InternShip Training Report
132 KV SubStation Kamalabad ,IESCO
Training Duration: Two Weeks ( 20 Aug 2015 to 04 Sep 2015 )
B.Sc Electrical Engineering
APCOMS affiliated with UET Taxila
Islamabad Electric Supply Company Limited ( IESCO )
1. 132 KV SubStation Kamalabad ,IESCO
The 132 KV substation Kamalabad was commissioned in the year 1957.
There are two main 132 KV bus incoming for the substation. These buses are:-
1. 132 KV Rawat Line
2. 132 KV Sandhani Line
2. SUBSTATION (Grid Stations)
Substation is an intermediate switching, transforming or converting station between the
generating station and the low tension distribution network situated generally the consumer’s
The sub-station receives power from the generating station by a single or more feeders at a
high voltage, transform the same to the different distribution voltages and sends to different
consumers through distribution network.
Substations are classified by two broad categories:-
1. According to the service requirement:
2. According to the constructional features:
3. CONDUCTORS USED IN SUBSTATION DESIGN
An ideal conductor should fulfill the following requirements:
a) Should be capable of carrying the specified load currents and short time currents.
b) 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.
c) Should be corona free at rated voltage.
d) Should have the minimum number of joints.
e) Should need the minimum number of supporting insulators.
f) Should be economical.
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.
STANDARD SIZES OF CONDUCTOR FOR 132 KV LINES
'Panther' ACSR having 7-strands of steel of dia 3.00 mm and 30-Strands of Aluminum of dia 3.00
4. SUBSTATION CHARACTERISTICS
1. Each circuit is protected by its own circuit breaker and hence plant outage does not
necessarily result in loss of supply.
2. 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.
3. 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.
4. Maintenance of a feeder or transformer circuit breaker involves loss of the circuit.
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.
When the energy is transformed into a higher voltage, the transformer is called step up
transformer but in case of other is known as step down transformer.
TYPES OF TRANSFORMERS
1. Power Transformer
It is used for the transmission purpose at heavy load, high voltage greater than 33 KV & 100%
efficiency. It also having a big in size as compare to distribution transformer, it used in
generating station and Transmission substation at high insulation level. They can be of two
types: Single Phase Transformers and Multi Phase Transformers.
2. Instrument Transformers
These transformers are used for the measurement purposes at that points where standard
voltmeters and ammeters cannot be used. They are of two types:-
a). CURRENT TRANSFORMER
A current transformer (CT) is used for measurement of
alternating electric currents. When current in a circuit is too
high to apply directly to measuring instruments, a current
transformer produces a reduced current accurately
proportional to the current in the circuit, which can be
conveniently connected to measuring and recording
instruments. A current transformer isolates the measuring
instruments from what may be very high voltage in the
b).POTENTIAL OR VOLTAGE TRANSFORMER
Voltage transformers (VT) (also called potential transformers
(PT)) are a parallel connected type of instrument transformer,
used for metering and protection in high-voltage circuits or
phasor phase shift isolation. They are designed to present
negligible load to the supply being measured and to have an
accurate voltage ratio to enable accurate metering. A potential
transformer may have several secondary windings on the same
core as a primary winding, for use in different metering or
3. On the basis of working
On the above basis, transformers are of two types:
Step up Transformer
Step down Transformer
4. Auto Transformers
An autotransformer is an electrical transformer with only
one winding. The "auto" prefix refers to the single coil acting on
itselfand not to any kind of automatic mechanism. In an
autotransformer, portions of the same winding act as both the
primary and secondary sides of the transformer. The winding has
at least three taps where electrical connections are made.
Autotransformers have the advantages of often being smaller, lighter,and cheaper than typical
dual-winding transformers, but the disadvantage of not providing electrical isolation.
5. Distribution Transformers
A distribution transformer is a transformer that provides
the final voltage transmission in the electrical power
distribution system, stepping down voltage to the level
used by customers.
6. CIRCUIT BREAKERS
A circuit breaker is an automatically operated electrical switch designed to protect an electrical
circuit from damage caused by overload or short circuit. Its basic function is to detect a fault
condition and interrupt current flow. Unlike a fuse, which operates once and then must be
replaced, a circuit breaker can be reset (either manually or automatically) to resume normal
operation. Circuit breakers are made in varying sizes, from small devices that protect an
individual household appliance up to large switchgear designed to protect high voltage circuits
feeding an entire city. There are different types of circuit breakers which are:-
1.Low-voltage circuit breakers
Low-voltage (less than 1,000 VAC) types are common in domestic,
commercial and industrial application, and include Miniature Circuit
Breaker (MCB) and Molded Case Circuit Breaker (MCCB).
2. Magnetic circuit breakers
Magnetic circuit breakers use a solenoid (electromagnet) whose
pulling force increases with the current. Certain designs utilize
electromagnetic forces in addition to those of the solenoid.
