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The characteristics of a multi-layered conductor, which was an assembly of Bi-2223 Ag-sheathed superconducting wires, were investigated. We confirmed that experimental value of the self-field loss in the multi-layer conductor was nearly identical with a theoretical value based on a critical-state model. In detail, the experimental result was well explained by the power law model assuming that the current-voltage characteristic of the high-Tc superconductor is well approximated as V∝In. We also investigated the AC current distribution of each superconducting layer using 50m/4-layer conductor. As measured with a Rogowski coil, it was found that an unbalanced current distribution occurred in the multi-layer conductor. It is possible to reduce the AC loss by suppressing unbalanced current distribution. As a solution, we suggested to adjust the pitch of the conductor in order to equalize the impedance of all the layers. For this we estimated the condition for a balanced AC current to each layer by calculation.

Self-field AC loss of cylindrical multi-layered conductor, which was an assembly of Bi-2223 Ag-sheathed superconducting wires, was investigated. The self-field loss of the conductor derived from the electric field measured by the 4-probe method could be explained theoretically by the critical-state model. It was also explained that the self-field loss could be measured by the pick-up coil which formed a loop surrounding all the superconducting wires in the conductor. To compare these methods, a cylindrical 2-layer conductor with two types of taps to detect the voltage was fabricated: one was a common voltage tap by solder and the other a pick-up coil surrounding all the superconducting part of the conductor. AC losses obtained by the two methods mentioned above were nearly identical when the transmission current was smaller than the critical current. Thus, we suggest that a pick-up coil is a useful tool for AC loss measurement of multi-layer conductors where no soldering is required.

In order to supply large amounts of electric power to meet increasing demand, a high-Tc superconducting cable compact enough to be applied to ducts without construction of new underground tunnels is desired. There are, however, many key technologies which are necessary to develop long high-Tc cables capable of large capacity in compact size. For the purpose of studying these technologies, two systems were developed and tested. At the first step, a 7m long, 3-phase 1kA superconducting cable system was developed to study issues concerning large current loading. The measurements of AC losses and inductance show that total AC loss of this cable was 3.5W/m/cct at 1kA loading, that hysteretic loss was dominant, and that the magnetic shields were effective enough to reduce the eddy current losses in thermally insulated pipe. As the next step, a 30m long, 66kV-1kA high-Tc superconducting cable system prototype was developed to study issues for long-length cables. In this system, an AC current of 40kV-1kA was successfully applied in sub-cooled liquid nitrogen (72K, 1.2kg/cm2abs) regardless of mechanical history, such as handling in the factory, transportation, laying and the axial force due to contraction during cool down.

We applied an electric-circuit model for the calculation of AC loss of a high-Tc superconducting power-cable conductor with four layers and interlayer insulation. This model is composed of the impedance of each layer, which has a resistive part and an inductive part. The resistance of each layer was calculated using the approximate value of the resistive voltage as a function of transport current of multifilamentary Bi-2223 wire. The inductance of each layer was considered to be one of two kinds: one depending on the self-field directed along the axis of the conductor, and one depending on the self-field directed along the circumferential direction of the conductor. From the results of the calculation using the electric-circuit model, it was found that both the calculated and experimental values of AC loss, as a function of transport current of the conductor, are nearly equal, and that the model explains the electromagnetic property of the conductor well. In calculating the current distribution of the conductor, it was also found that drift, in which almost all the current passes through the outer layer, occurs when the transport current is lower than the Ic-value of the conductor. The inductance that depends on the self-field directed along the circumferential direction of the conductor is dominant in the impedance of each layer. This value decreases with the increase in layer radius. Therefore, the impedance of the outer layer is decreased, and hence, the transport current passing through the outer layer is increased. The drift increases the AC loss of the conductor. In order to control the lack of balance of impedance amongst the layers, the inductance that depends on the self-field directed along the axial direction of the conductor, which is increased in the outer layer, must be increased. This value of inductance increases as the length of the spiral pitch of the high-Tc superconducting wires which make up the conductor decreases. The results of calculations of conductor AC loss as a function of the length of the spiral pitch of the wire showed that AC loss is markedly decreased when the length of the spiral pitch is less than 0.5m.

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.

Activity on the international standardization of the test methods for industrial superconductors is introduced, which is authorized by IEC/TC90 (Superconductivity). A brief history and strategy of the organization are explained. The substantial work involved to discuss and prepare the draft for each item is left to the respective international working group. At present, 8 working groups cover the field of superconductivity and related properties; terminology, critical current, residual resistance ratio, copper ratio, mechanical property and surface resistance. One of them, the test method of critical current for Cu/Nb-Ti composite superconductors, will soon be published as an international standard. The domestic activity in Japan supporting international standardization is supervised by the Japanese National Committee of IEC/TC90. The mission of the JNC is the following three activities; support of the international activity as a secretary country, translation of international standards to JIS and supervision of domestic activities. The actual test methods are proposed and carried out by the New Materials Center and Japanese Fine Ceramics Association. It is emphasized that establishing good liaison with groups such as VAMAS concerning the fundamental aspects of standardization is crucial to promote effective international collaboration.

In this study, ac losses in BSCCO silver-sheathed wires, both single- and multi-filamentary, were investigated by means of ac-magnetization and ac-transport techniques. It was shown that the longitudinal magnetization ac loss in a single-filamentary wire could be quantitatively explained by the simple sum of the magnetic hysteresis loss of the superconducting core and the eddy current loss in the silver-sheath, while those in multi-filamentary wires behaved quite differently, possibly being affected by electromagnetic coupling between filaments. On the other hand, the self-field ac loss of all the wires employed was found to be well described by a simple expression derived for a uniform superconducting cylinder based on Bean's model. Namely, the self-field ac loss was determined solely by Ic and Ip/Ic (Ip: peak value of the transport current), regardless of their cross-sectional structure, and the distribution of the current flow is considered to have an elliptical symmetry even for multi-filamentary wires, where the critical state is established from the outermost filaments toward those in the interior. It is suggested that the self-field loss of multi-filamentary wires can be reduced if the distribution of the current flow can be made equivalent for all filaments.

High critical temperature superconducting assembly of 50cm length have been developed using silver sheathed Bi1.8Pb0.4Sr2Ca2.2Cu3Ox superconducting tapes. A critical current of 150A at 77.3K was achieved over 40cm with the resistivity of 4×10-13Ω·m, which was much lower than the resistivity of silver, 3×10-9Ω·m. The average critical current of every 5cm was 177±9.4A. It suggests that the superconducting tapes of 10m length used for the assembly have a uniform transport property. Larger transport current will be attained by increasing the number of strand. Multifilamentary taped superconducting wire showed the good bending strain vs. critical current property. The critical current was decreased by 30% due to about 3% bending strain. Transport current properties of single core taped superconducting wires were stable for 50 times heat cycles between room temperature and 77.3K.

BiPbSrCaCuO silver sheathed high-Tc superconducting wires were fabricated through the solid reaction method. Maximum critical current density is 3.0×104A/cm2 at 77.3K in a zero magnetic field, 1.5×104A/cm2 at 0.1 Tesla and 3.3×103A/cm2 at 1 Tesla. We propose that the magnetic field dependence of Jc of the wire is dominated by the Lorentz force which works perpendicular to the c-axis, caused by the transport current with c-axis component crossing grain boundaries when the magnetic field was applied parallel to the plane of the wire. While, when the magnetic field was applied perpendicular to the plane, it was dominated by the pinning force in the grains.