A Plug-and-Play Switching Strategy for Seamless Transition Between Grid-Following and Grid-Forming Control Modes

The increasing penetration of renewable energy sources (RES) has significantly changed the grid dynamics, rendering the traditional grid-following (GFL) inverters insufficient to maintain stable operation under varying conditions. In this context, a plug-and-play switching unit (PnP-SU) is developed, which enables seamless transitions between GFL and grid-forming (GFM) control modes without altering their intrinsic structures. By reformulating conventional control loops into the PnP-SU framework, a unified implementation method applicable to various GFL and GFM strategies is proposed. Moreover, the stability of the resultant system is analyzed based on the switched system theory. It is confirmed that the overall stability is not altered by the proposed control, when both GFL and GFM modes are exponentially stable and the switching interval exceeds the settling time of the original controllers. Two representative pairs of GFL and GFM controllers without common control loops are employed to demonstrate the generality of the proposed method. Additionally, experimental tests are performed on a 3-kW three-phase hardware prototype with both single- bus and 14- bus power hardware-in-the-loop (PHiL) test systems. The experimental results verify that the proposed PnP-SU achieves smooth and stable mode transitions even during grid faults and transients, validating its effectiveness and general applicability for grid-connected inverters.