Research On Control Strategy Of The Permanent Magnet Direct-drive Wind Power System Based On MMC

Wind power generation is becoming a new direction for the world's energy industry.Therefore,countries around the world pay more and more attention to the wind power generation technology,and invested a lot of manpower and material resources to start researching wind power generation technology.The full-power converters of direct-drive permanent magnet synchronous wind turbine are the unique component to feed the energy into power grid,so it's play a curcial role in the system.Modular multilevel converter(MMC)as a novel kind of VSC-HVDC,because of the characteristics of good expansibility,high reliability and low output harmonic components,especially suitable for medium-voltage high-power occasions,and start to received the researchers' like.Therefore,for the better use of MMC applied to the field of wind power generation,it is urgent to solve the control method of converters based on MMC.This paper mainly studies the control strategy of direct drive wind power generation system based on MMC,the main contents are as follows:This paper,firstly,starting from the calculation of wind energy,the operating characteristics and power characteristics of the wind turbine are studied,and the main technical parameters of the wind turbine are introduced.Combining with the structure characteristics of the direct drive permanent magnet synchronous motor,deduced its mathematical model in different reference frame.And then according to the traditional PI control principle,put forward the basic control strategy of direct drive wind power generation system.And under Matlab/Simulink simulation software,set up its grid connection simulation model.Then for a direct-drive permanent magnet synchronous wind power system,in order to improve the capacity of the system and the shortage of the traditional control method,a proportional resonance control strategy based on modular multilevel system is proposed.This paper introduces the topology and basic working principle of MMC,analyzed its modulation principle and capacitance voltage equalization strategy,and deduced the mathematical model of MMC in different reference frame.Then,a proportional-resonant(PR)strategy is introduced in the ?? stationary reference frame.According to the characteristics of direct drive wind power generation system,designed the PI control strategy of machine side and PR control strategy of grid side converter based on MMC.Compared with the PI control method,this method can eliminate the static error of alternating current component and without tedious frame transformation,that is effectively reduced the difficulty of the control algorithm.A three-phase eleven-level MMC simulation model of direct-drive permanent magnet synchronous wind power generation system was established under Matlab/Simulink simulation software.The simulation results show that the PR control method of grid side based on ?? reference frame has strong dynamic stability and anti-interference performance,so that verified effectiveness of PR control strategy in this model.Finally,for the case of asymmetric fault,analyzed the system characteristics of the asymmetric fault and introduced fault phase sequences separator.In order to solve the problem that the direct drive permanent magnet synchronous wind power generation system has the ability of suppressing the failure when the wind farm is connected with the asymmetric fault,this paper proposed a control strategy to suppresses negative sequence current to zero which is based on ap reference frame to keep the system's dynamic stability and fault crossing capability.Then established the simulation model of the system in MATLAB/Simulink,and the validity of the proposed control strategy is verified.