Analytical discussion on dynamics of inverter-based resources under small-signal conditions

Grid integration of inverter-based resources (IBRs) and the consequent phase-out of synchronous machines cause a reduction in system strength and inertia, which may pose system instability challenges, including small-signal and inverter-driven instability. There is an ongoing discussion on the contributing factors to IBR stability, synchronism, and response specifications (e.g., response time, damping) under various grid conditions and characteristics. This paper presents an analytical discussion highlighting how grid characteristics such as short-circuit ratio, R/X ratio, and system loading conditions may affect IBR stability and synchronism under small-signal conditions. Furthermore, the contributing factors to IBR response characteristics imposed by the external grid are identified. The analytical discussion highlights the need to consider such factors when determining grid-code requirements for IBR response specifications and stability conditions.