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22633

Electrical engineering (Rashtrasant Tukadoji Maharaj Nagpur University)

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
Important Instructions to examiners:
1) The answers should be examined by key words and not as word-to-word as given in the model answer scheme.
2) The model answer and the answer written by candidate may vary but the examiner may try to assess the understanding level of
the candidate.
3) The language errors such as grammatical, spelling errors should not be given more Importance (Not applicable for subject
English and Communication Skills).
4) While assessing figures, examiner may give credit for principal components indicated in the figure. The figures drawn by
candidate and model answer may vary. The examiner may give credit for any equivalent figure drawn.
5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values may vary and there may be
some difference in the candidate’s answers and model answer.
6) In case of some questions credit may be given by judgement on part of examiner of relevant answer based on candidate’s
understanding.
7) For programming language papers, credit may be given to any other program based on equivalent concept.
8) As per the policy decision of Maharashtra State Government, teaching in English/Marathi and Bilingual (English + Marathi)
medium is introduced at first year of AICTE Diploma Programme from academic year 2021-2022. Hence if the students in first
year (first and second semesters) write answers in Marathi or bilingual language (English +Marathi), the Examiner shall
consider the same and assess the answer based on matching of concepts with model answer.
Q. Sub Marking
Answer
No. Q.N. Scheme
1. Attempt any FIVE: 10 Marks
a) Compare indoor & outdoor substation on the basis of the following points:
(i) Installation cost
(ii) Lightning protection
Ans:
Sr.
Particular Indoor Substation Outdoor Substation
No. 1 Mark each
Installation = 2 Marks
i. More Less
cost
Not required as all the Required as all the
Lightning
ii. equipment are installed inside a equipment are installed
protection
close building outdoor
b) State the functions of three phase distribution transformer.
Ans:
Functions of three phase distribution transformer: 1 Mark for
each of any
 It is a step down transformer.
two
 It transmits power to the secondary distribution circuit
= 2 Marks
 It reduces the voltage of the primary distribution circuit to a voltage level as
per the consumer requirement.
c) Differentiate between 11 kV & 33 kV substation on the basis of
(i) Location
(ii) Application
Ans:
Sr. 1 Mark each
Particular 11 kV Substation 33 kV Substation
No. = 2 Marks
i. Location Located at the end of primary Located at the end of
distribution system and the secondary transmission
start of secondary distribution system and the start of
system primary distribution system

