Time-Based Technical Requirements for Fast Frequency Response Robust Across Inertia Levels

As system inertia declines due to the increasing penetration of inverter-based resources, low-inertia systems face growing challenges in maintaining frequency stability. To address this issue, this paper proposes time-based technical requirements for fast frequency response (FFR) that explicitly incorporate resource-specific delays and activation times. Unlike conventional frequency-based approaches, the proposed framework ensures consistent energy delivery and predictable frequency support across heterogeneous resources and varying inertia conditions. Simulation studies on a multi-machine frequency response model demonstrate that the time-based approach achieves robust nadir performance under diverse inertia levels, while conventional methods exhibit significant variability. Furthermore, a delay compensation scheme is introduced to enhance reliability by mitigating interactions between FFR, primary frequency response, and load damping. The results highlight that time-based requirements provide a practical and effective foundation for enabling FFR to secure more reliable frequency stability in future low-inertia systems with high shares of inverter-based generation.