Effect of pulse duration on electrical tree and breakdown process of epoxy resin in LN2

Abstract

The international thermonuclear experimental reactor (ITER) project has attracted much attention in the world and its major part, namely the superconducting magnet system, uses epoxy resin as the basic insulation material. Epoxy resin needs to address the challenge of the liquid nitrogen temperature and the pulse voltage with changing duration due to the special operating environment. This paper investigates the effect of pulse duration on the characteristics of electrical tree growth in epoxy resin under low temperature. The tested samples were stressed with different pulse durations in a needle-plate geometry electrode system. The pulse duration was set to 50, 110 and 220 µs and the experimental temperature was −30, −90 and −196 °C. Fractal dimension, accumulated damage and expansion coefficient (D/L) are employed to characterize the electrical tree. The experimental results indicate that the typical structures of electrical tree are obviously different with the variations of low temperature and pulse duration. It is revealed that the increase of pulse duration promotes the growth of electrical tree under the same low temperature. In addition, larger pulse duration may lead to a more complex tree structure, which indicates the larger value of fractal dimension and accumulated damage. Meanwhile, obtained results show that large pulse duration plays an important role in promoting the electrical tree's growth processes, including propagation and breakdown characteristics.