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

Polyethylene has been one of the widely studied polymeric insulation materials. One of the major issues related to polymeric materials is the easy formation of space charge which may cause electric field enhancement. In this paper, a numerical simulation based on 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 this combined condition. The overall applied voltage, consisted of root mean square (RMS) values of 50 Hz AC voltage and DC voltage, is kept the same, while the DC component has been varied from 0 to 1. The simulated charge distributions present notable differences when DC offset is added compared with pure AC conditions. Besides, these differences become more significant when the offset ratio is increased. The total positive and negative charge amounts are calculated respectively by integrating the charge in the material, and a curve of net charge amount changing along with time is obtained.