Control Method for Hybrid GFM‐VSC and GFL‐VSC Systems Considering Mode Switching to Ensure the Existence of Stable Equilibrium Points During Faults

ABSTRACT The configuration of grid‐forming voltage source converter (GFM‐VSC) and grid‐following voltage source converter (GFL‐VSC) operating in parallel is becoming increasingly prevalent in modern power systems. However, the complex dynamic interactions between GFM‐VSC and GFL‐VSC introduce significant challenges to maintaining transient stability of hybrid systems under grid fault conditions. Specifically, the activation of the current limiter in the GFM‐VSC triggers transient mode switching, which complicates the coupled dynamics and may lead to the loss of stable equilibrium points (SEPs) during faults. To address this issue, this study first employs the phase portrait method to reveal how the GFM‐VSC's active power reference and the GFL‐VSC's current reference impact the transient dynamics of the hybrid system. Subsequently, to comprehensively analyze the post‐fault transient transition processes, the mode switching behaviors of the GFM‐VSC are systematically categorized into four scenarios based on the transition between current unsaturation mode (CUSM) and current saturation mode (CSM): continuous operation in CUSM, transition from CUSM to CSM, continuous operation in CSM, and recovery from CSM to CUSM. Based on this classification, the active power reference for each scenario is determined to prevent the absence of SEPs under fault conditions. Furthermore, the feasible range of the GFL‐VSC's output current reference is identified to ensure the existence of SEPs. Finally, a practical and easily implementable dynamic adjustment scheme for the active power and current references is proposed. By integrating adaptive control and a reserved power margin, the proposed scheme can withstand a 50% variation in line impedance and control delays of tens of milliseconds. Simulations in MATLAB/Simulink validate the theoretical analysis and the effectiveness of the proposed strategy.