aFRR: the automatic restoration loop

aFRR stands for automatic Frequency Restoration Reserve. After FCR has stopped the frequency from falling further, aFRR is the reserve that pulls it back to 50.00. It operates on a slower timescale (seconds to minutes), and unlike FCR (which each unit decides for itself based on local frequency), aFRR is dispatched from one central place: Svenska kraftnät’s control room.

If FCR is a reflex, aFRR is a coordinated response.

What aFRR actually does

flowchart TB
    A([Frequency stable at 49.85 Hz<br/>FCR is holding it])
    B([Svenska kraftnät calculates<br/>how much more power is needed<br/>updated every few seconds])
    C([Sends a signal<br/>to contracted aFRR units])
    D([Units adjust output<br/>frequency climbs toward 50.00])

    A --> B --> C --> D

    style A fill:#fecaca,stroke:#b91c1c,color:#7f1d1d
    style B fill:#fef3c7,stroke:#a16207,color:#713f12
    style C fill:#fed7aa,stroke:#c2410c,color:#7c2d12
    style D fill:#dcfce7,stroke:#15803d,color:#14532d

The signal is called the AGC signal (Automatic Generation Control). It is updated every 2 to 10 seconds. Each contracted unit receives its share of the signal in real time and adjusts its output accordingly.

aFRR is automatic in the sense that no human operator picks individual unit responses. The signal handles that. But it is centralised in the sense that the signal comes from one TSO computer, not from each unit’s local frequency measurement.

How big and how fast

The volume needed varies by hour, depending on the size of the largest credible imbalance, expected forecast errors, and the FCR reserves already procured.

How it gets bought

aFRR is procured both ahead of time (capacity) and at activation (energy).

flowchart TB
    A([Capacity auction<br/>D-1 or day-of<br/>SEK per MW per hour<br/>for being available])
    B([Energy bidding<br/>real-time merit order<br/>SEK per MWh<br/>when the signal activates you])

    style A fill:#fed7aa,stroke:#c2410c,color:#7c2d12
    style B fill:#fed7aa,stroke:#c2410c,color:#7c2d12

This two-part payment is normal for the deeper reserves. The capacity payment compensates the asset for being ready. The activation payment compensates for the energy it actually delivers.

A hydro plant contracted for 20 MW of aFRR up gets the capacity fee every hour, whether it is activated or not. When the AGC signal asks it to deliver 5 MWh of upward energy in a given hour, it also gets the activation payment for that 5 MWh.

Who provides aFRR

aFRR favours plants that can respond reasonably fast but for longer durations than a battery can typically sustain.

• Hydro. The dominant Swedish provider. Reservoir plants can sustain higher output for an hour or more.
• CHP and bioenergy. Some providers in Sweden.
• Large batteries (2 to 4 hour duration). Increasingly competing, as the European market design favours them.
• Industrial demand response via aggregators. Some pilots, growing.

A 1-hour FCR-D battery is too small for aFRR. A 4-hour battery starts to make sense, especially as the European aFRR market couples (see below).

The European coupling: PICASSO

Like day-ahead has SDAC and intraday has SIDC, aFRR has its own European coupling project: PICASSO (Platform for the International Coordination of Automated Frequency Restoration and Stable System Operation).

PICASSO lets aFRR bids in one country be activated from a TSO in another, as long as cross-border capacity allows. The Nordics are joining PICASSO in phases. This will widen the pool of aFRR providers and change the economics for Swedish suppliers.

What this means for an engineer crossing in

aFRR is more technically demanding than FCR. The asset has to listen for a real-time signal, sustain a multi-minute response, and report telemetry continuously back to Svk. Building dispatch software for aFRR is a step up in complexity from FCR-D.

The reward is access to a larger market, increasingly Europe-wide. As PICASSO rolls out, aFRR will be one of the most interesting growth areas in the Nordic reserve markets.

Next

aFRR runs for a few minutes. Then mFRR takes over for the longer haul. See mFRR: the manual reserve.