Fundamental properties of high-temperature SF₆ mixed with CO₂ as a replacement for SF₆ in high-voltage circuit breakers

Sulfur hexafluoride (SF₆) gas, used widely in high-voltage circuit breakers, can produce a remarkable greenhouse effect if released into the atmosphere. Fundamental properties of SF₆ mixed with CO ₂ at high temperatures are presented in this paper, considering their reduction of adverse impact on the environment as a replacement for pure SF₆ in high-voltage circuit breakers: to the knowledge of the authors, related data have not been reported in the literature. The species composition and thermodynamic properties (mass density, enthalpy and specific heat at constant pressure) are determined by the method of Gibbs free energy minimization, using standard thermodynamic tables. The transport properties, including viscosity, thermal conductivity and electrical conductivity, are evaluated by using the Chapman-Enskog method expanded up to the third-order approximation (second order for viscosity). Particular attention is paid to the establishment of a collision integral database related to a reacting species system containing basic elements of carbon, oxygen and sulfur as well as fluoride by the use of recent cross sections or interaction potentials available in the literature. The calculations, which assume a local thermodynamic equilibrium, are performed in the temperature range 300 to 30 000K for different pressures between 0.01 and 1.6MPa. The results obtained are compared to those of previously published studies, and the reasons for discrepancies are analysed. An evaluation of the current interruption performance by adding CO₂ to SF₆ is discussed from a microscopic point of view. The results provide reliable reference data for the simulation of switching arcs in high-voltage circuit breakers in SF₆-CO₂ mixtures.