Comparative Analysis of Damping Characteristics for Different Feedforward Strategies of LCL-Type Grid-Connected Inverters under Time-Varying Grid Impedance Conditions

Photovoltaic (PV) inverters serve as a critical interface between solar energy systems and grid, playing an indispensable role in PV systems. The T-type three-level PV inverter based on LCL filter can effectively suppress grid-connected current harmonics generated by the switching device turning on and off, thus improving power quality. However, the inherent resonance of the LCL filter poses significant challenges to system stability. While the existing point of common coupling voltage feedforward (PCCVF) and capacitor voltage feedforward (CVF) active damping (AD) control strategy can enhance system damping and suppress weak grid background harmonics, the grid impedance adaptation mechanism under the bridge-arm-side current feedback control architecture has not been clarified. This study develops an equivalent virtual impedance model for voltage feedforward to elucidate the damping characteristics of both feedforward strategies across varying grid impedances. Theoretical analysis and simulation results demonstrate that PCCVF excels in weak grid scenarios, whereas CVF is better suited for strong grid conditions. These findings provide a theoretical foundation for optimizing feedforward strategies in PV inverter engineering applications.