This paper presents a survey of real-world subsynchronous oscillation events associated with inverter-based resources (IBR) over the past decade. The focus is on those oscillations in the subsynchronous frequency range known to be influenced by power grid characteristics, e.g., series compensation or low system strength. A brief overview of the historical events is presented followed by detailed descriptions of a series of events. This paper also examines causation mechanisms and proposes future research directions to meet grid needs worldwide.

This letter proposes an adaptive hybrid grid-following (GFL) and grid-forming (GFM) control, which ensures small-signal stability of inverter-based resources with ultra-weak and very strong grid connections. The hybrid control offers mixed GFL and GFM characteristics by altering the grid synchronization and outer loop controls. The proportion of GFL or GFM features is adaptively adjusted by using the analogy of a hysteresis loop in response to the short-circuit ratio (SCR). Consequently, the GFL characteristics are gradually reinforced as the SCR increases. Impedance-based analysis and electromagnetic transient simulation results confirm that the proposed control enhances the system's small-signal stability from ultra-weak (SCR = 1) to stiff (SCR = 50) grids.