Effect of contact material on power frequency AC pre-strike characteristics of double-break vacuum circuit breaker after short-circuit current erosion

In capacitor bank switching — one of the most frequent duties of vacuum circuit breakers (VCBs) in power systems — phase-controlled closing is an effective means of suppressing the inrush current and the switching overvoltage. In service, however, VCBs must also interrupt power-frequency short-circuit currents, and the resulting arc erosion irreversibly degrades the contact surfaces, shifting the pre-strike behavior away from the non-eroded state on which the parameters of phase-controlled strategies are usually set. For double-break VCBs, which are increasingly employed at higher voltage levels, the power-frequency pre-strike characteristics after short-circuit current erosion remain insufficiently studied. This paper takes three contact materials, CuCr30, CuW, and CuW-WC, as the research objects. By employing a combined experiment of short-circuit current erosion and AC withstand voltage testing, the statistical distribution of pre-strike contact gap, voltage, and electric field strength after erosion is systematically investigated. The physical mechanism is further revealed through micro-morphology analysis. The results indicate that the ranking of characteristic gap and electric field strength is consistent across the three materials: CuCr30 exhibits the lowest dispersion and the highest insulation withstand capability, followed by CuW, while CuW-WC performs the worst. The continuous Cr skeleton of CuCr30 effectively suppresses molten copper migration, leading to a uniform and dense erosion layer. In contrast, CuW-WC significantly enhances field emission due to interfacial thermal mismatch and conductive contamination. The study definitively identifies CuCr30 as the preferred material for phase-controlled closing strategies, whereas CuW-WC is unsuitable for high-precision reactive power switching. It also points out that phase-controlled parameters require dynamic compensation based on a post-erosion statistical model, providing a theoretical basis for material optimization and long-term reliability of double-break vacuum circuit breakers.