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 to each layer by calculation.
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.
Tokyo Power Company and The Furukawa Electric Co., Ltd. are advancing development in the base, element and system aiming at the achievement of a compact high-Tc supercondunting cable of the 66kV class. The HTS wire used for the HTS cable must have a high critical current density (Jc), be flexible and easy to make in long lengths. The most advanced wire is Bi2223 silver sheath multi-filamentary wire judging from these requirements. We developed a Bi2223 silver sheath wire that has Jc>20kA/cm2 when long in length. Moreover, a wire with a filament twist, aiming at AC loss reduction, was developed, and a decrese in AC loss was comfirmed. To improve the mechanical properties of the wire, a multi-filamentary wire was made using Ag-Mg 0.2wt% alloy for the outside sheath. Concerning important AC losses for the HTS cable, a 50m-long conductor was manufactured and the AC losses were measured. As a result, it was comfirmed that AC losses could be decreased by making the current distribution of the each layer uniform. We newly proposed the UCD (Uniform Current Distribution) model, and succeeded in analyzing the AC losses of the UCD conductor by this model. Concerning the electrical insulation, which is an important cable component, the design data of an insulating paper impregnated by liquid nitrogen was able to be obtained by executing an electric insulation test using a model-cable. We manufactured a 66kV class-2kApeak prototype cable 5m long, and succeeded in making a load test. This prototype cable consisted of one HTS conductor that had electric insulation, a thermal insulated tube and two terminals that connected to both ends to do the load test.
We present the possible application of a compact high-Tc superconducting AC power cable for transmitting electric power in urban areas, reflecting its novel features such as extreme compactness for installation in underground ducts and considerably large-capacity rating comparable to present high-voltage cables. These new aspects are expected to bring a great cost benefit in the expansion of underground power transmission systems, primarily due to the fact there is no need to construct additional underground tunnels. We set up a model system using 66kV high-Tc superconducting cables with a 350-700MW capacity based on the typical skeleton of a present 275kV system. We investigated the stability of the model system, and found that stable power transmission is possible using a parallel capacitor within a rating capacity of a tertiary coil of transformers even though the high-Tc superconducting cable operates at a fairly low voltage and carrries large electric power. It was pointed out that the effective reactance of high-Tc superconducting cable is roughly an order of magnitude lower than that of present-day 66kV cable to the existence of a perfect shielding layer, which greatly contributes to the stability of the system.