HTS Roebel Cables: Self-Field Critical Current and AC Losses Under Simultaneous Application of Transport Current and Magnetic Field

Roebel cables made of high-temperature superconducting coated conductors are regarded as promising cables for winding applications in virtue of their large engineering current density and low losses. The composing meander-shaped strands are assembled very tightly into the cable, which results in a strong electromagnetic interaction between them. This interaction profoundly influences the effective self-field critical current (I-c) of the cable, which is much lower than the sum of the critical currents of the composing strands. In addition, the AC losses are influenced by the material's properties and by the geometrical configuration of the cable. Being able to predict the effective critical current and ac losses of such cables is very important for a proper design of applications: Due to the complexity of the cable's geometry and of the material's properties, this prediction can only be performed with advanced numerical tools. In this paper, we use finite-element-based models to compute the effective Ic and the ac losses of Roebel cables composed of 31 strands using tapes from two manufacturers. The AC losses are analyzed in the simultaneous presence of transport current and background perpendicular field proportional to the current, which mirrors the situation occurring in a winding. Our models include the angular dependence of J(c)(B, theta) at 77 K, which is very different for the two materials. By means of a successful comparison of the simulation results to experimental data obtained with a calorimetric method measuring the evaporation of liquid nitrogen, this work confirms the applicability and efficiency of our numerical techniques for simulating the electromagnetic behavior of Roebel cables and devices thereof.