System stability going forward

Ancillary services guarantee a secure and stable electricity supply. As the requirements of the energy transition are constantly changing,

dena is working on identifying technically and economically viable solutions in a timely manner.

Ancillary services

Ancillary services

Most people expect to have a secure and reliable electricity supply. However, what the end user doesn’t usually realise is that continuous operational adjustments are required in order to achieve this – these are referred to as ancillary services.

Ancillary services run regulatory operations in the background, performing multiple functions – monitoring, balancing and repairing the energy infrastructure.
In the event of a disturbance, ancillary services work to restore values such as voltage and frequency back to their normal range. To date, a major portion of these services have primarily been performed by conventional power stations, however, in future, renewable energy providers will also have to make a contribution towards grid stability. Furthermore, seamless coordination is required between grid and plant operators.
The ‘Ancillary Services’ Innovation Report presents the developments in the field of ancillary services over the past three years and specifies the action currently required to make the electricity grid sustainable for the future.

Important information about operating electricity grids

In order to guarantee a high level of quality, reliability and security of electricity transmission and distribution, the grid operators need to work continuously to keep the frequency, voltage and load of the grid operating equipment within the permitted tolerance limits or to return them to their normal range after a disturbance. These services, which are essential for maintaining a functioning electricity supply, are called ancillary services. These are split into four different ancillary services: operational management, frequency control, voltage control and system restoration.

Operational management: The grid operators are tasked with managing the secure operation of the electricity grid, continuously monitoring and controlling the network, including production and (to a limited extent) the load, for threshold violations (e.g. current flow overloads).

Frequency control: Frequency control is carried out by the transmission system operators. They are responsible for ensuring that electricity generation and consumption is precisely balanced at all times, which is an indispensable requirement for stable grid operation. For this purpose, the transmission system operators use the (thus far) inherent property of the system, the ‘instantaneous reserve’ and procure balancing energy through tender processes.

Voltage control: Voltage control refers to the responsibility of the transmission and distribution system operators to maintain the grid voltage in their allocated grid area within a permissible range to ensure voltage quality. A key factor in successful voltage control is the provision of reactive power from power generating plants and operating equipment.

System restoration: In the event of a widespread power failure, the transmission system operators, together with the distribution system operators, must be able to restore the electricity supply within a very short time. All necessary measures and strategies relating to this goal are grouped together in the ancillary service ‘system restoration’.

Grid operators have established a multitude of processes and products for providing the four ancillary services. These products are sourced from the grid operators’ operating resources, but also from grid users, i.e. from power generating plants or flexible loads.

The use of the term ‘product’ does not necessarily mean that remuneration is or should be provided for the ancillary service product. For example, remuneration is paid for the best-known ancillary service product, the operating reserve. In contrast, on the basis of stipulations in the technical connection conditions, reactive power is currently provided without remuneration.

Operational management is the responsibility of the respective grid operator, who must also consider the relevant required specifications of the respective upstream grid operators. The transmission system operators have the overarching responsibility of ensuring system stability and must also coordinate with the other transmission system operators involved in the integrated European grid.

The transmission system operators perform frequency control by maintaining a balance between electricity generation and consumption.

Voltage control refers to the responsibility of the transmission and distribution system operators to maintain the grid voltage in their allocated grid area within a permissible range to ensure voltage quality. This is carried out for each operator’s own grid level while taking the demands of the upstream and downstream grid levels into consideration at the same time.

In the event of a widespread power failure, the transmission system operators, in cooperation with the distribution system operators, must be able to restore the electricity supply within a very short time (ancillary service – system restoration).

The expansion of renewable energy, especially photovoltaic systems and wind-powered installations, changes the requirements of the power supply system. Large fluctuations in load flow situations and the increasing distances over which electricity must be transported change the demand for and ways of providing ancillary services as well as placing new demands on the operational management of the electricity grids. At the same time, operating times of conventional power stations are decreasing, meaning that, in future, they will be less and less available to provide ancillary services.

The use of storage systems by the energy producers generates greater flexibility, thus also creating new opportunities for grid management. Should the number of heat pumps and electric vehicles increase as part of the of the energy transition process, these new loads would need to be intelligently integrated into the electricity grids.

In spite of these changes in the electricity system, the provision of ancillary services must always be secure, reliable and economically viable. In order to ensure this, we must utilise new stakeholders’ capabilities and develop and adapt existing regulations appropriately.

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Our projects

Grid Flex Study

In its “Grid Flex Study", dena is investigating how storage facilities can be used to increase flexibility in the electricity grid. The aim is to identify operating models for storage facilities that are economically viable and which reduce the load on the electricity grid.


  • Project start: February 2016
  • Study duration: 12 months (until the end of January 2017)
  • The partners in the study are 23 companies from various industries (grid operators, energy providers, plant engineers and providers of storage facility solutions)

Ancillary Services Platform

In the future, renewable energy, storage facilities and other alternative providers of ancillary services can and must make a greater contribution towards the security and stability of our electricity system. With the aim of creating the necessary framework conditions in a swift and timely manner, the dena Ancillary Services Platform pools the results of individual activities in the field of future ancillary services, identifies areas in which action is required and promotes exchange between and communication with experts.


  • Project duration: September 2014 to September 2020
  • Partners of the platform: 22 companies from various industries (system operators, energy suppliers, system manufacturers) as well as politicians and associations

Ancillary services 2030

Ancillary services are required for a secure and stable power supply. The study analyses and evaluates technological alternatives for the provision of ancillary services in an energy system utilising a high percentage of renewable energy.


  • Publication: February 2014
  • Partners of the study: 16 companies
  • Agents: dena, Technical University of Dortmund / ef. Ruhr GmbH