Per Unit System MCQs

1. A 25 MVA, 33kV transformer has a pu impedance of 0.9. The pu impedance at a new base 50 MVA at 11 kV would be

  1. 12.2
  2. 10.4
  3. 16.2
  4. 14.4

Answer
Answer. c

2. The per-unit value of a 4 Ω resistor at 100 MVA base and 10 kV base voltage is

  1. 4 p.u.
  2. 2 p.u.
  3. 40 p.u.
  4. 0.4 p.u.

Answer
Answer. a

3. In a 3-phase, 5 kV, 5 MVA system, what is the base impedance?

  1. 5 ohms
  2. 50 ohms
  3. 0.5 ohm
  4. 500 ohms

Answer
Answer. a

4. The direct axis reactance Xd, of a synchronous generator, is given as 0.4 p.u. based on the generator’s name plate rating of 10 kV, 75 MVA. The base for calculation is 11 kV, 100 MVA. What is the pu value of Xd, on the new base?

  1. 0.279
  2. 0.578
  3. 0.412
  4. 0.44

Answer
Answer. d

5. For a given base voltage and base volt-amp, the per-unit impedance value of an element is X. What will be the per-unit impedance value of this element when the voltage and volt-amp bases are both doubled’?

  1. 2 X
  2. 4 X
  3. 0.5 X
  4. X

Answer
Answer. c

6. The per-unit impedance of an alternator corresponding to base values 13.2 kV and 30 MVA is 0.2 p.u. The p.u. value of the impedance for base values 13.8 kV and 50 MVA in p.u. will be

  1. 0.131
  2. 0.226
  3. 0.364
  4. 0.305

Answer
Answer. d

7. If a 250 MVA, 11/400 kV three-phase power transformer has leakage reactance of 0.05 per unit on the base of 250 MVA and the primary voltage of 11 kV, then the actual leakage reactance of the transformer referred to the secondary side of 400 kV is

  1. 0.8 Ω
  2. 0.0032 Ω
  3. 0.03125 Ω
  4. 32.0 Ω

Answer
Answer. d

8. The per-unit impedance of a circuit element of 0.15. If the base kV and base MVA are halved, then the new value of the per-unit impedance of the circuit element will be

  1. 0.075
  2. 0.15
  3. 0.30
  4. 0.60

Answer
Answer. c

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