Rresearch On Medium Voltage Multilevel Converter Used For Offshore Wind Power Genertation System

Power rising of signal wind turbine, developing of offshore wind farms, direct drive wind generation coming up from behind is shown as characteristics in the development of wind power generation. As the product of the above trend, direct-drive offshore wind power system will become a hot research issue in wind power field in the future. The larger power, higher voltage level and more extreme environments of the offshore wind generation system propose more stringent requirements to the wind power converter. However, the application of traditional two lever back to back in offshore wind power will produce excessive voltage and current stress to the converter, together with higher du/dt and EMS etc. The multilevel topology with smaller stress to single switch and better near-sinusoidal output waveform can meets the requirements of medium voltage system.This paper focuses on the application of multilevel converter topologies in offshore wind power generation system which is characterized by high power and medium voltage to start topology selection and simulation research.After reviewing kinds of multi-level structure used in wind electric field widely. Through compare between different structure, this paper combines multi-level structure and polyphase machine, and propose the scheme that combines six-phase pmsm and duplicate-busbar three-level PWM converter. Using multi-phase machine can promote single-machine capacity, which fit the tendency that wind driven generator is upsizing. Each converter on duplicate-busbar bear half of the system power, which will lower the converter’s sights. Three level structure can not only lower the voltage stress on each switch in medium voltage system, but also reduce the current and voltage distortion and filter capacity of two level converter under the same switching frequency.In consideration of both generator side and power grid side use three level converter, this paper build its math model in synchronous rotating reference frame. Research into three-phase three-level converter’s modulation mode and neutral-point voltage control scheme is conducted. After compare and analysis, three level SVPWM is applied and neutral-point voltage control scheme is through controlling positive and negative vectors’ action time. SVPWM is easy to be realized, and it has high DC use ratio. The applied neutral-point control scheme is simple and save extra hardware cost.The grid side converter is controlled with vector control scheme, and LCL filter is used instead of L filter and LC filter; At the generator side, the vector control of six-phase wind generator is equivalent to the control of three-phase permanent magnet synchronous generator. The whole system model of MATLAB is built to verify the feasibility and validity of the scheme proposed.The paper designs the main circuit and give out the parameter for5MW offshore wind power system, which make the research possess practical value and engineering significance.