Failure analysis of composite dielectric of power capacitors used in distribution systems

This paper describes the study of the reliability of capacitor units installed and operated in a distribution system during the period of 1980 through 1990. Failures of capacitor units in distribution substations can be detrimental to the supply of reliable power to consumers. To enhance utility reliability, failure analysis and rates, failure origin and physical damage causes were studied for these capacitor units. Two approaches, statistical and physical, were utilized in this study. In the statistical area, failure modes, reliability levels and failure causes are analyzed. The physical study mainly deals with the mechanism of deterioration of the composite dielectric. This paper models the capacitor's failure mathematical mode and calculates the capacitor failure rate. The results of the study on 2912 capacitor banks including 8736 capacitors installed at 153 distribution substations showed that the failure mode of capacitor units may be represented by a Weibull distribution and that each capacitor manufacturer has a different failure rate. Analysis showed that partial discharge properties are a critical indicator for capacitor failure mechanisms. Useful conclusions are presented both for power system operators and manufacturers. The methodology used in this study also applies to other equipment in the distribution system such as oil switches, transformers and insulators.