Computational investigation of arc behavior in an auto-expansion circuit breaker contaminated by ablated nozzle vapor

PC-based arc modeling taking account of the effects of nozzle ablation and the mixing process of PTFE vapor with SF₆ has been carried out for a full-scale auto-expansion circuit breaker with a moving contact. An equation to calculate the PTFE concentration in the mixture is introduced together with overall mass, momentum, and energy conservation equations for the mixture. The thermodynamic and transport properties of SF₆-PTFE vapor mixture calculated by the transport theory are given. An empirical relationship between radiation reabsorption factor and the relevant arcing parameters (nozzle size and instantaneous arcing current) has been employed to make predictions. The features of temperature, pressure, PTFE mass concentration, and flow fields during the whole arcing period are discussed. It is shown that the energy brought into the expansion volume by the hot PTFE vapor is mainly responsible for the pressure buildup. The propagation of the pressure wave in the expansion volume is responsible for the pressure peaks in the second loop of arcing. The predicted arc voltage and pressure variation are compared with the test results and with those predicted using the thermodynamic and transport properties of pure SF ₆. Results show that the use of the properties of SF₆ -PTFE vapor mixture substantially improves the agreement between the predicted and measured pressure rise in the expansion volume