Current-Only Fault Detection and Direction Scheme for IBR-connected Transmission Lines

This paper presents a current-only protection methodology for transmission lines connected to inverter-based renewable energy sources (IBRs) such as wind and photovoltaic (PV) plants. Conventional protection schemes often fail in IBR-dominated networks due to the current-limited and control-driven fault responses of power electronic converters. To address this challenge, a two-stage algorithm is developed. In the first stage, fault detection is achieved by applying a Savitzky-Golay (SG) differentiator to the sum of squared three-phase currents (SSC), forming a nonlinear fault detection index (FDI). In the second stage, fault direction is discriminated using a self-polarized directional index computed from the first and second discrete derivatives of the SSC signal. The scheme relies exclusively on local current measurements sampled at 10 kHz, requiring no voltage signals, synchronization, or communication links at the element level. Extensive simulations on a 500 kV transmission network and the IEEE 9-bus system, with different IBR configurations and hardware-in-the-loop (HIL) tests, confirm reliable operation under all fault types, fault resistances up to 100 Ω, and noise levels down to 10 dB SNR. The proposed scheme functions as a fast local fault-starting and directional element; for complete selective line isolation it feeds into a pilot protection scheme such as Permissive Overreaching Transfer Trip (POTT) or Directional Comparison Blocking (DCB) at the system level. Comparative evaluation with recent converter-interfaced line protection methods shows that the proposed approach offers superior speed, accuracy, and robustness while eliminating the need for voltage signals, inter-terminal synchronisation, or system-specific settings. The results demonstrate a practical and scalable solution for protection of the emerging converter-dominated power grid.