| Wind power has become an important way to solve the world’s energy shortage.Many countries pay more and more attentions to wind power generation. In the twomainstream wind turbine generation systems，the direct-driven permanent magnetsynchronous generator(PMSG) system has features of simpler structure, highergeneration efficiency and good operation reliability. The direct-driven PMSG systemhas become one primary technology direction of modern large variable-speedconstant-frequency(VSCF) wind turbine generation systems. Therefore, several keytechnologies of grid-connected converter for direct-driven PMSG system were deeplystudied in this dissertation. The main achievements of the research include：①From the point of view of system, mathematical model of the direct-drivenPMSG system is established. Several typical algorithms of maximum power pointtracking(MPPT) are compared. Considering the power-speed characteristics, a MPPTcontrol strategy based on the power signal feedback control is proposed in thisdissertation. The simulation model of a2MW direct-driven PMSG system is established.The rationality of the model and the validity of the proposed MPPT control strategy areverified through simulation studies.②Based on detailed analysis of the fluctuation mechanism for the dc-link voltageof full rated grid-connected converter, the coordinated control strategy of grid sideconverter and generator side converter is proposed to stabilize the dc-link voltage undernormal grid voltage conditions. The generator’s output active power information isintegrated into the control of grid side converter. According to the change of operationstate of generator, the active component of grid current is adjusted immediately, andtherefore, the active power delivered to the grid can timely track the generator’s outputactive power. Simulation and experimental results confirm the validity and effectivenessof the proposed control strategy.③In order to optimize the direct-driven PMSG system’s output power above therated wind speed, large inertia of this wind generator system, combined with generatorpower control and pitch control, are used to suppress the power fluctuation. When windspeed is under the rating value, rotating speed feedback control was adopted to achievethe maximum power point tracking. A dual-mode power control strategy is proposed forall the wind conditions based on the control modes mentioned above. Simulation results demonstrate that the output active power of the direct-driven PMSG system adopted theproposed dual-mode power control strategy is steadier compared to traditional controlscheme.④The influence of parameter errors on vector control system is analyzed. Asimple and effective online parameter identification algorithm is proposed to suitablefor engineering application. The algorithm based on compensating voltages-the currentPI regulator outputs, estimates the parameters to be identified quickly and accurately.Simulation and experimental results demonstrate that the current PI regulator outputsbecome to zero after the system is stable. Burden of current PI regulators are greatlyreduced that it can improve the control performance of the PMSG system.⑤A mathematical model of the grid side converter is established underunbalanced grid voltage conditions. Based on the model, the influence ofgrid-connected impedance on control strategy of grid side converter is analyzed. Anenhanced dc-link voltage control strategy is proposed to suppress the double supplyfrequency fluctuations in dc-link voltage. The calculation of current references for gridconverter is relatively simple and a solution to complicated matrix is avoided.Simulation results demonstrate that the double supply frequency fluctuations in thedc-link voltage can be effectively suppressed. The fluctuation amplitude of active powerand the current harmonics injected into the grid are also reduced by adopted theproposed strategy.⑥Taking unbalanced grid faults as research focus, an improved control strategyfor grid side converter with dc-link voltage soft control is proposed. The improvedcontrol strategy considering grid-connected impedance and grid current restrictions,takes no double supply frequency fluctuations in the active power delivered to the gridas the object. Based on this, a fault ride through control strategy is proposed for thedirect-driven PMSG system. In the grid fault process, the generator’s output activepower is reduced in proportion to retained grid voltage ensuring the roughly balance onboth ends of grid-connected converter. The proposed fault ride through control strategyis applicable to different grid fault types. A certain amount of reactive power can beinjected into the grid to support the power system. Simulation results are used tovalidate the proposed control strategy and demonstrate that fault ride through operationcapability of the direct-driven PMSG system is significantly enhanced. |
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