A pilot protection scheme for MMC-HVDC grids independent of line boundaries based on fault component integration

• Analysis of transfer functions reveals opposing polarity in faulted and healthy lines. • A direction criterion utilizing the accumulated line-mode components is introduced. • A pilot scheme is designed to eliminate dependency on CLR and enhance reliability. • Simulation results on an MMC-HVDC grid validate the method’s effectiveness. Implementing pilot protection schemes for high-impedance fault identification in MMC-HVDC grids can significantly enhance system reliability. Current protection strategies predominantly depend on accurately identifying fault-traveling wave arrival times, clearly defining the line protection boundary, and utilizing singular points in signals at specific instants. This paper investigates current limiting reactor configurations at line versus MMC branches, developing equivalent circuits for post-fault analysis in MMC-HVDC grids. Analysis of the transfer functions at different positions reveals that the faulted line and healthy lines connected to the common DC bus exhibit opposing current polarity during faults. This feature utilizes the grid’s topological constraints and is theoretically unaffected by line boundary elements and fault conditions. To mitigate the impact of multiple reflections, fault resistance, and noise environment, this paper expands on characteristic differences inside and outside the fault area and designs a pilot protection scheme by integrating the line-mode fault current, which can serve as a supplement or backup protection to the main protection. A four-terminal mesh MMC-HVDC grid is built in PSCAD/EMTDC. The simulation results demonstrate that the proposed scheme effectively performs protection across various fault conditions, system parameters, and noise environments.