Improved Circulation Suppression Strategy for MMC Considering Low Frequency Oscillation

When the modular multilevel converter works normally, there will be circulating currents between the bridge arms, and low-frequency oscillation circulating currents will occur under abnormal working conditions. A mathematical model of the MMC circulation is established, and the main frequencies of the circulation are derived. In order to better suppress the bridge arm circulation in the MMC, an improved circulation suppression strategy is proposed, which has a simple implementation principle and does not require phase decoupling links. The feasibility of improving the circulation suppression control strategy was verified on the RT-ALB simulation test platform. The simulation results show that the control strategy can simultaneously suppress the double-frequency circulation and the transient low-frequency oscillation circulation under abnormal conditions, and ensure the normal and stable operation of the MMC.