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.