Current disparity in multi-column surge arresters

High-voltage surge arresters for high-current, high-energy duty often comprise several parallel ZnO columns. The extreme nonlinearity of the voltage-current characteristics of these columns demands close matching. However, even initially well matched columns can be unbalanced by ageing, partial discharges, self inductance and thermal effects. Two test techniques are described to examine current disparity in a pair of parallel 15 kV rated surge arresters: these are a current-ratio method and a current-difference method. The tests allow three aspects of parallel operation to be investigated: the use of ballast resistors to minimise mismatch effects and to increase energy absorption rating; the influence of the arrester layout on current parity; and temperature-difference perturbation of transient characteristics.