100 % Converter-based Distribution Grids: Determining The Required Grid-forming Penetration to Ensure Small-Signal Stability

The massive integration of distributed energy resources in distribution grids is leading to local energy generation at the distribution grid level which represents a radical change in the traditional top-down structure of power systems. With the proliferation of battery energy storage systems, distribution grids can be islanded in order to enhance reliability. The formed islands will be in most cases, fully converter-dominated, raising the question of the required penetration of grid-forming converters for a stable operation. This paper presents a case study that identifies the required grid-forming penetration in a 100 % converter-based distribution grid. For that purpose, common grid-forming control strategies are compared and control interactions between grid-forming converters (GFMs) and grid-following converters (GFLs) are analyzed. The results indicate that the required grid-forming penetration strongly depends on the respective grid-forming control strategy, which is a consequence of various controller interactions between GFM and GFL.