Corona or partial discharge in rotating machines is a phenomenon caused by the ionisation of gas
molcules (usually air) in a strong electric field (Field gradient ³ 3 kV/mm). Corona usually refers to
visible luminous discharges whereas the term partial discharge is rather used for hidden discharges
in voids. The term partial discharge (PD) indicates that the insulation between conductors is
partially bridged and an exchange of electric charges is taking place. However named, the physical
cause and the effect on insulating materials are the same. PD can result from breakdown of gas in a
void, in an electric tree channel or along an interface. Usually PD does not cause immediate failure
of a rotating machine. But enduring PD is detrimental for insulating materials. Although mica, the
main insulating component in rotating high voltage machines, is corona-resistant, precautions must
be taken to prevent the onset of PD. PD induces several mechanisms which destroy the insulation: