1. What are the main two divisions of MAPS electrical system and what do you
understand by it?
The two main divisions of MAPS electrical system are
a. Main output system. Output system supplies power to the grid. Generated voltage
is stepped up to 220 kV from 16.5 kV and supplied to grid.
b. Station service system. This system supplies the load inside the power station.
The generated voltage is stepped down to 6.6 kV and 415 V from 16.5 kV and
supplies to auxiliary loads.
2. List out the components of station output system.
Main generator, Generator transformer, PT, CT, CVT, lightning arrestor, wave trap,
main 220 kV bus, transfer bus, SF6 circuit breakers and isolators, line protection
scheme, GT and Generator protection scheme, bus bar protection scheme etc.
3. Why earth switches are provided in 220 kV bays?
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.
4. What is the purpose of CVT (capacitance voltage transformer)?
Purposes of CVT are
a. To indicate if line is charged or not.
b. To synchronize grid with generator.
c. For power line communication and carrier tripping
.
5. What are the protections provided for 220 kV lines and bus bars?
a. Bus bar differential protection.
b. Distance protection.
c. Over current protection.
d. Earth fault protection.
6. What are the main sources of power supply to 6.6 kV buses?
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.
7. List some important loads to 6.6 kV buses.
a. Auxiliary transformers.
b. PHT motors.
c. BFP motors.
Question and answers Electrical Maintenance Unit
- 101 -
d. CEP motors.
e. CCW motors.
f. Chiller motors.
g. Pressuring pump motors.
8. What type breakers are provided in 6.6 kV buses?
ABB. Make, SF6 gas, 1250A and 2000A capacity circuit breakers are provided in
6.6 kV buses.
9. What is the difference in action of lock out pressure signal on 6.6 kV and 220 kV
breakers?
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.
10. For how long 220 V DC batteries can supply power UPS?
220 V DC batteries can supply Power UPS for 30 minutes. Within this time class III
power supply should be restored by DG’s.
11. What do you understand by station black out?
When class IV and class III power supply fails and DG’s cannot be started and also
this condition prevails for 5 minutes then it is called station black out condition.
12. What are the sources of power supply to class I bus?
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.
13. What are the lighting systems adopted in KGS?
There are two systems.
a. Normal lighting with class IV power supply.
b. Emergency lighting with class II power supplies and in control room with class I
power supplies.
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