Capacitor Monitoring And Fault Tolerant Control Of Modular Multilevel Converters Under Capacitor Aging

Modular multilevel converters(MMCs)have shown a very important prospect in the application of high-voltage and high-power systems due to its high efficiency,assembling and flexibility.The capacitor is one of the components with high failure rate in the MMC.The parameter changes of the capacitor in the MMC not only will cause the unbalance of the sub-modules’ power and loss in the arm,but also cause the unbalance of the arms’ power in the MMC,and therefore affects the performance of the MMC and the power quality of the grid.Hence,researching on the capacitor faults in MMC is of great significance to its reliability.To solve the capacitor faults in MMC,this paper will focus on the capacitor monitoring and the fault tolerant control.The aim of capacitor monitoring is to estimate the equivalent series resistance(ESR)and capacitance of Submodule(SM)capacitor online,so as to diagnose the fault capacitor in time.The aim of fault tolerant control is to eliminate the bad effects caused by the parameter change of fault capacitor before its replacement.First of all,this paper introduces the construction and operation principles of MMC,and then the mathematic model of MMC is obtained based on the Kirchhoff voltage and current law.Furthermore,the basic controls of MMC are analyzed,which include the double line frequency circulating current suppression control,the voltage balancing control and the general control.Secondly,the capacitor current of MMC is analyzed based on the mathematic model,which is used to simplify the capacitor model of MMC.Furthermore,the capacitor voltage and energy are analyzed,and two characteristic values are obtained which could sort the equivalent series resistances and capacitances within the arm.Based on the characteristic values,a sorting-based capacitor monitoring method is proposed,which only estimates the biggest equivalent series resistance and the smallest capacitance within the arm.Thirdly,based on the analysis of MMC equivalent circuit,it is illustrated that the capacitor fault will cause the fundamental component of circulating current in each phase.According to its dynamic equation,a suppression strategy based on the feedforward decoupling control is proposed to eliminate the fundamental component in the circulating current.Finally,the simulations and experiments are conducted,which both confirm the effectiveness of the capacitor monitoring method and the fault tolerant control.