Taketsune Nakamura

System Coordination of 2 GJ Class YBCO SMES for Power System Control

YBCO superconducting wire has a relatively low decrease in power distribution at high temperatures and under a high magnetic field. A high-intensity substrate is used for the wire, so the wire has high machine characteristics. Therefore, it is expected that this wire can be used for large-scale high magnetic field coils. Here, coordination between the SMES system for 100 MVA/2 GJ class load fluctuation compensating was conducted using IBAD/CVD-YBCO wire. The SMES system includes a toroidal type YBCO coil consisting of 180 compact, high magnetic field multi-unit coils, a large coil cooling system that uses the conduction cooling method, which does not use a refrigerant medium, and a multi-cell power converter that achieves multi-unit coil connection with relatively low current and low voltage. Studies were conducted for each individual device and for the whole system. Based on the study plan in this paper, it has become possible to develop and coordinate each device of the 100 MVA/2 GJ class power system load fluctuation compensation SMES system using YBCO wire, which up until now had seem impossible as an actual system.

Fabrication and Characteristics of HTS Induction Motor by the Use of Bi-2223/Ag Squirrel-Cage Rotor

HTS squirrel-cage induction motor was fabricated and tested in this study. Both of rotor bars and end rings were made of Bi-2223/Ag multifilamentary tapes in order to realize superconducting current loops, and the conventional (normal conducting) stator, 3-phase and 4-pole, was utilized. Rotating characteristics of the fabricated motor were tested for different input voltages at 60 Hz. The performances were also analyzed by means of the theoretical method based on the electrical equivalent circuit. It was shown that the minimum starting voltage was experimentally confirmed and agreed semi-quantitatively with the analysis result. The rotation at synchronous speed was realized by applying the load, at least, 1.5 Nm

Stress Tolerance and Fracture Mechanism of Solder Joint of YBCO Coated Conductors

YBCO coated conductors have been expected to be applied to superconducting magnetic energy storage (SMES) due to high critical current density under high magnetic field and possibility of reducing cooling cost. Solder joints are essential to fabricate a high Tc superconducting coil for SMES system which requires long length of coated conductors. Not only low joint resistance but sufficient mechanical strength is needed, since conductors are exposed to large electromagnetic force generated by large transport current and high magnetic field. In the present study, influence of tensile load on transport property through the joint was investigated. The solder joint with sufficiently low resistance of 5.3 nOmega was attained before loading. Such joint can carry the load up to 650 N without substantial degradation. For further applied load, degradation attributed to fracture at the edge of the conductor is firstly observed. Overall fracture is caused by delamination at the interface between YBCO and CeO2 . As a result, importance of interfacial strength between the superconducting and buffer layer is revealed to realize both low joint resistance and mechanical strength for solder joint.

Conceptual Design of HTS Coil for SMES Using YBCO Coated Conductor

High Tc superconducting (HTS) toroidal coil using YBCO coated conductor is designed for 70 MJ class superconducting magnetic energy storage (SMES) for power system control. Its configuration and shape are optimized by means of genetic algorithm (GA) to minimize the required length of the conductor. The optimization is performed for two kinds of constrains, i.e., maximum electric field or flux flow loss of the coil, which are calculated by means of finite element method (FEM). The FEM analysis considers quantitative current density (J)-electric field (E) expressions based on percolation transition model. It is shown that the great transport performance against magnetic field of YBCO coated conductor can realize a very compact SMES coil compared with an existing Nb-Tis one.

DC reactor effect on bridge type superconducting fault current limiter during load increasing

In high power applications, the fault current limiter has been discussed for many years because of some limitations of conventional circuit breakers. Many types of fault current limiter have already been introduced in papers. In this work, a simple bridge-type fault current limiter has been designed and constructed. The performances of the limiter have been tested successfully. In the bridge-type current limiter, a DC reactor appears in the line when the connected load is increasing. This causes a voltage drop across the load terminal during load changing. The DC reactor effect of the current limiter has been studied. Some experimental results regarding the reactor effect of the limiter have been considered and compared with the results obtained from computer simulation.

