CORRECTION FACTORS FOR POSITIVE DC VOLTAGES

Direct-current voltages are used mainly to test equipment utilized in HV dc transmission systems, as an alternative to ac for testing cables , capacitors, insulations in which internal discharges may lead to deg radation of the insulation under ac testing conditions, in investigati ons in discharge physics and dielectric behavior. For atmospheric air the voltage measurements under any conditions are related to the stand ard conditions employed the IEC-60 standard. It is demonstrated that t he IEC correction procedure is not adequate for dc voltage measurement s at high altitude; for example for rod-plane gaps there is a differen ce > 10% between experimental data obtained at 2240 m above sea level and those corrected with the IEC-60 procedure. This is mostly a conseq uence of the fact that the physical model employed, which is based on the field of propagation of streamers, fails to account for the combin ed effect of humidity and altitude. It is surmised that the propagatio n held of the prebreakdown streamers varies linearly with the air-dens ity. The experimental rod-plane results obtained by the authors are be tter modeled by an alternative semi-empirical model.