Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

The no-insulation (NI) winding technique is promising to be applied in the persistent-current mode (PCM) operation of high-temperature superconducting (HTS) coils to provide those advantages. To apply the NI PCM coil, its demagnetization behaviour (i.e., decay of persistent DC current) by external AC field, which occurs in maglev trains, electric machines, and other dynamic magnet systems, is essential to be understood. For this purpose, a 3D FEM model, capturing the full electromagnetic properties of NI HTS coils, is established. This work has studied three kinds of AC fields, observing the impact of turn-to-turn contact resistivity on demagnetization rates, which is attributed to current distribution modulations. Under transverse AC field, the lower contact resistivity attracts more transport current to flow in the radial pathway to bypass the ‘dynamic resistance’ generated in the superconductor, leading to slower demagnetization. Under axial AC field, the demagnetization rate exhibits a non-monotonic relation with the contact resistivity: (1) the initial decrease of contact resistivity leads to concentration of induced AC current on the outer turns, which accelerates the demagnetization; (2) the further decrease of contact resistivity makes the current smartly redistribute to avoid to flow through the loss-concentrated outer turns, thus slowing down the demagnetization. Under the rotating DC field, a hybrid of the transverse and axial fields, the impact of contact resistivity on the demagnetization rate exhibits combined characteristics of the transverse and axial components. Besides, quantitative prediction on the demagnetization rate of NI PCM coil under external AC field is instructive for practical designs and operations, which is tried by this 3D FEM model, and a comparison with experimental result is conducted.

Section: Introductionmentioning

confidence: 99%

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

Zhong,

Wu,

et al. 2024

AC loss calculation of no-electrical-insulation HTS magnets using a field-circuit coupling method

Wang,

Ma,

Zhou

et al. 2024

No-electrical-insulation (NEI) magnets gradually exhibit significant appeal due to their robust thermal stability and elevated mechanical strength. However, when exposed to AC conditions, the magnets would suffer more significant AC losses in dynamic electromagnetic devices, such as motors and maglev systems. Presently, the numerical methods for predicting the electromagnetic and loss behaviors of large-scale NEI magnets entail high computation costs due to the substantial degrees of freedom or complicated modeling strategies. Thus, we propose a fully finite-element method referred to the field-circuit coupling method to efficiently assess the overall behaviors of NEI magnets while preserving adequate accuracy. This method couples the T-A formula and the single-turn equivalent circuit through global voltage, to avoid the costly and complicated inductance calculation, and to simultaneously consider the induced current. By further integrating the homogenization method, the calculation speed can be increased up to ten times. Additionally, we study the critical current, electromagnetic and loss behaviors of the NEI magnets based on the proposed model. We identify some measurement methods that offer more precise estimations of the critical current and the turn-to-turn contact resistance of NEI magnets. Meanwhile, the results indicate the severe impact of high AC fields on the losses, and emphasize the importance of a reliable shielding structure for operational safety. Finally, the influence of turn-to-turn contact resistivity on the loss behavior is also investigated, which can provide valuable insights for the design of the NEI magnets in dynamic electromagnetic devices.

Fast and accurate 3D FEM model for electromagnetic simulations of no-insulation HTS coils based on polygon-anisotropic-resistivity

Zhong,

Wu,

Jin

2024

For no-insulation (NI) high-temperature superconducting (HTS) coils, a 3D electromagnetic model, which is fast and accurate, conducive to establish, and straightforward to muti-physics coupling, is still required. This paper introduces a polygon-anisotropic-resistivity (PAR) method for 3D FEM electromagnetic simulations of NI HTS coils. This model avoids dividing each tape into the specific HTS-tape layer and turn-to-turn contact layer, which yields: (1) a reduced computational burden; (2) improved convergence due to smaller element aspect ratios. The significance of the polygon-anisotropic-resistivity method lies in its indispensable role in achieving a 3D anisotropic-resistivity model with high computing speed and accuracy. The proposed PAR model is rigorously evaluated through three types of simulations: (1) charge and discharge tests; (2) AC losses of the NI coil subjected to AC fields with a DC current supply; (3) heat-triggered quench and recovery scenarios. For these simulations, the PAR model is validated by comparisons with the full-element model, namely, the 3D FEM model that explicitly incorporates each specific HTS-tape layer and turn-to-turn contact layer in the H-formulation model. Good consistency is observed. The computing speed of the PAR model is tested to be 12-38 times that of the full-element model with the same accuracy. The PAR model achieves a 40 % reduction in degrees of freedom compared to the full-element model, with the same mesh density along the HTS tape width and length, facilitating more precise and larger scale coil simulations within the same computational memory limits. Additionally, the PAR model entirely eliminates the inherent inaccuracies found in the conventional-anisotropic-resistivity 3D model, which stem from discrepancies between the arranged anisotropic-resistivity and the actual computed coil meshes. The proposed PAR model will enhance the prevalence of 3D electromagnetic analyses of NI HTS coils.