| With the increasingly serious global greenhouse gas emissions and environmental pollution,the interconnection of regional economy and the continuous development of energy complementarity and coordination,the power system is gradually developing in the direction of intelligence.Modular multilevel converter technology has efficient and fast power regulation capabilities,which can better adapt to the output fluctuation of renewable energy,thereby improving the reliability and stability of the power grid,which makes modular multilevel converter technology an inevitable trend of the global energy transition trend.This paper analyzes and conducts in-depth research on the voltage equalization and circulation problems generated by MMC(Modular Multilevel Converter)converters in operation.The topology of MMC,including the overall composition and sub-modules(SM),is introduced,and its working principle is explained in detail through examples.On this basis,the mathematical model of MMC is established,and the control methods of the respective electrical quantities are studied on the basis of the mathematical models,and the control strategy of the system is integrated,including the power outer loop and the current inner loop.Then,combined with the widely used CPS-PWM(carrier phase shift modulation strategy)and NLM(nearest level approximation modulation strategy),a hybrid modulation strategy is formed,and the working principle and control process of the hybrid modulation strategy are analyzed in detail.The superiority of the hybrid modulation strategy is verified by simulation.According to which submodules are triggered to keep the capacitor voltage relatively stable,the capacitor voltage fluctuation is analyzed in depth,and on the basis of the traditional submodule capacitor voltage equalization strategy,a grouping and sorting capacitor voltage equalization control algorithm is proposed,which reduces the amplitude of capacitor voltage fluctuation by grouping and taking the average direct insertion sorting algorithm,and also reduces the switching times and sorting operation time of IGBT.Simulation and experiments verify the effectiveness of the strategy.Aiming at the internal circulation problem of MMC,the circulation generation mechanism and circulation suppression principle of MMC are analyzed in detail,and the traditional circulation suppression strategy,that is,the PI circulation suppression strategy,is introduced,and the PI controller and PR controller are connected in parallel to form a quasi-PIR circulation suppression strategy,which can eliminate the coordinate transformation and decoupling control of the traditional PI controller design,and after using the improved circulation suppression strategy,The fluctuation of the bridge leg current decreases and the corresponding FFT(harmonic distortion rate)also decreases,the fluctuation range of the circulating current decreases,and the fluctuation of the capacitor voltage of the submodule is significantly reduced,and the simulation verifies that the method has a good circulating current suppression effect.The overall simulation diagram of MMC converter is constructed in Matlab/Simulink,and the simulation results of the output phase voltage and phase current and the corresponding FFT are compared and analyzed,which further verifies the superiority of the strategy used in this paper. |
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