Permittivity mismatch and its influence on ramp breakdown performance

Ramp breakdown tests remain a useful method for characterizing polymeric materials. In the current paper the effects of sample and oil permittivity and electrode radius on measured DC breakdown strength are explored, with the aid of finite element modeling. Localised field enhancement appears to explain apparent decreases in electrical breakdown strength whereas the shape parameter in the Weibull distribution appears to be related to the size of electrically stressed area relative to the electrode radius. Care needs to be taken to choose an oil of equal or higher permittivity than the sample to be tested and to use a fixed electrode geometry and a fixed voltage ramp rate to ensure such tests are valid for comparative studies.