Passivity Enhancement Method for Grid-Connected Inverter Based on the Fixed-Frequency Phase-Locked Loop in Weak Grid

In distributed generation system, the time-delayed phase-locked loop (TD-PLL) is a common method of grid synchronization in single-phase grid-connected inverters (GCIs). However, GCIs with TD-PLL are exposed to the greater risk of oscillatory instability in weak grid. In order to improve the adaptability of GCIs in weak grid, an inverter passivity enhancement method based on fixed-frequency improved second-order second-order generalized integral phase-locked loop (FF-ISO-SOGI-PLL) is proposed. Firstly, the influence of TD-PLL on the passivity of GCIs is analyzed by establishing a small signal impedance model. On this basis, the FF-ISO-SOGI-PLL with better stability and disturbance suppression ability is proposed, and the parameters of phase-locked loop (PLL) are optimally designed. The improvement effect of the impedance reshaping method based on FF-ISO-SOGI-PLL on the inverter passivity is analyzed. Finally, the effectiveness of the proposed method is verified by the comparative experiment.