A. Tomioka

Kyushu University, Hukuoka, Fukuoka, Japan

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Publications (41)31.98 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The upgrading plan of the Mini-RT which is an experimental device for plasma physics was initiated to replace the magnetically-levitated high temperature superconducting (HTS) coil by the new one wound with the latest REBCO tapes. The performance of the new HTS coil has been examined at the National Institute for Fusion Science. The coil was indirectly cooled to 35 K by forced flow of cold helium gas. The coil was successfully excited up to 100 A with a proper persistent current switch (PCS) operation. The central field given by the Hall probe was 0.21 T, which was the same with that expected by a numerical calculation. The time constant of the current decay during the persistent current mode was evaluated at the coil temperature of 36 and 41 K to be 306 and 228 h, respectively. These values are consistent with the estimation based on the measured joint resistances and they are much longer than that of the previous floating coil of Mini-RT wound 10 years ago (41 h before aged deterioration). By the test results, it has been confirmed that the promising performance of the manufactured REBCO floating coil has been achieved to further promote the Mini-RT project.
    IEEE Transactions on Applied Superconductivity 01/2014; 24(3):1-4. · 1.20 Impact Factor
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    ABSTRACT: We are developing an elemental technology for 66/6.9 kV 20 MVA-class superconducting power transformer with fault current limiting function. In order to obtain the characteristics of YBCO conductor when the AC over current supplied to the conductor, the model coils were manufactured with YBCO tapes and tested. Based on these results, we manufactured the 6.9 kV/2.3 kV 400 kVA-class YBCO model transformer with fault current limiting function and performed short-circuit test. At the 0.25 s after short-circuit, the short-circuit current of primary winding was limited to about 174 A for a prospective current of 559 A. It was consistent with the design. The I–V characteristics of the winding did not change before and after the test. We consider the model transformer to be able to withstand AC over-current with the function of current limiting. The results suggest the possibility to design YBCO superconducting transformers with fault current limiting function for practical power grid.
    Physica C Superconductivity 01/2013; 484:239–241. · 0.72 Impact Factor
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    ABSTRACT: In the Japanese national project to develop superconducting power applications using Y-based coated conductors, the present focus is on various aspects of technological development of superconducting power transformers. Since Y-based multi-filament wire contributes to reduction in AC loss of superconducting transformer coils, a 5mm-wide long-length multi-filament, between the filaments of 3 and 5, has been developing by a laser scribing method. In the case of the project target, a 66kV/20 MVA power transformer coils require a Y-based multi-filament wire between the lengths of 200m and 300m. It also must withstand over-current of about seven times the rated current, which is generated when a power system fault occurs. In this paper, over-current characteristics of model coil using Y-based 3-filament wire were evaluated.
    Physica C Superconductivity 01/2011; 471(21):1379-1380. · 0.72 Impact Factor
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    ABSTRACT: We designed and fabricated a 10 kVA single-phase transformer with RE1Ba2Cu3O7-δ (REBCO, RE:Rare Earth, Y, Gd, etc.) superconducting tapes. It had a 4-winding structure at the beginning. In this study, we removed the auxiliary windings and made a test as a general 2-winding transformer. Making repeated short circuit tests, we further investigated the response of the REBCO superconducting windings against a fault excess current. Although the short circuit current decayed satisfactorily, the whole of the REBCO superconducting windings did not shift to normal state even if the short circuit current exceeded the critical current all over the length of REBCO tapes. The induced normal resistance depended on the primary voltage. To study the phenomena, we derived the basic equations and made numerical simulations taking into account of flux-flow state, in which the equivalent resistance and generated heat are much smaller than those in normal state, and also the dependences of the critical current on temperature, applied magnetic field and the applied field angle to the tape face. It was clarified that short circuit current was limited not only by normal resistance but also by flux-flow one. The introduction of the concept of flux-flow state made it possible to quantitatively explain the behavior of the REBCO superconducting transformer at a sudden short circuit.
    IEEE Transactions on Applied Superconductivity 01/2011; 21(3):1405-1408. · 1.20 Impact Factor
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    ABSTRACT: We are developing elemental technology for 66kV/6.9kV 20 MVA-class YBCO power transformer. The YBCO transformer is considered to have a possibility to stabilize the power system by improving function of fault current limiting. Current limiting behavior functions over critical current flows. There is a possibility that superconducting characteristic may be damaged due to increase in temperature of YBCO tapes. Therefore, we have taken a measure to combine YBCO tape with CuNi tape or Cu Tape. We manufactured model coils using these conductors and conducted the AC over-current tests. The test current was two to seven times larger than the Ic of conductor and it was damped with time from its maximum value according to the generation of conductor resistance. We verified the effectiveness of current limiting characteristics.In these tests, the Ic of model coil did not degrade. We consider this conductor to be able to withstand AC over-current with the function of current limiting.
