Measurement of partial discharge current waveform in liquid nitrogen immersed butt gap

This paper describes the relation between a change of partial discharge (PD) pulse current and a appearance of bubbling in liquid nitrogen (LN2) immersed butt gap. AC voltage of 5-7.7 kV were applied on a butt gap model electrode system assembled with cylindrical aluminium ground electrode covered with two polyethylene terephthalate (PET) films facing edge to edge and outer PET film whose outside was coated with evaporated metal as a high voltage electrode. PD detection was carried out using a wide band current detection system that has a frequency range up to 250 MHz. The pulse width showed significant differences depending on the polarity of pulse current when the applied voltage was so high that bubbling appeared. When the applied voltage was negative polarity, characteristic pulse currents often appeared whose pulse width was large like as PDs appearing in gaseous nitrogen. Furthermore a few pulse currents appeared which had multi peaks in their current shape. These results suggest that when PDs progress through LN2 from the aluminium electrode to the PET film surface, bubbles appear and fill the butt gap and PDs change into gaseous PDs. Based on this result, the partial discharge measurement with high frequency range is suggested to be effective for the monitoring of discharge mode.