High-Frequency Inrush Current Prestrike Characteristics of Double-Break Vacuum Circuit Breakers
Geng Yun1, Yao Xiaofei2, Xu Minju2, Geng Yingsan2, Liu Zhiyuan2
1. School of Electrical Engineering Xi'an University of Technology Xi'an 710048 China; 2. State Key Laboratory of Electrical Insulation and Power Equipment Xi'an Jiaotong University Xi'an 710049 China
Abstract:Switching capacitor banks represent a critical and economical method for reactive power compensation in modern power systems. Vacuum circuit breakers (VCBs), valued for their superior arc-extinguishing capability and maintenance-free operation, are extensively deployed for frequent capacitor switching. However, the high-frequency, high-amplitude inrush current generated during capacitive closing operations induces severe contact erosion, compromising breaker performance and longevity. Phase-controlled closing technology offers a potential solution for inrush current suppression, but its effectiveness depends critically on the VCB's prestrike characteristics. This dependency is particularly complex and underexplored in double-break VCB configurations, where two interrupters are connected in series to enhance voltage-withstand capability and capacitive-current interruption performance. This paper presents a systematic experimental investigation of the high-frequency inrush-current prestrike characteristics of double-break VCBs, specifically the cup-type axial magnetic-field (AMF) and spiral-type transverse magnetic-field (TMF) contacts. An experimental platform was constructed to generate a characteristic damped oscillatory inrush current (peak: 5 kA or 20 kA; frequency: 4 250 Hz). Tests were conducted on two single-break configurations (VL_S1_A: AMF; VL_S2_T: TMF) and four double-break structures, i.e., AMF-AMF (VL_D1), AMF-TMF (VL_D2), TMF-AMF (VL_D3), and TMF-TMF (VL_D4). Each configuration underwent 80 closing operations. Key parameters measured included prestrike gap (dpre) and its statistical dispersion (σpre), inrush current interruption patterns, interruption time (tint), prestrike arcing time (ta), voltage distribution across breaks (VHpre, VLpre), prestrike field strength (Epre), and post-test contact surface field enhancement factor (β) derived from Fowler-Nordheim analysis to quantify erosion. High-speed imaging captured arc behavior, and ANSYS/Maxwell 3D simulations analyzed magnetic field distributions. Results demonstrate significant structural dependencies. Under 5 kA inrush conditions, the TMF single- break outperformed AMF, reducing the median prestrike gap (d50) by 47.4% (1.0 mm vs. 1.9 mm) and dispersion (σpre) by 28.6% (0.5 mm vs. 0.7 mm). The double-break configuration further improved performance. The TMF-TMF structure exhibits the most substantial enhancements: d50 reduced by 57.9% compared to AMF-AMF, dispersion by 71.4%, median interruption time (tint50) by 85.1% (0.10 ms vs. 0.67 ms for AMF single-break), and median arcing time (ta50) by 61.5% (0.30 ms vs. 0.78 ms for TMF-AMF). Voltage distribution analysis revealed that double-break structures with TMF contacts on the high-voltage side (TMF-AMF, TMF-TMF) promoted more balanced voltage sharing (approaching 64 for TMF-TMF) compared with AMF-dominated structures (e.g., 73 for AMF-AMF), reducing the probability of high-voltage-side prestrike initiation. Crucially, TMF contacts consistently exhibited superior resistance to surface erosion, and the growth rate K of the post-test field enhancement factor β was lowered (K≈2.2~3.0 for TMF-TMF contacts vs. 3.9~5.3 for AMF-AMF and up to 6.3 under 20 kA). Surface morphology inspection confirmed more severe localized melting pits in AMF slots than multiple shallow pits on TMF surfaces. This erosion mitigation in TMF structures is attributed to the transverse magnetic field, which generates a Lorentz force that drives high-speed arc rotation across the contact surface (simulations showed Bmax~44.63 mT at 4 250 Hz, with a Lorentz force of ~92.2 mN), thereby dispersing energy and preventing localized overheating. Conversely, AMF structures, despite promoting diffuse arcs, suffered from localized field enhancement due to slot geometry and less effective energy dispersal during the brief prestrike phase. Increasing the inrush peak to 20 kA markedly amplified instability and damage: d50 increased substantially (~3.5 mm for AMF-AMF), tint patterns shifted towards single long-duration arcing events (Type 1 probability rose to 76%), and β growth rates surged (K=6.3 for HV side AMF), indicating severe micro-protrusion formation and erosion. In conclusion, this paper quantifies the superior prestrike performance and contact-erosion resistance of double-break VCBs employing spiral TMF contacts. The TMF's ability to rotate the prestrike arc via a strong transverse magnetic field can minimize the localized energy density and promote the balanced voltage distribution in series breaks. This paper provides a reference for design optimization and reliable implementation of double-break VCBs in high-precision phase-controlled switching applications.
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