The effect of space charges on the conductivity of dielectrics under medium direct voltage stress conditions

Conduction and space charge theories in dielectrics under direct voltage conditions are reviewed and analysed. Published information on gas discharge processes in insulation, conductivity, discharge detection and measuring methods are also summarised. A combined technique whereby both discharges and current can be studied simultaneously is employed in the investigation of non-ohmic conduction in solid dielectrics at moderate d.c. stresses. Experimental evidence suggests that none of the conventional laws of conduction adequately explains the observed phenomena e.g. the thickness dependence of conductivity; and that the operating stress 'E' calculated as the ratio of the externally applied voltage to the thickness of the dielectric may not be the actual resultant stress. Some anomalies are explained on the basis of a new dielectric equivalent circuit, a modified Maxwellian 'n' strata dielectric and induced interfacial or space charges. The discharge behaviour of gaseous voids in dielectrics is analysed using an equivalent circuit incorporating the resistivity of the void gas. Extinction is explained partly by an extended Maxwellian polarisation phenomena....