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
ii. Applications i) To receive power from i) To receive power from
primary distribution system secondary transmission
and supply to consumer use. system and supply to
ii) To supply power to primary distribution system
commercial complex and for ii) To supply power to 11
residential use. kV feeder for commercial or
industrial use.
d) State the meaning of System Earthing.
Ans:
System Earthing:
(i) In System earthing, the neutral of the system is directly connected to earth with 2 Marks for
the help of the low resistance GI wire. The system earthing is also called the correct
Neutral earthing. answer
(ii) Such type of earthing is mostly provided to the system which has star winding.
For example, the neutral earthing is provided in the generator, transformer,
motor etc.
e) Justify the use of Earth Mat in 132 kV substation.
Ans:
Use of Earth Mat in 132 kV substation:
(i) Primary requirement of earthing is to have a low earth resistance. Substation
2 Marks for
involves many earthings through individual electrodes, which will have fairly
correct
high resistance.
answer
(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.
f) List any two advantages of GIS substation.
Ans:
Advantages of GIS substation:
(i) Space required is less i.e. 1/10th of conventional substation 1 Mark for
(ii) GIS are very safe & operating personnel are protected by the earthed metal each of any
enclosure. two
(iii) Most reliable compared to air insulated substation. advantages
(iv) Weight is very less due to aluminium enclosure used. = 2 Marks
(v) No pollution.
(vi) Dielectric strength of SF6 Gas is 3 times the dielectric strength of air.
(vii) Time of erection is less as assembled parts are used.
g) State the reason behind higher cost of GIS substation compared to ordinary
conventional substation.
Ans:
1 Mark for
Reason behind higher cost of GIS substation:
each of any
(i) In the GIS system, all the live components are enclosed in a grounded metal
two reasons
enclosure, and then the whole system is housed in a chamber full of gas, which
= 2 Marks
results in the increase in cost.
(ii) Whole substation is assembled in closed constructed building, which increases
the cost.
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
(iii) Care should be taken that no dust particles enter into the live compartments
which results in flashovers. So sealed compartments are required, which results
in the increase in cost.
(iv) SF6 gas pressure must be monitored in each compartment, reduction in the
pressure of the SF6 gas in any module results in flash overs. The monitoring
system results in the increase in cost.
(Or Any other equivalent answer)
2. Attempt any THREE: 12 Marks
a) Describe general safety rules to be followed to minimize the risk of electrical
hazards in substation.
Ans:
General safety rules to be followed to minimize the risk of electrical hazards in
substation:
i) Permits to be issued / cancelled while carrying out any electrical work.
ii) Use of PPEs, danger boards while working on line.
iii) SOP, SMP to be followed while carrying out maintenance activities.
iv) Understand the safety threats that abound in the work environment.
v) Know how each equipment functions, what parts are energized and what
1 Mark for
voltages, clearance requirements, risk involved and levels of protection are
each of any
required to work on the equipment.
four rules
vi) Be cautious even when around de-energized equipment.
= 4 Marks
vii) Conduct routine inspections to ensure that the area and equipment are safe for
use prior to each task.
viii) Maintain the prescribed clearance from each type of equipment, and handle
operations remotely if the option is available.
ix) Securing Battery Stores, Securing Perimeter Area, Restricting or Prohibiting
Visitor Access to such areas.
x) Enough Clearance from energized parts should be provided to avoid accidental
contact with them. If that can’t be met, live parts should be guarded or
enclosed.
(Or Any other equivalent answer)
b) Justify the use of CT & PT in 11 kV substation.
Ans:
Use of Current Transformer (CT) in 11 kV substation:
Metering and measurement:
It steps down the high line current to low value so that currents in HV circuit 2 Marks
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 2 Marks
help of standard low range voltmeter.
c) List out accessories used in 33 kV substation and state function of each.
Ans:
Accessories used in 33 kV substation:
i) Power transformers: These are essential for stepping down the secondary
transmission level voltage to primary distribution level voltage and distribute
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
energy / power among feeders.
ii) Bus-bars: A bus-bar in electrical power distribution refers to busbar formed
by using line conductors or thick strips of copper or aluminium in control
panel within the substation.
iii) Circuit breakers (33 kV and 11 kV): It is a switching device used to
interrupt / make not only the load current but also the much larger fault 1 Mark each
current, which may occur on a circuit. for any four
iv) Isolators or isolating switches (33 kV and 11 kV): These are mechanical accessories
switching devices capable of opening or closing a circuit: with their
1. When a negligible current is to be interrupted / made or function
2. Only a small charging current is to be interrupted or = 4 Marks
3. When no significant voltage difference exists across the terminals of each
pole.
v) Earthing switches: Earthing switches are provided at various locations to
facilitate maintenance. Main blades and earth blades are interlocked with both
electrical and mechanical means. The earthing switch must be capable of
withstanding short circuit current for short duration as applicable to the
isolator.
vi) Insulators: It provides adequate insulation for live parts, which is of prime
importance for obvious reasons of reliability of supply, safety of personnel
and equipment.
vii) Power and control cables: Power and control cables of adequate current
carrying capacity and voltage rating are provided at the substation. Power
cables are used for 33kV,11 kV or LT system to carry load current. The
control cables are required for operating and protection system connections.
viii) Control panel: Control panels installed within the building which provide
mounting for mimic bus, relays, meters, indicating instruments, indicating
lights, control switches, test switches and other control devices. The panel
contains compartments for incoming lines, outgoing lines, busbars with
provision for sectionalizing, relays, measuring instruments, etc.
ix) Lightning protection: Surge arrestors - Large over voltages that develop
suddenly on electric transmission and distribution system are referred to as
“surges” or “transients”. These are caused by lightning strikes or by circuit
switching operations. Surge arrestor is a protective device for limiting surge
voltages on equipment by discharging or bypassing surge current. The surge
arrestor which responds to over-voltages without any time delay is installed
for protection of 33 kV switchgear, transformers, associated equipment and
11 kV and 33 kV lines.
x) Instrument transformers (CTs and PTs): The substations have current and
voltage transformers designed to isolate electrically the high voltage primary
circuit from the low voltage measurement and protection circuit and thus,
provide a safe means of supply for indicating instruments, meters and relays.
xi) Earthing arrangements: Earthing must be provided for -
− safety of personnel,
− prevention of and minimizing damage to equipment because of flow of
heavy fault currents, and

