Investigation on critical breakdown electric field of hot carbon dioxide for gas circuit breaker applications

Sulfur hexafluoride (SF₆) gas is widely used in high-voltage circuit breakers, 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. The hot gas in the circuit breaker after current zero, with a complicated species composition caused by the dissociation and many other reactions, will lead to the electrical breakdown, which is one of the major concerns in assessing the arc interruption performance. Despite this, little research has been reported on the dielectric strength of hot CO₂. In this paper, the dielectric properties of hot CO₂ related to the dielectric recovery phase of the circuit breaker were investigated in the temperature range from 300 to 4000 K and in the pressure range from 0.01 to 1.0 MPa. Under the assumptions of local thermodynamic equilibrium (LTE) and local chemical equilibrium (LCE), the equilibrium compositions of hot CO₂ were obtained based on Gibbs free energy minimization. The cross sectionsfor interactions between electrons and the species are presented. The critical reduced electric field strength of CO₂ was determined by balancing electron generation and loss. These were evaluated using the electron energy distribution function (EEDF) derived from the two-term Boltzmann transport equation. The result indicates that unlike SF₆ or air, in hot CO₂ the reduced critical electric field strength does not change monotonically with increasing heavy-particle temperature from 300 to 4000 K. CO₂ has a superior dielectric strength to pure SF₆ above 2500 K at 0.5 MPa, which means it has the potential to improve the interruption performance of the circuit breakers, while reducing the global warming effect. Good agreement was found with published experimental results and calculations for CO₂ at room temperature, and with previous calculations for hot CO₂.