Low-frequency electromechanical oscillations (<2
Hz) 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 (>5 Hz) 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 EMTabc
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.