Rethinking Current Controller Design for PLL-Synchronized VSCs in Weak Grids

This paper revisits the design of the current controller for grid-connected voltage-source converters (VSCs), considering the dynamic impacts of the phase-locked loop (PLL), weak grids, and of voltage feedforward (VFF) control. First, a single-input single-output transfer-function-based model is proposed to characterize the interactions of control loops. It is analytically found that the proportional gain of the current controller essentially aggravates the instability effect of PLL in weak grids, while the cutoff frequency of the low-pass filter used with the VFF loop has a non-monotonic relationship with the PLL-induced instability. Then, based on these findings, a guideline for re-designing the current controller of PLL-synchronized VSCs is developed, which enables a co-design of the current controller and VFF controller. Lastly, simulation and experimental results confirm the validity of theoretical analyses.