Gaseous discharge phenomena in high-voltage d.c. cable dielectrics

Considerations of the short-time electric strength and of the conductivity/temperature/stress relationships show that both impregnated paper and polythene are likely to be satisfactory dielectrics for use in h.v. d.c. cables. From a.c. experience, however, it is known that polythene in particular is very vulnerable to damage caused by discharges in gas-filled cavities, and an assessment of the importance of this mode of deterioration under d.c. conditions is therefore necessary. Calculations have been made of the repetition rate of discharges in a gas-filled cavity in a dielectric subjected to a d.c. stress, since this is clearly a decisive factor determining the rate of deterioration. The repetition rate is shown to be a maximum when the surface conductivity of the dielectric is zero. Direct measurements of the repetition rate of discharges are described, and the measured rates are shown to be consistent with the calculated maximum rates. The effects of ripple voltages and polarity reversals are considered. From the predicted discharge repetition rates and from the results of accelerated life tests on samples containing cavities, a rough estimate of the effect of discharges on the probable life of an h.v. d.c. cable dielectric can be made. Polythene was selected as the example for study, on account of its known vulnerability to discharges, and it is shown that, with this material, discharge damage is unlikely to be a serious problem under d.c. conditions, provided that ripple voltages are not excessive and that very frequent reversals of polarity can be avoided.