Effects of the module voltage fluctuation on the output voltage of modular multilevel converter

In order to research the effects of the module voltage fluctuation on the output voltage of modular multilevel converter (MMC), the mathematical model of modular multilevel converter (MMC) was established and the analytical expression of the converter's output voltage was deduced on the condition that high frequency harmonics caused by modulation algorithm was neglected. And for the circulating current restraint control method which was often used, the expression of output voltage was simplified. The analysis shows that caused by the fluctuation of module capacitor voltage, there are odd harmonics in the output voltage. Moreover there is a deviation between the actual value and the reference value of the output voltage. In order to measure the value of the deviation, fluctuation coefficientof module capacitor voltage is defined, and it may draw a conclusion that the deviation is depended on the ratio of the fluctuation coefficientto the modulation degree of output voltage. To make sure that the converter can work properly, the ratio should be limited in a certain range. At last, experiment results demonstrate the validity of the theoretical derivation and analysis.