Adaptability enhancement of fractional-order LLCL-type grid-connected inverter to weak grid based on grid voltage weighted feedforward scheme

Grid-connected inverter with a novel fractional-order LLCL filter has the advantage of high grid current tracking accuracy and low total harmonic distortion without passive or active dampers, and the grid voltage full feedforward scheme is an effective method to improve the quality of grid current. However, due to the digital control delay, the suppression of the grid current harmonics is weakened, and the inverter output impedance has an additional negative phase shift, which deteriorates the stability of grid-connected systems under weak grid. In this paper, a weighted grid voltage fractional-order feedforward control scheme is proposed based on a quasi-proportional resonant (QPR) controller, which introduces two weighting factors in the full feedforward path of the grid voltage and compensates the phase lag of the output impedance by designing appropriate weighting factors. Then, an improved scheme combining the QPR controller with a phase compensator is presented. As a result, the suppression ability of grid current harmonics and the adaptability to grid impedance variations are enhanced. Simulation and experimental results verify the effectiveness of the proposed grid voltage fractional-order weighted feedforward scheme. Proposing a weighted grid voltage fractional-order feedforward control scheme based on a quasi-proportional resonant controller for a fractional-order LLCL-type grid-connected inverter under weak grid conditions, the scheme is improved by an insertion of a phase compensator. The suppression ability of grid current harmonics and the adaptability to grid impedance variations are enhanced.image