Development of REBCO superconducting transformers with a current limiting function

We designed and fabricated a 10 kVA single-phase transformer with RE ₁Ba₂Cu₃O_7-δ (REBCO, RE:Rare Earth, Y, Gd, etc.) superconducting tapes. It had a 4-winding structure at the beginning. In this study, we removed the auxiliary windings and made a test as a general 2-winding transformer. Making repeated short circuit tests, we further investigated the response of the REBCO superconducting windings against a fault excess current. Although the short circuit current decayed satisfactorily, the whole of the REBCO superconducting windings did not shift to normal state even if the short circuit current exceeded the critical current all over the length of REBCO tapes. The induced normal resistance depended on the primary voltage. To study the phenomena, we derived the basic equations and made numerical simulations taking into account of flux-flow state, in which the equivalent resistance and generated heat are much smaller than those in normal state, and also the dependences of the critical current on temperature, applied magnetic field and the applied field angle to the tape face. It was clarified that short circuit current was limited not only by normal resistance but also by flux-flow one. The introduction of the concept of flux-flow state made it possible to quantitatively explain the behavior of the REBCO superconducting transformer at a sudden short circuit.