Research On The Voltage Equalization Method Of Modular Multilevel Converter (MMC) Sub-modules

Modular multilevel converter(MMC)has become a research hotspot in the world's high-voltage DC transmission field with many advantages.And it is widely used in offshore wind power transmission,island power supply and large-scale new energy transmission projects.Compared with the traditional LCC topology,the MMC topology has advantages of low switching loss,good output waveform quality,and easy expansion.However,there are still many issues need to be resolved in engineering applications.For example,MMC transmission line fault identification,voltage balancing of sub-module capacitor,and interphase circulating current suppression.The research around the relevant key issues is significance to guide the practical application of the project.The MMC structure has the characteristics of mutual independence between the submodules.When the system is running,the charging time of each sub-module is different,and the capacitor parameters of the sub-modules are not the same.This will cause the sub-module capacitor voltage to be unbalanced,increasing the system's operating loss,distortion of voltage waveform,inter-phase circulation and harmonics will affect the stable operation of the system,and even crash and stop operation.Aiming at the problem of capacitor voltage balance of MMC sub-modules,this paper establishes the MMC mathematical model first,analyzes the principle of capacitor voltage imbalance in detail,and introduces the MMC modulation strategy in detail.Based on the analysis of the characteristics of MMC topology,a bidirectional self-balancing topology is proposed.This topology breaks the characteristics of the mutual independence between the sub-modules of the traditional MMC structure,and increases the energy path between the sub-modules.When the sub-module capacitor voltage is unbalanced,energy flows from the sub-module with a high capacitor voltage to the sub-module with a low capacitor voltage through the voltage balancing circuit,thereby achieving the effect of balancing the capacitor voltage of the MMC sub-module.