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Glossary · Energy industry & energy transition

Frequency Restoration Reserve (FRR)

frequency restoration reserve FRR balancing reserve frequency maintenance aFRR mFRR balancing energy electricity grid

The Frequency Restoration Reserve, FRR for short, is a form of balancing reserve. It is needed when electricity generation and electricity consumption no longer match. The EU legal framework, the Guideline on electricity transmission system operation (Regulation (EU) 2017/1485), describes FRR as active power reserves with two tasks: they are to bring the grid frequency back to its nominal value. In a synchronous area with several load-frequency control areas, they are also to bring the actual power exchange back to the scheduled value. In the continental European grid, the nominal value is 50 hertz. There, FRR gradually replaces the Frequency Containment Reserve (FCR), which is activated first, so that the latter is free again for new disturbances.

FRR acts on the instantaneous power balance. At every moment, the power fed in must equal the power withdrawn plus grid losses. If power is lacking, the frequency falls. If there is too much, it rises. Positive FRR compensates a deficit: a plant feeds in more or a consumer draws less. Negative FRR reduces a surplus: feed-in falls or consumption rises. Unlike FCR, which is provided symmetrically, aFRR and mFRR are procured separately as positive and negative products.

The reserve held available is measured as capacity in megawatts. This must be distinguished from the balancing energy actually delivered or absorbed when called upon, that is, electrical energy in megawatt hours. A provider initially commits only to keeping capacity available. Balancing energy arises only upon activation. A battery storage system, for example, can deliver several megawatts very quickly, but how long it can sustain this depends on its energy content. A power plant can deliver for longer, but, depending on the technology, reacts more slowly and has minimum loads or start-up conditions.

Distinction from FCR, aFRR and mFRR

In terms of timing, FRR comes after FCR. FCR reacts within seconds and in a decentralised manner: the plants measure the frequency themselves, throughout the entire synchronous area, regardless of where the imbalance arose. FCR arrests the frequency deviation but does not restore the balance of a particular zone. This task is taken over by FRR, namely in the control area that caused the imbalance.

In EU law, the central control variable is called the frequency restoration control error (FRCE). In a synchronous area with several load-frequency control areas, it is the area control error (ACE) of the respective area. If an area geographically coincides with the whole synchronous area, it is simply the frequency deviation. The aim of the frequency restoration process is to regulate the FRCE to zero within the frequency restoration time. Each transmission system operator of such an area must set up an automatic and a manual process. This gives rise to the division into aFRR and mFRR. The German transmission system operators determine the aFRR requirement from the control area balance, that is, the difference between scheduled and actual imports or exports, adjusted for the activated FCR.

  • aFRR (automatic FRR, formerly called secondary control reserve in Germany): A load-frequency controller at the transmission system operator calculates the requirement roughly every four seconds. It sends setpoints to the providers via a direct control-technology link. The first response must occur after 30 seconds, and full power must be available after five minutes at most. In the EU, the full activation time of aFRR is limited to a maximum of five minutes.

  • mFRR (manual FRR, formerly minute reserve): It compensates larger imbalances and replaces aFRR so that the latter is available again. The full activation time is 12.5 minutes. "Manual" does not mean telephone or fax. In Germany, activation takes place electronically via the Merit Order List Server (MOLS). Unlike aFRR, mFRR is needed only rarely, usually for a few quarter-hours.

For mFRR, the German transmission system operators distinguish two types of activation. Scheduled activation relates to a specific quarter-hour and is used preferentially. Direct activation is possible at any time, and its activation period ends with the following quarter-hour. Providers receive the activation message at least 7.5 minutes before the start of the activation period. At the start, 50 percent of the requested power must be available, and full power after five minutes at the latest. The regelleistung.net overview states a maximum activation duration of 60 minutes for both aFRR and mFRR, and 15 minutes for FCR.

FRR must also be distinguished from redispatch. Redispatch changes the dispatch of plants in order to resolve grid congestion. FRR compensates power imbalances that show up in the frequency and in the area control error. The same plant may be technically suitable for both, but products, triggers and rules differ.

Why FRR is needed

Electricity markets plan in time intervals, whereas physical operation runs continuously. Forecast errors for wind and solar power, power plant outages, unplanned load changes and schedule errors constantly ensure that feed-in and withdrawal do not match exactly. Responsibility for the reserve lies with the transmission system operators: they determine the requirement, check providers in a prequalification and activate the reserves. Providers can be conventional power plants, pumped storage, battery storage, industrial consumers or aggregators that pool smaller plants. Prerequisites are proven response time, controllability, metering, communication connection and availability.

Procurement and market in Germany

The four German transmission system operators procure FRR jointly via the regelleistung.net platform. There are two markets. On the capacity market, the provision is remunerated, that is, the readiness to keep power available. According to the transmission system operators' system description, awards are made on the previous day (D-1) in six four-hour blocks. On the energy market, quarter-hour products are traded, for which bids can be submitted until about 25 minutes before delivery. The energy actually activated is paid for. The energy market for aFRR and mFRR has existed in Germany since November 2020 in addition to the capacity market.

Activation is optimised Europe-wide. For aFRR, this happens via the PICASSO and IGCC platforms: PICASSO determines a marginal price at every activation, that is, about every four seconds, and IGCC nets opposing imbalances of several zones against each other. For mFRR, the MARI platform is responsible, with a settlement interval of 15 minutes. Germany procures aFRR capacity in the ALPACA cooperation, a voluntary collaboration under Article 33 of the EU guideline on electricity balancing. ALPACA goes back to the Austrian-German cooperation of 2020 and, since September 2025, also includes the Czech transmission system operator CEPS.

The market design creates incentives. Anyone holding reserve cannot sell this capacity elsewhere at the same time. A battery storage system weighs revenues from balancing reserve against intraday trading and arbitrage. An industrial plant checks whether a load adjustment fits with production. A power plant calculates fuel costs, minimum load, start-up costs and wear. Only prequalification, contracts, data flows, activation logic and settlement turn technical flexibility into a usable reserve.

Common misconceptions

  • FRR is not a stock of electricity. It refers first of all to power that can be activated. How long it is available depends on the plants: for storage, on the energy content; for loads, on process compatibility; for power plants, on availability and mode of operation.

  • A frequency deviation is not a national electricity shortage. The continental European grid is synchronously coupled, so a deviation affects the whole synchronous area. FCR is activated everywhere, whereas FRR is activated in the area that caused the imbalance. Settlement is carried out separately: the transmission system operators charge the costs of the deviations to the responsible balance groups via balancing energy. The FCR energy that flows across area borders is settled among the transmission system operators as unintended exchange.

  • More wind and solar power does not automatically mean more demand for every type of reserve. The demand depends on forecast quality, market time grids, plant behaviour, flexibility and European cooperation. Each load-frequency control block sizes its reserves under its own rules within the framework of the EU requirements.

  • FRR is not the same as security of supply. It contributes to short-term system security. It does not replace sufficient generation and grid capacity, fuel availability and long-term planning.

Role in a more flexible power system

Heat pumps, electric vehicles, electrolysers, storage and flexible production processes may in future provide more FRR. What matters is that they can be reliably activated. An electric car that happens to be plugged into a charging point is not yet a balancing reserve. That requires aggregation, controllability, metering and assured availability, which prequalification demands. Overall, FRR is the organised capability to restore the power balance of a control area after disturbances and to take over from the fast frequency support. The term is precise when understood as an operational instrument in the interplay of technology, market and grid responsibility. It becomes blurred when used across the board for storage needs or any kind of flexibility.

Sources

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Edited by eCMS on 12.11.2026 05:05

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