Treeing of Polyethylene at 77K

Ac tree starting voltage of low density polyethylene has been found to be higher at liquid nitrogen temperature (42kV) than that at room temperature (7kV) and it is almost equal to the positive impulse tree starting voltage. This improvement may be due to either the partial-discharge free contact of the interface between the needle electrode and the polyethylene and for the moderation of the intense field at the needle tip by injected space charges. The space charge injection was examined by evaluating the dependence of impulse tree starting voltage on the rise time of the impulse voltage, and the dc prestress effect on impulse tree starting voltage. It was estimated from the peculiar shape of the impulse tree that the injection depth of space charge is about 10 μm. The electroluminescence produced at the needle tip by ac voltage application was found to be completely different from the corona discharge in a void and it was explained by the radiative recombination process of space charges injected alternately.