Research On The Key Technologies Of Oscillating Buoy Wave Power Generation System

Growing concern over the exploitation of renewable resources is developed since the first oil crisis in 1973.Due to its advantages,high energy flux density,seldom affected by time limit,availability to implement power generation and supply with smaller volume,wave energy has drawn more and more interest from scholars and researchers all over the world.A permanent magnet linear synchronous generator(PMLSG)is adopted as power takeoff equipment by the studied oscillating buoy wave power generation system in this dissertation.The translator is driven by the connected heaving buoy and cutting magnetic lines of force,which leads to the power generation.The raw power energy is processed by the power converters before being supplied to load or connected to the grid.The key technologies researched in this dissertation include model building for the wave energy converter in time-and frequency-domain,optimal power extraction,power conversion and synchoronization of grid-connected power converters.First,the six-order state space model of the wave energy converter of the oscillating buoy wave energy converter is built based on the combination of the time domain and frequency domain model equations.Despite the simpleness of the frequency domain model,it does not take the nonlinear factors into consideration.The time domain model is appropriate because of its including nonlinear factors and close to the actual equipment.However,it needs to calculate the time-domain Green functions for every grid of the wet buoy surface at every moment.The workload is huge and impractical.The random waves are regarded as the linear superposition of a series of periodic functions by the impulse response method employed in this dissertation.The movement of the buoy can be considered as a comprehensive motion induced by a series of impluses.The frequency-domain model is established at the beginning and the frequency response curves of hydrodynamic parameters are obtained by the use of WAMIT software.The convolution part of the time-domain equation is replaced by a state space model,whose parameters are estimated by the curve fitting method based on the frequency responses of the hydrodynamic parameters.Eventually,the six-order state space model of the wave energy converter is acquired.In order to improve the energy extraction of wave power generation system and avoid damages caused by the uncontrolled free motions of the buoy,studies on the optimal power capture are necessary.The development of optimal wave power extraction in the last decades has been investigated and categorized in this dissertation.The working principles,parameters selection and simulation implements of these strategies are discussed.Based on the aforementioned achieved mathematical model,simulations under regular waves of different heights and periods using different methods are conducted and comparative diagrams of the absorbed average power and peak-to average ratio for absorbed power are achieved.The reasons for the results diffenrence are analyzed in detail.Conditioned optimal average power algorithom(COAPA)is proposed to solve that current algorithms are usually designed to improve the absorbed average power and blind to the correspondent huge cost ascribe to the high power generator and power converters.The absorbed average power can be optimized with the instantaneous power within the power limit by use of the presented strategy.In view of the good performance of approximate complex-conjugate control(ACC)algorithm under the small amplitude wave condition,a novel approach,optimal power selection method,is proposed.The control tactics is chosen among ACC algorithm,COAPA method and passive loading control approach according to the the presented process under the irregular wave condition and the absorbed average power is further optimized.Direct thrust control of PMLSG and direct power control of grid-connected converter strategies are adopted to implement power conversion.Because of the hysteresis modulations,the rapidity and robustness of the system is achieved.Besides the AC/DC conversion,the rectification part should also guarantee that the generator thrust follows the change of the reference computed from the optimal power extraction part.The direct thrust control system for PMLSG is designed to accomplish the task.The velocity of the generator is the same as that of the buoy.The reference thrust is yielded from the optimal power extraction process.After the utilization of isd=0 control scheme,the flux linkage reference is calculated.For the grid-connected voltage source inverter,the direct power control system is developed in this dissertation.According to the sector separation differing from the system adopting the conventional swithing table,two new swithing tables are proposed.The results of simulations and experiments show the fact that the systems employing the presented switching tables have better performance for both the steady state and dynamic response.Grid-connected power converters are very sensitive to the voltage disturbances.It is necessary for the synchronization between the power converters and the grid to secure the reliability of converters under unbalanced and distorted grid condition.A novel phase-locked loop(PLL)algorithm composed of multiple second-order generalized integrators(SOGI)is proposed.The new PLL,called MSOGI-PLL for abbreviation,has four SOGIs,namely the fundamental,the fifth,the seventh and the eleventh harmonic components.A frequency-locked loop is designed to implement frequency adaptivity.The harmonic decoupling network is introduced to separate the fundamental,the fifth,the seventh and the eleventh harmonic part from the original input signals.The phase angle of the positive sequence components for the extracted fundamental voltage is calculated and used as the synchronization signal.Simulations and experiments are carried out under different fault grid conditions.The results demonstrate that the proposed algorithm performs excellently in identifying the phase of the fundamental-frequency positive-and negative-components of the grid voltage accurately and rapidly,especially in the severely polluted grid case.