Nonlinear fillers in electrical insulation

The very low conductivities of modern insulating materials do not permit the dissipation of accumulated space charge. Space charge can therefore persist both within the bulk of an insulator and on its surface for long periods of time. This can lead to regions of intense electric field in the surrounding dielectric, producing localised breakdown resulting in the erosion of surfaces and penetration of the bulk by electrical trees. A nonlinear characteristic has been produced in the conductivity of polyester resin by the introduction of an appropriate quantity of particulate ZnO. The critical filler/resin ratio has been determined above which the insulation resistance is high at low fields but decreases as the electric field increases. Alternative fillers, such as Al2O3 and SiC, have been incorporated into the resin for comparative purposes and to elucidate the conduction mechanisms. The enhanced conductivity at high fields in ZnO-filled polyester permits the dissipation of accumulated space charge at the extremities of electric tree channels as they impinge on barriers of this material, thereby reducing the local electric field. As a result of this phenomenon, tree growth and barrier penetration are inhibited, leading to extended lifetimes.