Analysis of the Inner Unbalanced Current for Modular Multilevel Converter

Circulating components of the inner unbalanced current for modular multilevel converter will increase the power loss of the converter and reduce the utilization of the bridge arm capacities inevitably. Even worse, the distortion of the bridge arm current will be intensified, which will endanger the safe operation of the system. Among of the inner unbalanced current, the zero-sequence components cannot be offset by each other, resulting in DC-side voltage and the power injected into AC side fluctuation. Therefore, it is necessary to analyze the components of the inner unbalanced current so as to take effective control measures to suppress the circulating current, correspondingly. This paper presents the detailed components of the inner unbalanced current by means of the more accurate phase-unit instantaneous power analysis under different grid status. Moreover, it is pointed that, the output voltage of double-line frequency circulating current suppressing controller will result in quadruple-line frequency components emerging in the inner unbalanced current. A detailed three-phase MMC model is established in PSCAD/EMTDC simulation environment and the correctness of the proposed points gets verified.