A Single‐Ended Protection Method for Multi‐Terminal Flexible DC Grids Using the Current‐Limiting Reactor Voltage Energy Spectrum Matrix

Fast and accurate fault identification is crucial for the safe and stable operation of multi‐terminal flexible DC grids. Existing protection methods for flexible DC grids are sensitive to transition resistances, with limited engineering adaptability, making it difficult to balance speed and reliability. To address this issue, this paper proposes a single‐ended protection method based on the voltage energy spectrum matrix of the current‐limiting reactor. This method uses the difference of high‐frequency components of voltage on both sides of the current‐limiting reactor at the initial stage of fault, with the energy spectrum matrix similarity as the triggering criterion. Combined with the damping effect of the reactor, a fault criterion based on the high‐frequency and low‐frequency energy ratio is developed for both internal and external faults. The paper first analyses the voltage transient evolution of internal and external faults under the damping effect of the current‐limiting reactor. It then studies the time‐frequency characteristics of voltage signals under different operating conditions and, based on this, constructs a complete protection scheme. Finally, a multi‐terminal flexible DC system model is built in PSCAD/EMTDC for simulation verification. The results show that high‐frequency energy quickly concentrates at the early fault stage. The spectrum matrix is rapidly reconstructed, and the similarity between the two sides drops by more than 20%, which enables protecting fast starting. The high‐frequency energy ratio shows a clear difference between internal and external faults (more than 49%). The method can complete fault identification within 3 ms after fault inception. It remains reliable under a 300 Ω transition resistance and 30 dB noise. In addition, it is not sensitive to changes in the reactor inductance and can also effectively identify lightning disturbances. Therefore, it is suitable for fast and high‐reliability protection of long‐distance multi‐terminal flexible DC grids.