We are developing an elemental technology for 66/6.9kV 20MVA-class superconducting power transformer with fault current limiting function. In order to obtain the characteristics of YBCO conductor when the AC over current supplied to the conductor, the model coils were manufactured with YBCO tapes and tested. Based on these results, we manufactured the 6.9kV/2.3kV 400kVA-class YBCO model transformer with fault current limiting function and performed short-circuit test. At the 0.25s after short-circuit, the short-circuit current of primary winding was limited to about 174A for a prospective current of 559A. It was consistent with the design. The I–V characteristics of the winding did not change before and after the test. We consider the model transformer to be able to withstand AC over-current with the function of current limiting. The results suggest the possibility to design YBCO superconducting transformers with fault current limiting function for practical power grid.

We have been developing a light weight 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 2004. We performed the type-test, the system-test, and the vibration-test. Consequently, we could verify that the transformer satisfied the requirement almost exactly as initially planned. However, there have been raised some problems to be solved to put superconducting traction transformer into practical use such that AC loss of the superconducting tape must be lower and the capacity of the refrigerator must be larger. Especially it is the most important to reduce the AC loss of superconducting windings for lightweight and high efficiency. The AC loss must be reduced near the theoretical value of superconducting tape with multifilament. In this study, we fabricated and evaluated the Bi2223 tapes as introduced various measures to reduce the AC loss. We confirmed that the AC loss of the narrow type of Bi2223 tapes with twist of filaments is lower, and we fabricated windings of this tape for use in superconducting traction transformer.

We have developed the technology of the producing a HTS magnet for the power transformer. Three subjects have been mainly studied, high voltage technologies, large current and low AC loss technologies and sub-cooling system technologies to establish the technology of 66kV/6.9kV 10MVA class HTS power transformer.In order to verify the validity of elemental technologies, such as high voltage technologies, large current and low AC loss technologies and sub-cooling system technologies, single-phase 2MVA class 66kV/6.9kV prototype HTS transformer was manufactured and tested. In the load loss (AC loss) measurement, it was obtained that the measured value of 633W was almost corresponding to the calculated value of 576W at the rated operation of 2MVA. Moreover, the breakdown was not found all voltage withstand test. These test results indicate that elemental technologies were established for the development of 66kV/6.9kV 10MVA class HTS power transformer.

It is crucial to identify mechanical behaviors when developing a practical large-scale SMES (Superconducting Magnetic Energy Storage) featuring high reliability and high efficiency. A FEM (Finite Element Method) simulation model was developed to display the nonlinear mechanical deformation of a modified D-shaped elemental coil with a Rutherford type conductor in a 1 kWh/1 MW practical SMES. The model uses nonlinear mechanical characteristics of a conductor measured by bending test. The conductor's nonlinear mechanical characteristics are roughly explained by the frictional slip between two layers in the conductor. This paper describes two types of slip models between the conductor and the coil spacer, taking into account FEM simulation models, and examines the detailed mechanical behaviors of the coil. Analytical results on the nonlinear deformation and mechanical loss of the coil were compared to those actually measured in a 1 kWh SMES test. The FEM results almost coincided with the mechanical deformations of the coil measured in the test.

We investigated the possibility of applying high-T-C superconducting traction transformer to a rolling stock. In order to achieve a lightweight design, the traction transformer needs to have a special winding structure unlike the experimental high-T-C superconducting power transformers fabricated so far. The windings of the traction transformer need to be arranged as closely as possible in a high magnetic field. Then, we fabricated two high-T-C superconducting coils that simulated the winding of a high-T-C superconducting traction transformer. One is a multi-layer solenoid coil that simulated the primary winding of the transformer. This coil has five layers of a single Bi2Sr2Ca2Cu3Ox (Bi2223) superconducting tape. The other is a close solenoid coil that simulated the secondary winding of the transformer. This coil has one layer of eight parallel Bi2223 superconducting tapes. In this study, we measured the voltage-current, AC loss, current sharing and over current characteristics of these coils cooled by saturated and subcooled liquid nitrogen.

We designed and fabricated a 4 kJ conduction-cooled high-Tc superconducting (HTS) pulse coil. The coil is wound with an interlayer-transposed 6-strand parallel conductor which is composed of Bi-2223 silver alloy-sheathed multi-filamentary wires. We had developed a complete 3.6 MJ/1 MW low-Tc superconducting (LTS) SMES system for testing on a power line at Imajuku substation. Aiming at the feasible operation of SMES applying a HTS coil, we made a SMES system set-up in which HTS coils were serially connected to 3 LTS coils of the SMES. The SMES including the HTS coil was connected to Imajuku substation's power system, to made operational tests of compensation for load fluctuation at the 6 kV power line. The test results lead to the feasibility of the HTS SMES for practical use in future power systems.

