A Double-PLLs-Based Impedance Reshaping Method for Extending Stability Range of Grid-Following Inverter Under Weak Grid

Conventional vector current control scheme has been widely used in grid-connected inverter systems. However, it suffers from severe power limitation issues under weak grids. This article reveals that the coupling effect between the power delivery and the voltage at the point of common coupling causes the static power limit, which only depends on physical parameters of the system. When the stability of the control scheme is considered, negative resistance characteristics of the phase-locked loop (PLL) mainly result in the small-signal stability limit, which is called the dynamic power limit in this article. Because the small-signal stability limit can be improved by using different control methods, it attracts lots of research attention. In this article, a simplified d–q small signal impedance model is introduced to show the destabilizing factors. Then, an initial small-signal impedance reshaping method is proposed to counteract the major destabilizing factor caused by the PLL. Based on the initial impedance reshaping method, an improved control scheme is proposed. Through exhaustive mathematical analysis, it is proved that the dynamic power limit can be extended almost to the static power limit by using this proposed method. Finally, simulation and experimental results verify the effectiveness of the proposed method.