| With the rapid development of renewable energy utilization,HVDC transmission system has been widely used,and the reliability of its core device,Modular Multilevel Converter(MMC),has become a research hotspot.Each MMC bridge arm consists of nearly one hundred submodules connected in series,and the output voltage of MMC is adjusted by controlling the switching number of submodules.In order to ensure the capacitor voltage balance of submodules,MMC needs to adopt voltage sharing control.Due to the objective existence of capacitance tolerance,the charging and discharging working time of each sub-module is not equal,which leads to the difference of power loss of sub-modules,and then affects the reliability of different sub-modules.However,the existing reliability analysis of MMC usually does not consider the influence of voltage-balancing control.Therefore,based on the working principle and life prediction method of MMC,this paper analyzes the influence mechanism of voltage-balancing control on MMC reliability,puts forward a reliability analysis method of MMC considering voltage-balancing control,and studies the influence of voltage-balancing control on MMC device life under different capacitance errors.The main contents of this paper are as follows:Firstly,the research status of MMC reliability is reviewed,and the existing research is divided into mathematical statistical analysis and failure-based physical analysis,and their advantages and disadvantages are summarized.The complex relationship and research difficulty between device reliability analysis and circuit reliability analysis are pointed out,and the research significance of this paper is put forward.Secondly,the working principle of MMC,the voltage-balancing modulation strategy and the life prediction method of MMC devices are systematically introduced.Taking the commonly used voltage-balancing control strategy based on capacitor voltage sequencing as an example,this paper analyzes the relationship between the sub-module capacitance tolerance,capacitance charging and discharging speed,switching time and switching state,and qualitatively analyzes the influence of voltage-balancing control on sub-modules with different capacitance values,which provides a basis for the reliability analysis of MMC sub-module devices.Thirdly,the life prediction algorithm of MMC submodule based on recursive sequence is proposed.Specifically,the capacitance voltage and switching state of each sub-module at each control time are expressed by two recursion sequences,in which the capacitance voltage is a function of the previous switching state of the sub-module and the bridge arm current,while the switching state is a function of the sorting and modulation function of the capacitance voltage terms of all sub-modules of the bridge arm.The stress curve of each device in a long time scale can be obtained by alternately recursion of the two sequences,and the predicted life of each device in MMC sub-module can be calculated by combining the device thermal impedance model with the life prediction model.Finally,referring to the parameters of Xiamen flexible DC transmission system,the simulation study of MMC life prediction considering voltage sharing control is carried out.The influence of voltage sharing control on the reliability of MMC under different capacitance distribution is analyzed from the aspects of switching sequence,power loss,junction temperature or hot spot temperature,predicted life,etc.It is proved that the influence of voltage sharing control on the reliability of MMC submodule,and it is obtained that the greater the capacitance error,the shorter the total life of MMC.The influence of different voltage sharing control strategies on the reliability of MMC is compared,which provides a reference for the selection of voltage sharing control strategies. |
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