The ongoing transition towards power systems dominated by inverter-based resources calls for a robust understanding of their dynamic interaction with the grid across a wide range of frequencies. While requirements for damping of frequency–power oscillations at low frequencies and passive behavior in the harmonic domain are to some extend established, the intermediate “sub- and supersynchronous” frequency range is still characterized by unresolved questions and interpretation.

This contribution discusses frequency-domain observation methods that can be used to characterize the interaction between grid-forming inverters and network frequency variations. In particular, small-signal frequency perturbations of the grid are considered as a means to study the resulting active power response of converter-based resources. Such representations offer a unified view on dynamical effects traditionally addressed by time-domain stability concepts on the one hand and impedance-based approaches on the other.

Using generic simulations and exemplary control configurations, typical trends of inverter behavior over different perturbation frequency ranges are illustrated. The paper analyzes relationships between frequency and active power across low, medium, and high perturbation frequencies. The findings indicate that i) commonly proposed criteria such as “frequency and active power being anti-phase” are meaningful only up to a perturbation frequency limit, and that ii) classic passivity definitions very likely feature a lower bound. As a result, assessment approaches based on a single criterion or frequency range may not be directly transferable to adjacent domains.

From an OEM perspective, the paper outlines current challenges in defining clear, consistent, and verifiable requirements for damping and passivity in the sub- and supersynchronous frequency range. As a possible way forward, recently proposed frequency-domain specifications are discussed as a basis for harmonizing damping and passivity assessments across adjacent frequency domains. The paper aims to support the ongoing discussion on how frequency-domain specifications and assessment methods could be aligned in the future to ensure stable and predictable grid-forming behavior.