It draws the leading current to the line by reducing the reactive component of the circuit.
6. Circuit Breaker – The circuit breaker interrupts the abnormal or faults current to flow through
the line. It is the type of electrical switch which open or closes the contacts when the fault occurs
in the system.The outgoing feeder supplies the input power to the consumer end.
