We are developing RE1Ba2Cu3O7-δ (REBCO, RE:Rare Earth, Y, Gd, etc.) superconducting transformers with a current limiting function. The target is a 3φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. We designed and built two pieces of 1φ-6.9 kV/2.3 kV-400 kVA superconducting transformers with YBCO superconducting tapes. One was wound with a YBCO tape with a copper stabilizing layer with a thickness of 300μm. The other was 50μm in thickness of copper. Making sudden short-circuit tests with a 200 MVA short circuit generator, we investigated the response of the YBCO superconducting windings against the fault excess current over the critical current. Repeated sudden short-circuit tests revealed the quite different response between them. In the case of fully stabilized with copper 300μm thick, short circuit current flowed without decay. On the other hand, in the case of copper 50μm thick, short circuit current was reduced around three times as much as the rated current. Using the numerical simulation program, which we made up on the basis of the results of sudden short-circuit tests of a 10 kVA test transformer, we quantitatively investigated the transition phenomenon of REBCO superconducting windings to normal state through flux-flow state due to fault excess current. As a result, theoretical simulation explained experiment quantitatively and clarified how to control the short circuit current.
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