Fault behaviors of inverter-interfaced renewable energy sources (IIRESs) are primarily governed by the inverter control strategy. Uncertainties introduced by control response and network topology change pose challenges to the protection design of islanded microgrids. First, this paper establishes equivalent positive-sequence fault component (PSFC) models of IIRESs with and without activation of fault ride through (FRT) control. Their equivalent impedance angles are then analyzed under high- and low-resistance faults, revealing the dominant differences in fault direction features between forward and reverse faults. Subsequently, binary state fault direction indicators are constructed using the base frequency phase difference and high frequency cosine similarity between the PSFC voltage and current. A hybrid frequency direction feature-based protection scheme is then proposed, and its robustness against variations in FRT control response, fault scenarios, and network topology is demonstrated. The exchanging of binary state indicators between line terminals requires only low communication bandwidth. Finally, simulation results on a 10 kV islanded microgrid system validate the effectiveness of the proposed scheme under various conditions, including different fault resistances, fault inception angles, measurement noises, network topological changes, and IIRES control strategies.