The Deterioration and Breakdown of Solid Dielectrics by Internal Discharges at Cryogenic Temperatures

An investigation of internal discharge phenomena in artificial air-filled cavities in impregnated-paper, polyethylene, and polypropylene over a temperature range from 90 to 343 K is described. The technique used for the detection and measurement of internal discharges has a high resolution and sensitivity, and incorporates a multi-channel, pulse-height analyzer. It has been demonstrated that when the temperature is lowered, the repetition rate and the magnitude of internal discharges under direct voltages are greatly reduced. In addition, considerable improvement in the electric strength of the insulation is obtained with alternating, surge and direct-voltages, and with voltages with polarity reversal. The improvement measured with direct voltages is particularly marked. Higher working stresses might therefore be used for the insulation of electrical apparatus operating at cryogenic temperatures.