Stability Analysis and Frequency-Segmented Active Damping Method of Hybrid Grid-Following and Grid-Forming Inverter System Under Power Variations

Hybrid systems integrating grid-following (GFL) and grid-forming (GFM) inverters are increasingly deployed in renewable-energy-dominated power systems. However, impedance coupling between the two inverter types may induce low-frequency oscillations and high-frequency resonances, particularly under weak-grid conditions and varying power injections. This paper clarifies the stability mechanism of a hybrid GFL/GFM inverter system and develops a frequency-segmented active damping strategy. Small-signal impedance models are first derived for the GFL inverter, the GFM inverter, and the overall hybrid system, incorporating the control loops, digital delay, LC filters, interconnection branch impedances, and external grid impedance. Impedance decomposition, Bode plots, and Nyquist criteria are then employed to quantify the influence of power operating points and grid strength on system stability. The results indicate that increasing the GFL inverter output power weakens the stability margins in both low- and high-frequency ranges, whereas variations in the GFM inverter output power provide only limited impedance reshaping in the targeted oscillation bands. On this basis, a low-frequency damping loop is designed on the GFM inverter side, while a high-frequency damping loop based on capacitor-current feedback is implemented on the GFL inverter side. Simulation results confirm that the proposed strategy suppresses low-frequency oscillations and high-frequency harmonic components, maintains stable operation in the hybrid system under high GFL power injection, and reduces the THD of the PCC current from 14.52% to 0.62%.