An Optimized Voltage Balancing Strategy And Improved Sub Module Voltage Detection Method For Modular Multilevel Converter

In recent years,with several attractive features such as a modular design,the excellent quality of the output voltage waveforms,the capability of four-quadrant operation,low switch frequency,fault handling ability,modular multilevel converters(MMC)has been widely applied in the high voltage field.This thesis studies on the MMC through the theoretical analysis,software simulation and experiment.The research is focused on the optimization of the traditional voltage balancing strategy based on capacitor voltage sorting and the traditional detection method of sub module capacitor voltage.The detail content is as follows:Firstly,the basic structure and operation mechanism of MMC are introduced,and the basic principle of carrier phase-shift modulation(CPSPWM)and nearest level modulation(NLM)are analyzed.The reason why CPSPWM is applied to the case of less level and why NLM is applied to the case of more level are explained.Then,the balancing control strategy under two kinds of modulation is discussed.The shortcomings of the traditional voltage balancing strategy based on capacitor voltage sorting,such as high switch frequency and lagre sorting computation with excessive levels are analyzed,which are the theoretical preparations for the following paper.Secondly,in order to slove the problem of traditional voltage balancing strategy,an optimized voltage balancing strategy based on radix sort algorithm for MMC is proposed.Firstly of all,the principle of radix sort algorithm is introuced,which points out that using the radix sort for balancing control can reduce the sorting computation,and improve the speed of voltage balancing.Then,according to allowable fluctuation range of sub-modules(SM)capacitor voltage and the principle of radix sort algorithm,an optimized voltage balancing strategy is proposed,which can not only reduce the sorting computation,but also avoid the unnecessary switching operations and reduce the switching frequency by reducing the number of "collection" and "distribution" in radix sort algorithm.Simulation results verify the feasibility and efficiency of the optimized voltage balancing strategy.Thirdly,according to the problem which the traditional detection method of sub-modules capacitor voltage with the same number of sub modules increase the system cost and the burden of data acquisition system,an improved detection method of sub module capacitor voltage is proposed for NLM.The method divide the sub-modules into groups,each group has only a voltage sensor to correct the observer voltage,and the method for improving the number of corrections is proposed.Simulation results verify the feasibility and efficiency of the improved sub module capacitance detection method.Finally,the experimental platform of single-phase nine-level MMC is introduced,including the parameters of the capacitor and the inductance,the design of the power module and drive,the the control board of main chip(DSP+FPGA),as well as the software flow chart of DSP+FPGA.Then the optimized voltage balancing strategy is carried out on the experimental platform.Experimental results verify the feasibility and efficiency of the proposed optimized voltage balancing strategy.