Research On Sub-module Capacitor Voltage Ripple Suppression Of Modular Multilevel Converters With Active Power Filter Decoupling Circuit

With the improvement of voltage level and power capacity,modular multilevel converter(MMC)effectively avoids the direct series/parallel connection of power switching devices,and has the advantages of high modularity,black start capability and high output waveform quality.The MMC topology has been widely considered and applied in the field of power conversion such as high voltage flexible DC transmission system,medium voltage intelligent distribution system and medium/high voltage high power drive system.Compared with the line commutated converter(LCC)topology commonly used in traditional medium/high voltage and high power applications,the MMC topology eliminates the need for bulky output filter devices,and uses discrete sub-module suspension capacitors to replace traditional high voltage large capacity capacitors.In practical engineering application,in order to suppress the voltage ripple of sub-module capacitor,the metal polypropylene film capacitor with larger capacitance is generally selected,which accounts for about 1/3 of the total hardware cost and more than 50% of the total volume of the system device,which limits the further promotion of MMC topology.Therefore,it is of great significance to study the method of suppressing the MMC sub-module capacitor voltage ripple.Based on the mathematical model and working principle of MMC topology,from the two aspects of topology improvement and control method,this paper studies the formation mechanism of sub-module capacitor voltage ripple and the related theory and technology of power filter decoupling circuit control.Based on this,a new topology with inherent suppression of sub-module capacitor voltage ripple and corresponding modulation strategy and control method are proposed,which further improves and develops the new topology construction idea of MMC and the corresponding basic theory and method of optimal control.The main research contents of the paper are as follows:Firstly,the sub-module capacitor voltage ripple of the modular multilevel converter is analyzed.Based on the thorough investigation of the medium/high voltage and high-power power electronic converter,the application prospect of modular multilevel converter topology in the field of power conversion is expounded,and the restrictive reasons for its further promotion due to the large capacitor voltage ripple of sub-modules are summarized.From the perspectives of topology improvement and control strategy,the current research status of domestic and foreign scholars on suppressing capacitor voltage ripple of sub-modules is reviewed.Based on this,the mechanism of capacitor voltage ripple formation is revealed,and the decoupling technology of active power filter and its research status are introduced,which lays a theoretical foundation for the paper.Secondly,a middle sub-module multiplexing modular multilevel converter with active power filter decoupling circuit is proposed.Considering the particularity of the existing middle sub-module multiplexing MMC topology,a modulation strategy and sub-module voltage control method suitable for this topology is proposed.And based on this,a new middle sub-module multiplexing modular multilevel converter with active power filter decoupling circuit is proposed.In this topology,the active power filter decoupling circuit as a new control degree of freedom is introduced to realize the even frequency doubling power dissipation in each phase,which effectively suppresses the voltage ripple of the sub-module capacitor,and reduces the current stress and power loss of the power switch devices in the sub-module.Thirdly,a traditional modular multilevel converter with active power filter decoupling circuit is proposed.Aiming at the problem that the redundancy of middle sub-module is limited in the middle sub-module multiplex MMC topology,by analyzing the working principle of active power filter decoupling circuit and the switching mechanism of arm sub-module,a traditional modular multilevel converter with active power filter decoupling circuit is proposed.According to the circuit connection mode,the sub-modules in the arm are divided into the arm sub-modules and the middle sub-modules,and the switching process of the middle sub-modules is constrained.This topology does not need to reconstruct the traditional MMC topology,and the additional active power filter decoupling circuit has a lower coupling degree with the main circuit of MMC,which effectively reduces the control complexity.In addition,compared with the traditional MMC topology,the proposed topology has inherently smaller sub-module capacitor voltage ripple and sub-module power loss.Finally,a method for suppressing the sub-module capacitor voltage ripple of the modular multilevel converter with active power filter decoupling considering the voltage and current injection is proposed.A mathematical model of the traditional MMC topology with active power filter decoupling circuits connected across the midpoint of the upper and lower arms is established.By introducing high-frequency voltage and high-frequency current in the same phase into the bridge arm as two new control degrees of freedom,the control boundary of MMC is expanded,so that the fundamental frequency power of the main part causing the capacitor voltage ripple of the sub-module is converted into the high-frequency ripple power,so as to increase the charge and discharge frequency of the sub-module capacitor voltage,and the influence of injected high frequency voltage on the output characteristics of the converter is eliminated by introducing the active power filter decoupling circuit as a hardware control degree of freedom.Overall,in this dissertation,the modular multilevel converter is taken as the research object.Aiming at the unique characteristics of the sub-module capacitor voltage ripple,on the basis of summarizing the existing methods to suppress the sub module capacitor voltage ripple,the formation mechanism of the voltage ripple is explored.Combined with the active power decoupling circuit technology,a new class of MMC topologies and the modulation mode that inherently suppress the voltage ripple of the sub-module capacitor are proposed.Based on the mathematical model and working principle of the new topology,a method for suppressing the sub-module capacitor voltage ripple of the modular multilevel converter with active power filter decoupling considering the voltage and current injection is proposed,which can significantly reduce the capacitor voltage ripple of the sub-module.At the same time,the feasibility and effectiveness of the proposed improved topology and control strategy are verified by simulation and experiment on PLECS simulation platform and experimental prototype platform.