Investigation into Combined AC–DC Insulation Stress Evolution Mechanism and Combined Withstand Voltage Test Method for 40.5 kV Capacitor Bank Circuit Breaker Breaks

With the large-scale construction of renewable energy collection stations and distribution networks, the frequent switching of 40.5 kV reactive power compensation capacitor banks has become a routine operation. To accurately evaluate the insulation safety risk at the breaker break during the switching process of 40.5 kV capacitor bank circuit breakers, this paper comparatively investigates the dynamic transient processes and electric field evolution laws under four typical operating conditions: normal opening/closing, electrode plate short-circuit, non-synchronous opening/closing, and arc restriking. The microscopic failure mechanism under combined AC–DC stress is revealed, and a physical-equivalent combined AC–DC withstand voltage test scheme is proposed and experimentally validated. The results indicate that arc restriking introduces a secondary strong excitation during the unattenuated transient period, causing a surge in overvoltage on the capacitor side, making it the most severe condition inducing transient insulation degradation at the break. Insulation risk evaluation must be grounded on the true instantaneous potential difference between both sides of the break. Under the arc restriking condition, the voltages on both sides exhibit strong opposite-polarity superposition, resulting in a peak instantaneous potential difference of up to 129.0806 kV (busbar side: −49.4861 kV; capacitor side: +79.5945 kV), which substantially exceeds the 1 min power-frequency withstand voltage standard value (118 kV based on IEC 62271-100) for 40.5 kV circuit breaker breaks. Three-dimensional electrostatic field and space charge simulations further reveal the microscopic failure mechanism: under combined AC–DC stress, unipolar free charges undergo directional migration and steady-state accumulation at defects such as micro-protrusions on the contact surface on the DC residual voltage side, forming local high electric field regions and inducing field emission—an effect that cannot be effectively excited in traditional pure AC tests. Consequently, the non-physical extreme value superposition method is abandoned, and a combined AC–DC withstand voltage testing scheme comprising “AC power-frequency −49.4861 kV (peak) and DC +79.5945 kV” is recommended. This study provides deep theoretical insights and numerical evidence for evaluating the insulation margin and revising the type test standards of reactive power compensation circuit breaker breaks.