Impact of AC Voltage Control and Reactive Power Control on Transient Stability of Grid-Forming Converters With Current Saturation Under Phase to Ground Faults and Phase Jumps

Grid-forming (GFM) converters are increasingly deployed to support power system stability in low-inertia grids. However, the impact of voltage magnitude control strategies on transient stability under fault conditions remains insufficiently understood. We investigate the transient stability of a droop-controlled GFM converter with a circular current limiter, taking into account the choice of voltage magnitude control strategy. Mathematical models are derived for both normal operation and, more importantly, current saturation, and numerical examples are provided to illustrate the implications of the proposed models. In fault scenarios, voltage sag and phase jumps are analyzed separately. These theoretical results are further validated through simulations in Simulink. Our results necessitate the individual analysis of phase jumps and reveal that different voltage magnitude control strategies result in substantially different transient behaviors. In particular, reactive power control enables the converter to exit current saturation, whereas AC voltage control may prevent recovery. These findings provide insights for converter control design and enhance the robustness of GFM converters under grid disturbances. Furthermore, the analysis of phase jumps shows that, under AC voltage control, a larger phase jump angle does not necessarily lead to more adverse transient behaviors, and this reveals that the current Grid Code testing requirements for transient stability of GFM converters with respect to phase jumps, which rely on a maximum phase jump angle, are inadequate and should be revised.