Termination Methods for REBCO Tape High-Current Cable Conductors

Three types of joint termination methods for a 2G HTS cable have been developed: YBCO-BSCCO, YBCO-YBCO, and Folding-fan soldered terminations. For the YBCO-BSCCO terminator, each YBCO tape of a cable is connected to a BSCCO tape while the other ends of BSCCO tapes are bundled together and soldered inside a copper tube. The same termination was tested more than 30 times in liquid nitrogen and helium without any problems. It was operated at 10 kA in liquid helium and more than 2 kA in liquid nitrogen. The YBCO-BSCCO termination has been confirmed to be very easy to fabricate and convenient for a cable test since it is reusable. A YBCO tape termination method without BSCCO tape also has been proposed. Both terminations will be very useful for 2G HTS cables. These terminations are demountable if they are not soldered at the junction between tapes. Joint resistance is greatly reduced with increasing applied pressure especially for YBCO-YBCO contact joints. However, in the case of a YBCO-BSCCO joint, the BSCCO tapes may degrade at high pressure. The contact pressure should be optimized to obtain low joint resistance. If the contact surfaces of YBCO-BSCCO and YBCO-YBCO terminations are soldered for a permanent joint, the termination resistances are lower, and better uniformity will be achieved. Another termination is the Folding-fan termination. It is a soldered joint. Joint resistance distributions among cable tapes for these termination methods have been evaluated. Mechanical contact joint resistance distributions between YBCO-BSCCO tapes and YBCO-YBCO tapes have been measured under various contact pressures and contact surface conditions at 77 K. By optimization of the applied contact pressure and contact surface treatment techniques the contact joint resistance of a termination joint can be reduced and a better uniformity can be obtained. We also discussed effects on a cable performance due to no-uniformity of termination resistances, a loop resistance between two tapes in a termination of a cable, and a series operation of two 2G YBCO Twisted Stacked-Tape Cable (TSTC) conductors.本研究开发了三种适用于第二代高温超导（2G HTS）电缆的接头终端工艺：钇钡铜氧-铋系高温超导（YBCO-BSCCO）型、钇钡铜氧-钇钡铜氧（YBCO-YBCO）型以及折叠扇型钎焊终端。针对YBCO-BSCCO型终端，电缆的每一根钇钡铜氧（YBCO）带材均与一根铋系高温超导（BSCCO）带材连接，随后将所有BSCCO带材的另一端集束捆绑，并在铜管内部完成钎焊。该类终端已在液氮与液氦环境中完成30余次测试，未出现任何异常；其在液氦环境下可承载10 kA电流，在液氮环境下可承载超过2 kA电流。经证实，YBCO-BSCCO型终端制作简便且便于电缆测试，因其具备可重复使用性。此外，研究还提出了一种无需BSCCO带材的YBCO带材终端工艺。上述两类终端均对2G HTS电缆具有极高的应用价值。若未在带材的连接界面处进行钎焊，此类终端可实现拆卸。施加压力的提升可显著降低接头电阻，尤其针对YBCO-YBCO接触式接头。但对于YBCO-BSCCO接头而言，过高的压力可能会导致BSCCO带材性能劣化，因此需优化接触压力以获得低接头电阻。若对YBCO-BSCCO与YBCO-YBCO型终端的接触面进行钎焊以形成永久性接头，则可进一步降低终端电阻，并获得更优异的电阻均匀性。第三种终端为折叠扇型终端，属于钎焊式接头。研究人员对上述三种终端工艺下电缆各带材的接头电阻分布进行了评估；同时在77 K温度下，针对不同接触压力与接触面条件下的YBCO-BSCCO带材与YBCO-YBCO带材的机械接触式接头电阻分布开展了测试。通过优化接触压力与接触面处理工艺，可有效降低终端接头的接触电阻，并获得更均匀的电阻分布。本研究还探讨了终端电阻不均匀性、电缆终端内两根带材间的回路电阻，以及两根第二代高温超导钇钡铜氧叠带扭转电缆（TSTC）导体串联运行等因素对电缆整体性能的影响。