Experimental and theoretical study of decay and post-arc phases of a SF6 transfer arc in DC hybrid breaking

Abstract

During DC hybrid breaking, the transfer arc temperature decays quickly due to fast current commutation from the mechanical switch to the parallel connected branch, which may lead to deviation from thermal equilibrium. In this study, numerical investigation on a two-temperature calculation for SF6 transfer arc has been performed in order to study the arc’s behavior under the effect of thermal non-equilibrium. The free decay and post-arc phase have been studied. This model allows us to calculate electron and heavy-species temperatures, gas velocity, and electron number density. In addition, various energy exchange mechanisms including radiation, ohmic heating, and elastic collision energy are discussed. It has been predicted that the departure from thermal equilibrium appears near the arc edge and electrodes due to insufficient collision energy exchange between electrons and heavy species. Particularly the formation of a circular velocity vortex makes the gas temperature in the vortex zone exceed the arc core temperature at zero current, which does not favor the residual plasma withstanding the recovery voltage at post arc. After carrying out a simulated hybrid breaking experiment, the dynamic motion process of the transfer arc and post-arc current were determined experimentally and compared to theoretical results obtained from a two-temperature model. This comparison validated the model and showed that the presence of a vortex high-temperature zone is more prone to cause the breakdown.