Unit2 11KV Substation.

Q1) Draw the layout of a pole mounted 11kV/400 V substation and enlist any eight equipment of it.
S22
Ans:
Layout of a Pole Mounted 11kV/400 V Substation with Equipment:

Equipment:
1. Three phase distribution transformer.
2. DO fuses.
3. Lightning arrester.
4. Circuit breaker / Oil circuit breaker.
5. AB switch / Isolating switch.
6. Pin type insulators.
7. Current transformer.
8. Potential transformer.
9. 11kV overhead incoming conductors.
10. LV power cables.
11. Stay wire.
12. Earthing system.
13. DP structure.
14. Instrument transformers.
15. Metering and indication instruments.
16. Distribution panel board.
17. Capacitor bank.
18. Outgoing feeders (400 Volts).
19. Neutral grounding equipment.
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20. Control cables / wires.
Functions of different parts in pole mounted substation: W23
1. Lightning arrestors- It provides a low-impedance path to ground for the current from a lightning
strike or transient voltage and then restores to a normal operating condition.
2. Isolator- It is mechanical switching device operated on No load condition. It is capable of opening and
closing a circuit when small or negligible current is flowing.
3. Fuses/Air break switches- It is overcurrent protection device used to protect the circuit.
4. Transformer- It is used to step down 11kV voltage level to 400 V.
5. Circuit breaker- It is used as a protection and switching device to interrupt fault currents
automatically and may be used to switch loads on and off, or to cut off a line. Its basic function is to
interrupt the current flow and prevent the risk of fire during fault condition and during maintenance
activities.

Sketch the following layout Diagrams in 11 KV substation

(1) Floor mounted
(2) Pole mounted
Ans:
Layout Diagrams of 11 kV Floor mounted substation

Layout Diagrams of 11 kV Pole mounted substation:

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Q2) Enlist any eight routine tests to be carried out on 11 kV/ 400 V distribution transformers. S22
Ans:
Following are the routine tests that need to be carried out on 11 kV/ 400 V distribution transformers.
(Any eight)
1. Winding resistance test of transformer
2. Polarity and Transformer ratio test
3. Transformer vector group test
4. Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit
test)
5. Measurement of no load loss and current (Open circuit test)
6. Measurement of insulation resistance
7. Dielectric tests of transformer.
8. Tests on on-load tap-changer.
9. Oil pressure test on transformer to check against leakages past joints and gaskets
10. Impulse test
11. Temperature rise test

Q3) Write any six precautions to be taken while maintaining 11kV/400V distribution transformer.
S22
Ans:
i. Ensure all safety arrangement while working on electrical installation.
ii. Ensure that all tools & tackles are in good & working condition.
iii. Check and thoroughly investigate the transformer whenever any alarm or protection is operated.
iv. Check the protection system periodically.
v. Ensure every employee is familiar with the instructions for restoration of persons suffering from
electric shock.
vi. Trained the staff in operating the fire-fighting equipment.
vii. Always avoid un-balance loading on phase.
viii. Do earthing of all points before starting maintenance.
ix. Keep all spares away from dirt.
x. Work with full confidence.
xi. Ensure thorough and full cleaning of insulators, since partial cleaning is worse than no cleaning.
xii. Ensure perfect isolation of supply before commencement of maintenance work.
xiii. Put a caution board when on work.
Q4) State the functions of three phase distribution transformer. W22
Ans:
Functions of three phase distribution transformer:
 It is a step down transformer.
 It transmits power to the secondary distribution circuit
 It reduces the voltage of the primary distribution circuit to a voltage level as per the consumer
requirement.

Q5) Justify the use of CT & PT in 11 kV substation. S22

Ans:
Use of Current Transformer (CT) in 11 kV substation:
Metering and measurement:
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It steps down the high line current to low value so that currents in HV circuit can be measured with
the help of standard low range ammeter.
Use of Potential Transformers (PT) in 11 kV substation:
Metering and measurement:
It steps down the high voltage to low value so that it can be measured with the help of standard low
range voltmeter.