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.

A 60-kA high-temperature-superconductor (HTS) current lead has been fabricated and tested aiming at the application to a fusion magnet system. The HTS current lead consists of a forced flow cooled copper part and a conduction cooled HTS part. The HTS part is composed of 288 Ag-alloy sheathed Bi-2223 tapes, which are cylindrically arrayed on a stainless steel tube. For the Ag alloy, Ag–10at.%Au is used to obtain low thermal conductivity. Operation of a 60-kA current, which is the world record, was successfully achieved at a cooling condition of 20 K, 3.2 g/s for the copper part, and a low heat leak of 4.8 W at 4.2 K was demonstrated. This result indicates that the electric power of a refrigerator to cool the current lead can be reduced by 1/3 of that in a conventional current lead. In conclusion, technology of a large HTS current lead for fusion application is established.

We fabricated two high-Tc 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.

As the mechanical causes for instability and loss in superconducting coils become more pronounced as coils increase in scale, it is important to understand the physical behavior of such failure when developing highly reliable and efficient SMES's of a practical scale. However, it is difficult to understand the mechanical behavior of superconducting coils, especially the local deformation or displacement of coil components. It is helpful to use a model to simulate the mechanical behavior of superconducting coils when discussing the instability and mechanical loss caused by such mechanical factors. A simulation model was developed to show the nonlinear mechanical deformation of a modified D-shaped elemental coil with a Rutherford type conductor in a 1 kWh SMES. The nonlinear characteristics of the conductor were explained by the frictional slip between two layers in the conductor. FEM analysis conducted using this model explained well with the mechanical deformations measured in the coil.

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 transformer's applicability to a power grid.

A pair of 8 kA HTS current leads for an accelerator magnet test facility at KEK has been designed and fabricated. The HTS current lead is composed of a copper section in the high temperature region and a HTS section in the low temperature region. The copper section of the lead consists of copper wires bundled into a stainless steel pipe. The HTS section consists of 12 HTS tape units made by Bi2223/Ag-1 at% An tapes arrayed onto the outer surface of a stainless steel tube with a diameter of 54 mm. The whole length and the maximum diameter of the lead are 1465 mm and 71 mm, respectively. The entire length of the current lead is cooled with helium gas. The design heat load and the helium gas flow rate of the lead in a steady state 8 kA-operation are less than 0.2 W and 0.4 g/sec, respectively and temperatures of the high and low temperature ends of the HTS section under the condition are 50 K and 4.4 K, respectively. The thermal performance test was carried out, and the results agreed well with the calculated values. So, we verified the validity of the thermal analysis and design. The corresponding heat load is estimated to be 0.15 W. These values of 0.4 g/s and 0.15 W are only 89% and 1.5% of the conventional gas cooled current lead, respectively.

An AC current source of 1000 A class for AC transport measurements was designed and fabricated with using an oxide superconducting current transformer. Two cryocoolers were installed for cooling the transformer and a sample holder separately. A parallel conductor composed of six tapes was wound for a secondary winding and a transposition was performed to make the current distribution uniform in the parallel conductor. Temperatures of the transformer and the sample holder were controlled in the range of 35–50 and 25–77 K, respectively. The peak current in the secondary winding was over 1000 A in the frequency range of 1–75 Hz, when the primary peak current was 14 A.

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.

A 60kA HTS current lead has been designed for large fusion magnets such as ITER magnet. The actual refrigeration input power required to cool the current lead is specified to be reduced to one third that of the conventional copper lead. The HTS part of the 60 kA lead consists of 48 units installed with cylindrical array into the enter surface of a stainless steel tube with a diameter of 145 mm, Each unit is composed of six Bi2223/Ag-10at%Au tapes, and its cross-sectional dimension is 6.5 mm x 2.7 mm. The HTS part is cooled by conduction, and the warm and cold end temperature conditions of the HTS part are 50 K and 4.5 K, respectively. The copper part is cooled by helium gas, a flow rate of 3.9 g/s and the inlet temperature of 35 kA. The 60-kA lead has been designed in consideration of safety under the long discharge time condition of ITER-TF coil with a detection time of 2 sec. and a discharge time constant of 15 sec. For the purpose of verifying the reliability of the design for the lung discharge time, one unit sample has been fabricated and tested. The result indicates that the maximum temperature rise of the HTS part is less than 150 k for the ITER like-discharge from 1.25 kA. corresponding to 60 kA of the full lead with 48 units.

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 tr...