A simple protection evaluation method for no-insulation REBCO pancake coils during local normal-state transition

Abstract

No-insulation (NI) and metal-insulation (MI) winding techniques are promising for enhancing the thermal stability of REBCO pancake coils, toward the practical use of nuclear magnetic resonance and magnetic resonance imaging systems. The NI and MI REBCO pancake coils showed high thermal stability in many over-current tests. However, a disadvantage is a charging delay due to low turn-to-turn contact resistance. Therefore, many researchers have tried to increase turn-to-turn contact resistance to overcome the charging delay problem. Meanwhile, a partial element equivalent circuit (PEEC) model can accurately simulate the over-current and charging/discharging tests of NI REBCO pancake coils in detail. Using the PEEC model coupling with thermal finite element analysis (FEA), the thermal stability was evaluated during a local normal-state transition. From the simulation results, it was found that a best range of turn-to-turn contact resistance existed to keep a high thermal stability during a local normal-state transition. However, the PEEC and thermal FEA method requires a substantial amount of time with complicated simulation approach. In this paper, we propose a simple mathematical formulation to obtain an appropriate range of turn-to-turn contact resistance during a local normal-state transition, supposing a worst cooling condition. The simple computation is easy to use and timesaving to roughly estimate the operation reliability of NI REBCO pancake coils.

Turn-to-Turn Contact Resistance Measurement of No-Insulation REBCO Pancake Coil: External Field Dependence

“…The high contact resistance yields a short charging delay; however, too high contact resistance loses the stability. The influence of the turn-to-turn contact resistance on the thermal stability of REBCO pancake coils were discussed in [8], [9].…”

Database of the effect of stabilizer on the resistivity and thermal conductivity of 20 different commercial REBCO tapes

“…This could be too high for self-protection. According to the simple model from [26], the optimized turn-to-turn contact resistance should be at least one or two orders of magnitude lower than the SS co-winding value. In addition, unlike the charging decay and loss, the self-protection ability is not only dominated by the turn-to-turn contact resistance but is also closely related to the thermal conductivity of the REBCO tapes in both the circumferential and radial directions.…”

Effect of the SS304 coating layer on the electrical and thermal properties of REBCO commercial tape

“…Although co-winding with SS tape is considered to the best strategy for solving the charging delay problem in the NI process, the SS tape is too thick (30 µm) for commercial REBCO tape (90-100 µm), and significantly decreases the engineering critical current density (Je). However, simulations [32] have shown that the turn-to-turn electrical resistivity of cowound SS tape could be too high to hinder the self-protection ability.…”