Joint Transient Angle and Voltage Stability Analysis and Enhancement of Grid-Forming Converters

As an enabling technology in more-electronic power systems, grid-forming converters have been increasingly popular. However, their transient behavior under grid faults has not been fully explored. In particular, existing literature only investigates the transient angle or voltage stability problem separately, without considering the case where both events occur concurrently. To fill in the research gap, this article provides a comprehensive analysis of both transient angle and voltage stability problems. We first derive the existence condition for equilibrium points. Then, we disclose the attraction domain of the stable equilibrium point by use of the LaSalle invariant set theorem. The derived stable conditions can ensure the safe operation of grid-forming converters during transients. Moreover, we reveal that the active power reduction and reactive power injection both enhance transient angle and voltage stability. Nevertheless, the active power reduction deteriorates frequency regulation, while the reactive power injection increases converter current stresses. Further, to mitigate these negative effects, we propose an active and reactive power co-regulation method to enhance the transient stability of grid-forming converters. Finally, simulation and experimental results verify the theoretical analysis.