Application of partial discharge monitoring to impulse damaged polymeric Insulation

Impulse damage may occur to insulators within electrical systems due to lightning strikes, switching operations or self-clearing flashovers. This damage may not result in immediate failure of the system; however, progressive degradation can occur due to repeated events. If it is possible to develop economical systems to monitor the effects of impulse degradation of solid insulation, it may be possible to prevent costly failures leading to unplanned plant outage. The progression of the damage may become apparent through on-line partial discharge (PD) monitoring using radio frequency (RF) techniques, which potentially allow non-contact on-line monitoring of discharge activity occurring in electrical insulators. To determine the feasibility of correlating changes in PD activity and phase-resolved patterns with material degradation on the basis of RF measurements, a laboratory study was carried out using both the IEC 60270 conventional technique and RF methods to simultaneously measure PD activity in samples of polymeric insulation that had been subjected to controlled impulse voltages. The IEC method was used to provide a calibrated reference for pulses recorded using the RF technique. Model cylindrical insulators of diameter 25 mm and height 13 mm have been subjected to repeated impulse-driven surface flashovers created using a Marx Generator. Following the impulses, the PD activity on the samples at electric field strengths equivalent to those that would be experienced in service has been monitored. PD activity was observed in two of the five damaged samples (Torlon and polypropylene - the two most heavily damaged samples in terms of appearance), regular RF signals could be observed from the polypropylene sample, while the Torlon sample produced some RF data, but this was insufficient for analysis. This shows that the RF technique is applicable to this situation but much work is still needed before it becomes a reliable method.