A Novel Directional Element for AC Lines in Systems with Inverter-Based Resources

The fault response of inverter-based resources (IBRs) is strongly influenced by their control strategies, which may significantly change the directional information available to line relays during non-ground faults. As a result, conventional directional elements designed according to the fault characteristics of conventional power systems may exhibit poor adaptability in IBR-connected systems. In particular, zero-sequence directional elements cannot be applied to non-ground faults, whereas negative-sequence-based schemes may be adversely affected by the fault-control behavior of IBRs. To address this problem, this paper proposes a directional element for non-ground faults on AC lines in systems with IBRs. First, the positive-sequence measured impedance at the relay locations is analyzed under typical fault conditions, and the dependence of available directional information on the phase characteristic of the IBR fault current is clarified. Then, a control-protection coordinated method is introduced to regulate the fault-current phase of the IBR during faults so that stable and consistent positive-sequence directional features can be established at both line terminals. On this basis, a unified directional criterion is formulated. Finally, PSCAD/EMTDC simulations are carried out to verify the proposed method, and dynamic model experiments are conducted to validate its engineering feasibility. The results show that the proposed element correctly identifies the fault direction under both three-phase and phase-to-phase fault conditions. Additional tests considering measurement noise and opposite-side grid-strength variation further demonstrate the robustness of the proposed criterion. Compared with conventional directional elements, the proposed method improves the adaptability of non-ground fault-direction identification in IBR-connected AC lines.