In order to prevent a transmission line boundary having a weak high frequency from impairing single-ended transient-based relay protection, this paper proposes using the coordination of the current frequency domain features of the relay on both sides of the transmission line to achieve whole-line quick-action protection. In the case of an internal fault, since the frequency components of the electrical quantities in the frequency domain on both sides of the protected line are related to the distance from the fault point to two sides of the line, the frequency-domain current spectrum information on both sides of the line is very different and the fluctuation is strong. In the case of an external fault, the transient current at the protection point closer to the fault is transmitted through the entire length of the protected line and reaches the other side away from the fault. Therefore, the fluctuation in the current spectrum in the frequency domain on both sides is quite different. Thus, this paper proposes setting the fault criterion according to the difference in the frequency spectrum fluctuation degree of the frequency-domain current ratio on both sides when internal and external faults occur. After verification using PSCAD simulation, it was found that the protection principle proposed in this article can act quickly and correctly, and identify internal and external faults, under different fault types and fault locations, and is basically not affected by transition resistance.

A subsequent commutation failure (SCF) of the Line-Commutated Converter–High-Voltage Direct Current (LCC-HVDC) may occur during the recovery after the clearance of an AC fault, seriously threatening the safe and stable operation of the LCC-HVDC and the entire post-fault AC/DC hybrid power system. In this study, the mechanism of an SCF as affected by the transient stability was analyzed, and a DC power recovery speed control strategy is proposed as an additional form of control to prevent the occurrence of SCFs. First, the sending-end and receiving-end power systems were modeled as synchronous generators instead of ideal voltage sources, and the mechanism of an SCF as affected by the transient stability was analyzed and verified. Second, the ramp function was adopted to describe the recovery characteristic of DC power, and a model of its recovery was established. Then, the recovery speed control strategy is presented based on the mechanism analysis of SCFs, which can not only be used to avoid the occurrence of SCFs but also increase the transient stability margin of the sending-end power system. Finally, the effectiveness and robustness of the proposed control strategy were validated by using a hybrid electromechanical–electromagnetic model and the full electromagnetic model of the IEEE 39-bus asynchronous interconnection test power system. With the implementation of the proposed control strategy, the safe and stable operation of the LCC-HVDC and AC/DC hybrid power system can be guaranteed. The adaptive DC power recovery speed control strategy will be further investigated in future research work.