Kenji Shimohata

Research & development of superconducting fault current limiter in Japan

R&D of fundamental technologies for superconducting AC Power equipment (Super-ACE project) started as a national project, for a five-year plan since fiscal 2000 by MITI and NEDO. This project is to research and develop the basic technology of super-conductive cable, fault current limiter and transformer. This paper summarizes the research and development of following 2 types FCL. The R&D of the SN transition resistive type superconducting thin film fault current limiter aims to establish large-current/high-voltage technology by arranging films in multi-parallel/series, and increasing the current-carrying capacity through widening the area of the thin film of the current limiting element. The R&D of the reactor for a rectifier-type fault current limiter using coils of superconducting wire of the Ag sheath Bi2223 tape, aiming to apply it to high-voltage systems.

Design of a large current-type fault current limiter with YBCO films

Superconducting fault current limiters (FCLs) with YBCO films are under development in a national project in Japan. In this project, Mitsubishi Electric Corporation is in charge of the development of a large current-type FCL with a target capacity of 1 kA/200 V. To develop the FCL, numerical simulations on the combined thermal and electrical behavior of the device have been carried out considering inhomogeneity. The thermal stress of the FCL has also been studied by FEM using the calculated temperature profile. Here, the assumed size of a YBCO film on a sapphire substrate with a metal shunt layer is 30 mm in width and 100 mm in length. The critical current (I c ) of the film is assumed as 210 A +20% or -20%. The main results are as follows: (1) 16 films are arranged as a series of two stacks each consisting of eight films in parallel. (2) A polygonal arrangement is adopted for the YBCO films to obtain uniform current distribution. (3) The maximum temperature is 150 K and the thermal sink on the film effectively reduces temperature increase. (4) The thermal stress caused by the temperature gradient is <10 MPa. Therefore, mechanical breakdown is not a serious problem with this device.

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Abstract Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current ( I c ). (2) The current in a superconducting film was uniform when the current was much higher than I c . The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I c . (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.

Numerical analyses for ramp rate limitation from the standpoint of heat generation during current redistribution [in superconducting cables]

The ramp rate limitation (RRL) must be improved for large applications, for instance fusion machines. In superconducting multi-strand cables, adding to the coupling loss, the heat generation during current redistribution (moderation of a nonuniform current) causes temperature rises. Especially for cables in conduit-type conductors (CICC), the relation between the heat capacity of the coolant and the total heat dissipation determines the temperature rise. When this rises above the current sharing temperature, the conductor must quench. To establish stability against nonuniform current distribution, a small contact resistance between strands is preferable. However a smaller contact resistance results in a larger inter-strand coupling loss. Therefore, the contact resistance must be optimally designed to prevent the cable from RRL. In this study, the authors analyzed the current redistribution in a three-strand cable with electrical contact between strands. The heat generation due to: (1) normal resistance; (2) contact resistance between strands; and (3) terminal joint resistance were evaluated in the cases of a variety of contact resistances and cooling conditions. Finally, some of the particular phenomena reported as being found in experiments with multi-strand cables were simulated by analyses and then discussed.

A Conceptual Design of a Superconducting Magnet for a Magnetic Levitated Train Using a High Tc Oxide Superconducting Wire

A conceptual design of a superconducting magnet for magnetic levitated train (MAGLEV) using a silver sheathed wire of high Tc oxide superconductor is studied. In the conceptual design, the superconducting magnet is operated by persistent current at 20K. The efficiency and the reliability of refrigerator are high at 20K. The critical current density of 6x104 A/cm2 is required at 5T for the conductor. The coil is stable against disturbance, because the minimum quench energy is large. On the other hand, the quench propagation velocity of the coil is low. The coil needs active quench protection.

Application for Current Lead Using Bi-Based Oxide Superconductor

The long size superconducting plates for current lead were made via a few times of an intermediate pressing to (Bi,Pb)2Sr2Ca2Cu3Oy (2223) bulk samples. The critical current density was improved by the process, which was over 103A/cm2 at 77K, OT. The minimum contact surface resistivity was performed by sintering silver — films coated on the superconductor surface as the contact pads, which was about 10−6Ωcm2 at 77K, OT. We developed 500A — class current lead using 2223 bulk plates with 80K thermal anchor, and we could show that the thermal load from the lead drastically decreased under 20% in comparison with optimum designed metal leads with helium vapor cooling.

Research and development of slow response type 70 MW class superconducting generator with high-current density superconductors

Abstract Some 70 MW class superconducting generators have been developed in Japan to verify the basic technologies for a 200 MW class superconducting pilot generator. The manufacture of a 70 MW rotor with a slow response excitation began from 1992, and factory tests were conducted in 1996. Succeeding the factory tests, the rotor was combined to the stator with air gap windings at the verification test facility in Osaka power station, and verification tests were conducted in 1998. The advantages and reliability of superconducting generators had been clarified by these tests. In this paper, verification test results including stability of superconducting field windings and helium cooling characteristics are described. The main results are summarized as follows: (1) the advantages of the superconducting generators, such as increasing generation efficiency and improving power system stability were verified, and the operational reliability was confirmed by the long-term operation, (2) the superconducting stability against constant current and rapid current variations corresponding to the load conditions of the 200 MW class slow response pilot generator were confirmed to be satisfactory and (3) helium cooling characteristics such as cooling down and heat loss were confirmed to be also satisfactory.

Development of Superconducting Joint of NbTi Wire

The high performance of the superconducting joint of NbTi multifilament superconducting wires is achieved by using a sleeve that is reinforced and is tapered at the inner corner of the sleeve. The critical current of the joint is 1, 800A at 5T. This value is equal to the critical current of the superconducting wire. The superconducting joints are adopted as a superconducting magnet for synchrotron radiation. The critical currents of the joints between the wires for the coils are over 1, 100A at 5T for all joints, and those between the wires for the coil and persistent current switch exceed the quench current of the persistent current switch. By testing the superconducting joint using two filaments, it is suggested that the superconducting current passes at the joint when the interface of the NbTi filaments are slipped and deformed.

Calcination Conditions of Powder for AG-sheathed Bi-2223 Tapes

Precursor powder of a nominal composition of Bi2Ph0.44Sr2.2Ca2.4Cu3.4Ag0.5Oy by citrate method was calcined with two different processes that first heating temperatures were different. Intermediate phases formed during calcinations were analyzed by EPMA. In the calcination at 700°C~830°C, Bi2.0Pb0.03Sr0.1Ca1.3Cu0.1Ox formed first and turned into Bi-2212 phase, where Ca content was higher than Sr content, as increasing temperature. In the calcination at 750°C~830°C, the Bi-2212 phase, where Sr content was higher than Ca content, formed first. The Ca-rich Bi-2212 phase changed to the Bi-2223 at lower temperature than the Sr-rich 2212. Jc’s of Ag-sheathed tapes using the powder through the Ca-rich Bi-2212 were higher than the Sr-rich 2212.

Acoustic emission monitoring on a model field winding for the 70 MW class superconducting generator

Abstract Acoustic emission monitoring on a model field winding for the 70 MW class superconducting generator was performed during cooldown, excitation and quenches to diagnose the integrity of the model field winding and improve the reliability of the field windings for the generator. The results confirm that the acoustic emission signals are mainly due to conductor motions which cause premature quenching of the winding. It is therefore important to support the field windings as rigidly as possible to prevent conductor motions and to increase the stability of the field windings.