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
− improved reliability of power supply.
xii) Reactive compensation: It is always a good idea to ensure a power factor
correction for transformers. When they are operating on low load (e.g. during
the night) they absorb reactive power, which must be compensated to avoid
unnecessary loadings and losses.
xiii) DC supply arrangement: Station batteries supply energy to operate
protection equipment such as breakers and other control, alarm and indicating
equipment. The station batteries are a source for operating DC control system
equipment during system disturbances and outages. During normal conditions
the rectifier provides the required DC supply. However, to take care of
rectifier failure, a storage battery of adequate capacity is provided to meet the
DC requirements.
xiv) Auxiliary supply transformer: An Auxiliary Supply Transformer of
adequate capacity is required to be provided for internal use for lighting
loads, battery charging, oil filtration plant, etc. The supply should be reliable.
In a substation it is normally provided from a station transformer connected
on 33 or 11 kV bus bar.
xv) Fire-fighting system: In view of the presence of oil filled equipment in a
substation, it is important that proper attention is given to isolation; limitation
and extinguishing of fire so as to avoid damage to costly equipment and
reduce chances of serious dislocation of power supply as well as ensure safety
of personnel.
d) Describe procedure to undertake safe preventive maintenance in 132 kV substation.
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 ½ Mark for
line/equipment. If the operator has to work himself, he should avail self-line each of any
clear recording all operations and then only work. Self-line clears should be eight
limited to emergencies only. procedure
iv) Return of the “Permit to work” (by the same person who takes it) after ensuring points
that all equipment are free from men, material and ground. =4 Marks
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
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
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.
3 Attempt any THREE: 12 Marks
a) Justify the following factors governing the selection of the site of the electrical
substation: (i) Type of substation (ii) Atmospheric pollution.
Ans:
Factors governing the selection of the site of the electrical substation:
(i) Type of Substation:
The type of substation is important for its location. For example a step-up substation,
which is point where power from various sources (generators or generating stations) 2 Marks
is pooled and stepped up for long distance transmission, should be located as close to
the generating stations as possible to minimize the transmission losses.
Similarly a step-down substation should be located nearer to the load centre to
reduce transmission losses, cost of distribution system and better reliability of
supply.
(ii) Atmospheric Pollution:
Atmosphere around factories, which may produce metal corroding gases, air fumes, 2 Marks
conductive dust etc., and nearer to sea coasts, where air may be more humid and may
be salt laden, is detrimental to the proper running of power system and therefore
substations should not be located near factories or sea coast.
b) State any four advantages of routine maintenance of 11 kV substation.
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. 1 Mark for
2. Lowering the risk of breakdowns: each of any
Proper maintenance helps in lowering down the breakdown time. Breakdown affects four
the reliability adversely. advantages
3. Increased efficiency: = 4 Marks
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:
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
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.

c) Describe the procedure to perform voltage breakdown test on transformer oil.

Ans:
Voltage breakdown test on transformer oil:

1 Mark for
diagram

Procedure: -
 The sample of oil is taken from near the top & bottom of the transformer.
 In this kit, there are two electrodes separated by small gap of 2.5 mm or 4mm 3 Marks for
between them. procedure
 The cup (test cell) is filled with sample of oil to be tested up to about 1 cm above
the electrodes.
 The cup top is covered with clean glass plate.
 Now slowly raise the voltage between the electrodes till breakdown of oil occurs
or sparking starts between the electrodes. And note down voltage reading.
 Repeat the procedure by taking same sample of oil 3 to 6 times & Calculate
average of all results to finalize the breakdown voltage of oil sample.

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
d) State the meaning and functions of PLCC used in 132kV / 33 kV substation.
Ans:
PLCC:
PLCC means Power Line Carrier Communication. It refers to a technique in which 1 Mark for
power transmission line is used as communication line for transferring meaning
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 3 Marks for
electrical substations through power lines at high voltages, such as 110 kV, 220 any 3
kV, 400 kV. functions
2) Telephony: PLCC can be used for interconnecting private branch exchanges (Diagram is
(PBXs) for voice messaging. optional)
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.
4 Attempt any THREE: 12 Marks
a) Describe any four important safety practices followed during breakdown
maintenance in 11 kV substation.
Ans:
Important safety practices followed during breakdown maintenance in 11 kV
substation:
1) Complete isolation, discharging and effective grounding of the concerned 1 Mark for
circuit/equipment carried out before starting maintenance work. each of any
2) Use of rubber gloves/gantlets, while operating switches/links and during four
discharging and grounding. practices
3) Obtaining proper permit to work before commencing work on any = 4 Marks
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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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
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

