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 degradation of the insulation under ac testing conditions, in investigations in discharge physics and dielectric behavior. For atmospheric air the voltage measurements under any conditions are related to the standard conditions employed the IEC-60 standard. It is demonstrated that the IEC correction procedure is not adequate for dc voltage measurements at high altitude; for example for rod-plane gaps there is a difference > 10% between experimental data obtained at 2240 m above sea level and those corrected with the IEC-60 procedure. This is mostly a consequence of the fact that the physical model employed, which is based on the field of propagation of streamers, fails to account for the combined effect of humidity and altitude. It is surmised that the propagation field of the prebreakdown streamers varies linearly with the air-density. The experimental rod-plane results obtained by the authors are better modeled by an alternative semi-empirical model.