What is corona?

Corona  is a kind of variable power loss in transmission lines, caused by the ionization of air molecules near the transmission line conductors. These air molecules once ionized carry charge, in form of current with them, resulting in hissing or crackling sounds with  glowing streaks of red or blue light. These losses are variable because unlike losses due to resistive loads, these losses vary with rain, sunlight, atmosphere, concentration of molecules and types of molecules present at a particular place.

An important factor to reduce corona discharge is to increase the radius of conductor, but increasing conductor radius, increases cost of transmission. Sometimes losses can even be greater than 25kW/km.