The Effect of Polarity Reversals on the Dielectric Strength of Oil-Impregnated Paper Insulation for H.V.d.c. Cables

A laboratory investigation involving tests on models and on full-sized cables has been carried out into the behaviour of oil-impregnated paper insulation under the simultaneous action of a pre-stressing direct voltage and a voltage surge of either the same or opposite polarity. It has been found that the dielectric strength does not depend on the level or polarity of the pre-stressing direct voltage or on the duration of the surge, except when a series of reversed polarity surges is applied at intervals of a few seconds, progressively increasing the level until breakdown occurs. In this typical case reductions from 130 MV/m at zero d.c. to 45 MV/m at 100 MV/m d.c. have been observed and explained in terms of bubble formation and growth in the paper tape "butt-spaces" adjacent to the metallic conductors. This effect is entirely eliminated by increasing the oil pressure to suppress bubble formation.