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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.

This paper presents a single-ended fault location method for MMC-HVDC line. The method generates an active signal through converter control during fault ride through process. The fault location algorithm is based on parameter identification and distributed parameter line model. Simulation results has proved the high reliability and accuracy of the proposed method. The location can be calculated repeatedly and the accuracy of the location result is independent of higher sampling frequency. The impact of transition resistance can be eliminated with DC current control.

In AC/DC hybrid system, unsuccessful auto-reclosing in single-phase faults in AC side may cause continuous commutation failure, an adaptive single-phase reclosure scheme is proposed using the impedance property of the fault loop in transmission lines. The impedance is determined by the terminal-voltage of the fault phase and current of the ground capacitance. In transient faults with extinguished arc, the impedance is mainly determined by the ground capacitance of the fault phase. In permanent faults, it is mainly determined by the ground capacitance of the fault phase and transition resistance, which distincts from transient faults. The results show that the scheme is unaffected by fault location, line compensation and transition resistance and it is suitable for transmission lines with shunt reactors at one or both terminals.

In order to clarify the coupling relationship of fault information transmission characteristics in AC/DC hybrid system, a simplified model of DC system with electrical quantity transmission characteristics is established. Based on the switching function transfer theory, the law of fault information transmission on AC and DC sides is analyzed. On this basis, the dynamic phasor theory is introduced. Considering the steady-state, symmetrical and asymmetrical fault conditions, a simplified model of DC system is established based on the equivalent impedance of DC system and the control system. Combined with the simulation verification, the accuracy of the simplified model is verified by comparing with the detailed model. According to the general law of fault information transmission characteristics, the theoretical basis is provided for the follow-up research on the backup protection principle of AC/DC hybrid system.

Modular Multilevel Converter Based High Voltage Direct Current (MMC-HVDC) technology has been developed rapidly. However, the fault of DC line is the key problem of the safe operation of MMC-HVDC transmission system. In engineering practice, series small inductance is installed at both ends of MMC-HVDC transmission line, which creates favorable conditions to solve the problem of DC line fault in MMC-HVDC transmission system. In this paper, characteristics of various short-circuit faults that may occur on the DC line of MMC-HVDC system are analyzed firstly. According to analysis result, it is obtained that the polarity of the series inductance voltage is differrent and the change rate of the fault component of the series inductance voltage is also different when fault occurs on the DC line or not. Simulation results showed that the proposed protection principle was correct in PSCAD and MATLAB.

Current differential protection based on Kirchhoff's law has sensitivity problem due to the long-distance DC transmission lines with distributed parameter effect. In this paper, the charge of the DC transmission line under the situations of external and internal faults is analyzed. The feature is that the charge will be injected into the fault point from both sides of the DC line when the internal faults occur, and if the external faults occur, the distributed capacitance of the DC line will be discharged. Then a pilot protection principle based on charge balance is proposed using the polarity of the sum of the charge in a time period. The proposed principle is simple and independent of the parameters of the DC line, and it theoretically suits for both transient process and steady-state process. Finally, an MMC-HVDC transmission system is built in PSCAD/EMTDC, and the simulation results show that the method is reliable and not affected by fault resistance.