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Wind energy, as one of the most promising renewable energy sources, has been well developed in the past 30 years. However, the improvement in reliability and availability of wind turbine systems is still an important topic to both academia and industry due to the high operation and maintenance cost. As two of the key components of a wind turbine system, the wind generator and gearbox, have the longest downtime although they are less prone to failure than the power converters and their associated control units. Therefore, it is necessary to study the faults of wind generators and try to reduce their downtime by detecting their faults at an early stage and then implementing effective maintenance. Among all the faults of these wind generators, the winding fault, as the second most frequent fault, has attracted significant interest. It has been reported by the Electrical Apparatus Service Association (EASA) that there are five major winding faults, namely (1) inter-turn (turn-to-turn) short circuit (ITSC), (2) coil-to-coil short circuit, (3) open circuit of one phase, (4) phase-to-phase short circuit, and (5) coil-to-ground short circuit. Amongst all the faults, the ITSC fault, often regarded as one of the root causes of other winding faults, has attracted increasing attention from researchers over the last few years. This thesis will mainly focus on modelling and analysis of permanent magnet machines under ITSC fault for wind power application.

Offshore wind power has huge potential to be one of the main renewable energy sources. Larger wind turbines can significantly reduce the overall cost of offshore wind farms, making wind energy a competitive source of clean energy. However, because of the harsh environment, reliability and maintenance have been major challenges for the offshore wind market. The gearbox is considered to be one of the weakest links in the whole drive train system, creating serious reliability issues. A direct-drive system enables the complete elimination of the gearbox by directly coupling the generator with the shaft of turbine blades. Therefore, in recent years, high power direct-drive wind generators have become very popular especially for offshore wind applications. However, high power at low speed equates to high torque and hence increases the volume and mass of the direct-drive generators. It is broadly recognized that permanent magnet (PM) synchronous machines are the most attractive candidate for direct drive applications due to their high power density, efficiency and easy scalability. Although the high cost of permanent magnet machines has been a concern, these machines can still be competitive when the long-term costs including maintenance, efficiency and design simplifications are considered. In an attempt to further improve the torque density of the permanent magnet machines, Vernier machines have emerged as a potential candidate. Vernier machines have high torque density, simple structure and an inherently low torque ripple which are desirable features for direct-drive applications. However, the high torque density of Vernier machines comes as a trade-off with their poor power factors which result in increased power converter rating and cost. Hence it would be valuable to investigate the impact of the high torque density and the poor power factor features of Vernier machines on the overall system-level performance of the multi-MW direct-drive power train system. The objective of this research work is threefold. Firstly, to investigate the suitability of the conventional surface-mounted permanent magnet Vernier (SPM-V) machines for off-shore wind power application by comparing the overall system-level performance (including generator and converter) with the existing conventional SPM machines. This will include the scaling study of Vernier machines ranging from a few kWs to multi-MW power levels using analytical models, 2D FEA and experimental validations. Secondly, to propose a novel Vernier machine topology suitable for off-shore wind power applications that can improve the torque density compared to the conventional SPM-V machines. Finally, the proposed Vernier machine will be compared with the conventional SPM and SPM-V machines for the Levelized Cost of Energy (LCOE) considering the annual energy production, capital expenditure and operation cost for the lifetime of the wind turbine at a 10MW power level.

This thesis investigates some novel current harmonic injection methods to improve the electromagnetic performance of doubly salient synchronous reluctance machines (DS-SRMs). These machines will have different winding configurations, slot/pole number combinations and phase numbers. The theoretical analyses (both static and dynamic) are carried out based on Fourier Series analysis, and validated by 2-dimensional finite element method and also experiments using several prototype machines. Based on the analytical torque model in abc-axis frame, a powerful insight into the mechanism of torque generation of the DS-SRMs with pure sinewave current supply can be achieved. The electromagnetic torque (both magnitude and phase angle) produced by each order of inductance harmonic can be predicted, which allows us to obtain the dominant torque ripple components for such machines. Therefore, the appropriate current harmonic (3rd, 5th and 7th) can be injected to generate torque ripple components in order to compensate that produced by the fundamental current, and hence to achieve an overall reduced torque ripple. On the other hand, the average torque of the DS-SRMs can also be improved by properly selecting the current harmonics in terms of harmonic order, amplitude and phase angle. However, it is found that the current harmonics, although can improve torque performance, will often cause extra losses (both copper and iron losses) and undesirable distortion in the phase voltages, which could lead to negative impact on the machine efficiency and dynamic performance. Therefore, in order to fully evaluate the potential of the proposed harmonic current injection method, comprehensive studies about losses, efficiency and dynamic performances such as torque-speed curves of 3-phase and multi-phase DS-SRMs have been carried out. In order to simplify the investigation of dynamic performance analyses such as the torque speed curves and efficiency maps, novel analytical torque model in dq0-axis frame has also been proposed. The findings in this thesis can provide some useful guidelines for torque performance improvement of DS-SRMs using harmonic current injections.

This thesis describes an investigation of high frequency permanent magnet generators for use in a novel power generation system for aerospace applications. The system consists of a high frequency generator (in the 10s of kHz range) which feeds a full-wave rectifier to produce to a high frequency pulse train as input to a pulse-density modulated soft-switched converter. Various topologies of flux-switching, flux-reversal and Vernier machines are investigated using electric-circuit coupled finite element analysis. Having demonstrated the limitations of these topologies, a comprehensive design study into a single-phase, surface mounted permanent magnet machine based on a single turn serpentine winding is described. This study covers both internal and external rotor machines with pole numbers of 192 and 96 which correspond to generator fundamental frequencies of 32kHz and 16kHz at the rated speed of 20,000rpm. Several aspects of the machine design are optimised through extensive use of finite element modelling, including mechanical analysis of the rotor containment. This study includes a detailed consideration of iron loss, including consideration of iron powder based cores. This study has resulted in a down-selected design based on a low permeability but high resistivity powdered iron core. The manufacture of a demonstrator is described including the need to re-design the machine to employ ultra-thin Nickel Iron laminations because of the difficulties encountered in the machining of a powdered iron core. The performance of this Nickel Iron variant is investigated and a final design established. The numerous challenges involved in manufacturing this novel machine are described.