To protect expensive assets at substations,– installing metal oxide (MO) surge arresters in each of HV transformer bays .This program saw virtually all the 110 kV, 220 kV and 400 kV transformer bays equipped with arresters .

While the basic role of surge arresters is to protect electrical equipment against overvoltages, most of their service lives sees them function basically as insulators. Because of their unique structure (i.e. the MO discs have non-linear voltage/current characteristics and are connected in series), there is constant flow of current through the ground wire. Any deterioration in insulation characteristics will therefore be accompanied by increased total leakage current – especially its resistive component – at the applicable network voltage and ambient temperature. As a result, most diagnostic methods to monitor arrester condition are based on measurement of this parameter.

When sinusoidal voltage is applied to an arrester, total leakage current contains higher harmonics due to the non-linear resistance of the MO discs. The amount of these harmonics depends on the amplitude of resistive leakage current and the degree of non-linearity in the resistance of the varistors (also dependent on system voltage and temperature). This explains why the harmonic content of leakage current serves as an effective indicator of level of resistive leakage current. Apart from the non-linear resistance of MO arresters, total content of leakage current harmonics also includes that originating from the transmission network itself. Capacitive leakage current harmonics generated by system voltage are of the same order of magnitude as leakage current harmonics generated by the non-linear resistance of arresters. For example, typical peak values of capacitive leakage current are about 1 mA and 1% of the 3rd harmonics contained in network voltage produce a peak capacitive current of about 30 mA. Therefore, it is necessary to compensate for (i.e. eliminate) harmonics in leakage current generated by the network voltage to avoid risk of large errors when testing arresters.

Analysis of resistive leakage current data is made by comparing the results from testing arresters of the same type, same year of production and similar serial number. Moreover, since ageing of the MO varistors leads to higher resistive leakage current, it is necessary to perform testing at regular intervals to record such increases. The specific relationship between resistive leakage current 3rd harmonic and total resistive leakage current in LCM testing equipment is determined for the arresters produced by one manufacturer. Therefore, measurements are only valid for that particular supplier’s arresters. However, LCM testing equipment can also be used for testing arresters manufactured by other suppliers if one takes into account that results will not be the actual value of resistive leakage. Fortunately, this does not have much impact on the analysis assuming measurements are made at equal time intervals, while also monitoring any increase in resistive leakage current between measurements. Using this approach, results can be classified into analysis of measurements performed on arresters from one supplier and a similar analysis conducted on arresters from other suppliers.

During routine testing of two MO arresters installed in 110 kV transformer bays at different substations, it was discovered that resistive leakage current for each was over 10 times higher than the expected value. In both cases, in addition to testing these arresters, tests were also conducted on the current transformers (CT) by measuring insulation resistance, capacity and dielectric dissipation factor (tgδ). There was also analysis of dissolved gases in oil samples. Based on this investigation, significant weakening in their insulation was discovered and both CTs were taken out of service. Consequently, it was decided to also test the circuit breakers (single pole driving mechanism type) in these same 110 kV transformer bays. In the first case, testing showed that during the closing operation, one pole lagged the first by 260 ms while in the second case one pole was lagging the first by 140 ms. This happened to be the same pole where the increase in resistive leakage current was measured. Based on this, it was concluded that the reason for the increased resistive leakage current and the related weakening in insulation properties of both the arresters and the CTs was the appearance of overvoltages caused by the breaker poles lagging during switching operations on the unloaded transformer.

https://www.inmr.com/utility-program-to-monitor-condition-of-station-arresters/