    Physica C Superconductivity 01/2011; 471(21):1367-1373. · 0.72 Impact Factor
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    ABSTRACT: We are developing an elemental technology for 66 kV/6.9 kV 20 MVA-class power transformer with YBCO conductors. The protection of short-circuit technology is one of the elemental technologies for HTS transformer. Since short-circuit current is much higher than critical current of YBCO tape, there is a possibility that superconducting characteristics may be damaged during short-circuit period. We made a conductor to compose the YBCO tape with copper tape. We manufactured 6.9 kV/2.3 kV 400 kVA-class YBCO model transformer using this conductor and performed short-circuit current test. The short-circuit current of primary winding was 346 A which was about six times larger than the rated current. The I–V characteristics of the winding did not change before and after the test. We may consider this conductor withstands short-circuit current.Highlights► The 6.9 kV/2.3 kV 400 kVA-class single-phase YBCO model transformer with the YBCO tape with copper tape was manufactured for short-circuit current test. ► Short-circuit test was performed and the short-circuit current of primary winding was 346 A which was about six times larger than the rated current. ► The I–V characteristics of the winding did not change before and after the test. ► The transformer withstood short-circuit current. ► We are planning to turn the result into a consideration of a 66 kV/6.9 kV-20 MVA-class three-phase superconducting transformer.
    Physica C Superconductivity 01/2011; 471:1374-1378. · 0.72 Impact Factor
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    ABSTRACT: We have been developing a light weight and high efficiency superconducting traction transformer for railway rolling stock. We designed and fabricated a prototype superconducting traction transformer of a floor-mount type for Shinkansen rolling stock in 2004. We performed the type-test, the system-test, and the vibration-test. Consequently, we could verify that the transformer satisfied the requirement almost exactly as initially planned. However, there have been raised some problems to be solved to put superconducting traction transformer into practical use such that AC loss of the superconducting tape must be lower and the capacity of the refrigerator must be larger. Especially it is the most important to reduce the AC loss of superconducting windings for lightweight and high efficiency. The AC loss must be reduced near the theoretical value of superconducting tape with multifilament. In this study, we fabricated and evaluated the Bi2223 tapes as introduced various measures to reduce the AC loss. We confirmed that the AC loss of the narrow type of Bi2223 tapes with twist of filaments is lower, and we fabricated windings of this tape for use in superconducting traction transformer.
    Journal of Physics Conference Series 01/2010; 234(3).
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    ABSTRACT: We designed and fabricated a 7.5 kW-360 rpm synchronous motor with a superconducting rotor. The rotor was 6-pole type and has no iron core. The armature winding was wound with copper wires. 6 pieces of racetrack-shaped field coils were wound with 5 mm wide RE<sub>1</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7-delta</sub> (REBCO, RE:Rare Earth) superconducting tapes produced by an ion beam assisted deposition method and a pulsed laser deposition method. The total length of REBCO tape was 400 m per a field coil. We confirmed the good transport properties of the respective field coils up to 80 A at 40 to 50 K before and also after the assembly to a motor. The maximum magnetic field in the field coils was 0.67 T. The superconducting rotor was cooled down to around 40 K by forced-flowed helium gas and the completed motor was tested in a similar way to conventional motors. As a result of a load test, the designed operation of 7.5 kW at 360 rpm was verified for the rated field current of 60 A. Even when the field current was increased up to 70 A at 40 K, the temperature at each part of the superconducting rotor was held constant and the quite stable operation up to 11 kW at 360 rpm was also achieved.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: In Japan we started a national project to develop a 66/6.9 kV–20 MVA transformer with REBCO superconducting tapes in 2006. This paper gives an overview of progress of the development of superconducting transformers in Japan and also describes the fundamental technologies studied before now to realize a 66/6.9 kV–20 MVA transformer as follows. To reduce the ac loss in REBCO superconducting thin tapes, authors proposed a new method different from the conventional technique of reducing the ac loss in superconducting multifilamentary wires. It consists of scribing process into a multifilamentary structure by laser or chemical etching, and a special winding process. Making a multilayered solenoidal coil with laser-scribed REBCO tapes, we verified the ac loss reduction in proportion to a filament width even in coil configuration. In addition, to realize a current capacity more than the rated secondary current of 2.4 kA, we first investigated the workability of REBCO tapes in the actual winding process with forming a transposed parallel conductor, where REBCO tapes were bent edgewise at transposing points. Making a test coil of a 24-strand parallel conductor, we verified no degradation of the critical current and nearly uniform current distribution among the tapes. The result suggests the applicability of the method of enhancing the current capacity by forming a parallel conductor with REBCO tapes. Further, to realize the dielectric strength regulated for the Japanese standards, i.e. lightning impulse withstand level of 350 kV and excess ac voltage of 140 kV, we made test coils and carried out dielectric breakdown tests. As a result, we got hold of the required insulation distance at the important points from the viewpoint of insulation design.
