Origin of anomalous instability of grid-forming converters tied to stiff grid

Grid-forming (GFM) converter is believed to be highly promising in the future power systems, due to its ability of providing voltage and frequency support. However, some recent studies have shown that the GFM converter may suffer from instability in stiff grids, which seriously hampers its application. In this paper, the mechanism of this anomalous effect is studied by using the small-signal stability analysis in detail. First, a detailed state-space model of a single converter grid-tied system is established from the first principle, and by using the participation factor analysis, the interaction between the terminal voltage loop and the power synchronization loop is identified as the major cause for the system instability. Then relying on a reduced-order model containing only these two controls and using two classical analytical methods including the Routh criterion analysis and the Phillips- Heffron model of complex torque analysis, the origin of this anomalous instability of GFM converters tied to stiff grid coming from the negative damping provided by the terminal voltage loop is well uncovered and the critical grid strength is well predicted. In addition, these results may provide ideas for subsequent control optimization and stability improvement of GFM converters under various situations.