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Introduction:
The course Power System Protection covers almost everything related to protection system in power
system including standard lead and device numbers, mode of connections at terminal strips, color
codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power
system protection relays and schemes including special power system protection schemes like
differential relays, restricted earth fault protection, directional relays and distance relays etc. The
details of transformer protection, generator protection, transmission line protection & protection of
capacitor banks are also given. It covers almost everything about protection of power system.
The switchgear testing, instrument transformers like current transformer testing voltage or potential
transformer testing and associated protection relay are explained in detail. The close and trip, indication
and alarm circuits different of circuit breakers are also included and explain.
Objective of Power System Protection:
The objective of power system protection is to isolate a faulty section of electrical power system from
rest of the live system so that the rest portion can function satisfactorily without any severer damage
due to fault current.
Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit
breakers automatically open during fault condition due to its trip signal comes from protection relay.
The main philosophy about protection is that no protection of power system can prevent the flow of
fault current through the system, it only can prevent the continuation of flowing of fault current by
quickly disconnect the short circuit path from the system.
History:
Power Grid Company of Bangladesh Ltd. (PGCB) was formed under the restructuring process of
Power Sector in Bangladesh with the objective of bringing about commercial environment including
increase in efficiency, establishment of accountability and dynamism in accomplishing its objectives.
PGCB was incorporated in November 1996 with an authorized capital of Tk.10 billion. It was entrusted
with the responsibility to own the national power grid to operate and expand the same with efficiency.
Pursuant to Government decision to transfer transmission assets to PGCB from Bangladesh Power
Development Board (BPDB) and Dhaka Electric Supply Authority (DESA), PGCB completed taking
over of all the transmission assets on 31.12.2002. PGCB expanded its network and capacity manifold
and operating those efficiently and effectively.
A public limited company. Incorporated through sponsorship of chairman, BPDB and its six members.
76.25% ownership with BPDB & 23.75% with general public.
Its Head Office is at Institution of Engineers of Bangladesh Bhaban (New), the 3rd and 4th floor, 8/A
Ramna,Dhaka-1000,Bangladesh.

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To plan, promote, develop, operate and maintain an integrated and efficient power transmission system
network in all its aspects including planning, investigation, research, design and engineering,
preparation of preliminary feasibility and detailed project reports, construction operation and
maintenance of transmission lines, substations, load dispatch centers and communication facilities and
appurtenant works, co-ordination of integrated operation of regional, national and international grid
systems, providing consultancy services in power systems field, execution of turnkey jobs for other
utilities / organization, wheeling of power, purchase and sale of power.
Brief on Substation:
A substation is a part of an electrical generation, transmission, and distribution system. Substations
transform voltage from high to low, or the reverse, or perform any of several other important functions.
Between the generating station and consumer, electric power may flow through several substations at
different voltage levels.
Substations may be owned and operated by an electrical utility, or may be owned by a large industrial
or commercial customer. Generally substations are unattended, relying on SCADA for remote
supervision and control.
A substation may include transformers to change voltage levels between high transmission voltages
and lower distribution voltages, or at the interconnection of two different transmission voltages. The
word substation comes from the days before the distribution system became a grid. As central
generation stations became larger, smaller generating plants were converted to distribution stations,
receiving their energy supply from a larger plant instead of using their own generators. The first
substations were connected to only one power station, where the generators were housed, and were
subsidiaries of that power station.
Types of Substation:
According to service requirements substation may be classified into-
1. Transformer Substation
2. Switching Substation
3. Power factor correction Substation
4. Frequency changer Substation
5. Converting Substation
6. Industrial Substation
According to constructional features the substations are classified as-
1. Indoor Substation
2. Outdoor Substation
3. Underground Substation
4. Pole mounted Substation

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KHULSI GRID 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.

4. Page 4 of 15
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.
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 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. 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 replaced the first
capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the
secondary terminals.
Instrument Transformer:
Instrument transformers are used to step-down the current or voltage to measurable values.
They provide standardized, useable levels of current or voltage in a variety of power monitoring and
measurement applications. Both current and voltage instrument transformers are designed to have
predictable characteristics on overloads. Proper operation 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

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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 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
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 but also 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

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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.
In an ideal transformer, the secondary voltage vector is exactly opposite and 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 secondary voltage vector is displaced from the primary voltage vector.
Bus Bar:
The bus is a line in which the incoming feeders come into 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 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. 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 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.

