TRANSFORMERS
• Give transformer nameplate details of GT, SUT, UAT, SET, 415 V Aux transformer
and Lighting transformer.
GT SUT UAT SET 415V Aux trans. L Trans.
USI 5210 5120 5220 4120 5242 5231
Make Telk Telk BHEL BHEL EE Square
Automation
STD IS – 2026 IS– 2026
Type WFOC Oil immersed Oil immersed DRY RESIN
CAST DRY RESIN CAST DRY RESIN CAST
Cooling OFWF ONAF / ONAN ONAF / ONAN AN AN AN
VA 260/260 MVA 35/20/20/12 MVA
24.5/14/14 MVA
35/20/20 MVA
24.5/14/14 MVA
3150 kVA 1800/1200 kVA 250 kVA
Volts 235/16.5 kV 220/6.9/6.9/11
kV
16.5/6.9/6.9
Kv
16500/575
V 6600/435 V 415/415V
Amps 639/9098 A 64/1172/440 A
91/1675/629 A
1266/858A
1676/1172 A 157.5/2400 A 630/250,125A
No of φ 3 3 3 3 3 3
Frequency 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz
Impedance 13.13 (14) % 9.75% / 18.82% 10 ±10% HV
22 ±10% LV
Vector YNd11 Yn yno yno
(d1)
D yn1 yn1 Dyn Dyn11 Dyn11
Oil 42000 Lt. 25260 Lt. 19750 Lt.
Tap change Off load ON load HV ON load HV
Tapchange% 10 steps of 2.5 % ϒ12% in 1.5% steps ϒ12% in 1.5% steps
• What is the use of Tertiary winding?
Star connected circuit, which has an isolated neutral there can be no zero sequence
components. Since the zero sequence components are by definition in time phase
with another their sum can not be zero at the junction point as per kirchoff’s law. It
follows that there are limitations upon the phase loading of a bank of transformers
connected in star – star unless the neutral points are connected to the source of power
in such a manner that the zero sequence components of current have a return path or
unless the transformer are provided with tertiary winding.
• What is E/F current limit for SUT and UT?
400a limited by 10 ohms resistor.
• What is the coverage of differential protection for SUT?
Covers from 230 kV bushing to 6.6 kV breaker end.
• What are the advantages of dry type transformer?
No fire hazard.
It can be mounted indoor.
• During unit operation, can we parallel 2 SUT?
No, due to switchgear limitation.
• Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to
400 A?
a) To reduce burning and melting in faulted switchgear or machine.
b) To reduce mechanical stresses in equipment.
c) To reduce the electrical hazards by stray ground fault currents in the ground
return path.
d) To reduce momentary line voltage dip due to ground fault.
e) The current is limited to 400a, that is ¼ th of the load current to reduce the size of
the screen in 6.6 kV XLPE (cross-linked polyethylene). Therefore the cost of the
cable decreases.
• During unit operation can we have one UT feeding both unit 6.6 kV loads?
No, logically prevented.
• During unit operation, can we parallel UT & SUT continuously?
No, due to switchgear limitation.
• What is the design basis of 6.6 kV aluminium bus bars?
a) Temperature rises not exceed 90 ºC.
b) Withstand short ckt stresses.
c) Take care of thermal expansion.
• Why 2 types of earth fault relays in 6.6 kV side of transformers?
I – Trips 6.6 kV breakers only. It gives primary protection for 6.6 kV bus bars.
I1 – Trips the both HT and LT breakers. It acts as a backup to ref and also acts as
backup to bus bar earthfault relay.
• Why core balance CT is preferred over residual connected CT’s to sense earth fault
in 6.6 kV feeders?
a) To avoid relay mal-operation due to CT saturation
b) Better sensitivity is got.
c) High pickup and TMS avoided in IDMT earth fault relay.
• How selection of cooling fluid in GT done?
a) There are 5 factors are there.
b) Density
c) Coefficient of thermal expansion
d) Viscosity
e) Specific heat
f) Thermal conductivity.
