Electrical Breakdown Due to Discharges in Different Types of Insulation

By comparing the propagation of electrical trees in polyethylene with the evolution of artificial air-filled cavities in epoxy, it is possible to envisage the dielectric breakdown as a treeing process. Experiments were carried out on epoxy resin specimens with voids of 0.250 mm thick and 3 mm in diameter. The propagation of electrical trees was studied in low density polyethylene in which a needle was inserted, a metallic plane in close contact with the polymer being the grounded electrode. For a cavity, the delocalization of discharges was observed by recording the corresponding pulses against time and was confirmed by optical observations. On the other hand, the study of the partial discharge characteristics of electrical trees has shown the existence of several distinct phases. They give a good idea of the nature and the localization of trees. To take into account the decentralization of the discharges, the classic equivalent circuit of a cavity has been modified. The model explains the behavior of the discharge number but does not take into account the permanent change in the dimensions and the electrical characteristics of the medium, particularly in the case of treeing.