Q6) State any four advantages of routine maintenance of 11 kV substation. W22

Ans:
Advantages of Routine Maintenance of 11 kV substation:
1. Lengthening life of substation equipment:
Keeping substation equipment in good condition allows it to run longer, lowering costs.
2. Lowering the risk of breakdowns:
Proper maintenance helps in lowering down the breakdown time. Breakdown affects the reliability
adversely.
3. Increased efficiency:
Routine preventive maintenance of substation helps to operate the substation more efficiently.
4. Decreased unplanned downtime:
Downtime can be significantly reduced by scheduling maintenance in advance with a proper
maintenance plan.
5. Promote health and safety:
Preventive maintenance can improve the health and safety of your substation and working personnel.
6. Boost customer satisfaction:
Ensuring equipment is always running as efficiently as possible directly translates to cost savings and
increased revenue, but it also helps to create a strong brand image and boost customer satisfaction.
7. Saving of money:
All of the reasons listed above lead back to one ultimate benefit of preventive maintenance: it saves
money. Unplanned maintenance typically costs 3 to 9 times more than planned maintenance.
OR
1) Routine maintenance helps to prevent minor faults from developing into major breakdown.
2) Routine maintenance helps to reduce breakdown period.
3) Routine maintenance helps to keep the machine in good working condition by reducing wear and tear.
4) Routine maintenance helps to provide greater safety & protection to the workers.
5) Routine maintenance helps to use less standby equipment.
6) Routine maintenance helps to increase life of equipment.
7) Routine maintenance helps to avoid inconvenience caused due to equipment failure.
8) Routine maintenance helps to increase productivity.
9) Routine maintenance helps to determine the need for major & minor repairs.
10) Routine maintenance helps to develop maintenance schedule at low cost.

Q7) Describe any four important safety practices followed during breakdown maintenance in 11
kV substation. W22 ,W23
Ans:
Important safety practices followed during breakdown maintenance in 11 kV substation:
1) Complete isolation, discharging and effective grounding of the concerned circuit/equipment carried
out before starting maintenance work.
2) Use of rubber gloves/gantlets, while operating switches/links and during discharging and grounding.
3) Obtaining proper permit to work before commencing work on any line/equipment.
4) Return of the “Permit to work” (by the same person who takes it) after ensuring that all equipment are
free from men, material and ground.
5) Authorization of persons by name to work on particular clause of
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line/equipment.
6) Efficient Supervision by a qualified and responsible person while carrying out works to adjacent
energized circuits.
7) Locking and hanging “men–on-line” boards on switches controlling circuits declared safe for work.
8) During shut-down the working area should be marked with Red Ribbon / Rope. While working on
elevated positions, safety belts and ropes should be used. If a ladder is being used, ensure that it does not
fall or reach outside the working area marked with Red Ribbon / Rope.
9) Before lowering the oil in a transformer for and internal inspection, it is advisable to take oil samples
from top and bottom for testing.
10) The following safety measures are recommended before entering a transformer for an internal
inspection.
11) Men working in or on open transformer must remove all objects from their pockets. All tools must be
attached to suitable hand lines so that the tools may be withdrawn if accidentally dropped inside the
transformer.
12) Boxes or approved containers of a convenient size to hold loose nut bolts and other small objects
must be provided and used when working on the top of a transformer which is open.
13) Training related to safety provided before starting maintenance work.
14) Ensuring the use of personal protective equipment such as helmet, goggles, gloves, ear muffler etc.
15) Interlocks should not be bye-passed unless it is very essential. Written permission should be obtained
from the departmental head - incharge of the station. Extra precaution should be taken by all the parties
during each case.
16) No unsafe operation will ever be permitted. Feedback regarding unsafe operation / condition should
be taken into consideration with proper spirit and review should be made to avoid accident.
17) Operations and maintenance manuals/manufacturer’s instructions for different equipment should be
available and should be referred to before taking out equipment for maintenance.
18) Operations should be carried out as per operating instructions. This will help in carrying out
operations safely and maintaining uniformity. In case of any modifications / changes in the layout,
operating instructions should be reviewed and all personnel are apprised of the modifications / new
instructions.
19) Only authorized persons shall be allowed to carry out operation and maintenance.
20) Metal ladders should not be used in switchyard.
21) Adequate number of first aid and firefighting equipment shall be maintained.
22) Every person should be familiar with firefighting and should know how to operate firefighting
equipment, so that fire can be extinguished promptly minimizing damage.
23) In the event of fire on electrical installation, the affected part shall be immediately switched-off and
isolated from all the sources.
24) First aid and artificial respiration chart should be exhibited.
25) While opening the isolator, confirm that it is not carrying load current. Do not break load current on
isolator. Similarly, isolator should not be closed on load. However certain isolators are designed to
change / discharge transformer (without load) and small length of overhead lines or cables.