• What are the ranges in which each type is effective?
ONAN – Natural cooling – up to 15 MVA.
ONAF – Air forced radiators cooling – 10 to 100 MVA depending on availability of
area.
OFWF = oil forced and water forced used in more than 100 MVA.
• Why off load tap changer was chosen for GT?
Because our plant works on base load always.
• What are the advantages of OFWF?
Ensure the differential temperature between top and bottom of transformer is
minimum and Effect of ambient air temperature is minimum.
• What is the type of lightning arrestor for GT?
Zn O (zinc oxide) types.
• What is the purpose of header breaker in water circuit?
The header breaker ensures oil pressure greater than that of water pressure always.
Therefore there is no leak of water into oil.
• Why thermosyphon filter required?
To keep required dryness/improve dryness of the transformer insulation, internal part
of transformer. When transformer operates, due to pressure head between top and
bottom small quantity of oil flows through filters (absorbent material activated
alumina grade g-80 removes the moisture from oil). Absorbent material remove
slag, acids, peroxides, ionic impurities from oil, which otherwise accelerate
against of oil. Absorbent unit is reactivated at regular intervals.
• What is the purpose of pronol conservator (KAPP)?
Flexible separator avoids direct contact with atmosphere. Efficient barrier between
oil and air. Ensures the protection against water vapour, suppression of gas bubbles
formation in the oil.
• Why main generator/UT is not provided with separate overfluxing protection?
Since GT is provided with overfluxing protection, it is adequate to protect main
generator / UT also. Main generator can withstand higher degree of overfluxing. If a
generator CB is used, separate overfluxing protection is essential for main generator.
• What is the advantage of Pressure relief device in TELK type GT over explosion
vent of BHEL, even though in both cases oil will be expelled out during sudden
pressure rise?
During internal fault, the internal pressure rise is relieved by the expelling out of oil
through Pressure relief device /explosion vent. However the Pressure relief device
closes back when the pressure drops. Hence the oil exposure to atmosphere is
minimised, thus saving large quantity of costly transformer oil from oxidation and
moisture absorption. Fire hazard due to transformer oil does not exist after the
closure of Pressure relief device.
• To reduce tower-footing resistance, which are better to use a) chemical, b) ground
rods, c) counter poise?
B & C
• Why tap changer is kept at neutral end?
a) To reduce insulation cost of tap changer.
b) But reactance changeover the tap range increases.
• Why guard connection is given for megger?
For true measurement of IR value of HV to earth of a transformer, connect line to
HV, earth to transformer tank and guard to LV. Therefore leakage current from HV
to LV is not included.
• Why lighting isolation transformer is req.?
a) 3 wire to 4 wire conversion, since neutral is required for lighting load.
b) Prevents transfer of E/F currents
c) Reduces the fault level on secondary side and permits use of small sized cables /
CB’s / fuses.
• Why neutrals are solid grounded above 33 kV?
a) Less transient over voltage due to arcing grounds.
b) Voltage of phases are limited to phase to ground voltage. (No neutral shifting)
c) Allows graded insulation of transformer (low cost)
d) Fast E/F protection.
• Why SET is chosen as Dyn 11?
To have smooth commutation in generation in between stator and rotor.
• Why all 415V transformers are chosen Dyn 11? What are the protections provided
for the 415V transformers?
a) To facilitate interchange.
b) To have momentary parallel during changeover.
Protections
a) Door interlock to trip HT and LT breakers.
b) LT breaker can on only after HT breaker is in on position.
c) Instantaneous O/C and inverse O/C (50 + 51).
d) Instantaneous E/F (50N).
e) IDMT E/F and restricted E/F (51N + 64).
f) Winding temp high trip (140°C trip and 130°C alarm)
• What is the instrument name used for thermograph?
Infrared camera.
• Why neutrals are solid grounded below 600v?
Human safety
Permits enough E/F current because ground resistance is large in less than 415v,
hence fast fault clearance,
Equipment safety against over voltage.
