A Setting-less and Boundary-element-independent Directional Pilot Protection for Multi-terminal HVDC Systems

The application scenarios of HVDC systems are becoming increasingly diverse, featuring complex and variable topologies as well as boundary conditions. Existing single-ended protection methods for DC lines cannot ensure selectivity of operation in the absence of line boundaries. The pilot protection can identify faults without relying on line boundaries, but it generally depends heavily on precise system parameters or requires a cumbersome threshold value selection process. This study analyses the transient impedance phase-frequency characteristics of DC systems after faults under three typical scenarios, i.e., no boundary, DC reactor boundary, and DC filter boundary. Based on the polarity characteristic of high-frequency transient power during forward and backward faults, a novel directional pilot protection for DC lines is proposed. Compared to existing methods, this approach does not depend on system parameters or boundary elements, and eliminates the need for threshold setting, significantly reducing the complexity of engineering implementation and maintenance. Finally, a prototype of the fast directional pilot protection device is developed based on the proposed method, and extensive hardware-in-loop testing is conducted on the real time digital simulator (RTDS) platform to validate its operation performance.