Subsynchronous oscillations (SSO) have re‑emerged as a critical stability concern in modern electric power systems with grid forming (GFM) control. Traditionally, associated with interactions between synchronous generators (SGs) and series‑compensated transmission networks, SSO was categorized into subsynchronous resonance (SSR), subsynchronous torsional interaction (SSTI), and subsynchronous control interaction (SSCI). However, the rapid growth of inverter‑based resources (IBRs) in form of GFM-STATCOMs and E-STATCOMs has fundamentally reshaped grid dynamics.

This paper first provides a comprehensive and unified review of SSO phenomena in both conventional and converter‑dominated power systems. It then evaluates modern mitigation and damping analysis approaches, and practical considerations needed to guide engineers and researchers in assessing SSO risk in emerging converter‑dominated grids. Finally, by employing the network frequency perturbation (NFP) concept, it offers a P/f and Q/f evaluation and analysis on the Test Signal Injection approach for analyzing three installed real-case GFM-STATCOMs and its role to mitigate the risk of SSOs and damping torque improvement. Finally, technical lessons learned from the Landesbergen field case are utilized to examine the damping behavior of GFM solutions in practical grid applications. The study highlights the role of different control loops in shaping the damping of torsional modes and provides a systematic basis for configuring GFM controllers to achieve desired damping performance and enhance system stability.