• What are the advantages of ungrounded system?
Supply is maintained even with fault on one line
Less interference to communication lines because of absence of zero sequence
currents.
• Why resistance grounding preferred for less than 33 kV and more than 415 V?
a) To limit the earth fault current for equipment safety else, high short ckt forces
dislocate in windings/bus bars etc,
b) Over voltage due to arcing ground reduced
c) Permits earth fault protection (not possible in ungrounded system)
• What is meant by tan-delta measurement?
It is the tan of the angle between the capacitive current and the total current.
Ir
Ic Ic - capacitive current
I Ir - resistive current
I - total current
As the value of tan delta increases the resistive component of the current in
increasing. Hence it shows a weak insulation.
• What is the vector group of GT, UT, SUT?
Yd11
Dy1
Yy0
• Why all the transformers are having different vector group?
UT and SUT are getting paralleled at 6.6 kV bus. Hence they should have voltage of
same phase relationship. This is achieved by assigning different vector group to the
transformers.
• What are the built in protections for transformers?
a) Buchholz relay
b) Explosion vent or relief valve
c) Gas operated relay for on load tap changers.
• Why water pressure is kept below the oil pressure? How it is maintained?
Incase of a heat exchanger tube failure the water should not go inside the
transformer. For this purpose the oil pressure is kept above the water pressure.
• What is the requirement oil in a transformer?
Oil is used removal of heat produced in the transformer and also as insulating
medium.
• What is meant by over fluxing of transformer?
When the voltage is increased and the frequency is reduced the transformer will draw
high magnetising current. This will result in higher core loss and subsequent heating
of core and ultimate failure of transformer. Hence over fluxing protection is provided
for the transformer.
• What type oil pumps are used?
Canned rotor pumps.
• What is oil reclaiming and reconditioning?
In reclaiming process the oil treated to remove all its impurities like acidity, sludge,
sediments, moisture etc. The treated oil will be in par with the new oil. In
reconditioning process (filtering of oil) only moisture and suspended impurities and
sediments are removed.
• Why there is no mixing of oil of tap changer and transformer?
When the tap changing takes place arc is struck between the contacts. Due to this the
oil inside the tap changer will be highly carbonised. If both oil get mixed up the
quality of transformer tank oil will come down. This is not advisable. Hence both
oils are kept separately.
• Why the tap changers are always connected to HV side of the transformer?
During tap changing action the load current has to be shifted from one tap to another
tap. In case HV wining the load current will be less. Hence lesser arcing will take
place.
• What is the purpose of conservator?
To accommodate the change in volume of oil during increase in temperature.
• Why the neutral is earthed through earthing resistance in case of UT and SUT?
This is done to limit the earth fault current.
• Why REF is provided in the LV side of SUT and UT?
The LV sides of the two transformers are earthed through the resistance. This will
limit the flow of current in case of LV earth fault. Hence the differential protection
may not act for a LV earth fault. Hence ref protection is provided.
• Why twin secondary SUT?
As per is, the rating single secondary power transformer is limited to 25MVA (6.6
kV) or 40 MVA (11 kV), in order to limit the 3 phase symmetrical fault level with in
26-40 kA (contribution from grid and local machines)
• How 6.6 kV-bus supply was chosen?
11 kV was rejected in view of the high insulation cost with 11 kV motors.
3.3 kV was rejected, since max motor size with 3.3 kV bus is limited to 2 MW. But
we are having the motors having rating more than 2 MW, which cannot suit to 3.3
kV bus. 6.6 kV bus we can start upto 5 mw size motor.
• Why oil transformers are out door?
Oil fire point = 170 ºC easy catching of fire.
• What are the I.S used in transformers?
IS – 1866 FOR MAINTENANCE AND SUPERVISION OF OIL
IS – 10593 FOR GAS ANALYSIS
IS – 1886 FOR INSTALLATION AND MAINTENANCE
• When oil filteration is required?
On reweaving oil test results.
