Study On Electromagnetic Characteristics Of CICC Based On Quasi-isotropic HTSStrands

With the continuous improvement of the preparation technology of the second generation high temperature superconducting(HTS)tape,the REBCO tape has been widely used because of its high critical current density,critical magnetic field and excellent axial tensile property.In addition,the content of precious metal silver is reduced in the preparation process,so the cost reduction makes commercial production possible.Especially when the magnitude of external magnetic field is high,the REBCO tape becomes the first choice in the application of large low-temperature high magnet.However,in order to meet the requirement of current carrying capacity,multiple superconducting conductors/cables need to be combined into cable-in-conduit conductor(CICC)to wind the magnet coil in the preparation process of large magnet.At present,researches of North China Electric Power University have studied the critical current,temperature distribution and electromagnetic stress distribution characteristics of quasi isotropic superconducting strand(q-is),but the relevant researches on the CICC designed based on Q-IS have not been conducted yet.Therefore,this dissertation proposes a CICC designed based on Q-IS to study the effects of temperature,external magnetic field intensity and external magnetic field direction on the critical current.In addition,screening current can be induced in the magnet during excitation/demagnetization and then generates screening magnetic field,which has an effect on the distribution characteristics of magnetic field.At the same time,mechanical stress will be generated on CICC and each Q-IS when the magnet coil is winding,which will influence the electrical characteristics.To sum up,this dissertation conducts a systematic research on the contents of above quasi-isotropic strand not yet carried out.A CICC based on quasi-isotropic HTS strands is proposed.Then,the formulas of critical twist pitch and critical bending radius are deduced according to the plane geometric relationship of this CICC.Furthermore,a sharing current method and copper terminal ends fixing technology suitable for the long-distance preparation of CICC are proposed.In the finite element simulation software,a two-dimensional plane model of partially-filled superconducting CICC with the same structure as the experimental sample is established,and its critical current is calculated by using the self-consistent model.Accordingly,the experimental sample of dummy cable is fabricated and the experimental bench is built.The effect of different transmission current rates on the contact resistance of copper terminals is analyzed,and the critical currents of the sample are tested under the conditions of 77K and self-field.The verified model is applied to the calculation of the critical current of all superconducting CICC at different temperatures,and the effects of the direction and the intensity of external magnetic field on the critical current of conductor are discussed.For HTS strands with different stacking forms,the effects of external magnetic field direction,external magnetic field intensity and excitation rate on the stability and distribution characteristics of screening magnetic field with time are studied,and the isotropic characteristics of screening magnetic field are analyzed.For strands with different plane structures,H algorithm and T-A algorithm are used to calculate the screening magnetic field,respectively.On this basis,strand samples with different stacking forms are fabricated,and the experimental bench is built.The experimental measurements are carried out by using Hall sensors,and the experimental verifications are carried out under the same condition as simulations.According to the single strand model,the screening magnetic field calculation model of CICC is established,and the stability and distribution characteristics of screening magnetic field are preliminarily discussed,which lays a foundation for its future application in the preparation of magnets.The critical current degradation characteristics of strands in CICC under different bending forms are studied.The current distribution of strands under different deformation and different bending radii are calculated by simulation.Then,the experimental mold is designed and the experimental bench is built.After completing the research on strands,the end joint of strand is proved to be subjected to stress,which may cause a certain effect on the current transfer length and contact resistance.Firstly,the current transfer process,judgment standard and experimental method are introduced.Afterwards,the change process of strand’s current transfer length with different transverse pressure is explored,and the contact resistance of the joint is calculated according to the relationship between contact resistance and current transfer length.All superconducting strands are fabricated and verified by relevant experiments.Finally,the method can be applied to the contact resi stance calculation of the proposed CICC end joint.