Small-signal stability and interaction analysis of grid-forming and grid-following converters in converter-dominated power systems

The increase of converter-interfaced generation lead to a power system with different penetrations of grid-forming converters, grid-following converters, and synchronous generators. The converter control loops are prone to different degrees of interaction with synchronous machines, passive elements of the grid, and other converters. This paper addresses the interactions and identifies those which are prone to instability. To this end, we present a mathematical model of a system containing a converter with grid-forming virtual synchronous machine (VSM) control, a converter with grid-following control, a Thévenin equivalent or synchronous generator, as well as Pi lines and a load. Eigenvalue and participation factor analyses, performed in Matlab, allow the investigation of interactions between the elements of the power system. We outline that the interactions between the current controls of both converters and the quasi stationary electrical model of the converter with VSM control can induce instabilities, particularly in the cases of a weak grid or a high share of gridforming control. In addition, we demonstrate that the Thevenin equivalent neglects significant behavior of a converter dominated power system.