Stochastic analysis of partial discharge patterns deriving from electrical tree measurements

The partial discharge height distributions resulting from tree-growth tests performed by needle-plane electrodes in specimens of EVA copolymer, at different levels of constant voltage and room temperature, are studied. The tests are performed on specimens displaying either an intimate contact of the needle with the copolymer or an artificial void at the needle tip. It is shown that the stochastic behavior of the pulse charge height distribution depends on test conditions, but generally fits well a two-parameter Weibull function when either partial discharges occur in the void or tree grows into the polymer in the absence of artificial void. A five-parameter Weibull function can be used to fit data which are obtained when the detected partial discharges are due to both tree growing into the polymer and partial discharge evolution into the artificial void. Estimation of the Weibull parameter values, as well as of other statistical indexes, can describe well the observed partial discharge phenomena and provide useful diagnostic tools