Effects of Thermal and Electric Field Distribution on Tree Growth in EPDM for HVDC Cable Accessories

In this article, electrical tree growth characteristics of ethylene propylene diene monomer (EPDM) are studied under combined dc and ac voltages with different temperature gradients (Ts). The temperature difference between needle tip and ground electrodes varies from -90 degrees C to 90 degrees C, forming a T within EPDM insulation. Variance is used as a new method to indicate the overall length of the electrical tree. Experimental results show that T can accelerate the growth of electrical tree. Electrical tree tends to grow toward the ground electrode under negative T, while it distributes around needle electrode under positive T. Specific electric field distribution, determined by dc conductivity, is employed to explain the treeing process under different Ts. It is concluded that temperature-dependent conductivity results in weaker electric field near needle electrode and a quicker decreasing speed of electric field as it goes ahead to ground electrode under positive T, thus leading to the electrical trees being more concentrated to the needle tip.