Study on current discrepancy and redistribution of HTS non-insulation closed-loop coils during charging/discharging and subsequent transient process toward steady-state operation

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Owing to the presence of joint resistance and flux creep, high-temperature superconducting (HTS) coils without a power supply inevitably suffer from current decay. A flux pump is a voltage supply that requires connections with smaller footprints and a lower heat load than traditional current leads. In this study, we explain the principle of the upper limit for the output current of the traveling wave flux pumps. Based on this principle, a miniaturized linear flux pump device was developed. With narrow and misaligned iron teeth, elaborate 3D geometry of the iron pieces, and optimized driving current waveform, the miniaturized flux pump can support more than 120 A output current with only a 10 mm wide HTS tape and a compact size of 4.6 × 4.6 × 3.4 cm. Our experimental results show that the critical current of the HTS tape has a significant effect on the flux pump output. An HTS tape with a larger critical current supports a higher maximum transport current, whereas an HTS tape with a smaller critical current requires less applied current for positive output. Finally, excitation tests on HTS coils were performed. Charge/active discharge and field supplement experiments were done on a maglev HTS racetrack coil of 0.4 H, where charging/field supplement capbility of the miniaturized flux pump were demonstrated up to 46.8 A (close to the critical current of the coil). It has also been proved that the flux pump can work together with an external power supply with persistent current switch. The miniaturized flux pump can also independently charge an HTS coil of 60 μH to 91.6 A, which is the critical current of the coil at a low voltage criterion.

Section: Hts Coil Excitation Experimentsmentioning

confidence: 73%

Section: Hts Coil Excitation Experimentsmentioning

confidence: 99%

Section: Hts Coil Excitation Experimentsmentioning

confidence: 99%

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Miniaturized HTS linear flux pump with a charging capability of 120 A

Chen

et al. 2023

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“…The authors did not avoid the non-uniform ρ t2t for this NI coil sample even though this sample is well-wound with constant pre-stress and strongly compacted with the Kapton over-banding. In previous experiments, Lu et al have derived the non-uniform ρ t2t distribution that the innermost and outermost turns have the largest ρ t2t values through refined voltage measurements for different sections of their NI coil [61]. Wang et al have found that experimental results agree better with the simulation results setting larger ρ t2t value for the outermost turns [62].…”

Section: Simulation Analysismentioning

confidence: 99%

Time-variant magnetic field, voltage, and loss of no-insulation (NI) HTS magnet induced by dynamic resistance generation from external AC fields

Zhong

et al. 2023

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High-temperature superconducting (HTS) coils serving as DC magnets can be operated under non-negligible AC fields, like in synchronous machines of maglev trains and wind turbines. In these conditions, dynamic resistance generates in HTS tapes, causing redistribution/bypassing of the transport current inside the no-insulation (NI) coil and its unique operational features. This issue was studied by experiments on an NI coil with DC current supply put into external AC fields. Due to the current redistribution induced by dynamic resistance, the central magnetic field and voltage of the NI magnet initially undergo various transient processes, and eventually exhibit a stable central magnetic field reduction and a DC voltage. These time evolutions have implications for a time-varying torque and loss of an HTS machine. These time evolutions are strongly affected by the contact resistivity distribution, and whether it is the first time of the NI magnet being exposed to the AC field, showing several qualitatively different waveforms (e.g., some are even non-monotonic with time). The magnitudes of the stable central field reductions, and their observed linear correlation with the DC voltages are found to be decided by the local contact resistivity of the innermost and outermost several turns. It is also noted that the non-insulated turn-to-turn contact help lessening the loss induced by the dynamic resistance. A numerical model is established to analyse/explain those experimental results by observing the microscopic current distribution. Two risks of quench are noticed: (i) azimuthal current of the middle part turns increases as the AC field is applied; (ii) a concentration of radial current is observed near the terminals of the NI coil.

“…Additionally, the alternating background magnetic field also induces radial currents in the NI coils or similar cables, which leads to inter-turn losses [18,19], distinct dynamic resistance distribution from the insulated coils [20][21][22], and the redistribution of coil currents and spatial magnetic fields [23]. In closed-loop NI coils during the excitation process, non-uniform distribution of circumferential and radial currents and their continuous redistribution processes will be experienced after turning the persistent current switch to be superconducting and lead to a misunderstanding of the field decay rate [24]. In this condition, accurate prediction of the operating current at stable persistent current mode is significantly affected by TTRD.…”

Section: Introductionmentioning

confidence: 99%

A non-destructive method for detecting turn-to-turn resistivity distribution in NI REBCO coils

Wu,

Lu,

Zhong

et al. 2023

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A non-destructive method is proposed for detecting the turn-to-turn resistivity distribution (TTRD) of non-insulation (NI) coils made of REBCO tapes. In conventional designs, TTRD is often estimated to be a constant, while it is actually non-uniform. However, it is crucial to detect more accurate TTRD of NI coils as it determines the behaviour of NI coils and may lead to peculiar phenomena such as local reverse currents.The proposed approach involves acquiring the temporal change of voltage distribution during an excitation/demagnetization process, which is subsequently incorporated to a system of ordinary differential equations established derived from an equivalent circuit model. A genetic algorithm (GA) is then employed to fit the collected time-varying voltage data and generate the results of “measured” TTRD. The system of equations can actually be numerically solved. The solved time-varying TTRD results are averaged over the measuring period, which serve as the initial value of GA fitting, and accelerate the fitting process.Virtual measurements were performed on an artificially established mock coil, demonstrating high accuracy in reproducing the predetermined TTRD. Furthermore, an actual measurement was also conducted on a single-pancake coil, however with unknown TTRD, using 8 voltage measurement points during a demagnetization process. The measured TTRD was incorporated into the equivalent circuit model to predict the temporal changes in voltage and magnetic field of the coil under additional excitation/demagnetization conditions. By comparing the predicted results with the experimental data, a high level of agreement was observed, thus confirming the potential application of the proposed method.