Common-Mode Voltage Suppression Method for MMC under Submodule Unified Pulse Width Modulation

Submodule unified pulse width modulation (SUPWM) is attractive for modular multilevel converter (MMC) because of better output harmonic characteristics and fewer comparison units. However, there are also drawbacks to SUPWM that come to light. When the conventional SUPWM is employed in the MMC, the discontinuous switching action of the three-phase submodules will result in a large common-mode voltage on the AC side of the MMC. The common-mode voltage can produce serious electromagnetic interference. In addition, in the field of AC drive applications, the common-mode voltage can also damage the insulation of the motor windings and increase the mechanical wear of the bearings. Therefore, the common-mode voltage in SUPWM should not be neglected. To solve the problem, a phase-shifting control method is proposed. The proposed method can eliminate the common-mode voltage generated by SUPWM by changing the phase of the triangular carriers. The effectiveness of the proposed method is verified by Matlab/Simulink simulation.