Inertia Market Part 2: Brownfield vs Greenfield: How Digital Twins Enable BESS Participation in the German Inertia Market

The FNN now gives a clear direction for new stations (greenfield), but it also opens the door for existing stations (brownfield) to be upgraded and pre-qualified for the inertia market. For co-location projects – Wind + BESS or PV + BESS with true GFR capability – I see very strong chances to secure the “basic” inertia revenue (availability ≥ 30 %) and then earn additional upside by operating with higher availability.

Grid-forming BESS sits naturally at the forefront of fulfilling the inertia requirements. That is attractive from a financial perspective because it creates a relatively stable capacity-type revenue stream in the model. Keeping permanent headroom for inertia can even have a positive side effect on degradation by reducing the effective DoD and avoiding deep cycles.

Some papers point to potential negative impacts from fast services due to current peaks and thermal stress. My working hypothesis is: as long as continuous and short-term C-rate limits are respected, temperatures are controlled, and SOC is kept in a mid-band, the additional ageing from inertia pulses will be limited and could be compensated – or even over-compensated – by the inertia income. In the best case, inertia revenues could pay for the “premium” long-life cells (Li-rich cathodes, extended cycle-life LFP, etc.). But this is a hypothesis that must be validated with proper mission-profile and degradation studies, not marketing slides or just my opinion.

Inertia is only for GFR BESSs. Pre-qualifying an existing BESS from GFL to GFR is not just a click in the EMS screen. It is essentially changing the identity of the station from being current source to be voltage source or the “big brother” that shapes and stabilises the grid

This change affects everything: short-circuit behaviour, SCR sensitivity, protection philosophy, interaction with nearby converters and synchronous machines. The FNN is very explicit: this is a “wesentliche Änderung” which requires new tests, new models and a new certificate as a netzbildende Typ-2-Einheit .

Unlike pure GFL, in GFR mode the full RMS/EMT package becomes mandatory. You must prove that the station can:

behave as a controlled voltage source,
operate in weak-grid conditions,
synchronise cleanly with nearby grid elements and the wider system,
and provide the contracted Momentanreserve without instability.

In other words: a digital twin is no longer “nice to have”, it is necessary.

Digital twin: three layers for German prequalification

I like to think of the digital twin in three layers:

Offline RMS and EMT
RMS (PSS®E, PowerFactory) for the wider grid and multi-second dynamics.EMT (PSCAD) for the station and the local grid around the POC and the benchmarking step to make sure RMS and EMT tell a consistent story for the relevant events.
HIL (Hardware-in-the-Loop)
Controller-HIL on RTDS to connect the real PPC/EMS and control hardware to the EMT grid model.This is where you check that the real code behaves like your “ideal” control blocks in PSCAD.
PHIL (Power-Hardware-in-the-Loop)
Though expensive, it is the most convincing layer.A real converter or string is connected via a power amplifier to the simulated grid to test current limits, DC-link behaviour and short-term overload in realistic inertia scenarios.

The measurement side of these tests aligns naturally with FGW TR3 , and the modelling side with FGW TR4. Together they form the German context in which the FNN Hinweis and the TSOs will judge whether your BESS is truly netzbildend and inertia-capable.

Digital twin to prequalify existing station for inertia

For new stations, this is a relatively straightforward path: you design hardware and controls with GFR + inertia in mind, build the digital twin in parallel, validate via HIL/PHIL and then go through the FNN/FGW certification chain.

For existing stations, the story is more challenging:

You start from a built asset, not from a clean target.
Your PCS hardware, transformer and protection are already fixed.
You must work “inside” those limits and ask: how much GFR and how much EMom is realistic?

If your PCS has sufficient overload margin and is by design capable of GFR/VSM control, and if the financial analysis looks promising, then it is worth pursuing. But always remember: you are starting from an existing asset, not from a blank sheet aiming at “perfect compliance”.

So ask yourself, very honestly:

How much headroom can you reserve for inertia within your business case?
Can the PCS deliver the required dP/df without current saturation or thermal issues?
Can your grid-forming controls interact cleanly with nearby grid elements without mis-sync or oscillations?

If the answer is “yes”, then a reasonable retrofit path could look like this:

Install a PMU at the POC (or at another node that sees the relevant grid dynamics).This gives you real disturbance data as input for test cases and for tuning the models.
Use the OEM models as a baseline and adapt them to mimic the behaviour of your station. In my experience, this is the most challenging part. If the station is originally designed as GFL, you have to add the VSM/GFM layer, tune it, and then verify the combined behaviour in simulation.
Run offline RMS/EMT simulations. I personally prefer offline EMT here, because you have “unlimited” processing time compared to HIL. You can explore many scenarios, parameter sets and fault locations without being constrained by real-time execution.
Respect the DC side: the DC bus is not an infinite bus. It can easily become the bottleneck in the station’s inertia response.Therefore, use HIL/PHIL to test the real station (or at least a representative string) and verify that current limits, DC-link dynamics and protection behave as expected under worst-case inertia events.
Follow the FNN path to become a grid-forming station.This path includes not only inertia but also black-start capability and islanding behaviour.Once you add black start and GFR into your portfolio, you are moving closer to the KRITIS universe and should expect additional requirements, including information-security obligations (ISO 27001 and related standards).

For me, the inertia market is not “free extra revenue”, it is a filter that separates marketing slides from real engineering. Whoever is willing to invest now in grid-forming capability, EMT models, digital twins, HIL/PHIL and the FNN/FGW path will not only secure a new stable cashflow, but will also define how the German grid looks in 2030+. And this is exactly where I see BESS – especially in hybrid plants – playing a leading role, if we are honest about the limits, transparent about the risks, and serious about the physics behind that small word: inertia.

Originally published on LinkedIn Pulse. Open the original →