HTS Magnets: Modelling, Charging and Large Scale Applications
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The advent of High Temperature Superconductors (HTS) has resulted in increased research into potential industrial applications that best exploit the superconducting behaviour at relatively high cryogenic temperatures. In the superconducting state, HTS have the ability to transmit power with negligible ohmic losses. This lossless transmission presents HTS as an ideal conductor choice in a myriad of engineering application areas such as high-field magnets for medical imaging and nuclear fusion. Despite the promise, the low industrial maturity of HTS has limited their adoption. In addition, cooling them to cryogenic levels requires careful design considerations. This thesis addresses these issues by exploring the use of HTS in charging devices (flux pumps) that can power up a magnet as well as the ensuing applications of large-scale magnets for medical imaging and smart grid applications.
Chapters 1 and 2 present the basics of superconductivity, HTS flux pumps and magnets. Chapter 3 looks at a key component of HTS transformer-rectifier flux pumps: the HTS switches. These are used as an electromotive force (emf) source that cycle between an off-state with an average voltage and an on-state with zero resistance. The commutation between the on- and off-state is achieved by applying an AC magnetic field to a DC-current carrying HTS tape. An FEM model is developed to present design considerations of these switches to improve operating performance. The findings are verified using an experimental study, followed by an analysis into HTS switches made with artificially pinned HTS tapes.
Chapter 4 builds on Chapter 3 and on previous SPICE modelling work of transformer-rectifier flux pumps to develop a novel MATLAB/Simulink model. The model is verified using the SPICE benchmark and an experimental analysis. The model is then used to investigate new HTS flux pump topologies such as the full-wave (4-switch and centre tapped), 2-transformer, and 2-load device, with the arrangements compared using metrics such as switching efficiency, relative pumping speed, pumping capability and the number of switches required per load. The switch enhancements studied in Chapter 3 are studied with the MATLAB model and compared with an experimental analysis. Moreover, a means of regulating the current using the modulation of the switching duty cycle is presented.
Chapter 5 studies an industrial use-case of HTS flux pumps: the operation of HTS magnets for Magnetic Resonance Imaging (MRI.) An existing baseline design for an optimised magnet that provides a highly uniform field using the genetic algorithm is replicated. This model is subsequently used to perform a sensitivity analysis to help guide the design process and bridge the gap between design and construction. The operation of the coil is designed using a detailed thermal analysis that considers both the transient and steady-state cooling phase. A load-line analysis is also presented to determine operating points (current and field) of the coil. The chapter concludes with an active shimming approach to improve field homogeneity, with a predicted homogeneity of ∼ 0.304ppm over a 10cm DSV obtained – an improvement over previously reported values.
Chapter 6 looks at another upcoming application of HTS magnets – high-power energy storage devices in smart grids. This is studied in detail by proposing a novel 2-stage allocation and energy management algorithm that can run a combination of HTS and other energy storage devices in smart grids with stochastically uncertain load and generation profiles. The management algorithm is tested using a time-sequential Monte Carlo analysis that not only shows the optimality of the proposed algorithm but also demonstrates the placement of HTS energy storage. Despite the low capacity factor and high capital cost, HTS energy storage is key in enhancing overall system reliability and expected displacement of carbon generation, evaluated using capacity credits.