High Stable Current Control of LCL-Filtered Energy Storage Converters with Grid Impedance Estimation

Under weak grid conditions, variations in the equivalent grid impedance adversely affect both the current control loop performance and the dq-axis current coupling exacerbated by the grid impedance in LCL filtered energy storage converters, thereby limiting their stability, dynamic response, and output current quality. To address this issue, this study proposes a grid-impedance-estimation-based linear active disturbance rejection control strategy (GIE-LADRC). This method integrates the online estimated grid impedance into the design of the LADRC compensation factor, enabling the factor to better approximate the actual system gain under impedance variations in weak grids. This enhances the ability of the Linear Extended State Observer (LESO) to observe and suppress disturbances caused by impedance. In practice, LADRC first provides approximate decoupled control of the dq-axis currents. Subsequently, under this decoupled condition, a reactive power variation strategy that maintains a constant d-axis current reference is employed to obtain an online estimate of the equivalent grid impedance, which has minimal impact on the converter's active current output during the estimation process. The stability of the proposed method under the complete current control framework is evaluated through impedance model-based stability analysis. Finally, experimental results from a 4 kW prototype confirm that GIE-LADRC can maintain high-quality current output even in very weak grids with a SCR below 2.