Improved Design of PLL Controller for $LCL$-Type Grid-Connected Converter in Weak Grid

When LCL -type converter is attached to weak grid, its current control and phase-locked loop (PLL) will interact with each other, via the point of common coupling voltage. Unfortunately, the conventional PLL controller design methods are mainly for PLL independent systems, regardless of the aforesaid interaction. As a consequence, PLL dynamic might deteriorate the grid current control and even leads to instability problem. For this issue, this article proposes an improved design of PLL controller parameters to mitigate the negative effect of PLL on the current control. First, the small-signal model of the current control system considering PLL effect is established, and the system stability with the conventional PLL controller design method is analyzed. Then, an improved design of PLL controller parameters is proposed, and the design guideline is given in detail. With the method, not only the dynamic and static response performance of PLL independent system can be maintained, but also the negative influence of PLL dynamic on the current control can be effectively reduced in weak grid. Moreover, the grid current control has a strong robustness to the grid impedance variation. Finally, the proposed method is validated by the simulation and experiment.