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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.
c. Vacuum 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.
VCB, vacuum is used as the arc quenching medium. Since vacuum offers the highest insulating
strength, it has far superior arc quenching properties than any other medium. VCB are being employed
for outdoor application ranging from 22kv to 66kv.
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

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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 of moisture, 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.
Tap changing:
Off-circuit designs (NLTC or DETC)
In low power, low voltage transformers, the tap point can take the form of a connection terminal,
requiring a power lead to be disconnected by hand and connected to the new terminal. Alternatively,
the process may be assisted by means of a rotary or slider switch.
Since the different tap points are at different voltages, the two connections cannot be made
simultaneously, as this would short-circuit a number of turns in the winding and produce excessive
circulating current. Consequently, the power to the device must be interrupted during the switchover
event. Off-circuit or de-energized tap changing (DETC) is sometimes employed in high voltage
transformer designs, although for regular use, it is only applicable to installations in which the loss of
supply can be tolerated. In power distribution networks, transformers commonly include an off-circuit
tap changer on the primary winding to accommodate system variations within a narrow band around
the nominal rating. The tap changer will often be set just once, at the time of installation, although it
may be changed later during a scheduled outage to accommodate a long-term change in the system
voltage profile.
On-load designs (OLTC)
Also called on circuit tap changer or On Load Tap Changer (OLTC). For many power transformer
applications, a supply interruption during a tap change is unacceptable, and the transformer is often
fitted with a more expensive and complex on-load tap-changing (OLTC, sometimes LTC) mechanism.
On-load tap changers may be generally classified as either mechanical, electronically assisted, or fully
electronic.
Isolator:

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The use of thisisolatoristoprotectthe transformerandthe otherinstrumentinthe line.The isolatorisolates
the extravoltage tothe groundandthusany extravoltage cannotenterthe line.Thusanisolatorisusedafter
the bus also for protection.
Control and Relay Panel:
The control and relay panel is of cubical 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 for terminating 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 ph system). 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:
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. Numerical relay
ii. Distance relay

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iii. Differential relay
iv. Directional over current relay
i. Numerical 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
detector with 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 be compared
to the setting values input into the relay. With the microprocessor relay, various curves or multiple
time-delay settings 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 a combination of both voltage and current. The voltage 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

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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 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 product of the magnitude of current, voltage, and the phase angle
between them or to the product of two currents and the phase angle between them. The value of this
product necessary 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. Microprocessor relays 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

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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 explains how to specify such equipment. It has Single 100% battery and 100% charger,
Low capital cost, No standby DC System outage for maintenance. Need to isolate battery/charger
combination from load under boost charge conditions in order to prevent high boost voltages.
II. 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.
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, (5-10) Amp
Boost: 180 V, 30Amp
Capacitor Bank:
The demand of active power is expressing Kilo watt (kW) or megawatt (mw). This power should be
supplied from electrical generating station. All the arrangements in electrical pomes system are done
to meet up this basic requirement. Although in alternating power system, reactive power always comes
in to picture. This reactive power is expressed in Kilo VAR or Mega VAR. The demand of this reactive
power is mainly originated from inductive load connected to the system. These inductive loads are
generally electromagnetic circuit of electric motors, electrical transformers, inductance of transmission
and distribution networks, induction furnaces, fluorescent lightings etc. This reactive power should be
properly compensated otherwise, the ratio of actual power consumed by the load, to the total power
i.e. vector sum of active and reactive power, of the system becomes quite less. This ratio is alternatively
known as electrical power factor, and fewer ratios indicates poor power factor of the system. If the
power factor of the system is poor, the ampere burden of the transmission, distribution network,
transformers, alternators and other equipment’s connected to the system, becomes high for required

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active power. And hence reactive power compensation becomes so important. This is commonly done
by capacitor bank.
Functions of Associated System in Khulshi Substation
Functions of Associated System in Khulshi Substation is as shown below in table-1
Table-1 Functions of Associated System in Substation
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 outdoor substation 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
- control cables
- 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.

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7. PLCC system
power line carrier
communication system
For communication, telemetry, tele-control, power line carrier
protection etc.
8. Telephone, telex,
microwave, OPF
For internal and external communication
9. Auxiliary standby power
system
For supplying starting power, standby power for auxiliaries.
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.

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Cnclusion
The Electrical and Electronic Engineering Department, organized an industrial visit to PGCB sub-
station Khulshi branch (132/33/11 KV) was very informative. From this visit, we got the information
and practical knowledge about Power Distribution and Transmission. This visit was extremely
beneficial in familiarizing the different equipments and working of a substation. We got the idea how
to read the single line diagram of power substation using different symbols used in diagram. We
cleared out practical knowledge of transformer as how it step down voltage 132 KV to 33 KV. We
understand the reason behind using stones/gravel in the substation which was to reduce the step
potential and touch potential when operators work on switch yard and eliminate the growth of small
weeds and plants inside the switch yard.
We hope that this visit will help us in our future practical life and bring a positive change in our
thinking and practical behavior regarding Education and specially Engineering.