Anisotropic monoblock model for computing AC loss in partially coupled Roebel cables

Abstract

When exposed to time-dependent magnetic fields, REBCO Roebel cables generate AC loss resulting from both magnetic hysteresis and induced inter-strand coupling currents. Until now, the AC loss has been computed in a two-dimensional approximation assuming fully coupled or decoupled strands, and a finite inter-strand resistance could be simulated only with three-dimensional models. In this work, we propose a homogenization procedure that reduces the three-dimensional geometry of the Roebel cable to two dimensions, without ignoring connections between the strands. The homogenized cable consists of two parallel ‘monoblocks’ with an anisotropic resistivity. The proposed model enables computation of AC coupling loss without the need for complex three-dimensional simulations. For experimental validation, a Roebel cable with soldered strands was prepared. The inter-strand resistance was determined by applying a transverse current and measuring the voltage profile. Additionally, the AC magnetization loss of the cable was measured in fields of 1 to 50 mT with frequencies of 1 to 2048 Hz using a calibration-free technique. With the measured inter-strand resistance as input parameter, the monoblock model gives a good estimate for the AC loss, even for conditions in which the coupling loss is dominant.

Calculation and measurement of coupling loss in a no-insulation ReBCO racetrack coil exposed to AC magnetic field

“…This model represents the coil as a homogeneous material with an anisotropic resistivity. Such a model was first proposed for pancake coils by Mataira et al [25] and has also been used to describe AC loss in Roebel cables [26]. An advantage is that the three-dimensional coil geometry can be reduced to two dimensions, thereby reducing computation time.…”

“…A method proposed by Brandt et al [32,33] is used to find a system of ordinary differential equations that describes the current densities. The application of this method is described in detail in a publication about ReBCO Roebel cables [26], which in the cross-section also appear as two side-by-side stacks of tapes, although the orientation is different. Below, a shortened derivation will be given to enable the reader to make their own implementation.…”

Influence of Eddy Current Losses in Non-Superconducting Layers of HTS in Superconducting Electrical Machines

“…However, an important design alteration has been made. The model developed by Otten et al (2020) [17] indicated that coupling losses in the non-insulated Roebel cable could be high since the coupling losses scale with the square of the frequency and amplitude of the magnetic fields [18]. Hence, tapes in the same turn should be insulated from one another under these conditions, for example, with a polyimide varnish coating [19].…”

Numerical Modeling of AC Loss in HTS Coated Conductors and Roebel Cable Using T-A Formulation and Comparison With H Formulation

“…Amongst them, the finite element method (FEM) with different formulations has been widely accepted and intensively used for its advantage in tackling complex geometries, compatibility with multidisciplinary simulations and transferability in the scientific and industrial communities [10]. In particular for HTS cables, FEM models in 2D with simplified geometries or modified equations, and 3D with full structures were developed, including Roebel [18]- [21], CORC® [22]- [24] and TSTC [25], [26].…”