Impedance modeling of three-phase grid-connected inverters and analysis of interaction stability in grid-connected system

In a distributed generation system, the stability of grid-connected inverters is directly related to the reliable operation of the grid-connected system. The impedance-based analysis method can be employed to effectively study the interaction stability between grid-connected inverters and grid, which means that it is necessary to obtain the impedance modeling of grid-connected inverters for the analysis of impedance stability. Based on the three-phase LCL-type grid-connected inverter, the harmonic linearization method was adopted in this paper to analyze the frequency characteristic of Phase-Locked Loop (PLL), and a design approach of PLL regulator parameters was proposed. Meanwhile, the impacts of the factors that include PLL and digital control delay on the impedance characteristic were considered, and the positive-sequence and negative-sequence impedance model of the grid-connected inverter was built by combining the harmonic linearization and symmetrical component methods. Then, the frequency characteristic of PLL was verified by the simulation results and the output impedance model of the grid-connected inverter was verified by the experimental results, which effectively proved the correctness of the theoretical analysis. Finally, based on the output impedance model of the grid-connected inverter, the impedance-based analysis method was adopted to make a theoretical analysis and experiment validation of the interaction stability of the grid-connected system.