Space charge behaviours in polyethylene under combined AC and DC electric fields

Abstract

Polyethylene is one of the widely studied polymeric insulation materials, which have been used extensively for cable insulation. One of the major issues related to polymeric materials is its easy formation of space charge, a high chance to cause electric field distortions. This phenomenon is more significant under high voltage direct current (HVDC) stresses. Space charge can also be observed under high voltage alternative current (HVAC)stresses but with much less intensity due to the limited charge injection period and the effect of charge recombination caused by the constantly variance of the external fields. When considering the situation of an AC voltage combined with a DC offset, a possible scenario in HVDC technology, there was little research on charge dynamics in the insulation in terms of both experimental and simulation work. In this paper, a numerical simulation based a bipolar charge injection/transport model is used to obtain characteristics of space charge in polyethylene under the combined AC and DC high voltage at room temperature. The bipolar charge injection/transport model, which is widely used in HVDC space charge simulation, is applied in the combined conditions. The overall applied voltage, consisted of root mean square (RMS) values of the AC voltage and DC voltage, is kept the same, while the DC component’s voltage ratio and AC component’s frequency are changed respectively, to illustrate their effects on the space charge dynamics within the insulation under combined electric fields. The simulated charge distributions present notable differences when DC offset is increasingly added in, while relatively small differences when AC component’s frequency altering, especially for the cases whose frequency exceeding 0.5 Hz

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