Calculation of the interruption capability of SF6-CF4 and SF6-C2F6 mixtures. I. Plasma properties

For a gas used in circuit breakers, the extinction capability corresponds roughly to the rate of evolution of the electrical resistance during the arc decay. This property is very prominent for SF ₆ which explains its wide use in high-voltage circuit breakers. But in severe winter conditions, this gas may partially liquefy, and some mixtures of SF₆ with other gases are considered. The present work deals with a calculation of the extinction capability of SF₆-CF₄ and SF₆-C₂ F₆ mixtures. This first paper consists of the study of the material functions of the plasma assumed to be in local thermal equilibrium (LTE). The equilibrium composition and the thermodynamic properties have been calculated from the partition functions. The transport coefficients have been computed by the method of Chapman-Enskog and the net emission coefficient obtained assuming isothermal and homogeneous plasma. CF₄ and C₂F₆ present dissociation phenomena analogous to those of SF₆ which leads to similar variations of mass density, specific heat, and thermal conductivity. The electrical conductivities are practically identical for all the mixtures. The net emission coefficient is enhanced by the presence of carbon in spite of a strong absorption of the resonance lines