In the small-signal synchronisation stability analysis of grid-following converters (GFL)

the detailed interactions among multi-loop control mechanisms are often oversimplified or neglected

leading to significant modelling inaccuracies and potential stability assessment failures. To address this issue

this paper presents a motion-equation-based model for GFL converter that comprehensively incorporates multi-loop control dynamics

including the current control loop

AC voltage control loop

DC-link voltage control loop and PLL. Based on the proposed model

the coupling characteristics of multi-loop control are explicitly analysed and visualised in terms of system damping. The analysis reveals that low-frequency oscillations in GFL converters can be attributed to unintended negative damping induced by multi-loop control couplings. To mitigate this issue

an improved additional damping control (IADC) strategy is proposed

which enhances the synchronisation stability via effectively compensating for the negative damping caused by multi-loop interactions. Finally

the simulation and experiment studies are presented to validate the proposed analysis results and the effectiveness of the IADC strategy.