Unified Directional Element for Transmission Networks Considering Various Controllers of Inverter-Based Resources

Fault currents contributed by inverter-based resources (IBRs) differ fundamentally from those of conventional sources. As a result, many traditional protection functions, such as directional elements, may fail to operate correctly when faults are predominantly supplied by these IBRs. This paper analyzes the relative angle between positive-sequence voltage and current under symmetrical faults and evaluates its variation at both near-end and far-end buses. In addition, the relative angle between negative-sequence voltage and current is analytically derived for fault currents supplied by different IBR control strategies. Based on this analysis, a unified directional element is designed that operates reliably for both symmetric and asymmetric faults, across a wide range of fault locations and resistances, and independently of the IBR controller employed. By addressing the unique challenges presented by IBRs, this research aims to enhance the reliability and effectiveness of protection systems in modern power grids. Time-domain simulations demonstrate the accuracy of the proposed directional element in correctly identifying fault directions across a wide range of fault locations and resistances, as well as under different IBR control strategies.