A comprehensive study of common mode voltage (CMV) reduction and neutral point (NP) potential balance for a back-to-back three-level (3L) NPC converter is proposed in this paper. Based on phase-disposition PWM (PDPWM), a zero-sequence voltage (ZSV) injection method is adopted. The ZSV is divided into two parts, ZSV for CMV reduction and ZSV for NP potential balance. The specific selection method is discussed in detail in this paper. After using the proposed ZSV injection method, the CMV can be reduced to 1/6 of the DC bus voltage, and the NP potential fluctuations can also be suppressed. Simulation and experimental results verify the effectiveness of the proposed method.

Modular multilevel converter (MMC) is an attractive topology for medium- and high-voltage applications due to the merits of high modularity, easy scalability, high-quality output voltage, and fault tolerance of submodules. This letter focuses on the control of a three-phase MMC under the single arm fault condition, which can further improve the fault tolerance ability of the MMC. According to three essential operating constraints of the MMC, this letter proposes a feasible arm voltage and current configuration method for the single arm failed MMC, which can guarantee its stable operation with reduced output power. Several operating limitations are also discussed. Experiment results based on a prototype with 4 half-bridge submodules per arm are presented to verify the proposed arm fault tolerance method.