Impact of inverter and grid conditions on impedance estimation of distance protection in grid-following and grid-forming inverter systems

Traditional protection systems, historically designed around the electromechanical behaviour of synchronous generators, are increasingly challenged by the growing penetration of inverter-based resources (IBRs) and their relatively small fault current. This paper presents a comparative assessment of distance protection performance in the presence of either a grid-following (GFL) inverter with fault ride-through capability or a grid-forming (GFM) inverter employing virtual-impedance-based current limiting. A systematic parameter-sweep study is conducted in which two parameters of the IBR (the pre-fault active power and the current-limit) and three parameters of the grid (the X/R ratio of the grid (line) impedance, the pre-fault grid voltage, and grid frequency) are varied individually. For each parameter choice, the impedance seen by a distance protection relay during a balanced fault is evaluated. The results demonstrate that the pre-fault active power operating point strongly influences impedance estimation accuracy for the GFL inverter, while variations in grid voltage and frequency have only a limited effect on impedance estimation for both types of inverter. The contrasting sensitivities observed between GFL and GFM operation reveal fundamental differences in their interaction with distance protection, providing insight that can inform protection-aware planning and design of IBR-dominated power systems.