A Stratified Voltage Balancing Control Method of Sub-module Capacitor Voltage for Modular Multilevel Converter

To improve the sorting process and reduce the switch frequency of sub-modules in the capacitor voltage balancing control of modular multilevel converter(MMC),a stratified voltage balancing control method of sub-module capacitor voltage for high voltage level MMC system is proposed.Firstly,the capacitor voltage container is determined according to the maxi and min capacitor voltages.The number of containers is discussed according to the time complexity of the algorithm and the effect of balancing control.Secondly,all the sub-modules are arranged into the corresponding containers according to their capacitor voltages.In accordance with an arm current and the number of sub-modules needed to be inserted in the arm,the correct submodules are chosen for insertion via the stratified capacitor voltage balancing control and a proposed optimal process.Finally,a criterion for resetting the containers is presented to reduce the amount of calculation.If the voltage variation of any inserted sub-module is less than the voltage interval of container,then the voltage margins of containers remain unchanged.Otherwise,they should be reset.Simulation results of a 21-level MMC in PSCAD/EMTDC show that the proposed controller is able to guarantee excellent voltage balancing results and high efficiency of sorting rate.Furthermore,the proposed method will significantly decrease the switching losses in insulated gate bipolar transistor(IGBT)in comparison with the bubble sort method.