New approach to breakdown voltage estimation after interruption of capacitive currents

The interruption of capacitive currents is a very common switching case, unlike the breaking of short-circuit currents. The current involved is quite small and therefore easy to interrupt. However, a recovery voltage higher than 2 per unit appears across the contacts half a cycle after current zero. This circumstance inevitably increases the risk of restrikes, which may have rather undesirable effects, such as voltage escalation. Combined electric field and flow field simulations are widely used as a tool for evaluation of the dielectric performance of high voltage SF6 circuit breakers in these conditions. The computational fluid dynamics (CFD) simulation starts from the completely closed contact positions and, therefore, it takes CPU time of several hours running a workstation. In this paper, in order to shorten the calculation time to several minutes, a new numerical method for calculating the coupled fields has been developed. The new method uses simulation software for HV SF6 circuit breakers based on an integral-physical enthalpy flow arc model to obtain boundary conditions for CFD simulation. This way, a higher precision CFD simulation combined with electric field calculations can start at any given moment during recovery voltage after the current reaches zero and cover only a short interval of time proximate to an increased risk of restrikes. The new approach additionally provides a full correlation between all design parameters of the circuit breaker (including its operating mechanism) and estimated breakdown voltage during the interruption of capacitive currents.