DC fault protection algorithms of MMC HVDC grids: fault analysis, methodologies, experimental validations and future trends

To protect the converters and minimize the power transmission interruption during dc line faults, it is necessary to detect the faults at ultra high speed for the modular multilevel converter (MMC)-based HVdc grids. This article reviews the state-of-the-art of dc fault protection methods of MMC-HVdc grids, and summarizes the underlying principles of each method. On this basis, the dc fault characteristics analysis in terms of modal-domain, time-domain, and frequency-domain are analyzed, which direct the protection design. Typical boundary protection and nonboundary protection schemes are reviewed. The advantages and disadvantages of existing fault protection methods are compared. A two-terminal MMC-HVdc prototype is developed to test the effectiveness of three fault protection methods. Comprehensive quantitative assessments of the protection methods discussed above are carried out in a four-terminal MMC-HVdc grid. Finally, the future trends of the protection schemes are discussed and the findings are concluded.