DC Electrical Tree Characteristics in Epoxy Resin at Cryogenic Temperature

Epoxy resin has been widely used in the cable terminations and current lead insulation of a HVDC high temperature superconducting (HTS) power transmission system. The electrical tree is one of the main reasons for the long-term degradation of electrical insulations. This paper presents the DC electrical tree characteristics and partial discharge patterns during the growth of the trees of the pure epoxy resin at room and cryogenic temperatures. The results show that the inception probability and tree lengths under positive voltages are higher than negative voltages at both temperatures. The cryogenic temperature inhibits the initiation and growth of the electrical tree in the epoxy resin. The partial discharge measurement is performed using similar test conditions as that of the electrical tree measurement. Significantly higher partial discharges are observed during the voltage-rise and voltage-drop stages. Moreover, the higher amplitude partial discharges with lower repetition rate occur at the cryogenic temperature than the room temperature. In addition, the tree shape of the DC trees is also affected under the cryogenic temperature. During the electrical tree propagation at the room temperature, more number of lateral branches could be observed than in LN₂.