Aiming at the hybrid bipolar DC transmission system, combined with the principle of model identification and current polarity protection, a new principle of longitudinal protection of transmission lines is proposed. This paper analyzes the fault network and finds :when the front end of the inverter side fails, the fault model is equivalent to the C model, and when the back side of the inverter side fails, the fault model is equivalent to the RL model; when the front end of the rectification side fails, the rectification side fault current has a positive polarity, and when the rectification side back side fails, the rectification side fault current has a negative polarity. In conclusion, a failure in the area, namely the inverter side front and rectifier side front fault at the same time, the fault model matching C model, there was a positive polarity fault current at the same time; Outside of a failure, namely the inverter side front and rectifier side front no fault at the same time, inverter side failure model does not match the C model, or rectifier side fault current with a negative polarity. This distinguishes between external and internal faults in the DC line. The simulation results show that the new principle can compensate for the main protection as an improvement of the backup protection.