Sudden Short-Circuit Test of 22-kV YBCO Triaxial Superconducting Cable

For the power line between a power generator and a step-up transformer, isolated power bus (IPB) is generally used. The present IPB has a normal cylindrical conductor made of aluminum or copper at the center of the outer jacket and generates very huge Joule loss. To decrease it, superconducting buses have been developing. In this study, a 22-kV 3-kA triaxial superconducting bus was designed and fabricated with YBa2Cu3O7-d (YBCO) tapes and the various kinds of properties were investigated. Previously, the ac and lightning impulse withstand voltage test, critical current Ic test, and ac loss measurement were conducted and it was shown that the developed triaxial superconducting cable met the designed specifications. In this paper, a sudden short-circuit test of a 2-m-long triaxial cable was carried out by using a 200-MVA short-circuit generator. For protection, each phase of the cable was connected to a XLPE cable with a length of 3 m and a cross-sectional area of 250 mm² in parallel. Three-phase short-circuit current of 25 kA was applied to every phase of the cable bath-cooled with liquid nitrogen at 77 K for 2 s. As a result, the following tremendous phenomenon was demonstrated. Any phase of the triaxial cable kept a flux-flow state and did not quench. Short-circuit current was shared between the superconducting cable and the connected XLPE cable, and the branch current ratio varied even during one period of 60 Hz. The numerical simulation was also conducted to explain the phenomenon theoretically. That should contribute to the instantaneous recovery to a normal operation after a short-period interrupt of the circuit.