A Novel FLL for Single-Phase Grid-Connected PV Inverter under Weak Grid

The weak grid with high line impedance and low short-circuit capacity leads to a large number of background harmonics and fluctuation of the grid voltage, and it is difficult for the inverter to synchronize with the weak grid. In order to overcome these problems, a novel fast and accurate frequency-locked loop(FLL) method is proposed. Firstly, an improved quadrature signal generator(QSG) is utilized to effectively eliminate the impact of the DC offset on the accuracy of frequency and phase detection. Secondly, low-pass filters(LPF) with specific harmonic filtering capabilities are used so that the system has good dynamic response speed and anti-interference ability. Finally, the grid frequency is calculated by the Teager energy operator(TEO) algorithm without any parameter adjustment, and use Park transform to obtain grid phase information. It can be known from the modeling that the FLL system is a type I system with an excellent dynamic response speed and stability margin. The simulation results verify the effectiveness of the proposed method when the grid voltage fluctuates or distorted.