Impacts of the Reactive Power Control on the Small-signal Stability of Grid Forming Inverters
Nabil Mohammed, Weihua Zhou, Behrooz Bahrani
Abstract
Grid forming inverters (GFMIs) are emerging as promising alternatives to address grid instability challenges, offering vital ancillary services to power systems, including inertia support, frequency support, and oscillation damping. Therefore, understanding the control mechanisms and stability characteristics of GFMIs is crucial. This study investigates two control approaches for regulating the output reactive power used by virtual synchronous generator (VSG)-based GFMIs. The first method (without reactive power controller (without RPC)) sets the voltage reference for the voltage loop as a constant value pre-computed using power flow equations, while the second method (with RPC) explicitly specifies the desired reactive power and automatically generates the voltage reference for the voltage loop. This study thoroughly examines the robustness of these two control methods under varying grid conditions using frequency domain analysis (Bode and Nyquist plots) and time domain verification based on the EMT model simulated in MATLAB/Simulink to analyze small-signal stability and low-frequency oscillations. Additionally, this study offers recommendations for enhanced stability, highlighting the importance of selecting the appropriate reactive power control strategy for VSG-based GFMIs, particularly in strong grids, to ensure stable and reliable grid operation.