The RTRI manufactured a prototype superconducting power transformer (25kV) for rolling stock (Fig. 1) using windings made from bismuth-base superconducting tape wire immersed in liquid nitrogen. For Shinkansen use, the winding voltage is set at 25kV (primary), 1,200V x 4 windings (secondary) and 440V (tertiary) (Table 1). It is a core-type transformer with solenoid coil windings that reduce AC loss. The core is placed in a normal-temperature space to reduce the load on the refrigerator.
      The body is roughly 1.2m wide, 0.7m deep and 1.9m high (excluding the compressor). The weight is 2.3t (with the total weight of refrigerator and compressor assumed to be 0.6t) against the targeted weight of 2.1t. As the first stage of development, the RTRI designed it to be placed on the floor.
      The type test stipulated by JIS for rolling stock power transformers showed that the maximum capacity of the transformer in the superconducting state was approximately 3.5MVA. A current of 750A can flow in the secondary winding, which is equivalent to 4MVA in the total capacity. No abnormalities were observed in the 10-minute 42kV AC withstand voltage test or the 150kV lightning impulse test prescribed by JIS, demonstrating that there are no problems with electrical insulation. AC loss was estimated to be 7.9kW at 4MVA, according to an AC loss test. The RTRI will continue research and development on such subjects as the reduction of AC loss and development of a lightweight large-capacity refrigerator.
      This study was promoted with a subsidy from the Ministry of Land, Infrastructure and Transport.

Fig. 1 Appearance of the prototype superconducting power transformer

Table 1 Dimensions of the prototype superconducting power transformer

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