AC Loss Research on Conductor on Round Core (CORC) Cables and Multifilamentary Magnesium Diboride (MgB2) Wires for All-superconducting Rotating Machines

<p><strong>All-superconducting rotating machines, which utilize superconducting materials for both field and armature windings, offer significant advantages of high-power density, lightweight, and high efficiency, making them promising candidates for future all-electric aviation applications. AC loss, as a main heat source that greatly influences its performance and efficiency, is one major challenge hindering its practical applications. Under this condition, selecting appropriate superconducting materials to minimize AC losses is one of the critical issues. Conductor on round core (CORC) cables assembled with multiple HTS coated conductors, are one promising candidate for the field windings, due to their excellent current-carrying capacity and robust mechanical strength. Low-cost, round magnesium diboride (MgB2) wires are one promising material for the armature windings considering their multifilamentary structure, fine filament size and tight twist. However, AC loss behaviors of both candidates operated at realistic conditions have not been fully explored and remain unclear for its real applications.</strong></p><p>This thesis aims to conduct numerical research in AC loss of these two candidates for their applications in aviation when operated in realistic conditions. In order to obtain a systematic understanding of AC loss behaviors and provide valuable references for the design of practical HTS applications, the following research questions are addressed: how the AC loss behaves in the CORC cables when transporting DC current and exposed to AC magnetic field; what the analytical equations for AC loss are for the spirally wound CORC cables; how the AC loss changes when the operation temperatures and the number of tapes and layers of CORC cables vary; how the AC loss behaves in the multifilamentary MgB2 wires when carrying AC current under AC field; and how the variation of operational conditions impact AC loss, such as the frequency, the twist pitch, the resistivity of matrix and the filament size?</p><p>Firstly, three-dimensional (3D) simulations of a spiral tape and two HTS CORC cables (one-layer Cable A and two-layer Cable B) carrying DC current exposed to an AC field were carried out to explore their AC loss characteristics, with the external AC magnetic field amplitudes up to 500 mT. The experimental Ic(B, T, ��) and n(B, T, ��) of a HTS coated conductor from SuperOx are interpolated into models and the operation temperatures are at 77.5 K, 50 K and 30 K, respectively. The generated AC loss, termed as total loss, comprises magnetization loss due to the shielding currents induced by the applied AC magnetic field and dynamic loss from dynamic resistance due to the interaction of the DC current and AC magnetic field. The simulation results show that the total loss of all models above 100 mT increases as temperature decreases from 77.5 K to 30 K when the current-carrying level is less than 30% of their respective critical currents. When the current-carrying level is more than 50% and the field amplitudes are greater than 100 mT, the onset of flux-flow loss at 77.5 K and 50 K leads to a surge in total loss. This is because the transport current exceeds their respective field-dependent critical currents. When compared to those observed in the spiral tape and one-layer Cable A, the total loss of the two-layer cable B becomes lower due to shielding by the second layer. Through simulations, we also uncovered that the dynamic resistance of the spiral tape and each tape of Cable A can be analytically predicted by considering the geometric coefficient 2/π, whereas this finding is not suitable for Cable B due to the shielding effect. Then, the dependence of transport critical current Ic and associated power-law index (n-values) on magnetic field and temperatures of four non-magnetic, multifilamentary MgB2 wires, Ic(B, T) and n(B, T), were measured over a wide range of temperatures (15 – 35 K) and fields (0 – 7 T). Three twisted 54-filament MgB2 wires are manufactured by Hyper Tech Research, USA, with diameters 0.48 mm, 0.48 mm and 0.70 mm respectively, one non-twisted 18-filament MgB2 wire with diameter 0.83 mm is from Sam Dong, South Korea. In order to reduce the heating issues at the current contacts, a newly devised sample mounting method is used for the transport Ic measurements. Among these four wires, only one Hyper Tech wire with a 10 mm twist pitch and 0.48 mm diameter has been fully tested without temperature rise at all