Research On Control Strategy Of Grid-side Converter In Direct-drive Wind Power System

In recent years, with the gradual depletion of fossil fuels, people began to look into the development and utilization of new energy sources. As an important part of the new energy field, wind power technology have made great progress. In wind power system the Stand-alone capacity and total installed capacity will continue to improve, but at the same time, the phenomenon of abandoned wind power rationing is also very serious. How to provide high-quality power for grid has become an important part of wind power system studies. As a connection device between wind turbines and grid, wind power converter has become increasingly prominent.In the direct-drive wind power generation system, the converter can be divided into the generator-side converter and grid-side converter. Generator-side converter acts as the main control of motor, grid-side converter stabilizes the DC bus voltage and controls the power flow direction. The grid-side converter control strategy is a priority in this thesis.The topology structure and operation principle of the diode-clamped three-level inverter is analyzed and mathematical models in different coordinate systems are established in the thesis.The converter control strategy is researched based on the mathematical, and the voltage and current double closed-loop control strategy is focus. Voltage loop achieves a stable DC voltage and two-way flow of power, the current loop to achieve active component and reactive components are independently controlled. The implementation procedure of SVPWM is described detail, including the judgment of reference vector working region basic vector selection calculation and distribution of basic vector duration. Simulation of control strategy and SVPWM is established in the thesis. To realize two-way flow of energy, simulations of the rectifier and inverter paper respectively are established.The necessity to add dead time is discussed, and the hazards of dead-band effects are studied in this thesis. In order to eliminate the harm caused by the dead time, a dead zone compensation is studied. To verify the dead-time compensation strategy, no dead time simulation, adding the dead time simulation and add dead-time compensation simulations are contrasts in the inverter simulation model. The SVPWM overmodulation strategy, especially the dual-mode overmodulation strategy is studied, and to verity the strategy simulation is established.The diode-clamped three-level converter neutral- point potential imbalance is a priority in the thesis. The neutral- point potential imbalance caused by circuit theory and devices work is analyzed, and the causes of neutral- point potential fluctuations are detailed studied. The control strategies based on vector control factors and the method based on virtual vector are focuses on this thesis. The method based on small vector control can be applied directly to traditional three-level SVPWM modulation. The method based on virtual vector needs to study the corresponding virtual vector modulation strategy. Two different control strategies are comparative analyzed in the thesis. A 2.5MW direct drive wind power system grid-side converter prototype is used to verify part of the control strategy, and grid-side converter of double-fed wind power generation system is used as a DC source in inverter experiment.