Natsuro Hobara

Development of 66 kV/750 A High-T/sub c/ superconducting fault current limiter magnet

One of the items included in the Super-conductive AC Equipment (Super-ACE) project, being performed from 2000 to 2004, is a development of a 66 kV/750 A high-T/sub c/superconducting (HTS) fault current limiter (FCL) magnet. This research focuses on fundamental technical items essential for a 66 kV class fault current limiter, that is, high current capacity, high voltage insulation and sub-cooled nitrogen cooling. This paper describes the design of the magnet and the test results obtained so far. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of unit-coils wound with Bi2223 tapes. The rated current of each coil is about 125 A at 70 K, and so the total current capacity of the magnet is 750 A. The insulation voltage of the magnet is of the 66 kV class. By the end of FY2002, three sets of the unit coils were set connected in the cryostat and some evaluation tests were implemented as a milestone of the program. In the cooling down test, sub-cooled nitrogen of 65 K was obtained with homogenous temperature distribution in the cryogen. In the overvoltage test, no breakdowns were observed in the case of applying both ac voltage of 140 kV for one minute and lightning impulse voltage of 350 kV. Voltage-current characteristics of the coils were measured in sub-cooled nitrogen. The rated current of 375 A was successfully obtained for both direct and alternate current tests. All the targets constituting the milestone of the 66 kV/750 A magnet development were achieved.

Development of RE123 coated conductor by ISD method

Abstract High rate deposition of HoBa2Cu3O7−δ (HoBCO) layers using the pulsed laser deposition method were experimented for long high temperature superconducting (HTS) coated conductor. Laser conditions and oxygen ambient during HoBCO deposition were precisely inspected. This revealed that the deposition rate of HoBCO reached to 5 μm/min. Even at such high deposition rate, the Jc of HoBCO films remained over 105 A/cm2 on the biaxial buffered substrate fabricated by the inclined substrate deposition (ISD) method. Moreover continuous tape production equipments were installed to make a long and flexible coated conductor by ISD method. These equipments will enable us to fabricate of over 100 m long coated conductor. 55 m long coated conductor were fabricated by using these equipments as our first trial of long length fabrication. Evaluation of Jc distributions of 55 m coated conductor revealed that all parts of 1 m in the 55 m conductor had the Jc over 104 A/cm2.

66 kV-class high-T/sub c/ superconducting fault current limiter magnet model coil experiment

As a part of the Super-conductive AC Equipment (Super-ACE) project being performed from 2000 to 2004, an AC magnet for a fault current limiter is being developed. This research focuses on fundamental technical items essential for a 66 kV class fault current limiter magnet wound with high-T/sub c/ superconductors operating at around 65 K. These items are high current capacity, high voltage insulation and sub-cooled nitrogen cooling. This paper describes experimental results obtained for a model coil wound with four-parallel Bi2223 tape conductors. Voltage-current characteristics, equal current distribution among tape conductors and overvoltage behavior are evaluated.

YBa2Cu3O7−δ thin-film-seeded Nd1+xBa2−xCu3O7−δ thick-film grown by liquid phase epitaxy

Abstract The initial stage of YBa 2 Cu 3 O 7− δ (YBCO) thin-film-seeded Nd 1− x Ba 2− x Cu 3 O 7− δ (NdBCO) thick-film-growth was studied using a vertically dipping experiment. The boundary between the liquid-untouched and the liquid-dipped regions were carefully observed under different conditions. Through a kinetic factor control, the low peritectic temperature (1010 °C) of YBCO can act as seed for growing NdBCO LPE thick films at a high processing temperature about 1055 °C. The YBCO grains partly decomposed into a Y 2 BaCuO phase with a Ba–Cu–O melt and partly retained. The retained YBCO grains are seeds for the LPE-grown NdBCO thick film, evident from the same orientation between YBCO and NdBCO grains and the Y segregated region detected from the initially grown NdBCO grains by EPMA.

Development of Coated Conductors in Japan

The recent achievements of critical current densities in excess of 1MA/cm2 at 77K in YBCO films deposited over appropriate bi-axially textured buffer layers/metal substrates have stimulated interest in the potential future applications of coated conductors at liquid nitrogen temperature and high magnetic fields, which are called as the next/second generation tape conductors. Several different processes for obtaining the bi-axially textured buffer layers/metal substrates as well as thicker film deposition processes and non-vacuum processes are reviewed. Under the Ministry of International Trade and Industry (MITI) of Japan administration through the New Energy and industrial Technology Development Organization (NEDO) as collaborative research and development of fundamental technologies for superconductivity applications, Superconductivity Research Lab. was given a responsibility of carrying out research and development of the coated conductors starting from the fisical year of 1999. This paper reviews the research goals of the project including the recent development of the related research by individual research group.

