ESP Unit 4 (132KV Substation)

Q1) Write any four needs of 132/33 kV substation. S22

Ans:
Needs of 132/33 kV Substation:
1. For decreasing the voltage level from 132 kV to 33 kV for secondary transmission.
2. To receive power from primary transmission system and supply it for secondary transmission.
3. Provides facility for switching when power is required to be turned “ON” or “OFF”.
4. To satisfy / supply the new load growth.
5. To accommodate new generation i.e., there is a need of collector substation to tie solar / wind
generators etc. and connect them to the power grid.
6. To maintain reliability requirements in case of fault etc. on the other substations / lines.
7. To provide overall support to the power flow system i.e., for better management of overall power
flow system.
8. To connect communication signals to the circuits.
9. For regulating voltage to compensate for system voltage changes.
10. To measure electric power quantities flowing in the circuits.
11. Substations provide the necessary real-estate / infrastructure to install transformers and associated
substation equipment required for power transmission and distribution.

Q2) Draw and explain single line diagram of 132kV/33kV substation indicating major
equipment. S22
Ans:
Single line diagram of 132kV/33kV substation:
(Sample diagram is shown, any equivalent diagram is acceptable)
1. 132kV Incoming lines:
i) Two Incoming lines of 132kV voltage level.
ii) Capacitive Voltage Transformer is connected to both the lines.
iii) Major equipment like Lightning arrestor, Wave trap, Current Transformer, Circuit Breaker,
Isolator on both side of breaker are connected in each bay.
2. 132kV Busbar:
i) Two bus system is present.
ii) Major equipment like, Power Transformer, Current Transformer, Circuit Breaker, Isolator on both
side of breaker are connected in each bay
3. 132/33kV Transformer:
i) 132/33kV Transformer is used to step down the voltage level.
ii) Isolators with Earthing are provided on both side of transformer.
4. 33kV Outgoing lines ( Feeders):
i) Eight outgoing 33kV feeders are connected to 33kV Busbar which is common to all feeders.
ii) Supply will provide to all types of consumers through these feeders.

Q3) Suggest the suitable method of neutral grounding in 132/33 kV substation with its any two
specific reasons. S22
Ans:
Suitable Method of Neutral Grounding in 132/33 kV Substation with Specific Reasons:
1. Generally, one neutral ground is provided at each voltage level for better safety, operation,
protection and proper discrimination.
2. The neutral grounding is provided at source end and not at load end for more effective protection.
3. The neutrals of auxiliary supply generators (if used) are grounded through resistance grounding to
limit the stator fault current.
4. The neutrals of synchronous motors and synchronous capacitors (if used) are grounded through
reactance grounding to limit the earth fault current.
5. For all circuits / equipment / machines (e.g. star point of distribution transformer, current
transformer, potential transformer etc.) which has the rated voltage in the range of 3.3kV to 33 kV
resistance or reactance grounding is used because by adjusting the value of resistance / reactance the
arcing grounds can be minimized.
6. Each major “Bus - Section” is provided with neutral grounding for safety and stability.
7. For all circuits / equipment / machines (e.g. star point of distribution transformer, current
transformer, potential transformer etc.) which has the rated voltage of below 600 Volts and above 33
kV solid or effective grounding is used because effective grounding limits the voltages of healthy
phases to line-to-neutral values in the event of ground faults.

Q4) Illustrate any eight reasons of major fire risks within 132 kV/33 kV substation. S22
Ans:
Reasons of Major Fire Risks within 132 kV/33 kV Substation:
1. Overloading of substation equipment e.g. transformers / cables / conductors etc. for long time,
increases temperature & create the risk of fire.
2. The majority of fires are caused due to selection of incorrect rating of the switchgears or use of
incorrect rating MCCB / fuses or incorrect setting of safety relays for substation equipment / system.
3. If there is damage / deterioration in insulation, in any substation equipment / system a short circuit
may occur causing fire.
4. Loose connections / poor joints in cables, conductors, bus-bars etc. may cause overheating,
sparking & lead to a fire risk in substation.
5. Failing to replace worn out / defective etc. equipment / cables associated with substation causes
the electrical fire.
6. Storage of highly flammable liquids in the substation is the ultimate risk of fire.
7. Faulty / out dated electrical equipment, insulators, auxiliaries, supports etc. are a common cause of
electric fire.
8. Fire may be caused if the substation is not maintained properly and regularly.
9. Use of poor quality of material, non ISI mark equipment, under rating equipment, under rating
conductors etc. are the risk of fire.
10. If clearances are not maintained as per voltage level & as per IE rules then there is strong
possibility of sparking, which is the risk of fire in the substation.
11. Incorrect insulation level / improper insulation of substation system / equipment is the risk of fire.
12. Use of equipment which are not as per the specifications is the risk of fire.
13. With so many people working with substation on a daily basis, human error can be a common
factor in the causes of fires by using equipment / system incorrectly, not reporting faulty machinery
etc.
14. Lack of basic fire safety training / unknown about the correct procedures to extinguish the
electrical fire to concern staff is the risk of fire.
15. Faulty / inoperative / inaccessible / insufficient / non availability of proper fire fitting equipment
in the substation is the risk of fire.
16. Fire risk is there if incompetent / unqualified / untrained / unskilled staff is working in substation,
which are incapable to handle the fire risks.

