Abstract: In the photovoltaic medium-voltage DC collection system, the short circuit fault and overcurrent force the converter to quickly lock, and cause the entire system to stop. To solve this problem, this paper realizes the identification of fault lines through the incipient fault lines. However, there is no clear boundary between incipient faults and insulation defects, so it is difficult to set the threshold value. Therefore, this paper proposes a method for incipient fault diagnosis of transmission lines based on the mapping relationship between carbonization channel resistance and temperature. The experimental fitting model of incipient fault is established, and the dielectric carbonization channel resistance is selected as the fault criterion. Considering the system voltage control constraints, the mapping relationship between the cable interlayer temperature difference and the carbonization channel resistance is analyzed to determine the initial boundary of the incipient fault and solve the problem of the unclear boundary between the incipient fault and the insulation defect. The simulation results show that the proposed method can accurately calculate the resistance value of the dielectric carbonization channel, identify and isolate the fault line, and has a specific anti-noise ability, which guarantees the continuous and safe operation of the flexible DC system.