Insulation characteristics of epoxy insulator with internal void-shaped micro-defects

Presently, an increasing number of gas insulated switchgear (GIS) has been operated for longer than 30 years, which is generally considered their service life. The degradation in solid insulators is considered to be one of the key factors determining the service life of such highly aged GIS in terms of insulation. Under normal circumstances, GIS insulators have outstanding electrical performance and insulation failure is considered unlikely within the design life of 30 years or so, provided the units conform and have passed the partial discharge (PD) test. However, assuming operation beyond 30 years, the insulation characteristics must be clarified in case ultra-microscopic defects undetectable via the PD test are present inside insulators. Accordingly, in the preceding study, micro-defect models in three types of shape (void, crack, and delamination) with an apparent PD of 1 pC in an actual 550 kV-GIS spacer, were designed and produced. Of those three, the present study covered void-shaped defects and obtained breakdown voltage-time (V-t) characteristics through an electric field (EF) acceleration test. These V-t characteristics were extrapolated to evaluate the breakdown risk in 30 to 50 years operation, which is considered the design life of GIS. As a result, void-shaped defects of about 1 pC were evaluated as highly unlikely to cause any problem according to the 50% value of the V-t characteristics. Given the relatively large dispersion among the breakdown times in the EF acceleration test results, a study was subsequently conducted with this dispersion taken into account. The probability of breakdown within 50 years was evaluated at less than 0.1%, assuming an operating EF of 4 kVrms/mm. Furthermore, when considering the location parameter that does not result in breakdown within a certain period, the probability of breakdown was considered to be much smaller. In fact, since the declining gradient of the V-t characteristics is considered smaller in the long time region, evaluation by extrapolating the V-t characteristics in the present study is considered sufficiently safe. The Vt characteristics obtained have made it possible to evaluate the period to breakdown and the breakdown risk where ultra-microscopic voids are present.