Research On Fault Diagnosis And Fault Tolerance Of MMC Sub-module

Modular multilevel converter(MMC)is a new type of voltage source converter which has a highly modular structure,it can output a high level voltage and has a low switching frequency,these advantages make MMC become more widely used in the high-voltage and high-power situation.At the same time,MMC contains a large number of sub-modules,which not only increases the system failure rate,but also to sub-module fault detection more difficult,in order to quickly maintain the system,reduce costs and improve its stability,timely detection and positioning failure sub-modules and take reliable fault tolerant measures is an urgent problem to be solved.The traditional fault sub-module detection method is to determine the state of the capacitor voltage by installing the voltage sensor in the sub-module.In the case of high-level MMC,the number of bridge arm sub-modules can reach hundreds of giant,which will increase the cost and the complexity of the system.This thesis focuses on the analysis of the circuit work and the fault form of the MMC sub-module IGBT fault,put forward a method to detect and diagnose the MMC sub-module fault only by collecting the output voltage of each bridge arm and combining the sub-module switching signals.This method only needs a small number of sensors,reducing costs.In this thesis,Firstly,analyzed the working principle of MMC by the topology and mathematical model,and then studied several modulation strategies which commonly used in MMC,the most common carrier phase shift pulse width modulation(CPS-PWM)strategy and the nearest level modulation(NLM)strategy are analyzed emphatically.Then,combined with the operation principle and fault characteristic of MMC sub-module,a sub-module fault detection method which only needs a few sensors is proposed for the most common IGBT open circuit and short circuit fault in sub-module.The method combines the switch signals of the IGBT in sub-module to separate the output voltage of each sub-module from the single-level arm voltage and then judges the fault conditionaccording to the change of the output voltage of sub-module.Built a 7-level MMC fault simulation platform based on MATLAB/Simulink,the proposed method is simulated and the validity of the method is verified.Then,this thesis introduces the common fault tolerant control schemes of MMC fault,such as symmetric bypass,neutral point drift and redundant backup,and analyzes the implementation principles of these methods.Finally,the fault tolerance schemes are compared in terms of fault tolerance,software complexity,hardware complexity and hardware cost.