3.Thermal magnetic circuit breakers
Thermal magnetic circuit breakers, which are the type found in
most distribution boards, incorporate both techniques with the
electromagnet responding instantaneously to large surges in
current (short circuits) and the bimetallic strip responding to less
extreme but longer-term over-current conditions. The thermal
portion of the circuit breaker provides an "inverse time" response
feature, which trips the circuit breaker sooner for larger over
Three-pole common trip breaker for supplying a three-
phase device. This breaker has a 2A rating. When
supplying a branch circuit with more than one live
conductor, each live conductor must be protected by a
breaker pole. To ensure that all live conductors are
interrupted when any pole trips, a "common trip"
breaker must be used. These may either contain two or
three tripping mechanisms within one case.
5. Sulfur hexafluoride (Sf6) high-voltage circuitbreakers
A sulfur hexafluoride circuit breaker uses contacts
surrounded by sulfur hexafluoride gas to quench the arc.
They are most often used for transmission-level voltages
and may be incorporated into compact gas-insulated
6.Oil circuit breakers
A high-voltage circuit breaker in which the arc is drawn in oil
to dissipate the heat and extinguish the arc; the intense heat
of the arc decomposes the oil, generating a gas whose high
pressure produces a flow of fresh fluid through the arc that
furnishes the necessary insulation to prevent a restrike of the
The arc is then extinguished, both because of its elongation
upon parting of contacts and because of intensive cooling by
the gases and oil vapor. They are further of two types: Bulk Oil
Circuit Breaker (BOCB) and Minimum Oil Circuit Breaker
7. Air circuit breakers
Rated current up to 6,300 A and higher for generator
circuit breakers. Trip characteristics are often fully
adjustable including configurable trip thresholds and
delays. Usually electronically controlled, though some
modelsare microprocessor controlled via an integral
electronic trip unit. Often used for main power
distribution in large industrial plant, where the breakers
are arranged in draw-out enclosures for ease of
Isolator can be rewritten as isolator is a manually operated
mechanical switch which separates a part of the electrical
power system normally at off load condition.
8. BUS BARS
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.
The following are the important bus bars arrangements used at substations:
Single bus bar system
Single bus bar system with section alisation.
Duplicate bus bar system
An electrical insulator is a material whose internal electric charges do not flow freely, and
therefore make it very hard to conduct an electric current under the influence of an electric
field. 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
. Shackle insulator
In early days, the shackle insulators were used as strain insulators. But now
a day, they are frequently used for low voltage distribution lines. Such
insulators can be used either in a horizontal position or in a vertical
. Pin type insulator
As the name suggests, the pin type insulator is mounted on a pin
on the cross-arm on the pole. There is a groove on the upper end of
the insulator. The conductor passes through this groove and is tied to
the insulator with annealed wire of the same material as the
conductor. Pin type insulators are used for transmission and
distribution of electric power at voltages up to 33 kV. Beyond
operating voltage of 33 kV, the pin type insulators become too bulky
and hence uneconomical.
For voltages greater than 33 kV, it is a usual practice to
use suspension type insulators shown in Figure. Consist of a
number of porcelain discs connected in series by metal links in
the form of a string. The conductor is suspended at the bottom
end of this string while the other end of the string is secured to
the cross-arm of the tower. The number of disc units used
depends on the voltage.
. Strain insulator
A dead end or anchor pole or tower is used where a straight
section of line ends, or angles off in another direction. These
poles must withstand the lateral (horizontal) tension of the
long straight section of wire. In order to support this lateral
load, strain insulators are used.
For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators. However,
for high voltage transmission lines, strings of cap-and-pin (disc) insulators are used, attached to
the crossarm in a horizontal direction. When the tension load in lines is exceedingly high, such
as at long river spans, two or more strings are used in parallel.
position. They can be directly fixed to the pole with a bolt or to the
In a power system it is inevitable that immediately or later some failure does occur somewhere
in the system. When a failure occurs on any part of the system, it must be quickly detected and
disconnected from the system. Rapid disconnection of faulted apparatus limits the amount of
damage to it and prevents the effects of fault from spreading into the system. For high voltage
circuits relays are employed to serve the desired function of automatic protective gear. The
relays detect the fault and supply the information to the circuit breaker.
The electrical quantities which may change under fault condition are voltage, frequency,
current, phase angle. When a short circuit occurs at any point on the transmission line the
current flowing in the line increases to the enormous value. This result in a heavy current flow
through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the
trip circuit of the breaker making the circuit breaker open and isolating the faulty section from
the rest of the system. In this way, the relay ensures the safety of the circuit equipment from
the damage and normal working of the healthy portion of the system.
Relay works on two main operating principles:-
RELAY USED IN CONTROLLING PANEL OF SUBSTATION
A differential relay is one that operates when vector difference of the
two or more electrical quantities exceeds a predetermined value. If this
differential quantity is equal or greater than the pickup value, the relay
will operate and open the circuit breaker to isolate the faulty section.
