aFRR, the reserve in the middle

aFRR (automatic Frequency Restoration Reserve) is the second-tier balancing product on the European grid — fully activated within roughly 5 minutes of a frequency deviation, with the TSO sending a closed-loop control signal that the BSP follows on a 4-second cycle. aFRR brings the system back to 50 Hz after FCR has contained the initial deviation. Estonia connected to the EU PICASSO aFRR-energy platform on 9 April 2025.

FCR catches the fall. mFRR cleans up afterwards. aFRR is what happens in between — and it is arguably where the cleverness of the European balancing system actually lives. (Sister posts on FCR and mFRR for context.)

When a large generator trips offline, frequency starts dropping immediately. FCR responds in seconds, holds the line, prevents collapse. But FCR is a stopgap. The system also needs to be brought back to its nominal 50 Hz, and to the cross-border flows that the day-ahead schedule said would be happening. That restoration job belongs to aFRR. It activates inside thirty seconds, ramps in over the next five minutes, and pulls the area back into spec while the slower mFRR queue is being assembled behind it.

The mechanics are different from FCR in a way that matters. FCR units sense frequency locally and respond on their own droop curve. aFRR units sit and wait for a signal. Every four seconds the TSO’s Automatic Generation Control (AGC) computes the area control error, the gap between actual frequency and 50 Hz combined with the deviation of cross-border flows from schedule, and dispatches a corrective setpoint to each participating BSP, proportional to its committed capacity. The asset must follow that signal accurately. Control errors are measured and penalised.

Procurement is split into two products. BSPs sell capacity (the obligation to be available when called) and activation energy (the MWh actually delivered when the signal arrives). Both are priced separately, both are typically procured for upward and downward regulation independently. In Estonia, Elering auctions both daily. An asset that can ramp in both directions, a battery being the obvious case, can sell into both products and double its addressable revenue.

Cross-border, aFRR has been quietly transformed by PICASSO, the European platform for exchanging aFRR activations between control areas. Instead of each TSO activating only its domestic reserves, PICASSO pools the merit order across participating countries and activates the cheapest available MW continent-wide. It is a quietly enormous market-coupling project, with member TSOs joining in waves, and it changes the price formation for activation energy substantially.

What makes an asset suitable? Controllability on a four-second timescale, accurate signal-following, and the comms infrastructure to receive setpoints reliably. Batteries excel because their response is essentially instantaneous and their tracking error is tiny. Modern hydro and gas turbines participate too, and demand-response aggregations are starting to. The economic case for batteries is interesting: aFRR rewards control accuracy more than raw capacity, which is exactly the dimension on which batteries beat thermal plants. Activation prices during system stress can be very high, but activations are intermittent, so revenue depends on both capacity payments and the frequency of stress events.

Estonia’s aFRR market in its current form launched in April 2025, when Elering joined the PICASSO platform and the Baltic capacity market opened for aFRR. Battery participation has grown quickly since. Volton is a certified BSP at Elering and trades aFRR daily for the assets in our portfolio, alongside other balance service provider duties. Following a four-second signal cleanly, day after day, is less glamorous than chasing day-ahead spreads, but it is where a meaningful share of the value of a flexible asset is realised.