Critical current asymmetry in HTS switches using iron-core electromagnets

Abstract

Iron-core electromagnets are used widely as applied-field switches in HTS flux pumps. It was recently reported that the critical current (I c) of the HTS wire is suppressed by up to 45% in the air-gap of the iron-core. Here, it is reported that the self-field I c of a single tape (unifilar) is suppressed from 357 A to 195 A while in the air-gap of the iron-core, agreeing closely with the previous study. This is extended by showing the I c response to perpendicular magnetic field is asymmetric, with peak I c of 249 A found to be at 0.16 T of applied field. This effect can be eliminated using an anti-parallel current configuration (bifilar), which is shown experimentally to recover close to the no-core self-field I c of 357 A with no observed asymmetry. The results of both types of switch configuration are closely replicated in finite-element modelling using COMSOL Multiphysics®. This is caused by the asymmetric interaction between the self-induced magnetic field of the HTS switch element and the magnetic circuit of the iron-core electromagnet.

Dynamic resistance and voltage response of a REBCO bifilar stack under perpendicular DC-biased AC magnetic fields

“…REBCO coated conductors (CCs) are becoming the preferred wire choice in many high-temperature superconducting (HTS) applications, such as field-triggered persistent current switches (PCSs) [1][2][3][4][5][6][7][8], flux pumps (FPs) [9][10][11][12], fault current limiters (FCLs) [13][14][15], electrodynamic suspension systems [16][17][18] and linear synchronous motors [19,20]. In these HTS applications, a REBCO CC carries DC currents under AC and DC magnetic fields.…”

“…These previous studies have not examined the influence of the waveforms and frequencies of AC magnetic fields on the dynamic resistance and voltage response when CCs are exposed to a wide range of AC and DC magnetic fields. The remaining research gaps need to be addressed to underpin practical applications, such as AC-fieldtriggered HTS switches [1][2][3][4][5][6][7][8] and transformer-rectifier FPs [9][10][11][12], where the waveform and frequency of the external magnetic fields influence their maximum output currents and stability.…”

“…A number of these HTS switches and FPs have the configuration of placing a HTS wire in a narrow airgap of a C-shaped iron-core magnet [2,7,12,35]. However, the selffield critical current of the single CC within the iron-core air gap is suppressed by approximately 45%, which directly influences the switching performance and efficiency [7,12,40].…”

“…A number of these HTS switches and FPs have the configuration of placing a HTS wire in a narrow airgap of a C-shaped iron-core magnet [2,7,12,35]. However, the selffield critical current of the single CC within the iron-core air gap is suppressed by approximately 45%, which directly influences the switching performance and efficiency [7,12,40]. On the other hand, a bifilar stack of two antiparallel REBCO CCs was used as the switching element, which is preferable for gapped-iron-core-based HTS PCSs and HTS FPs.…”

“…On the other hand, a bifilar stack of two antiparallel REBCO CCs was used as the switching element, which is preferable for gapped-iron-core-based HTS PCSs and HTS FPs. Such a 'noninductive' structure of bifilar stacks effectively minimizes the self-induction of the switch and substantially increases the self-field critical current of the CC, which provides more stable load fields and significantly improves the efficiency of HTS switches and FPs [1,7,12,40].…”