Abstract: Protection for DC transmission lines is one of the key problems that urgent to be solved in constructing high-voltage and large-capacity VSC-based DC grid in the future. Aiming at the prominent problem that the current communication technology is difficult to meet the requirements of real time transmission of high-speed sampling data proposed by traditional differential protection based on travelling-wave， a new differential protection scheme based on equivalent travelling-wave for VSC-based DC grid is presented by this paper. As a backup protection for primary protection based on single-terminal electrical quantities， a small amount of exchanged modulus maxima which can represent the critical information of the original fault-current travelling-wave are used to reconstruct the equivalent fault-current travelling-wave which can perfectly reproduce the original fault-current travelling-wave in this scheme. As a result， not only the communication data needed to construct the differential protection criterion is greatly reduced， but also the reliable and fast identification of fault location and different types of faults in the internal area could be realized. Finally， a four-terminal loop VSC-based DC grid model is built in PSCAD/EMTDC， and the differential protection scheme based on equivalent travelling-wave is simulated and verified.

Key words: VSC-based DC grid, equivalent travelling-wave, differential protection, fault identification, wavelet transform, backup protection