Numerical modeling of SF6 thermal plasma generated during the switching process

Computational fluid dynamics (CFD) has been extended to the fields of the electric and electronics as well as the mechanics because of increasing of computer power and developing of numerical schemes. The thermal plasma inevitably established by separating the contacts makes a complex interaction with surroundings, it is not easy to analyze the phenomena neither experimentally nor theoretically. Nowadays, the operating energy used to move the contacts is required as low as possible, because it is the key factor to compact the integral system and to increase the reliability. Therefore, it is indispensable to analyze the thermal plasma conditions of circuit breakers using CFD. The thermal plasma characteristics could be obtained with computing flow field and electromagnetic field simultaneously. In this paper, we have simulated the switching process of the thermal expansion SF6 gas circuit breaker (GCB) that is combination of two interruption techniques, the self-expansion by the arc itself and the arc rotation by coils. In order to calculate the governing equations describing the dominant physical processes accurately, the commercial CFD code, PHOENICS, which is customized to allow the inclusion of arc plasma modeling by Fortran subroutines externally has been used. Through this work, we have found that the energy produced by the arc causes the pressure-rise in the chamber during high current period. And as the current falls near current zero, furthermore, the gas flows back from this region over the arc and should extinguish the arc just after current zero. The results have been compared with various dimensions and verified with the measured arc voltage.