Statistical analysis of in service failed epoxy resin bushings in a 50 kV switchgear assembly

In the 1960´s up to the 1980´s a very compact oil-filled switchgear assembly was installed in the 50 kV networks of different utilities in the Netherlands. Per bay the switchgear consists of different oil-filled compartments where the high voltage parts are supported and connected from one compartment to the other by epoxy resin bushings. The population of this type of bushing is relative large. The number of failures however is low, and most of them occurred in the early nineties. This initiated a concern about the possibility of an oncoming re-investment wave due to replacement of this type of circuit breaker. In this contribution a statistical analysis of failure data was performed for failure prediction in the future. From the statistical analysis it can be concluded that the bushings are in the wear-out life period of the bathtub curve. However, the relative low value of the shape parameter indicates that the ageing process is in its early stage. If the failure rate function obtained from the analysis is compared with the population in service, than the total population can be stated as young. With the analysis an estimate is made of the expected number of failures in coming years, which can help the asset manager to determine if action has to be taken to meet the business policy