Q8) Write the name of relevant fire-fighting equipment used in case of power transformer fire and
justify its use. W22
Ans:
Relevant fire-fighting equipment used in case of power transformer fire:
1. Deluge System: This system uses open spray heads attached to a piping system connected to a water
supply through a valve that is opened by means of a detection system installed in the same area as the
spray heads. When the valve opens, water flows into the piping system and discharges through all spray
heads attached to the system. This type of system uses high volumes of water which may lead to other
post fire clean up issues or contaminated runoff.
2. Fixed Water Spray System: This system is similar to a deluge system; however, the water discharge
points are designed to create a spray pattern unique to the particular area or equipment being protected.

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The water spray head locations and spray pattern are tailored to the asymmetrical shape of the equipment
being protected. The added benefit of the equipment specific design is the control of fire spread through
increased wetting of the equipment under the influence of the fire. Electrical transformers containing oil
are commonly protected by fixed water spray systems.
3. Water Mist System: This system is similar to the fixed water spray system with an added benefit of
using significantly less water by employing special discharge heads creating water droplets called mist.
A water mist system is generally defined by the droplet size created. Droplet sizes typically are less than
1000 microns and delivered by a high-pressure pump. The water droplets create a mist that allows a
given volume of water to create a larger surface area exposed to the fire. Smaller droplets facilitate larger
heat absorption to cool the fire.
4. Pre-action Water Mist System: This system uses automatic sprinklers attached to a piping system
containing air with a supplemental detection system installed in the same areas as the sprinklers. System
activation may require both a signal from the detection system and the heat activation of a sprinkler. Pre-
action systems are used to protect areas where the risk of false discharge or leakage must be kept to an
absolute minimum. This system has recently seen increased occurrence of use in transformer fire
suppression because of water conservation, greater immunity to false initiation and greater ability to
control fire spread.

Q9) Describe the procedure to measure the insulation resistance of distribution transformer using
neat labelled diagram. W22
Ans:
Procedure to measure Insulation Resistance of Distribution transformer:
1) Do not disconnect the ground connection to the transformer tank and core. Make sure that the
transformer tank and core are grounded.
2) Disconnect all high-voltage, low-voltage, and neutral connections, lightning arresters, fan systems,
meters, or any low-voltage control systems that are connected to the transformer winding.
3) Before beginning the test, jumper together all high-voltage bushings, making sure that the jumpers are
clear of all metal and grounded parts. Also jumper together all low-voltage and neutral bushings, making
sure jumpers are clear of all metal and grounded parts.
4) Use a mega-ohmmeter (megger) with a minimum scale of 20,000 MΩ.
5) Resistance measurements are then made between each set of windings and ground. The windings that
are to be measured must have their ground removed to measure their insulation resistance.
6) Mega-ohmmeter reading should be maintained for 1 min. For three-winding transformers, the test
should be made as follows:
7) High to low, tertiary and ground (H-LTG), shown in Figure (a).
8) Tertiary to high, low and ground (T-HLG).
9) Low to high, tertiary and ground (L-HTG), shown in Figure (b).
10) High, low and tertiary to the ground (HLT-G).
11) High and tertiary to low and ground (HT-LG), shown in Figure (c).
12) Low and tertiary to high and ground (LT-HG).
13) High and low to tertiary and ground (HL-TG).

