Seamless Transition Control Method of Grid-connected Inverters under Unbalanced Voltage Conditions

The transition between Grid-Forming (GFM) and Grid-Following (GFL) modes is critical for adapting to changing power grid conditions. These transitions are essential for protecting grid-connected inverter systems and ensuring reliable power supply. However, abrupt changes during the transition process can disrupt both internal and external control loops, potentially causing voltage and current distortions that jeopardize system stability. This challenge is particularly significant when transitioning between GFL and GFM modes under unbalanced load conditions. To address this, this paper proposes a seamless control strategy that enables dynamic and stable transitions between GFL and GFM modes, even under unbalanced three-phase voltage conditions. The proposed approach is based on three key criteria that ensure stability during transitions across various grid scenarios. Additionally, a simple yet effective method for extracting positive and negative sequence components using digital filters is introduced to facilitate the seamless control implementation. The effectiveness of the control strategy is validated through extensive simulations and experiments using a 300 kVA hardware setup. The results demonstrate successful seamless transitions under unbalanced conditions while maintaining a total harmonic distortion (THD) of less than 3%. This method significantly enhances the stability and reliability of power systems, especially in environments characterized by varying grid strengths.