    Physica C Superconductivity 01/2009; 469:1726-1732. · 0.72 Impact Factor
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    ABSTRACT: We are developing a 7.5 kW-360 rpm synchronous motor with a superconducting rotor. The rotor is 6-pole type and has no iron core. The armature winding is wound with copper. The designed operating temperature is 40 K. It will be cooled by forced-flowed helium gas. We have finished the winding of the first superconducting field coil with YBCO superconducting tapes fabricated by IBAD-PLD method. It is a 62-layer solenoidal coil. The total length of YBCO tape is 400 m. The magnetic field in the field winding is about 0.4 T at maximum and that in the gap is 0.2 T. We cooled it down to 20 K by G-M type cryocooler and investigate the transport properties in advance of the winding of the other field coils. We verified the stable operation up to 100 A at 40 K against the rated current of 50 A. In addition we prepared the cryogenic rotor system and installed 6 pieces of dummy copper coils instead of superconducting coils. We cooled it down to 30 K by forced-flowing helium gas and tested it by rotating up to 360 rpm. We verified the satisfactory good mechanical and thermal operation.
    IEEE Transactions on Applied Superconductivity 07/2008; · 1.20 Impact Factor
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    ABSTRACT: Implementation of a conventional current lead with a pulse tube refrigerator has been validated to be working as an Integrated Current Lead (ICL) system for the Superconducting Magnetic Energy Storage (SMES). Realization of the system is primarily accounted for the flexibility of a pulse tube refrigerator, which does not posses any mechanical piston and/or displacer. As for an ultimate version of the ICL system, a High Temperature Superconducting (HTS) lead links a superconducting coil with a conventional copper lead. To ensure the minimization of heat loads to the superconducting coil, a pulse tube refrigerator has been upgraded to have a second cooling stage. This arrangement reduces not only the heat loads to the superconducting coil but also the operating cost for a SMES system. A prototype two-stage pulse tube refrigerator, series connected arrangement, was designed and fabricated to satisfy the requirements for the ICL system. Operation of the first stage refrigerator is a four-valve mode, while the second stage utilizes a double inlet configuration to ensure its confined geometry. The paper discusses the optimization of second stage cooling to validate the conceptual design
    03/2008;
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    ABSTRACT: We have strategically promoted R&D activity for the development of elemental technologies in order to put into practical HTS power transformers applying YBa2Cu3O7 (YBCO) coated conductors, focused on AC loss reduction in HTS coils. This paper shows AC loss reduction in model coils of YBCO tapes divided into five filaments by laser ablation. At first experiment, magnetization loss of YBCO tape with five filaments and YBCO tape with non-divided were firstly measured using a pick-up coil technology. Then, AC loss measurement is performed using two types of model coils of YBCO tapes with five filaments and non-divided, by two different technologies: an electric technique (i.e. 4-probe) and a calorimetric technique (i.e. boil-off). The results of the experiment show that the AC loss in the model coil of YBCO tapes with five filaments was reduced to one-fifth when compared with model coil of YBCO tapes with non-divided.
