DC line protection is one of the key problems in the development of multi-terminal flexible DC power grid. This paper proposes a pilot protection method based on included angle cosine of fault current component to solve the existing problems of the pilot current differential protection. The included angle cosine is calculated by using the fault current component at both terminals of the line to identify internal and external faults. When internal faults occur

the directions of fault current component are opposite

and the included angle cosine is negative. When external faults occur

the directions of fault current component are same

and the included angle cosine is positive. In the proposed protection method

the voltage gradient is used to rapidly detect faults

and the ratio of the voltages of positive pole and negative pole is adopted to identify the fault pole. The simulation results show that the proposed protection principle can identify internal and external faults reliably. In addition

the proposed method is able to respond to faults with high transition resistance and is independent of distributed capacitive current.