Development of 66 kV/750 A High-Tc superconducting fault current limiter magnet

One of the items included in the Super-conductive AC Equipment (Super-ACE) project, being performed from 2000 to 2004, is a development of a 66 kV/50 A high-T-c superconducting (HTS) fault current limiter (FCL) magnet. This research focuses on fundamental technical items essential for a 66 kV class fault current limiter, that is, high current capacity, high voltage insulation and sub-cooled nitrogen cooling. This paper describes the design of the magnet and the test results obtained so far. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of unit-coils wound with Bi2223 tapes. The rated current of each coil is about 125 A at 70 K, and so the total current capacity of the magnet is 750 A. The insulation voltage of the magnet is of the 66 kV class. By the end of FY2002, three sets of the unit coils were set connected in the cryostat and some evaluation tests were implemented as a milestone of the program. In the cooling down test, sub-cooled nitrogen of 65 K was obtained with homogenous temperature distribution in the cryogen. In the overvoltage test, no breakdowns were observed in the case of applying both ac voltage of 140 kV for one minute and lightning impulse voltage of 350 kV. Voltage-current characteristics of the coils were measured in sub-cooled nitrogen. The rated current of 375 A was successfully obtained for both direct and alternate current tests. All the targets constituting the milestone of the 66 kvn50 A magnet development were achieved.