Revisiting transient synchronization stability of grid-forming MMC inverters: interactions between reactive power control and current limitations

• Theoretical analysis reveals that the influence of Reactive Power Control Loops (RPCL) on the transient stability of Grid-Forming MMCs is highly dependent on the activation of circular current limiters. • Contrary to cases without current limitations, increasing the droop coefficient when current limitations are triggered can enhance transient stability. • A nonlinear adaptive droop coefficient is proposed, which incorporates exponential and logarithmic terms to enhance transient performance during current limitations. Grid-forming (GFM) capable modular multilevel converters (MMCs) are crucial in flexible distribution networks to regulate voltage and frequency, especially given the increasing integration of distributed power sources. Existing research has independently investigated how reactive power control loops (RPCL) and current limitation link affect the system transient stability. However, the interactions between these two critical components have been largely overlooked. To further investigate how the interaction between RPCL and current limitation links affect the system transient stability, this study demonstrates that the influence of RPCL on transient stability varies significantly based on whether the current limitation is triggered. Subsequently, this paper employs the equal area criterion (EAC) and the Lyapunov direct method (based on LaSalle’s invariance principle) to characterize the domain of attraction of the system, analyze its behavior under faults, and comprehensively evaluate the effects of RPCL and current limitation link interactions on the GFM MMC system. An adaptive droop coefficient applicable to faults of varying severity is also designed. Finally, the effectiveness and correctness of the theoretical analysis and the proposed method are verified via MATLAB/Simulink.