Increasing penetration of inverter-based resources (IBRs) undermines the performance of conventional protection systems since IBRs’ fault characteristics are fundamentally different from those of synchronous generators. In this paper, a PMU-based backup protection method is proposed for power transmission systems with high penetrations of IBRs. The method involves replacing all IBRs and the line suspected to be faulty by proper nodal current sources. For accurately detecting the faulted line from the set of candidates, a residual-based index is proposed, utilizing the concept of superimposed circuits and the weighted least-squares method. The method's robustness in the face of influential factors, including input errors, settings, numbers, and locations of IBRs is investigated through extensive simulations on the IEEE 39-bus test system.@en