Draining of oil for maintenance
Topping up of transformer oil
• Why oil filteration is required?
To remove water, sediments, sludge etc.
• What are the types of oil used for in transformer for cooling?
Paraffin based and naphtha based (in INDIA)
• What are the types of bushing used in transformer?
Condenser type bushing
Porcelain type bushing
• What are the precautions to be taken while terminating the bushings?
Contact surfaces with intermediate plates,
Mating surfaces should be identical.
• How bushings are terminated inside the transformers?
By grooving method or by binding wire method.
• Why ICT are used?(INTERPOSING CT)
To correct the system primary CT errors in case of high current faults out side CT
zone (ICT’s primary CT is 800/1, but in fault current may go to thousands of amps.
This ICT will take care of those errors.
a) Matching the ratios.
b) Matching the phase angle differences.
• How CT is connecting in ckt?
If the primary of CT is delta connected load the CT will be in star connection and
vice versa. This is because to have square root 3 time compensation.
• What type of gasket and adhesive are used in transformer?
Gasket – Neoprene based rubberised cork type RC70-C. (IS4253)
Adhesive –Dunlop adhesive S-758
These are recommended by TELK
• What are the precautions to store the Gasket?
a) Stress free storage
b) No folding
c) No reuse
d) Replace with same thickness
• What is the in built protection for transformer?
PRV to protect from over pressurization of tank due to the release of gases, oil etc.
This is the replacement for the explosion vent.
• Why UT, SUT secondary is rated for 6.9kV where as bus voltage is 6.6kV?
The no load secondary 6.9kV voltage level adequately takes into account voltage
drop during loaded condition to cater station buses at 6.6kV level.
• Why our GT having off load tap changer?
Because our station is base load station.
• Why vector group of SUT is chosen as Yn-Yo-Yo?
To facilitate momentary paralleling of SUT with UT on 6.6kV buses.
• Grounding of various transformers.
GT HV solidly grounded
LV (delta)
UT HV (delta)
LV cast stainless steel 9.95 ohms 400A for 10 seconds.
SUT HV solidly grounded
LV cast stainless steel 9.95 ohms 400A for 10 seconds.
• What are the protections for GENERATOR TRANSFORMER?
a) Differential protection
b) Restricted earthfault protection
c) Backup earthfault protection
d) GT phase back up protection
e) Overfluxing protection
f) Oil surge (gas) protection
g) High winding temperature and oil temperature protection.
• What are the protections for SUT?
a) Over current protection for phase and earth fault
b) Differential protection
c) HV and LV restricted earthfault protection
d) HV side directional back up over current protection for phase and earth fault.
e) LV back up over current and earth fault protection
f) Over fluxing protection
g) Buchholz and high oil, winding temperature protection.
• What are the protections for UT?
a) Differential protection
b) LV restricted earthfault protection
c) LV back up earthfault and over current protection
d) Buchholz and high oil, winding temperature protection.
• What is the purpose of carona ring?
To minimize the arcing current during switching operations of disconnecting
switches.
• What are the various tests on transformers?
a) Tan delta and capacitance dissipation factor
b) Tests on cooling fans
c) Tests on OLTC
d) Vector group test
e) Short circuit test
f) Open circuit test
g) Insulation resistance test
h) Turns ratio test
i) Winding resistance test.
• Why input transformer of PUPS module 1 is delta-delta and module 2 is delta-star?
With the help of this arrangement, combined DC output from both chargers is
equivalent to that from a 12-pulse rectifier. Advantage of 12-pulse rectifier is that the
mains current is fairly close to sine wave. Harmonics injected into system by rectifier
are low. The phase angle difference 30-degree between module 1 output and module
2 output give 12-pulse output.
https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg RRB JE, SSC AE/JE UPSSSC JE, SSC JE, CIVIL ENGINEERING MCQs, ELECTICAL ENGINEERING MCQs, preavious year quesion papers, dmrc, lmrc, drdo,rrb ntpc, ntpc, pgcil, dsssb, states board, GATE IES EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA
• Give transformer nameplate details of GT, SUT, UAT, SET, 415 V Aux transformer
and Lighting transformer.