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Q10) Draw and explain working diagram of earth tester. S22
Ans:

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Earth tester is an electrical measuring instrument used to measure the resistance between any two points
of the earth. It is also called earth resistance tester.
Construction and Working of Earth Tester-
i) Earth Tester consists of a dc generator, a rotating current reverser, rectifier, and a PMMC instrument to
measure resistance. The schematic diagram of the earth tester for measuring the resistance of earth soil is
shown in above figure.
ii) The current reverser is used to convert dc to ac so that the earth resistance test can be done with
alternating current. The ac supply from the current reverser is given to the earth electrode that is under
test.
iii) The rectifier is used because the tester is equipped with a moving coil instrument to measure
resistance. Thus the pressure and the current coil of the instrument need dc supply. The rectifier converts
alternating current coming from the electrode to direct current and supplied to the current coil.
iv) The pressure or potential coil is connected across the generator and it measures voltage applied. The
current coil is connected in series with the dc generator and it measures current through the electrode.
Both current reverser and rectifier are connected with the help of two commutators mounted on the shaft
of the generator. Two pairs of fixed brushes are provided with each commutator to transfer power from
the rotating shaft to the stationary parts.
v) The earth tester is provided with four terminals C1, C2, P1, and P2. The terminals C1 and C2 are the
current terminals while terminals P1 and P2 are known as potential terminals. The electrode under test
i.e., earth electrode E is connected to the terminals C1 and P1 by shortening them. The two auxiliary
electrodes P (potential electrode) and C (current electrode) are connected to the terminals P2 and C2
respectively.
vi) The potential and current coils of the PMMC instrument are connected such that, the voltage drop
between electrodes E and P is applied across the potential coil and current through the current coil
depends upon the resistance of the earth.
When the generator is driven at its rated speed, the pointer of the moving coil instrument deflects and
the earth tester indicates the resistance of the earth. The deflection of the pointer is given by the ratio
of voltage across the potential coil and current of the current coil.

Q11) Compare equipment earthing and system earthing on any four points.
Ans:

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Q12) Explain the procedure to be followed for shut down of 11 kV substation and power lines.
Ans:
Procedure to be followed for shut down of 11 kV substation and power lines.
a) Announcement of shut-down:-
When feeder lines, transmission lines, or power transformers require maintenance, it is essential to
prepare a maintenance program and fix-up the date for arranging it. This type of program needs a long
period of shut-down; which will affect the working of H.T. consumers as well as L.T. consumers. As per
the terms and conditions of H.T. supply, it is essential to inform the date and time of shut-down for such
works hence every H.T. consumer is informed in advance about such shut-down; It is also necessary that
such long period shut-downs are declared in papers for information of all L.T. consumers fed from lines
or sub-stations, etc.
b) Obtaining a permit to work:-
When the work is to be carried out on live lines or equipment or in the vicinity of live equipment, a
shutdown is required. The person-in-charge of such maintenance work should apply, on the prescribed
form to the officer-in-charge of the installation, or sub-station who will then arrange to effect the shut-
down and will issue a “permit-to-work” which will consist of the official authority for carrying out the
work.
c) Isolation of lines/system:-
When an authorized person in the sub-station receives a request in the prescribed form, from an
authorized person for shut-down of a particular line, equipment, etc. he has to observe the following
procedure:
1. Clearly identify the circuit breakers or control switches to be opened to shut down a proper section of
lines etc.
2. Trip off the circuit breaker of the required section, lock it in the off position (open), the isolating
switches in the circuit, if fuses are there they should be removed, and the neutral link if any should be
removed.
3. Close the earth isolator so that the line will be earthed.
4. On the circuit breaker or switch handle a warning board “men-on-work” should be hung.
5. Make sure that the supply is disconnected.
6. Earth the equipment properly at receiving end side, either by permanent earth or temporary earth.
7. The “Permit to work” is then issued to the authorized person

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Q13) Give rating and type of following components used in pole mounted sub- station of your
institute campus .3 Phase transformer, CT, PT, LA, DO, CB

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