NONLINEAR FILLERS IN ELECTRICAL INSULATION

The very low conductivities of modern insulating materials do not perm it the dissipation of accumulated space charge. Space charge can there fore persist both within the bulk of an insulator and on its surface f or long periods of time. This can lead to regions of intense electric field in the surrounding dielectric, producing localised breakdown res ulting in the erosion of surfaces and penetration of the bulk by elect rical trees. A nonlinear characteristic has been produced in the condu ctivity of polyester resin by the introduction of an appropriate quant ity of particulate ZnO. The critical filler/resin ratio has been deter mined above which the insulation resistance is high at low fields but decreases as the electric field increases. Alternative fillers, such a s Al2O3 and SiC, have been incorporated into the resin for comparative purposes and to elucidate the conduction mechanisms. The enhanced con ductivity at high fields in ZnO-filled polyester permits the dissipati on of accumulated space charge at the extremities of electric tree cha nnels as they impinge on barriers of this material, thereby reducing t he local electric field. As a result of this phenomenon, tree growth a nd barrier penetration are inhibited, leading to extended lifetimes.