Research On Topology And Control Strategy Of Hybrid Modular Multilevel Converter With Cascaded Full Bridge

Modular Multilevel Converter(MMC)is widely used in HVDC transmission because of its modularity,low switching frequency and low harmonics.However,when it runs at low frequency,the fluctuation of capacitor voltage increases,which hinders its application in the field of high-power motor drive.The scheme of injecting high-frequency common-mode voltage and high-frequency circulating current can reduce the fluctuation of capacitor voltage,but at the same time it will bring high-amplitude common-mode voltage on the motor side,which will bring hidden dangers to the safe operation of the motor.In this paper,a hybrid modular multilevel converter with cascaded full bridge(HMMC-CFB)is used to solve the common-mode voltage problem.Firstly,the topological structure of half-bridge and full-bridge sub-modules is analyzed,which lays a foundation for the discussion of DC side pre-charging start-up.Then the topology of HMMC-CFB is analyzed,and a mathematical model for analysis is established.Taking the low-frequency output of the AC side as an example,the working principle of the high-frequency common-mode voltage is studied in detail.The DC side pre-charging process of HMMC-CFB is divided into three stages.The influence of current limiting resistance on uncontrollable charging stage is discussed.In the controllable charging phase,the existing open-loop and closed-loop charging strategies are analyzed and improved.Aiming at the problem of high time complexity of sorting algorithm in traditional capacitor voltage equalization,this paper proposes a module input process based on Top-K algorithm and a hybrid capacitor voltage equalization strategy with random number,which reduces the time complexity of the algorithm and the switching frequency.In order to reduce the number of voltage sensors and reduce the complexity of the system,the HMMC-CFB capacitor voltage discrete observer is designed to observe the capacitance voltage of all modules of each bridge arm.At the same time,the passivity theory is adopted,and the observation is given from the perspective of stability.The instrument error system is consistently asymptotically stable observer gain method,Finally,the model is built in Matlab/Simulink to verify the above control strategy.