Study of Shock Wave Development in Cold Flow of Gas Circuit Breaker for Improving Capacitive Current Switching

Capacitive current switching performance of SF₆ circuit breaker is determined by combined action of gas flow and electric field in interruption chamber. Computational fluid dynamics method is used to study the development of shock waves inside a SF₆ circuit breaker under the condition of capacitive current switching. It was found that a shock wave exists and travels with the pin contact during the contact opening. Existence of shock waves reduces the local gas density around contact head, while the contact head is also stressed by high electric field at the same time. If the region of low gas density coincides with the region with high electric strength, local dielectric performance will be significantly reduced. Using E/ρ as the dielectric performance index, when the local value of E/ρ exceeds the gas breakdown criterion, it will result in restrike to the capacitive current switching. The causes of high E/ρ are studied by analyzing the shock waves during the sequential periods of contact travel. Since the gap between the pin contact and nozzle throat strongly affects the strength of shock wave, the behavior of gap is studied with the calculation of E/ρ to find its influence on capacitive current switching.