Leakage current based online condition monitoring of metal oxide surge arresters with harmonics compensation

Abstract

Metal oxide surge arresters (MOSA) are used as overvoltage limiting devices to protect the transmission and distribution system from high voltage surges caused by severe lightning and switching operations. Their health and hence condition monitoring are vital to ensure the reliability of the power system. Any deterioration of the non-linear properties of a MOSA is known to cause a corresponding increase in the resistive leakage current, especially the odd harmonic components of the resistive current. The arrester resistive leakage current level is a known and reliable indicator of MOSA deterioration, provided it can be successfully extracted from the arrester total leakage current. The currently available resistive leakage current extraction techniques are divided into either system voltage independent or system voltage dependent. Even though the extraction is made simpler by having a simultaneous arrester voltage signal, the voltage measurement is either difficult or prohibitive. Several previously proposed voltage independent extraction techniques suffer from inaccuracies. In addition, the presence of harmonics in the supply voltage also affects the accuracy of the extraction techniques. The research aims to improve the accuracy of the MOSA resistive leakage current extraction by means of a modified shifted current method-circuit based method (MSCM-CBM) hybrid technique, a thumb-rule based technique for compensating the effects of voltage’s harmonics, and a condition monitoring technique using the resistive fifth harmonic leakage current, IR5th, as an ageing indicator of MOSA. The MSCM-CBM hybrid method was developed and tested on Simulink software and then experimentally applied on 120 kV rated MOSAs. Results show that the resistive current extracted by the proposed hybrid method is 3.2 % more accurate than that for the modified shifted current method. The execution of the proposed thumb-rule technique is based on the determination of the total harmonic distortion of both the system voltage and the MOSA total leakage current. The proposed thumb-rule technique is at least 10.5 % more accurate than the existing compensation techniques. The condition monitoring technique of MOSA based on the resistive fifth harmonic is found to be less sensitive towards the voltage harmonics as compared to that using the third harmonic current. Results show that the proposed fifth harmonic-based ageing indicator of MOSA is 1.6 % more accurate than that for the existing third harmonic-based indicator. It is also found that the trend of IR5th variation with respect to the applied arrester terminal voltage and arrester ageing is similar to that of IR3rd. The performance of the proposed IR5th based condition monitoring of MOSA was also validated using the maximum temperature and power loss measurements. The improved accuracies provided by the MSCM-CBM based hybrid method, rule of thumb-based compensation and IR5th based condition monitoring may provide a solution for better and more efficient MOSA condition monitoring.