GT SUT UAT SET 415V Aux trans. L Trans.
USI 5210 5120 5220 4120 5242 5231
Make Telk Telk BHEL BHEL EE Square
Automation
STD IS – 2026 IS– 2026
Type WFOC Oil immersed Oil immersed DRY RESIN
CAST DRY RESIN CAST DRY RESIN CAST
Cooling OFWF ONAF / ONAN ONAF / ONAN AN AN AN
VA 260/260 MVA 35/20/20/12 MVA
24.5/14/14 MVA
35/20/20 MVA
24.5/14/14 MVA
3150 kVA 1800/1200 kVA 250 kVA
Volts 235/16.5 kV 220/6.9/6.9/11
kV
16.5/6.9/6.9
Kv
16500/575
V 6600/435 V 415/415V
Amps 639/9098 A 64/1172/440 A
91/1675/629 A
1266/858A
1676/1172 A 157.5/2400 A 630/250,125A
No of φ 3 3 3 3 3 3
Frequency 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz
Impedance 13.13 (14) % 9.75% / 18.82% 10 ±10% HV
22 ±10% LV
Vector YNd11 Yn yno yno
(d1)
D yn1 yn1 Dyn Dyn11 Dyn11
Oil 42000 Lt. 25260 Lt. 19750 Lt.
Tap change Off load ON load HV ON load HV
Tapchange% 10 steps of 2.5 % ϒ12% in 1.5% steps ϒ12% in 1.5% steps
• What is the use of Tertiary winding?
Star connected circuit, which has an isolated neutral there can be no zero sequence
components. Since the zero sequence components are by definition in time phase
with another their sum can not be zero at the junction point as per kirchoff’s law. It
follows that there are limitations upon the phase loading of a bank of transformers
connected in star – star unless the neutral points are connected to the source of power
in such a manner that the zero sequence components of current have a return path or
unless the transformer are provided with tertiary winding.
• What is E/F current limit for SUT and UT?
400a limited by 10 ohms resistor.
• What is the coverage of differential protection for SUT?
Covers from 230 kV bushing to 6.6 kV breaker end.
• What are the advantages of dry type transformer?
No fire hazard.
It can be mounted indoor.
• During unit operation, can we parallel 2 SUT?
No, due to switchgear limitation.
• Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to
400 A?
a) To reduce burning and melting in faulted switchgear or machine.
b) To reduce mechanical stresses in equipment.
c) To reduce the electrical hazards by stray ground fault currents in the ground
return path.
d) To reduce momentary line voltage dip due to ground fault.
e) The current is limited to 400a, that is ¼ th of the load current to reduce the size of
the screen in 6.6 kV XLPE (cross-linked polyethylene). Therefore the cost of the
cable decreases.
• During unit operation can we have one UT feeding both unit 6.6 kV loads?
No, logically prevented.
• During unit operation, can we parallel UT & SUT continuously?
No, due to switchgear limitation.
• What is the design basis of 6.6 kV aluminium bus bars?
a) Temperature rises not exceed 90 ºC.
b) Withstand short ckt stresses.
c) Take care of thermal expansion.
• Why 2 types of earth fault relays in 6.6 kV side of transformers?
I – Trips 6.6 kV breakers only. It gives primary protection for 6.6 kV bus bars.
I1 – Trips the both HT and LT breakers. It acts as a backup to ref and also acts as
backup to bus bar earthfault relay.
• Why core balance CT is preferred over residual connected CT’s to sense earth fault
in 6.6 kV feeders?
a) To avoid relay mal-operation due to CT saturation
b) Better sensitivity is got.
c) High pickup and TMS avoided in IDMT earth fault relay.
• How selection of cooling fluid in GT done?
a) There are 5 factors are there.
b) Density
c) Coefficient of thermal expansion
d) Viscosity
e) Specific heat
f) Thermal conductivity.
