General information on control reserve - technical aspects

A permanent balance between electricity generation and demand is an important precondition for the stable and reliable operation of the grid. Ensuring that their customers are supplied with power in a reliable fashion is the foremost of TSOs' responsibilities. For the purpose of maintaining the above-mentioned balance between supply and demand, TSOs procure control reserve (also known as balancing power).

A need for control reserve arises as soon as the current feed-in differs from current consumption. Deviations are caused by fluctuations on the consumer side in the supply and consumption behaviour or on the generation side due to disturbances (e.g. power outages). A lack of generation capacity (or excess of power consumption) is expressed as a frequency drop, an excess of generating capacity (or lack of power consumption) as a frequency increase in the total electric energy supply system in Europe.

The objective of control reserve activation is, on the one hand, to maintain the system frequency within a narrow range around its target frequency of 50 Hz and, on the other hand, to eliminate regional deviations in the balance from their reference value. For this purpose, different types of control reserve have to be deployed in a coordinated fashion for a dynamic and chronological interaction.

Based on the valid rules of ENTSO-E (European Network of Transmission System Operators for Electricity), the German TSOs procure the following types of control reserve:

Primary control reserve:

  • Provided according to the solidarity principle by all TSOs synchronously connected within the ENTSO-E area
  • Automatic and complete activation of primary control reserve within 30 seconds
  • Period per incident to be covered: 0 < t < 15 min

Secondary control reserve:

  • energy balance of the control area and frequency control
  • immediate automatic activation by the concerned TSO
  • complete activation within five minutes (at most)

Minute reserve (tertiary control reserve):

  • The activation is based on MOLS by a Merit-Order-List (electronical activation) since 2012
  • Complete activation within fifteen minutes
  • Period per incident to be covered t > 15 min to 4 quarter hours or up to several hours in case of several incidents



For further information see the ENTSO-E-Operation Handbook.