Dynamic Interactions Between Inverter-Based Resources and Synchronous Generators: A Comparative Study for Cascading Failure Risk

The increasing integration of inverter-based resources (IBRs) alongside traditional synchronous generators (SGs) presents new challenges for maintaining stability and resilience in modern power systems. Conventional cascading failure frameworks, originally developed for SG-dominated grids, often fail to capture the unique dynamics of mixed SG-IBR systems. This paper addresses this gap by developing an analytical framework based on a unified second-order swing-equation model for SGs, grid-following (GFL) inverters, and three grid-forming (GFM) control strategies: droop, virtual synchronous machine (VSM), and dispatchable virtual oscillator control (dVOC). This approach enables direct comparison across technologies and incorporates eigenvalue-based analysis of oscillation frequency and damping ratio, as well as Critical Clearing Time (CCT) estimation using the Equal Area Criterion (EAC). The analytical insights are validated through electromagnetic transient (EMT) simulations of a 16-bus, 230 kV system in EMTP-RV, evaluating the dynamic behavior of SGs, GFLs, and GFMs. Results show that while GFLs maintain stability under strong grid conditions, they become unstable in weak grids. SGs offer longer CCTs due to their high inertia but may exhibit underdamped responses. Various GFM strategies significantly enhance stability in weak grids, particularly when combined with GFLs; however, their effectiveness diminishes under strong grid conditions or when the system contains a high proportion of GFMs, leading to dynamic instability and reduced CCTs. Additionally, improperly tuned VSM controllers may introduce low-frequency oscillations. These findings highlight the importance of coordinated control design and careful tuning to ensure robust and resilient operation in mixed SG-IBR power systems.