Modeling of Converter Synchronization Stability Under Grid Faults: The General Case

The synchronization dynamics of grid-connected power converters are known to have a tremendous impact on transient stability and fault ride-through performance under grid faults. Up till now, the modeling of converter synchronization stability, which has paved the way for numerous enhanced control methods, is developed around the assumption that the grid fault is symmetrical. However, this is rarely the case. Moreover, grid codes require dual-sequence current injection during asymmetrical faults, which implies that the previously developed models are no longer valid during unbalanced conditions. To address these issues, this article identifies the necessary stability conditions during asymmetrical conditions and presents a quasi-static large-signal reduced-order model of a grid-following converter for analyzing its synchronizing interaction with the external network during symmetrical and asymmetrical grid faults. The modeling approach is developed and tested for three different short-circuit faults: A single line-to-ground fault, a double line-to-ground fault, and a line-to-line fault. The accuracy of the proposed model is verified through detailed simulation studies and experimental tests. Thus, this model can be used to assess the transient synchronization stability of grid-following converters during any type of grid fault, and due to its low-order representation, it may be well applicable for large-scale power system studies.