A protection strategy for multi-terminal hvdc grids based on mechanical dc circuit breakers

A protection strategy for multi-terminal HVDC (MTDC) grids based on AC/DC converters without fault blocking capability such as half-bridge MMC converters is presented in this paper. The key components of the proposed strategy consist of DC breakers located at each converter station DC side and at each transmission line end, all of them based on low-speed mechanical DC breaker technology with no fault current limiting devices. A primary protection sequence as well as a back-up sequence are developed and are presented. The proposed protection strategy is compared to a protection strategy using the same philosophy and based on IGBT-CBs and Fast DC switches. Off-line simulations of both protection strategies are performed in EMTP-RV® and results are depicted and compared. Advantages and features of the proposed protection strategy are described.