Abstract
Non-insulation high-temperature superconducting coils provide a much lower risk of burnout in fault/abnormal conditions, such as hot-spot quench and overcurrent. This study employs an equivalent circuit grid model, coupled with magnetic field calculation and the E–J power law of superconductors, to deeply and systematically investigate the overcurrent charging process in a double-pancake non-insulation coil. An evident saturation of the magnetic field in the axial direction of the coil was observed and verified by experiments. Experimentally, the entire process, including the behavior of the magnetic field, was consistent with the numerical results. Based on the verified model, two main points were addressed: (1) Transient current distribution inside the coil during overcurrent charging was studied. Potential quenching risks were found to be at the innermost and outermost turn near the electrodes, as well as the pancake-to-pancake connection part. (2) Magnetic field saturation, which is a unique phenomenon in non-insulation superconducting coils during overcurrent charging, was studied in detail and first quantitatively defined by a new concept “converged load factor”. Its relationship with turn-to-turn resistivity was revealed.
Highlights
One of the advantages of non-insulation (NI) high-temperature superconducting (HTS) coils is the capability to operate under the fault conditions with a much lower risk of burnout [1]
Several studies have focused on the performance of NI–HTS coils operating under overcurrent conditions
The partial element equivalent circuit (PEEC) [15] or equivalent circuit grid (ECG) [16], coupled with a thermal model, which was proposed to numerically analyze the behaviors of NI coils during overcurrent conditions, suggested that a considerable amount of Joule heat is generated near the outer electrode, initiating the quench propagation and ascribing the magnetic field saturation to a local decrease in critical current [17]
Summary
One of the advantages of non-insulation (NI) high-temperature superconducting (HTS) coils is the capability to operate under the fault (typically overcurrent and hot-spot quench) conditions with a much lower risk of burnout [1]. The partial element equivalent circuit (PEEC) [15] or equivalent circuit grid (ECG) [16], coupled with a thermal model, which was proposed to numerically analyze the behaviors of NI coils during overcurrent conditions, suggested that a considerable amount of Joule heat is generated near the outer electrode, initiating the quench propagation and ascribing the magnetic field saturation to a local decrease in critical current [17]. For the aforementioned ECG model [16], A1 is always a non-singular square matrix, and unlike the previously proposed method [16], the radial current vector J is selected as the state variable, and the spiral current vector I can be derived, as shown by Equation (5). The geometry of the coil in profiles of current distribution [24,25] in the radial direction was enlarged for better illustration
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