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.

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

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 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 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. Therefore it is timely 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 cable.

https://iopscience.iop.org/article/10.1088/0953-2048/27/9/093001/pdf

RBCO (R = Y or rare earth element) coated conductors (CC) are the most promising HTS materials for future high field coils operated at moderately high temperature (40-50 K). Coils are planned for the second generation of fusion reactors (DEMO, "DEMOnstrator") and beyond. A ROEBEL bar concept for a high current (kA-class) low AC loss cable is the most suitable assembling technique for conductors in magnet windings due to the flat rectangular cross section. The presented RACC-cable technique (RACC=ROEBEL assembled coated conductors) works with pre-shaping of tapes into strands with the ROEBEL specific meander geometry. The usually very good bending properties of the CC support the assembling procedure of the RACC-cable. We report on a 16 strand RACC-cable with 19 cm transposition length made from CC material from the commercial supplier SuperPower which reached 1020 A transport critical current at 77 K (Jeng = 11.3 kAcm-2). The basic properties of the virgin YBCO tapes and the shaped strands like orientation and field dependent transport currents, current homogeneity and bending effects, were investigated and correlated with the measured properties of the RACC-cable. Calculation of the self field effects by means of a model adapted to the specific RACC-cable geometry and in particular taking into account the current distribution in the cable, explained the 30% current reduction in the cable quantitatively.

ROEBEL Assembled Coated Conductors (RACC): Preparation, Properties and Progress

/pdf/a-review-of-commercial-high-temperature-superconducting-2o02lszkmx.pdf

A review of commercial high temperature superconducting materials for large magnets: from wires and tapes to cables and conductors

https://iopscience.iop.org/article/10.1088/1741-4326/aad835/pdf

High temperature superconductors for fusion magnets

Superconducting transformers are an important innovation for future power transmission and transportation systems. Powerful, lightweight, energy-saving and environmentally friendly they offer enormous benefits compared to their conventional counterparts. Siemens is developing a 1-MVA demonstrator transformer for laboratory testing, exhibiting innovative features like horizontal design, cabled-conductor windings and a closed cooling cycle with sub-cooled nitrogen. Being one of the most promising applications Siemens has started a programme towards the development of on-board transformers for electrical rail vehicles. This paper summarises world-wide efforts in the development of superconducting transformers and reports on the progress achieved at Siemens.

W. Knorr

Papers

Development and application of superconducting transformers