Review of sub-synchronous oscillation (SSO): from historical challenges to modern inverter-based renewable energy sources

Sub-synchronous oscillations (SSO) pose significant challenges to the stability and reliability of modern power systems, especially in grids with high penetration of renewable energy sources such as wind and solar power. This paper provides a comprehensive review of SSO phenomena, including their evolution, mechanisms, and impacts on power system stability. A detailed classification of SSO is presented, covering grid-level, device-level, and control strategy-dependent oscillations, along with renewable-energy-specific interactions. Notably, it is found that GFL inverter-based sources are highly prone to SSO due to their PLL-based synchronization, whereas GFM inverters are generally more stable, offering improved damping and inertia-like responses. This study highlights recent SSO incidents, such as the 2009 Texas PV case and the 2019 Hornsea wind event, to underscore practical implications. Additionally, a taxonomy of SSO types and inverter technologies is provided. For instance, GFL systems in weak grids with SCR < 3 showed higher oscillation risks. The insights gained from this review suggest that robust control strategies and coordination between GFL and GFM units are essential. These findings pave the way for enhanced grid resilience and provide numerical indicators to guide future renewable deployment in weak grid conditions. While GFM inverters are generally more stable than GFL inverters, it is important to note that they are not entirely immune to SSO. Under certain conditions – such as improper tuning of droop or virtual inertia parameters, lack of coordination with GFL inverters, or interaction with mechanical systems, GFM inverters themselves can become a source of SSOs. Therefore, robust and well-coordinated control design is essential even for GFM-based systems to prevent unintended dynamic instabilities.