Research On Fault-tolerant Control Without Redundant Sub-modules And Circulating Current Suppression In The MMC System

Modular multilevel converter(MMC),an innovation type of multilevel converter structure,is widely used in the field of high-voltage and high-power HVDC.Due to multiple sub-modules structure of the MMC system,the operational stability of the system might be threaten when failures appearing in sub-modules and the redundant sub-modules running out of power.Meanwhile,the inherent interphase circulating current will increase the loss of the system and the capacitance voltage fluctuation of sub-modules.Therefore,the realization of the fault tolerant control of the MMC system without redundant sub-modules and the research of the circulating current suppression technology are very significant to improve the stability and reliability of the MMC system.First,this thesis introduces the topological structure,working principle,mathematical model and control strategy of MMC,and compares the advantages and disadvantages of a number of modulation strategies.The working characteristics of the asymmetric MMC system after fault removal are analyzed,and a new fault-tolerant control strategy without redundant sub-modules is proposed.The control strategy can achieve the function of fault ride-through when multiple sub-modules fail in the same phase or different phases.Second,to restrain the interphase circulating current of the system,the PI circulating current suppression controller and the quasi-PR circulating current suppression controller are designed and built,in which the PI circulating current suppression strategy can be applied to the three-phase symmetric MMC system.Nevertheless,the quasi-PR circulating current suppression strategy is suitable for use in single-phase MMC system and three-phase asymmetric operating system.Aiming at the circulating current suppression problem of the MMC system with a large number of sub-modules,an improved finite control set model predictive control strategy is proposed.This control strategy is unnecessary to set the weight coefficients and has a fixed switching frequency,which contributes to the stable operation of the system.In addition,this control strategy can track the currents in AC side effectively and balance the sub-module capacitor voltage.The correctness of this control strategy is verified by using simulation software MATLAB/Simulink.Finally,a set of MMC hardware platform is built.The hardware platform is introduced in the aspects of sub-module and control board,and the influence of the dead time on the MMC system is analyzed and verified through experiments.