Common-Mode Voltage of Back-to-Back Hybrid Modular Multilevel Converter under Third-Harmonic Injection

Modular multilevel converter (MMC) has been increasingly attractive in the medium voltage motor drives, while the limitation of its development is the large capacitor ripple at low motor speed. To solve this problem, hybrid modular multilevel converter (HMMC) topology was proposed, and this topology was extended to back-to-back HMMC (BTB-HMMC) topology for four-quadrant application. However, the BTB-HMMC topology would introduce high common-mode voltage on the motor, which is a great challenge to the insulation of the motor. This paper analyzes the common-mode voltage generated by the BTB-HMMC under the third harmonic injection. An improved BTB-HMMC topology is introduced to reduce the common-mode voltage. Moreover, the capacitor voltage ripple is analyzed under the third harmonic injection. Simulation results are included in this paper to verify the analyses.