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
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.
b) Determine the rating of LA, CT, PT and DO fuse for mounting of 500 kVA, 11
kV/0.4 kV plinth mounted substation.
Ans:
Rating of LA:
Voltage rating = 1.1× 11×0.8 = 9.68KV
Power frequency spark over voltage = 1.5×9.68 = 14.52KV
Nominal discharge current = 5kA
Rated voltage: 12 kV
Continuous Operated voltage: 10kV
Long duration discharge class 2
High current impulse withstand value 100KA
Energy absorption capacity: 6KJ/KV (Fpr 3 sequential shot)
Rating of CT:
Normal system voltage (KV rms): 11 KV
Highest system voltage (KV rms): 12 KV
Frequency: 50Hz
Impulse withstand voltage: 75 KV peak
Insulation level: 12 Kv
1 Mark for
One minute power frequency dry withstand voltage:
minimum
a) primary (KV r.m.s.): 28 KV
two ratings
b) secondary (KV r.m.s.): 03 KV
of each
Transformation ratio (CT Ratio):
device
a) For Consumer Metering : 100/5 A, 50/5 A & 15/5 A
= 4 Marks
b) For Feeder Metering: 200/5 A
Rated output (VA burden): 10 VA
Class of accuracy: 0.5 S
Short time thermal current rating: 6.4 KA for one second for CT ratio 5/5 A & 15/5
A, 18 KA for one second for CT Ratio 50/5 A & 100/5 A 18 KA for 3 seconds for
CT ratio 200/5 A
Rating of PT:
Normal system voltage (KV rms): 11 KV
Highest system voltage (KV rms): 12 KV
Nos. of phases: Three
Impulse withstand voltage (KVP): 95
One minute power frequency dry withstand voltage (on assembled CT-PT set)
i) primary (KV r.m.s.): 28
ii) secondary (KV r.m.s.):03
Frequency: 50Hz
Transformation ratio (PT Ratio): 11 KV/ 110V
Rated output (VA burden): 30 VA per phase
Class of accuracy: 0.5 S
Temp. rise over max. ambient temp: Within limits of IS:3156/1992
Ratio error (Max.): Within limits of IS:3156/1992
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
Rating of DO fuse:
Type: Expulsion type drop out, outdoor type
Mounting: Vertical
Standard specification: IS 9385/1983
System voltage: 12/24 KV, 36KV
Rated current: 100/200 A
Insulation level: As per IS standard
Fuse Carries: SRBP/ Fibre tube
Operation: In service the fuse carrier is latched in
OR Any other Equivalent
c) Write the name of relevant fire-fighting equipment used in case of power
transformer fire and justify its use.
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 Any one to
post fire clean up issues or contaminated runoff. be
2. Fixed Water Spray System: This system is similar to a deluge system; however, considered
the water discharge points are designed to create a spray pattern unique to the
particular area or equipment being protected. The water spray head locations and 1 Mark for
spray pattern are tailored to the asymmetrical shape of the equipment being name
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. 3 Marks for
Electrical transformers containing oil are commonly protected by fixed water spray justification
systems. = 4 Marks
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.
(Or Any other equivalent)
d) List any four important safety practices followed during routine maintenance of GIS
substation.
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
Ans:
Safety practices followed during routine maintenance of GIS substation:
1) The installation and commissioning should be carried out by qualified and
authorized personnel. 1 Mark for
2) Define and discuss in advance the maintenance to be performed and the relative each of any
hazards. four
3) Use parts only supplied by Original Equipment Manufacturer (OEM). practices
4) Treat everything or each equipment is live = 4 Marks
5) It is essential to make sure that the equipment is to be earthed on all sides of
work zones.
6) Check that the personnel operating the apparatus have instruction manual with
them.
7) Dis-connector switch can be operated only when VCB is off.
8) At a time, Dis-connector switch can be operated either electrically or
mechanically but not simultaneously.
9) Use of overalls, jumpers and coats having metal buttons, metal strap and similar
metal fittings should be avoided.
10) Do not wear suspenders and arm bands with metal buckles or other metal parts.
These might come in close proximity to live parts and cause serious, if not fatal,
injury.
11) Think carefully before you act:
i) Make sure you are right, watch out for the other man and make sure he is
right.
ii) Never speak to any person working upon live mains or apparatus, doing the
work is aware of your presence.
12) Dangerous Areas: When working in areas which contain or may contain live
mains and danger notice plates, barriers, rails or other guarding arrangement for
the working area. Do not store materials within high voltage enclosures or low
voltage areas.
13) The internal parts need cleaning during periodic maintenance under a clean and
dry environment.
14) Working in Damp Situations: Extra precautions should be taken when working
in abnormally damp area.
(OR Any other Equivalent)
e) Elaborate the causes of fire in GIS substation.
Ans:
Causes of fire in GIS substation:
1) Particle or moisture contamination inside the compartment causes flash overs.
2) A leak in gas-insulated equipment means there is less gas to protect it from the 1 Mark for
effects of electrical arcs. The results are lost signals, short circuits, malfunctions, each of any
and, ultimately equipment failure that compromise safety and operations. four causes
3) The SF6 can be easily exploded into different decomposition gas products when = 4 Marks
subjected to electrical discharge, such as electric arc, spark.
4) Negligence of maintenance causes minor faults to develop and causing fire.
5) Short circuit and too much overloading, which causes heating and ultimately fire.
6) Protective devices not in good working condition, fail to detect fault, causing fire.
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
7) Failure of protective relaying circuit causes faults to persist and ultimately fire.
8) Unskilled persons employed for sensitive circuits cannot maintain it properly and
so faults can be developed ultimately resulting fire.
(OR Any other Equivalent)
5 Attempt any TWO: 12 Marks
a) Describe the procedure to measure the insulation resistance of distribution
transformer using neat labelled diagram.
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:
3 Marks for
7) High to low, tertiary and ground (H-LTG), shown in Figure (a).
procedure
8) Tertiary to high, low and ground (T-HLG).
+
9) Low to high, tertiary and ground (L-HTG), shown in Figure (b).
3 Marks for
10) High, low and tertiary to the ground (HLT-G).
diagram
11) High and tertiary to low and ground (HT-LG), shown in Figure (c).
= 6 Marks
12) Low and tertiary to high and ground (LT-HG).
13) High and low to tertiary and ground (HL-TG).

