Advanced optical PD diagnostics of GIS at onsite testing and service including transient overvoltages

VFTO generated by switching of disconnectors and circuit breakers. Therefore, further investigations were initiated, regarding a thorough root cause analysis and searching an adequate diagnostic method, e.g. optical PD measurements, also applicable under transient voltage stress.

The outcome of the route cause analysis was that the failures were caused by field distortions, due to fixed protrusions or discharges at micro-defects, triggering a breakdown by the VFTO.

Based on the experience gained with the optical PD (OPD) method in the past, this method was further developed and applied during field tests as well as lab investigations. During field tests,

OPD signals were acquired under VFTO and LI voltage stress, which were partly already detected by previous OPD measurements at AC voltage, but not by UHF measurements. During lab investigations the performance of the OPD system at transient overvoltages was studied in more detail, in particular the development of optical discharge signals at VFT as well as LI voltages. The investigations confirmed the ability of the OPD system to detect field distortions particularly critical under VFT voltage stress and not detectable by the UHF method.

Furthermore, the paper shows that OPD diagnostic can provide information in addition to the conventional PD diagnostic methods, as the OPD method can help to detect critical PD defects due to its sensitivity and ability working even under transient voltages.