Temperature Gradient Affecting Electrical Tree Growth in EPDM for HVDC Cable Accessories

As the main insulating material of HVDC cable accessories, ethylene propylene diene monomer (EPDM) is subjected to the temperature gradient (ΔT) in actual operation, which threatens the insulation safety. In this paper, the electrical trees at ΔT varying from -90 to 90 °C are studied under 8 kV, 15 Hz harmonic superimposed 10 kV DC voltage within EPDM. The tree length, treeing time, factual coefficient and accumulated damage are used to describe the characteristics of the electrical trees. Temperature and electric field distribution are discussed to explain the tree structure. It is concluded that both negative or positive ΔT can promote the growth of electrical tree. Under negative ΔT, the stronger electric field near needle electrode and higher temperature of ground electrode lead to longer electrical tree channels and treeing time. However, under positive ΔT, the increased temperature results in larger damage area while the weaker electric field strength leads to shorter tree channels which are around the needle tip.