Effects of charge accumulation in a dielectric covered electrode system in air

The effect of space and surface charges on the increased insulation performance of a dielectric-covered electrode system in air during negative impulse voltage stress has been investigated. Results obtained from high-speed photographing, using a CCD camera, and the corresponding current measurements of the discharge phenomena in an air-insulated, dielectric-covered electrode system during negative lightning impulse voltage are presented. The correlation between the measured current pulses and the photographs of light events in the air-gap was found to be clear. Measurements revealed two different phenomena: at the front (voltage rise) of the impulse voltage, a negative discharge pulse emerged; while at the tail (voltage decay) of the impulse voltage, a sequence of small positive discharge pulses occurred. It is believed that discharges at the front cause heavy and distinct ionization in the air-gap which deposits charge, surface as well as space charges, at the covered electrode interfaces. The discharges at the tail correspond to re-distributions of the accumulated charge when the applied voltage vanishes. The discharges are mainly of the barrier discharge type.