Finite-element computation of capacitance networks in multiple-electrode systems: application to ZnO surge arresters

In the low-conduction regime, the capacitive current in ZnO surge arresters is dominant, about 40 times greater than the resistive current. Therefore, the voltage distribution along surge arrester columns is mainly determined by the location and shape of the arrester terminals and the floating electrodes formed by the arrester flanges and the metallised surfaces of the ZnO elements. Terminal current and voltage measurements are not usually sufficient to quantify the effect of location and shape of the various electrodes. A method to determine the capacitance network that exists in systems with multiple-electrode configurations is described. The method is based essentially on the numerical electric field computation using commercially available finite-element software. The modelling of a distribution polymeric zinc-oxide surge arrester in the low-conduction regime is used for illustration. The arrester is modelled in terms of capacitances based on its geometric and dielectric properties using the finite-element method. A full equivalent capacitance network, which takes into account the ZnO material properties and the stray capacitances to the floating electrodes, is derived, A simplified method requiring only a single finite-element computation to obtain a reduced capacitance network is also described.