Efficient Capacitor Voltage Balancing Method for Modular Multilevel Converter Under Carrier-Phase-Shift Pulsewidth Modulation

The modular multilevel converter (MMC) is an attractive converter topology for medium-/high-voltage applications. The voltage balancing for floating capacitors of submodules in MMC under carrier-phase-shift pulsewidth modulation (CPS-PWM), is an important research focus. However, the existing capacitor voltage balancing methods under this modulation mode have limited robustness and may not be applied to different working conditions. In this article, the disturbance and unbalance mechanisms of capacitor voltages in MMC system under CPS-PWM are theoretically revealed and solved. The disturbance metrics for capacitor voltage are extracted by using a proposed asynchronized sampling mode. Based on the characteristics of the extracted disturbances in nonsynchronized sampling mode, a novel capacitor voltage balancing method is proposed. The proposed balancing method is highly efficient for different working conditions of MMC, and is easy to be designed/realized. Finally, a flexible mission-profile emulator for the test of MMC is designed and built, with comprehensive validations to verify the effectiveness of the proposed capacitor balancing method under different operating conditions of MMC.