Investigating the Effects of Altitude (Air Density) on the HVDC Breakdown Voltage of Rod-Plane Air Gaps
This paper is on a study of altitude correction factors on short air gap DC breakdown voltage. Tests were conducted on small rod-plane air gaps in a pressure vessel. The air gaps used varied between 100 mm and 300 mm in length and pressure was drawn in the pressure vessel to give relative air densities as low as 0.53. The pressure was drawn from an ambient pressure of about 835 mbar. The range of pressures used made it possible to simulate relative air densities corresponding to altitudes of 1700 m to 4500 m. The results obtained showed a decrease in breakdown voltage as pressure decreased, but only up to a threshold point beyond which an increase in breakdown voltage at very low pressures was observed. The results were corrected using IEC 60060-1 and were also compared to predicted breakdown voltages proposed by Calva et al. The corrected results according to the IEC 60060-1 standard did not resemble the trend of a decrease in breakdown voltage as relative air density decreases. When compared to the predicted breakdown voltages according to the method by Calva et al, the experimentally obtained results did relate to the predicted breakdown voltages. Even though the predicted flashover voltages were higher, the trend in the breakdown voltage agreed with the ones obtained using Calva et al but only until the pressure threshold was reached. A potential exists for the method proposed by Calva to be improved for small air gaps.