AC losses in multifilamentary HTS tapes can be classified into hysteresis loss, coupling loss, and eddy current loss from the viewpoint of their generation mechanism. Operating electromagnetic conditions significantly influence their AC loss characteristics. From the viewpoint of the major magnetic field component generating them, they can be classified into magnetization loss, transport loss, and total loss. Dividing a superconductor into fine filaments, twisting the filament bundle and increasing transverse resistivity effectively reduce magnetization loss and total loss when the external magnetic field is relatively large, while they do not effectively reduce transport loss. General analytical expressions for the total loss in HTS tapes carrying AC current in an AC magnetic field have not been derived. Numerical electromagnetic field analysis based on the finite element method is a powerful tool to study total loss theoretically. In the magnetic field parallel to the tape's wide face, twisting can reduce the AC loss in Bi2223 tapes with pure silver matrix, while increasing transverse resistivity is required essentially for the AC loss reduction in a perpendicular magnetic field. Recently, twisted multifilamentary Bi2223 tapes with pure silver matrix were fabricated, and the reduction of magnetization loss was proved experimentally in a parallel magnetic field. Furthermore, transverse resistivity was increased successfully by the introduction of a resistive barrier between filaments.
The AC loss in high-Tc superconductors near the glass-liquid transition temperature, Tg, is studied numerically and theoretically. It is shown that the AC loss in a slab sample under a DC bias magnetic field depends noticeably on the angular frequency, ω, and hence, cannot be described by the usual critical state model using critical current density, Jc, defined with the aid of the electric field criterion. However, the AC loss including the flux flow loss is shown to be describable quantitatively by present handy theoretical expression, which has the same form as the well-known expression based on Bean's critical state model except that Jc is replaced by the effective critical current density, Jce(ω).
The possibility of reducing AC losses in superconducting Bi-2223 tapes with the aid of the reversible motion of flux lines is investigated. It was found that the AC loss was reduced by reducing the size of superconducting filaments, and the behavior was explained by the theoretical model of Campbell. To get a sufficient reduction of AC losses at 77.3K from the prediction using the critical state model, it is necessary to reduce the filament thickness below 2.5μm.
The AC transport loss properties under self-fields at 77K have been investigated for simple cable conductors composed of 4 or 8 pieces of Ag-sheathed Bi2223 multifilamentary tape with different filament arrangements. The AC transport losses per cycle are nearly independent of frequency from 30 to 200Hz for each conductor, indicating that the main contribution to losses comes from the hysteresis loss in the superconductor regardless of filament arrangements in the tape strand and the arrangement of tape strands in the conductors. The loss values of conductors strongly depend on the filament arrangements in the tape strands. The conductors composed of tape strands with sectioned filament arrangements have lower losses than those without sectioned filament arrangements, because the field-free core (FFC) in the former is sectioned into some parts in the filament group. The AC transport losses of cable conductors are also influenced by the number of tape strands. The loss values are decreased by increasing the number of tape strands, which is explained by calculated results for density distributions of AC transport losses per cycle in a tape strand. These results show that improvement in the filament arrangements in tape strands and the arrangement of tape strands in conductors are quite important to suppress the losses generated in cable conductors under AC current transmission.
Ag-sheathed Bi-2223 round strands for high-temperature superconducting (HTS) transposed cable conductors were developed. The critical current density (Jc) at 77K, 0T reached 2.5×108A/m2 in a short-length sample and 1.6×108A/m2 in a 50m-long sample. Several analyses were done to evaluate the basic properties of round strands, such as Bi-2223 volume fraction and crystal alignment, and magnetic field dependence of the Jc. Bi-2223 volume fraction was determined by magnetic susceptibility measurement using a SQUID magnetometer. It was confirmed that the Bi-2223 volume fraction was closely correlated with Jc. The crystal alignment was observed by SEM in the cross section of the round strand. It was found that the crystal alignment angle decreased with increasing Jc. The outgrowth feature of the crystals was also found to be decreased in the high-Jc sample. The crystal alignment angle in the longitudinal direction of the round strand was determined by two different Jc-B curves, in which magnetic fields were applied parallel and perpendicular to the longitudinal directions. An assumption was employed in the analysis that Jc was suppressed only by the c-axis component of the applied magnetic field. It was also found that the crystal alignment angle in the longitudinal direction of the round strand was decreased with increasing Jc.
Bi2Sr2CaCu2Ox (Bi-2212) superconducting wires have been fabricated successfully without using Ag. Commercial Ni tapes were used for the substrate. After heat treatment in air to form a NiO layer on the Ni tape, Bi-2212 precursor was coated and heat-treated with a conventional slow cooling step. The Bi-2212 superconducting tape exhibited high critical current and critical current density: 184A and 1.2×105A/cm2 at 4.2K and 10T, respectively. Since Ni is much less expensive than noble metal of Ag and the whole process is simply carried out in air, the present method is promising in terms of cost.
The superconducting fault current limiter is expected to be the first application of high-TC superconductors to electric power systems. To analyze the performance of the magnetic shielding-type superconducting fault current limiter, the dependence of the voltage-current (E-J) characteristics of high-TC superconducting bulk on temperature and magnetic field have been measured. Short-circuit fault tests with a small limiter model have been carried out. A computer program based on the finite element method was used for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The computer program, considering the voltage-current (E-J) characteristic, successfully simulated the electromagnetic behavior in the fault current limiter test.