Development Of Large Capacity DC Switch For Quench Protection System Of Superconducting Magnet In Fusion Devices

CRAFT(Comprehensive Research Facility for Fusion Technology)is one of the 13th Five-Year Plan for specific national scientific development project,and DC 100kA/10kV superconducting magnet quench protection system is one of its important subsystems.In this paper,the main switch of quench protection system in CRAFT project is developed.Firstly,based on the analysis of the topology and working principle of the quench protection system circuit of superconducting magnet,the function and principle of the DC switch of the quench protection system are described,and the structure scheme of mechanical bypass switch parallel vacuum switch meeting the requirements of CRAFT quench protection system is proposed.The transfer process of 100kA breaking current of the system is studied,in addition,the influence of switch stray parameters and branch parameters on current transfer time and post-arc recovery characteristics of vacuum switch are also studied,so the parameter configuration of quench protection system is optimized.According to the design requirements of large capacity DC switch,the key technologies such as contact and current carrying,lightning impulse withstand voltage,bus bar temperature rise control and operating mechanism stress of mechanical bypass switch are studied from the aspects of current sharing,dynamic stability,thermal stability and over-voltage.Combined with simulation and actual component testing,the structure optimization and finalization of each component of the switch are completed,and good test results are obtained;The key technologies such as performance of vacuum interrupter,operating mechanism and magnetic field distribution of vacuum switch are studied from the aspects of fracture form,driving and control mode and contact structure,and the design of vacuum switch with series double fracture structure is completed.The electromagnetic repulsion driving mode is proposed,and the coaxial connecting rod and innovative buffer holding part with four spring are designed,which can solve the difficulty of rapidity and synchronization breaking action,and the magnetic shielding cover is designed to reduce the risk of high current breaking failure under the uneven longitudinal magnetic field distribution in the contact gap.In addition,based on the engineering design of vacuum switch,the prototype manufacturing is completed and a series of technical requirements are passed.At the same time,the optimal contact opening distance theory for vacuum arc zero crossing is proposed,and the minimum opening distance parameters of high current breaking of vacuum switch prototype are determined,which provides the control sequence basis for the breaking test circuit,and the 100kA DC breaking is successfully completed with the artificial zero crossing circuit,which also provides a reference basis for the parameter design of the converter circuit of the quench protection system,and improves the reliable breaking theory of the vacuum switch.Considering the progress of technology,this paper proposes a new design scheme of quench protection system that may be applied in the future-a hybrid quench protection system composed of DC mechanical switch with three-level contact structure and fully-controlled power electronic devices,which avoids the use of complex artificial zero crossing commutation technology and the possible risk of arc re-ignition on arcing contact,it has the advantages of reducing system volume and improving system reliability.Finally,the principle prototype of the new hybrid DC circuit breaker is developed,and the expected satisfactory results are obtained in the relevant current breaking test.The large capacity DC switch designed and developed in this paper fills the gap in the field of DC switch equipment with relevant high parameters in China,and provides important technical support for the safe and reliable operation of the superconducting magnet quench protection system.