The ageing in metal oxide surge arresters (MOSAs) is monitored by measuring their leakage current parameters. Particularly, the third harmonic component of resistive leakage current is directly correlated with the ageing and degradation of the arrester. However, the third harmonic of resistive current (I_R3rd) is also affected by the third harmonic component in the grid's voltage. Therefore, I_R3rd based monitoring may lead to an inaccurate condition assessment of MOSA when the supply voltage comprises third harmonic content. Hence, this paper introduces the fifth harmonic of resistive current (I_R5th) as a new ageing indicator for MOSA. This study has been done experimentally on five samples of aged polymeric MOSAs rated at 120kV. Results have shown that the I_R5th exhibits similar sensitivity towards ageing as compared to the I_R3rd. Additionally, it has been found that the variation in I_R5th with the fifth harmonic voltage is smaller than the variation in I_R3rd with the third harmonic voltage. Therefore, the following technique can support the online condition monitoring of MOSAs in a more realistic way than the existing third harmonic current measurement technique.

通过测量漏电流参数来监测金属氧化物避雷器 (MOSA) 的老化情况。尤其是阻性漏电流的三次谐波分量与避雷器的老化、劣化直接相关。然而，电阻电流的三次谐波 (I _R3rd ) 也受电网电压中三次谐波分量的影响。因此，当电源电压包含三次谐波含量时，基于I _R3rd的监测可能会导致 MOSA 的状态评估不准确。因此，本文介绍了电阻电流的五次谐波（I _R5th）作为MOSA 的新老化指标。这项研究是在五个额定电压为 120 kV 的老化聚合物 MOSA 样品上进行的。结果表明，I与 I _R3rd相比，_R5th表现出类似的老化敏感性。此外，已经发现I _R5th随五次谐波电压的变化小于I _R3rd随三次谐波电压的变化。因此，以下技术可以比现有的三次谐波电流测量技术更现实地支持 MOSA 的在线状态监测。