Voltage source converter-based multi-terminal DC transmission (VSC-MTDC) system has long-term development prospects, but its protection scheme has poor anti-interference ability. A double-ended protection principle with strong anti-interference ability is given. First, virtual measured wave impedance expression is obtained through equivalent circuit of VSC-MTDC system, and its amplitude characteristics are analyzed accordingly. Then, changes in virtual measurement wave impedance (VMWI) are obtained by analyzing reflection coefficient. Finally, Euclidean distance with weighted is introduced to characterize fault. Theoretical analysis and simulation verify anti-interference ability of the proposed scheme. Results show the proposed scheme has strong anti-interference ability. Results indicate the scheme enhances anti-interference ability and applicable scope of existing scheme.

Hybrid DC transmission technology is used in new energy transmission due to its advantages of large capacity and long distances. Line-commutated converter-voltage source converter-multi-terminal DC (LCC-VSC-MTDC) is one of the more important development directions in future power grids. But the multi-terminal high voltage direct current system has problems of inconsistent boundary characteristics, inconsistent control, and fault response characteristics, which places higher requirements on the protection scheme. A protection scheme based on the undistorted factor is proposed to ensure the safe operation of the system. First, the inconsistent line mode voltage attenuation characteristics of different fault locations are used to construct the undistorted factor. Subsequently, the undistorted factor is used to identify fault. The advantage of this scheme is that it remains unaffected by boundary elements, control strategies, and distributed capacitance. Finally, the LCC-VSC-MTDC model verifies the correctness and superiority of the proposed protection scheme.

Due to the advantages of LCC-VSC (line-commutated converter-voltage source converter) three-terminal DC technology in terms of new energy consumption, long-distance transmission, reliability and economy, China has successively constructed a number of hybrid DC transmission projects. However, there is very little research on the protection principles of this topology. And there are problems, such as different control strategies, different line boundary elements, and different operating time requirements. A technology based on the Gini impurity of line mode voltage is proposed as the main protection scheme. This protection principle uses Gini impurity to describe the degree of confusion of fault information caused by boundary elements and further identifies internal and external faults. Finally, different faults are set to verify the reliability and superiority of the proposed protection scheme. A large number of results show that the protection scheme based on Gini impurity can identify the fault type within 1ms under an interference of 600 $\mathrm{\Omega }$ resistance and 10dB noise.

Multi-terminal direct current based on a modular multilevel converter (MMC-MTDC) has received more attention due to the advantages of large capacity and high flexibility. However, the current traveling wave protection schemes adopted by the MMC-MTDC DC project are not sufficiently sensitive to high-impedance faults. To solve this problem, this paper first analyzes the characteristics of the first traveling wave of the fault on the DC transmission line and derives the expressions of the first traveling wave of the fault when the DC transmission line has an internal and external fault. Subsequently, the line frequency characteristic is used to simplify the traveling wave expression and construct the fault coefficient. Finally, the principle of single-ended traveling wave protection based on the fault coefficient is proposed. Simulation results show that the protection principle has a strong ability to withstand transition resistance and noise interference.