Based on the proposal of IEC/TC90, we have been standardizing the method for the copper-to-superconductor volume ratio (copper ratio) of Cu/Nb-Ti composite superconductors. There are many methods for measuring copper ratio. As a result of comparison studies for each method, however, it has been confirmed that the weight method is very reliable. Therefore, we investigated various restricting conditions in the weight method such as the weight of test samples, dissolving method, etc. We considered the reduction of scattering in measuring values by the weight method and determined a procedure to confirm the appropriateness of these restricting conditions. Upon completing these steps, a round-robin test (RRT) was carried out by eight organizations participating in this project. We present a commentary on the test method for copper ratio in this paper.
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.
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