Effects of Ambient Temperature on Electrical Tree in Epoxy Resin under Repetitive Pulse Voltage

Chosen as an important insulation material in high voltage direct current (HVDC) cable terminal, epoxy resin bears the harsh environment of high temperature and changing pulse power. This paper reports on investigations into the effect of high temperature on characteristics of electrical tree growth. Samples made of epoxy resin and equipped with a needle-plate geometry electrode system were stressed with pulse voltage whose amplitude was 12, 14 and 16 kV and frequency was 200, 300 and 400 Hz. The ambient temperature was set to 60, 90 and 120 degrees C. Four types of typical tree growth process were partitioned to better describe the different growth characteristics. Typical morphology, tree length, fractal dimension, accumulated damage and expansion coefficient were employed to characterize the electrical tree. The initiation and breakdown characteristics were also analyzed. The results indicate that temperature, pulse frequency and pulse amplitude affect electrical tree growth. Higher temperature would lead to more complex tree morphology and promote the growth rate of electrical tree. When reaching the glass transition temperature, which is 93 degrees C in this paper, the change of internal state of epoxy resin would cause some special characteristics of tree growth. In addition, inception probability increased with temperature rising, whereas the time to breakdown decreased. Meanwhile, obtained results show that pulse frequency and pulse amplitude also play an important role in promoting electrical tree growth processes, including initiation, propagation and breakdown stages.