Yuki Kudo

Improvement of maximum working voltage of resistive fault current limiter using YBCO thin film and metal thin film

We have investigated the resistance of current limiting elements in order to improve the working voltage. The element consisted of YBCO thin film on sapphire substrate and metal film on AlN substrate. The working voltage was improved by optimizing the resistance of metal film, and the high value of 83 V/cm was obtained. Furthermore, a module composed of 12-connected elements in series was assembled. The current limiting properties of this module were measured using a capacity discharge circuit. The short circuit current of 1.74 kA was reduced to 460 A, the voltage drop of was 5.45 kV peak, and the dissipated energy was 15 kJ.

Crucial role of sustainable liquid junction potential for solar-to-carbon monoxide conversion by a photovoltaic photoelectrochemical system

Solar energy conversion to carbon monoxide (CO) is carried out using a wired photovoltaic photoelectrochemical (PV PEC) system under simulated solar light irradiation. The PV PEC system promotes CO generation from carbon dioxide and water with approximately 2.0% solar-to-CO conversion efficiency (ηCO) for 2 h. This is achieved via contributions from electrolyte conditions, which generate a sustainable liquid junction potential, in addition to the combination of efficient visible light absorption by a triple-junction amorphous silicon PV cell with high electrode activities with low overpotentials. Estimations of energy conversion efficiency based on the electrochemical properties of the PV cell and electrodes exhibit that the liquid junction potential makes a huge contribution to ηCO. Moreover, the liquid junction potential created by bubbling two kinds of carrier gases produces sustainable chemical bias. This system may contribute to new strategies for the development of sustainable artificial photosynthesis.

Performance verification of a practical fault current limiter using YBCO thin film

Aiming at a practical use of superconducting fault current limiter, a newly designed device which consists of YBCO thin films and normal conducting films has been developed. The configuration allows increase of the resistance of the YBCO film without excess heating and partial overvoltage. To verify the current limiting performance and the reliability of the device, an analogue simulator test with a condition equivalent to a typical distribution system has been conducted. The developed device can limit a fault current to a target level within a half cycle and the increasing speed of the resistance is fast enough to mitigate the current overshoot after the fault. The series and parallel connection of the unit devices are feasible to attain large capacity. These test results prove that the proposed device has sufficient performance for applying to an actual power system.

Improvement of In-Plane Alignment of Grains in YBCO Films on Ag Tapes

The in-plane alignment of grains in YBCO superconducting films deposited on poly-crystalline Ag tapes by the ionized-cluster-beam-deposition method was investigated. All of the YBCO films obtained had c-axis orientation. It has been clarified that the in-plane alignment depends on the crystal orientation of the Ag tapes. Namely, the in-plane alignment of YBCO films was improved when Ag (110) was aligned in the tape surface. On the other hand, when the crystal orientation of the Ag tapes was random, the YBCO films did not show any alignment in the tape surface. It is considered that the growth mechanism of YBCO films is explained by the near-coincident site lattice model between YBCO and Ag lattices.

Measurement of lateral current redistribution of fault current limiter using YBCO thin film

The current redistribution during the limiting phase of a FCL element using YBCO thin film was studied. The FCL element which we have been studying consists of YBCO thin film and nickel film shunt resistor and they are connected in parallel by indium bridges. When a normal zone appears in YBCO thin film due to the over-current, the current transfers from YBCO thin film to nickel film shunt resistor through indium bridges. We measured the change in the distribution of magnetic flux produced by the current carried by the FCL element using an array of pick-up coils. From the pick-up coil signals, the change in the lateral current distribution in the FCL element was calculated assuming an electrical circuit model of the FCL element.

Application of Superconducting Films to Fault Current Limiter

The applicability of high-T c-superconducting films to a fault current limiter (FCL) without liquid nitrogen ( L-N2) was investigated. The FCL element consists of a 200-nm-thick YBa2Cu3O7-X (YBCO) film having a current-limiting area 4 mm wide and 90 mm long. It is cooled by a refrigerator. When a voltage of 100 Vrms (150 A) was applied at 77 K, the YBCO film transitioned to a resistive state at 13 A and the current was limited to 7 A in less than 0.5 ms. At lower temperature, 74 K, the transition current increased to more than 30 A, while the limited current remained less than 9 A. Thus, it was considered that a superior FCL system without L-N2 could be obtained.

Properties of Ag-Cu alloy substrate for Y-Ba-Cu-O superconducting tape

Textured structure, surface morphology and tensile strength of Ag-Cu alloy tapes and Ag-Cu/Ni alloy clad tapes are studied by using XRD, pole figure, SEM and AFM. An addition of Cu was effective for improving surface smoothness; [210] textured structure, on which YBCO film aligns in-plane, was obtained in Ag-Cu tape. Clad tapes which have 2-4 times higher proof stress 60.2 than pure Ag at 700/spl deg/C were obtained. These clad tapes are expected to be useful for obtaining low-cost and high-J/sub c/ YBCO superconducting tape.

New Metallic Substrate for YBCO Superconducting Tape

New Ag alloys (Ag-Cu alloy, Ag-Cu/Ni clad alloys) for YBCO superconducting tape were investigated with respect to the mechanical tensile strength, crystal orientation and superconducting property. These alloys were obtained by cold roll in the size of 10 mm wide and 0.1 mm thick. The crystal orientation of these as-rolled alloys showed Ag(110) parallel to the tape surface and changed to (210) after annealing below 700°C×30min. The proof stress σ 0.2 for these clad alloys was more than 80 MPa, which is about 4 times larger than that of pure Ag. Pole figure of YBCO film deposited by PLD at 700°C: showed in-plane alignment with a FWHM of 22°. The Jc value of 1.4×l05A/cm2 was obtained at 77 K.

Fault Current Limiter Using YBa 2 Cu 3 O 7−x Superconductive Film

The aim of this study is to investigate the applicability of high-Tc-superconducting films in a fault current limiter (FCL) without liquid nitrogen (L-N2). The FCL element consists of a 200 nm-thick YBCO film having a current limiting area 4 mm wide and 90 mm long. A half-cycle-alternating voltage (50 Hz) was applied to the above element which had been cooled by a refrigerator. When a voltage of 100 Vrms (150A) was applied at 77K, the YBCO film became resistive at 13 A and the fault current rapidly decreased to 7 A in less than 0.5 ms. At lower temperature, 74K, the transition current increased to more than 30 A, while the limited current remained less than 9 A. These results suggest that a superior FCL system without L-N2 will be obtained.

Quench Properties and Fault Current Limiters of YBCO Thin-Film Superconductors

We measured the current dependence of quench propagation velocities in strip-shaped YBCO thin films and the current-limiting properties of fault current limiters consisting of a YBCO thin film and a silver thin film. The estimated quench propagation velocities were about 10-300 cm/sec at 77 K. The current-limiting properties were calculated assuming the homogeneous heating up of a fault current limiter (FCL) by an energy dissipation. The calculated current- limiting properties were similar to the experimental result. Further, two-series connected FCLs were switching simultaneously. From these results, it was confirmed that the FCLs were switching to the normal state uniformly.