Q5) State purpose of circuit breaker, isolator, and earthing switch. Explain their operational
co-ordination in substation. S22
Ans:
i) Purpose of circuit breaker:
An electrical circuit breaker is a switching device which can be operated manually and automatically
to interrupt or make the load current as well as the much larger fault current, which may occur on a
circuit.
OR
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage
caused by an over current or short circuit. Its basic function is to interrupt current flow to protect
equipment and to prevent the risk of fire.
ii) Purpose of Isolator:
Isolators are used to disconnect the transformers, circuit breakers, and bus bars for repair and
maintenance. They are operated under no current conditions (no load) only since they don’t have
rated making or breaking current capacity.
iii) Purpose of Earthing switch:
Main function of earth switch is to ground the isolated bus/conductor. Even after isolator operation
there may be some residual charges on the bus which may harm the personnel going for maintenance.
So before commencing maintenance we have to ground the isolated bus too to avoid any
mishappenings. So earth switch provides extra safety to the working personnel.
iv) Operational coordination between circuit breaker, isolator & earthing switch:
i) During opening of circuit breaker the sequence of operation is as below-
1. Open Circuit breaker
2. Open Isolator
3. Close earth switch
ii) During closing of circuit breaker the sequence of operation is as below-
1. Open earth switch
2. Close isolator
3. Close circuit breaker
Q6) State the function of the following in 132 kV/33 kV sub-station. CVT, HT fuse, Wave trap,
PLCC, Insulator, Isolator. S23
Ans:
1) Function of CVT:
Capacitor Voltage Transformer is used in 132kV/33kV substation for two purpose:
(i) As instrument transformer to step down 132kV line voltage to safe value for measurement to
supply to voltmeter.
(ii) As coupling capacitor in PLCC to facilitate injection (or couple) of carrier signals into lines.
2) Function of HT Fuse:
HT fuse is used to protect transformer windings in the event of short circuit fault. The short-circuit
current flows through the fuse element and produces large heat to melt it. Thus open circuit or
disconnect the winding.
3) Function of Wave-trap:
(i) A wave trap is a Prevention device that is mounted on the transmission line to trap the high
frequency communication signals sent on the line between two substations and prevented from
entering the power system components in the substation.
(ii) These high frequency communication signals are diverted to the telecom/tele protection panel in
the substation control room (through coupling capacitor and LMU).
(iii) This is relevant in Power Line Carrier Communication (PLCC) systems for communication
among various substations without dependence on the telecom company network.
4) Function of PLCC:
Power Line Carrier Communication (PLCC) is used for following purposes:
(i) Power-line carrier communication (PLCC) is mainly used for telecommunication, and tele-
monitoring between electrical substations through power lines at high voltages, such as 110 kV, 220
kV, 400 kV.
(ii) PLCC can be used for interconnecting private branch exchanges (PBXs).
(iii) Carrier protection scheme for transmission line.
(iv) Telemetering of electrical quantities such as kW, kVA, kVAR.
5) Function of Insulators:
Insulators are used to provide insulation between live conducting parts of substation and earthed
parts. Insulators can be –
(v) line insulators supporting and insulating lines from supporting towers or structures,
(vi) Supporting insulators to support various devices or components in substation such as isolators,
circuit-breakers, CTs, PTs etc.
6) Function of Isolators:
Isolators are used to disconnect the transformers, circuit breakers, and bus bars for repair and
maintenance. They are operated under no current conditions (no load) only since they don’t have
rated making or breaking current capacity.

