Analysis of the dielectric strength of an SF6 circuit breaker

A computer model for predicting the time evolution of the dielectric strength of SF₆ following the interruption of a small capacitance current in a puffer-type circuit breaker is presented. This study was motivated by experimental results which show that the insulating properties of the gas in the transient regime are of the same order as in the static regime, and that this is the case, even if the pressure increases in the cylinder during opening. The proposed model for the breakdown simulation is based on a criterion related to a critical E/N in the gas. The electric field E is computed by a finite-difference method, while the number N associated with the density of the fluid is calculated by a time-accurate finite-volume Euler solver. The simulation considers the unsteady nature of the problem, including moving elements. The results explain important details of the influence of the compressible flow on the dielectric strength. It is shown that the formation of a shock wave in front of the electrode has a major influence on the transient breakdown voltage