Research On Fault Detection Technology Of Modular Multilevel Converter (MMC)

Modular multilevel converter(MMC)is a new type of converter topology,due to a high degree of modularity,good output power quality and low voltage requirement for the power device,it has been extensively applied in HVDC transmission,grid connection of new energy power generation and VSC-MTDC fields in these years.The large amount of sub-modules increases the possibility of fault,so rapid diagnosis and accurate location of fault sub-module is the important guarantee for uninterrupted operation of the converter.This thesis first introduces the operating principle,modulating strategy and capacitor voltage balancing algorithm of MMC,and adopts improved carrier phase shift pulse width modulation as the system control strategy.Aiming at the adverse effects on fault detection caused by inner circulating current of MMC,the mechanism of circulating current is analyzed,and parallel quasi-PR controller is designed to suppress the second harmonic component and fourth harmonic component.Then several common types of sub-module fault and redundant fault-tolerant control schemes are introduced,aiming at sub-module fault,the fault features are analyzed,and a fault diagnosis method based on arm current distortion is proposed in this thesis,by which the fault arm and fault type can be determined accurately.Using the input and output variables of MMC system,an adaptive observer for sub-module capacitor is designed,the observability of the system has been verified and the parameters of the observer have been calculated.According to the changes of the mathematical model of MMC after fault,by comparing the residual between the observed value and measured value of capacitor voltage,the fault sub-module can be located accurately.The simulation platform is carried out using the simulation software MATLAB/Simulink,the validity of the circulating current suppression strategy,fault diagnosis and fault location algorithm is verified by the simulation results.In the end,the MMC prototype hardware architecture is designed and the selection principle of main circuit parameters is analyzed.A sub-module of MMC is designed in detail and tested in laboratory,the experiment result shows the realization of fundamental functions.