Research on Single-Ended Protection for Multi-Terminal Flexible DC Lines Based on Line-Mode Reverse Traveling-Wave Covariance

Multi-terminal HVDC transmission lines contain a T-zone, where no explicit line boundary element exists. Conventional traveling-wave (TW) protection schemes mainly identify faults by utilizing the attenuation characteristics of TWs caused by transmission lines and boundary elements. Therefore, their performance is sensitive to transition resistance and sampling frequency, and they cannot be directly applied to fault identification in the T-zone. To address this issue, this paper proposes a single-ended directional protection scheme based on the normalized covariance of line-mode backward TWs. The proposed method constructs the protection criterion according to the overall waveform variation in line-mode backward TWs. It does not rely on boundary-effect-induced wave attenuation or the extraction of specific high- or low-frequency components. Fault regions are identified using only local measurements collected by the protection devices installed on both sides of the T-zone, and the fault pole is determined by the bipolar voltage ratio. A ±500 kV three-terminal MMC-based multi-terminal DC system is established in PSCAD to evaluate the proposed scheme. Simulation results show that the proposed method identifies the fault region within a 0.5 ms data window. It can tolerate transition resistances up to 300 Ω and 40 dB Gaussian white noise. The proposed scheme features fast operation, low computational complexity, and high robustness.