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The document provides important instructions for examiners assessing candidates' answers in the Summer 2023 examination for Electrical Substation Practices. It emphasizes evaluating understanding over exact wording, allowing for variations in answers, and considering responses in both English and Marathi. Additionally, it outlines specific topics and questions related to electrical substations, including safety rules, equipment types, and maintenance procedures.
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0% found this document useful (0 votes)
26 views25 pagesSummer 2023
The document provides important instructions for examiners assessing candidates' answers in the Summer 2023 examination for Electrical Substation Practices. It emphasizes evaluating understanding over exact wording, allowing for variations in answers, and considering responses in both English and Marathi. Additionally, it outlines specific topics and questions related to electrical substations, including safety rules, equipment types, and maintenance procedures.
Uploaded by
rohitkale6806Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF or read online on Scribd
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISO/IEC -27001 - 2013 Certified)
Important Instructions to examiners:
SUMMER — 2023 EXAMINATION
Subject Name: Electrical Substation Practices 22633: ES)
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) Incase 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
conce,
8) As a the policy decision of Maharashtra State Government, teaching in English/Marathi and
Biingual (English + Marathi) medium is introduced at frst year of AICTE diploma Programme from
academic year 2021-2022. Hence if the students 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.
Sub]
Q}a Answer Marking
No N. Scheme
1. |__| Attempt any FIVE of the following: 10 Marks
T. | a) | State the value of earth resistance as per IE rule for:
1. 11 kV substation (small)
2. Power station (32, 220kV)
Ans:
Values of earth resistance as per IE rules:
1. 11 kV substation (small): 2 ohm 1 Mark each
2. Power station (32,220kV): 0.5 ohm = 2 Marks
T. |p | State the need of 33/11kV substation.
‘Ans:
Need of 33/11kV substation: 1 Mark for
1, 33/11 KV substation is used to step down the 33KV voltage level to 11 kV. each of any
2. It is used to distribute power from one substation to another. two needs
3. It is used to isolate lines in case of faults and maintenance = 2 Marks
4. These substations act as a point of voltage regulation
T. | ¢) | List any two safety rules followed during breakdown maintenance on LIkV
substation.
‘Ans:
Safety rules to be followed during breakdown maintenance on L1kV substation:
1. Ensure complete isolation, discharging and effective grounding of the concemed 1 Mark for
circuit/equipment before starting work. each of any
2. Use rubber gloves/gantlets, while operating switches‘links and during discharging | two safety
and grounding, ules
3. Obtain proper permit to work before commencing work on any line/equipment. = 2 Marks
4, Return the “Permit to work” (by the same person who takes it) after ensuring that
all equipment are free from men, material and ground,
Page 1of 24 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISO/IEC -27001 - 2013 Certified)
SUMMER — 2023 EXAMINATION
Model Answer ~ Only for the Use of RAC Assessors
Subject Name: Electrical Substation Practices
22633: ESP
5. Al the activities must be supervised by a qualified and responsible person while
carrying out works.
6. Use of “Safe for work” boards on equipment d
Locking and hanging “men-on-line” boards on
declared safe for work.
8. Working area should be marked with Red ribbon/rope. While working on elevated
positions, safety belts and ropes should be used. Use of proper PPEs should be
lared safe for work
ches controlling circuits
done
T.| d) State any one type of battery trouble, iis cause, maintenance, remedy to be taken,
Ans:
Sr.] Type of Battery Cause Maintenance/ remedy to be
No. Trouble taken
1. [Excessive gassing and Betting of floating [Reduce the floating voltage to a
rogressive increase in Woltage high lower value and check the
Fei gravity during accuracy of the volimeter in the
loat charging charger if nevessary
2. [Battery voltage falling fi) Loose connections i) Tighten the loose
100 rapidly on ii) Corroded onnections.
ischarge jerminals ii) Corroded parts should be
bleaned with hot water and
poated with jelly
3, (Continuous Towering of fi) Leakage of i) Replace container
lectrolyte level. lectrolyte level immediately in case of leakage.
ii) Loss of water in _{ii) Adjust the floating voltage
1 electrolyte by too
igh floating voltage
r excessive charging.
4, (Continued low reading [internal short circuit. [Examine the plates for accidental
f specific gravity or pontacts or missing or sheared
Jow open circuit
oltage in any cell.
5 Ee flowing in cell [Damp atmosphere in [improve ventilation in the battery
lue to increase in attery room causing oom,
lectrolyte level even _kondensation in the
vithout the addition of fell covers of spray
Klistiled water. estors.
6. [High voltage pen circuit in the |i. Check the Voltage at control
censing circuit leads. | _ board terminals.
i. Check pulse generator cireuit
for internal shoring.
