PD pulse burst characteristics of transformer oils

Partial Discharge (PD) characteristics of four typical transformer oils, having respective viscosities of 3.5, 9.1, 13.0, 18.3 cSt at 40°C, were examined under ac conditions, using a needle-to-plane electrode system. Wide and narrow bandwidth measurements were carried out to determine simultaneously the apparent charge transfers associated with the overall PD pulse bursts in the oil as well as that of the individual discrete PD pulses within the PD pulse bursts themselves. Based on the apparent charge transfer value, the size or depth of the PD initiating microcavities in the field direction within the oils, producing the first detected discrete PD pulse, were estimated to be in the order of 2 μm. The recurrence rate of the PD pulse bursts was found to increase with voltage above the PD inception voltage; this increase was accompanied by both an increase in number of discrete PD pulses and their amplitude within the pulse burst itself. These increases were reflected by a substantial rise in the apparent charge transfer per PD pulse burst. The charge transfer levels of the PD pulse bursts associated with the highly pressurized gas microcavities were of the same order of magnitude as those produced within the normally much larger macroscopic cavities that exist under atmospheric pressure in conventional oil-impregnated insulating systems of power transformers.