3-Phase Transformer MCQ

1. A three-phase power transformer is provided with star-delta connections. In order to protect against fault, the connection for current transformer should be in

  1. Star-star
  2. Delta-star
  3. Delta-delta
  4. Star-delta

Answer. b

2. A 3-phase transformer has its primary connected in delta and secondary in star. Secondary to primary turns ratio per phase is 6. For a primary voltage of 200 V, the secondary voltage would be

  1. 2078 V
  2. 693 V
  3. 1200 V
  4. 58 V

Answer. c

3. Three-phase step-up transformer installed at the power house, just before the commencement of transmission line, have

  1. Delta-Delta connection
  2. Star-Star connections
  3. Delta-Star connections
  4. Star-Delta connections

Answer. c

4. Which of the following combination of 3-phase transformers can be successfully operated in parallel

  1. Δ-Y and Δ-Y
  2. Y-Y and Δ-Y
  3. Δ-Δ and Δ-Y
  4. Y-Δ and Δ-Δ

Answer. a

5. A delta/delta connected three-phase transformer can be connected in parallel with

  1. Star/delta
  2. Delta/Delta
  3. Star/star
  4. Star/zigzag

Answer. b

6. Which type of connection in a 3-phase transformer is used for the substation end of the transmission line?

  1. Star-star
  2. Star-Delta
  3. Delta-Delta
  4. Delta-Star

Answer. b

7. Star-star power transformer is protected by current transformer having

  1. Star/star connnections
  2. Delta/delta connections
  3. Star/delta connections
  4. Delta/star connections

Answer. b

8. Which type of connection is employed for current transformers for the protection of star-delta connected 3-phase transformer?

  1. Delta-Delta
  2. Star-Star
  3. Star-Delta
  4. Delta-Star

Answer. d

9. In electrical power transmission, the _______ configuration of transformers are extensively used.

  1. Neither Y/Δ and Δ/Y
  2. Y/Δ
  3. Both Y/Δ and Δ/Y
  4. Δ/Y
Answer. b

10. The Δ connection has the advantage of restricting the third harmonic components currents to

  1. Within star connection
  2. Within delta connection
  3. Within both star and delta connections
  4. Neither star nor delta connections
Answer. b

11. Consider the following statements regarding three-phase transformers in Open-Delta (V-V) connections:

  1. Being a temporary remedy when one transformer forms of Delta-Delta system is damaged, and removed from service.
  2. The Volt Ampere (VA) supplied by each transformer is half of the total VA, and the system is not overloaded.
  3. An important precaution is that load shall be reduced by \sqrt{3} times in this case.

Which of the above statements are correct

  1. i and iii only
  2. i and ii only
  3. i, ii and iii
  4. ii and iii only
Answer. a

12. A 3-phase transformer has 420 and 36 turns on the primary and secondary windings respectively. The supply voltage is 3300 V. The secondary line voltage on no-load when the windings are connected in star-delta is nearly

  1. 1908 V
  2. 22260 V
  3. 490 V
  4. 164 V
Answer. d

13. Consider the following statements concerning the utility of mesh-connected tertiary windings in star-star transformers:

  1. It is used to suppress harmonic voltages.
  2. It is used to allow flow of earth fault current for operation of protective devices.
  3. It facilities supply of single-phase loads.
  4. It provides low-reactances paths for zero-sequence currents.

Which of these statements are correct?

  1. i, ii, iii and iv
  2. i, ii and iii only
  3. iii and iv only
  4. i, ii and iv only
Answer. a

14. The impedance of a Δ/Y, 11000 V/400 V, transformer of capacity 100 kVA, on its name plate data base is (0.02 + j0.07) pu. The ohmic impedance pu phase referred to the primary (11000 V) side is

  1. (0.02+ j0.07) Ω
  2. (0.55 + j1.925) Ω
  3. (72.6+ j254.1) Ω
  4. (42+ j147) Ω
Answer. d

15. In which one of the following sets of 3-phase transformer connections will zero sequence current be present in the transformer windings?

  1. Primary in star, neutral grounded; secondary in star, neutral not grounded.
  2. Primary in star, neutral grounded; secondary in delta.
  3. Primary in star, neutral not grounded; secondary in star, neutral grounded.
  4. Primary in star, neutral not grounded; secondary in delta.
Answer. b

16. Which 3-phase connection can be used in a transformer to introduce a phase difference of 30° between its output and corresponding input line voltages?

  1. Star-delta
  2. Star-star
  3. Delta-zigzag
  4. Delta-delta
Answer. a

17. When one transformer is removed from a Δ-Δ bank of 30 KVA transformer, the capacity of the resulting 3-phase transformer corresponding input line voltages?

  1. 17.3 kVA
  2. 11.5 kVA
  3. 25.9 kVA
  4. 20 kVA
Answer. a

18. A Δ/Δ connected transformer is connected to V/V connected transformer. The ratio of VA rating of V/V connected transformer and Δ/Δ connected transformer is

  1. 57.7%
  2. 100%
  3. 75%
  4. 50%
Answer. a

19. If the waveform of the voltage impressed on the primary of a Y-Δ bank contains 5th harmonics, what are the waveforms of the resultant voltages of the primary and the secondary?

Primary Secondary
a. Peaked Peaked
b. Peaked Flat-topped
c. Flat-topped Peaked
d. Flat-topped Flat-topped
Answer. b

20. Three single phase 11000/220 V transformers are connected to form 3-phase transformer bank. High voltage side is connected in star, and low voltage side is in delta. What are the voltage ratings and turns ratio of 3-phase transformer?

  1. 19052/220 V, 50
  2. 19052/220 V, 50\sqrt{3}
  3. 11000/381 V, 50\sqrt{3}
  4. 11000/220 V, 50
Answer. a

21. Possible three-to-three phase transformer connection for parallel operation is

  1. Δ-Y to Δ-Y
  2. Δ-Δ to Δ-Y
  3. Y-Y to Δ-Y
  4. Δ-Y to Y-Δ
Answer. a
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