Comprehensive understanding of DC pole-to-pole fault and its protection for modular multilevel converters

A modular multilevel converter (MMC) is one of the promising voltage source converter topologies in the field of high-voltage direct current (HVDC) transmission. MMCs are vulnerable to the direct current (DC) short-circuit faults due to the high discharge current stress. This study first introduced the simplified equivalent circuits of MMCs under both normal and DC fault operation conditions. Then the MMC fault spread loops and fault energy distribution principles are presented. Moreover, a detailed comparative study of the existing DC fault handling schemes, which are classified as the rigid blocking and flexible blocking methods are carried out. It provides a technical reference for further study on the MMC DC fault protection schemes for the HVDC transmission system.