In order to improve the reliability of DC line protection, it is necessary to study the principle of back-up protection，which can distinguish transient and steady process after fault. Principle of fault whole-time Differential protection based on polarity characteristics of current fault component difference is proposed. By analyzing the characteristics of the current fault components in transient-state (current fault components in the non-response state, the dynamic regulation state) and the steady-state of the fault, during the whole process of the fault, it is found that the fault component difference between rectifier side and inverter side satisfies the positive polarity characteristics when internal fault occurs. But it is not satisfied when external fault occurs. The protection principle criteria satisfying the characteristics of the whole process of the fault is established to realize the protection action. The simulation results show that the principle can operate correctly under various working conditions, and is less affected by the harmonics of fault characteristics of AC system. It has strong resistance to transition resistance and anti-interference ability.

DC line, whole process of fault, back-up protection, fault component