Effect of valve element characteristics, stray capacitance and voltage harmonics on surge arrester diagnostics
Fernando, Sudath Namal
Metal Oxide Surge Arresters (MOSA) degrade in service under the influence of constant working voltage, internal partial discharges, non-uniform voltage distribution, possible moisture ingress and the occurrence of transient or sustained over-voltage; all of the above result in an increased value of resistive leakage current. The degradation of MOSA in service is diagnosed by using the third harmonic content of the resistive portion of the arrester leakage current as an indicator. This thesis investigates errors in the diagnostic indicator due to the hysteresis character of the valve elements, the effect of stray capacitance and voltage harmonics. A method is proposed to assess the discerning ability of the diagnostic indicator due to hysteresis character of the valve elements and voltage harmonics. Effect of stray capacitance on the diagnostic indicator is investigated by carrying out simulations in PSCADTM using a model developed in the present work. The simulation results show that the diagnostic indicator is influenced by hysteresis character of the valve elements, stray capacitance and voltage harmonics. The diagnostic indicator is affected by the location of the aged valve element in the arrester column.