Grid State-Aware Adaptive Switching Between Grid-Following and Grid-Forming Modes Enabled by Fast Online Impedance Estimation

Changes in grid strength can impair the stability and dynamic performance of converters operated in fixed grid-following or grid-forming modes. Adapting the mode requires impedance estimation and preparation of the incoming controller. This paper proposes a single-converter strategy that combines impedance estimation using a notch–bandpass compound filter, hysteretic mode selection, and bidirectional controller state preparation. The estimator identifies resistance and inductance from the injected frequency response and reconstructs the fundamental frequency short-circuit ratio. The switching strategy coordinates grid strength requests with phase/amplitude synchronization and state initialization to reduce transfer transients. Conditional error and synchronization analyses establish the requirements linking estimation to mode execution. With matched measurement channels, 86 fixed-mode simulation configurations give a maximum final-interval short-circuit ratio error of 2.32 percent. In shared-input noiseless tests, the estimator has a median settling time of 130.85 milliseconds as measured from the network change command. At 20 kW, the converter peak current in a common transfer window from grid-following to grid-forming decreases from 217.41 to 45.83 A relative to direct transfer, a 78.9 percent reduction. These results support the proposed integration of online grid strength estimation and prepared mode transfer in the evaluated single-converter system.