Challenges and Opportunities of Permanent Magnet Vernier Machines for Offshore 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.