Effects of Temperature Gradient on Electrical Tree Growth with Harmonic Superimposed DC Voltage

Ethylene propylene diene monomer (EPDM) is the main insulating material of HVDC cable accessories. In actual operation, temperature gradient $(\Delta T)$ and harmonic voltage are significant problems that threaten the insulation safety. In this paper, electrical trees within EPDM at $\Delta T$ varying from -90 to 90°C are studied with 8kV, 50Hz harmonic superimposed -10 or 10kV DC voltage. Thermal and electric field distribution are discussed to explain the tree structure under different $\Delta T$. It is concluded that both negative and positive $\Delta T$ can promote the growth of electrical tree, but lead to different tree characteristic; including length, accumulation damage and treeing time. Under positive $\Delta \mathrm{T}$, increased temperature and decreased electric field near needle tip cause the electrical trees shorter but produce more deterioration countertraded in the high temperature area. With harmonic superimposed negative DC voltage, electrical trees show denser branches but slower growing rate, which may be because of the properties of charge carriers with different polarities that more electrons can be injected and form a space charge layer under negative DC voltage.