Transient Stability Analysis and Damping Control of Grid-Following Inverters Considering DC-Link Voltage Control and Its Output Limit

For grid-following inverters, grid voltage sag may induce the loss of synchronization under weak grids. Numerous efforts have been devoted to the transient instability dominated by phase-locked loops, while treating the dc input of inverter as an ideal constant voltage source. However, the dc input is usually connected to the photovoltaic array. The active current reference generated by the dc-link voltage control is not constant, and it may reach its software upper limit under grid fault. The impact of dc-link voltage control and its output limit on the transient stability has been ignored in most existing studies. Therefore, a nonlinear model is established considering the coupling of dc-link voltage control with its output limit and phase-locked loop. It is found that there is still a risk of transient instability after triggering the limit, while using the traditional reduced-order model may give incorrect stability analysis results. Then, the influence of different parameters with current limit is discussed, followed by the stability-oriented controller parameter design. A damping control method is further proposed for the transient stability enhancement. Finally, the correctness of the above analysis is verified through experiments.