Reliability Study Of Modular Multilevel Converter

In the face of the large transmission span of China’s power grid,voltage source converter based high voltage direct current transmission(VSC-HVDC)is now widely used,with modular multilevel converters(MMC)becoming the most popular converter technology.The reliability of MMC is now a hot topic of research and an important factor in analyzing the economics and stable operation of converters.In response to this,this thesis investigates the reliability of conventional MMC converters as well as new hybrid converters.Firstly,the topology and operating principle of MMC converter are discussed,and the changing law of AC and DC side voltage and current of the converter in normal operation and in case of fault is analyzed.The operating mode of the half-bridge submodule(HBSM),the basic unit of the converter,is explained,and the operating mechanism of a typical submodule with active DC short-circuit elimination function is studied in depth.The research results provide new ideas for the upgrade of MMC converter topology.Secondly,based on the analysis of the indexes required for constructing the reliability function,the reliability principle and influencing factors of the MMC converter are studied.A unified reliability function is proposed for different submodule topologies,and a corresponding bridge arm reliability model is proposed for different submodule redundancy configuration strategies;the reliability model of the MMC converter as a whole is obtained based on the construction of the submodule and bridge arm reliability functions,which provides a mathematical tool for the subsequent study of the reliability of the MMC converter composed of new submodule topologies.Again,the reliability of the self-blocking submodule converter is studied and analyzed.On the basis of comparing the introduced submodule topologies,the self-blocking submodule structure(SBSM)is proposed;the structural composition and working principle of SBSM are described,and the corresponding submodule reliability model is established;the reliability functions of the converter under redundant passive and active configurations are established using the Gamma distribution and k/n(G)model respectively;practical examples are used to explore the influence of different redundant configuration.The effect of different redundancy configurations and different redundancy degrees on the reliability is investigated.Finally,a hybrid MMC converter that combines economy and reliability is proposed;based on the analysis of the new submodule used,the diode clamp with bidirectional switch submodule(DCBSSM),the quantity share of different submodules in the hybrid MMC is calculated;a reliability model for the hybrid MMC is developed,and the IGBT utilization rate is used to evaluate the redundancy of different submodules.The impact on the reliability of the hybrid converter with different redundancy levels and different submodule reliability is investigated in combination with practical examples,and the optimal redundancy configuration of the hybrid MMC is finally obtained...