This paper studies protection principles for a series multi-terminal DC system. The difference between conventional HVDC systems and series multi-terminal HVDC (MTDC) is analyzed from protection aspect. The problem of conventional protection principle applied in serial MTDC has been investigated. In order to supply main and back up protection with high reliability, high sensitivity and fast operation speed, a travelling wave protection solution based on the rate of changes in the local measured signals together with modified DC filter configuration is proposed in this paper which can be used as the main protection of the transmission line in series MTDC grid. Simulation on PSCAD model verifies the validity of the proposed solutions.