Impulse dielectric strength characteristics of liquid-impregnated pressboard

The impulse dielectric strength of pressboard impregnated with transformer mineral oil and transformer Askarel and a variety of other liquids having differing chemical constitution is reported. It is emphasized that the decreasing dielectric strength with increasing multiple cycles of alternating current is responsible for the high impulse ratio of mineral oil-impregnated pressboard and that this effect is diminished greatly in the case of transformer Askarel, which has a low impulse ratio. Investigation of polarity effects with different electrodes has indicated that breakdown of the Askarel-impregnated pressboard is initiated in the liquid at the edge of the positive electrode.