Principle, control and comparison of modular multilevel converters (MMCs) with DC short circuit fault ride-through capability

Lack of DC short circuit fault blocking and ride through capability is one of main issues in applications of Modular Multilevel Converter (MMC) to actual HVDC transmission system. Recently, several topologies have been proposed to provide DC short circuit fault blocking capability and/or DC fault ride through capability. In this paper, the operation principles, functionalities, and characteristics of several topologies are compared. And, it's revealed that the conventional leg capacitor energy balancing strategy by regulating DC component of leg current fails under DC short circuit fault. A corresponding new leg capacitor energy balancing method by common mode voltage injection is proposed. Validity of the proposed method is verified by computer simulation.