This set of Transformers Multiple Choice Questions & Answers focuses on “Generator and Power Station Transformers”.
1. Generator transformers are ________________
a) Step-up transformers
b) Step-down transformers
c) Auto-transformers
d) One-one transformers
View Answer
Explanation: Generator transformers are employed in generating stations to connect the power station to the transmission system. Generator transformers step up the generator output at low voltage to the voltage at which the transmission system operates.
2. In the CCGT, how many step-up transformers will require?
a) 3
b) 4
c) 5
d) 6
View Answer
Explanation: In the case of combined cycle gas turbine (CCGT) plants this can mean having four step-up transformers, three associated with the gas turbines (each rated about 150 MW) and one with the steam turbine (rated about 250 MW) on a single unit.
3. Which of the following is one of the criteria of selecting particular generator transformer?
a) Low HV voltage
b) Low LV currents
c) High impedance
d) On-load tap-changer
View Answer
Explanation: An on-load tapchanger is required to allow for variation of the HV system volts and generator power factor. LV volts will generally remain within š5% under the control of the generator automatic voltage regulator (AVR).
4. Generator transformers can undergo sudden load-changes.
a) True
b) False
View Answer
Explanation: Generator transformers may be subjected to sudden load rejection due to operation of the electrical protection on the generator. This can lead to the application of a sudden overvoltage to the terminals connected to the generator.
5. Station transformers are generally used for ____________
a) Providing generator voltage to transmission
b) Providing power to load from transmission
c) Isolating DC
d) To supply power section auxiliary
View Answer
Explanation: The station transformer generally supplies the power station auxiliary system for starting up the boiler/turbine generator unit or gas turbine/generator and for supplying those loads which are not specifically associated with the generating unit, for example lighting supplies, cranes, workshops and other services.
6. Which of the following does not follow the criteria of station transformer?
a) LV at 11 kV
b) HV at 275-400 kV
c) Low impedance
d) On-load tap-changer required
View Answer
Explanation: According to the UK standards, Impedance must be such that it can be paralleled with the unit transformer at 11 kV to allow changeover from station to unit supplies and vice versa without loss of continuity and without exceeding the permissible fault level for the unit and station switchgear this usually means that it is about 15%.
7. Operating load factor of station transformer must be _________
a) low
b) high
c) zero
d) infinite
View Answer
Explanation: Operating load factor is low, i.e. for much of its life the station transformer will run at half-load or less. Load losses can therefore be relatively high, but fixed losses should be as low as possible.
8. For a unit transformer HV voltage must be _____________
a) 400 kV
b) 200 kV
c) 24 kV
d) 100 kV
View Answer
Explanation: The HV voltage is relatively low, being equal to the generator output voltage, i.e. usually between 11 and 23.5 kV. The LV voltage is usually 11 kV nominal, although on some combined cycle gas turbine stations 6.6 kV is used to supply the unit auxiliaries.
9. What voltage of On-load tap-changer is required for unit transformer?
a) 11 kV
b) 23 kV
c) 400 kV
d) Not required
View Answer
Explanation: Paralleling of unit and station transformers during changeover of station and unit supplies can result in a large circulating current between station and unit switchboard. This generally adds to the unit transformer load current, and subtracts from that of the station transformer.
10. On-load power factor for generator transformer is __________
a) high
b) low
c) can’t define
d) zero
View Answer
Explanation: in the case of the generator transformer, operating load factor is high, so that load losses and no-load losses should both be as low as is economically practicable. (Except in some nuclear stations, where two fully rated unit transformers are provided per unit for system security purposes.)
Sanfoundry Global Education & Learning Series – Transformers.
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