    Physica C Superconductivity 01/2008; 468:1731-1733. · 0.72 Impact Factor
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    ABSTRACT: We have been developing a lightweight and high-efficiency superconducting traction transformer for railway rolling stock. We designed and fabricated a prototype superconducting traction transformer of a floor-mount type for Shinkansen rolling stock. In this study, we present test results such as type test, system test and vibration test. We performed the type test in accordance with JIS, E5007. In this test, we measured the basic electrical characteristics. In the system test, we tested the transformer with a dynamic simulator for rolling stock, as a converter connected at secondary winding, and verified the operating characteristics. We performed vibration tests in accordance with JIS, E4031, vibrating the transformer on a shaking table with three-dimensional movement. As a result, we could verify that the transformer satisfied the requirement almost exactly as initially planned.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: We developed a 15 kW synchronous motor with a fixed superconducting field winding. It was 8-pole type. Each field coil was a racetrack-shaped single pancake wound with a YBCO superconducting tape. It was fabricated by IBAD-PLD method and 10 mm in width. The rotating armature was composed of a copper winding and an iron core. The field coils were conduction-cooled down to 20 to 30 K through a copper pipe attached to the copper coil flange, in which helium gas was forced-flowed. The completed motor was first tested in the Suzuka factory of Japan motor and Generator Co. Ltd. before shipping as usual. Then it was moved to the test site of the Nagasaki shipyard of Mitsubishi Heavy Industry Ltd. and installed into a ship propulsion test system, which had a propeller with a diameter of 0.5 m. We verified the quite stable underwater operation and the output power of 15 kW-360 rpm as designed.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: It is highly expected that superconducting transformers, which are characterized by incombustibility, light weight, and compactness, will be used practically for equipment in substations and office buildings sited in urban areas. In recent years, remarkable improvements in the performance of YBaCuO (YBCO) tape wire have been seen. This kind of tape wire has the property that its critical current is large in a highly magnetic field; and, its use is expected to reduce AC loss by applying slit processing on the tape wire, contributing to future cost reductions. In addition, the development of efficient, economical cryogenic units is also expected. The authors have developed a conceptual design for a 66kV/20MVA power transformer using YBCO tape wires, or an YBCO power transformer. As a result, we were able to identify the transformer’s optimal system structure including coil winding comprised of coils and tape wires; and, we confirmed that the devised transformer can made to be both compact and highly efficient.
    Physica C Superconductivity 01/2007; 463:1233-1236. · 0.72 Impact Factor
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    ABSTRACT: The authors have designed and fabricated a 6.9kV–500kVA 3-phase HTS reactor, based on the elemental technologies achieved in the previously developed single-phase HTS transformers, which were focused on the reduction of AC loss for enhancing efficiency as well as the creation of a disk winding method for coil downsizing. This paper describes the results obtained from an experiment conducted to verify the performance of the HTS reactor and to identify its characteristics when connected to the grid.
    Physica C Superconductivity 01/2007; 463:1237-1239. · 0.72 Impact Factor
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    ABSTRACT: We designed a floor type single-phase 4 MVA superconducting traction transformer for Shinkansen rolling stock. In this study, we fabricated a prototype superconducting traction transformer based on this design. This transformer of the core-type design has a primary winding, four secondary windings and a tertiary winding. The windings are wound by Bi2223 superconducting tapes and cooled by subcooled liquid nitrogen. The core is kept at room temperature. The cryostat is made of GFRP with two holes to pass core legs through. The outer dimensions are about 1.2m × 0.7m × 1.9m excluding the compressor. Its weight is 1.71t excluding that of refrigerator and compressor. The transformer was tested according to Japanese Industrial Standards (JIS)-E5007. We confirmed that the performance of transformer has been achieved almost exactly as planned. The rated capacity is equivalent to 3.5MVA in the superconducting state.
    Journal of Physics Conference Series 07/2006; 43(1):841.
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    ABSTRACT: We fabricated a cryocooler-cooled Bi2223 superconducting pulsed coil and experimentally studied the thermal runaway in dc or ac operation. We carried out numerical simulation of thermal properties of the coil in order to explain the thermal runaway of the coil. Firstly, we analyzed the total heat generation of flux-flow losses and ac losses inside the winding from the experimental results of the external field losses and the E-J characteristics for the Bi2223 strands. Secondly, we numerically simulated the thermal properties by using a 2-dimensional heat conduction equation with axial symmetry. The numerical simulation shows the relation between the initiation of thermal runaway and the temperature distribution with highly concentrated heat source in the winding. We have a semi-quantitative agreement between the numerical results and the experimental ones for the condition of the thermal runaway
    IEEE Transactions on Applied Superconductivity 07/2006; · 1.20 Impact Factor
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    ABSTRACT: We studied the thermal and electromagnetic behavior of a conduction-cooled superconducting pulse coil. It was wound with a 4-strand parallel conductor composed of silver sheathed Bi2223 multifilamentary tapes and impregnated with an epoxy resin. We first investigated the temperature dependence of the thermal runaway current in dc operation. It was higher than the critical current by several tens amps at any temperature from 40 to 90 K. In order to study the mechanism of the thermal runaway, we carried out the numerical simulation of the coil behavior. We found out that the thermal runaway was caused when the heat generation at the winding end close to the cryocooler head where the flux-flow loss is the largest due to perpendicular magnetic field exceeded the cooling through thermal diffusion though the thermal runaway of the coil started at the other end of the winding. If the transport current is smaller than the thermal runaway current at the coil temperature just after applying current, the temperature distribution in the longitudinal direction of the coil varied into a monotonous gradient and the steady thermal flow was established. After that the coil temperature decreased to the balanced one
    IEEE Transactions on Applied Superconductivity 07/2006; · 1.20 Impact Factor
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    Advances in Science and Technology 01/2006; 47:204-211.