Impedance Remodeling Control Strategy of Grid-Connected Inverter Based on Feedforward Voltage under Weak Grid

Under weak grid conditions, the negative impedance characteristics introduced by the phase-locked loop (PLL) in the frequency range above the fundamental frequency can reduce the phase of the output impedance of grid-tied inverter systems, thereby severely affecting the stability of the system. Additionally, grid voltage feedforward control has gained widespread attention in grid-tied inverter control due to its simplicity, effectiveness, and ease of implementation. Among them, research on grid-tied inverter impedance reshaping has been widely applied. In order to seek a simple and effective control strategy to improve the stability of the system, this paper focuses on single-phase LCL-filtered grid-tied inverters under weak grid conditions. By performing system control diagram equivalence transformation, the grid voltage feedforward function that eliminates the impact of the PLL on the system's output impedance is derived. Subsequently, a grid voltage feedforward control-based grid-tied inverter impedance reshaping control strategy is proposed. Furthermore, the optimization design of the grid voltage feedforward function is achieved using function approximation and multi-objective constraint methods, making the proposed control strategy more feasible and universally applicable. The analysis of the robustness of grid-tied inverter systems based on grid voltage feedforward control shows that whether using function approximation or multi-objective constraint methods, this control strategy can reshape the inverter output impedance, improve the phase angle at the crossover frequency, and ensure that the system has a phase margin greater than 30° under weak or fragile grid conditions. It guarantees the robustness of the system without additional phase compensation stages, and has negligible impact on the fundamental current tracking performance of the system. Simulation and experimental verification demonstrate that, regardless of whether function approximation or multi-objective constraint methods are used, the grid current quality of the system based on grid voltage feedforward control has been significantly improved. The grid-tied point voltage and current exhibit no phase deviation, satisfying the requirement of the unity power factor grid connection. Additionally, the system exhibits excellent dynamic performance, effectively expanding the adaptability range of grid-tied inverter systems considering the PLL under weak grid conditions. The optimization design method of the feedforward function based on function approximation facilitates the acquisition of control parameters, greatly improving the feasibility of the grid voltage feedforward control strategy. However, its effect on reshaping the phase characteristics of the system's output impedance is reduced. On the other hand, the method based on multi-objective constraints can adjust and supplement the constraint conditions according to different engineering standards and specifications, enabling the free design of the feedforward function and enhancing the universality of this control strategy. Its drawback lies in high computational complexity and feedforward function parameter acquisition. In practical engineering, the method selection can be made according to actual circumstances. © 2024 China Machine Press. All rights reserved.