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Description
This paper presents an approach to calculate the thermal quench temperature of non-insulation(NI) REBCO coils and predict the thermal quench upper limit of large - scale engineering coils. This method serves as a guiding principle for the design of coil and magnet structures. Given a specific quench margin, it safeguards the coil from damage and ensures excellent thermal stability.
Quench remains a formidable challenge in the application of high - temperature superconducting(HTS) tapes. When the conducting current surpasses the tape's critical current, the resistance of the superconducting layer escalates rapidly, resulting in the loss of superconducting properties. In contrast to insulated coils, NI coils exhibit remarkable thermal stability and self - protection capabilities. In the event of a quench, the current at the quench site can bypass the quenched region via the inter - turn contact resistance, enabling automatic current sharing. This substantially reduces the current capacity of the quench region and the local heat generation within the coil, thereby expediting the recovery of the tape during the quench process. Nevertheless, due to the presence of inter - turn branches, the current distribution within NI coils becomes more intricate during the quench process, rendering it arduous to measure the current distribution and its variations experimentally.
To investigate and forecast the operating characteristics of NI coils, different equivalent circuit models have been proposed. The Partial Element Equivalent Circuit (PEEC) model divides each turn of the coil into multiple units along the circumferential direction, which offers high - precision calculations but demands a substantial amount of time. The Moderate equivalent circuit model aggregates several turns of the coil into a single unit. Although its calculation accuracy is marginally lower, it features a significantly faster calculation speed. In this study, equivalent circuit models are integrated with the finite - element simulation models of the magnetic field and thermal field to simulate the electro - magnetic - thermal characteristics of the coil during the quench process. Since the Moderate model is incapable of simulating point - quench, a point - quench in the PEEC model is manifested as the entire - unit quench of the corresponding unit in the Moderate model.
By comparing the simulation results of the two models, it is evident that under identical quench conditions, the maximum temperature and steady - state average temperature of the coil obtained through the Moderate model are slightly higher than those of the PEEC model, while the remaining magnetic field retention rate is marginally lower. However, the overall trends of temperature and magnetic field variations are entirely consistent. In terms of calculation speed, the Moderate model is nearly 12 times faster than the PEEC model. For large - scale engineering coils, the PEEC model is more complex and exerts extremely high demands on computing resources. Employing the Moderate model to predict the engineering thermal quench upper limit can significantly conserve computing resources and time in practical use.
Consequently, this paper advocates that the Moderate equivalent circuit model be coupled with the finite - element simulation model, and the entire - unit quench be utilized to approximate the point - quench within the unit for simulating and deriving the thermal quench upper limit of large - scale engineering coils. Based on this upper limit, the thermal and mechanical structures can be designed to ensure the thermal stability of large - scale engineering coils.
