Fault Characteristic Analysis And Fault Tolerant Control Strategy For Sub Module Of Modular Multilevel Converter

Compared With the traditional voltage source converter(Voltage source converter,VSC),the modular multilevel converter(Modular Multilevel Converter,MMC)has advantages of owing a modular structure,good harmonic characteristics,fault recovery capability,and unbalanced operation,thus having a broad application prospect in the field of new energy access asynchronous,passive power grid interconnection,power supply,and city distribution network,which also attracted attention both in industrial and academic circle.This paper mainly analyzes the fault characteristics of MMC sub module and designs a faulttolerant control strategy with the following content:The paper analyzes the working principle of MMC,the working principle of half bridge sub module and the basic control strategy of MMC-HVDC system;a mathematical model of MMC in rotating coordinate system is deduced,and the controller of MMC-HVDC system is designed.On this basis,the paper introduces two commonly used modulation strategy of MMC,and builds a simulation analysis of 9 level based on carrier phase shift modulation strategy and 21-level using the nearest level modulation of multilevel in Simulation platform;finally,a sub module capacitor voltage balancing control strategy has been designed accordingly based on two different modulation strategiesDue to the structural characteristics and working principle of MMC,the existence of circulation is inevitable,which cause a series of problems such as an increase of system loss.This paper deducted the continuous function model of MMC,based on which the forming mechanism and characteristics of MMC circulation was analyzed.Concerning the circulation problem in MMC,a decoupling controller was designed using the principle of coordinate transformation,and the validity of circulation suppressor was verified by building 21 level MMC-HVDC system in Simulation platform.The paper analyzes the open circuit fault of the internal power switch tube in MMC sub module theoretically.The operation characteristics of MMC sub module after fault are deduced in detail by using the average switching function model.The fault tolerance schemes of three redundant modules are simulated and analyzed respectively,and the advantages and disadvantages of the three fault-tolerant schemes are compared.Finally,a kind of loop suppressor based on quasi PR controller is designed to restrain the circulating current of MMC after bypass,so as to realize the fault-tolerant operation after sub module fault.