Switchgear and Protection Part-1

Switchgear and Protection

1. Short-circuit currents are due to________.

a. Single-phase to earth fault

b. Phase-to-phase fault

c. All the three phases-to earth fault

d. All the three-phases short-circuited

e. Any of the above

 

2. The most serious consequence of a major uncleared short-circuit fault could be________.

a. Blowing of fuse

b. Fire

c. Heavy voltage drop

d. None of the above

 

3. Which of the following results in a symmetrical fault?

a. Single-phase to earth

b. Phase-to-phase

c. All the three phases to earth

d. Two phases to earth

 

4. Which portion of the transmission system is more prone to faults?

a. Underground cable

b. Overhead lines

c. Transformer

d. Alternator

 

5. Which portion of the power system is least prone to faults?

a. Overhead lines

b. Transformers

c. Switchgear

d. Alternators

 

6. The magnitude of fault current depends upon________.

a. Total impedance up to fault

b. Voltage at the fault point

c. Load current being supplied before occurrence of fault

d. Both (a) and (b)

 

7. The most common type of fault is________.

a. Single-phase to ground

b. Phase-to-phase

c. Two-phase to ground

d. Three-phase to ground

 

8. The maximum short-circuit occurs in the case of__________.

a. Single-line-to-ground fault

b. Line-to-line fault

c. Double-line-to-ground fault

d. Three-phase bolted fault

 

9. For a fault at the terminals of a synchronous generator, the fault current is maximum for a________.

a. 3-phase fault

b. 3-phase to ground fault

c. Line-to-ground fault

d. Line-to-line fault

 

10. A 10 kVA, 400 V/200 V single-phase transformer with 10% impedance, draws a steady short-circuit current of_______.

a. 50 A

b. 150 A

c. 250 A

d. 350 A

 

11. A transformer rated for 500 kVA, 110 kV/0.4 kV has an impedance of 10% and is connected to an infinite bus. The fault level of the transformer is________.

a. 500 kVA

b. 5000 kVA

c. 500 MVA

d. None of these

 

12. Series reactors are used to_______.

a. Bring down the fault level withing the capacity of the switchgear

b. Improve the voltage regulation

c. Improve the power factor of the power system

d. Improve the transmission efficiency

 

13. Current limiting reactors maybe________.

a. Air-cooled air-cooled

b. Oil immersed magnetically shielded

c. Oil immersed non-magnetically shielded

d. Any of the above

 

14. The symmetrical components are used in the fault analysis because________.

a. The number of equations becomes smaller

b. The sequence networks do not have mutual couplings

c. The results are required in terms of symmetrical components

 

15. In a star-connected system without neutral grounding, zero-sequence currents are_______.

a. Zero

b. Phasor sum of phase currents

c. Same as rms value of phase currents

d. Same as peak value of phase currents

 

16. A balanced 3-phase system consists of_______.

a. Zero-sequence currents only

b. Positive-sequence currents only

c. Negative-and zero-sequence currents

d. Zero, negative and positive sequence currents

 

17. The positive sequence current of a transmission line is___________.

a. Always zero

b. 1/3 of negative sequence current

c. Equal to negative sequence current

d. 3 times negative sequence current

 

18. In case of an unbalanced star-connected load supplied from an unbalanced 3-phase, 3-wire system, load currents will consist of _________.

a. Posistive-sequence components

b. Negative-sequence components

c. Zero sequence components

d. Only (a) and (b)

 

19. For an unbalanced fault, with paths for zero-sequence currents, at the point of a fault______.

a. The negative and zero sequence voltages are minimum

b. The negative and zero sequence voltages are maximum

c. The negative sequence voltage is minimum and zero sequence voltage is maximum

d. The negative sequence voltage is maximum and zero sequence voltage is minimum

 

20. Zero sequence fault current is absent when fault is_______.

a. Single-line to ground fault

b. Line-to-line ground fault

c. Double-line to ground fault

d. Line-to-line

 

21. The current of a single-phase load drawn from a 3-phase system has_________.

a. Zero-sequence component of current as zero

b. Negative sequence component of current more than positive sequence component

c. Positive, negative and zero sequence components equal

d. Negative sequence component of current less than positive sequence component

 

22. Zero sequence currents can flow  from a line to transformer bank if the windings are in______.

a. Delta/delta

b. Star/Grounded star

c. Delta/star

d. Grounded star/delta

 

23. Negative sequence reactance of a transformer is_________.

a. Smaller than the positive sequence reactance

b. Larger than the positive sequence reactance

c. Equal to the positive sequence reactance

 

24. If all the sequence voltages at the fault point in a power system are equal, then the fault is a_________.

a. Double-line to ground fault

b. Line-to-line fault

c. Line-to ground fault

d. Three-phase fault

 

25. Fuses have got advantages of________.

a. Cheapest type of protection

b. Inverse time current characteristic

c. No maintenance

d. Current limiting effect under short-circuit conditions

e. All of the above