5.1 Introduction

High voltage systems are often subject to transient overvoltages of internal or external origin. The resultant surges travel along the transmission line and can cause damage to unprotected terminal equipment. Corona losses and the earth return path can attenuate and distort the surges, but the magnitude of the surge may still exceed the insulation level of the equipment. Surge arresters provide a limitation of the overvoltage to a chosen protective level. The superiority of the recently developed zinc oxide (ZnO) material over earlier silicon carbide (SiC) renewed interest and boosted the use of surge arrester protection.

The ideal surge arrester would be one that would start conduction at a voltage level at some margin above its rated voltage, hold that voltage level with as little variation as possible for the duration of the overvoltage surge and cease conduction as soon as the voltage across the arrester returns to a value close to the rated voltage. Such an arrester would therefore conduct only that current required to reduce the surge voltage to the arrester protective level, and absorb the energy that is associated with the overvoltage. The basic non-linear formula that relates voltage V and current I in a surge arrester is given by:

(5.1)

where k is a constant and ? is the coefficient of non-linearity.

Surge arresters are used for protection of power system equipment against surge overvoltages because they offer low protection levels and permit the reduction of insulation levels,...