Possible contributions of MT-DC systems with grid-forming properties to AC system stability by providing inertia

∗ inertia. The proposed approach couples the AC frequency and the DC voltage via an 𝑢𝑢DC

(𝑓𝑓AC )droop control, allowing the energy stored in DC capacitors and cables to be exchanged with the

AC system during frequency events. A modified hybrid AC/DC benchmark system with reduced rotating inertia is used for RMS-simulations. The influence of DC-droop parameters on frequency behavior and DC voltage limits is analyzed, and the effect of a DC-chopper for overvoltage protection is evaluated. Simulation results show that an appropriate tuning of the

DC voltage and frequency droop parameters can effectively reduce the frequency nadir, limit the RoCoF, and provide additional damping after disturbances. While the DC-chopper successfully prevents DC overvoltages, its parametrization strongly affects the energy balance between the DC system and synchronous generators. Overall, the results demonstrate that MTHVDC systems with Grid-Forming control can support AC frequency stability by utilizing DCside momentary reserves, highlighting their potential role in low-inertia power systems.