A decoupling strategy for average value control and ripple suppression of capacitor voltages in modular multilevel converters based on notch filters

Abstract

Modular multilevel converters (MMC) have become an emerging hotspot in the field of medium-voltage drives. A major challenge is the large voltage ripple of sub-module capacitors at low rotor speeds with high torque. Therefore, it is necessary to achieve ripple suppression while maintaining the average values of the capacitor voltages at their reference. However, the average value control and ripple suppression of the capacitor voltages have a coupling effect during the circulating current control loop, which reduces the performance. This paper proposes a decoupling strategy based on notch filters (NF). First, the coupling problem existing in the traditional control strategy is analysed by theoretical analysis and simulation. Then, the principle and parameter design method of the improved decoupling strategy are introduced. Finally, the performance of the decoupling strategy is experimentally verified using a small prototype.