Control and analysis of module capacitor voltage divergence of Modular Multilevel Converter

Modular Multilevel Converter(MMC) has the advantages of high quality output voltage waveform and the total harmonic distortion(THD) of voltage and current is small. Apart from these features, its active power and reactive power can be controlled independently and its module cascaded design enhance the robustness of the converter. Hence, this topology is being increasingly applied to the field of flexible High Voltage Direct Current(HVDC). But there is a serious problem after the MMC´s charging process has been completed. Module capacitor voltage will be divergent due to the dispersion parameters of self-created energy module. This paper proposes a circuit topology to solve this problem. Besides, a theoretical analysis and calculation has been made about the value of the power resistor which offers reference to system design. Also, this paper has made an optimization to the sampling frequency. Using the proposed control frequency and the value of power resistor calculated can not only ensure module capacitor voltage convergence but also reduce switching frequency and system losses. The proposed algorithm in this paper can extend the life of switching device. Finally, the effectiveness of the proposed topology and the correctness of the analysis and optimization to the system are verified by simulation.