As the share of renewable, converter-based generators is rising and synchronous generators are phased out, grid-forming converters need to provide essential grid stabilising ancillary services.

Compared to centralised synchronous machines in high-voltage grids, the effects of decentralised grid-forming converters in medium-voltage grids remain insufficiently analysed.

This study investigates five representative synthetic, geographically referenced German medium-voltage grid models, selected from 2,789 models using clustering. Dynamic simulations examine the response of medium-voltage grids to fault events under varying combinations of grid-forming converter shares and coupling strengths at the point of common coupling to the high-voltage grid.

In some voltage ride through cases, lasting power and angle oscillations occurred when grid-forming converters were present.

For the investigated models, voltage-dip propagation and the activation of protection functions differed between grid classes.

During three-phase short circuits, stronger coupling was associated with improved frequency minimum and frequency rate of change values.

Increasing the grid-forming converter share from zero to forty percent produced smaller improvements, most notably under weak coupling conditions.

These results are conditional on the selected converter controls and simplified high-voltage-grid representation.

Further validation with alternative controls and more detailed external-grid models is required.