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
b) Draw neat labelled single line diagram of 33kV substation and describe the
functions of circuit breaker and isolator.
Ans:
Function of Circuit breaker:
1) A circuit breaker is a switching device built ruggedly to enable it to interrupt/
make not only the load current but also the much larger fault current, which may
occur on a circuit. 1½ Mark
2) A circuit breaker contains both fixed contacts and moving contacts. The purpose
of circuit breakers is to eliminate a short-circuit that occurs on a line.
3) Circuit breakers are found at the arrivals and departures of all lines incident on a
substation.
4) When the circuit breaker is closed these contacts are held together. The mode of
action of all circuit breakers consists in the breaking of the fault current by the
separation of the moving contacts away from the fixed ones. An arc is
immediately established on separation of the contacts. Interruption of the current
occurs after the arc at these contacts is extinguished and current becomes zero.
Function of Isolator:
1) Isolators are mechanical switching devices capable of opening or closing a
circuit:
− when a negligible current is broken or made, or 1½ Mark
− only a small charging current is to be interrupted, or
− when no significant voltage difference exists across the terminals of each pole.
2) Isolators can carry current under normal conditions and short circuit currents for a
specified time. In open position, the isolator should provide an isolating distance
between the terminals.
Single line diagram of 33kV substation:

3 Marks for
labeled
diagram

(OR Any other equivalent single line diagram)

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
c) Describe the causes of hot spot formation in transformer and state the methods of
identification.
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 1 Mark for
losses (hysteresis and eddy losses) inside the CRGO lamination stack. Losses will each of any
not be uniform in the core and the joints between limbs and yokes will have more three causes
losses (10-20 %) than the average unit weight loss (w/kg). Hence the temperature = 3 Marks
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 1 Mark for
temperature in the transformer tank and the temperature of the top oil, the each of any
temperature rise of the top oil in the tank and the ambient temperature, the sum is three
equal to the hot spot temperature. methods
2) Other method is using fibre optical temperature sensors positioned at the = 3 Marks
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.

