Influence of harmonics in system voltage on metal oxide surge arrester diagnostics

As a metal oxide surge arrester (MOSA) ages, its resistive leakage current increases with accompanying increase in the third harmonic component which is therefore widely used as an indicator of ageing. Since the system voltage may contain harmonics, principally the third, researchers have developed a technique to account for the third harmonic component of capacitive current in deriving the resistive third harmonic current from the leakage current. However, the influence of voltage harmonics on the magnitude of the resistive current has not been considered which aspect is explored in this paper. It is shown that even nominal amounts of the third, fifth, seventh and eleventh system voltage harmonics have considerable influence on the third harmonic component of arrester resistive current. In fact the third harmonic resistive current of an aged MOSA under sinusoidal voltage is comparable to that obtained with an unaged MOSA subjected to harmonic contaminated voltage.