MMC Circulating Current Suppression Strategy Under Unbalanced Power Grid

When a modular multilevel converter (MMC) operates under unbalanced grid voltage conditions, it generates positive-sequence, negative-sequence, and zero-sequence components in the arm currents, which increase system losses and disrupt normal operation. To tackle this problem, this paper introduces a method that employs a second-order generalized integrator (SOGI) to extract the second- and fourth-frequency components from the arm currents. Subsequently, an enhanced quasi-proportional integral resonant (PIR) controller is applied to individually suppress these circulating currents. The approach is validated through MATLAB simulations. The results demonstrate that the proposed suppression method quickly reduces the secondand fourth-frequency circulating currents and significantly minimizes submodule voltage fluctuations, confirming its effectiveness under unbalanced grid conditions.