MCQ on Corona Effect in Transmission Lines

1. Corona is

(a) partial breakdown of air.
(b) complete breakdown of air.
(c) sparking between lines.

2. Which of the following statements is true regarding corona ?

(a) Corona takes place at a voltage lower than breakdown voltage.
(b) Corona takes place at a voltage higher than breakdown voltage.
(c) Corona is a current phenomenon.
(d) Corona increases the transmission line efficiency.

3. Which of the following statements regarding corona are true ?

1. It causes radio interference.
2. It attenuates lightning surges.
3. It amplifies switching surges.
4. It causes power loss.
5. It is more prevalent in the middle conductor of a transmission line employing a flat conductor configuration.

Select the correct answer using the codes given below. Codes:
(a) 1, 3, 5
(b) 2, 3, 4
(c) 1, 2, 4, 5
(d) 2, 3, 4, 5

4. Corona is accompanied by

(a) violet visible discharge in darkness
(b) hissing sound.
(c) vibration.
(d) power loss.
(e) radio-interference.
(f) ozone.
(g) all of the above.

5. Ozone effect can be detected by

(a) presence of ozone detected by odor.
(b) hissing sound.
(c) faint luminous glow of bluish color.
(d) all of the above.

6. Corona loss increases with

(a) decrease in conductor size and increase in supply frequency.
(b) increase in both conductor size and supply frequency.
(c) decrease in both conductor size and supply frequency.
(d) increase in conductor size and decrease in supply frequency.

7. The good effect of corona on overhead lines is to

(a) increase the line carrying capacity due to conducting ionized air envelope around the conductor.
(b) increase the power factor due to corona loss.
(c) reduce the radio interference from the conductor.
(d) reduce the steepness of surge fronts.

8. Compared with a solid conductor of the same radius, corona appears on a stranded conductor at a lower voltage, because stranding

(a) assists ionization.
(b) makes the current flow spirally about the axis of the conductor.
(c) produces oblique sections to a plane perpendicular to a axis of the conductor.
(d) produces surfaces of smaller radius.

9. Corona loss can be reduced by the use of hollow conductors because

(a) the current density is reduced.
(b) the eddy current in the conductor is eliminated.
(c) for a given cross section, the radius of the conductor is increased.
(d) of better ventilation in the conductor.

10. Bundled conductors reduce

(a) surface electric stress of conductor.
(b) increases the line reactance.
(c) decreases the line capacitance.

11. Corona loss in a transmission line is dependent on

(a) diameter of the conductor.
(b) material of the conductor.
(c) height of the conductor.

12. Corona occurs between two transmission conductors when they

(a) have high potential difference.
(b) are closely spaced.
(c) carry DC power.
(d) both (a) and (b).

13. In humid weather, the corona occurs at a voltage

(a) much less than that needed in fair weather.
(b) much higher than that needed in fair weather.
(c) equal to that needed in fair weather.
(d) none of the above.

14. The effect of corona is

(a) increased energy loss.
(b) increased reactance.
(c) increased inductance.
(d) all of the above.

15. The charging current in a transmission line increases due to corona effect because corona increases

(a) line current.
(b) effective line voltage.
(c) power ions in lines.
(d) the effective conductor diameter.

16. The chances of occurrence of corona are maximum during

(a) humid weather.
(b) dry weather.
(c) winter.
(d) hot summer.

17. Corona is likely to occur maximum in case of

(a) distribution lines.
(b) transmission lines.
(c) domestic wiring.
(d) service mains.

18. Corona is affected by

(a) size of conductor.
(b) shape and surface condition of the conductor.
(c) operating voltage.
(d) all of the above.

19. The only advantage of corona is that it

(a) produces a pleasing luminous glow.
(b) makes line current sinusoidal.
(c) works as a safety valve for surges.
(d) ozone gas is produced.

20. Presence of ozone owing to corona

(a) improves the pf.
(b) reduces the pf.
(c) corrodes the material.
(d) improves regulation.

21. What is the approximate breakdown strength of atmospheric air at N.T.P.?

(a) 0.3 kV/cm.
(b) 1.0 kV/cm.
(c) 3 kV/cm.
(d) 30 kV/cm.

22. The dielectric strength of air under normal condition is about

(a) 100 kV_p/cm
(b) 21.1 kV_p/cm
(c) 30 kV_p/cm
(d) 200 kV_p/cm

23. Disruptive corona begins in smooth cylindrical conductors in air at NTP if the electric field intensity at the conductor surface goes up to

(a) 21.1 kV_(rms)/cm.
(b) 21.1 kV_(peak)/cm.
(c) 21.1 kV_(average)/cm
(d) 21.1 kV_(rms)/m.

24. The dielectric strength of air is

(a) proportional to barometric pressure.
(b) proportional to absolute temperature.
(c) inversely proportional to barometric pressure.
(d) none of the above.

25. Visual critical voltage is

(a) lower than disruptive critical voltage.
(b) higher than disruptive critical voltage.
(c) equal to critical voltage.
(d) none of the above.

26. Critical voltage limit of a transmission line is increased by

(a) increasing the radius of the conductors.
(b) increasing the spacing between conductors.
(c) reducing the spacing between conductors.
(d) reducing the radius of the conductors.

27. The maximum permissible value of fair weather corona loss for an HV line is

(a) 0.6 kW/3-phase km.
(b) 1.2 kW/3-phase km.
(c) 0.3 kW/3-phase km.
(d) 2.4 kW/3-phase km.

28. Corona losses are minimized when

(a) conductor size is reduced.
(b) smooth conductor is used.
(c) sharp points are provided in the line hardware.
(d) current density in conductors is reduced.

29. The corona loss on a particular system at 50 Hz is 1 kW/km per phase. What is the corona loss at 60 Hz in kW/km per phase?

(a) 0.83
(b) 1.0
(c) 1.13
(d) 1.2

30. Corona loss can be reduced by using

(a) solid conductor of diameter ‘d’.
(b) hollow conductor of diameter ‘d +δd’ .
(c) bundle conductor.
(d) both (a) and (b).
(e) both (b) and (c).