Unified RES Inverter Control for Seamless Transition Between Grid-Forming and Grid-Following Modes

With the increasing integration of renewable energy sources (RESs), traditional power systems based on synchronous generators (SGs) are evolving into power electronics-based systems. This transformation introduces more complex grid dynamics. To maintain system stability and reliability under varying grid and source conditions, the grid-side converters (GSCs) of RESs must adapt their control strategies, operating either in grid-following (GFL) or grid-forming (GFM) modes. However, switching between these modes can cause abrupt voltage and current transients due to changes in control loops or converter operation. Although several seamless transition methods have been proposed, they often require zero power transfer conditions or involve complex control structures that are difficult to implement in practice. To overcome these limitations, this paper proposes a unified control (UC) strategy that enables GSCs to transition smoothly between GFL and GFM modes under any power transfer condition. The transition is achieved simply by adjusting two coefficients in the control loops, ensuring ease of implementation. Experimental results are presented to validate the effectiveness of the proposed approach.