Impulse corona inception in dielectric covered rod-plane air gaps

Positive corona inception in dielectric covered rod-plane air gaps stressed by lightning and switching impulse voltages is investigated. Corona inception probability distributions were obtained and the salient characteristics of the corona discharge, namely inception time and voltage, were measured as influenced by the gap length, waveshape of the applied impulse voltage and by the cover material, namely PTFE and epoxy resin. The simple air gaps with a bare rod were regarded as reference. The corona inception voltage increases significantly due to the insulating cover, especially for the PTFE cover under switching voltages. Electric field computations were performed. The corona inception field is, contrary to the inception voltage, lower for the dielectric covered rod-plane than simple rod-plane air gaps; an assisting mechanism for initiatory electrons production is suggested to explain this behaviour.