Justify the use of Earth Mat in 132 kV substation. W22

Ans:
Use of Earth Mat in 132 kV substation:
(i) Primary requirement of earthing is to have a low earth resistance. Substation involves many
earthings through individual electrodes, which will have fairly high resistance.
(ii) If these individual electrodes are inter linked inside the soil, forming earth mat, it increases the
area in contact with soil and creates number of parallel paths.
(iii) Hence the value of the earth resistance using earth mat will be much lower than the individual
value. So earth mat is used in 132 kV substation.
Q7) Explain with neat sketch functioning of S22
i) Wave trap
ii) PLCC
Ans:
i) Function of Wave trap:

1. A wave trap is a Prevention device that is mounted on the transmission line to trap the high
frequency communication signals sent on the line between two substations and prevented from
entering the power system components in the substation.
2. These high frequency communication signals are diverted to the telecom/tele protection panel in
the substation control room (through coupling capacitor and LMU).
3. This is relevant in Power Line Carrier Communication (PLCC) systems for communication among
various substations without dependence on the telecom company network.
ii) Function of PLCC

1. Power-line carrier communication (PLCC) is mainly used for telecommunication, and tele-
monitoring between electrical substations through power lines at high voltages, such as 110 kV, 220
kV, 400 kV.
2. PLCC can be used for interconnecting private branch exchanges (PBXs).
3. Carrier protection scheme for transmission line.
4. Telemetering of electrical quantities such as kW, kVA, kVAR.
Q8) State the meaning and functions of PLCC used in 132kV / 33 kV substation. W22
Ans: PLCC:
PLCC means Power Line Carrier Communication. It refers to a technique in which power
transmission line is used as communication line for transferring communication / telephone signals
from one substation to the other.
Function of PLCC:

1) Telecommunication & Telemonitoring: Power-line carrier communication (PLCC) is mainly

used for tele-communication, and tele-monitoring between electrical substations through power lines
at high voltages, such as 110 kV, 220 kV, 400 kV.
2) Telephony: PLCC can be used for interconnecting private branch exchanges (PBXs) for voice
messaging.
3) Line Protection: For the purpose of carrier aided protection, PLCC channels use modulation
schemes namely the Amplitude modulation (AM) for blocking schemes and Frequency Shift keying
(FSK) for unblocking, permissive and direct-trip schemes.
4) Telemetering: To monitor electrical quantities like voltage, current, power (kW, kVA, kVAR)
etc. at remote locations.

Q9) Describe the procedure to locate and record the hot spots in a 132 kV/33 kV sub-station.
S23
Ans: Procedure to locate and record the hot spots in a 132 kV/33 kV sub-station:
1. A Typical substation (Switchyard) consists of CT (Current Transformer), PT (Power Transformer),
CB (Circuit Breakers), Surge or Lightning Arrester, Busbars, Earth Connectors, Potential
Transformer, etc. These components are connected that form “joints”. Normally, over a period, these
joints develop hot spots which require timely and daily monitoring. So during preventive
maintenance activities, plan is made to monitor all the joints, critical lines and equipment for
hotspots. These hot spots may cause the breakdown of the substation which may result in a shutdown
of plants which is dependent on power from the substation. It may also cause accidents. So, it is
important to monitor “hot spots” in a substation regularly to prevent such breakdowns or accidents.
2. It is done with the help of an infrared camera. At some substations, an infrared camera is fixed at
one location, and it covers a certain area of the substation. Hence to cover the complete substation,
multiple cameras are fixed at different suitable locations.
3. At some substations, a handheld portable infrared camera is used. This portable camera is moved
around, and the complete substation is scanned manually for thermal anomalies.
4. Using a thermal camera, which converts invisible infrared radiation into clear images from which
temperatures can be read, it is possible to identify components that are overheating and those that are
abnormally cool. The images from the camera can be displayed on a monitor in real time or stored
for later analysis.
5. For substations, the recommended procedure is to start with the exterior using a thermal imager.
Scan the transmission line feeding the station, the circuit from the transmission line, high side
insulators (arresters), and then check on specific components. For example, on a transformer, look at
the bushings, the tap changer tanks, and so on. Recording the results and trending them over time can
provide invaluable additional information. When a hot spot is identified, repairs can be carried out
during a scheduled shutdown. This avoids an unplanned shutdown and is an effective form of
preventative maintenance.

