The deterioration and breakdown of solid dielectrics by internal discharges at cryogenic temperatures

An investigation into internal discharges in artificial air-filled cavities in impregnated-paper, polythene and polypropylene ueing a multi-channel, pulse-height analyser is described. On lowering the temperature of test samples from 343 K down to that of liquid nitrogen the discharge repetition rate and discharge magnitude under alternating-and direct-voltages are greatly reduced. The electric strength with power-frequency-alternating-, direct-and lightning surge-voltages with polarity reversal, is appreciably increased when the temperature is lowered, the improvement being particularly marked with direct voltages. Operating the electrical apparatus at cryogenic temperatures increases the working voltage, the electric strength and the life time of the electrical insulation.