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Ancillary Services - Frequency Restoration Reserves

Written by
Tomás Oliveira

Reserve power is a critical component of a reliable electrical grid system, and it is categorized into primary reserve, secondary reserve, and tertiary reserve based on their activation times and the duration of activation. These frequency restoration reserves ensure that the grid can respond to sudden changes in supply and demand, maintaining a stable and reliable supply of electricity. 

6 min
6th Sep, 2023


Frequency Containment Reserve (FCR), Automatic Frequency Restoration Reserve (aFRR) and Manual Frequency Restoration Reserve (mFRR) and Restoration Reserves (RR) are specific categories of reserve power that play a vital role in maintaining grid stability, especially in response to sudden changes in frequency.

In order to maintain the frequency within specific limits, Transmission System Operators (TSOs) implement balancing services which they obtain from providers known as Balancing Service Providers (BSPs). While the requirements may vary from one country to another, following the harmonization of balancing markets in Europe, BSPs are now obligated to ensure they deliver the reserve service within a 5-minute time frame (referred to as Full Activation Time). After an initial 30-second period, aFRR takes over as the secondary reserve, while the tertiary reserve, known as mFRR, provides support or partially takes over from aFRR after approximately 12.5 minutes. Here's how they relate to the broader concept of reserve power:


Frequency Containment Reserve (FCR):

As the primary reserve, the FCR seeks to maintain equilibrium between power generation and consumption within the Synchronous Area, with the objective of ensuring the operational reliability of the Synchronous Area's power system and stabilizing the system frequency at a constant value

  • Activation Time: Primary reserve units are the quickest to respond to sudden changes in demand or supply. They are typically required to respond within seconds, usually less than 10 seconds.
  • Duration of Activation: Primary reserves are designed to provide a short-term burst of power for a limited time, usually up to 15 minutes.
  • Operation: Primary reserve power is generated by power plants (usually gas turbines or hydroelectric generators) that are already online and synchronized with the grid but operating below their maximum capacity. When there is a sudden increase in demand or a generator falls offline, primary reserve units can quickly increase their output to stabilize the grid.


aFRR (Automatic Frequency Restoration Reserve):

Serving as the secondary frequency control mechanism, aFRR is responsible for fine-tuning the active power output to rectify frequency deviations and bring interchanges with other systems back to their designated levels following an imbalance, ultimately returning the frequency to its target value.

  • Activation time: aFRR, also known as secondary reserve or spinning reserve, is the fastest-responding reserve category. It is activated automatically in response to deviations in grid frequency, typically within 30 seconds.
  • Duration of activation: aFRR provides a short-term burst of power for a limited time, usually up to 12,5 minutes.
  • Operation: When there is a sudden increase or decrease in electricity demand or supply, grid frequency may deviate from its nominal value (e.g., 50 Hz or 60 Hz). aFRR consists of power resources (e.g., gas turbines, hydroelectric generators) that are already online and synchronized with the grid but operating below their maximum capacity. These resources can quickly increase their output or reduce their generation to help restore the grid's frequency to its nominal value.


mFRR (Manual Frequency Restoration Reserve):

In its role as the tertiary frequency control, mFRR's responsibilities encompass reinstating the primary and secondary frequency control reserves, managing congestion within the transmission network, and restoring both frequency and interchanges to their specified target values.

  • Activation Time: mFRR, as the name suggests, is manually activated by grid operators, and it typically has a longer activation time compared to aFRR.
  • Duration of Activation: mFRR is designed to provide power for a longer duration, often several hours.
  • Operation: mFRR can include various resources that are not actively synchronized with the grid but can be started and brought online relatively quickly. This category may also encompass demand response programs and energy storage systems, where certain loads can be manually curtailed or increased to provide additional supply or demand to restore grid frequency. Grid operators initiate mFRR when aFRR and other reserves are insufficient to address frequency deviations.

Replacement Reserve (RR):

RR is tasked with replenishing or reinforcing the necessary level of frequency restoration reserves and addressing system imbalances, including maintaining generation reserves through active power reserves. Restoration Reserves are vital for ensuring the rapid recovery of grid operations and minimizing the impact of large-scale outages on communities, industries, and essential services. These reserves require careful planning, coordination, and resources to address the unique challenges posed by the restoration phase of grid operation.

  • Activation Time: Restoration Reserves are typically activated in the aftermath of significant disruptions, such as large-scale blackouts or cascading failures. The activation time for RR can vary significantly, depending on the complexity of the restoration process and the extent of the damage.
  • Duration of Activation: RR may need to be available for an extended period, potentially spanning several hours to days, as restoring a large-scale grid system can be a complex and time-consuming task.
  • Operation: Restoration Reserves encompass a wide range of resources and strategies. This category includes power sources and equipment that are used to rebuild critical grid infrastructure, reroute power flows, and bring offline components back into service. It may involve mobilizing emergency generators, repairing transmission lines, and coordinating the synchronization of power plants and substations to gradually restore power to affected areas. The goal of Restoration Reserves is to systematically recover the grid's functionality and gradually return power to all affected customers, ensuring a safe and stable restoration process.


Ancillary Services - Frequency Restoration Reserves



These reserve categories are crucial for maintaining the stability and reliability of the electrical grid. Transmission system operators carefully manage and coordinate the activation of these reserves to ensure that they can quickly respond to disturbances and maintain the balance between supply and demand. This helps prevent blackouts, voltage fluctuations, and other disruptions in the power supply, ultimately ensuring a continuous and reliable source of electricity for consumers.

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