Seamless transition between grid forming and grid following inverters based on online grid impedance estimation

The rapid increase in the penetration of inverterbased resources (IBRs) into the distribution network causes thepower system to behave as a weak grid and often alters system strength. IBRs operate as either grid-following (GFL) or grid forming (GFM) inverters, differing in both control stability and inertia support capabilities under varying operating conditions. To maintain system stability across different grid strengths, IBRs need to support smooth switching between GFL and GFM modes. This paper presents an automatic smooth-switching control strategy based on online grid impedance estimation to enhance operational stability. Ordinary Least Squares (OLS) regression is used to estimate grid impedance, and hysteresis switching is applied to prevent continuous switching when grid strength fluctuates near boundary conditions. The effectiveness of the proposed control strategy is verified through case studies under varying grid strengths.