Condition Monitoring of Metal Oxide Surge Arresters – Case Study

Lightning overvoltage is frequent phenomenon in power system which cannot be avoided, however damaged caused by it can be controlled to certain extent. Metal Oxide Surge Arresters (MOSA) are connected in the system for the protection of transmission and distribution system against lightning over voltages. During normal operating condition MOSA acts as an insulator whereas, it offers conductive path during overvoltage. Hence, MOSA is a core protective device for transmission & distribution system against lightning over voltages. It consists of zinc oxide (ZnO) elements which has non-linear V-I characteristics. ZnO elements are connected in series and fitted in ceramic or polymer based housing. Due to ageing effect MOSA degrades under influence of continuous operating voltage, pollutants and surge voltages. Degradation of MOSA increases the leakage current flowing through it which results increased in temperature of surge arrester. The resistance of zinc oxide elements decreases as temperature increases. As a results leakage current increases, which again increase the temperature of a MOSA. This results in to thermal runaway conditions for MOSA. Once, it reaches to thermal runaway condition it cannot return to normal working condition. This condition is a primary cause of premature failure of surge arresters. So, condition monitoring of surge arresters should be done at periodic intervals. Various Offline and online condition monitoring techniques are available for health assessment of MOSA. Offline condition monitoring techniques are not much popular as it requires to remove surge arrester from the system, which requires system shutdown. Hence, online condition monitoring techniques are more adopted and hence much popular. ERDA has expertise of various condition monitoring techniques of MOSA and conducted online health monitoring of various ratings of MOSA up to 400 kV. The amplitude of total leakage current (IT) and amplitude of resistive leakage current (IR) have been considered as indicators for surge arrester condition monitoring. This paper presents ERDA’s experience of the condition monitoring of MOSAs with case studies.