Shinji Torii

Superconducting Fault Current Limiter Design Using Parallel-Connected YBCO Thin Films

Superconducting fault current limiters (SFCLs) are able to reduce fault currents to an acceptable value, reducing potential mechanical and thermal damage to power system apparatus and allowing more flexibility in power system design and operation. The device can also help avoid replacing circuit breakers whose capacity has been exceeded. Due to limitations in current YBCO thin film manufacturing processes, it is not easy to obtain one large thin film that satisfies the specifications for high voltage and large current applications. The combination of standardized thin films has merit to reduce costs and maintain device quality, and it is necessary to connect these thin films in different series and parallel configurations in order to meet these specifications. In this paper, the design of a resistive type SFCL using parallel-connected YBCO thin films is discussed, including the role of a parallel resistor and the influence of individual thin film characteristics, based on both theory and experimental results.

Theoretical and experimental study on AC loss in HTS tape in AC magnetic field carrying AC transport current

AC loss characteristics of high temperature superconducting tape in external AC magnetic field carrying AC transport current were theoretically and experimentally studied. We measured the AC transport current and magnetization losses in a conventional Bi2223/Ag multifilamentary tape in the external AC magnetic field carrying the AC transport current. The measured AC magnetization loss, AC transport current loss and total AC loss were compared with the numerical results. The measured AC loss characteristics of the tape can not be explained by the critical state model. Numerical calculations on the AC losses in the tape were performed by using our analytical model introducing the electric field and current density relation locally determined in the filamentary region of the tape. The numerical losses by our model reasonably agree with the measured data.

Transport current properties of Y-Ba-Cu-O tape above critical current region

YBCO tape is highly promising as a 2nd-stage HTS conductor for power applications, and development of long tape is proceeding worldwide. Generally, HTS tape conductors have broad transition characteristics ranging from the superconducting state to the normal state, so they rarely quench just above the critical current defined for a usual electrical field or resistivity. However, such characteristics make the stability criteria complex when these tapes are considered for power applications. To estimate the stability criteria of HTS tapes, we prepared YBCO tapes with YSZ textured buffers by the IBAD method. The thickness, width and length of the YBCO layer are 1 /spl mu/m, 10 mm and about 140 mm, respectively, and tapes are coated with Ag of various thicknesses as a stabilizer and protector. Transport current properties are measured in a wide range of current above the critical current defined by 1 /spl mu/V/cm. The establishment of criteria to define the stability of HTS tapes will be necessary for their applications in power apparatuses.

Tests for Conceptual Design of 6.6 kV Class Superconducting Fault Current Limiter With YBCO Thin Film Elements

A superconducting fault current limiter (SFCL) for 6.6 kV and 400 A installed in a cubicle for a distribution network substation was conceptually designed. The SFCL consists of parallel- and series-connected superconducting YBCO elements and a limiting resistor. Before designing the SFCL, some tests were carried out. The width and length of each element used in the tests are 30 mm and 210 mm, respectively. The element consists of YBCO thin film of about 200 nm in thickness on cerium dioxide (CeO2) as a cap-layer on a sapphire substrate by metal-organic deposition with a protective metal coat. In the tests, characteristics of each element, such as over-current, withstand-voltage, and so on, were obtained. From these characteristics, series and parallel connections of the elements, called units, were considered. The characteristics of the units were obtained by tests. From the test results, a single phase prototype SFCL was manufactured and tested. Thus, an SFCL rated at 6.6 kV and 400 A can be designed.

Development and electrical test of a 30 kVA class fully superconducting generator

We are developing a 4 pole 50 Hz 30 kVA class fully superconducting generator to investigate the characteristics of superconducting armature windings subject to a rotating magnetic field. Compared with other fully superconducting generators that have been developed and tested, our machine is designed to obtain a higher armature current. It is generally observed in coils wound from AC superconducting cable that the AC quench current of the coil is much lower than its DC quench current. Countermeasures against these AC current degradation phenomena were developed and applied to the AC cable and armature winding of our generator. Results of the electrical tests of the generator are presented and the validity of the countermeasures is discussed. >

3-D FEM Analysis of Effect of Twist Pitch and Shielding Layer on Multi-Layered HTS Power Cable

3-D finite element analysis of current distribution taking account of the nonlinear E-J characteristic is carried out. In order to study the ac loss of multi-layered HTS power cable, the analysis region is considerably reduced by modeling spirally wound superconducting tapes as conductors having an anisotropic conductivity and using the periodic boundary condition. By using this model, the effect of the twist pitch on the current distribution of multi-layered HTS power cable with a shield layer is illustrated and the structure with low ac loss is discussed

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.

Numerical evaluation of measured AC loss in HTS tape in AC magnetic field carrying AC transport current

Numerical study to evaluate the validity of the measurement method of AC loss in high temperature superconducting tapes in external AC magnetic field carrying AC transport current was done. The AC transport current loss and the magnetization loss in a conventional Bi2223/Ag-sheathed multifilamentary tape in the external AC magnetic field carrying the AC transport current were measured by the four terminal method and the pick-up coil method respectively. Numerical calculations on the AC losses in the tape were performed by using our previously developed analytical model. The measured losses were compared with the analytical results and the errors of the AC loss measurement were evaluated.

Measurement of AC losses of superconducting cable by calorimetric method and development of HTS conductor with low AC losses

To achieve compact and highly efficient HTS power transmission cables, reducing and evaluating AC losses is important. Furukawa is developing conductors with low AC losses of 1 W/m at 3000 Arms for the 66/77 kV class HTS cables as a part of the Super-ACE project. Specifically, we aim to achieve a superconducting conductor with minimum AC losses by optimizing the winding pitches and twisted filaments in the tape. The electrical method is generally used to evaluate AC losses in HTS cables. However, measures of AC losses were uncertain using only the electrical method of evaluation. Therefore, the calorimetric device was developed and was used for determining AC losses. Consequently, results from the two methods almost agreed. Moreover, AC losses in the fabricated HTS conductors were measured at the lowest levels ever achieved.

Ultrafine multifilamentary Nb Ti wires with Cu Si alloy matrix

Abstract The Cu  Si alloy has been proposed as a new matrix material for filamentary Nb  Ti wires in a.c. use. The Cu  Si alloy shows appropriate mechanical and electrical properties, and is economically more favourable than the Cu  Ni alloy matrix used currently. Moreover, the addition of Si to Cu prevents the formation of intermetallic compounds around the filaments. After extensive investigations on CuSi alloy as a matrix material, an ultrafine multifilamentary Nb  Ti/Cu − 2.5 wt% Si composite wire has been successfully fabricated. The d.c. superconducting properties of the wire were adequate for use in electrical apparatus. A preliminary study has revealed that the a.c. loss of the new wire is equivalent to that of a typical ultrafine multifilamentary Nb  Ti/Cu  Ni composite wire.