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
6 Attempt any TWO: 12 Marks
a) Illustrate the procedure to carry out preventive maintenance of 33kV/11kV air break
circuit breaker.
Ans:
Procedure to carry out Preventive Maintenance of 33kV/11kV Air Break 1 Mark for
Circuit Breaker: each of any
1. Isolate properly the air break circuit breaker or take it from the circuit away.
six steps
2. Use a vacuum cleaner for thorough de-dusting (dry cloth can be used also). = 6 Marks
3. Insulation: Remove and clean interphase barriers. Clean all insulating materials
with vacuum and/or using cleaning agent.
4. Check control terminal blocks / wires for any damage or sparking or looseness.
Use WD-40 cleaning agent for cleaning purpose.
5. Contacts: Ensure that all contacts are clean, smooth, and in proper alignment.
6. On silver contacts, discoloration is not usually harmful unless caused by
insulating deposits. Clean silver contacts with alcohol or silver cleaner using non-
abrasive cloths.
7. If possible, a contact resistance test should be performed to determine the quality
of the contacts. Measure contact resistance of power terminals (under test mode).
8. Draw out contacts on the circuit breaker and the stationary contacts in the
cubicle should be cleaned and inspected for overheating, alignment, and broken
or weak springs. Coat contact surfaces with contact lubricant to ease mating.
9. Arc Interrupter: Clean all ceramic materials of loose dirt and examine for signs
of moisture, make sure the assemblies are clean and dry. Examine for cracked or
broken pieces. Dirt and arcing deposits may be removed by light sanding, do not
use emery cloth or wire brushes which may leave conductive residue behind.
Repair or replace as necessary. Examine arc chutes for dirt and/or dust
accumulations and clean as necessary
10. Check air puffer for proper operation.
11. Operating Mechanism: Inspect for loose, broken, worn or missing parts (consult
manufacturer’s schematics for required parts). Examine for excessive wear of
moving parts. Observe that operating mechanisms function properly without
binding, hanging, or without delayed action.
12. Ensure that spring pressures are maintained according to manufacturer’s
specifications.
13. Manually close breaker to check for proper wipe, contact pressure, contact
alignment, and to ensure that all contacts make at approximately the same time.
14. Auxiliary Devices: Inspect operating devices for proper operation and general
condition. Ensure all indicating devices are fully functional and properly set.
15. Measure breaker closing time and opening time (it must match the original
value) during preventive maintenance of breakers.
16. Check-up of all mechanical components.
b) Describe “Safety practices” followed during breakdown maintenance of 132kV
substation.
Ans:
Safety practices to be followed during breakdown maintenance of 132kV
substation:
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
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 1 Mark for
operating limits of different equipment such as breakers, transformers, Isolators, each of any
CTs, PTs, etc. A person should be allowed to operate or take over the equipment 6 practices
only after he has acquired adequate knowledge of the equipment. = 6 Marks
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.
c) Describe the importance of partial discharge monitoring system for the GIS
substation.
Ans:
Importance of partial discharge monitoring system for the GIS substation:
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)
________________________________________________________________________________________________________________________________
WINTER – 2022 EXAMINATION
Model Answer
Subject Name: Electrical Substation Practices 22633: ESP
1) Finding a problem quickly: The monitor will identify a problem in its earliest
stages of defect development. This will provide one with sufficient information
as to the growth rate and the severity of the defect.
2) Providing information as to which phase the defect is in and generally
what type of defect: Defects such as Corona Surface Discharge, Void type of 1 Mark for
defect (Insulation Delamination), Conductive tape deterioration (Slot each of any
Discharge), loose high voltage connection that is arcing, are informed. It is six points
even possible to localize even further the location of the defect. = 6 Marks
3) Since no labor is required to perform the tests, continuous monitoring allows
the use of limited resources for finding solutions to problems instead of finding
problems.
4) Reducing unnecessary maintenance because the monitor will be constantly
testing and will have accurate data on which to base decisions.
5) Collecting more accurate data as tests are conducted under real operating
condition.
6) Requiring no outage to perform the test, therefore there is no loss of asset
productivity.
7) No introduction of infant mortality failure patterns via more invasive testing
procedures.
8) Reduction of forced outages and increased safety of personnel. One will always
be aware of conditions and/or problems.
9) Correlation of other dynamics such as temperature, humidity and load current
to PD activity, which provides additional insight for diagnostics. There will be
no need to go to several sources and gather the information.
10) Provides the opportunity for remote diagnostics. The expert does not need to
come out to the field for basic diagnostics. A site visit by an expert will be the
exception and not the rule. This can be done my emailing data to an expert or
perhaps have a modem connected to the monitor so the expert can dial-in and
upload the information for analysis and even provide a special test if necessary.
11) Evaluation of a piece of equipment is based on its own history and not by
comparison to other equipment. This will make the detection of subtle
problems easier.
12) Easily monitor worsening conditions so one can defer repairs and allow time to
plan an outage.

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