Structural Design,Simulation And Performance Testing Of Binary Current Leads For Superconducting Levitation Magnets

The high-temperature superconductor(HTS)has been wildly used in superconducting magnetic levitation train due to its high critical temperature,large engineering critical current density and critical stress.In order to improve the performance of the on-board HTS magnets,they are usually cooled to the temperature of 20 K-40 K.However,the current leads that charge the HTS magnets need to work from room temperature to the low temperature environment.Under such a large temperature range,the traditional current leads will bring a great heat load to the system,which will not only affect the cooling process of the cryogenic system but also the stability of HTS magnets.In order to reduce the heat load of the system as well as the production cost of current leads,a pair of binary current leads with detachable structure were fabricated to replace the traditional current leads in this thesis.The binary current lead is composed of conventional lead and HTS lead.Compared with the Bismuth-based tapes,YBCO tapes are more suitable for preparing HTS lead,since they are less affected by magnetic fields and have lower thermal conductivity.Therefore,the HTS lead was made of second-generation coated conductor YBCO tapes,while the conventional lead was made of oxygen-free copper with RRR=50.In this thesis,the binary current leads used in the superconducting levitation magnets are taken as the research object,and the research method combining the analytical calculation,multi-physics finite element simulation and experimental test was used to study the structural design,electromagnetic characteristics and temperature characteristics of the binary current leads.Firstly,from the perspective of reducing heat load,based on the theory of heat transfer,the optimal length-sectional area ratio of the copper lead with minimum heat load was optimized;the steady-state mathematical model of the copper lead was established and the finite difference method was used to calculate the steady-state temperature distribution and heat load of the copper lead.From the perspective of safety,the shape and cross-sectional area of the shunt for HTS lead was optimized based on the quench propagation theory of HTS tape to reduce the risk of HTS lead being burned.From the perspective of economy and practicability,the joint resistance of different lap joint modes of HTS tape were compared and analyzed.It was found that the joint resistance of parallel welding of Pb₃₆Sn₆₄ was 1/17of that of stack welding.And the welding mode between HTS tape and copper cap/stainless steel shunt was determined.Secondly,based on the COMSOL Multiphysics finite element simulation software,a three-dimensional model of the binary current lead was established,and the temperature characteristics and electromagnetic characteristics of the binary current lead were further analyzed in this part.The load line method was used to predict the current carrying capacity of HTS lead in liquid nitrogen environment.Meanwhile,the effects of magnetic fields,cooling failure at high temperature end,overcurrent and cooling contraction on the current carrying capacity and temperature distribution of HTS lead were analyzed.The simulation results show that the current carrying capacity,heat load and safety of binary current leads can meet the design requirements.Finally,according to the design and simulation results,the production and performance test of binary current leads were completed.The current carrying capacity of HTS leads were tested in liquid nitrogen as well as the operation performance of the binary current leads.The experimental results show that the current carrying capacity of HTS leads exceeded 600 A.Besides,in order to verify the safe operation performance of HTS leads in an environment of20 K-40 K,a cryogenic test platform was designed and built.Cooperated with the cryogenic vessel in the platform,the detachable structure of binary current leads was implemented and some related experiments were completed,such as the no current carrying cooling test,current carrying test and the cooling failure test at high temperature end of the HTS leads.The experimental results show that the binary current leads with detachable structure have stable excitation performance and significantly lower heat load;At the same time,the design rationality of the shunt were further verified by the cooling failure test at high temperature end of the HTS leads.