In overhead transmission line, when an alternating potential difference is applied across two parallel conductors, potential gradient is set up in the air. This potential gradient has maximum value at conductor surfaces. The free electrons and ions are set in motion under the influence of potential gradient. If we increase the applied voltage, free electrons velocity also increases and when potential gradient reaches maximum value about 30KV per cm, the free electrons and ions attain enough force to dislodging electrons from adjacent molecule. These dislodged electrons and ions further collide with other molecules producing more ions. This process continues by this way, which either form corona or completely breakdown the dielectric medium between two conductors.
The ionization is generally observed by a violet luminous glow around the conductor.

The violet glow phenomenon with hissing noise and production of ozone gas in an overhead transmission line is known as corona.

Smooth or rough surfaces, where corona is greater?

In A.C transmission line, the corona glow in smooth surfaces uniformly throughout the length, but the brightest part of the glow is visible near the rough surfaces. That means at the rough points, corona glows brighter.

What happens in case of HVDC?

In case of HVDC transmission system, the corona glow is uniform through the positive wire. And the negative conductor has spotty glow. That means with d.c voltage, there is difference in the appearance of corona of the two wires.

In d.c field negatively charged particles move from negative pole conductor to positive pole conductor continuously. Similarly positively charged ion moves towards negative pole conductor. Thus movement of charged particles will be characterized by electric field distribution, rather than the surface gradient. It is also being noted that corona loss increases in d.c system so badly in bad weather like a.c system.

Factors affecting corona

The phenomenon corona is affected by some factors. They are
1) Effect of frequency: Corona loss is directly proportional to the system frequency.
2) System voltage: Corona loss increases rapidly with increasing system voltage.
3) Atmosphere: As corona is formed by ionization of air surrounding the conductor, so atmosphere plays a great role on corona.

In stormy weather corona loss is much more than fair weather as number of ion is more available in stormy weather.

4) Effect of conductor diameter: If we increase the conductor radius, electric field intensity reduces, which reduce the corona loss.
5) Spacing between conductors: if spacing between conductors is very large as compared to their diameter, there is no corona effect.
6) Effect of load current: If higher load current flows, it heats the conductor surfaces, which prevents deposition of dew or snow, reduces corona loss.
7) Effect of conductor’s surfaces: roughness of conductor’s surface results higher corona loss.
8) Effect of rain drops: Rain drop and dust deposition increases corona loss.

What is critical disruptive voltage and visual critical voltage?

the minimum phase voltage at which corona starts, is known as critical disruptive voltage. That means this is the voltage at which free electrons and ions attains enough force to dislodge from adjacent molecule. The breakdown strength is 30 KV/ Cm. (maximum at 25˚c)
The minimum phase voltage at which corona glow appears all along the line conductor is known as visual critical voltage.

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