How 6.6 kV-bus supply was chosen?

How 6.6 kV-bus supply was chosen?
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1 Answer

Answer :

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. 
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Related questions

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Last Answer : a) Temperature rises not exceed 90 ºC. b) Withstand short ckt stresses. c) Take care of thermal expansion. 

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Last Answer : Closing coil and Trip coil 2 supply from one source. Trip coil 1 supply from one source. Protections supply from one source. 

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Last Answer : a. Unit transformer which steps down the generated voltage to 6.6 kV from the generator. b. Start up transformer, which steps down the grid voltage to 6.6 kV. 

What are the protections provided for 220 kV lines and bus bars?

Description : What are the protections provided for 220 kV lines and bus bars? 

Last Answer : a. Bus bar differential protection. b. Distance protection. c. Over current protection. d. Earth fault protection. 

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Last Answer : Answer: C

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Last Answer : Answer: B

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Last Answer : Answer: C

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Last Answer : Answer: D

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Last Answer : Answer: C

What are the sources of power supply to class I bus?

Description : What are the sources of power supply to class I bus? 

Last Answer : a. Through control UPS 240 V AC. b. Through control UPS 220 V DC backed by 220 V batteries. c. Through control UPS 48 V DC backed by 48 V batteries. 

What are the types of cables used in 6.6 kV and 415 V system voltages?

Description : What are the types of cables used in 6.6 kV and 415 V system voltages? 

Last Answer : In 6.6 kV system AC (unearthed) grade having stranded aluminium conductors cables are used. Their insulation's are as follows. 1. XLPE - Crossed linked polyethylene insulation. 2. FRLS PVC - Fire ... V grade copper or aluminium stranded cables are used. Their insulations are HR PVC and FS type. 

What are the interlocks between 415 V and 6.6 kV switch gear?

Description : What are the interlocks between 415 V and 6.6 kV switch gear? 

Last Answer : 6.6 kV Breaker closed cannot rack in or out. Service lever cannot move while breaker closed. Breaker cannot be closed in in-between position. 415 V Breaker door cannot be opened when breaker is ... . Breaker cannot be closed in in-between position. Breaker closed cannot be rack in or out. 

What is the difference between lockout of 6.6 kV and 415 V CB’s?

Description : What is the difference between lockout of 6.6 kV and 415 V CB’s?

Last Answer : Voltage levels. Manual / auto reset. Lockout relays of 415 V breakers are all Electro-magnetic type. There is no mechanical latch. But in 6.6 kV it is of mechanical latch type relays.

Why core balance CT is preferred over residual connected CT’s to sense earth fault in 6.6 kV feeders?

Description : Why core balance CT is preferred over residual connected CT’s to sense earth fault in 6.6 kV feeders?

Last Answer : 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. 

Why 2 types of earth fault relays in 6.6 kV side of transformers?

Description : Why 2 types of earth fault relays in 6.6 kV side of transformers?

Last Answer : 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. 

During unit operation can we have one UT feeding both unit 6.6 kV loads?

Description : During unit operation can we have one UT feeding both unit 6.6 kV loads? 

Last Answer : No, logically prevented.  

Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to 400 A?

Description : Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to 400 A?

Last Answer : 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 ... screen in 6.6 kV XLPE (cross-linked polyethylene). Therefore the cost of the cable decreases. 

What is the difference in action of lock out pressure signal on 6.6 kV and 220 kV breakers?

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Last Answer : When lock out signal comes to 6.6 kV breakers the breaker will trip. Where as in case of 220 kV breakers the breaker will not trip. If the breaker is open it will be open only and can not be closed. If it is in closed condition it will be closed.

What type breakers are provided in 6.6 kV buses?

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List some important loads to 6.6 kV buses.

Description : List some important loads to 6.6 kV buses. 

Last Answer : a. Auxiliary transformers. b. PHT motors. c. BFP motors.  d. CEP motors. e. CCW motors. f. Chiller motors. g. Pressuring pump motors. 

What is the minimum voltage required for starting of 6.6 kV motors?

Description : What is the minimum voltage required for starting of 6.6 kV motors?

Last Answer : Minimum 80% of rated voltage.

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Last Answer : (A) 435.6 MW

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Description : As per chart on energy consumption the supply unit of furnace oil is ……… a. kWh c. kV b. kg. d. Watts

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Description : As per pie chart on energy consumption the supply unit of the electricity is……….. a. kWh c. kV b. kg. d. Watts

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Description : 99. As per chart on energy consumption the supply unit of furnace oil is ……… a. kWh c. kV b. kg. d. Watts

Last Answer : kg.

95. As per pie chart on energy consumption the supply unit of the electricity is……….. a. kWh c. kV b. kg. d. Watts

Description : 95. As per pie chart on energy consumption the supply unit of the electricity is……….. a. kWh c. kV b. kg. d. Watts

Last Answer : kwh

The dielectric strength of a vacuum or dry air is approximately ________________. A. 450 volts/inch B. 1000 volts/inch C. 20-kv/inch D. 50-kv/inch

Description : The dielectric strength of a vacuum or dry air is approximately ________________. A. 450 volts/inch B. 1000 volts/inch C. 20-kv/inch D. 50-kv/inch

Last Answer : Answer: C

If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

Description : If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

Last Answer : A. As we know the Voltage & current relation for transformer-V1/V2 = I2/I1 We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps. By putting these value on Relation540/11000= I2/2334 So,I2 = 114.5 Amps

Why 1.1 kV grade cables used for 415V?

