分布式能源的广泛接入使得柔性直流电网成为未来电网发展趋势。直流电网系统阻抗低，故障电流上升速度快且峰值大，因此直流断路器需要在几个毫秒内完成故障电流的开断。快速机械开关作为混合式直流断路器通流支路的核心元件，包含灭弧单元、驱动单元、缓冲单元与保持单元。在张北±500kV直流电网示范工程中，选用电磁斥力机构作为驱动与缓冲单元，选用双稳弹簧保持机构作为保持单元。双稳弹簧保持机构可以满足单断口100kV，开距30mm的快速机械开关技术需求，然而当将其应用于较低电压等级、较小开距工况时，双稳弹簧保持机构出现保持不可靠、机械制造困难的缺陷。本文对双稳弹簧保持机构、碟簧保持机构以及单稳/双稳永磁保持机构进行了对比，对四种保持机构在小开距工况下的可靠性进行了分析，并对他们在不同开距工况下的适用性进行了对比。本文对四种保持机构进行了有限元建模或数学建模，比较其静态特性，得出永磁保持机构具有最优的操作功特性的结论，同时永磁保持的静态保持力出力能力强，在小开距工况下具有优良的适用性。在此基础上，本文分别采用有限元法与等效电路法进行了快速机械开关模型搭建以分析其动态特性，通过调整仿真驱动电路参数弥补了永磁保持机构较大的附加运动质量对机械开关动态特性的影响。仿真表明采用永磁保持的快速机械开关能够在中开距（13mm）与小开距（6mm）工况下快速建立耐压开距，满足快速分闸需求。本文搭建了由真空灭弧室、斥力驱动机构与单稳态永磁保持机构组成的快速机械开关样机，分别采用斥力驱动与永磁驱动进行了6mm、13mm两种开距工况下的分合闸试验。样机试验验证了仿真模型的可靠性，表明了此种快速机械开关具有基于斥力驱动的快速分闸功能、基于永磁驱动的慢速分合闸功能以及基于斥力-永磁双驱动模式的快速合闸功能。根据仿真分析与样机试验，本文给出了不同开距工况下保持机构的选择建议：小开距工况(5~10mm)下应当选用永磁保持机构；中大开距工况(10~30mm)下，根据保持力需求与合闸速度需求，灵活选用双稳弹簧或永磁保持机构。
The widespread access of distributed energy makes the flexible DC grid a future development trend of the grid. The low impedance of the DC system leads to fast rising velocity of the fault current and also, large current value. Consequently, DC circuit breaker needs to complete the interruption of the fault current within a few milliseconds. The fast mechanical switch, as the core component of the current-passing branch of the hybrid DC circuit breaker, includes an arc extinguishing unit, a drive unit, a buffer unit and a holding unit. In the Zhangbei ±500kV DC power grid project, the electromagnetic repulsion mechanism is selected as the driving and buffer unit, with the bistable spring holding mechanism selected as the holding unit.The bistable spring holding mechanism can meet the technical requirements of fast mechanical switching with a single fracture of 100kV and an opening distance of 30mm. However, when it is applied to a lower voltage level and a small opening distance, the bistable spring holding mechanism appears to be unreliable. The bistable spring holding mechanism, the disc spring holding mechanism, and the monostable/bistable permanent magnet holding mechanism are compared in the paper to analyze the reliability and applicability of them under different working conditions and opening distances.Finite element modeling and mathematical modeling of four kinds of holding mechanisms are carried out in this paper to compare their static characteristics. It is concluded that the permanent magnet holding mechanism not only has the best performance of holding force output, but its static holding force has the strongest stability with the change of the opening distance. Permanent magnet holding mechanism is the only type feasible for small opening distance. Furthermore, the finite element method and the equivalent circuit method are utilized in the paper to model the ultrafast mechanical switch and analyze its dynamic characteristics. It’s shown that the permanent magnet holding mechanism is inferior to others due to its large additional motion mass.By upraise the ability of the drive circuit, the permanent magnet holding mechanism is proved applicable under the working conditions of the middle opening distance (13mm) and small opening distance (6mm).In this paper, a fast mechanical switch prototype composed of a vacuum interrupter, an electromagnetic repulsion mechanism and a monostable permanent magnet holding mechanism is built. The prototype test of opening and closing under the working conditions of 6mm and 13mm are carried out, respectively. Combining the fast-driving mode of the electromagnetic repulsion mechanism and the slow-driving mode of permanent magnet mechanism, the prototype is proved to be able to realize the functions of slow closing and fast/slow opening, improving the mechanical life of the switch in a great way.According to the simulation analysis and prototype test, this paper gives suggestions for the selection of the holding mechanism under different working conditions of opening distance: under small and medium open distance conditions(0~20mm), the permanent magnet holding mechanism should be selected; under the large opening distance condition(20~30mm), according to different technical requirements, bistable spring or permanent magnet holding mechanism should be chosen according to the technical requirements of holding force and closing velocity.
