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Modular multilevel converter (MMC) has been widely used in VSC-HVDC system for its smooth output waveform, low harmonic content and low switching loss. But compared with the conventional LCC-HVDC system, modular multilevel converter has a short development history, few engineering applications, high failure rate and lacks of very effective protection strategies, which limits its further development. Therefore, this paper firstly introduces topological structure of MMC and its working principle. Then the commonly used control policy of dual-terminal MMC-HVDC project is elaborated. Finally, the transient electrical characteristics of the typical faults are detailedly analyzed through the combination of theoretical analysis and simulation, such as three phase short circuit of converter valve, single phase grounding of converter valve, and the bridge arm short circuit. The conclusion provides an essential theory for the research of the converter protection strategy in the MMC-HVDC project.

HVDC converter transformer oil-flow relay plays an important role in realizing non-power protection of converter transformer on-load tap changer, which can act accurately and directly affect the safe and stable operation of the whole HVDC program as well as the converter transformer itself. In this paper, the research revolves the hidden dangers of HVDC converter transformer and its identification. The paper begins with the analysis of the converter transformer operating principles of two German companies: MR and EMB. Then, the current application status and related inspection requirements of oil-flow relay are expounded. Finally, pertinent hidden danger identification and modification schemes are provided based on analysis of the danger in actual operation of oil-flow relay and its causes. The feasibility and effectiveness of the scheme are verified by the actual operation on site.