Abstract: DC protection is essential for the safety of multi-terminal DC (MTDC) transmission grid. Traveling wave protection is fast enough to satisfy the speed requirement of DC protection, however, has defects of high sampling rate, low reliability and low sensibility when line boundary is limited. In this paper, the new regional directional pilot protection scheme is proposed, in which extended boundaries consisting of the converter and the reactors at each line of converter are used. Firstly, the fault characteristics of voltage on both sides of extended boundaries are analyzed. The different fault characteristics between backward and forward faults caused by the obviously attenuation characteristics of extended boundaries on voltage time domain waveform are used to realize the fault identification, in which the directional pilot protection is developed. Busbar fault identification can also be realized. In addition, a regional protection scheme is formed by coordination between adjacent protections. Then the influence of multiple reflected and refracted traveling wave on the fault characteristic is analyzed, and the protection operation time is settled. Finally, simulations and comparisons are performed. The proposed scheme has a characteristic of low requirements for sampling rate, and has a high sensibility and reliability under large fault resistance, strong noise, and relatively low reactor.