Prediction of reduced critical electric field strength of hot dry air in the temperature range 300–5000 K at 0.1 MPa for medium-voltage switchgear

Abstract

The dielectric breakdown properties of dry air, which is widely used as the insulation gas in switchgears of electric distribution systems, are determined for dry-air switchgear analysis and design. In this study, a two-term Boltzmann analysis was applied to investigate the dielectric breakdown properties of dry air with increasing temperatures (300–5000 K) at atmospheric pressure (0.1 MPa). According to the changes in the component ratios of dry air, the collision cross sections of high-mole-fraction decomposition species were considered. In order to analyze the contribution of each dry-air decomposition species to dielectric properties, we compared the dielectric properties calculated with and without considering small amounts of the species, such as Ar and CO2, in actual dry air. The results show that reliable calculations can be obtained for dry air at atmospheric pressure and temperatures up to 5000 K only by considering species with mole fractions exceeding 10−2 in the composition. Based on the calculations, it is observed that the reduced critical dielectric strength of dry air does not significantly change for temperatures up to 2000 K. This means that the dielectric properties of dry air show temperature independence over a wider temperature range than those of SF6.