Research On Modulation Strategy And Sub Module Fault Of Modular Multilevel Converter

Modular Multilevel Converter(MMC)has the advantages of low switching frequency flexible controlled,low loss,good quality of output waveform,small impact on the converter when a fault occurs in DC side,etc.It has a broad application prospects in the field of high voltage direct current transmission,power quality governance and so on.In recent years,it has become a research focus in the field of high voltage power electronic conversion.In this paper,as a study object,we comprehensive study for the two problems of Nearest Level Modulation strategy and Sub modulefault diagnosis.Firstly,introduced basic structure,operation principle,mathematical model,control strategy and other basic theory of Modular Multilevel Converter,it can establish the system models and provide the foundation for the later research.Secondly,we studied for the Nearest Level Modulation,considering these shortcomings,such as when this modulation strategy under the condition of low level output current cannot precise tracking of reference voltage,bad waveform,etc.,we proposed an improved Nearest Level Modulation,which can controlled the error current within the set of the ring wide range,and analyzed ring width of current hysteresis controller,derived the mathematical relationship between the ring width and the equivalent switching frequency,and verified on Matlab/Simulink software.Finally,Sub modules fault reasons for fault classification.Several faults which occur frequently in the sub module are analyzed in detail.Constructed sliding mode observer for the MMC by using the design method of the sliding mode observer,used the bridge arm circulating current and the sub module capacitor voltage as the observation variables,and studied an method for diagnosing open circuit fault of sub module,the method does not need to add additional measuring points,can quickly detect and determine the location of the fault,then isolate the faults,and has good robustness.The validity of this method is verified on Matlab/Simulink software.In the end,introduced the hardware and software of MMC converter system experimental system in detail,and the improved modulation strategy is verified by experiments.Subsequently,summarizedthe work of the paper,and put forward some future research directions.