Angular dependenceof the magnetisation AC loss: coated conductors, Roebel cables and double pancake coils

TL;DR

This paper uses simulations to analyze how the angle of magnetic fields affects AC loss in coated conductors and Roebel cables, revealing that AC loss depends on more than just perpendicular magnetic components and can be significantly reduced with optimal cable design.

Contribution

It introduces a detailed simulation approach considering anisotropic field dependence and compares different numerical methods to understand AC loss mechanisms in Roebel cables.

Findings

AC loss depends on more than just perpendicular magnetic field components.

AC loss can be reduced by over an order of magnitude with optimal cable orientation.

Simulations show significant reduction in AC loss for fields below 7 degrees from strand surface.

Abstract

The AC loss in ReBCO coated conductors is large in situations when the conductors are subjected to a considerable magnetic field, like in rotating machines, transformers and high-field magnets. Roebel cables can reduce the AC loss in these cases. However, computer simulations are needed to interpret the experiments, understand the loss mechanisms, reduce the AC loss by optimising the Roebel cable and design the cryogenic system. In this article, we simulate and discuss the AC loss due to an applied magnetic field making an arbitrary angle with the cable and taking into account a realistic anisotropic field dependence of the critical current density. We study the AC loss in the superconductor parts for the limits of very high coupling currents and completely uncoupled strands. The simulations for the uncoupled case also describe a double pancake coil with no transport current. For the simulations, we use two different numerical methods with complementary strengths. This serves as a mutual check of the correctness of the simulation results, which agree to each other. Opposite than expected, we found that the AC loss does not only depend on the perpendicular component of the applied magnetic field. We also found that the AC loss for applied fields with an orientation below 7 degrees with the strands surface is reduced more than one order of magnitude comparing to an untransposed cable. Therefore, we recommend to use Roebel cables for windings with important parallel components of the magnetic field, such as transformers and high-field magnets.