Stability Analysis of LCL-Type Grid-Connected Inverter Under Single-Loop Inverter-Side Current Control With Capacitor Voltage Feedforward

When single-loop inverter-side current control is used in the LCL-type inverters, there may be more than one stable region with regard to computation delay in control path. The system is stable if computation delay is small enough, but it, named as the first stable region, may be too narrow to finish the control codes in high-frequency system. On the other hand, the second stable region is highly sensitive to the grid impedance variation and is hardly applicable in weak grids. To deal with the situation, this paper investigates the influence of the capacitor voltage feedforward on the system stability considering flexible computation delay in feedforward path. It is found that the capacitor voltage feedforward is able to suppress the resonance of the LCL filter. However, a new resonance may arise if computation delay is not carefully handled. The influence of computation delay in both forward path and feedforward path on system stability is systematically analyzed. The analytical results show that after applying feedforward control with optimized delay, the system stability is greatly improved and is not sensitive to the grid impedance variation. The simulation and experiment results verify the analytical results.