A Self-Tuning Notch Filter based Hybrid Active Damper for LCL-Type Grid-Tied Inverters with Adaptability to Weak Grids

$LCL-$ filters are widely used in grid-tied converters for improved attenuations and lower costs. To prevent instability, the $LCL$ resonance peak needs to be damped by the active dampers that avoid the dissipations but increase the control complexity. In the weak grid, the harmonic currents and the grid impedance variations make the control become more complicated and difficult. The influence of the harmonic currents and the grid impedance in parallel system is researched in this paper. Then, a self-tuning notch filter (STNF)is introduced, and the resonance peak identification process implemented in digital controller is proposed in detailed. The consumption and tracking error of the STNF are analyzed guaranteeing an effective design. By adopting the STNF, the local maximum gain frequency is estimated online in response to the grid inductance variations. The construction as well as optimal design principles of the hybrid active damper are proposed in detail. To further reject the harmonic voltage effects, a PR regulator with a harmonic compensation controller is adopted. A step-by-step example design of the controller is illustrated in detail. The online implementation in DSP controllers and the digitization influence of the proposed method are also amply discussed. Finally, simulation and experimental results verify the effectiveness of the proposed method.