Corona Discharge occurs when you get a high enough potential at a point to ionize the air around that point.
Air molecules are a reasonable insulator however the do contain quite a few free electrons.
Get those electrons exited by a high changing voltage and these 'bump' into neutral molecules causing them to lose or gain electrons. As the lose or gain electrons they become -ve and +ve ions giving of light. Most of the time you see a blue glow at the point of isonization as this is due to the prescence of -ve ions. Also Ozone is produced at the point.
Now give that isonized air a path to flow to (a nearby earth point) and voila discharge. The higher the voltage the further this distants should be between points to prevent discharge (or you could increase the insulation resistance)
CORONA is a luminous and sometimes audible, electric discharge caused by ionization of surrounding high-voltage conductors...its most harmful effect is the production of ozone and nitrogen oxides which causes deterioration of the organic materials used in insulation...
if for example...say in a system and corona tracking is not found and corrected..the buildup of leakage paths can result in a line to line or line to ground flashover...the corona discharge and surface leakage at the barrier were probably due to a build up of conductive deposits on the cable sheath and the insulating barrier..
and don't forget...an annoying problem associated with corona is radio or communication interference..
there should be some good books on your library that will give you more descriptions and maybe pictures of this phenomena..
