Like almost every other power system, frequency stability in Germany used to be an almost automatic by-product of synchronous generators. As coal, nuclear and gas units are decommissioned and converter-based renewables dominate, that “free” buffer shrinks. The RoCoF rises, oscillatory interactions become more likely in weak pockets of the grid, and the system becomes more sensitive to large disturbances and system splits. This is exactly the risk picture that motivated the German Roadmap Systemstabilität“system stability”

The roadmap is explicit: by 2035 the German system must remain secure with very high shares of renewables, which won't be possible without big and efficient fleet of GFR-Storage units (Storage Type 2).

The system stability “ Systemstabilitätsbericht 2025“ report in 2025 acknowledges the importance of GFR-BESS in providing system stability by providing many services and among others Inertia. Two main documents followed this report to regulate the inertia market BNetzA decision BK6-23-010 (22 April 2025) – for the market design and just last month the FNN Guideline “Technical requirements for grid-forming capabilities including provision of inertia” was published.

In contrast to some other codes, the FNN guideline takes a clear control-theoretic stance. Here, “inertia” is not defined as “X MW within Y milliseconds”, but as the terminal behaviour of a grid-forming (GFM) unit modelled as a voltage source behind an effective impedance – a Thevenin equivalent that must satisfy stringent dynamic requirements.

So while the Nordic FFR product focuses on very fast active-power response (full activation typically within 700 m.s. –1.3 s) to support frequency, the FNN framework defines inertia differently and requires grid-forming behaviour. Germany intentionally goes further: it does not just want fast response, it wants converter fleets that collectively behave like synchronous machines in terms of small-signal stability and RoCoF ride-through. This is a very healthy signal that Germany is preparing its grid for much higher IBR (inverter-based resources) and a deeper green transition.

For BESS inertia is a very good and promising market, since it is more about readiness and availability than MWs and SOC. We are still waiting for the regional price lists (to be published next Jan. 2026), yet it is a new secured revenue stream that reduces risks on the financial models and make the project more interesting from an engineering perspective and more attractive for the grid operator at all voltage levels.

TSOs are expected to define Beschaffungsregionen for Momentanreserve „zones for inertia“. This reflects the fact that local RoCoF and oscillations matter, so:

· Large HV-connected BESS close to the system’s centre of inertia (COI) have a strong influence on system-wide RoCoF and are more favourable as first responder to RoCoF recovery.

· Smaller MV-connected BESS can significantly improve local RoCoF and damping in weak grid pockets, where disturbances occur and DSOs struggle with stability.

In practice, the binding constraint for BESS in the inertia market is less the total MWh installed, and more:

· the ability to pass the grid-forming and damping tests ,

· SOC / power-headroom management to satisfy energy Vorhaltung and availability, and

· robust aggregation concepts across multiple converters and sites.

In the second part of this article I will try to explain the testing and pre-qualification process for the inertia market based on the available documents and standards. But already now the message for BESS developers is clear: design your BESS stations future-proof , and enjoy the “magic” of batteries and power electronics.

See you in the next part.

Originally published on LinkedIn Pulse. Open the original →