Partial discharge oscillograms and pulse distribution in transformer oil-pressboard insulation

Partial discharges cause degradation of insulation systems and a study of their behaviour can give useful information regarding the state of the insulation system. Partial discharge tests are therefore, carried out as quality checks for the insulation system. A model transformer insulation system consisting of pressboard and transformer oil with uniform field electrode arrangement was tested for its behaviour under partial discharges. The system was allowed to age until failure of the sample occurred in the form of a puncture. Records of partial discharge distributions were acquired along with oscillograms. Oscillograms help in the traceability of the measurement when compared with distributions. A comparative study of the effect of amplifier setting and the pulse repetition rate on the partial discharge distributions and the oscillograms acquired during ageing of the sample has been presented. It is shown that by appropriate threshold on the distribution, distinctive features can be extracted.