A Critical Review of Modular Multilevel Converter Configurations and Submodule Topologies from DC Fault Blocking and Ride-Through Capabilities Viewpoints for HVDC Applications

Modular multilevel converters (MMCs) based on half-bridge submodules (HBSMs) are unable to prevent the AC side contribution to DC side fault currents, thus necessitating circuit breakers (CBs) for protection. A solution to this problem is using submodules (SMs) that are capable of blocking the flow of current from the AC grid to feed the DC side fault. The full-bridge submodule (FBSM) is one type of fault blocking SM where the presence of two extra switches ensures that in the event of a DC fault, the reverse voltage from the FBSM capacitor is placed in the path of the AC side current feeding the DC side fault through the antiparallel diodes. However, the additional semiconductor switches in the FBSMs increase the converter cost, complexity, and losses. Several SM configurations have been proposed in recent years that provide DC fault blocking capability with lower losses and device counts than those of FBSMs. Besides, many of the proposed hybrid converter configurations that combine different topologies to optimize converter performance are also capable of providing DC fault blocking. Furthermore, certain SM topologies are capable of riding through DC faults by remaining deblocked and operating in static synchronous compensator (STATCOM) mode to provide reactive power support to the AC grid. In this paper, noteworthy SM and MMC configurations capable of DC fault blocking and ride-through are reviewed and compared in terms of component requirements, semiconductor losses, and DC fault handing capability. The review also includes a discussion on control strategies for MMC arm/leg energy balancing during STATCOM operation.