Masayuki Konno

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.

Preliminary tests of a 500 kVA-class oxide superconducting transformer cooled by subcooled nitrogen

The authors have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in sub-cooled nitrogen. The primary and secondary windings are three- and six-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. In the parallel conductors, the strands are transposed several times for uniform current distribution among them. A transformer, cooled by liquid nitrogen of 77 K, was steadily operated with a 500 kVA secondary inductive load. The efficiency in full operation at 77 K was 99.1 %, even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. They installed the transformer in a continuous flow system of sub-cooled nitrogen as a fundamental step for compact superconducting transformers operating in sub-cooled nitrogen with a single-stage refrigerator. Short-circuit tests of the transformer were also performed in a region of temperature below 70 K. The transformer was operated with no quenching up to a level of critical current at 66 K, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3 % in the sub-cooled nitrogen.

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.

Experimental results of the Nb3Sn demo poloidal coil (DPC-EX)☆

Abstract The aim of the development of DPC-EX is to demonstrate the applicability of an Nb 3 Sn conductor to pulsed coils for tokamak fusion machines. The DPC-EX, whose inner diameter is 1 m, consists of two double pancakes fabricated by a react-and-wind technique. The conductor is a forced cooled, flat, cable-in-conduit conductor. The DPC-EX has been installed between two Nb-Ti demo poloidal coils (DPC-U1 and U2). In the series operating mode (DPC-EX, DPC-U1 and U2), DPC-EX was ramped up to 17 kA in 1 s and ramped down to zero in 1 s after a flat top time for 1 s, without normal transition. The maximum magnetic field and the maximum pulse field were 6.7 T and 6.7 T s −1 , respectively. The current density in the winding was 37.2 A mm −2 at an operating current of 17 kA. The ratio of a.c. losses to the stored energy was ≈ 0.14% during pulsed operation. After more than 50 cycles of pulsed operation, no damage could be found in the DPC-EX. The stability test indicates that DPC-EX has a high stability margin. These results demonstrate the possibility of high field (12–14 T) poloidal coils, as required in the FER and ITER.

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.

Production and Test of a REBCO Superconducting Synchronous Motor

We designed and fabricated a 7.5 kW-360 rpm synchronous motor with a superconducting rotor. The rotor was 6-pole type and has no iron core. The armature winding was wound with copper wires. 6 pieces of racetrack-shaped field coils were wound with 5 mm wide RE1Ba2Cu3O7-delta (REBCO, RE:Rare Earth) superconducting tapes produced by an ion beam assisted deposition method and a pulsed laser deposition method. The total length of REBCO tape was 400 m per a field coil. We confirmed the good transport properties of the respective field coils up to 80 A at 40 to 50 K before and also after the assembly to a motor. The maximum magnetic field in the field coils was 0.67 T. The superconducting rotor was cooled down to around 40 K by forced-flowed helium gas and the completed motor was tested in a similar way to conventional motors. As a result of a load test, the designed operation of 7.5 kW at 360 rpm was verified for the rated field current of 60 A. Even when the field current was increased up to 70 A at 40 K, the temperature at each part of the superconducting rotor was held constant and the quite stable operation up to 11 kW at 360 rpm was also achieved.

Test results of high temperature superconductor current lead at 14.5 kA operation

High temperature superconductor (HTS) current leads have been developed for the International Thermonuclear Experimental Reactor (ITER) magnet system, which are required not only to reduce the lead heat leak but also to maintain safety in a fault condition. A pair of 10-kA class HTS current leads was fabricated and tested. The lead consists of a copper part and an HTS part. The HTS part is composed of 192 Bi-2223 silver-alloy sheathed tapes in a cylindrical array on a stainless steel tube. Thermal performance and stability were tested. The current leads could carry up to 14.5 kA by placing magnetic materials between the HTS elements, which were installed to reduce the perpendicular magnetic field in the HTS elements.