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We have designed and constructed a 500 kVA-class oxide-superconducting power transformer. The windings are cooled by liquid nitrogen or subcooled nitrogen in a G-FRP cryostat of 785 mm in diameter and 1210 mm in height, that has a room-temperature space for an iron core with the diameter of 314 mm. The primary and secondary windings are three-strand and six-strand parallel conductors of a Bi-2223 multifilamentary tape with silver sheath, respectively. The strand 0.22 mm thick and 3.5 mm wide has 61 filaments with no twisting. The ratio of superconductor is 0.284. In the parallel conductors, the strands are transposed five times in each layer for a uniform current distribution among them. It was proved that the transformer has the rated capacity of 500 kVA by means of two-h short-circuit test and half-h no-load test in liquid nitrogen of 77 K. The efficiency is estimated as 99.1% from a core loss of 2.3 kW and a thermal load of 2.2 kW in coolant. The latter is composed of AC losses in windings and heat leakage from the cryostat and current leads, and is multiplied by a refrigeration penalty of liquid nitrogen, 20. Load test was also performed up to 500 kVA. The transformer was furthermore operated in subcooled nitrogen at 66 K with no quenching up to a critical level, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3% in subcooled nitrogen. Measured a.c. loss in both windings are well explained by a theoretical prediction with the “critical state model”. We also discuss prospective applications of the parallel conductors composed of advanced HTS multifilamentary tapes to a.c. windings with large current capacity.

We have designed and constructed a 500kVA-class oxide-superconducting power transformer. The windings are cooled by liquid nitrogen or subcooled nitrogen in a G-FRP cryostat of 785mm in diameter and 1, 210mm in height, that has a room-temperature space for an iron core with the diameter of 314mm. The primary and secondary windings are three-strand and six-strand parallel conductors of a Bi-2223 multifilamentary tape with silver sheath, respectively. The strand of 0.22mm thick and 3.5mm wide has 61 filaments with no twisting. The ratio of superconductor is 0.284. In the parallel conductors, the strands are transposed five times in each layer for a uniform current distribution among them. It was proved that the transformer has the rated capacity of 500kVA by means of two-hours short-circuit test and half-hour no-load test in liquid nitrogen of 77K. The efficiency is estimated as 99.1% from a core loss of 2.3kW and a thermal load of 2.2kW in coolant. The latter is composed of AC losses in windings and heat leakage from the cryostat and current leads, and is multiplied by a refrigeration penalty of liquid nitrogen, 20. Load test was also performed up to 500kVA. The transformer was furthermore operated in subcooled nitrogen at 66K with no quenching up to a critical level, that is equivalent to 800kVA. The efficiency estimated was improved to 99.3% in subcooled nitrogen. Measured AC loss in both windings are well explained by a theoretical prediction with the ‘critical state model.’ We also discuss prospective applications of the parallel conductors composed of advanced HTS multifilamentary tapes to AC windings with large current capacity.