Corona discharge also known as corona effect is  an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor that is electrically charged. It will occur in high voltage systems unless sufficient care is taken to limit the strength of the surrounding electric field.

It can also produce a violet glow, production of ozone gas around the conductor, radio interference, and electrical power loss.

The corona event occurs naturally due to the fact that air is not a perfect insulator , containing many free electrons and ions under normal conditions.

Two factors are important for corona discharge to occur:

1. Alternating electrical potential difference must be supplied across the line.
2. The spacing of the conductors must be large enough compared to the line diameter.