As the share of renewable, converter-based generators increase the stability and reliability, concepts for grids must be rethought. Distributed and renewable generation from solar photovoltaic and wind power must be combined with other technologies such as battery storage and demand management to balance demand and supply continuously. The components of such a mix include conventional grid-following converters (GFLC) and grid-forming converters (GFMC). The latter type can emulate inertia, while the currently more common GFLCs lack such ability by design.

Previous grid events, e.g., the blackout on the Iberian Peninsula 2025 and the attack on the distribution grid of Berlin 2026, have shown the importance of grid stability and reliability. Distribution grid stability is influenced by multiple parameters. This work investigates the impact of GFMC shares, grid strength at the connection point to the higher voltage grid, and the grid topology on frequency stability.

To this end, we utilize synthetic medium voltage grid models of Germany that are derived using openly available data and an ant-colony optimization algorithm. The synthetic grids are clustered to five representative categories differing in their topology. The higher voltage grids are simplified by a transformer, a subtransmission line, and a voltage source with an internal impedance. Adapting the internal impedance of the voltage source renders different Short-Circuit Ratios and, hence, different grid strengths at the connection points of the medium voltage grids. To implement the dynamic character into the stationary synthetic grids, their distributed generators are equipped with dynamic models to account for either GFLC or GFMC. The investigated operation scenarios differ in terms of the electricity generation provided by GFMC (0%, 20%, and 40% of the grid’s demand) while the remainder is supplied by GFLC.

By testing different events, e.g., under- over-voltage, short circuits, or frequency ramps, initiated by the external voltage source, we evaluate the dynamic response of medium voltage grids in different scenarios. To this end, we analyse active and reactive power flows at the medium voltage/high voltage transformer as well as generator tripping. We identify critical scenarios for a stable medium voltage grid and derive recommendations for grid operators. Additionally, an automation framework is created which can be used to aid the simulation process in future scientific work and enable upscaling. Hence, this work introduces an essential step towards a stable operation of future networks dominated by converter-based generation.