A Software-Based Fault-Tolerant Strategy for Modular Multilevel Converter Using DC Bus Voltage Control

A software-based method is proposed in this article to enhance the fault-tolerant capability of modular multilevel converters (MMCs). In the proposed approach, dc bus voltage is considered as a controllable parameter in a way to maximize the line-to-line voltage amplitude at the postfault condition. In comparison with the previous works, the proposed method maintains a larger line-to-line voltage in various fault states. Several drawbacks such as high amount of common-mode voltage, power-factor limitations, and electromagnetic interference problems existing in the previous works are alleviated in the proposed method. Another advantage of the proposed method is that the line-to-neutral voltages of the MMC remain balanced in the postfault condition. This feature makes it possible to utilize third-harmonic injection technique to maintain larger line-to-line voltage after the fault occurrence in the MMC. Moreover, the proposed method can be applied to both three-phase and single-phase systems. A general formulation of the proposed method is derived, and at the end, comparative simulation and experimental results are provided to validate the effectiveness of the proposed method.