aFRR (Automatic Frequency Restoration Reserve)
Also known as: Secondary Reserve, Secondary Control Reserve
A frequency regulation product that automatically restores grid frequency to its nominal value (50 Hz in Europe) after the initial containment response. Activated on a timescale of 30 seconds to several minutes. Part of the broader FRR product family defined by ENTSO-E.
In energy storage
BESS must pass a TSO prequalification test demonstrating full activation within 5 minutes to participate in aFRR. Bids can be symmetric (equal up and down regulation) or asymmetric, which allows operators to adjust offered capacity based on state of charge. Participation typically requires reserving an energy buffer — commonly 15 to 60 minutes of capacity depending on the TSO — at both ends of the SoC range to ensure sustained delivery.
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