.Over Current Relay
This type of relay works when current in the circuit exceeds the
predetermined value. The actuating source is the current in the circuit
supplied to the relay from a current transformer. These relay are used
on A.C. circuit only and can operate for fault flow in the either direction.
This relay operates when phase to phase fault occurs.
.Earth Fault Relay
This type of relay sense the fault between the lines and the earth.
It checks the vector sum of all the line currents. If it is not equal to
zero, it trips.
This type of relay is in the conjunction with main relay. When
main relay sense any fault in the system, it immediately
operates the trip relay to disconnect the faulty section from
An auxiliary relay is used to indicate the fault by glowing bulb or
showing various flags.
11. PROTECTION OF SUBSTATION
A lightning arrestor is a device used in power systems
and telecommunications systems to protect
the insulation and conductors of the system from the
damaging effects of lightning. The typical lightning
arrester has a high-voltage terminal and a ground
terminal. When a lightning surge (or switching surge,
which is very similar) travels along the power line to the
arrester, the current from the surge is diverted through
the arrestor, in most cases to earth.
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. Various
protection methods used for transformers are:-
Buchholz relay is a safety device mounted on some oil-filled powertransformers and reactors,
equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is
used as a protective device sensitive to the effects of dielectric failure inside the
equipment.Depending on the model, the relay has multiple methods to detect a failing
transformer. 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.
Depending on design, a second float may also serve to detect
slow oil leaks.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 before the fault causes additional
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.
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.
When the transformer is in operation heat is generated due to iron losses the removal of heat is
There are several types of cooling methods, they are as follows:
1. 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
2. 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.
3. Oil natural cooling
Medium and large rating transformers 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.
4. Oil blast cooling
In this type of cooling, forced air is directed over cooling elements of transformers
immersed in oil.
5. 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.
6. Forced oil and water (OWF) cooling
In this type of cooling oil flow with water cooling of the oil in external water heat
exchanger takes place. The water is circulated in cooling tubes in the heat exchanger.
12. Three Phase Transformer Connections
Three phase transformer connections In three phase system, the three phases can be
connected in either star or delta configuration. In case you are not familiar with those
configurations, study the following image which explains star and delta configuration. In any of
these configurations, there will be a phase difference of 120° between any two phases.
Windings of a three phase transformer can be connected in various configurations as (i) star-
star, (ii) delta-delta, (iii) star-delta, (iv) delta-star, (v) open delta and (vi) Scott connection. These
configurations are explained below.
Two transformers are used and primary and secondary
connections are made as shown in the figure below. Open delta
connection can be used when one of the transformers in Δ-Δ bank
is disabled and the service is to be continued until the faulty
transformer is repaired or replaced. It can also be used for small
three phase loads where installation of full three transformer bank
is un-necessary. The total load carrying capacity of open delta
connection is 57.7% than that would be for delta-delta connection.
Star-star connection is generally used for small, high-voltage transformers. Because of star
connection, number of required turns/phase is reduced (as phase voltage in star
connection is 1/√3 times of line voltage only). Thus, the amount of insulation required is
The ratio of line voltages on the primary side and the secondary side is equal to
the transformation ratio of the transformers.
Line voltages on both sides are in phase with each other.
This connection can be used only if the connected load is balanced.
This connection is generally used for large, low-voltage transformers. Number of required
phase/turns is relatively greater than that for star-star connection.
The ratio of line voltages on the primary and the secondary side is equal to the
transformation ratio of the transformers.
This connection can be used even for unbalanced loading.
Another advantage of this type of connection is that even if one transformer is disabled,
system can continue to operate in open delta connection but with reduced available
The primary winding is star star (Y) connected with grounded neutral and the secondary
winding is delta connected.
This connection is mainly used in step down transformer at the substation end of the
The ratio of secondary to primary line voltage is 1/√3 times the transformation ratio.
There is 30° shift between the primary and secondary line voltages.
Delta-Star OR Delta-Wye(Δ-Y)
The primary winding is connected in delta and the secondary winding is connected in star
with neutral grounded. Thus it can be used to provide 3-phase 4-wire service.
This type of connection is mainly used in step-up transformer at the beginning of
The ratio of secodary to primary line voltage is √3 times the transformation ratio.
There is 30° shift between the primary and secondary line voltages.
Above transformer connection configurations are shown in the following figure.
13. CAPACITOR BANK
The load on the power system is varying being high during 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 economic 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 such device can be capacitor bank.
The capacitors draw a leading current and partly or completely neutralize the lagging reactive
component of load current.
Main functions of Capacitor Bank are:-
Supply Reactive Power
Improve Terminal Voltage
Improve Power Factor
Now from this report one can conclude that electricity plays an important role in our life. That’s
why various protective measures are taken to protect the substations from various faults and
its smooth functioning. At the end of the training, I came to know about the various parts of
substations and how they are operated