  Grid-forming (GFM) capability requirements are increasingly imposed on
grid-connected voltage-source converters (VSCs). Under large grid disturbances,
GFM-VSCs need to remain stable while providing GFM services. Yet, such
objectives, as pointed out in this paper, inherently lead to conflicting
requirements on the dynamics of internal voltage source (IVS) of GFM-VSCs,
i.e., the fast IVS dynamics is needed to avoid the loss of synchronism with the
grid, whereas the slow IVS dynamics is preferred for maintaining GFM
capability. To tackle this challenge, an adaptive fast/slow IVS control is
proposed, which switches GFM-VSC between fast and slow IVS dynamics based on
system needs. The proposed method enhances the transient stability of GFM-VSC,
whilst maximizing its capability of providing GFM service. Further, the
approach is robust to different grid strengths and different types of grid
disturbances. The experimental results verify the theoretical findings and the
effectiveness of the proposed control method.
