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As one of the programs in the Ministry of Economy, Trade and Industry (METI) R&D project on coated conductors, we developed a three-phase 6.6 kV superconducting fault current limiter (SFCL) and conducted some evaluation tests. The developed SFCL mainly comprised a set of three-phase current-limiting coils installed in a sub-cooled nitrogen cryostat with a Gifford-McMahon (GM) cryocooler, circuit breakers, and a sequence control circuit. Two tapes were wound in parallel in each limiting coil to obtain a rated current of 72 A rms. AC characteristics of each coil were measured, and relevant performance metrics were obtained. The whole system was installed in a cubicle. Short circuit experiments were then conducted with a short circuit generator. In a three-line ground fault test, the SFCL successfully restricted a short circuit current of over 1.56 kA to about 840 A with an applied voltage of 6.6 kV. The system integration ability and the obtained data show the promise of this approach for practical implementation. The SFCL was ready for user field tests.

This paper describes the design and test results of a 6.6 kV-class superconducting fault current limiter (FCL) coil wound with YBCO tapes. YBCO tapes, with stainless-steel lamination, were prepared for an experimental FCL coil. The main feature of this FCL coil is winding of multiple YBCO tapes in parallel in order to increase the rated current. To obtain a rated current up to several hundred amps, four tapes, electrically insulated, were wound in parallel to form a non-inductive coil. The coil specifications, such as winding pitch, number of turns, and transpositions, were carefully designed by numerical simulation to equalize the current distribution in each tape. The fault current limiting performance of the FCL coil was evaluated through over-current tests using a capacitor bank. The FCL coil successfully suppressed a fault current of 10.4 kA to below 2.1 kA.

This paper describes the design of a 66 kV high-Tc superconducting fault current limiter (FCL) magnet and the test results obtained. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of coils wound with Bi2223 tape. The rated current is 750 A. The insulation voltage of the magnet is of the 66 kV class. In the final year of the project, all six sets of the coils are set connected in the cryostat and evaluation tests have been implemented. In the cooling test, sub-cooled nitrogen of 65 K was obtained, with homogenous temperature distribution in the cryogen. The rated current of 750 A was successfully obtained for both direct and alternating current tests. In addition, the magnet passed the simultaneous current and voltage application test. The dielectric test results showed that the magnet satisfied the insulation for 66 kV apparatus in the Japanese Electrotechnical Committee Standard. Finally, the magnet was combined with a rectifier bridge circuit to form an FCL. In a fault current limiting test with a short-circuit generator, the FCL successfully limited short circuit current up to 20 kV applied voltage. The obtained result was in good agreement with the design and the Electro-Magnetic Transients Program (EMTP) analysis

A 66 kV-class power lead insulation system has been developed for high-Tc superconducting fault current limiters (SCFCL). The power leads are insulated with ethylene-propylene rubber without organic filler, and have excellent crack resistance. This system was evaluated by an insulation model and was shown to have strong enough electrical insulation properties for 66 kV-class equipment. To minimize insulation layer thickness, the dielectric characteristics of this EPR were investigated in detail. The specimens were made 0.3-10 mm thick: near the thickness of an actual power lead insulation layer. The breakdown stress was measured at room temperature and at liquid nitrogen temperature. The V-t characteristic was also measured. Based on the study, the power lead insulation model was constructed and its electrical insulation properties were evaluated

A series of experiments on the instability resulting from mechanical disturbance at the coil surface of a small racetrack coil is described, along with a preventive measure against its instability. Epoxy-impregnated racetrack coils sometimes experience premature quenches due to frictional heat produced by coil slides at fixtures. The first experiment confirmed coil slides during coil charging. These slides were about 10 mu m, an equivalent of 20 mJ in fractional heat generation. One effective preventive measure against this mechanical disturbance is the utilization of a thermal barrier method. The thermal barrier is an insulation layer at the interface between the coil and the fixtures. The second experiment examined the thermal barrier effect on the stability margin on the racetrack coil. A thicker insulation layer substantially increased the coil stability margin. The margin increased from 105 mJ to 200 mJ by thickening the insulation layer from 0.36 mm to 1.00 mm. The two experiments showed that the racetrack coil was stabilized if the thickness of the insulation layer exceeded 0.20 mm. Charging a racetrack coil with a 0.36 mm-thick insulation layer confirmed this criterion. >

Stabilization of a persistent current switch with a parallel connection is described. The current transfer behavior for a seven-strand conductor, wound as a persistent current switch, was studied. Each strand current was measured by using Rogowski coils. In the current ramp, negative current flowed into the center strand, which was inverted to a positive direction in about a 2-min time constant. Strand quench was induced by a heater wound in each strand. When one of the outer strands was quenched by its heater, the current was mainly transferred to the nearest three strands including the center strand in about 2 ms. These experimental results agreed well with the calculated results considering the self/mutual inductance for a parallel conductor and the joint resistance. The transferred current of several tens of amperes, which flowed with a few orders of 10000 A/s ramp rate, resulted in the degradation of the whole switch quench current from the conductor critical value. >