Electro-mechanical analysis of Roebel cables with different geometries

Abstract

For future fusion applications, superconducting motors and generators, HTS Roebel cables with high effective current densities are desired. Due to the enormous Lorentz forces of fusion magnets or the centrifugal forces of rotors, these cables must also be able to withstand high mechanical loads. Coated conductor material can sustain high tensile loads; however, the meander structure needed for Roebel assembled cables concentrates mechanical stress at the crossing sections. This weakens the tape mechanically and makes it prone to failure in these regions. The increase in mechanical sensitivity depends on geometric parameters such as angle, width and inner radius of the Roebel structure and can be reduced by an optimized geometry. In this paper Roebel cables with varying geometries were investigated mechanically and electrically. Firstly, different meander structure geometries were analyzed using 3D finite element method (FEM) models to evaluate their resistance to tensile loads and the minimal possible transposition length. Secondly, the electro-magnetic properties of the 2D cross section of a Roebel cable were investigated with FEM simulations. These simulations were used to derive an improvement of the meander structure which permits almost the same twist pitch, but increases the mechanical stability and the current carrying capabilities of the whole Roebel assembled cable.

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

“…The inter-strand gap is strongly correlated to the assembling procedure, being narrower for hand-made cables (KIT) and wider in the case of fully automatic process (IRL). Numerical calculations on the geometry with the best mechanical performance and current capacity [21] confirmed the set of parameters usually used at KIT: an inner radius Ri = 2 mm (the stress hot spot in the tape), a strand-edge clearance of 1-2 mm for WT = 10-12 mm and no clearance for WT =4 mm (which needs a sufficient inter-strand gap) and a crossover angle of 30 degrees. According to the calculations presented in [21], a sharp outer edge of the crossover section and an increase of the inner radius leads to a reduction of the von Mises stresses under tensile loads.…”

“…Numerical calculations on the geometry with the best mechanical performance and current capacity [21] confirmed the set of parameters usually used at KIT: an inner radius Ri = 2 mm (the stress hot spot in the tape), a strand-edge clearance of 1-2 mm for WT = 10-12 mm and no clearance for WT =4 mm (which needs a sufficient inter-strand gap) and a crossover angle of 30 degrees. According to the calculations presented in [21], a sharp outer edge of the crossover section and an increase of the inner radius leads to a reduction of the von Mises stresses under tensile loads. In the Roebel cables from IRL some parameters were chosen differently, in order to be better compatible with the assembling machinery, namely: a larger clearance width Wc between the two sides of the cable and a smaller number of strands per cable length unit [17].…”

“…Mechanical induced effects from applied strain have been the subject of finiteelement method investigations [21,25], which identified the inner radius of the crossover section as the critical point with the peak values of mechanical stresses. It has also been found that a sharp outer edge of the crossover section, as in the KIT cables, reduces the peak strain [20].…”

Dipole Magnets above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…Understanding how strain develops in REBCO layer during the cabling and magnet winding processes will be critical [32,104,106,[108][109][110]195,196]. The design tools based on the understanding of the conductor mechanics will help guide the development of more flexible round REBCO conductors and the optimized REBCO tapes with thinner substrates and narrower widths [34,38,160].…”

Electromagnetic analysis of high current carrying superconducting cable consisted of YBCO coated conductor

“…3. The current distributions of conductor layers are influenced by the structure parameters of the Roebel cable [2]. A Roebel cable consisted of YBCO coated conductors is designed using COMSOL Multiphysics as well.…”