Reaction force control implementation of a linear generator in irregular waves for a wave power system
Most designs for wave energy converters include a hydraulic (or pneumatic) interface between the wave device and the generator to smooth electricity production, but a direct drive power take-off system is a possible way of increasing the power transfer efficiency and the reliability, which was first adopted by Archimedes Wave Swing. Direct drive wave energy systems normally include a low speed linear generator directly coupled with the wave device. With no mechanical interface, the mechanical energy loss and maintenance requirements can, in theory, be significantly reduced. To maximize the energy capture, the motion of the wave energy converter must be controlled to achieve mechanical resonance so that the velocity is in phase with the incoming waves. So far, a number of control methods have been proposed, but few of them have been tested experimentally. For direct drive linear generators in real sea conditions, reaction force control is shown to be an effective way to achieve control where knowledge of future wave could not be required. Different reaction force control methodologies are suggested where the force is provided directly from the linear generator. Among these methodologies, complex conjugate control is regarded as the optimal control and can be used to achieve mechanical resonance. When resonance occurs, some system parameters such as the system excursion and required power take-off force become extremely large, and may exceed the design parameters. In this thesis, the system is modelled under reaction force control taking into account practical considerations which are based on design parameters. A novel control scheme for a direct drive linear generator to achieve such reaction force control in irregular waves is proposed, where a voltage-source rectifier is employed as the bridge between the linear generator and the dc bus. The application of linear generator in real wave conditions not only has inherent advantages, but also present a big challenge for controller design in order to obtain maximum power production. For a linear generator in real sea states, reaction force control idea can be implemented to adjust the velocity of motion, hence to maximize the power production, where the required currents in the generator coils to provide the desired force are constantly varying in frequency and amplitude. The control strategy of the active rectifier is developed based on the derived three-phase currents and the dynamic response of the system to determine varying modulation indices. The unknown situations and some unmeasurable parameters in the system degrade the performance of the control system, hence the current feedback and PI controller are both adopted to reject the effect of the disturbance. Simulation verifications are included for the proposed control idea.