250+ TOP MCQs on Switchgear and Protection – 1 and Answers

Power Systems Multiple Choice Questions on “Switchgear and Protection – 1”.

1. The factor which influences the acr de ionisation dominantly ________
A. line voltage
B. magnitude of transient fault current
C. speed of reclosure
D. all of the mentioned

Answer: A
Clarification: The line voltage determines the arc de ionisation in a circuit breaker.

2. A three phase transformer having a line voltage ratio of 400/33000 V is connected in the star-delta. The CTs on the 400V side have a CT ratio of 1000/5. What must be the ratio of CTs on the 33000 side?
A. 7/5
B. 5/7
C. 3/5
D. 5/2

Answer: A
Clarification: As the power levels remain same at the two sides of transformer,
√3*400*1000 = √3*33000*IL2
IL2= 400/33
Current through the secondary of CT on the primary side = 5A
Current through the pilot wire = 5√3 A
So CTs on the secondary side being star connected will have 5√3 A.
CT ratio on 33000V side = 400/(33*5√3 ) = 7/5.

3. A three phase transformer having a line voltage ratio of 400/33000 V is connected in the star-delta. The CTs on the 400V side have a CT ratio of 1000/5. What will be the current through the pilot wire?
A. 5√3 A
B. 5/√3 A
C. 5 A
D. 1/5 A

Answer: A
Clarification: As the power levels remain same at the two sides of transformer,
√3*400*1000 = √3*33000*IL2
IL2= 400/33
Current through the secondary of CT on the primary side = 5A
Current through the pilot wire = 5√3 A.

4. The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What percentage of the winding is protected against L-G faults?
A. 76.4
B. 77.8
C. 73
D. None of the mentioned

Answer: D
Clarification: The given system will not protect for the LG faults, instead it will protect for earth faults.

5. The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What percentage of the winding is protected against earth faults?
A. 76.4
B. 77.8
C. 73
D. None of the mentioned

6. The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What percentage of the winding is protected against L-G faults?
A. 76.4
B. 23.6
C. 73
D. None of the mentioned

Answer: B
Clarification: The primary CT current for the relay operation = 1.5*1000/5 = 300 A
% winding unprotected = (Is*RV*100%)
= (300*5√3)/11000*100
= 23.6 %.

7. The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What should be the minimum value of the earthing resistance to protect 90% of winding?
A. 2.12 Ω
B. 1.12 Ω
C. 4.24 Ω
D. 3.24 Ω

Answer: A
Clarification: % winding protected = 76.4 %
resistance required to protect 90% of the winding
10 = Is*RV*100
R = 2.12 Ω.

8. While a transformer is energised, the differentially connected relay will
A. mal operate
B. not mal operate
C. mal operate for lagging loads
D. never mal operate

Answer: A
Clarification: There will be flow of inrush current at the energization of transformer which will let relay may operate.

9. If a transformer is provided with differentially connected relay. To prevent the mal operation of the relay, the relay relay operating coil is biased with ______
A. 3rd harmonic
B. 2nd harmonic
C. 7th harmonic
D. 5th harmonic

Answer: D
Clarification: The restraining coil will be biased not the operating coil. The transformer having heavy inrush current has 2nd harmonic components.

10. If a transformer is provided with differentially connected relay. To prevent the mal operation of the relay, the relay restraining coil is biased with ______________
A. 3rd harmonic
B. 2nd harmonic
C. 7th harmonic
D. 5th harmonic

Answer: B
Clarification: The transformer having heavy inrush current has 2nd harmonic components.

11. The frequency of the carrier in the case of carrier-current-pilot scheme is in the range of _________
A. 50 kHz-500 kHz
B. 1 kHz-10 kHz
C. 25 kHz-50 kHz
D. 15 kHz-25kHz

Answer: A
Clarification: The carrier frequency has to be high in the case of carrier-current-pilot scheme.