Degradation of polymeric insulation due to photoemission caused by high electric fields

The mechanism of electrical tree initiation in polyethylene subjected at AC, DC, and half-rectified AC voltages is studied by an optical technique which is at least two orders of magnitude more sensitive than partial discharge detection. For DC voltage up to 46 kV, no light emission or tree inception was detected in the polymer containing the normal and decreased concentrations of air in its free volume. For AC and half-rectified AC voltages, tree inception only occurred if light, in the visible and ultraviolet ranges, was detected. Photodegradation of the polymeric insulation by ultraviolet radiation can occur when the voltage applied to the polymer exceeds the threshold voltage of light inception such as during switching and lightning overvoltages. This can account for the gradual degradation of the polymeric insulation which ultimately leads to the formation of an electrical tree. By comparing the light emitted from the polymer subjected to AC voltage having half-cycles of different amplitudes to DC superimposed on AC it is shown that the space charge injected into the polymer causes the zero-crossing of the local field in the polymer. This is required for light emission and subsequent tree inception