Long time range breakdown due to field distortion in polyethylene under temperature gradient

Space charge distribution in a low-density polyethylene film subjected to dc step voltages was measured by PEA method under a temperature gradient. The 100 μm-thick film was inserted between a 333 K (60 °C ) upper electrode and a 278 K (5 °C) lower electrode. A number of space charge injected from the upper electrode was accumulated in the bulk of the film, so that the field distribution in the film was strongly distorted. The breakdown time lag distribution in the film showed that the time lag consisted of a formative time lag for the field distortion due to space charge accumulation and a statistical time lag for the initiation of electrical avalanche. The mean breakdown time lag in cathode-heating mode was much longer than that in anode-heating mode though the maximum field strength in cathode-heating mode was higher than that in anode-heating mode. These results indicated that there was remarkable polarity effect in the long time range breakdown under the temperature gradient.