Transient Overcurrent Suppression in Fault Recovery Stage for Grid Forming Converter Based on Postfault Active Angle Control

The grid forming (GFM) converter may undesirably output a higher postfault current than the fault duration current, which would cause it to face the severe risk of overcurrent or even damage after the grid fault has already been cleared. The existing methods mainly focus on the fault duration current limitation, while the tailored strategy for transient overcurrent suppression in the fault recovery stage is still missing. This article describes the analytical relationship between transient current and grid voltage considering power angle swing effect in GFM converter. On this basis, it reveals that the grid voltage recovery under a significant power angle value is the root cause of the recovery overcurrent (ROC) phenomenon. To solve this issue, a postfault active angle control (PAAC) strategy is proposed and designed without the need for additional fault grid detection methods. This strategy may help the GFM converter effectively avoid the ROC problem by reshaping the power angle trajectory at the fault recovery stage without compromising resynchronization capability with the postfault grid. Furthermore, the effect of parameters in PAAC control is assessed analytically, and its design criteria are also discussed in detail. Finally, the correctness of the theoretical analysis and the feasibility of the PAAC strategy are both verified in the experimental tests.