Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

TLDR

High‑temperature superconductors are used in AC devices such as motors, generators, transformers, and fault‑current limiters, where low AC loss and high current capacity are essential; Roebel cables, invented in 1914 for copper, have been adapted to REBCO coated conductors to meet these demands, and extensive research has been published on their design and performance. This review aims to synthesize the accumulated research on REBCO Roebel cables and provide a concise, user‑friendly guide for developers, manufacturers, and operators. Roebel cables achieve low loss by fully transposing multiple strands so that each carries current along the entire cable length, a design that has been implemented in REBCO conductors to deliver high transport currents, low AC loss, and sufficient flexibility for practical applications.

Abstract

Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables.