Calculation of Nozzle Ablation During Arcing Period in an SF₆ Auto-Expansion Circuit Breaker

The nozzle ablation process is described as two phases of heat and ablation in the interruption for an SF₆ circuit breaker in this paper. Their mathematical models are established with the Fourier heat conduction differential equation respectively. The masses of nozzle ablation with different arc durations and arc currents are calculated through the model of the nozzle abla¬tion combined with an MHD (magneto-hydrodynamic) arc model. The time of the temperature rise on the inner surface of the nozzle under a given energy flux and of reaching the pyrolysis temperature under different energy fluxes is respectively analyzed. The relations between the mass of nozzle ablation and breaking current and arc duration are obtained. The result shows that the absorbing energy process before the nozzle ablation can be neglected under the condition of the energy flux entering into nozzle q>10⁹ W/m² . The ablation is the severest during the high-current phase and the ablation mass increases rapidly with the breaking current and with arc duration respectively.