Electrical Tree Performance in Epoxy Resin under Low-Frequency Bipolar Square- Wave Voltage

Epoxy resin has been widely used as the main insulation in power electronic transformers, which, however, inevitably suffer from the detrimental effect of electrical trees. In this paper, bipolar square-wave voltage is applied to the epoxy resin to initiate electrical trees, with the effect of waveform parameters and ambient temperature on electrical tree characteristics explored. All trees observed demonstrate branch-like structures. The electrical tree length, width, extension factor, fractal dimension, and accumulated damage increase with voltage amplitude and frequency, while breakdown time decreases. In addition, the results of gas chromatography show that thermal decomposing occurred during the breakdown of the epoxy resin, inducing small-molecule gases including acetylene, hydrogen, and carbon monoxide. This indicates that the initiation voltage of the electrical tree at 500 Hz decreases from 8 kV to 6 kV when the temperature is elevated from 20°C to 80°C. The results of surface potential decay testing shows that the trap energy depth and the charge mobility are increased at higher temperatures, which also leads to an easier charge injection and results in an enhanced electrical tree evolution.