7. [Cow outpu Mickering [input to the main |i. Check SCRs if SCRs are
oltage (hunting) ctifier low or in only | healthy, check the fuses.
ne or two main SCRs fi. If low output is noticed with
conducting or if | increase in load then check’
yy of the rectifier fuse] the current limit circuit. The
is blown or filter bank | setting of potentiometer may
s out of order. have been disturbed and this,
2 Marks for
any one
trouble and
its cause,
remedy Page 2of 24 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISO/IEC -27001 - 2013 Certified)
SUMMER - 2023 EXAMINATION
Subject Name: Electrical Substation Practices 22633: ESP
have to be re-adjusted. Also
check the filter bank.
Te) | State the type of LA used for 132 KVB3KV sub-station, Give iis rating.
Ans:
Types of Lightning arrestors used for 132 kV/33 kV sub-station:
1. Electrolytic arrestor | Mark for
2. Valve type Lightning arrestor any two
3. Thyrite type Lightning arrestor types
4. Oxide film Lightning arrestor +
5. Metal Oxide Lightning arrestor | Mark for
any two.
Rating of LA used for 132 kV/33kV: ratings
1. Voltage Rating of LA = Line to line voltage x 1.1 x coefficient of earthing = 2Marks
132x 11x08
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
5x 116.16
= 17424kV
3) Rated Discharge current = 5-10 kA
1} | Name the material used in a fire-fighting equipment for GIS.
Ans:
Material used in a fire-fighting equipment for GIS: 2. Marks for
1. Dry powder: Ammonium phosphate, sodium bicarbonate, potassium bicarbonate, or | any one
a combination of these chemicals, graphite or sodium chloride-based formulations. } material
2. Carbon dioxide (C02)
3. Water
4, Inert gases: Nitrogen, Argon
T]g) | List any two properties of the SF6 gas usedin GIS.
Ans:
Properties of SF6 gas used in GIS: 1 Mark for
5. Itis extremely chemically stable each of any
2. [tis non-flammable. two
3. Itis highly electronegative properties
4. Ithas excellent dielectric property of approximately 2.5 times more than air, five | =2Marks
times the density of atmospheric air at mean sea level.
2|___| Attempt any THREE of the following: 12 Marks
a) | Explain any four factors to decide the selection of site for the sub-station.
Ans:
1) The site should be as near the load center as possible
2) The site should be far away from obstructions, to permit easy and safe | Mark for
approach/termination of high voltage overhead transmission lines. each of any
3) The Site should have sufficient land area. four factors
4) The site should have Necessary clearances for electrical safety. = 4 Marks
Page 3of 24 SER = EXAMINATION
Subject Name: Electrical Substation Practices
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISOMEC - 27001 - 2013 Certified)
Model Answer - Only for the Use of RAC Assessors
5) Access to maintain large apparatus such as transformers.
6) The site must have room for expansion due to load growth or planned transmission
additions,
7) Environmental effects like drainage, noise and road traffic effects should be taken
into account.
8) Grounding must be taken into account to protect passers-by during a short circuit in
the transmission system.
9) ‘The substation site must be reasonably central to the distribution area to be served
b) | Draw a neat sketch of I1kV/400V, 250 kVA pole mounted sub-station. State
function of each part.
‘Ans:
2 Mark
for
diagra
n
Marks
for
functio
nof
any
four
parts
= 4 Marks
Functions of different parts in pole mounted substation:
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.
©) | Describe the procedure to locate and record the hot spots in a 132 kV/33 kV sub-
station.
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
Page 4 of 24 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISO/IEC -27001 - 2013 Certified)
SUMMER - 2023 EXAMINATION
Subject Name: Electrical Substation Practices 22633: ES)
and daily monitoring, So during preventive maintenance activites, 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 | 2 Marks for
is important 10 monitor “hot spots” in asubstation regularly to prevent such | description
breakdowns or accidents. of location
2. Itis 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 | 2 Marks for
to cover the complete substation, multiple cameras are fixed at different suitable description
locations. of recording
3. At some substations, a handheld portable infrared camera is used. This portable of hotspots
camera is moved around, and the complete substation is scanned manually for | =4 Marks
thermal anomalies
4. Using a thermal camera, which converts invisible infrared radiation into clear images
fiom which temperatures can be read, itis 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.
4) | Compare equipment earthing and system earthing on any four points.
Ans
Sr “ Equipment Earthing System Earthing
1. [Equipment earthing means connecting Bystem earthing means connecting a 1 Mark for
fiead part (the one which does not carry kurrent carrying part/ neutral of the system || each of any
plectric current under normal condition) fo earth. four points
0 earth
2. Equipment earthing is done fo protect _ System earthing is done to protect the
human being fromelectric shock. __ ower system from malfunctioning.
3. fn normal condition the earthing wire is [in normal condition neutral wire will have
t zero potential, it does not carry any ome potential. Itis always charged
eurrent, but in case of failure of
insulation, it carries the current to
round.
4. Following are the methods of Following are the methods of