Investigation of pre-strike characteristics in asynchronous closing of double-break vacuum circuit breakers based on field emission current

To investigate the influence of asynchronous closing strategies on the pre-strike behavior of double-break vacuum circuit breakers, a series of DC closing experiments were conducted using two commercial 12 kV vacuum interrupters. The field emission current (I_pᵣ_e), pre-strike gap (d_pᵣ_e), and floating terminal voltage (U_pᵣ_e) were systematically measured under various closing sequences, and their statistical characteristics were analyzed using a three-parameter Weibull distribution model. Six asynchronous closing conditions were tested across DC voltage levels of 20 kV, 40 kV, and 60 kV. The results indicate that a clear linear negative correlation between I_pᵣ_e and d_pᵣ_e is observed only under synchronous closing. As the advance closing time of the high-voltage side increases, the voltage division ratio on the low-voltage side rises, leading to a significant increase in the median d_pᵣ_e (up to 1.40 mm) and a substantial decrease in I_pᵣ_e (as low as 0.90 mA), accompanied by reduced dispersion in both parameters. The correlation between U_pᵣ_e and I_pᵣ_e remains weak overall, suggesting that pre-strike behavior is predominantly governed by the physical gap distance, with limited influence from variations in floating potential. These findings demonstrate that by controlling the asynchronous timing of contact closure, the capacitive voltage distribution and division ratio can be effectively tuned to suppress fluctuations in field emission current and enhance insulation stability. This provides a theoretical foundation and empirical data to support the development of phase-controlled closing strategies for double-break vacuum circuit breakers.