Characterisation of partial discharges in oil impregnated pressboard insulation systems

Partial discharge (PD) measurement and interpretation is one of the most useful diagnostic tools for quality assurance testing during the design, manufacture and life assessment of electrical equipment. Pressboard-oil insulation continues to be a major component in the insulation design of EHV transformers. As tests on practical systems are not always feasible due to cost and time factors, model systems tests are gaining importance. In the present work a model insulation system consisting of oil impregnated pressboard with uniform field electrode arrangement has been tested for its behaviour under PDs. It is shown that by defining a modified scale parameter for the Weibull distribution it is possible to take into account the effect of both smaller and larger magnitude PD pulses. The modified scale parameter serves as a good index for characterising PD behaviour in the oil impregnated pressboard insulation system studied.