Corona is a discharge caused by electrical overstress. Corona is a luminous discharge due to ionization of the air surrounding a conductor around which exists a voltage gradient exceeding a certain critical value.
For transmission line studies, the insulating material in which the discharge occurs is the air adjacent to conductor or insulator surfaces, when the electrical stress at these surfaces exceeds the critical value.
With the increased use of high-voltage transmission lines and the probability of going to still higher operating voltages, the common aspects of corona (radio influence and corona loss) have become more important in the design of transmission lines.
In the early days of high-voltage transmission, corona was something which had to be avoided, largely because of the energy loss associated with it. In recent years the RI (radio influence) aspect of corona has become more important.
In areas where RI must be considered, this factor might establish the limit of acceptable corona performance. Under conditions where abnormally high voltages are present, corona can affect system behavior.
It can reduce the overvoltage on long open-circuited lines. It will attenuate lightning voltage surges and switching surges. By increasing the electrostatic coupling between the shield mire and phase conductors, corona at times of lightning strokes to towers or shield wires reduces the voltage across the supporting string of insulators and thus, in turn, reduces the probability of flashover
and improves system performance.
On high-voltage lines grounded through a ground-fault neutralizer, the inphase current due to corona loss can prevent extinction of the arc during a line to ground fault.
At a given voltage, corona is determined by conductor diameter, line configuration, type of conductor, condition of its surface, and weather. Rain is by far the most important aspect of weather in increasing corona.
Hoarfrost and fog have resulted in high values of corona loss on experimental test lines. However, it is believed that these high losses were caused by sublimation or condensation of water vapor, which are conditions not likely to occur on an operating line because the conductor temperature would normally be above ambient.
For this reason, measurements of loss made under conditions of fog and hoarfrost might be unreliable unless the conductors were at operating temperatures. Falling snow generally causes only :a moderate increase in corona. Also, relative humidity, temperature, atmospheric pressure, and the earth’s electric field can affect corona, but their effect is minor compared to that of rain. There are apparently other unknown factors found under desert conditions which can increase corona.