Experimental study on field emission current distribution between contacts of vacuum circuit breakers during DC closing

Under the background of high-voltage direct current (HVDC) transmission and renewable energy integration, the vacuum circuit breaker (VCB), as a key device for control and protection, plays a vital role in determining the insulation performance during the initial closing process, which directly impacts the stability and safety of system operation. However, current research predominantly focuses on alternating current (AC) conditions, with limited experimental evidence and statistical modeling concerning pre-strike behavior under direct current (DC) conditions. To address this gap, this study investigates single-break vacuum interrupters (VIs) with two typical contact structures: axial magnetic field (AMF) cup-type contacts and transverse magnetic field (TMF) spiral-type contacts. A dedicated DC dynamic insulation performance test platform was developed, and closing experiments were conducted at voltage levels of 20 kV, 40 kV, and 60 kV. The field emission current and contact gap data during the pre-strike stage were systematically recorded. The results show that the pre-strike current increases with voltage, accompanied by significant changes in pre-strike gap distribution. Both current and gap data conform to a three-parameter Weibull distribution. A strong negative linear correlation was observed between emission current and pre-strike gap, indicating that the effective electric field between electrodes plays a decisive role in electron emission behavior. These findings provide new experimental evidence for understanding the initiation mechanism of vacuum breakdown and offer theoretical and data support for insulation evaluation and phase-controlled closing strategy optimization of VCBs.