The role of negative ions in the propagation of discharges across insulating surfaces

Abstract

The relative importance of atmospheric negative ions in corona formation and breakdown in a radial electric field has been investigated, with special reference to effects occurring at an insulating surface of a cycloaliphatic polyurethane with dolomite filler. The inception times, corona charge, light emission (by UV photography) and sparkover voltages under lightning impulses have been recorded under natural atmospheric conditions and under the influence of negative ions introduced from an auxiliary corona. The presence of excess negative ions on the surface is shown to increase both the charge injected and the mean radius of the corona, attributable to an augmentation of streamer discharges. In similar experiments in air, the excess ions cause a transition to a `glow' discharge. Effects of negative ions on sparkover are not significant on the insulator surface, but they cause a small increase in air. Comparisons between the two cases lead to the conclusion that the most important effect of the ions on the surface is to provide `seed' electrons for streamer propagation, following ion detachment in the field of advancing streamer tips.