A Sub-module Grouping Strategy For Reducing The Number Of Voltage Sensors In Modular Multilevel Converters

As the industry continues to develop steadily and sustainably at present,power electronic devices in the field of high-voltage and high-power has been used increasingly widespread.However,using small and medium-sized power devices through multi-level technology is one of the best choice to solve high-voltage high-power applications,when the modern switching devices in the voltage and power has not been significantly improved.In this context,with its low harmonic output performance,easy expansion of the voltage level and excellent redundancy,Modular Multilevel Converter(MMC)has been paid close attention by experts and scholars both at home and abroad since it was put forward.After more than ten years of development,MMC has made great progress in the modulation strategy,the suppression of sub-module capacitor voltage fluctuations and the reduction of the bridge arm circulation.Nevertheless,its high manufacturing price is one of the main factors that hinder its large-scale commercial application.This paper studied on sub-module grouping control to reduce the numbers of voltage sensor and then achieve the purpose of improving its economy in the end.Firstly,this paper introduced the significance of the current research on the modular multilevel converter and pointed out the advantages of MMC compared to the general converter.Then,application of MMC in power system and industrial production is summarized and its characteristics is expounded.Furthermore,the current research status of sub module grouping and the direction of reducing voltage transformer is also analyzed.Secondly,the grouping rules for MMC is emphatically introduced.This part first introduces the topology and working principle of MMC,and then described the rules of sub module grouping in detail.In this rule,the number of sub-modules for the bridge arm 2N,according to 20,21,22,……2N-1,20,divide into N group and each group only installs one voltage sensors.The sub-module grouping rules are adjusted to fit all the projects when the number of the arm of the sub-module is not 2N.Finally,the sub-module group voltage measurement method is designed for the sub-module grouping structure.Thirdly,the corresponding control strategy is designed for the proposed structure,which includes modulation strategy,sub-module capacitor voltage balancing strategy and sub-module inter group equalization strategy.In modulation,each sub-module group as a whole,inputs or removals simultaneously.The sub-module capacitor voltage balancing strategy and sub-module inter group voltage balancing strategy is used to balance each sub-module capacitor voltage.Finally,simulation and experimental verification for the proposed sub-module grouping structure and control strategy are presented.For the simulation,in the MATLAB/Simulink environment,a three-phase 33 level model is built to simulate the steady state and the transient state of power change.In the experiment part,the low voltage low power single-phase 9 level experiment platform is introduced,which includes the structure of the experiment platform,the design of control circuit and the design process of the control algorithm for the experimental platform.Then,simulation and experimental results verify the advantages and practicability of the proposed sub-module grouping and control strategy.