Study on the effects of winding pre-tension force on electromagnetic performance of pancake coils

Abstract

The winding process plays a critical role in determining the turn-to-turn contact resistance in high-temperature superconducting (HTS) no-insulation (NI) pancake coils. Previous studies typically employed a specific contact resistivity to characterize the turn-to-turn contact resistance of the NI pancake coil. However, in practice, the turn-to-turn contact resistivity changes with many factors, such as the winding pre-tension force and anisotropy of tapes. In this paper, the turn-to-turn contact stress under different winding pre-tension forces is analyzed from both theoretical and numerical simulation perspectives. An analytical expression for turn-to-turn contact stress is derived based on the elastic theoretical and Hakiel models, followed by the formulation of an expression for stress-dependent contact resistance. Based on this, the equivalent circuit model is employed to investigate the internal current distribution, terminal voltage, charging delay of the coil under different winding pre-tension forces, as well as the current distribution of the coil with a local defect. The results indicate that the winding pre-tension forces have a significant effect on the turn-to-turn contact resistance and electromagnetic performance of pancake coils. Further, the effects of HTS tape anisotropy and bobbin stiffness on the terminal voltage of the pancake coil are also examined. The results indicate that winding core stiffness has a negligible effect, while HTS tape anisotropy significantly affects the terminal voltage. This study provides a theoretical basis for the preparation and practical application of pancake coils.