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Modeling of Grid-Connected Single-Phase LCL Converter Considering Line Impedance Effect and Its Improvement on System Stability and Current Distortion with DDC

In this article, a conventional single-phase LCL converter with PI control and active damper is first reviewed and modeled, and a direct digital controller for the converter to improve system stability and grid-current distortion is presented. Due to line impedance effect, the converter connected to the grid may cause stability problems, especially under weak grid conditions. This article investigates the grid-impedance effect in terms of short-circuit-ratio (SCR) and conducts stability analysis of the converter under various SCRs. Then, a direct digital controller (DDC) for the single converter is proposed to stabilize the system. The modeling and proposed approach are verified with simulated and experimental results, and moreover, they will be extended to investigate 10 converter units connected in the same grid.