Detection of impulse current caused ageing in valve element of a distribution class metal oxide surge arrester

One of the factors which causes ageing of the valve elements in Metal Oxide Surge Arresters (MOSA) is the passage of current impulses. This paper reports the results of experiments conducted with the valve elements of distribution class MOSA. The valve elements were subjected to a serious of impulse current tests comprising of a specified number of “shots” at different current levels up to and including 70 kA. The progressive change in the element characteristics was assessed by application of six diagnostic methods. It is shown that impulse current caused ageing results in an increased magnitude of resistive current flow. As ageing progresses the resistive current becomes asymmetrical. Although the ageing is irreversible it is possible to maintain symmetry to the resistive current if the valve element is subjected to impulse current passages in the opposite direction. The effectiveness of the diagnostic methods in detecting impulse current caused ageing is discussed