Seamless Transition Between Grid-Following and Grid-Forming Modes for Grid-Tied Virtual Synchronous Generator with LCL Filter

The ongoing transition from conventional grids to power-electronic-dominated grids (PEDGs) has significantly increased the complexity of the grid's dynamic characteristics, particularly in terms of the short-circuit ratio (SCR) and inertia requirements. In order to adapt to the dynamic grid conditions, seamless transition of distributed energy resources (DERs) between grid-forming (GFM) and grid-following (GFL) modes becomes essential. However, traditional mode-transition approaches typically rely on multi-stage procedures using cascaded PI-based control loops designed for single-variable regulation, such as controlling either current or voltage at one point of the LCL filter. Such methods are unable to coordinate multiple control objectives, leading to power instability, transient fluctuations, and current surges during mode transitions. This paper proposes a fast and direct seamless transition technique between GFM and GFL modes for DERs equipped with LCL filters without the need for intermediate stages. The GFM operation is governed by a virtual synchronous generator (VSG) control scheme. The proposed method employs a model predictive controller (MPC) that acts as the lowest-level control layer with two selective cost functions. One function regulates active and reactive power during GFL operation, while the other one ensures output voltage regulation during GFM operation. The effectiveness of the proposed approach has been demonstrated through real-time simulation under both high and low SCR conditions.