A single-ended fault location method based on active boundary control for multi-terminal flexible DC grids

In modular multilevel converter (MMC)-based multi-terminal flexible DC (MTDC) grids, fault current limiting suppresses post-fault electrical transients and weakens available fault characteristics, thereby reducing the sensitivity and reliability of conventional fault location methods. To address this issue, this paper proposes a single-ended fault location method based on active control and boundary self-validation. First, a coordinated MMC control framework integrating fault current limiting and characteristic-frequency signal injection is developed to construct controllable electrical features after fault occurrence. Then, an active boundary control strategy is proposed at the converter where fault model selection is required. By coordinating with a local directional criterion, different terminal responses are constructed for different fault regions, enabling invalid location results caused by incorrect fault model selection to be rejected. On this basis, frequency-domain fault location models are established for both pole-to-ground (PTG) and pole-to-pole (PTP) faults, with detailed discussion on parameter selection. The fault distance and transition resistance are identified through multi-frequency residual minimization, after which the relative residual variation is used to validate the location result and determine the final fault distance. Simulation results on a three terminal flexible DC system demonstrate that the proposed method can accurately locate PTG and PTP faults on different line sections under various fault distances and transition resistances, with fault location errors below 1%. The proposed method enables single-ended fault location without inter-station communication and can potentially serve as a backup protection function, thereby facilitating faster post-fault restoration in MTDC grids.