Eigenvalue Analysis of IBR Instabilities in Power System

The global transition towards renewable energy has led to a significant increase in the integration of Inverter-Based Resources (IBRs), such as solar, wind, and battery storage systems, into power grids. While this shift is crucial for reducing carbon emissions and fostering sustainability, it also introduces challenges to grid stability due to the replacement of conventional synchronous generators with IBRs. Unlike traditional generators, IBRs lack inherent mechanical inertia, relying instead on advanced control systems to interface with the grid. This fundamental difference raises concerns about small-signal stability, particularly in low-inertia grids with high renewable penetration.

This thesis explores the small-signal stability impacts of IBRs in modern power systems, focusing on grid-following (GFL) and grid-forming (GFM) inverters. The study is conducted on two systems: the IEEE 9-bus test network and the South Island Power System (SIPS) of New Zealand, a large-scale renewable-rich grid. Eigenvalue analysis and participation factor evaluation are employed to assess oscillatory modes, damping characteristics, and the dynamic behavior of power systems under varying IBR penetration levels.

Key findings reveal that GFM inverters significantly enhance stability by emulating the behavior of synchronous generators, particularly under high renewable penetration, whereas GFL inverters struggle in weak grids due to phase-locked loop (PLL) synchronization limitations. Hybrid configurations of GFL and GFM inverters demonstrate complex interactions, highlighting the need for coordinated control strategies. The analysis of SIPS further validates the benefits of GFM inverters in improving damping, thereby supporting their suitability for high-renewable systems. This research emphasizes the critical role of advanced inverter control strategies and configurations in addressing stability challenges in low-inertia grids. While the scope is primarily limited to first assessment of small-signal stability, the study recommends pathways for further exploration, including the use of dynamic simulations, impedance-based methods, and the impact of network topology on stability. The findings contribute valuable insights for designing robust, renewable-integrated power systems capable of meeting the demands of a sustainable energy future.