Research On Multilevel Converters For Direct Driven Wind Generation System

Among the current mainstream wind power systems, the direct driven wind power system become an important area of research following double-fed wind power system due to its superiorities on simple structure, high energy conversion efficiency, high reliability, and flexible power control ability, etc. As interface between direct drive wind power generation system and grid, the choice of power converter's structure is critical to the variable speed constant frequency operation and performance. However, the expert do more research on the converter of double-fed wind power system than that of direct drive wind power system at home and abroad. The power converter and control technologies of direct drive wind power system are still heavily relied on importing in our country, which significantly restricts further development of wind energy industry. Therefore, this dissertation gives theoretical study on full-scale grid-connected power converter in direct drive wind power system, and the simulations demonstrate the correctness of the research results.The dissertation firstly summarizes the art-of-the-state of wind power system. The structure and features of constant speed constant frequency and variable speed constant frequency wind power system are analyzed and compared; A detailed description on the topology of variable speed constant frequency wind power system is given; The dissertation also reviews the topology and control methods of grid-connected converter.Second, the dissertation gives a comparative analysis on the topology and characteristics of typical grid-connected converter program. Considering the switching frequency and the level of pressure values of current switching devices, a grid-connected program in line with requirements of high power direct drive wind power system is proposed that combined multilevel cascaded converter with direct-drive based system. The program make use of low-speed multi-phase permanent magnet synchronous generators for each unit in series to provide independent DC power supply. The scheme combines the inherent advantages of multilevel cascaded converter high-quality, high-power output characteristics with that of direct drive wind power.Third, the dissertation analyzes the topology, operation principle and control methods of the multilevel converter. The actual working conditions for this scheme may arise issues such as the imbalance of DC side voltage. By analyzing the output waveform under carrier phase-shifted pulse-width modulation method for the different depths of the output waveform, solutions is proposed that stable output voltage in the case of DC voltage unbalance and verified by simulation as well.Finally, the calculation formula of instantaneous power is derived under the three-phase static coordinate system and the two-phase synchronous rotating coordinate system. According to this mathematical model we established, the current control method is proposed and verified by simulation.