AC creeping discharges propagating over solid–gas interfaces

This study is aimed at the characterisation of surface discharges propagating over different solid materials namely glass, bakelite and epoxy resin immersed in three gases and their mixtures (namely SF₆, N₂ and CO₂, SF₆/N₂ and SF₆/CO₂) under AC voltage (50 Hz) in a point-plane electrode system. It is shown that the stopping length of discharges (L_f) increases linearly with the applied voltage and decreases when the gas pressure is increased. L_f is longer in CO₂ and N₂ than in SF₆ [L_f(N₂) > L_f(CO₂) > L_f(SF₆)]; and the increase of SF₆ content in the investigated mixtures leads to a significant decrease of L_f. Also, for a given gas, L_f is slightly longer with glass and bakelite than that with epoxy resin indicating that the higher the dielectric constant, the longer the discharge is. Thus, the density of the discharge branches and L_f reduce when the thickness of insulators increases and/or their dielectric constant decreases. Such observations evidence the implication of capacitive charge effect and electric field in the development of creeping discharges.