Derong Qiu

Experimental and Numerical Study of Quench Characteristics of Nonuniform REBCO-Coated Conductors

This study numerically and experimentally analyzed the quench propagation characteristics of nonuniform REBa2 Cu3O7-δ (REBCO, RE = rare earth) coated conductors (CCs) at the self-field and under a nearly adiabatic conditions. Four nonuniform REBCO CCs cases were studied: a copper-laminated sample with a point defect, a bare sample with a point defect, a copper-laminated sample with a regional defect, and a bare sample with a regional defect. The finite element method was used with the numerical computational models for these four nonuniform cases. The temperature and voltage profiles obtained from the numerical models were compared with experimental data. The quench parameters, including the peak temperatures in the defect region and normal zone propagation velocity, were obtained at various operation currents (from 36% Ic to 112% Ic). Of the four cases, the bare sample with the regional defect had the highest peak temperature at the same time and percentage of critical current (Ic). The results will be useful for the quench detection and protection design of REBCO CCs for use in applications.

Coil Voltage Pulse at the Beginning of the Fast Discharge Operation of No-Insulation REBCO Coils

This paper presents a study on the coil voltage of the no-insulation (NI) high-temperature superconductor (HTS) pancake coils during fast discharging operations. Fast discharging tests are performed on both a double pancake (DP) and an single pancake (SP) NI coil. The measured coil voltages are compared with that from an equivalent circuit model, which has been widely used in previous studies. A voltage pulse is observed on both the NI coils at the beginning of the fast-discharging operation. It is much higher than that from the modeling. The peak value of the voltage pulse increases linearly with the initial current. Comparison tests show that the coupling iron core has little influence on this coil voltage pulse though the inductance of the coils is enlarged seven times by coupling iron core. This coil voltage pulse shows that, during the fast discharging operation, the NI coil has a much higher turn-to-turn resistivity at the beginning stage than that in following process. Since the coil voltage pulse can be 600 times of the modeling estimation, it may be a potential risk to the coil protection system and measuring instruments. Therefore, special attentions are required on this aspect in the design of NI magnets.

Feasibility Study of Closed-Loop No-Insulation Coil to Shield Alternating Magnetic Field

No-insulation (NI) coil made of 2G high-temperature superconductor tape is widely used in a dc magnet due to its excellent performance of engineering current density, thermal stability, and mechanical strength. While the charging/discharging delay of the NI coil has become the biggest obstacle to applying it into an ac power device. In this paper, as a preliminary study, we test the capability of the NI coil to shield alternating magnetic field (AMF). When shielding AMF, the NI coil works in inductive mode such that each turn in the NI coil is induced at the same time and there might no longer exist any delay caused by turn-to-turn current path. To verify this hypothesis, three closed-loop coils, two NI coils, and an insulated (INS) coil are fabricated. The result of the contrast experiments shows that the NI coil could shield all the alternating magnetic flux through it, independent of turn-to-turn resistance and frequency of the background AMF. This capability is promising for the NI coil to be applied in the inductive superconducting fault current limiter (iSFCL) as the secondary winding, which can shield all the magnetic flux generated by the primary winding to keep the impedance of the iSFCL at a very low level during normal the operation of the power system.

Experiment and Numerical Analysis on Magnetic Field Stability of Persistent Current Mode Coil Made of HTS-Coated Conductors

High-temperature superconducting (HTS) coils made of coated conductors that can operate in persistent current mode (PCM) are regarded to be promising in MRI/nuclear magnetic resonance and Maglev system. The temporal stability of the magnetic field trapped by the PCM coil is a key issue, which significantly determines the imaging quality of MRI and dynamic behavior of Maglev. This study focuses on the temporal stability of the trapped magnetic field in the double-slit HTS PCM coil, which is magnetized by a field cooling (FC) method at 77 K. The magnetic field decay behavior under different initial fields and FC ramp rates are systematically studied. The experiment results indicate that higher initially trapped field will lead to faster decay, but if the initially trapped field is lower than a certain value, this trend is no longer obvious. The FC ramp rate has a little impact on the field decay. Finally, a numerical model based on R–L circuit and E–J equation is established to fit the decay process. It is found that if the operating current is lower than 60% of the coil critical current, good temporal stability could be achieved. Another long-term experiment is performed that a stability of 8.1 ppm/h is achieved after 19 days’ decay.