Description : Why 1.1 kV grade cables used for 415V?

Last Answer : To take care of the both earthed / unearthed systems.  

What are the levels of SF6 gas in 220 kV breaker and their significance?

Description : What are the levels of SF6 gas in 220 kV breaker and their significance?

Last Answer : 7 kg/cm2 - normal pressure 5.2 bar - alarm 5.0 bar - closing/tripping operation blocked. 

What is the type of 220 kV circuit breaker?

Description : What is the type of 220 kV circuit breaker? 

Last Answer : 220 kV, SF6 breaker, single pole, puffer type. 

What are the main parts of 220 kV Circuit Breaker?

Description : What are the main parts of 220 kV Circuit Breaker? 

Last Answer : Pole column filled with SF6 Pneumatic drive system with compressed air circuit Control cubicle unit 

Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.

Description : Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.  

Last Answer : 220 kV SF6 Circuit Breaker 1. Make - ABB 2. Air pressure blocking a. Close Block - 17.3 bar. b. Open block - 16.7 bar. c. Auto reclose block - 19 bar. 3. SF6 pressure block a. Alarm - 5. ... , storage and erection. Rated voltage : 216 kV rms. Operating voltage : 184 kV rms. 

Why resistance grounding preferred for less than 33 kV and more than 415 V?

Description : Why resistance grounding preferred for less than 33 kV and more than 415 V?

Last Answer : 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)

Why neutrals are solid grounded above 33 kV?

Description : Why neutrals are solid grounded above 33 kV? 

Last Answer : 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 the earthing transformer primary voltage is 16.5 kV rated in main generator even though actual voltage during the E/F is root 3 times less?

Description : Why the earthing transformer primary voltage is 16.5 kV rated in main generator even though actual voltage during the E/F is root 3 times less?

Last Answer : The transformer should not saturate during E/F otherwise it will cause ferroresonance with the GT winding capacitance. Dangerous O/V and neutral shifting will occur. During loss of load or field ... saturation. Saturation can also occur due to point on wave of application causing flux doubling. 

In 220 kV switchyard why lightning arrestor should be properly grounded?

Description : In 220 kV switchyard why lightning arrestor should be properly grounded?

Last Answer : a. During lightning, surges should be discharged to ground, otherwise it will puncture the equipment insulation and it is possible only when lightning arrestor is grounded properly.  b. In order to make it effective ... kV. Soil resistivity a should be minimum and may be it is 3.5 ohm per meter.  

Why earth switches are provided in 220 kV bays?

Description : Why earth switches are provided in 220 kV bays?  

Last Answer : When bay CB trips, both end (station and grid) CB will trip. The earth switches are provided because the grid will always be alive so to prevent any shocks to the operator or maintenance personnel who is working on the line or bay due to accidental energizing of the bus.  

If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

Description : If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

Last Answer :  As we know the Voltage & current relation for transformer-V1/V2 = I2/I1 We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps. By putting these value on Relation540/11000= I2/2334 So,I2 = 114.5 Amps

For LV applications (below 1 kV), ______________ cables are used. (A) Paper insulated. (B) Plastic. (C) Single core cables. (D) Oil filled.

Description : For LV applications (below 1 kV), ______________ cables are used. (A) Paper insulated. (B) Plastic. (C) Single core cables. (D) Oil filled.

Last Answer : Ans: C For low voltage applications single core cables are suitable.

Oil-filled cable has a working stress of __________ kV/mm (A) 10 (B) 12 (C) 13 (D) 15

Description : Oil-filled cable has a working stress of __________ kV/mm (A) 10 (B) 12 (C) 13 (D) 15

Last Answer : Ans: D This is defined by dielectric strength of mineral oil i.e. 15 kV/mm.

The generation voltage is usually (A) between 11 KV and 33 KV. (B) between 132 KV and 400 KV. (C) between 400 KV and 700 KV. (D) None of the above.

Description : The generation voltage is usually (A) between 11 KV and 33 KV. (B) between 132 KV and 400 KV. (C) between 400 KV and 700 KV. (D) None of the above.

Last Answer : (A) between 11 KV and 33 KV.

Why 110V has been chosen in MCC cell?

Description : Why 110V has been chosen in MCC cell? 

Last Answer : To isolate control circuit from power circuit for Human safety at control circuit side.  

Why vector group of SUT is chosen as Yn-Yo-Yo?

Description : Why vector group of SUT is chosen as Yn-Yo-Yo?

Last Answer : To facilitate momentary paralleling of SUT with UT on 6.6kV buses.  

Why all 415V transformers are chosen Dyn 11? What are the protections provided for the 415V transformers?

Description : Why all 415V transformers are chosen Dyn 11? What are the protections provided for the 415V transformers?

Last Answer : 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) ... and restricted E/F (51N + 64). f) Winding temp high trip (140°C trip and 130°C alarm) 

Why SET is chosen as Dyn 11?

Description : Why SET is chosen as Dyn 11? 

Last Answer : To have smooth commutation in generation in between stator and rotor.  

Why off load tap changer was chosen for GT?

Description : Why off load tap changer was chosen for GT?

Last Answer : Because our plant works on base load always.