• What are the ranges in which each type is effective?
ONAN – Natural cooling – up to 15 MVA.
ONAF – Air forced radiators cooling – 10 to 100 MVA depending on availability of
area.
OFWF = oil forced and water forced used in more than 100 MVA.
• Why off load tap changer was chosen for GT?
Because our plant works on base load always.
• What are the advantages of OFWF?
Ensure the differential temperature between top and bottom of transformer is
minimum and Effect of ambient air temperature is minimum.
• What is the type of lightning arrestor for GT?
Zn O (zinc oxide) types.
• What is the purpose of header breaker in water circuit?
The header breaker ensures oil pressure greater than that of water pressure always.
Therefore there is no leak of water into oil.
• Why thermosyphon filter required?
To keep required dryness/improve dryness of the transformer insulation, internal part
of transformer. When transformer operates, due to pressure head between top and
bottom small quantity of oil flows through filters (absorbent material activated
alumina grade g-80 removes the moisture from oil). Absorbent material remove
slag, acids, peroxides, ionic impurities from oil, which otherwise accelerate
against of oil. Absorbent unit is reactivated at regular intervals.
• What is the purpose of pronol conservator (KAPP)?
Flexible separator avoids direct contact with atmosphere. Efficient barrier between
oil and air. Ensures the protection against water vapour, suppression of gas bubbles
formation in the oil.
• Why main generator/UT is not provided with separate overfluxing protection?
Since GT is provided with overfluxing protection, it is adequate to protect main
generator / UT also. Main generator can withstand higher degree of overfluxing. If a
generator CB is used, separate overfluxing protection is essential for main generator.
• What is the advantage of Pressure relief device in TELK type GT over explosion
vent of BHEL, even though in both cases oil will be expelled out during sudden
pressure rise?
During internal fault, the internal pressure rise is relieved by the expelling out of oil
through Pressure relief device /explosion vent. However the Pressure relief device
closes back when the pressure drops. Hence the oil exposure to atmosphere is
minimised, thus saving large quantity of costly transformer oil from oxidation and
moisture absorption. Fire hazard due to transformer oil does not exist after the
closure of Pressure relief device.
• To reduce tower-footing resistance, which are better to use a) chemical, b) ground
rods, c) counter poise?
B & C
• Why tap changer is kept at neutral end?
a) To reduce insulation cost of tap changer.
b) But reactance changeover the tap range increases.
• Why guard connection is given for megger?
For true measurement of IR value of HV to earth of a transformer, connect line to
HV, earth to transformer tank and guard to LV. Therefore leakage current from HV
to LV is not included.
• Why lighting isolation transformer is req.?
a) 3 wire to 4 wire conversion, since neutral is required for lighting load.
b) Prevents transfer of E/F currents
c) Reduces the fault level on secondary side and permits use of small sized cables /
CB’s / fuses.
• Why neutrals are solid grounded above 33 kV?
a) Less transient over voltage due to arcing grounds.
b) Voltage of phases are limited to phase to ground voltage. (No neutral shifting)
c) Allows graded insulation of transformer (low cost)
d) Fast E/F protection.
• Why SET is chosen as Dyn 11?
To have smooth commutation in generation in between stator and rotor.
• Why all 415V transformers are chosen Dyn 11? What are the protections provided
for the 415V transformers?
a) To facilitate interchange.
b) To have momentary parallel during changeover.
Protections
a) Door interlock to trip HT and LT breakers.
b) LT breaker can on only after HT breaker is in on position.
c) Instantaneous O/C and inverse O/C (50 + 51).
d) Instantaneous E/F (50N).
e) IDMT E/F and restricted E/F (51N + 64).
f) Winding temp high trip (140°C trip and 130°C alarm)
• What is the instrument name used for thermograph?
Infrared camera.
• Why neutrals are solid grounded below 600v?
Human safety
Permits enough E/F current because ground resistance is large in less than 415v,
hence fast fault clearance,
Equipment safety against over voltage.
• What are the advantages of ungrounded system?
