AC losses in power transmission cables composed of coated conductors could be potentially small. A strategy to approach their potentially small AC loss was studied. AC losses in mono-layer conductors for cables were calculated numerically in order to show the principle for AC loss reduction: use of narrower coated conductors and/or decrease in space between conductors reduce the magnetic field component perpendicular to the wide face of coated conductors, and they are effective for AC loss reduction. This principle was confirmed experimentally by using short monolayer conductors. Based on the principle, 1 kArms-class three-layer conductors were fabricated, and AC loss of 0.054 W/m at 1 kArms was achieved in a three-layer conductor whose outer diameter is 19.6 mm. The influence of a magnetic substrate on the AC losses in a single coated conductor and a mono-layer conductor for a cable was studied numerically.

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Two types of four-layer cables composed of coated conductors, with practical cross-sectional geometries and various critical currents, were designed. The AC losses in the cables were numerically calculated using a numerical model employing a one-dimensional FEM (1D model). The AC loss of each layer in the four-layer cable with w = 4.0 mm was compared with Norris's analytical values, Q NS , for a wide range of I t /I c . The AC loss in the first (innermost) layer was smaller than its Q NS , whereas the losses in the outer layers were close to, or much larger than, their Q NS . These results indicate that, in multi-layer cables, the AC loss in an outer layer increases because of the magnetic field generated by the current in the inner layers, and the entire AC loss of the cable is dominated by the AC losses in the outer layers. The AC losses in the four-layer cables with w = 3.3 mm, which had smaller gaps between the conductors, were smaller than those for the cable where w = 4.0 mm. These results showed the effectiveness of using narrower conductors and decreasing the gaps to reduce AC loss in multi-layer cables composed of coated conductors. The AC losses of the four-layer cable with w = 3.3 mm and I cc /w = 500 A cm −1 at 1 and 3 kArms were much smaller than the measured AC losses in cables fabricated with YBCO-coated conductors or BSCCO tapes. The AC loss in a four-layer cable with practical cross-sectional geometries should approximately be within an order of magnitude of their Q NS .

Two-dimensional electromagnetic field analyses were undertaken using two representative cross sections of two-layer cables consisting of coated conductors with magnetic and non-magnetic substrates. The following two arrangements were used for the coated conductors between the inner and outer layers: (1) tape-on-tape and (2) alternate. The calculated magnetic flux profile around each coated conductor was visualized. In the case of the non-magnetic substrate, the magnetic field to which coated conductors in the outer layer are exposed contains more perpendicular component to the conductor wide face (perpendicular field component) when compared to that in the inner layer. On the other hand, for the tape-on-tape arrangement of coated conductors with a magnetic substrate, the reverse is true. In the case of the alternate arrangement of the coated conductor with a magnetic substrate, the magnetic field to which the coated conductors in the inner and outer layers are exposed experiences a small perpendicular field component. When using a non-magnetic substrate, the AC loss in the superconductor layer of the coated conductors in the two-layer cables is dominated by that in the outer layer, whereas the reverse is true in the case of a magnetic substrate. When comparing the AC losses in superconductor layers of coated conductors with non-magnetic and magnetic substrates in two-layer cables, the latter is larger than the former, but the influence of the magnetism of substrates on AC losses in superconductor layers is not remarkable.