Katsuyuki Kaiho

Study of Superconducting Fault Current Limiter Using Vacuum Interrupter Driven by Electromagnetic Repulsion Force for Commutating Switch

Using a high-temperature superconductor, we constructed and tested a model superconducting fault current limiter (SFCL). The superconductor and the vacuum interrupter as the commutation switch were connected in parallel using a bypass coil. When the fault current flows in this equipment, the superconductor is quenched and the current is transferred to the parallel coil because of the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter. Due to the opening of the vacuum interrupter, the current in the superconductor is broken. By using this equipment, the current flow time in the superconductor can be easily minimized. On the other hand, the fault current is also easily limited by large reactance of the parallel coil

Characteristics analysis of transformer type superconducting fault current limiter

The transformer type superconducting fault current limiter, which is made up of a series transformer and a superconducting current limiting device, has many advantages such as the design flexibility of the current limiting device. However, the design strategy for determining the superconducting current limiting device ratings and the transformer ratings is unclear because the relations between the current limiting characteristics and these ratings are not analyzed enough. In this paper, the relations between the current limiting characteristics and these ratings are analyzed theoretically. The experimental results of the transformer type superconducting fault current limiter are also shown.

Degradation mechanism of Nb 3 Sn composite wires under tensile strain at 4.2K

Bronze-processed Nb 3 Sn composite wire conductors exhibit changes in their superconducting parameters when strained in tension. This paper describes a detailed study of the effect of strain on critical current and an analysis by optical and SEM techniques of crack formation in the Nb 3 Sn layer under strain. The effect of strain history on both reversible and irreversible changes in critical current and the roles of differential thermal contraction induced residual strains and of Nb 3 Sn cracking are discussed.

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.

Current limiting characteristics of transformer type superconducting fault current limiter

The transformer type superconducting fault current limiter is composed of a series transformer and a superconducting current limiting device. The primary winding of the transformer is connected in series to the power transmission line, and the secondary winding is short-circuited through the superconducting current limiting device whose normal resistance plays an important role in limiting the fault current. According to our previous investigations, the basic characteristics and the design indexes of the transformer type superconducting fault current limiter were clarified. In this paper, the results of the current limiting test of a 5 kVA class transformer type superconducting current limiter are analyzed in detail, and the limiting characteristics such as the dependency of the initiation current of the limiting operation on the fault phase angle are shown.

Temperature Characteristics of Superconducting Thin-Film Fault Current Limiting Elements Using High-Resistivity Alloy Shunt Layers

We have been investigating a superconducting fault current limiter (FCL), in which YBCO superconducting thin films with Au-Ag alloy shunt layers are used. The ability of the films to withstand high electric fields (> 40 Vpeak/cm) enables the total length of FCL elements to be reduced, thus greatly reducing the cost of FCLs. In this paper, we report the temperature characteristics of Au-Ag/YBCO composite films with sapphire substrates. Temperature of the film was estimated as average temperature from voltage and current in the films during the over-current period of 0.1 s using the relation between their resistance and temperature. The effect of thickness of the substrate was examined. Temperature data were analysed with the relation between the maximum temperature and joule heating during the over-current period. Data on recovery time are also shown.

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.

500 V/200 A fault current limiter modules made of large-area MOD-YBa2Cu3O7 thin films with high-resistivity Au–Ag alloy shunt layers

We developed 500 Vrms/ 200 Arms superconducting thin-film fault current limiter (FCL) modules that can withstand high electric fields (E>30 Vrms cm−1) by using large-area YBa2Cu3O7 (YBCO) thin films with high-resistivity Au–Ag alloy shunt layers. Au–Ag alloy films about 60 nm thick were sputter-deposited on YBCO/CeO2/sapphire films (2.7 cm × 20 cm) prepared using a fluorine-free MOD method. Each 20 cm long Au–Ag/YBCO film was then divided into three segments (each ~5.7 cm long) by four Ag electrodes deposited on the Au–Ag layer, resulting in an effective length of 17 cm. The 500 V/200 A FCL modules were then fabricated by first connecting two of the segmented films in parallel using Ag-sheathed Bi-2223 superconducting tapes and then connecting in parallel an external resistor and a capacitor for each segment to protect the Au–Ag/YBCO film from hot spots. Switching tests using a short-circuit generator revealed that all the modules carried a superconducting ac current of ≥237 Arms and that modules prepared with YBCO films having a relatively homogeneous critical current Ic distribution successfully withstood ≥515 Vrms for five cycles without any damage. These results demonstrate that (a) the FCL modules fabricated here successfully achieved the rated current of 200 Arms and rated voltage of 500 Vrms and (b) total area of the YBCO films on sapphire substrates required for the 500 V/200 A (100 kV A) module was less than one-third that for conventional thin-film FCL modules that use gold shunt layers, leading to the significantly reduced cost of thin-film FCLs. Film damage due to hot spots depended on the difference in Ic between the two parallel-connected films and on the inhomogeneity of the Ic distribution in the film, and is most probably due to nonlinear current flows at the moment of quenching that cause local overheating.

Low-cost and high-power-density resistive fault-current limiting elements using YBCO thin films and Au-Ag alloy shunt layers

We propose a new design for the high-temperature superconducting thin-film faultcurrent limiter (FCL), which uses high-resistivity Au-Ag alloy shunt layers instead of the pure gold (or silver) shunt layers conventionally used. An FCL element (5 mm wide and 40 mm long) with a YBCO thin film (THEVA) and a parallel inductively-wound shunt resistor successfully withstood very high electric field (> 44 Vpeak/cm) for 5 cycles (0.1 sec) after switching, and achieved a very high switching power density, ~2.0 kVA/cm2. We confirmed similar maximum tolerable electric field (>40 Vpeak/cm, limited by power supply) in a larger sample (1 cm ? 6 cm). The composition of our FCL element is very simple, and the achieved power density is more than five times higher than conventional devices, which leads to a dramatic reduction in the amount of expensive superconducting thin films. We made a conceptual design and cost estimation of our FCL elements used in a typical 6.6 kV FCL.

Diagnosis of ITER's large scale superconducting coils using acoustic emission techniques

In 2000, Japan Atomic Energy Research Institute (JAERI) and its collaboration team accomplished many kinds of experiments under the magnetic field of 13 T for the ITER Project. The target coils are the central solenoid (CS) model coil and the CS insert coil. In 2001, the test using both the CS model coil and the toroidal field (TF) insert coil was carried out and successfully finished. During the experiment, we have measured the change in the amount of mechanical disturbances inside the coil using acoustic emission (AE) technology. In this paper, we report the general trend of AE characteristics obtained in the experiments for two years. That is to say, as for the CS model coil, we investigated the training characteristics of the CS model coil that experienced one cooling cycle from 4.2 K to room temperature. As a result, we confirmed the training effect of the CS model coil wound by forced flow CIC conductors. On the other hand, as for the insert coil, some peculiar AE signals were observed during the CS insert coil cyclic test. On this matter, a re-examination was carried out.