Kizen Sakaki

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.

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.

Development of HTS current leads for 1 kWh/1 MW module type SMES system. I. Design study

We have been developing high-temperature superconducting (HTS) current leads for a 1 kWh/1 MW module-type SMEs. Each module of a module-type SMES requires a pair of current leads. Therefore, we employed bulk HTS in order to reduce the heat load of the current leads. It is important that HTS current leads for SMES be reliable. The HTS current leads described in this paper have been designed to minimize the heat load and to maintain a high level of reliability. The HTS current leads are designed to hold the heat load at the cold-end terminal to less than 0.1 W. They are also designed with safety leads to bypass current in the event the HTS is quenched and with metal superconductors to assure the continuation of SMES operation even if the HTS should fail or deteriorate in performance. This paper describes an optimal design and the results of a heat load evaluation of HTS current leads for SMES.

Development of REBCO Superconducting Transformers With a Current Limiting Function—Fabrication and Tests of 6.9 kV-400 kVA Transformers

We are developing RE 1 Ba 2 Cu 3 O 7-δ (REBCO, RE: Rare Earth, Y, Gd, etc.) superconducting transformers with a current limiting function. The target is a 3φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. We designed and built two pieces of 1φ-6.9 kV/2.3 kV-400 kVA superconducting transformers with YBCO superconducting tapes. One was wound with a YBCO tape with a copper stabilizing layer with a thickness of 300 μm. The other was 50 μm in thickness of copper. Making sudden short-circuit tests with a 200 MVA short circuit generator, we investigated the response of the YBCO superconducting windings against the fault excess current over the critical current. Repeated sudden short-circuit tests revealed the quite different response between them. In the case of fully stabilized with copper 300 μm thick, short circuit current flowed without decay. On the other hand, in the case of copper 50 μm thick, short circuit current was reduced around three times as much as the rated current. Using the numerical simulation program, which we made up on the basis of the results of sudden short-circuit tests of a 10 kVA test transformer, we quantitatively investigated the transition phenomenon of REBCO superconducting windings to normal state through flux-flow state due to fault excess current. As a result, theoretical simulation explained experiment quantitatively and clarified how to control the short circuit current.

Fabrication of inner secondary winding of high-T/sub C/ superconducting traction transformer for railway rolling stock

We studied the possible application of high-T/sub C/ superconducting traction transformer to railway rolling stock, and designed an iron core type two-leg 4 MVA superconducting traction transformer for Shinkansen. This traction transformer has a primary winding, four secondary windings and a tertiary winding. The four secondary windings are independent of each other. Two secondary windings are arranged around each leg of the iron core and placed inside and outside the primary and tertiary windings. In this study, we fabricated a high-T/sub C/ superconducting coil whose form and dimension are the same as those of the inner secondary winding. This coil can be used as the inner secondary winding of the experimental high T/sub C/ superconducting traction transformer that will be fabricated in the future. The inner secondary winding has 96 turns and a layer with eight parallel Bi2223 superconducting tapes. Transposition among the superconducting tapes is performed 15 times every six turns. We measured the voltage-current and AC loss characteristics when it was cooled in saturated and sub-cooled liquid nitrogen.

Fabrication of winding model of high-T/sub c/ superconducting transformer for railway rolling stock

We fabricated two high-T/sub c/ superconducting coils that simulated the winding of a traction transformer for railway rolling stock. The multi-layer solenoid coil to simulate the primary winding of the transformer had five layers with a single Bi2223 superconducting tape. The closed solenoid coil to simulate the secondary winding of the transformer had one layer with eight parallel Bi2223 superconducting tapes. We measured the voltage-current, AC loss and current sharing characteristics of these coils cooled in saturated liquid nitrogen at 77 K. As a result, we concluded that the multi-layer solenoid coil is applicable to the primary winding and the closed solenoid coil is also applicable to the secondary winding.

Superconducting magnet system for high power gyrotron

A superconducting magnet system was designed and fabricated for a 1-MW gyrotron in a frequency range of 110-140 GHz. The system consists of superconducting magnets, a cryostat, a two-stage cooler, and power supplies. The diameter of the gyrotron bore is 220 mm. A magnetic field of 5.0 T in the resonance region was obtained with a 340-mm bore superconducting split coil. A magnetic field of 0.25 T in the gun region was obtained with two solenoid coils. A persistent current switch, removable current leads, and a two-stage cooler were adopted to decrease the heat load of liquid helium. The superconducting magnet system has successfully achieved a 110% rated current without quenching.<<ETX>>

Test Results of the HTS Current Leads for 1kWh/1MW Module Type SMES System

We have developed two pairs of HTS current leads for 1kWh/1MW module type SMES system (which we call “ESK” for experimental SMES of Kyushu Electric Power Co., Inc.) in order to reduce the heat load from the current leads to the cryostat.

Interface Detail Design for ITER Coil System

Publisher Summary The ITER superconducting magnet system requires 45 pair of current leads and cryolines for supply of electricity and forced-flow supercritical helium at 4.5K and 6.0 bar. The interface components between coil system and operating facilities such as electricity supply system and cryogenic plant consist of feeder and cryoline, coil terminal box and cryostat feedthrough. These components satisfied the required design condition. This chapter introduces their feasibility and provides detail about design results. The superconducting(SC) magnet system requires interface components between the SC coils and the operating facilities. They consist of the feeders and cryolines, the coil terminal boxes and the cryostat feedthroughs. The conceptual design of the coil terminal boxes is completed and their installation in the tokamak hall pit is acceptable. Accordingly, the length of the feeders and cryolines is shorter and a neutron and radiation shield is needed around the cryogenic feedthrough to prevent the neutron and radiation through the penetration of the cryostat making the coil terminal box radioactive. The SC busbar design takes into consideration manufacturing. Force-cooled current leads are used for connection between normal busbar and the SC busbar on consideration of space requirement. However, removal of its heat leak to the main coolant flow is needed.