EMT — RMS Modeling Trade-Off for IBR-Driven Sub-Synchronous Oscillations

Low-frequency electromechanical oscillations ($< $ 2Hz) are time-separated from faster network dynamics, allowing network dynamics to be safely neglected in positive-sequence RMS (RMS$+$) studies. However, with increasing shares of inverter-based resources (IBRs), sub-synchronous oscillations (SSOs) occur at higher frequencies ($>$ 5Hz) within the electromagnetic timescales. The shift challenges using RMS$+$ tools for planning IBR-dominated grids, as the time-scale separation is no longer as distinct as in synchronous machine-based systems. This paper demonstrates that relying on RMS$+$ studies in high-IBR scenarios can lead to erroneous conclusions about SSO, including a false assurance of stability. We explain how the interaction between IBR control and network dynamics affect the damping of SSO. This highlights the need for EMT-$dq$ with network dynamics in a synchronously rotating reference frame as a middle ground between EMT-$abc$ (point-on-wave) and RMS$+$ for studying IBR-driven SSO. EMT-$dq$ is computationally simpler than EMT-$abc$ and allows frequency-domain analysis for deeper insight and effective SSO mitigation. This emphasizes the necessity of established (rather than bespoke) EMT-$dq$ tools to quickly screen SSO-prone scenarios for detailed investigation in EMT-$abc$.