Instability Caused by Integration of IBRs Under Strong Grid Connections — A Practical Case Study on Large-Scale Energy Storage Systems

Inverter-based resources (IBRs) with grid-following (GFL) control have been known to cause converter-driven stability issues under weak grid conditions. However, this study investigates a practical scenario involving large-scale energy storage systems (ESSs) using GFL control under strong grid conditions. These ESSs generate a 150 Hz oscillation in the d-q coordinates while providing both capacitive and inductive reactive power support to a bulk power system. To preserve manufacturer confidentiality, the stability analysis is conducted by focusing on the equivalent dynamics of ESSs rather than getting involved in their internal parameters. The results show that dynamic interactions among multiple ESSs intensify as their scale increases. Although each individual ESS is designed to be stable, instability occurs from the superimposed effect of ESSs. A damping index is proposed and indicates that the superimposed effect always induces negative damping to the system stability, and its magnitude grows as the ESS scale extends. Finally, key factors are identified, mitigation strategies are proposed, and the conclusions are validated. This study provides practical insights into instabilities caused by large-scale ESSs even under strong grid conditions, and emphasizes the importance of accounting for grid-side dynamic interactions in large-scale IBRs.