Harmonics and Stability Analysis of Single-Phase Grid-Connected Inverters in Distributed Power Generation Systems Considering Phase-Locked Loop Impact

In the distributed power generation systems (DPGSs) based on the renewable energies, the stability and harmonics of the grid-connected inverter are seriously affected by the uncertainties of the grid at the point of common coupling (PCC). More importantly, as several inverters are usually tied at the PCC together, the interactions among the inverters and the grid impedance can cause more serious harmonics and instabilities at the low frequencies. Hence, the phase-locked loop (PLL) affecting the low-frequency behaviors must be considered. This study aims to provide a comprehensive study of the stability and harmonics of single-phase inverters in the weak grid. Modelings of single-inverter and multiple-inverter systems considering the PLL are established. The impacts of different PLLs on the low-order current harmonics and stability are then studied. It has been proved that the maximum ability of inverters in suppressing the low-order grid current harmonics closely relates to the PLL, even if a harmonic resonant controller and/or PCC voltage feedforward is used. Thus, using different PLLs yields quite different performances. Simulation and experimental results will be given to demonstrate the analysis, which is helpful for the readers to design a proper PLL for DPGSs.