Efficient finite element analysis of electromagnetic properties in multi-layer superconducting power cables

It is highly beneficial to reduce AC losses in multi-layer superconducting power cables. For this purpose, each superconducting layer is wound with a different twist pitch on a former so as to balance current distribution among layers. Finite element analysis makes it possible to show electromagnetic properties in superconducting layers visually, which is useful for discussing methods to enable further loss reduction. However, making a numerical analysis model of a multi-layer cable which is faithful to its geometry is highly complicated and troublesome as the model requires a correction each time the set of twist pitches needs to be changed. The authors have developed a new method to analyze such a cable by introducing anisotropy on the conductivity. In this paper, the method and the effects are discussed and compared with experimental results.

Dielectric characteristics of 66 kV/750 A high-T/sub c/ superconducting fault current limiter coil

An insulation system for the 66 kV class high-T/sub c/ superconducting fault current limiter (FCL) coil has been developed. The breakdown characteristics were investigated for three kinds of model coils in 77 K liquid nitrogen and 65 K sub-cooled liquid nitrogen at atmospheric pressure. For the model coils, the breakdown tests at AC voltage and lightning impulse voltage in 77 K liquid nitrogen were carried out. The breakdown voltage in 65 K sub-cooled liquid nitrogen confirmed that the breakdown voltage is almost equivalent to 77 K liquid nitrogen. To verify this insulating capability, performance tests with lightning impulse voltage of positive and negative 350 kV were carried out in the 8-layer coil. A power lead that was insulated by ethylene propylene rubber (EPR) without filler was constructed based on the results of the R&D. The insulation systems of these power leads were evaluated for crack resistance and electrical insulation properties. No cracks occurred after seven trials of thermal cycling. The power leads withstood AC 140 kVrms for 1 min and the lightning impulse of positive and negative 350 kV three times each.

Preparation of MgO films on metal substrate as a buffer layer for liquid phase epitaxy processed RE123 coated conductor

Abstract The fabrication process of in-plane aligned MgO films on metal substrate as a buffer layer for liquid phase epitaxy (LPE) processed RE123 (RE123: RE=Y, Sm, Nd…) coated conductor was developed using the inclined substrate deposition (ISD) method with electron-beam evaporation. The texture development was investigated and a growth mechanism of in-plane alignment was discussed. Although the in-plane alignment of MgO can be obtained due to ISD, the reaction between metal substrate and high temperature melt cannot be prevented by the layer because of the presence of column spacings. Then, a two-step deposition process was developed for LPE method. The first layer was deposited by the ISD to obtain the in-plane alignment and the second layer was deposited at high temperature to realize the full coverage which is necessary to prevent the reaction between metal substrate and MgO-saturated Ba–Cu–O melt. This two-step deposited MgO buffer layers realize both the in-plane alignment and prevention of the reaction, and make the LPE process enable to apply for coated conductors.

Spiral Pitch Adjusted Multi-layer Conductor with Alloy-sheathed Wire

Current distribution of spiral pitch adjusted multi-layer conductor was investigated with rogowski coils. Condition of the spiral pitch to suppress the unbalanced current distribution was determined by circuit model regarded the multi-layer conductor as a circuit network with mutual and self inductances. The conductor was fabricated with alloy-sheathed tapes. It was confirmed unbalanced current at each layer was reduced and the conductor loss was reduced in spiral pitch adjusted conductor.

Sub‐Cooled Nitrogen Cryostat for 66 kV/750A Superconducting Fault Current Limiter Magnet

As a part of the Super‐conductive AC Equipment (Super‐ACE) project presently being performed, an AC magnet for a fault current limiter (FCL) is being developed. The goal of the project is the development of 66 kV/750A High Tc Superconducting (HTS) FCL magnet, which is composed of six HTS coils operating at around 65 K. This paper describes a design of a sub‐cooled nitrogen cryostat for the FCL magnet. Three sets of Gifford‐McMahon (GM) cryocooler were used for cooling liquid nitrogen in the cryostat. Experimental results of cooling down and temperature stability during current flowing tests of the magnet were reported in this paper. The sub‐cooled nitrogen of 65 K was successfully obtained in the vessel with 2.6 m3 in volume. And the temperature uniformity was observed in both of the cooling down process and the coil energizing process.