Surface damage of polymer dielectric under repetitive frequency bipolar square-wave voltage

The interface is the starting point for insulation damage, especially at high frequency bipolar voltages. The electric field distortion and energy release during charge injection and extraction at the dielectric/electrode interface significantly change the molecular structure, affecting the dielectric properties, and leading the surface layer a weak point of insulation. In this paper, the damage of polymer dielectric under uniform electric field of repetitive frequency bipolar square-wave voltage was studied. The polyethylene was chosen as the sample since it degrades more easily under the electric field. By comparing the test results of the polyethylene samples before and after the electrical aging process, it was found that the density of the shallow charge traps increased, and the dc breakdown strength decreased significantly while the ac (50 Hz) breakdown voltage was almost unchanged. It proved that the damage of polymer dielectric under the repetitive frequency bipolar square-wave voltage starts at the surface of the dielectric. In addition, it means that dc breakdown strength can be a key feature to characterize the degree of dielectric damage after serving under repetitive frequency bipolar square-wave voltage. The conclusion has also been proved by the electrical aging experiments of epoxy resin and polyimide. This study lay a preliminary foundation for further systematic research on the dielectric damage mechanism under repetitive frequency bipolar square-wave voltage.