Q10) Describe the causes of hot spot formation in transformer and state the methods of
identification. W22
Ans:
Causes of hot spot formation in transformer:
1) The winding is heated up by I2R losses, the core is heated up due to the core losses (hysteresis and
eddy losses) inside the CRGO lamination stack. Losses will not be uniform in the core and the joints
between limbs and yokes will have more losses (10-20 %) than the average unit weight loss (w/kg).
Hence the temperature hot spot in the core will be near these core joints. In three-phase three limbed
cores, this hot spot will be in the middle of the top yoke above the middle limb. In other types of
cores, the hot spot will be slightly down in the upper part of the middle limb portion.
2) Moisture and oxygen cause the oil to decay much faster than the normal rate and form acid and
sludge. Sludge settles on windings and inside the structure, causing transformer temperature to rise.
If temperature increases, then conductor resistance increases and consequently transformer Output
voltage and load voltage decreases. So, under voltage occurs if transformer temperature rises.
Moisture lowers the dielectric strength of oil. Thus, insulating property decreases. So, breakdown
voltage also decreases with increase of moisture content in oil, Moisture raises the temperature and
hence dielectric power factor and increases the risk of thermal breakdown of solid insulation.
3) When the windings are loaded, leakage flux from the windings under the rated load will impinge
on the outermost packets of the core, heating the surface due to the eddy current losses. This
temperature rise can be a few kelvins to several tens of kelvins over the adjacent oil depending on the
transformer winding, core, and tank shielding design. This creates hot spots.
4) In short, the real core hot spot temperature is a combination of the hot spot from the main flux,
core surface heating from winding leakage flux, and temperature rise of the oil around the area of the
surface hot spot.

Identification of Hotspot in transformer-

There are few methods to identify and measure the hotspot temperature.
1) The conventional method of finding the hotspot temperature is indirect calculation method.
Adding the temperature difference between the hot spot temperature in the transformer tank and the
temperature of the top oil, the temperature rise of the top oil in the tank and the ambient temperature,
the sum is equal to the hot spot temperature.
2) Other method is using fibre optical temperature sensors positioned at the predicted hotspot of the
windings. The thermal sensors, attached to the end of optical fibre are usually placed between the
insulated conductor and spacer, and their signals via optical fibre transmitted out of the tank.
3) When hotspots due to faults are created in the transformer winding, they lead to formation of
hydrocarbon gases (H2, CH4, C2H4, C2H6, and C2H2), whereas faults involving the solid insulation of
the transformer result in the formation of oxide gases (CO and CO2). Oil samples are taken from
transformers and are analyzed for the existence of these gases. Then the approximate location of hot
spots is found by using infrared images of the external surfaces of the power transformer.
Q11) State the type of LA used for 132 kV/33kV sub-station. Give its rating. S23
Ans:
Types of Lightning arrestors used for 132 kV/33 kV sub-station:
1. Electrolytic arrestor
2. Valve type Lightning arrestor
3. Thyrite type Lightning arrestor
4. Oxide film Lightning arrestor
5. Metal Oxide Lightning arrestor
Rating of LA used for 132 kV/33kV:
1. Voltage Rating of LA = Line to line voltage x 1.1 x coefficient of earthing
= 132 x 1.1 x 0.8
= 116.16 kV
(Assuming coefficient of earthing equals 0.8 for the effectively earthed system)
2) Power frequency spark over voltage = 1.5 ×Voltage rating of LA
= 1.5 x 116.16
= 174.24 kV
3) Rated Discharge current = 5-10 kA

Q12) Define the terms Touch potential, Step potential, Mesh Potential and Transferred
Potential in associated with substation. S22
Ans:
i) Touch Potential: The potential difference between any item of metalwork and the soil around it
(creating a hand-to-feet voltage difference) during the time that fault current flows.
ii) Step Potential: Step potential in a particular direction is defined as the potential difference
between two points a meter apart.
iii) Mesh Potential: The maximum touch voltage within a mesh of a ground grid is called Mesh
Potential.
iv) Transferred Potential: The potential which is transferred into or out of the substation from or to
a remote point external to the substation site is called transferred potential.

