PD signatures of wedge type discharges in transformer insulation

Discharge signatures have been studied in a combined transformer oil/impregnated paper insulation system subjected to AC high voltage. A semi-homogeneous sphere-plane electrode configuration was used to simulate wedge discharges in a power transformer. The moisture level of the impregnated paper was varied between 0.3% and 9.6%. At the highest moisture level no discharges were observed before breakdown. Below a threshold near 100 pC the discharges were almost harmless to the insulation. The variation of statistical parameters with experimental conditions was thoroughly investigated. It was found that the statistical moments skewness and kurtosis were very unstable and consequently not suitable for automatic pattern recognition.