The Current Unbalance in Stacked REBCO Tapes — Simulations Based on a Circuit Grid Model

Unlike low-temperature superconducting cables, there is so far no perfect solution for REBCO coated conductors to form a fully transposed high-current cable. Every REBCO cable concept must import a stack of tapes to achieve an operating current as high as tens of kiloamperes. The stacked REBCO tapes, no matter whether they are twisted or not, however, have a nature of nontransposing and, therefore, could result in current unbalance. In this article, the current unbalance and the related electrical characteristics of a cable made of 40 stacked REBCO tapes are studied with an electrical circuit simulation. The differences in splice resistances and tape inductances that are both related to the nontransposed structure of a REBCO stack are considered. Results show that for a 40 cm long termination, a proper method to keep the contact resistivity between each tape and the copper termination around 10 nΩ·m is crucial to totally avoid current unbalance lowering the cable's performance. Surprisingly, the intertape current transfer is found to be able to further exacerbate local high current though it does make the overall distribution more balanced. The inductance difference induced current unbalance is only important if local defects exist at long REBCO tapes, which on the other hand can be cured by good intertape current transfer. For a fast-charging rate of 1 kA/s, the intertape contact resistivity should also be low to a level of 10 nΩ·m to ensure a short current transfer length of around 1 m.