Abstract With the advantages of modularization, flexible structure and convenient expansion, modular multi-level converters (MMC) become the core topology under the condition of high voltage and high power. In low frequency status, the problems, such as unbalanced energy distribution in arms, severe pulsation of sub-module capacitor voltage in MMC variable-speed drives system, will exist. These problems will not only affect the running ability of full speed area, but threaten the system safety. Therefore, a full speed area running method based on common voltage and bias voltage control for MMC variable-speed drives system is proposed. The proposed method can suppress the pulsation of sub-module capacitor voltage by controlling instantaneous power of each arm. Firstly, the mathematical model of MMC system was built to analyze the influencing factors for pulsation of sub-module in low-frequency. Then, the low frequency controller and on-line link of mode switch were designed for variable-speed drives system. Last, the comparisons of the simulations and experiments verified the proposed control strategy. The results show that the control strategy proposed can effectively suppress the pulsation of MMC sub-module capacitor voltage, accomplish smooth switch during different frequencies, reduce system loss, and hence improve the system output quality and the safety performance of MMC system.
Yin Shi,Tan Guojun,Fang Lei等. Variable-Frequency Control Strategy of Modular Multilevel Converter[J]. Transactions of China Electrotechnical Society, 2016, 31(20): 139-150.
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2016, Vol. 31 
