Hi,
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)
Any help ?, yes no let me know.
Regards