Research On Over-current And Mechanical Characteristics Of The Second Generation High Temperature Superconducting Strands

With the improvement in properties of high temperature superconducting (HTS) material, the engineering application of superconductors is developing rapidly. A cabling method of isotropic strands made from coated conductors is proposed to improve current-carrying capacity. With a compact design, the superconducting occupation space ratio and engineering critical current density of the isotropic strands can be extremely high. Based on the above advantages, superconducting strands have broad application prospects and they are possibly applicable for DC transmission cables, large-scale magnets and superconducting fault current limiter, etc.In this thesis, the structure design of superconducting strands with round and square cross-section is introduced firstly. The symmetric stacking method of the ReBCO tapes is described. In consideration of better thermal conductivity and mechanical property, we fill the interspace between the superconducting core and metallic sheath with different structures of metal material. The laser welding technique adopted in the preparation of copper, aluminum and stainless-steel sheath has also been presented in detail. To realize the large-scale production of superconducting strands, we have divided the automated production line in several parts and introduce respectively. One strand sample with copper sheath is demounted and the critical current of ReBCO tapes are tested to clarify the safety of the processing method. The experimental results prove that the preparing process does not result in any damage to the ReBCO tapes.Different two-dimension models of superconducting strands are established with the magnetic field simulation software according to the actual sizes. The critical current of superconducting strands is calculated based on the empirical formula and simulating results of magnetic field. The E-I curves of strand samples with different sheaths are also tested and the critical current can be determined by removing the influence of current transferring voltage. The experimental results show the validity of the simulation model and the calculation method, which can provide references for superconducting strands with different materials and specifications.The quench behavior of superconducting strands with different sheaths subjected to over current at power frequency and liquid nitrogen are experimentally studied. Current over critical current is applied to the strand samples with duration of 700 ms and the voltage, current and temperature rise along the strand samples are recorded during the pulsing over-current test. Based on the E-I curves, the resistance of sample strands and phase angle between voltage and current are also obtained and analyzed. After each pulsing current, the critical currents and n-values of the sample strands are retested to evaluate over-current withstanding ability. The results show that quench characteristics of the strand samples with different sheaths are very different from each other. Additionally, the strand sample fabricated with copper sheath has stronger over-current withstanding capacity and that with stainless-steel sheath has better current-limited property among different sheath, that is, copper, aluminum and stainless steel..The mechanical characteristics of superconducting strand, including bending and tensile, are studied. The deformation of ReBCO tapes inside the bended superconducting strand is theoretically analyzed. Strand samples with different bending radii are prepared and tested to study the influence of bending radius on critical current and the minimum bending radius of superconducting strand. The critical current and the n value of strand samples under tensile stress are tested to evaluate the influence of tensile stress on the critical current. The results shows that the minimum bending radius of superconducting strand is determined by the side bending properties of ReBCO tapes used in the fabrication, and the critical current and the n value illustrate a reducing trend with the decrease of bending radius and the increase in tensile stress.