Impedance-Based Small-Signal Harmonic Stability Analysis of a STATCOM in Power Systems with Renewables

With the increasing penetration of inverter-based resources (IBRs), maintaining converter-driven stability has become a critical challenge. Static synchronous compensators (STATCOMs) are widely deployed for voltage regulation through rapid injection or absorption of reactive power, making their capability crucial in IBR-dominated systems. However, harmonic resonance between STATCOMs and the grid is a critical concern, since interactions between converter control dynamics and grid impedance can span from sub-synchronous to harmonic frequencies. Most existing impedance-based studies focus on grid-following inverters and seldom capture the combined influence of communication delay, current-controller gain, and grid impedance; they do not address passive-filter-based mitigation. This paper examines the converter-driven stability of a grid-connected STATCOM using impedance-based small-signal analysis, considering inner and outer control loops, communication delay, and grid impedance. Using a modified IEEE nine-bus system, we investigate resonance migration and stability sensitivity to control delay and the current-controller proportional gain, and evaluate a C-type passive filter that improves delay robustness by damping the dominant current-control-loop resonance. The framework is validated through nonlinear electromagnetic-transient (EMT) simulations in MATLAB/Simulink, corroborating the small-signal delay-stability margin within 19.7%.