Effects of PLL in the Damping Unit of a Gird-Connected VSG

As a promising control method for grid-forming converters, the virtual synchronous generator (VSG) control strategy was developed quickly using different damping units. A straightforward approach is to emulate the dynamics of a synchronous generator (SG), where a phase-locked loop (PLL) is employed to increase the transient damping ratio by measuring the angular frequency of the grid. Ignoring the effects of PLL, this damping method can effectively improve the active power stability of a grid-connected VSG. However, the impact and the mechanism of the introduced PLL are not distinct. Considering the effects of PLL, the dynamic model of a grid-connected VSG is established in this paper. Further investigations reveal that the PLL will introduce a pair of unexpected eigenvalues, which will trigger instability when the bandwidth of the PLL is higher than a critical value. Finally, the theoretical analysis is verified by simulation results.