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.