Abstract:The design and analysis of modular multilevel converter (MMC) is usually carried out under the condition of power grid balance, but the AC power grid is unbalanced in actual operation. The imbalance of the power grid will cause the DC components of the circulating current of the MMC bridge arm to be no longer equal, including the positive sequence, negative sequence, and zero sequence components of the double frequency, which will increase the system loss and affect the system performance. In this paper, based on the mathematical model of MMC and the instantaneous power of phase units, the generation mechanism of circulating current was analyzed, and the equivalent circuit of each component of circulating current was obtained when the power grid is unbalanced. Then, a circulating current suppression strategy based on second order generalized integrator (SOGI) was proposed under unbalanced power grid. This strategy can suppress the second harmonic component of three-phase circulating current and the unequal DC component separately. Based on SOGI, the proposed strategy uses proportional integral (PI) control to extract the positive sequence and negative sequence second harmonic components of bridge arm circulating current. Meanwhile, quasi proportional resonance (PR) is applied to control the zero sequence second harmonic component of the circulation. Finally, a 217 level MMC system model was built in PSCAD/EMTDC platform, and through simulation experiments, the circulation suppression strategy proposed in this paper was compared with three typical strategies, i.e. the traditional circulation suppression strategy, the circulation strategy controlled by quasi proportional resonator and the circulation suppression strategy controlled by proportional integral resonator (PIR) . The experimental results showed that in the case of single-phase non-metallic ground fault, compared with the other three circulating current suppression strategies, the proposed strategy can suppress the second harmonic component of circulating current to 0.000 6 kA and reduce the harmonic distortion rate to 0.03%, which verifies its superiority. Furthermore, in the case of two-phase non-metallic ground fault, the proposed strategy can suppress the second harmonic component of the circulating current to 0.003 9 kA and reduce the harmonic distortion rate to 0.22%. The simulation results verify the effectiveness of this method.