DEVELOPMENT OF A 500 KVA-CLASS OXIDE-SUPERCONDUCTING POWER TRANSFORMER OPERATED AT LIQUID-NITROGEN TEMPERATURE

We have designed and constructed a 500 kVA-class oxide-superconducting power transformer. The windings are cooled by liquid nitrogen or subc ooled nitrogen in a G-FRP cryostat of 785 mm in diameter and 1210 mm i n height, that has a room-temperature space for an iron core with the diameter of 314 mm. The primary and secondary windings are three-stran d and six-strand parallel conductors of a Bi-2223 multifilamentary tap e with silver sheath, respectively. The strand 0.22 mm thick and 3.5 m m 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 m eans of two-h short-circuit test and half-h no-load test in liquid nit rogen of 77 K. The efficiency is estimated as 99.1% from a core loss o f 2.3 kW and a thermal load of 2.2 kW in coolant. The latter is compos ed of AC losses in windings and heat leakage from the cryostat and cur rent leads, and is multiplied by a refrigeration penalty of liquid nit rogen, 20. Load test was also performed up to 500 kVA. The transformer was furthermore operated in subcooled nitrogen at 66 K with no quench ing up to a critical level, that is equivalent to 800 kVA. The efficie ncy estimated was improved to 99.3% in subcooled nitrogen. Measured a. c, loss in both windings are well explained by a theoretical predictio n with the ''critical state model''. We also discuss prospective appli cations of the parallel conductors composed of advanced HTS multifilam entary tapes to a.c. windings with large current capacity. (C) 1998 El sevier Science Ltd. All rights reserved.