Supply is maintained even with fault on one line
Less interference to communication lines because of absence of zero sequence
currents.
• Why resistance grounding preferred for less than 33 kV and more than 415 V?
a) To limit the earth fault current for equipment safety else, high short ckt forces
dislocate in windings/bus bars etc,
b) Over voltage due to arcing ground reduced
c) Permits earth fault protection (not possible in ungrounded system)
• What is meant by tan-delta measurement?
It is the tan of the angle between the capacitive current and the total current.
Ir
Ic Ic - capacitive current
I Ir - resistive current
I - total current
As the value of tan delta increases the resistive component of the current in
increasing. Hence it shows a weak insulation.
• What is the vector group of GT, UT, SUT?
Yd11
Dy1
Yy0
• Why all the transformers are having different vector group?
UT and SUT are getting paralleled at 6.6 kV bus. Hence they should have voltage of
same phase relationship. This is achieved by assigning different vector group to the
transformers.
• What are the built in protections for transformers?
a) Buchholz relay
b) Explosion vent or relief valve
c) Gas operated relay for on load tap changers.
• Why water pressure is kept below the oil pressure? How it is maintained?
Incase of a heat exchanger tube failure the water should not go inside the
transformer. For this purpose the oil pressure is kept above the water pressure.
• What is the requirement oil in a transformer?
Oil is used removal of heat produced in the transformer and also as insulating
medium.
• What is meant by over fluxing of transformer?
When the voltage is increased and the frequency is reduced the transformer will draw
high magnetising current. This will result in higher core loss and subsequent heating
of core and ultimate failure of transformer. Hence over fluxing protection is provided
for the transformer.
• What type oil pumps are used?
Canned rotor pumps.
• What is oil reclaiming and reconditioning?
In reclaiming process the oil treated to remove all its impurities like acidity, sludge,
sediments, moisture etc. The treated oil will be in par with the new oil. In
reconditioning process (filtering of oil) only moisture and suspended impurities and
sediments are removed.
• Why there is no mixing of oil of tap changer and transformer?
When the tap changing takes place arc is struck between the contacts. Due to this the
oil inside the tap changer will be highly carbonised. If both oil get mixed up the
quality of transformer tank oil will come down. This is not advisable. Hence both
oils are kept separately.
• Why the tap changers are always connected to HV side of the transformer?
During tap changing action the load current has to be shifted from one tap to another
tap. In case HV wining the load current will be less. Hence lesser arcing will take
place.
• What is the purpose of conservator?
To accommodate the change in volume of oil during increase in temperature.
• Why the neutral is earthed through earthing resistance in case of UT and SUT?
This is done to limit the earth fault current.
• Why REF is provided in the LV side of SUT and UT?
The LV sides of the two transformers are earthed through the resistance. This will
limit the flow of current in case of LV earth fault. Hence the differential protection
may not act for a LV earth fault. Hence ref protection is provided.
• Why twin secondary SUT?
As per is, the rating single secondary power transformer is limited to 25MVA (6.6
kV) or 40 MVA (11 kV), in order to limit the 3 phase symmetrical fault level with in
26-40 kA (contribution from grid and local machines)
• How 6.6 kV-bus supply was chosen?
11 kV was rejected in view of the high insulation cost with 11 kV motors.
3.3 kV was rejected, since max motor size with 3.3 kV bus is limited to 2 MW. But
we are having the motors having rating more than 2 MW, which cannot suit to 3.3
kV bus. 6.6 kV bus we can start upto 5 mw size motor.
• Why oil transformers are out door?
Oil fire point = 170 ºC easy catching of fire.
• What are the I.S used in transformers?
IS – 1866 FOR MAINTENANCE AND SUPERVISION OF OIL
IS – 10593 FOR GAS ANALYSIS
IS – 1886 FOR INSTALLATION AND MAINTENANCE
• When oil filteration is required?
On reweaving oil test results.
Draining of oil for maintenance
Topping up of transformer oil
• Why oil filteration is required?
