The Garber Current Pattern: An Additional Contribution to AC Losses in Helical HTS Cables?

TL;DR

This paper investigates AC losses in helical HTS cables with a focus on the Garber current pattern, using 3D simulations to analyze how current profiles and losses depend on design parameters.

Contribution

It introduces a detailed 3D simulation study of AC losses in helical HTS cables considering the Garber pattern, highlighting the effects of pitch angle and frequency.

Findings

Confirmed the non-trivial current profiles due to Garber pattern

Identified an optimal pitch angle for minimizing AC losses

Showed that current profiles are strongly frequency-dependent

Abstract

Conductors made of high-temperature (HTS) wires helically wound in one or more layers on round tubes (CORT) are compact, flexible, and can carry a large amount of current. Although these conductors were initially developed for DC applications, e.g. in magnets, it is worth considering their use for AC, e.g. in underground cables for medium voltage grids and with currents in the kA-range. In these cases, the major challenge is reducing AC losses. In contrast to a straight superconducting wire, in a helical arrangement, due to superconducting shielding, the current does not follow the direction of the wires, but takes a non-trivial zig-zag path within the individual HTS wires (Garber pattern). This includes current components across the thickness of the superconducting layers, so that the often used thin-shell approximation does not hold. In this contribution, we studied a one-layer three-wire CORT by means of fully three-dimensional simulations, based on the H-formulation of Maxwell's equations implemented in the commercial software package COMSOL Multiphysics. As a result of our simulations, the peculiar current profiles were confirmed. In addition, the influence of current, pitch angle, and frequency on the AC losses was studied. We found an optimum for the pitch angle and that the current profiles strongly depend on frequency.