Space charge measurement of cross-linked polyethylene at low temperatures

This study describes a modified pulsed electroacoustic (PEA) system, which can measure the space charge distribution in dielectric films at low temperatures. The temperature around the sample is controlled by the liquid nitrogen, an electric heating coil as well as a PID temperature controller. The space charge distribution of DC stressed cross-linked polyethylene (XLPE) with the thickness of ~300 pm is investigated at -20 °C, -40 °C and the ambient temperature (20 °C). The experiment results show that with the decrease of temperature, the accumulated space charges decrease in the same applied electric fields. Specifically, there are apparent homocharges accumulated in the vicinity of the electrodes at the ambient temperature. In addition, more positive charges disperse deeper in the bulk of the XLPE film than the negative charges, which demonstrates a stronger positive charge injection than negative charge injection. However, the injected negative charges always dominate in XLPE under various electric fields at -20 °C. Whereas almost no distinguished space charge can be observed in XLPE film during short circuit when it is subjected at -40° C. Besides, in order to investigate the space charge dynamics in XLPE at low temperatures, a simple charge detrapping model is used to analyze the composition of the trapped charges. The traps in XLPE is divided into two types, one is the trap with shallow depth, and the other is the trap with deep depth. It is found a large part of the accumulated charges at the ambient temperature are trapped in deep traps, whereas a large part of the accumulated charges at -20°C are trapped in shallow traps, which is predictable to some extent. Because the detrapping rate of the trapped charges increases with the increase of temperature. In addition, the variation of the depths of the two kinds of traps at different temperatures is also discussed.