A modular multilevel converter (MMC) is one of the promising voltage source converter topologies in the field of highvoltage 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.

This paper proposes a super capacitor energy storage-based modular multilevel converter (SCES-MMC) for mine hoist application. Different from the conventional MMCs, the sub-modules employ distributed super capacitor banks, which are designed to absorb the regenerative energy of mine hoist and released in the traction condition, so as to improve energy utilization efficiency. The key control technologies are introduced in detail, followed by analysis of the configuration and operation principles. The feasibility of the proposed SCES-MMC topology and the control theory are also verified. Simulation results show that SCES-MMC can adapt to the variable frequency speed regulation of the motor drive, which shows good application prospects in the future for mediumand high-voltage mine hoist systems.

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