Analysis of the Impact of Inverter-Based Resources on Distance Relay During Balanced Faults

The increasing penetration of inverter-based resources (IBRs) raises concerns about the impact of reduced short-circuit capacity on transmission-line protection. In Japan, fault ride-through requirements are specified, but the fault-current characteristics of commercially deployed three-phase photovoltaic inverters have not been sufficiently clarified. This paper first evaluates the fault-current characteristics of seven three-phase photovoltaic inverters used in Japan through balanced-fault voltage-step tests. Measured instantaneous voltage and current signals are processed with a relay-based RMS algorithm to obtain voltage magnitude and active/reactive current components, thereby characterizing the magnitude and response speed of the supplied fault current. The results show clear differences among inverter types. The paper then analyzes, using a separate test configuration representing an IBR connected to a tapped branch of a transmission line, the effect of these characteristics on the apparent impedance seen by a grid-side distance relay. The resistance component of the apparent impedance generally becomes smaller than the true line impedance, whereas the reactance component can become either larger or smaller depending on inverter type, system short-circuit capacity, and inverter power factor. These findings indicate that distance-relay settings should take inverter-specific fault-current characteristics into account, with attention to both overreach and underreach.