Power System MCQ With Answers PDF

1. The capacitance of a cable depends upon the

(a) length of the cable.
(b) relative permittivity of dielectric used in cable.
(c) ratio of sheath diameter and core diameter.
(d) all of the above.

2. The charging current drawn by the cable

(a) lags behind the voltage by 90°.
(b) leads the voltage by 90°.
(c) leads the voltage by 180°.
(d) none of the above.

3. In underground cables, the electrostatic stress is

(a) maximum at conductor surface and minimum at the sheath.
(b) minimum at conductor surface and maximum at the sheath.
(c) same at the conductor and sheath.
(d) zero at the conductor as well as on the sheath.

4. In a cable of conductor diameter ‘d’ and overall diameter with dielectric material ‘D’, the maximum dielectric stress.

(a) occurs at the conductor surface and is proportional to d.
(b) occurs at the conductor surface and is proportional to 1/d.
(c) occurs at the middle of the dielectric and is proportional to 1/D.
(d) occurs at the outer surface of the dielectric and is proportional to D.

5. To obtain the minimum value of stress in cables, the ratio (R/r) should be

(a) 2.13
(b) 2.718
(c) 1.96
(d) 1.5

6. The surge impedance of a 50 miles long underground cable is 50 Ω. For a 25 miles length it will be

(a) 25 Ω
(b) 50 Ω
(c) 100 Ω
(d) none of these.

7. The breakdown of insulation of a cable can be avoided economically by using

(a) insulation layers of different dielectrics.
(b) inter-sheath.
(c) either (a) or (b).
(d) none of the above.

8. Capacitance grading of cable means

(a) use of dielectrics in different concentrations.
(b) introduction of capacitances at various lengths of cable to counter the effect of inductance.
(c) use of dielectrics of different permittivities.
(d) grading according to capacitance per km length of the cable.

9. Grading of cables

(a) reduces insulation cost and increases current rating.
(b) reduces insulation cost but decreases current rating.
(c) increases both.
(d) none of the above.

10: The inter-sheaths in cables are used to

(a) provide proper stress distribution.
(b) minimize the stress.
(c) use inferior insulation.
(d) provide protection against moisture and voltage surges.
(e) provide protection against current and voltage surges.

11. The desired overall diameter of the conductor without increasing its x-sectional area can be had by

(a) using aluminum core instead of copper.
(b) stranding the copper conductors around a hemp centre.
(c) stranding the copper conductors over a lead tube.
(d) any of the above.

12. In a 3-core cable, the capacitance between two conductors (with sheath earthed) is 3 µF. The capacitance per phase will be

(a) 1.5 µF
(b) 3 µF
(c) 6 µF
(d) 12 µF

13. Underground cables are laid at sufficient depth so as to

(a) minimize temperature stresses.
(b) minimize effects of shocks and vibrations owing to passing vehicles etc.
(c) avoid being unearthed easily owing to removal of soil.
(d) both (a) and (c).

14. In case the communication cables are to be laid parallel to power cables the distance between the two should be at least……..so that there is no interference.

(a) 0.5 m
(b) 2.0 m
(c) 4.0 m
(d) 0.05 m

15. While crossing the road the cable should be

(a) buried in trenches.
(b) surrounded by sawdust to absorb vibrations.
(c) laid in conduits or pipes.
(d) none of the above.

16. Consider the following statements :

1. The insulation resistance of cable will increase if the length of cable is increased.
2. For the same overall diameter of cable, the grading of cable will increase the safe working voltage.
3. The normal operating temperature of PVC cable is 70°C.
4. The thermal resistance of soil increases as the moisture content of soil increases.

Of these statements
(a) 1 and 2 are correct.
(c) 3 and 4 are correct.
(b) 2 and 3 are correct.
(d) 1 and 4 are correct.

17. The source(s) of heat generation in cables is/are

(a) copper loss in conductor.
(b) dielectric losses in cable insulation.
(c) losses in metallic sheathings and armoring.
(d) all of the above.

18. A cable carrying ac has

(a) leakage losses only.
(b) hysteresis losses only.
(c) hysteresis and leakage losses only.
(d) hysteresis, leakage and friction losses.

19. Dielectric hysteresis loss in a cable varies as

(a) impressed voltage.
(b) (impressed voltage)².
(c) (impressed voltage)^1/2.
(d) (impressed voltage)^3/2.

20. The current carrying capacity of cables in dc is more than that in ac. It is mainly due to

(a) smaller hysteresis losses.
(b) absence of harmonics.
(c) absence of ripples.
(d) none of the above.