Fast, sensitive, and selective protection principles are one of the major challenges in the feasibility of modular multi-terminal (MMC) high voltage direct current (HVDC) grids. Rate of change of voltage (ROCOV) and transient-based solutions are the traditional and widely accepted protection principles. Despite the speed and practicality of these solutions, they generally suffer from sensitivity and selectivity issues, particularly when dealing with high-resistance faults and low-size current limiting inductors (CLIs). To improve upon these methods, this paper proposes a new primary protection method that utilizes a selective drop rate of fault-generated voltage traveling waves (TW) to detect internal DC line faults. This is achieved by a comprehensive analysis of the line-mode fault-generated voltage (LFGV) under various internal and external fault scenarios. As the key fault characteristics, the proposed method exploits the minimum points of initial LFGV and the corresponding time to form the basis of the proposed protection method. The effectiveness of this approach is evaluated using a four-terminal MMC-HVDC grid in PSCAD/EMTDC. Compared to ROCOV and transient-based solutions, the proposed method identifies internal faults up to 1250 Ω with fast response, while maintaining its practicality and independence to CLI size.

Wavelet transform has proven to be a capable tool for protection purposes in high voltage direct current (HVDC) transmission lines due to its desired speed and accuracy. However, the need to enhance the WT-based protection methods in terms of sensitivity and selectivity is of interest. This paper proposes a new non-unit WT-based protection method with adaptive threshold setting. According to the improved time-domain analytical approach, line-mode fault-generated voltage traveling wave is adopted to identify the internal faults. The simulation results for a multi-terminal modular multilevel converter-based HVDC grid in PSCAD/EMTDC corroborate accurate and fast internal faults detection of the proposed method, up to 850 Ω, i.e., almost three times larger than conventional schemes. In addition, the reliable performance of the presented method in a noisy environment, using relatively low sampling frequencies, and different sizes of current limiting inductors is demonstrated in the presented analysis. The generality of the presented analytical approach ensures that the proposed protection method can be extended to more complex HVDC grids.