Processes leading to discharges in oil-impregnated paper

Stress-induced gas formation in oil-impregnated paper, and therefore the onset of ‘growing’ discharges, can be due to residual traces of moisture in the paper or to local breakdown of the oil, the latter process becoming predominant when the paper is very dry. A study of breakdown processes in viscous liquids has shown that electrons and ions can cause local vaporization of the liquid, and breakdown proceeds through ionization within the vapour.