Reaction force control implementation of a linear generator in irregular waves for a wave power system
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Date
29/11/2012Author
Li, Bin
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Abstract
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.