Under weak grid conditions

grid-following inverters (GFL) are prone to sub/super-synchronous oscillations

whereas grid-forming inverters (GFM) exhibit better adaptability to weak grids. The parallel operation of GFL and GFM can enhance the stability of grid-connected systems

but the configuration cost increases with installed GFM capacity. To improve the stability of parallel GFL/GFM inverter systems under weak grid conditions while reducing the required capacity of GFM

an adaptive control strategy based on impedance detection is proposed. Firstly

the q-axis impedance mathematical models for both GFL and GFM are established. The Nyquist stability criterion is used to analyze the relationship between GFL output oscillations and q-axis impedance under weak grid conditions. System stability at different short-circuit ratios and different GFM capacity configurations is also evaluated. Subsequently

an adaptive control strategy is proposed to reshape the GFM q-axis impedance when the installed GFM capacity is low

thereby enhancing the stability of the parallel system over a range of weak-grid strengths. The effectiveness of the proposed strategy is verified using MATLAB/Simulink. The research results demonstrate that the proposed control strategy can be adopted to effectively improve the operational stability of the parallel system under different weak grid conditions when the installed GFM capacity is low.