Akira Tomioka

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.

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.

Development of a 15 kW Motor With a Fixed YBCO Superconducting Field Winding

We developed a 15 kW synchronous motor with a fixed superconducting field winding. It was 8-pole type. Each field coil was a racetrack-shaped single pancake wound with a YBCO superconducting tape. It was fabricated by IBAD-PLD method and 10 mm in width. The rotating armature was composed of a copper winding and an iron core. The field coils were conduction-cooled down to 20 to 30 K through a copper pipe attached to the copper coil flange, in which helium gas was forced-flowed. The completed motor was first tested in the Suzuka factory of Japan motor and Generator Co. Ltd. before shipping as usual. Then it was moved to the test site of the Nagasaki shipyard of Mitsubishi Heavy Industry Ltd. and installed into a ship propulsion test system, which had a propeller with a diameter of 0.5 m. We verified the quite stable underwater operation and the output power of 15 kW-360 rpm as designed.

Current distribution in superconducting parallel conductors wound into pancake coils

We made a preliminary investigation of the applicability of high-T/sub c/ superconducting parallel conductors to pancake coils. For the sake of a uniform current distribution and low AC loss, the constituent strands need to be transposed so as to be inductively equivalent with each other. We adopted an interdisk transposition where the strands are not transposed inside a single-pancake coil but only at the joint between the pancake coils. The fabrication process is simple. We have only to fabricate the same double-pancake coils and connect the strands individually with transposition outside the winding. We searched theoretically for the optimum transposition in the case of 3-strand and verified the theoretical result by using small test coils wound with NbTi 3-strand parallel conductors for convenience.

Tests of Superconducting Traction Transformer for Railway Rolling Stock

We have been developing a lightweight and high-efficiency superconducting traction transformer for railway rolling stock. We designed and fabricated a prototype superconducting traction transformer of a floor-mount type for Shinkansen rolling stock. In this study, we present test results such as type test, system test and vibration test. We performed the type test in accordance with JIS, E5007. In this test, we measured the basic electrical characteristics. In the system test, we tested the transformer with a dynamic simulator for rolling stock, as a converter connected at secondary winding, and verified the operating characteristics. We performed vibration tests in accordance with JIS, E4031, vibrating the transformer on a shaking table with three-dimensional movement. As a result, we could verify that the transformer satisfied the requirement almost exactly as initially planned.

Test results of a HTS power transformer connected to a power grid

Abstract A 22 kV/6.9 kV–1 MVA high-Tc superconducting (HTS) power transformer has been developed as a prototype with single-phase part of a 3 MVA HTS power transformer. The prototype unit is cooled by a continuous subcooled liquid nitrogen (LN2) supply system with cryocoolers. During the field tests, the HTS transformer was connected to a distribution line at Imajuku substation (Kyushu Electric Power Co., Inc.) in Fukuoka, and inrush-current test and a long-term operation test were implemented. The test results demonstrated the HTS power transformers applicability to a power grid.

Development of a 3 -66/6.9 kV-2 MVA REBCO Superconducting Transformer

We have designed and fabricated a 3Φ-66 kV/6.9 kV2 MVA transformer with RE1Ba2Cu3O7-δ(REBCO, RE:Rare Earth, Y, Gd etc.) superconducting tapes. It is a 1/10 model of a 3Φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. The superconducting windings were reduced only in current capacity by reducing the number of tapes in parallel conductors. In the primary side, a single REBCO tape with a width of 5 mm was cylindrically wound into 8 layers. In the secondary one, an 8-strand parallel conductor was wound similarly into 2 layers, where each strand was transposed 15 times per one layer. The REBCO tapes for the secondary winding were also scribed by laser into a 3-filament structure to reduce the ac loss. The windings for 3 phases were installed into a GFRP cryostat which had an elliptic-cylinder-shape and three cylindroid bore for an iron core at room-temperature. A Ne turbo-Brayton refrigerator with a cooling capacity of 2 kW at 65 K was developed and located close to the transformer. The windings were cooled with subcooled liquid nitrogen at 65 to 70 K, which was forced-flowed by a pump unit between the transformer and the refrigerator. The completed transformer was first tested in liquid nitrogen at 77 K according to the domestic regulation for conventional transformers. The load loss, i.e., ac loss of the windings, was 26.9 W for the rated operation. The dielectric strength was also verified by applying 350 kV impulse voltage and 140 kV ac voltage for 1 minute.

Development of a REBCO Superconducting Transformer With Current Limiting Function

A single-phase 10 kVA transformer was designed and fabricated with RE1Ba2Cu30O7-δ (REBCO, RE: Rare Earth) superconducting tapes to quantitatively investigate the transition phenomenon of REBCO superconducting tapes to normal state due to a fault excess current. The voltage ratio was 1:1 and the rated voltage was 393 V. It had four windings. The primary and secondary windings had auxiliary windings in addition to main ones respectively. The main and auxiliary windings were connected in parallel in the both sides. The addition of the auxiliary windings made it possible to observe the induced normal resistances in the respective main ones individually. First usual performance tests were carried out and a stable operation as designed was confirmed. In steady state almost all of the current flowed in the main windings due to the small leakage reactance between them. At a sudden short-circuit test, the main windings quenched due to excess current over the critical current and the current was reduced to one fortieth as compared with the case of no current limiting function. It was clarified the whole of the main windings did not shift to normal state though the current exceeded the critical current all over the length and the induced normal zone was almost proportional to the primary voltage.

Fabrication of Superconducting Traction Transformer for Railway Rolling Stock

We designed a floor type single-phase 4 MVA superconducting traction transformer for Shinkansen rolling stock. In this study, we fabricated a prototype superconducting traction transformer based on this design. This transformer of the core-type design has a primary winding, four secondary windings and a tertiary winding. The windings are wound by Bi2223 superconducting tapes and cooled by subcooled liquid nitrogen. The core is kept at room temperature. The cryostat is made of GFRP with two holes to pass core legs through. The outer dimensions are about 1.2m × 0.7m × 1.9m excluding the compressor. Its weight is 1.71t excluding that of refrigerator and compressor. The transformer was tested according to Japanese Industrial Standards (JIS)-E5007. We confirmed that the performance of transformer has been achieved almost exactly as planned. The rated capacity is equivalent to 3.5MVA in the superconducting state.

AC loss properties of a 4 kJ conduction-cooled pulse coil wound with a Bi2223 6-strand parallel conductor for SMES

We designed and fabricated a 4 kJ conduction-cooled superconducting pulse coil with a 6-strand interlayer-transposed parallel conductor composed of Bi2223 multifilamentary tapes. We adopted the helium gas forced-flow cooling system where the helium gas flowed inside a copper pipe soldered with the flanges of brass. We succeeded in the continuous pulse operation with an amplitude of 500 A-1.6 T at a sweep rate of 140 A/s at 30 K as designed. Even in the ac operation with an ac loss of 120 W, the difference in temperature inside the winding was only 5 K and it was possible to hold the coil temperature around 30 K. In this paper, we report the design and the test results of the coil system from the aspect of ac loss and thermal properties.