We have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in subcooled 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 a uniform current distribution among them, First, the transformer cooled by liquid nitrogen of 77 K was steadily operated with a 500 kVA secondary inductive load, The efficiency in the full operation at 77 K attains to 99.1% even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. We installed the transformer in a continuous now system of subcooled nitrogen as a fundamental step for compact superconducting transformers operating in subcooled 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 subcooled nitrogen.
