Voltage distribution on ZnO polymeric arrester under pollution conditions

At working voltage levels and under the knee of breakdown, the voltage distribution along ZnO arresters is affected by stray capacitances. The existence of any stray capacitance will cause a non-uniform voltage distribution. When ZnO arrester operates under polluted conditions, it can be stressed with severe non uniform field distribution which could cause external breakdown, internal partial discharges and a rise in the temperature of the varistors. In order to obtain the voltage distribution in the ZnO arrester, a full equivalent circuit, which takes into account the ZnO material properties and the stray capacitances, is derived and implemented in the EMTP-ATP package in order to simulate the internal and external leakage currents flowing through the surge arrester with and without pollution under power frequency voltage. When subjected to fog conditions, pollution layers on the arrester housing will become conducting and could result in a significant redistribution of the voltage. This paper presents an equivalent circuit for a ZnO surge arresters under polluted conditions. The voltage distributions under various scenarios of pollution and applied voltage are computed. Areas of highly stressed regions were determined which could be used to predict design weaknesses.