To remove water, sediments, sludge etc.
• What are the types of oil used for in transformer for cooling?
Paraffin based and naphtha based (in INDIA)
• What are the types of bushing used in transformer?
Condenser type bushing
Porcelain type bushing
• What are the precautions to be taken while terminating the bushings?
Contact surfaces with intermediate plates,
Mating surfaces should be identical.
• How bushings are terminated inside the transformers?
By grooving method or by binding wire method.
• Why ICT are used?(INTERPOSING CT)
To correct the system primary CT errors in case of high current faults out side CT
zone (ICT’s primary CT is 800/1, but in fault current may go to thousands of amps.
This ICT will take care of those errors.
a) Matching the ratios.
b) Matching the phase angle differences.
• How CT is connecting in ckt?
If the primary of CT is delta connected load the CT will be in star connection and
vice versa. This is because to have square root 3 time compensation.
• What type of gasket and adhesive are used in transformer?
Gasket – Neoprene based rubberised cork type RC70-C. (IS4253)
Adhesive –Dunlop adhesive S-758
These are recommended by TELK
• What are the precautions to store the Gasket?
a) Stress free storage
b) No folding
c) No reuse
d) Replace with same thickness
• What is the in built protection for transformer?
PRV to protect from over pressurization of tank due to the release of gases, oil etc.
This is the replacement for the explosion vent.
• Why UT, SUT secondary is rated for 6.9kV where as bus voltage is 6.6kV?
The no load secondary 6.9kV voltage level adequately takes into account voltage
drop during loaded condition to cater station buses at 6.6kV level.
• Why our GT having off load tap changer?
Because our station is base load station.
• Why vector group of SUT is chosen as Yn-Yo-Yo?
To facilitate momentary paralleling of SUT with UT on 6.6kV buses.
• Grounding of various transformers.
GT HV solidly grounded
LV (delta)
UT HV (delta)
LV cast stainless steel 9.95 ohms 400A for 10 seconds.
SUT HV solidly grounded
LV cast stainless steel 9.95 ohms 400A for 10 seconds.
• What are the protections for GENERATOR TRANSFORMER?
a) Differential protection
b) Restricted earthfault protection
c) Backup earthfault protection
d) GT phase back up protection
e) Overfluxing protection
f) Oil surge (gas) protection
g) High winding temperature and oil temperature protection.
• What are the protections for SUT?
a) Over current protection for phase and earth fault
b) Differential protection
c) HV and LV restricted earthfault protection
d) HV side directional back up over current protection for phase and earth fault.
e) LV back up over current and earth fault protection
f) Over fluxing protection
g) Buchholz and high oil, winding temperature protection.
• What are the protections for UT?
a) Differential protection
b) LV restricted earthfault protection
c) LV back up earthfault and over current protection
d) Buchholz and high oil, winding temperature protection.
• What is the purpose of carona ring?
To minimize the arcing current during switching operations of disconnecting
switches.
• What are the various tests on transformers?
a) Tan delta and capacitance dissipation factor
b) Tests on cooling fans
c) Tests on OLTC
d) Vector group test
e) Short circuit test
f) Open circuit test
g) Insulation resistance test
h) Turns ratio test
i) Winding resistance test.
• Why input transformer of PUPS module 1 is delta-delta and module 2 is delta-star?
With the help of this arrangement, combined DC output from both chargers is
equivalent to that from a 12-pulse rectifier. Advantage of 12-pulse rectifier is that the
mains current is fairly close to sine wave. Harmonics injected into system by rectifier
are low. The phase angle difference 30-degree between module 1 output and module
2 output give 12-pulse output.
https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg RRB JE, SSC AE/JE UPSSSC JE, SSC JE, CIVIL ENGINEERING MCQs, ELECTICAL ENGINEERING MCQs, preavious year quesion papers, dmrc, lmrc, drdo,rrb ntpc, ntpc, pgcil, dsssb, states board, GATE IES EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA
No comments:
Post a Comment