Double-Loop Circulating Current Suppressing Strategy for Modular Multilevel Converter under Unbalanced Conditions
Zhang Chen1, Ye Hua1, Wei Lingxiao2, Wang Chunyi3
1. Key Laboratory of Power System Intelligent Dispatch and Control of Ministry of EducationShandong University Ji’nan 250061 China; 2. State Grid Beijing Electric Power Company Beijing 100031 China; 3. State Grid Shandong Electric Power Company Ji’nan 250001 China;
Abstract：The inner circulating current of modular multilevel converter (MMC) will increase the power loss and reduce the capacities utilization of the bridge inevitably. Even worse, the bridge current distortion will be intensified, which will endanger the safe operation. Therefore, it is necessary to suppress the inner circulating current. Via the more detailed phase-unit instantaneous energy analysis, this paper reveals that the quadruple-line frequency components of the inner circulating current will be increased due to the interaction between the circulating current at double line-frequency and the output voltage of the circulating current suppressing controller. Then, a suppression-enhanced double-loop circulating current suppressing strategy is proposed. Moreover, by reasonably tuning the controller parameters, the distribution of closed-loop poles can be improved, the system stability as well as the suppression ability can be enhanced significantly. A detailed three-phase MMC model is established in PSCAD/EMTDC environment, the correctness and effectiveness of the proposed strategy get verified.
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2019, Vol. 34 
