Fault Mechanism Analysis and One-terminal Protection Method of Flexible DC Transmission Line under the Influence of Multi-type Faults

Multi-terminal flexible DC transmission line protection faces various fault types and is susceptible to factors such as transition resistance and noise. To address this, a fast-action protection method for the entire flexible DC line using single-ended voltage gradient energy values is proposed, which can protect the entire length of the line based solely on single-ended measured voltage. The voltage fault characteristics inside and outside the flexible DC line are analyzed, and an improved voltage gradient algorithm is introduced to construct a protection initiation criterion. Furthermore, a full-line fault protection criterion based on energy value comparison is constructed, and fault pole selection is achieved based on the different ranges of positive and negative energy ratios under unipolar or inter-pole faults. Simulation verification shows that the proposed method can quickly identify faults inside and outside the zone, determine the fault pole, and exhibits good noise immunity and high resistance tolerance.