When the electric field around a conductor is high enough, atoms are excited and air molecules close to the conductor are ionised. When an excited atom returns to its original state (electrons move back to their valence bands) the excess energy is released in the form of a photon. This light is visible to the human eye.
Corona is a phenomenon inherent to transmission lines of 88 kV and above. Corona is self-sustaining and occurs when the critical field strength in a non-uniform field is exceeded. In a uniform field, corona cannot occur, without a flashover taking place.
The term surface gradient or gradient is used to quantify the electric field at the conductor. Corona occurs when the electric field, at the conductor surface, reaches a certain critical value, called the corona inception gradient.
The corona inception gradient is a function of air pressure, temperature, conductor radius and the surface condition of the conductor. The roughness factor is a critical parameter and varies between 0.6 and 0.85 for stranded conductor of AC lines (on DC lines it could be as low as 0.5 when insects and dust are collected).
The corona effects that are considered in transmission line designs are power loss, electromagnetic interference (EMI) and audible noise. Corona can can lead to damage to composite insulators (Corona and Composite insulators)Examples of corona produced on high voltage equipment can be seen above.
Corona Produces amongst other things: