When we apply an alternating potential difference across two conductors whose spacing is large as compared to their diameters, then the atmospheric air surrounding the conductors is subjected to electrostatic stresses. At low p.d. between the conductors, there is no change in the condition of the air around the conductors.

However, if we increase the voltage gradually, a stage is reached when a hissing noise is heard, and there appears a glow of violet color around the conductor. This phenomenon is known as the corona effect in transmission lines. It is always accompanied by the production of ozone gas.

The glow is since the air around the conductor becomes conducting due to the electrostatic stresses. If p.d. is further increased, the intensity of sound and light will increase till flashover occurs between the conductors due to the breakdown of air insulation.

Corona Effect in Transmission Lines

So the phenomenon of hissing noise, violet glow, and production of ozone gas in an overhead transmission line conductor is known as the corona effect.

If the conductors are uniform and smooth, the corona glow will be consistent throughout the length; otherwise, the rough points will appear brighter.

In the case of the DC system, the glow due to the corona effect in electrical transmission lines will be different in the two conductors. The positive conductor has a smooth and bright glow, whereas the negative conductor has a spotty glow.

Causes of Corona Effect in Transmission Line

If the spacing between the conductors is small, flashover may take place between the conductors without any hissing noise and glow. It is because the distance between the conductors being smaller; there is no time for the glow to occur. Following are the leading causes of corona effect in transmission lines:

Conductors shape: The corona effect in transmission lines depends upon the shape and conditions of the conductor. The rough and irregular surface will give rise to more corona. It is because the unevenness of the surface decreases the value of breakdown voltage. Similarly, a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor having a smooth surface.

Atmosphere: Since corona is caused due to ionization of air surrounding the conductor, therefore, it is affected by the physical state of the atmosphere. In the stormy weather, the number of ions is more than normal, and corona occurs at much less voltage as compared with fair weather.

Spacing between conductors: If the spacing between the conductors is made very large as compared to their radii (r), there may not be any corona effect. It is because larger spacing reduces the electrostatic stresses at the conductor surface.

Line voltage: Line voltage significantly affects the corona effect. If it is low, there is no change in the condition of air surrounding the conductor and hence no corona. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, the corona effect appears.

Advantages of Corona Effect

• Due to the formation of the corona, the air surrounding the conductor becomes conducting. It increases the virtual diameter of the conductor. The increased diameter decreases the electrostatic stresses between the conductors.
• The corona effect reduces electrostatic stresses between the conductors. This reduces the probability of flashover and improves system performance.
• Corona reduces the effects of transients produced by lightning.

Disadvantages of Corona Effect

• The corona effect in electrical transmission lines is accompanied by a loss of energy, which is dissipated in the form of light, heat, sound, and chemical action.
• On the formation of the corona, ozone gas is produced, which chemically reacts with the conductor, causing corrosion.
• The current drawn by the line due to corona loss is non-sinusoidal, and hence non-sinusoidal voltage drop occurs in the line. This may interfere with the neighboring communication lines.

Methods of Reducing Corona Effect

It has been seen that intense corona effects are observed at a working voltage of 33 kV or higher. Therefore, careful designs should be made to avoid any corona on the substations or bus-bars rated at 33 kV or higher. Otherwise, highly ionized air may cause flashover in the insulators or between the phases, causing considerable damage to the equipment. The following methods can reduce the corona effect in transmission lines:

By increasing the conductor size: By increasing conductor size, the voltage at which corona occurs is raised, and hence corona effects are considerably reduced. It is due to this reason that ACSR conductors which have, a larger cross-sectional area, are used in transmission lines.

By increasing the spacing between the conductors: By increasing the spacing between conductors, the voltage at which corona occurs is raised, and hence corona effect can be eliminated. However, spacing cannot be increased too much; otherwise, the cost of supporting structure may increase too much.

 

 

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).

Thanks for reading about the “corona effect in electrical transmission lines.”

 

 

• Skin & Proximity Effect in Overhead Lines
• Efficiency | Voltage Regulation of Transmission Line
• Types of Electrical Cables
• Basics of HVDC Transmission System

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