Non-unit protection scheme for DC line using line mode fault impedance

• Proposes a non-unit protection scheme based on the root mean square of line-mode fault impedance derived from frequency-domain analysis. • Develops a rigorous quantitative framework to analyze line-mode fault characteristics using a frequency-dependent transmission line model • Demonstrates fast and reliable fault identification without communication or threshold tuning, with strong robustness to noise and fault resistance. • Validates the scheme on a four-terminal MMC-HVDC system, achieving accurate protection with low computational burden. To ensure the secure operation of MMC-based DC grids, this paper proposes a non-unit DC line protection scheme based on line-mode fault impedance. A quantitative analysis framework is developed using a frequency-dependent transmission line model to characterize the transient behavior of line-mode voltages and currents at the protection terminal. The analysis reveals that the line-mode fault impedance (LFI) reflects the transient equivalent impedance seen from the measurement point, and exhibits distinct dynamic characteristics for internal and external faults. Then the root mean square of the line-mode fault impedance (RLFI) is introduced as a robust fault detection criterion. The proposed scheme is validated in a four-terminal MMC-HVDC test system under various fault conditions, including different fault resistances, noise disturbance, and abnormal sampling. Simulation results show that the method enables fast and accurate fault identification with low computational complexity.