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A circulating current suppressing controller (CCSC) is presented to eliminate the inner balancing currents in modular multilevel converter (MMC). Based on the double-fundamental-frequency, negative-sequence rotational frame, the three phase alternative circulating currents are decomposed to two DC components, and are limited to a low level by a pair of proportional integral controllers. This control strategy does not affect the MMC outer dynamics in the ac side. Simulation results based on PSCAD/EMTDC demonstrated the circulating current suppressing effect of the proposed CCSC and proved that the distortion of the arm current is mitigated significantly.
Tu et al. (Mon,) studied this question.