Protection System of Multi-Terminal MMC-based HVDC Grids: A Survey

Fast development of High Voltage Direct Current (HVDC) technology has been remarkable over the past decades. Multi-terminal HVDC grids based on Voltage Source Converters (VSC) are promising solution to connect multiple asynchronous power grids, as well as integrate large-scale renewable energy sources e.g. offshore wind farms. New generation of the VSCs, Modular Multilevel Converter (MMC), offers considerable benefits. However, protection has always been a great challenge for the VSC-HVDC transmission systems, in particular, for the multi-terminal DC grids. This paper presents an insightful survey considering protection strategies for MMC-HVDC grids. The study sheds light on state-of-the-art of protection scheme including fault identification and interruption methods for the MMC-HVDC systems. Moreover, a short discussion regarding HVDC grid topologies and control has been presented to investigate their impacts on protection and security of the VSC-HVDC grids. Investigation of Fault-Ride-Through (FRT) capability is vital to be included in protection coordination strategies under DC fault conditions. Possibility of having different DC FRT capabilities for different structures of the MMCs is investigated. In addition, pros and cons of protection strategies are presented and discussed comprehensively.