Chemically non-equilibrium model of decaying N2 arcs in a model circuit breaker

Nitrogen gas has been investigated as one of the candidate substitutes for SF₆ in a high-voltage circuit breaker (HVCB) and also in a low-voltage interrupter. In this paper, a chemically non-equilibrium model was established to investigate N₂ arc plasmas in the decaying phase during current interruption in a model circuit breaker. Unlike the conventional model assuming local thermodynamic equilibrium, i.e. both chemical equilibrium and thermal equilibrium, in this work a chemically non-equilibrium model was developed for N₂ arc plasmas. Thermal non-equilibrium effects were neglected, meaning a one-temperature model was adopted. The developed model took into account 5 species such as N₂, N, N₂⁺, N⁺ and e^-, and 22 chemical reactions including electron impact ionizations, heavy particles impact dissociations and their backward reactions. Temperature dependent reaction rates were used for all considered reactions. The species composition in N₂ arc plasma was calculated by solving the mass conservation equation of each species considering diffusion, convection and reaction effects. Then the influence of the chemically non-equilibrium composition on the arc behavior was calculated by updating the thermodynamic and transport properties at each iterative step. Finally, for the decaying N₂ arc plasma under a free recovery phase, the time evolutions were derived in the profiles of the temperature and the number densities for each species. The results in this work were compared with the calculated results based on the chemical equilibrium assumption.