Q13) Illustrate with relevant figures the following: S23

(i) Touch potential
(ii) Make Mesh potential
(iii) Transfer potential
Ans:
i) Touch Potential (or voltage): The potential difference between any item of metalwork and the
soil around it (creating a hand-to-feet voltage difference) during the time that fault current flows.
ii) Mesh Potential (or voltage): Mesh potential is defined as the largest potential difference between
metallic objects connected to the grid, and soil within the grid, under worst-case fault conditions.
iii) Transferred Potential (or voltage): The potential which is transferred into or out of the
substation from or to a remote point external to the substation site is called transferred potential.
Q14) Describe procedure to undertake safe preventive maintenance in 132 kV substation. W22
Ans:
Procedure to undertake safe preventive maintenance in 132 kV substation:
i) Complete isolation, discharging and effective grounding of the concerned circuit/equipment before
starting work.
ii) Use of rubber gloves/gantlets, while operating switches/links and during discharging and
grounding.
iii) Obtaining proper permit to work before commencing work on any line/equipment. If the operator
has to work himself, he should avail self-line clear recording all operations and then only work. Self-
line clears should be limited to emergencies only.
iv) 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.
v) Authorization of persons by name to work on particular clause of line/equipment. An approved list
Permit To Work (P.T.W.) on equipment should be issued or taken only by the persons who are
directly in-charge of the lines or equipment concerned. Helpers should work under the guidance of
higher category officials only, except in isolated stations, where selected men may be authorized by
name, when necessary, and considered quite justified by the Executive Engineer on recommendation
of the Assistant Engineer.
vi) Efficient Supervision by a qualified and responsible person while carrying out works to adjacent
energized circuits.
vii) Use of “Safe for Work” boards on equipment declared safe for work.
viii) Locking and hanging “men–on-line” boards on switches controlling circuits
declared safe for work.
ix) 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.
Q15) Prepare a tabular form to show maintenance schedule of a power transformer above 1000
kVA capacity. S23
Ans:
Maintenance schedule of a power transformer:
Q16) State any one type of battery trouble, its cause, maintenance, remedy to be taken. S23
Ans:

Q17) Describe “Safety practices” followed during breakdown maintenance of 132kV

substation. W22
Ans:
Safety practices to be followed during breakdown maintenance of 132kV substation:
1. Properly isolate the equipment under breakdown.
2. Do the local earthing.
3. No unsafe operation should be permitted.
4. Interlocks should not be bye-passed unless it is very essential.
5. Operation & maintenance staff should be authorized to work on the breakdown equipment.
6. Operation & maintenance staff should be familiar with the station layout and operating limits of
different equipment such as breakers, transformers, Isolators, CTs, PTs, etc. A person should be
allowed to operate or take over the equipment only after he has acquired adequate knowledge of the
equipment.
7. Operations and maintenance manuals/manufacturers instructions for different equipment should be
available and should be referred to before carrying breakdown maintenance.
8. It is the responsibility of the supervisor to interpret correctly and explain safety rules and
regulations to all the persons concerned and ensures that they thoroughly understand the same.
9. Supervisor shall guard against the use of defective safety appliances, tools and materials.
10. In case of any emergency, in which quick action is necessary in order to safeguard life or
property, persons are authorized to take necessary action, which is thoroughly understood by them.
Under no circumstances attempt shall be made to carry out operations which are not safe.
11. All persons must use the standard protective equipment intended for the job.
12. All protective equipment should be periodically tested.
13. Metal ladders should not be used in switchyard.
14. Adequate number of first aid and fire-fighting equipment shall be maintained.
15. Every person should be familiar with fire-fighting and should know how to operate fire-fighting
equipment, so that fire can be extinguished promptly minimizing damage.
16. In the event of fire on electrical installation, the affected part shall be immediately switched-off
and isolated from all the sources.
17. First aid and artificial respiration chart should be exhibited.
18. While working on the breaker, its operating mechanism should be de-energized such as
discharging spring, release air pressure etc.
19. Transformer should be discharged and grounded from all sides (windings). Neutral grounding of
the transformer should not be treated as grounding.
20. Current transformer secondaries should never be left open circuited.
21. After disconnection of capacitor bank, it should be allowed to discharge through discharge PT for
about 5 minutes. The bank should be grounded with hot stick before commencing the work.
22. Apparatus, frame work and other non-current carrying metal parts associated with power system
are to be effectively grounded.

Q18) Illustrate standard procedure to be carried out of Break Down Voltage (BDV) test on
power transformer oil. S22
Answer:
Standard procedure to be carried out of Break Down Voltage (BDV) test on power transformer oil is
as given below-
1. Collect the 300-400 ml oil sample of transformer oil from the bottom valve of the transformer in a
glass or plastic vessel of the BDV testing kit.
2. Pour the oil sample in oil pot of the machine.
3. Keep the oil pot in open air for 5 Minutes so that air bubble if any gets out. Further, the vessel
should be disturbed to get out of the air bubbles and for homogeneous distribution of impurities.
4. Now put the oil sample in the machine.
5. Increase the KV at the rate of 2 KV/Sec and observe the oil sample through a glass window.
6. Continuously observe the oil sample through an inspection window while increasing the voltage.
7. Note down the KV at which sparking is observed in the oil. The voltage at which sparking
observed is the breakdown voltage of oil.
8. Repeat the step No. 4, 5, 6 and 7 for the same sample for six times and note down the breakdown
KV in each case.
9. Take the average of BDV values observed in step 7 and 8.
10. If Breakdown value of oil is above 30 KV, transformer oil is good.