Thermodynamic and Electrical Properties of SF₆-N₂ Thermal Plasmas

Abstract

High-voltage circuit breakers (HVCB) generally act in pure SF₆ under pressure of a few bars. the working of this kind of circuit-breaker consists first to create an electric arc due to the separation of the contacts and second to rapidly extinguish this arc by a high blowing during the current-zero phase of the alternative current. for a given gas, the rate of transition from the electrically conducting medium (the arc plasma) to a dielectric medium (the cold non-ionised gas) is called the interruption capability. SF₆ has a very high interruption capability because of its properties as a thermal plasma (thermal conductivity, radiation losses) and of its dielectric properties as an electro-negative gas. Nevertheless under severe weather conditions (external temperature of the order of-30°C) SF₆ may partially liquefy and thus may loss a part of its interrupting properties. for this reason SF₆-containing mixtures were considered for replacing pure SF₆ in HVCB. Among them SF₆-N₂mixtures are attractive because nitrogen plasmas have a high thermal conductivity. Recently, environmental and safety considerations related to green house effect and to by-product toxicity tend also to decrease the use of SF₆ in gas insulated systems and in HVCB.