Stress relief by non-linear fillers in insulating solids

Discharge degradation and tree growth arise from the creation of intense electric fields at the tips of conducting inhomogeneities, discharge channels and sites of stress concentration produced by partial discharge activity. The stress concentration produces localised breakdown, creating cavities and tree channels. If these points of stress concentration could be avoided, the lifetime and reliability of the insulation could be increased. Furthermore, tristate breakdown across insulating surfaces, such as GIS spacers, is thought in many cases to be triggered by trapped surface charge which distorts the applied field causing unpredicted zones of electrical stress and subsequent breakdown of the insulating surface. Both of these phenomena would benefit from an element of conductivity to dissipate space charge accumulations. One approach is to introduce a non-linear characteristic to the conductivity of the insulation, so that high fields produce a local raising of conductivity and hence a redistribution of charge, so preventing local intrinsic breakdown stresses being achieved. This paper describes a series of experiments in which the inclusion of ZnO as a filler in polyester resin produces a field dependent conductivity which precludes the build up of significant bulk and surface charge and the accompanying degradation of insulation integrity.