Non-Intrusive Battery-Backed Grid-Forming Configuration for Grid-Following Photovoltaic Plants Retrofit

Existing grid-connected photovoltaic (PV) systems with grid-following (GFL) control rely on phase-locked loop (PLL)-based grid synchronization, which may become unstable under weak grid conditions. By contrast, grid-forming (GFM) retrofits are regarded as promising solutions for improving the stability of GFL-based PV integration. However, existing retrofit methods often require comprehensive replacement or substantial modification of the existing GFL controller or hardware, thereby increasing retrofit costs and maintenance complexity. To address these issues, a non-intrusive configuration with a battery-backed GFM branch comprising a charging rectifier and a GFM inverter is proposed for retrofitting implemented GFL PV power plants in this paper. With the proposed configuration, the existing GFL PV inverters retain their original hardware and GFL control. The charging rectifier branch absorbs surplus PV power from the PV inverter output, whereas the proposed battery-backed GFM branch is connected at the point of common coupling (PCC) to provide voltage- and frequency-forming supports. To utilize the available branch capacity under normal grid conditions, selected auxiliary functions, i.e., harmonic compensation and Volt–VAR support, are incorporated subject to the available inverter operating margin. The effectiveness of the proposed configuration and the corresponding control scheme is validated through different operating scenarios, including PV/load mismatch charging, nonlinear load harmonic compensation, reactive load Volt–VAR support, grid frequency and grid voltage disturbances under different grid strength conditions, and combined dynamic operating disturbances at weak grids.