Research On Control Methods Of Converter For Direct-drive Permanent Magnet Synchronous Wind Power Generation System

Direct-drive permanent magnet synchronous wind power generation has wideapplications and prospects for development. As one of the main technical methods towind power utilization, wind power network technology became the focus of allresearchers. Full-power converters are the unique part to feed the energy into powergrid, and their control performance and reliability play a crucial role in the system. Inorder to meet the improvement of interconnection technology and the increase ofconverter power level, the performance for control methods of full-power convertersare much higher. Full-power converter was selected as study object in this dissertation,and related technologies of sensorless control, harmonic compensation, circulationcurrent control and software phase-locked were researched deeply, which haveimportant academic significance and engineering value. The main works are asfollows：(1) The mathematical models of permanent magnet synchronous generator, PWMconverter and the DC bus were analyzed. The control strategies of machine-sideconverter and grid-side converter were described. A sensorless identification algrithmbased on optimal bounded ellipsoid was proposed, which can produce accurate ro torposition and speed signals. The simulation results show it has better identificationeffects when motor operation status and system parameter changed. On the simulatedplatform for dual-PWM direct-drive permanent magnet synchronous wind powergeneration system, experiments were done. And the results indicate the correctnessand feasibility of all kinds of control strategies.(2) Mathematical models for PWM converter of machine-side and grid-side wereconducted under two-phase static coordinate. Inorder to solve the problem thattraditional PI control algorithm cannot realize zero error adjustment of AC signal, PRcontroller was introduced to converter current regulation in direct-drive permanentmagnet synchronous wind power system. A harmonic compensation method based onPR controller was proposed. The compensation methods for machine-side andgrid-side were deeply discussed respectively. The model and simulated platform werebuilt, and the simulation and experiment results indicate that harmonic current can becompensated perfectly by using this method, thus the power quality can be enhanced.The harmonic compensation method is simple, convenient and can effectively reduce the number of coordinate transformations and alos it is easy to be engineeringimplemente.(3) Based on the establishment of mathematical model for converter underparallel topology, the analysis of circulation current in converter under direct andinterleaved parallel topology. A novel control method based on improved SVWPMmodulation algorithm was proposed by introducing the zero-sequence current closedloop, and the circulation current controller was designed. By using traditional andimproved SVPWM modulation algorithm respectively, the simulation of operationperformance for interleaved parallel converter was conducted, and the simulatedplatform was built. The simulation and experiment results demonstrate thatzero-sequence circulation current exists in parallel converters, but some higherharmonic can offset each other in interleaved parallel system, the power quality canbe enhanced. By using improved SVPWM algorithm, the voltage utilization ratio canbe raised, the circulation current can be suppressed effectively, and the oscillation oflow frequency circulation can be weakened. Meanwhile, the converters can operateindependently.(4) The advantages and disadvantages of several software phase-locked methodsbased on traditional single synchronous reference coordinate, symmetrical elementsingle synchronous reference coordinate, improved single synchronous coordinate anddecoupled double synchronous coordinate were studied. Their performance wassimulated at different voltage, frequency and phase. For the defects of softwarephase-locked methods mentioned above at unbalance and deterioration of grid voltage,a novel software phase-locked method was put forward. By using all-pass filter andsimple mathematic calculation, positive-sequence voltage component in asymmetricpower grid under d-q synchronous rotating reference coordinate was extracted, andthen the phase can be extracted to realize phase locking. Its working principle wasstudied in detail. The simulation and experiment results indicate that the novelphase-locked loop shows better performance in case of unbalance and dete rioration ofgrid voltage.