Partial discharge pulse epoch distribution in dielectric liquids and impregnated papers under AC conditions

A comparative study has been carried out on the partial discharge pulse epoch distribution characteristics of a dielectric liquid and of a bushing having a paper insulation impregnated with the same liquid. At the partial discharge inception voltage, the discharge pulse epochs in the dielectric liquid were found to be concentrated at and close to the applied voltage peaks, in contrast, the discharge epochs in the impregnated papers appeared at and in the vicinity of the voltage zeros thereby implying a normal discharge process within cavity type occlusions. The dielectric liquid-impregnated-paper insulation in the bushing did not show any liquid phase like discharge pulses even at applied voltages substantially above its operating value.