Kazuhiro Kajikawa

Development of a 22 kV/6.9 kV single-phase model for a 3 MVA HTS power transformer

We have developed a 22 kV/6.9 kV HTS single-phase transformer cooled by liquid nitrogen for field test, which is a practical model for the single-phase part of a 3 MVA HTS power transformer. First, we numerically simulated electromagnetic, mechanical and thermal conditions of the windings in accidental cases of short-circuit and lightning impulse, and considered the winding structure withstanding the severe loads. We constructed a small-sized model coil of Bi-2223 Ag/Mn-sheathed tapes and confirmed applicability of the design concept for the overcurrent and high-withstand-voltage tests. We designed and constructed a single-phase HTS transformer on the basis of the model-coil-test results. The primary and secondary windings are transposed parallel conductors of two and six Bi-2223 Ag/Mn tapes, respectively. The same tests for the HTS transformer as for usual oil-filled ones indicated the reliable operation and high performance. The field test in a distribution grid of Kyushu Electric Power Co. included in-rush-current test and long-term operation of the transformer cooled by a continuous supply system of subcooled liquid nitrogen with cryocoolers.

AC loss properties of a 1 MVA single-phase HTS power transformer

We designed and built a single-phase 1 MVA-22/6.9 kV HTS transformer with the multi-layered cylindrical windings composed of Bi2223 parallel conductors. In advance of the design, the AC loss induced in the windings was estimated on the basis of the observed results in a strand. A subcooled liquid nitrogen cryogenic system with the corresponding cooling capacity was developed and attached to the transformer. The actual AC loss was measured by an electrical method. It was a great part of the total heat load and dominated the temperature rise of subcooled liquid nitrogen. We discussed the validity of the present estimation procedure of the AC loss in the windings as compared with the observed results.

Numerical evaluation of AC losses in HTS wires with 2D FEM formulated by self magnetic field

The ac losses of high critical-temperature superconducting (HTS) wires are numerically calculated by means of a finite element method (FEM), which is formulated with a self magnetic field due to an induced current as unknown. The numerical model is straight HTS wires carrying an alternating transport current in an external ac magnetic field perpendicular to the wire axis. In this situation, the electromagnetic field around the wires is given by two-dimensional (2D) Maxwells equations. It is also assumed that the transport property is represented by either the critical state model or the power-law model, in which the electric field is proportional to the power of the current density. The obtained losses are compared with conventional theoretical curves in several simple geometries.

Development of a 1 T cryocooler-cooled pulse coil with a Bi2223 superconducting parallel conductor for SMES

The authors designed and fabricated a 1 T cryocooler-cooled pulse coil operating at 40 K. A 4-strand interlayer-transposed parallel conductor composed of Bi-2223 rectangular cross-sectional multifilamentary wires was adopted to realize a uniform current distribution and to reduce the AC loss density down to the level of that of a single strand. The pulse coil is a 16-layer solenoidal one with an inner diameter of 52 mm, an outer diameter of 111 mm and a height of 120 mm. The heat drains of AlN plates, which are insulators, are arranged between layers for the cooling of the heat due to the AC loss by heat conduction. They could continuously operate the coil in a triangular waveform mode with an amplitude of 1 T and a frequency of 1 Hz. The AC loss was 10.6 W and the other thermal load was 13 W.

A High-Resolution 1.3-GHz/54-mm LTS/HTS NMR Magnet

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently in the final stage at the Massachusetts Institute of Technology Francis Bitter Magnet Laboratory. Its key component is a three-coil (Coils 1-3) 800-MHz HTS insert comprising 96 no-insulation (NI) double-pancake coils, each wound with a 6-mm-wide GdBCO tape. In this paper, after describing the overall 1.3-G system, we present innovative design features incorporated in 1.3 G: 1) an NI winding technique applied to Coils 1-3 and its adverse effect in the form of charging time delay; 2) persistent-mode HTS shims; 3) a “shaking” magnet; and 4) preliminary results of Coil 1 operated at 4.2 K.

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.

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AC losses are analytically and numerically evaluated in thin superconducting wires with cross sections of various shapes for the application of either transport currents or transverse magnetic fields perpendicular to the broadest face. It is assumed that their electromagnetic response is determined by Beans critical state model, in which the critical current density is independent of the magnitude of local magnetic field. New theoretical expressions of the AC losses are derived for a rhomboid strip, whose sheet critical current varies from centre to edges linearly, and they are compared with conventional results for rectangular and elliptical strips. The transport current losses of wires with finite thickness are also calculated numerically by evaluating the penetration of magnetic flux into them on the basis of the minimization of magnetic energy in addition to the perpendicular field losses. These obtained results are compared with each other for different shapes of cross sections, and the influence of sheet critical current distribution along the wire width on the AC losses is discussed.

Wire

We theoretically investigated the adequate geometrical configuration of a saddle-shaped pick-up coil for the accurate measurement of the ac losses in oxide superconducting thin wires with large anisotropy. Defining the ratio of the apparent signal due to the magnetic moment in sample wires, which is detected by a saddle-shaped pick-up coil, to the actual one as a detection ratio, we calculated the dependences of the detection ratio on the height, width and length of the straight and opening parts of a saddle-shaped pick-up coil. We found out that it is possible to obtain the actual magnetization without compensation by supposing that the effective length of sample wires is equal to that of the straight part of a saddle-shaped pick-up coil and by considering that the signal which is detected by the opening part compensates for the shortage of the signal which is detected by the straight part. Finally, the recommendable dimensions of a saddle-shaped pick-up coil for the measurement of ac losses in currently developed oxide superconducting wires were concretely shown.

AC loss evaluation of thin superconducting wires with critical current distribution along width

The hysteretic ac loss of a current-carrying conductor in which multiple superconducting strips are polygonally arranged around a cylindrical former is theoretically investigated as a model of superconducting cables. Using the critical state model, we analytically derive the ac loss Qn of a total of n strips. The normalized loss Qn∕Q1 is determined by the number of strips n and the ratio of the strip width 2w to the diameter 2R of the cylindrical former. When n⪢1 and w∕R⪡1, the behavior of Qn is similar to that of an infinite array of coplanar strips.

Theoretical investigation on the detection ratio of the magnetization in superconducting wires by a saddle-shaped pick-up coil

Application of an external AC magnetic field parallel to superconducting tapes helps in eliminating the magnetization caused by the shielding current induced in the flat faces of the tapes. This method helps in realizing a magnet system with high-temperature superconducting tapes for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) applications. The effectiveness of the proposed method is validated by numerical calculations carried out using the finite-element method and experiments performed using a commercially available superconducting tape. The field uniformity for a single-layer solenoid coil after the application of an AC field is also estimated by a theoretical consideration.