Control of a Variable Speed Generator to Optimise Output from a Heaving Wave Energy Device

This study investigates the power generation system and control of an isolated wave device in which heave oscillation of a float drives a flywheel which in turn rotates the shaft of an induction generator. This represents a single unit within a Manchester Bobber wave power device; a concept which comprises a closely spaced array of heaving floats. The hydrodynamics have been tested at several scales, 1:100th and 1:70th scale in university test facilities and at 1:10th scale in a large outdoor test tank to evaluate a non-linear model of device response. However, limited research has been undertaken on the power generation system or on control in order to maximise energy extraction from irregular wave-fields. In particular it is important to understand how the torque-speed curve of the generator influences performance and loading. To this end, a model has been developed using EMTDC (ElectroMagnetic Transient including DC) to simulate the coupled electro-mechanical system of a single drive-train when subject to irregular wave-forcing. This is a development of a model of the float hydrodynamics and mechanical system (by Stansby et al.) in which the generator was modelled as a constant torque machine. Sensitivity of device performance to the induction generator torque speed curve is investigated. It is shown that considerable increase of power capture can be obtained by appropriate selection of the generator characteristics but that this is sometimes offset by increased mechanical loads. Specifically, high rates of change of torque may occur. However, these may be reduced by using a proportional-integral controller whilst maintaining similar performance. A brief study of annual output is conducted for a range of sites to assess the variation of performance with deployment location.