A Solution for Voltage Balancing in MMCs Through a Single-Cell Injection Method

The modular multilevel converter (MMC) has become a promising topology for high and medium-voltage applications in recent years. However, its operation poses technical challenges in implementing the control system, including balancing submodule (SM) capacitor voltages and suppressing circulating currents. The imbalance among the SM capacitor voltages that is caused by the active power flow in the MMC must be mitigated to ensure its operation. In addition, the presence of circulating currents leads to additional power losses, increases thermal stress on the power devices, and shortens their lifetime. This article introduces a closed-loop control strategy for the MMC to balance the voltage across the SMs simultaneously, limit capacitor voltage ripple, and reduce circulating currents by injecting a low-frequency alternating signal in a rotating manner to a SM of each converter arm. The primary advantage of the proposed solution is that it achieves all the stated objectives while requiring minimal implementation effort compared to existing alternatives. The results of experimental tests on a five-level MMC are presented to prove the effectiveness of the proposed technique and compare its performance with existing approaches.