Investigation on the reduced critical breakdown field of hot CO2 gas and CO2-based mixtures

Sulfur hexafluoride (SF₆) gas is widely used in high-voltage circuit breakers (HVCB), but due to its high global warming potential, substitutes are being sought. CO₂ has been investigated as a candidate based on its arc interruption performance and has been used in some practical products. However, at room temperature its dielectric properties is inferior to both SF₆ and air. The goal of this work is to investigate the reduced critical breakdown fields of the hot CO₂ and CO₂-based mixtures, i.e., CO₂, CO₂/O₂, CO₂/CH₄, CO₂/He and CO₂/N₂. In this paper, the dielectric properties of hot gases were investigated at elevated temperatures. The point that the fundamental data for this calculation, which are the electron scattering cross sections, should be carefully selected is stressed in this paper. The data used in present calculation were validated by the published experimental results and calculations. Then species compositions of hot gases were obtained based on Gibbs free energy minimization. The reduced critical electric field strength (E/N)cr of gases was derived from the calculated electron energy distribution function (EEDF) by solving the Boltzmann transport equation. The results show that the admixture of O₂ and CH₄ leads higher reduced critical breakdown field, which makes these two mixtures have the potential to further improve the dielectric properties of the CO₂ in the practical use.