Publications (54)43.59 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: Superconducting magnets for MRI need uniformity of produced magnetic field within ppm order in time and space. To develop the superconducting magnets for MRI operating in subcooled liquid nitrogen at 65 K, two pieces of small test coils were fabricated with Re1Ba2Cu3O7-δ (REBCO, Re: Rare Earth such as Y, Gd, and Eu) superconducting tapes. One was wound with a nonscribed tape and the other was with a laser-scribed tape into a four-filament structure. Each test coil was inserted inside a NbTi superconducting magnet. After bath cooling, the test coil with subcooled liquid nitrogen and applying a dc bias magnetic field of 0.5-2 T, transport current was applied step by step, and the variation of magnetic field at the coil center was observed by a Hall probe with high accuracy. Although magnetic field followed the variation of transport current with a delay in the respective coils, the drastic difference between them was revealed. It was demonstrated that the influence of the induced shielding current on the produced magnetic field was drastically reduced by scribing. Not only the magnitude of shielding current but also the decay time constant was reduced by scribing. The produced magnetic field of the test coil with the scribed tape reached to a target one in a short time as programmed. Scribing REBCO tapes brought about faster relaxation of magnetization.
- [Show abstract] [Hide abstract] ABSTRACT: We have designed and fabricated a 3Φ-66 kV/6.9 kV2 MVA transformer with RE1Ba2Cu3O7-δ(REBCO, RE:Rare Earth, Y, Gd etc.) superconducting tapes. It is a 1/10 model of a 3Φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. The superconducting windings were reduced only in current capacity by reducing the number of tapes in parallel conductors. In the primary side, a single REBCO tape with a width of 5 mm was cylindrically wound into 8 layers. In the secondary one, an 8-strand parallel conductor was wound similarly into 2 layers, where each strand was transposed 15 times per one layer. The REBCO tapes for the secondary winding were also scribed by laser into a 3-filament structure to reduce the ac loss. The windings for 3 phases were installed into a GFRP cryostat which had an elliptic-cylinder-shape and three cylindroid bore for an iron core at room-temperature. A Ne turbo-Brayton refrigerator with a cooling capacity of 2 kW at 65 K was developed and located close to the transformer. The windings were cooled with subcooled liquid nitrogen at 65 to 70 K, which was forced-flowed by a pump unit between the transformer and the refrigerator. The completed transformer was first tested in liquid nitrogen at 77 K according to the domestic regulation for conventional transformers. The load loss, i.e., ac loss of the windings, was 26.9 W for the rated operation. The dielectric strength was also verified by applying 350 kV impulse voltage and 140 kV ac voltage for 1 minute.
- [Show abstract] [Hide abstract] ABSTRACT: We are developing RE1Ba2Cu3O7-delta (REBCO, RE: Rare Earth, Y, Gd, etc.) superconducting transformers with a current limiting function. The target is a 3 phi-66 kV/6.9 kV-20MVA one for a distribution power grid. We designed and built two pieces of 1 phi-6.9 kV/2.3 kV-400 kVA superconducting transformers with YBCO superconducting tapes. One was wound with a YBCO tape with a copper stabilizing layer with a thickness of 300 mu m. The other was 50 mu m in thickness of copper. Making sudden short-circuit tests with a 200 MVA short circuit generator, we investigated the response of the YBCO superconducting windings against the fault excess current over the critical current. Repeated sudden short-circuit tests revealed the quite different response between them. In the case of fully stabilized with copper 300 mu m thick, short circuit current flowed without decay. On the other hand, in the case of copper 50 mu m thick, short circuit current was reduced around three times as much as the rated current. Using the numerical simulation program, which we made up on the basis of the results of sudden short-circuit tests of a 10 kVA test transformer, we quantitatively investigated the transition phenomenon of REBCO superconducting windings to normal state through flux-flow state due to fault excess current. As a result, theoretical simulation explained experiment quantitatively and clarified how to control the short circuit current.
- [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.
- [Show abstract] [Hide abstract] ABSTRACT: High-temperature superconductor technology has become remarkably advanced, and components such as current leads and magnets have been explored for their applicability to fusion. Especially, REBa2Cu3O7-δ (RE = rare-earth: REBCO) tape has been developed in research fields other than fusion, because of superior performance of REBCO in high magnetic fields. Here, we fabricated a REBCO coil with a radius of 150 mm and a total coil current of 55.2 kA, to be applied as an internal floating coil in Mini-RT, a plasma experimental device. This newly fabricated REBCO coil has replaced the Bi2Sr2Ca2Cu3O10 (BSCCO) coil developed in 2003, and has improved coil performance and plasma operation. A REBCO tape of width 4.3 mm was employed, and a 0.1 mm-thick copper laminate was attached for the countermeasure at the quench. In addition, the REBCO tape was wrapped in a polyimide sheet for electric insulation, increasing the total thickness of the REBCO tape to 0.27-0.28 mm. A persistent current switch was constructed from a small-turn REBCO winding with a heater formed from a stainless steel sheet. Solder joint of two REBCO tapes over a 30 mm length were employed, by overlapping their copper laminates. Persistent current, with a decay time ∼320 h, has been successfully achieved at 25 K operation temperature. This decay time corresponds to a resistance of ∼125 nΩ, roughly consistent with the total resistance of the seven joint sections in the persistent current circuit.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] ABSTRACT: RE1Ba2Cu3O7-delta(RE: Rare Earth, Y, Gd and so on, REBCO) superconducting tapes have high performance in critical current density, J(c), property even in high magnetic field and at liquid nitrogen temperature. We have proposed the ac loss reduction method of REBCO superconducting tape, which are the combination of a laser-scribing into a multifilamentary structure and a special winding process. Using the ac loss reduction technique, we could realize REBCO superconducting transformers with the advantage of light weight, compactness and high efficiency in comparison to conventional one. Further we aim to add a current limiting function to REBCO superconducting transformers. Recently we manufactured a 10kVA four-winding transformer with REBCO tapes. We carried out a sudden short circuit test and investigated the response against the fault excess current. We developed a numerical analysis program so as to quantitatively simulate the phenomena taking into account of a flux-flow resistance. As a result, it was shown that the whole of the REBCO winding did not shift to normal state even though the current exceeded the critical current all over the length. In this paper, we report the outlines. (C) 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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.
- [Show abstract] [Hide abstract] 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
Fuji Electric Co.Edo, Tōkyō, Japan
Kyushu UniversityHukuoka, Fukuoka, Japan