The influence of metal vapors resulting from electrode evaporation in a thermal puffer-type circuit breaker

Abstract

The present work is a computational investigation of thermal plasmas in SF 6 switchgear, which burns in a mixture of working gas and PTFE and metal vapors resulting from nozzle ablation and electrode erosion, respectively. In order to solve the concentration of PTFE and electrode vapors conservation equations for the PTFE vapor and the metal vapor are solved together with the governing equations for mass, momentum, and energy of the gas mixture under the assumptions of local thermodynamic equilibrium (LTE) and local chemical equilibrium (LCE). The influence of metal vapor due to electrode evaporation on the arc behavior in SF 6 switchgear can be explained with some important parameters indicating the characteristics of SF 6 arc plasma. First, temperature in arc column is considerably different from that of the case without contact evaporation. The contamination by metal vapors broadens the arc radius and lowers the core temperature when compared to the case without evaporation. Second, the concentration of PTFE vapor from nozzle surfaces can more easily move to the expansion volume during high current period due to the existence of metal vapors near electrodes. Finally, the electrical conductivity in the arcing zone can be higher and may negatively affect the interruption limit of switchgears due to the increasing rate of breakdown by the metal vapors between the electrodes.