A multi-vendor protection strategy for HVDC grids based on low-speed DC circuit breakers

A protection strategy for Multi-terminal HVDC (MTDC) grids based on AC/DC converters without fault handling 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 (overhead line or cable) end, all of them based on low-speed mechanical DC breaker technology with no fault current limiting devices. The primary protection sequence includes steps of fault detection, AC sources contribution suppression, faulty transmission line discrimination and isolation, and power flow restoration. A back-up sequence is also proposed in case of breaker operation failure which is able to be performed in the same time duration than the primary protection sequence. The design requirements of the DC breakers have been verified and are consistent with both the primary and back-up sequences. The concept of the proposed strategy and its associated key components are described in the paper and validated by off-line simulations performed in EMTP-RV®.