Toshitada Onishi

A proposal of fast self-acting and recovering magnetic shield type superconducting fault current limiter and the analyses of their characteristics ☆

Abstract A high-performance magnetic shield type superconducting fault current limiter (MSFCL) has been proposed. It will be fast activated automatically when a short circuit failure occurs. And it will be recovered to superconducting state in a short period of time after the fault is cleared. The designed magnetic shielding body consists of a large number of the flat Bi2223 rings. They are stacked with a small gap between each ring and hence the cooling surface is increased by about 10 times that of an usual tubular shielding body. The toroidal coils (TCs) will be wound around the rings so that a magnetic field may be applied to the rings in a circumference direction. The thin heaters similar in form to the flat ring will also be stuck on both sides of each ring. It is shown that the rings will be automatically heated quickly up to Tc or more and accordingly a faster current limiting action will be realized by these two additional effects. A fairly fast recovery to a superconducting state after a fault is also clarified.

Design and current-limiting simulation of magnetic-shield type superconducting fault current limiter with high Tc superconductors

The continuous development of electric power utilities has led to the increase in the problems concerning fault currents. The use of fault current limiters, which suppress fault currents below a prescribed level, has been examined in many places. The authors have studied a superconducting fault current limiter that is based on the magnetic shielding effect of superconductors. In this paper, we derive a requirement for leading to a flux-jumping inside a ceramic superconductor at liquid nitrogen temperature, Next, two kinds of magnetic-shield type superconducting fault current limiters are designed, and their current-limiting simulations are tried using a computer.

An experimental study on a fast self-acting magnetic shield type superconducting fault current limiter

The current limiting characteristics in a fast self-acting magnetic shield type superconducting fault current limiter were investigated experimentally and compared with the computer simulations. One of the features of the fault current limiter is that the superconducting magnetic shielding body is automatically heated up by the heaters glued on it when a fault occurs. It is revealed that the superconductor is heated up in a short period of time after a fault occurs. Hence current limiting characteristics of high performance are realized. The experimental results are compared with the computer simulations.

Investigations on current limiting performances in magnetic shield type high Tc superconducting fault current limiter

Abstract In a magnetic shield type superconducting fault current limiter, the flux flow resistivity ϱ f has to be increased to improve its current limiting characteristics. We studied the effect of ϱ f on current limiting characteristics with both experiments and computer simulations by applying 1 kHz a.c. or pulse currents to a toroidal coil wound on the superconducting tube and by changing ϱ f . It has been made clear that the impedances are considerably increased to about eight times an initial value at 1 kHz a.c. current of 200 A. Computer simulations suggested that such a large increase of impedance will arise from a flux flow resistivity of one order of magnitude larger than the one expected in an ordinal flux flow state in a magnetic field.

Thermal design and performance tests of a current limiter with a conduction cooled Nb/sub 3/Sn screen

We examined the possibility of the practical use of a conduction cooled magnetic shield type fault current limiter from the view point of a thermal design. The thermal calculation was carried out with a small model using the Nb/sub 3/Sn wire with low AC loss. We found that the temperature increase of the small model was insignificant. We also confirmed the current limiting operation in an experiment. The conceptual design and the thermal calculation of the conduction cooled fault current limiter for a distribution power system is also discussed in this report.

Stress Evaluation in a Bulk High-Temperature Superconductor during Field-Cooling Magnetization

Stresses in a bulk high-temperature superconductor are numerically evaluated during the magnetization process by field cooling. Shielding current distributions are obtained through a macroscopic numerical simulation with the Maxwell equations and the critical state model. The stress distributions are obtained through numerical analysis with the finite-difference method. Two numerical codes are combined in the iterative calculation. Peak values of the maximum hoop, radial and shear stresses are obtained under different conditions during the magnetization.

Thermally and magnetically stable flux flow state in type‐II superconductor

Behaviors of flux flow states under thermal or magnetic disturbances have been studied using a Pb–In–Sn superconductor with the property ∂JC/∂T>0. Existence of the previously reported stable flux flow states has been confirmed clearly. When heating the sample the flux flow voltages decrease stably to zero (recovery of superconducting state). Also, flux flow states are found to be stable against magnetic field sweeping. They further indicate that magnetization will increase with increasing field sweep rates.

Stability tests against thermal disturbances on rotating superconducting field windings

Abstract A superconducting test rotor was constructed and the transient stabilities of the field windings were studied. They were evaluated at various centrifugal accelerations up to 2000 g and it was found that the field windings with bare strands were extremely stable against transient disturbances, and that the insulator coating on the strands might be very detrimental from the viewpoint of both steady state and transient stabilities. It was also clarified that both the minimum film boiling and maximum nucleate boiling heat fluxes were considerably increased, even inside a slot under large centrifugal acceleration, and that these increases contributed to the improvement of stability of the windings. It was shown that even a small cooling channel inside the windings could be effective for cooling.

DC and pulse operations of 4 MJ pulsed superconducting magnet and its stress analysis

4 MJ pulsed magnet was tested in dc and pulse operating modes, It was successfully charged up to 5940 A ( 4.6 MJ, 6.9 T) at 0.15 T/sec and discharged at 3.5 T/sec without quenching. Strains due to hoop stress were measured and analysed using a spring model. It is pointed out that they do not necessarily propagate from turn to turn and hence the strains measured at the outer support band may be small. A residual compressive force in the magnet windings was also analysed. Ac losses were measured using a 3 MJ pulsed magnet having almost the same structure and conductor as 4 MJ magnet and shown to be fairly small.

Stability of a shorted Nb/sub 3/Sn coil cooled by a refrigerator for a magnetic shield type fault current limiter

A stabilizing method for a shorted Nb/sub 3/Sn coil, which will be cooled by a refrigerator at around 10 K and be applied to a magnetic shielding body for a magnetic shield type fault current limiter, is proposed. The stabilizing method is realized by short-circuiting the terminals of the limiting coil through a semiconductor device and keeping the voltage across the Nb/sub 3/Sn coil constant. The stability is studied in a single layer Nb/sub 3/Sn coil operated in a constant AC voltage mode in liquid helium or helium gas. The method has shown to be quite effective for stabilization.