Impedance Modeling and Stability Analysis of Single-Phase Grid-Connected Inverters with Open-Loop Grid Synchronization

Fast and precise acquisition of grid voltage phase is of vital importance to grid-connected inverters. Among grid synchronization scheme, the open-loop synchronization (OLS) has attracted much attention recently due to the elimination of feedback loop. Note that recent studies focus on analyzing and improving the performance of open-loop grid synchronization itself. But, since the open-loop synchronization owns a strongly nonlinear structure (even with no feedback loop), it is difficult to apply the widely-used impedance modeling method to facilitate further investigation of the stability and performance of grid-connected inverter affected by such open-loop synchronization. Therefore, to cope with this issue, this study establishes a simplified s-domain model of open-loop grid synchronization, and the output impedance model of the overall grid-connected inverter system is obtained. Then, the accuracy of the proposed model is verified, and the established model of output impedance is used to analyze the stability of the grid-connected inverter under weak grid. Selected waveforms are provided to verify the correctness. The results of analysis are helpful to the stability analysis and parameter design of grid-connected inverter in the case of complex grid.