Robust Inverter Synchronisation for Weak Grid Based on Impedance Estimation

The growing integration of distributed renewable energy resources interfaced with current power systems through electronic converters presents significant challenges for system monitoring, stability, control and protection. Grid impedance plays a critical role in the operation and stability assessment of grid-connected inverter systems. The proposed technique in this manuscript presents a real-time grid impedance estimation method based on the discrete Fourier transform and recursive least squares. The proposed method is integrated with the Advanced Angle Estimation Kalman Filter using a Linear Quadratic Regulator current controller (AAEKF-LQR) that uses the estimated grid impedance to evaluate an instantaneous grid phase angle. The simulation results in this paper confirm that the proposed impedance estimation method interacts effectively with the AAEKF-LQR controller, maintaining stable system performance under weak grid conditions. The approach also demonstrates the ability to deliver fast and accurate impedance estimation during operational variations in grid conditions, thereby supporting stable inverter operation. Additionally, the method exhibits strong robustness against external oscillation disturbances originating from a synchronous machine interfaced through the point of common coupling in a grid-following configuration with weak grid conditions to achieve significant improvements in oscillation damping.