Impact of Grid-Forming Inverters on Protective Relays: A Perspective for Current Limiting Control Design

Grid-forming (GFM) inverters can significantly alter the fault characteristics of power systems, posing challenges to the proper function of protective relays for transmission lines. This article gives a holistic analysis of the interaction between GFM inverter-based resources (IBRs) and the supervising elements in protective relays, including directional and phase selection elements. It is revealed that the current limiting control (CLC) that is based on the current reference saturation method adversely affects the performance of supervising elements that rely on the negative-sequence quantities. In contrast, adopting highly inductive virtual impedance in the CLC enables a reliable operation of such elements. This finding provides insights into the design of CLC for GFM IBRs from a protection perspective. It is further found that even with a highly inductive virtual impedance, the altered virtual impedance dynamics introduced by the CLC can still lead to malfunctions of the incremental quantity-based supervising elements. These theoretical findings are corroborated by simulations and controller hardware-in-the-loop (CHIL) tests.