temperatures down to 15 K. Thus, the full dataset of this wire is used in AC loss simulation of MgB2 wires later. For other three wires, the sudden temperature rise at low fields and low temperatures results in missing Ic(B, T) data, we thus established an empirical expression to estimate the magnetic field- and temperature-dependent critical current density. This is crucial for aviation applications employing MgB2 wires, as the target operating temperature is 20 K, which is achieved using liquid hydrogen – considered as an efficient coolant. Additionally, all wires have negligible difference in in-field transport Ic when exposed to increasing or decreasing magnetic fields, which is critical for lowering AC loss because the additional losses from magnetic sheaths can be eliminated. Next, 3D simulations of magnetization loss of twisted 2-filament and 54-filament wires with a non-magnetic matrix at 20 K were presented to analyse the loss behaviours and estimate the loss values under realistic conditions, with AC field amplitudes between 0.1 T and 2 T and frequencies ranging from 50 Hz to 200 Hz. The measured Jc(B, 20 K) and n(B, 20 K) are used in simulations. Starting from a simple 2-filament wire, the operational frequency, the twist pitch, the filament size, the matrix resistivity, and inter-filament gap have been varied to systematically study their impacts on magnetization loss and its loss components (hysteresis loss, coupling loss and eddy currents). The simulation results show that the 2-filament wire with a 5 mm twist pitch and a higher resistivity matrix operated at 50 Hz has the lowest magnetization loss through decoupling the filaments. In this context, for the 54-filament MgB2 wire, only the magnetization loss of a 5 mm twist pitch and a higher resistivity matrix wire operated at 50 Hz is obtained in order to estimate the potential minimum loss values. Simulation shows that the hysteresis loss of the 54-filament wire can be well predicted by the analytical hysteresis loss equation for a cylindrical superconductor multiplied by 54 (the number of filaments) when Ba ≤ 0.5 T. The deviation at high-B is attributed to the field-dependent Jc(B) in our simulation while the analytical curve assumes a constant Jc0. Good agreement is also observed between the simulated coupling loss and the analytical coupling loss equation from Wilson for a circular-arranged multifilamentary superconducting wire. Finally, 3D total AC loss simulations on a non-magnetic 54-filament MgB2 wire carrying AC current and exposed to AC field operated at 20 K, targeting for the real armature winding applications, were presented, where the current-carrying level, the twist pitch and the operational frequency were varied. The transport loss without field and magnetization loss without current were also investigated to provide comparisons for the total AC loss. Simulation results show that the total AC loss values in the wires with different twist pitches agree well with the sum of magnetization loss without current and transport loss without field for all different current levels when Ba ≤ 1 T. This suggested that total AC loss in an MgB2 wire carrying an AC current exposed to an AC magnetic field can be accurately predicted by knowing magnetization loss without current and transport loss without field values that are more easily obtained at low-B. At 2 T, the simulated total AC loss experiences a surge this may be due to the quench effect. In addition, with the increase in twist pitch and frequency, a significant increase of total AC loss at high fields is observed. Simulation results also estimate the achievable total AC loss values of a 5 mm twist pitch wire, operated at 200 Hz, are below 4.46 W/cm3 under i = 0.3 and Ba ≤ 0.5 T, where Qm0 is dominant and Qt0 makes a minor contribution. This work presents a pioneering study of the AC loss analysis on the HTS CORC cables and multifilamentary MgB2 wires, operated under realistic conditions for all-superconducting rotating machines in all-electric aircraft. For CORC cables carrying DC current exposed to AC field, the influence of operational temperature, the structure of CORC cables on total loss and dynamic resistance, as revealed in this thesis, contributes to the design of field windings in all-superconducting rotating machines. For multifilamentary MgB2 wires carrying AC current and subject to AC field, the quantitative estimation of AC loss at 20 K, considering the variations in twist pitch, operation frequency, the resistivity of matrix, also provide valuable insights for the real-world applications as well as in the manufacturing of MgB2 wires.</p>