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Glossary · Energy industry & energy transition

Frequency Containment Reserve

Frequency Containment Reserve FCR primary control frequency maintenance balancing reserve power system

Frequency Containment Reserve, abbreviated FCR, is called Frequenzhaltungsreserve in German. It refers to the fastest standardised form of balancing reserve in the alternating-current grid. It reacts automatically and in a decentralised manner to deviations of the grid frequency from its setpoint. If the frequency falls because less is generated than consumed, FCR units increase their feed-in or reduce their consumption. If the frequency rises, they reduce generation or absorb additional electricity. FCR thus limits the frequency deviation after a disturbance. However, it does not on its own bring the frequency back to the setpoint.

In the continental European interconnected grid, the nominal frequency is 50 hertz. The frequency indicates the instantaneous balance between feed-in and withdrawal in the entire synchronously coupled system. If a large power plant suddenly fails, power is missing. In the first moments, the rotating masses of the synchronous machines release kinetic energy and thus slow the frequency decline. Immediately afterwards, FCR must take effect.

Technical requirements: characteristic and timing behaviour

The European framework is set by Regulation (EU) 2017/1485, the guideline on electricity transmission system operation (SO GL). Under it, FCR units and FCR groups must activate their reserve via a proportional controller that responds to the frequency deviation. In the synchronous area of Continental Europe, full activation is reached at a deviation of ±200 millihertz, i.e. at 49.8 or 50.2 hertz. Binding minimum requirements apply to the timing behaviour. Activation must not be artificially delayed. For a deviation of 200 mHz or more, at least half of the FCR capacity must be available after 15 seconds at the latest and the entire FCR capacity after 30 seconds at the latest. Between 15 and 30 seconds, the power output must increase at least linearly. For smaller deviations, the same timing behaviour applies proportionally.

A worked example illustrates the characteristic: A plant with 5 MW of FCR provides around 2.5 MW of additional power at 49.9 hertz, i.e. a deviation of 100 mHz. At 49.8 hertz or lower, the full 5 MW is provided. FCR is therefore stated in megawatts, because it describes callable power and not a quantity of energy. How much energy actually flows depends on the duration and magnitude of the deviation. If the full 5 MW were needed for a quarter of an hour, that would amount to 1.25 MWh. In normal operation, the frequency usually fluctuates only slightly around 50 hertz, and the units continuously regulate in both directions.

Sizing in the synchronous area

FCR is sized jointly for the entire synchronous area. For Continental Europe, the SO GL sets a reference incident of 3000 MW, in each of the positive and negative directions. This corresponds roughly to the simultaneous failure of two very large power plant units. The transmission system operators of the synchronous area determine the required FCR quantity at least once a year and allocate it among the individual operators. FCR is thus measured locally and provided locally, but acts across the entire interconnection. Each unit responds to the same common frequency, no matter where the outage occurred.

Procurement in Germany

The four German transmission system operators tender FCR via the joint platform regelleistung.net. This takes place within the cross-border FCR cooperation, in which further European transmission system operators participate. The tender runs daily, and bids are possible until 8:00 a.m. on the previous day. Each day is divided into six products of four hours each. Unlike aFRR and mFRR, FCR is a symmetric product: a bidder that is awarded a contract must be able to deliver positive and negative balancing power to the same extent. The minimum bid size is 1 MW. Remuneration follows the uniform pricing method (pay-as-cleared), so all successful bidders receive the highest price still accepted. Only the capacity held available is paid for. There is no separate balancing energy market with energy prices for FCR, as there is for aFRR and mFRR.

Participation is preceded by prequalification. In it, the provider demonstrates technically that its unit or group reliably meets the requirements. For plants with a limited energy reservoir, the German prequalification conditions additionally require energy storage management. In the normal state, the marketed FCR must be able to be delivered in a secured manner continuously for two hours in a positive or negative direction. The July 2024 version provides for operating point shifts combined with compensating energy transactions for this purpose.

Anyone holding FCR available must operate its plant with headroom upwards and downwards. A thermal power plant then does not run, for example, at its maximum load. A storage unit keeps its state of charge in the middle range. This headroom causes opportunity costs, because the capacity cannot be marketed elsewhere at the same time. Double marketing of the same capacity is not permitted.

Terminological distinctions

In German usage, FCR is usually equated with Primärregelleistung (primary control power) or primary control. This is factually correct. FCR did not replace primary control with a different function but is the European designation for the same reserve quality. This designation makes the function clearer: the deviation is contained, not eliminated.

  • Instantaneous reserve and inertia: Rotating synchronous machines release their kinetic energy without measurement and control. This acts even before FCR and dampens the rate of change of the frequency. FCR, by contrast, is a controlled response to the measured frequency. A battery storage system can deliver FCR without inherently offering synchronous inertia. Conversely, a synchronous generator with inertia is not automatically prequalified for FCR.

  • aFRR and mFRR: The frequency restoration reserves take over after FCR. They are dispatched centrally by the transmission system operators and return the frequency and the exchange balances between control areas to their setpoints. Their activation times are longer: the German market description cites 5 minutes for aFRR and 12.5 minutes for mFRR, compared with 30 seconds for FCR.

  • Redispatch, grid reserve and capacity reserve: Redispatch and grid reserve resolve spatial grid congestion, while the capacity reserve safeguards the adequacy of generation. FCR, by contrast, concerns only the short-term power balance of the synchronous area.

Providers and new technologies

In the past, primary control power came mainly from thermal power plants and hydropower plants. Today, battery storage systems in Germany provide a large share of FCR. Controllable industrial loads and pooled smaller plants are also candidates. Batteries can change their output very quickly and precisely. Their limit is energy content: positive FCR discharges the storage unit, negative FCR charges it. Without state-of-charge management, it could not sustain the reserve during prolonged deviations. For the validity of this statement, it is decisive that storage units must provide proof for this.

Consumers provide positive FCR by reducing their load and negative FCR by increasing it. Electrolysers, refrigeration plants or certain industrial processes may be suitable, for example. The prerequisite is that the process tolerates rapid changes and that the power can be measured. Plants with power electronics, such as photovoltaics or wind energy, can respond to the frequency through suitable control. In the event of overfrequency, they can curtail output. Positive reserve, by contrast, requires that they run throttled and hold back power. Because FCR is procured symmetrically in Germany, they must be able to handle both directions. Whether this is economical depends on prices.

Common misunderstandings and context

FCR is not a particular class of plant and not a reserve power plant, but a function. A hydropower plant, a battery storage system and a controllable load can deliver the same system service. FCR is also not synonymous with security of supply. It safeguards the frequency over seconds to minutes. Whether enough firm capacity is available on a low-wind winter evening is not something it answers. Likewise, FCR is only a very fast and strictly standardised subset of flexibility. Load shifting over hours or seasonal storage do not fall under it.

Overall, FCR shows that in the power system, time scales, response speed and verifiability also count, not just energy quantities. Markets trade electricity in quarter-hours and hours, but the frequency changes within seconds. FCR closes this gap as a distinct, technically precisely defined product. When new providers take the place of conventional power plants, prequalification, monitoring and a balanced market design become all the more important.

Sources

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Edited by eCMS on 12.11.2026 05:05

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