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

Abstract

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.

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

“…The HTS coil was connected to a DC power source, and its central axial field was measured by a Gauss meter and Hall sensor. The test rig was operated in liquid nitrogen at 77 K. In our previous work [111], this coil has been put on the same test rig for a driven-mode operation under transverse AC fields up to 51 mT and 50 Hz; the measured output voltage with time is very stable without any rise observed (see figure 11 of [111]), indicating that the temperature rise of this coil is subtle, and the test rig has basically satisfied cooling efficiency. First, the HTS coil was excited by the DC power source and a thermal persistent-current switch for a PCM operation [10].…”

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

“…A portion of the current in the NI coils will be forced to flow into the inter-turn paths during unsteady conditions such as excitation, demagnetization and quench, resulting in delayed charging and discharging [13], complicated quality degradation in the magnetic field [14,15] and stress concentration problem [16,17]. 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].…”