Dependence of the critical current density of Bi-2223 superconducting tape on the angle of external magnetic field is investigated theoretically and experimentally. The model proposed here is based on an effective coherence tensor model which can deal with the anisotropy of the coherence length, and has been derived so as to be applicable for the arbitrary angle of the external field. The resulting mathematical expression of the angular dependence of the critical current density shows that the critical current density is dependent only on the normal component of the magnetic field to the tape surface (i.e., the component perpendicular to the ab plane). The measurements of critical current density were made based on the four-point DC method, and the results show the validity of the theoretical considerations.
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.