Research On Capacitor Voltage Control Strategy Of Modular Multilevel Converter

As a new topology,The modular Moudular multi-level Converter(MMC)circuit structure is characterized by high modularity and symmetry.By increasing the number of cascade modules,the transmission voltage level and transmission power can be extended.The output voltage has low harmonic content and good power characteristics.MMC has high engineering practice value and research prospect.Through the literature analysis of MMC research status,this paper first discusses the research background and significance,as well as the progress and problems in MMC research.Then the structure of MMC and the topological characteristics of its sub-modules are analyzed,and the operation mechanism of MMC and its sub-modules is studied.Two Modulation modes commonly used in MMC,carrier phase shift Modulation and Nearest Level Modulation(NLM),are discussed and simulated.Based on the existing research theory,the voltage fluctuation of the submodule is the main cause of the phase circulation.In this paper,a new voltage equalization strategy is proposed.The capacitive voltage equalization strategy of the submodule based on the bidirectional threshold approximation of NLM is proposed.Then,the circuit model of MMC precharging process is simplified and analyzed in this paper,and it is demonstrated that the capacitor can not be effectively charged to the rating value without the control of precharging,so that it cannot enter the normal operation.The characteristics of MMC precharging are studied,and the controllable precharging strategy for ac output with no load and with load is proposed.The simulation model is built to verify the important role of current limiting resistance and the effectiveness of the two charging strategies.Finally,MMC circulation inhibition is studied.The mathematical model of the converter is derived,the mechanism of the circulation is analyzed,the main harmonic components of the phase circulation are analyzed by using the instantaneous power theory,the design of the circulation suppressor is analyzed by means of dq decoupling,and the circulation suppression PI controller is combined with the equalizing pressure strategy to eliminate the circulation and realize the equalizing pressure.