Corrigendum: Three-dimensional model for numerical electromagnetic field analyses of coated superconductors and its application to Roebel cables

We developed a novel model for numerical electromagnetic field analysis of assembled conductors, considering their three-dimensional structure. Since the superconductor layer of a coated conductor is very thin, we applied a thin-strip approximation, where the current flows only in a direction parallel to the wide face of the coated conductor. Although the amount of computation was reduced by the application of the thin-strip approximation, the three-dimensional geometry of each strand was retained in the modeling: the superconductor layer of each strand was modeled as a thin curved strip that follows the three-dimensional shape of the strand. The electromagnetic interaction between strands was considered in the modeling. The established model was then applied to a high-Tc superconductor (HTS) Roebel cable in which coated conductors are interlaced as strands and exhibit a complex three-dimensional structure. Analyses were performed for an HTS Roebel cable in typical cases: carrying ac current, exposed to an ac magnetic field, and carrying ac current while exposed to an ac magnetic field. The current profile and distribution of ac loss, that is, power dissipation on the wide face of the strand, were observed.

Theoretical analysis of a YBCO squirrel-cage type induction motor based on an equivalent circuit

A HTS induction motor, with a HTS squirrel-cage rotor, is analysed using an electrical equivalent circuit. The squirrel-cage winding in the rotor consists of rotor bars and end rings, and both are considered to be made of YBCO film conductors. A wide range of electric field versus current density in YBCO film is formulated based on the Weibull function, and analysed as a non-linear resistance in the equivalent circuit. It is shown that starting and accelerating torques of the HTS induction motor are improved drastically compared to those of a conventional induction motor. Furthermore, large synchronous torque can also be realized by trapping the magnetic flux in the rotor circuit because of the persistent current mode.

Proposal of saturated DC reactor type superconducting fault current limiter (SFCL)

Abstract A new superconducting fault current limiter (SFCL) using DC biased saturated core have been proposed in this paper. All existing saturated core SFCL are AC type. The proposed SFCL is a DC reactor type where all the windings carry DC current. The B – H curve of a magnetic circuit shows that less inductance value can be achieved in saturated region and very high value in unsaturated region. The reactance window between the saturated and unsaturated region depends on magnetic material, core size and its biasing conditions. The proposed type of SFCL can be designed efficiently taking the full advantage of the reactance window in current limiting mode. The advantage of this type of SFCL over the existing DC reactor type is that, it shows very low value of inductance at normal operation and very high value during fault time. On the other hand, the existing DC reactor (bridge) type SFCL shows a fixed value of inductance both in normal and fault time. Some simulation results are presented here to show the performance of the proposed SFCL.

Trial Test of Fully HTS Induction/Synchronous Machine for Next Generation Electric Vehicle

Our group has tried to introduce High Temperature Superconducting Technology to electric vehicle (EV) drive motor, and developed High Temperature Superconducting Induction-Synchronous Machine (HTS-ISM). In this paper, 20 kW class HTS-ISM were designed and manufactured with DI-BSCCO wires. First, we developed and tested partial superconducting HTS-ISM (HTS rotor windings with copper stator) for clarification of the performance. We could observe the efficiency at 92.3% even for the light load. Furthermore, we successfully fabricated and conducted no-load test for fully superconducting HTS-ISM. It was verified that stable synchronous rotation at 1200 rpm was succeeded, which is the world-first success of achieving the rotation at over 1000 rpm for fully superconducting motor. Our results are one of the big steps for realizing the HTS-EV.

Proposal of a Fully Superconducting Motor for Liquid Hydrogen Pump With MgB

The outline design of a fully superconducting motor for liquid hydrogen pump with a magnesium-diboride (MgB2) superconducting wire is carried out to present various advantages arising from its prospective performances. The squirrel-cage rotor winding composed of superconducting loops with the MgB2 wire enables us to operate the motor not only in a slip mode but also in a synchronous rotation mode, and consequently the rotor winding loss can be suppressed drastically. Furthermore, it would be expected that the stator winding loss becomes smaller by using the MgB2 wire compared with familiar normal metals as typified by a copper. The time evolution of magnetic field distribution around the stator winding is obtained by means of a finite element analysis in order to estimate the AC loss and the primary circuit resistance.