Partial discharge behaviour in oil-impregnated insulation

The insulation within an oil-insulated power transformer consists of mainly organic materials including mineral oil, cellulose paper and pressboard layers. The presence of moisture, coupled with high operating temperature in the insulation reduces the dielectric strength of the cellulose paper, the pressboard and the transformer oil. This paper reports results of an experimental investigation of partial discharge behaviour in a laboratory model of transformer oil-impregnated insulation at high temperature and with different levels of moisture contamination of both the oil and paper. The experiments were performed in the laboratory with an environment and configuration similar to practical operating conditions in a transformer. The partial discharge patterns together with inception and extinction data were recorded and correlated with moisture and temperature effects and age of insulation. The thermal generation of gas bubbles and, moisture levels and their associated discharge characteristics were also recorded and analysed. The effect of aged and new transformer oil on partial discharge activity patterns was also investigated.