Effect of pulse frequency on tree characteristics in epoxy resin under low temperature

The superconducting magnet system is the key part of international thermonuclear experimental reactor (ITER) device which has attracted much attention in many countries. Due to its excellent performance in many aspects, epoxy resin has been selected as the base material of superconducting magnet insulation. Considering the unique operating environment, epoxy resin faces the challenge of liquid helium temperature as well as pulse voltage. The influence of pulse frequency and polarity on the electrical tree characteristics in epoxy resin under low temperature is investigated in this paper. Samples made of epoxy resin were stressed with the different frequencies in the needle-plate geometry electrodes. The frequency of pulse voltage varied from 100 to 500 Hz with positive and negative polarities. The experimental temperature was set to -30, -90 and -196 °C. Accumulated damage and expansion coefficient (D/L) are employed to characterize the electrical tree. The experimental results indicate that low temperature, pulse frequency and polarity have an influence on treeing process. The typical tree structures in epoxy resin are obviously different with the variation of low temperature and pulse frequency. It is revealed that the low temperature is resistant to the growth of electrical tree and there are more small branches generating from the main branches with the decrease of temperature. However, when under the same low temperature, the tree initiation and propagation processes are accelerated by the higher pulse frequency. The density of the tree branches increases with the increase of pulse frequency. The accumulated damage presents a similar trend corresponding to branches density. Meanwhile, electrical tree tends to grow faster with the positive pulse than negative pulse under the low temperature. Moreover, the higher value of D/L at lower temperature implies that tree branches extend at a wide range direction.