Numerical Study of AC Loss in a Saddle Coil Carrying DC and Experiencing AC Field Utilizing T-A Formulation

AC loss estimation in REBCO high-temperature superconductor (HTS) coils is crucial in ac applications. The finite-element method (FEM) modeling is an efficient and widely-used approach to simulate ac loss in HTS coils. However, such models can become time-consuming and resource-intensive due to the high aspect ratio and anisotropic field-angle-dependent critical current properties of REBCO tapes. In addition, HTS coils with complex geometry, such as saddle-shaped coils, further increase the complexity of models because the 3D rotational asymmetry must be considered. This work presents a detailed demonstration of a 3D 10-turn saddle coil to overcome these simulation challenges. More importantly, the loss behaviors in an HTS saddle coil carrying currents under external magnetic fields are characterized for the first time. Frenet-Serret equations are utilized within the finite element analysis software to construct the 3D structure of a 10-turn saddle coil wound with REBCO tape. The dimensions of the coil are approximately 200 mm in length and 70 mm in width. The T-A formulation is employed, and measured I-c (B,theta) data is interpolated in the FEM model to simulate the electromagnetic behaviors of HTS tapes. The critical current of the saddle coil, along with magnetization loss at various magnetic field amplitudes, dynamic resistance, and total loss at different dc levels, are analyzed. Additionally, a race-track coil model with a similar size to the saddle coil is constructed and simulated for comparison purposes. Simulation results indicate that for both types of coils, the loss is mainly generated in the straight parts rather than the curved end portions.