Nonlinear Behavior and Transient Stability of Grid-Following Converters Using Direct Power Control under Weak Grid

Ensuring reliable operation of renewable energy sources requires robust grid-connected converters. Depending on the choice of synchronization methods, grid-connected converters may exhibit distinct nonlinear behavior that plays a vital role in determining their transient stability. Recently, the grid-voltage-modulated direct power control (DPC) has been proposed as an alternative to the conventional phase-locked loop (PLL) to enhance the dynamic response of the grid-following converter (GFLC). However, existing studies have primarily treated the DPC-based GFLC as a linear system. In this paper, we investigate the nonlinear behavior of this converter under weak grids using a large-signal model based on double reference frames. Our findings reveal that the DPC-based GFLC demonstrates sustained oscillation. Interestingly, the stable periodic orbit observed does not arise from a Hopf bifurcation but rather a saddle-node bifurcation of periodic orbits. This critical bifurcation is characterized by the coexistence of a stable periodic orbit and a stable equilibrium point, resulting in a sudden contraction of the converter's stability region. Furthermore, we provide a comparison between the nonlinear behavior of PLL-based GFLCs and DPC-based GFLCs. To validate our findings, we present full-circuit simulations and laboratory experiments.