Multifunctional grid-connected inverters are able to modulate their output current by utilizing the harmonic detection

therefore are commonly used to improve the power quality in strong grids. However

under weak grid conditions and grid impedance fluctuations

significant background harmonics of grid voltage will be generated

distorting the output current of these inverters. The background harmonic problem under weak grid can be effectively suppressed by using existing capacitor voltage feed-forward control strategy

but the existence of feedforward path will worsen the system stability. To address these issues

this paper proposes a weighted resonant capacitive voltage feedforward control strategy

which can effectively suppress the background harmonics and improve the system stability. Firstly

a multifunctional grid-connected inverter control system based on the existing capacitive voltage feedforward control strategy is mathematically modeled

and the system stability is analyzed by using the magnitude/phase-frequency characteristic curve. Secondly

a PR controller is connected in series in the second-order feedforward path to reshape the magnitude/phase-frequency characteristic curve of the system

so as to improve the power quality

suppress the harmonics and improve the system stability. At the same time

the coupling effect between the system output impedance and the grid impedance is taken into consideration

and the impedance analysis method is used for system impedance modelling to analyze the causes of instability of the multifunctional grid-connected inverter system with improved control strategy under a weak grid. On this basis

a weighted resonant capacitive-voltage feedforward control strategy is proposed. A simulated annealing algorithm is used to calculate the weighting factor and the proportional feedforward and second-order feedforward paths are introduced to further improve the robustness of the system under impedance fluctuations. Finally

the effectiveness of the proposed control strategy under impedance fluctuation is verified by simulations.