Research On Nonlinear Control Strategy Of Modular Multilevel Converter

With the development of power electronic devices,Modular Multilevel Converter(MMC)has been widely used in high-voltage and high-power system,such as Flexible HVDC transmission system,because of its flexibility,easy cascade and good output characteristics.In this paper,the work are fourfold i.e.,the working principle of basic topology,systematic nonlinear control strategy,modulation strategy and circulating suppression.(1)MMC system model is established under abc and dq frames respectively to analyze the system operating principle.(2)Combining the application background,the systematic controller is designed,which is composed of PI control in the outer loop and a passive controller in the inner loop.And when the load changes,the inner ring adopts PI+RVCC controller.(3)The carrier phase shift pulse width modulation technique is adopted and the working mechanism and realization method of N+1 and 2N+1 phase voltage are discussed respectively.In Simulink,1～10 submodules(SMs)single-phase MMC models are set up,and the similarities and differences between the two modulation methods are simulated and compared.(4)According to the double frequency negative sequence circulation in the converter,a circulating suppression strategy based on quasi-proportional resonance(QPR)controller is designed.1～ 4 SMs three-phases MMC models are set up in Matlab.The correctness and effectiveness of the control method are verified via simulation.Finally,a three-phases MMC grid-connected model with 4 SMs is improved in Simulink,and the time-control of the control strategy is verified by the integrated system-level passive control and circulation suppression.The effect of converter performances with N+1 和 2N+1 level voltage modulation are further compared as well.