Fault Recovery and Transient Stability of Grid-Forming Converters Equipped with Current Saturation

When grid-forming (GFM) inverter-based resources (IBRs) experience large grid disturbances (e.g., short-circuit faults), the current limiter may be triggered and GFM IBRs enter the current saturation mode, inducing nonlinear dynamical behaviors and imposing great challenges to the post-disturbance transient angle stability. This paper presents a systematic study to reveal the fault recovery behaviors of a GFM IBR and identify the risk of instability. The impact of the angle of the magnitude-saturated current on the post-fault recovery and transient stability is also investigated. The selection of the angle of magnitude-saturated current significantly influences the post-fault behaviors while a few additional dynamical conditions that have a substantial impact are also identified. It is found that the system may follow multiple post-fault recovery trajectories depending on those conditions: 1) Convergence to the normal stable equilibrium point (SEP), 2) convergence to the saturated stable equilibrium point (SSEP), and 3) divergence (instability). To examine the models' accuracy, several cases are simulated.