A Cutting-Edge Dual-Mode Seamless Switching Method With Fast Phase Locking for Grid-Connected Inverter Under Distorted Grid Conditions

To ensure the operation quality of the grid-connected inverter (GCI) when there are significant fluctuations in the short-circuit ratio (SCR), the dual-mode control has been proposed to select the grid-following inverter (GFLI) or grid-forming inverter (GFMI) according to the SCR value. However, this method is only applicable to ideal conditions and fails when the grid is distorted. In this article, a novel seamless switching method suitable for distorted grid conditions is proposed. The proposed method employs a Kalman filter to extract the fundamental components of the point of common coupling (PCC) voltage and grid-injected current and utilizes harmonic voltage observation (obtained by subtracting the fundamental voltage observation from the PCC voltage) as the feedforward for pulse modulation to eliminate the influence of grid background harmonics. Meanwhile, the fundamental voltage observation is used as the input of the synchronous reference frame phase-locked loop (SRF-PLL) to ensure phase synchronization before and after switching. The proposed approach does not require any change in the structure of the PLL, does not need to extract each harmonic separately, and does not increase the computational burden. The simulation and experimental results demonstrate that the proposed inverter control method can achieve seamless switching under various strong and weak-grid conditions.