| The modular multilevel converter(Modular Multilevel Converter,MMC)has become a research interest especially in the field of high voltage and high power converter for its advantages such as low switching loss and its independence on the series of devices.In this paper,the working principle and control strategy of modular multilevel power conversion system are studied.This paper summarizes the research status and analyses working principle of multilevel power conversion system,gives the schematic diagram of the simplified MMC topology based on the analysis of the MMC circulation as well as summarizes the operating characteristics of MMC.The carrier phase shift SPWM,carrier disposition SPWM and nearest level modulation technology are three typical modulation methods which would be compared from the implementation method,the harmonic suppression effect and sub module capacitor voltage effect by Matlab/Simulink simulation environment test,combining simulation results and the environmental application of these three control strategies.Considering the influence of voltage fluctuation of bridge capacitor on the output waveform,the MMC voltage balance control is studied,focusing on to solve the complexity of traditional capacitor voltage sorting method.This paper do discuss about innovation,optimal control of bridge capacitor voltage,and simulation in the Matlab/Simulink environment and the simulation results show that the optimization method has good voltage and current output waveform and low voltage distortion rate.In this paper,the closed loop negative feedback of the modular multilevel converter,the negative feedback control of MMC in two application scenario,the passive inverter circuit and the active inverter circuit is analyzed.The control objective of MMC is to make the output voltage waveform as close as possible to the sine wave.The key of MMC is to realize the control of the AC side current of the converter.Based on the theoretical analysis and simulation,the simulation experiment of the modular multilevel converter system is carried out.MMC in the passive inverter simulation results show that MMC for passive inverter has good output voltage and current in rectifier unit power factor;MMC example simulation shows that it can achieve a stable DC output;MMC in STATCOM simulation example,current output waveform with reactive power control when MMC is lower the bridge capacitor voltage fluctuation and better;the voltage control,MMC can maintain the voltage of the common coupling point stability.The research on control strategy provide a reference to the implementation of the project,conferring to the MMC device and the required output characteristics to select the appropriate control strategy.Therefore an optimization algorithm is proposed to solve the complexity problem of MMC in high level case sorting method and improve the speed of operation of the processor. |
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