Adaptive smooth-switching control strategy for soft open points based on dynamic short-circuit ratio identification
This paper aims to address stability issues from dynamic short-circuit ratio (SCR) variations caused by high distributed energy penetration. The limitations of soft open points using single control strategies, which are challenged by varying grid strengths, are overcome and severe transients experienced during direct control mode switching are mitigated. An adaptive smooth-switching control strategy based on dynamic SCR identification is proposed. First, small-signal models under two control modes are established, and the stable operating boundaries are analyzed. Second, a nonintrusive impedance identification method using secondary-side virtual disturbance is introduced for real-time SCR estimation. Finally, continuous phase angles and current references during transitions are ensured by a switching mechanism incorporating phase pre-synchronization and state tracking. Simulation and experimental results demonstrate that real-time SCR variations are accurately identified by the proposed method. Disturbance-free, zero-deviation smooth switching between grid-following and grid-forming modes during transitions across strong and weak grids is enabled, whereby severe voltage/current transients and power fluctuations are effectively eliminated. The need for extra hardware injection is eliminated by the nonintrusive SCR identification via virtual disturbance. Furthermore, combined with the pre-synchronization loop, an active, impact-free transition mechanism is provided, by which the adaptive support capability and operational robustness of flexible distribution networks are significantly enhanced.