Control energy


Control energy refers to the electrical energy or powerthat is required in a control area to compensate for unforeseen fluctuations in supply and demand that could otherwise jeopardize the stability of the electricity grid.
In order to permanently stabilize the electricity grid, the German transmission system operators (TSOs) are responsible for keeping the frequency of the German electricity grid constant at 50Hz. The TSOs monitor and operate the extra-high and high-voltage lines for transporting electricity over long distances.

If fluctuations have to be balanced, control energy can be used to feed electricity into the grid as well as to withdraw it from the grid.Ifmore electricity is fed into the grid to compensate for a grid frequency that is too low, this is referred to aspositive control energy. If thefeed-inisthrottled toreduce the grid frequency, this isnegative control energy.

In order to keep the grid frequency stable at all times, there are 3 types of control energy that can be used for different scenarios. These are shown in the following infographic.

In the event of a fluctuation, the primary control system takes effect first and balances the grid frequency. This works automatically and without communication with the power plant operators.
If the primary control power is not sufficient to compensate for the fluctuation, secondary control is called up.
The minute reserve is used for fluctuations in the grid frequency that occur over slightly longer periods of time.

How is control energy accessed?

A provider ofprimary control power must be able to guarantee the agreed power within 30 seconds and for a maximum of 15 minutes from the start of the control process. Thanks to its fast response time and ability to provide positive and negative control power, a large-scale battery storage is particularly suitable for the provision of primary control power.  

So what happens to the storage when the primary control power is called?

If the grid frequency drops below 50Hz, the storage system feeds into the grid. Depending on the level of the deviation, only part of the storage system's output is called up. As can be seen in the diagram below, the full marketed capacity of the storage system is called up from a deviation of 49.8Hz.
If the grid frequency is higher than 50Hz, the large battery storage system draws power from the grid. Here too, the power drawn depends on the level of fluctuation. From a deviation to 50.2Hz, the storage system stores at full power.  

Who is allowed to participate in the balancing energy market?

In order to participate in the primary control power market, the systems must be prequalified by the responsible transmission system operator. For this purpose, the system runs through certain load curves to ensure that it meets the requirements of the control energy. Thus, the system completes the so-called double hump curve and must feed into the grid twice in the positive direction and twice in the negative direction (PQ run) in order to complete the prequalification process vis-à-vis the TSO.

How big is the market for primary control power?

The tender for primary control power runs over 4-hour time slots. For these four-hour periods, 1450 MW of primary control power is tendered for the ENTSO-E grid (Association of European Transmission System Operators). Within Germany, the figure is 650 MW.
Currently, 450 MW of battery storage is prequalified for primary control power in Germany. The further expansion of large-scale battery storage systems means that they will dominate the balancing energy market in the future. In addition to balancing energy, large-scale battery storage systems can also be used for a whole range of other applications. As part of a well-coordinated multi-use strategy, large-scale battery storage systems can develop their full potential.  

How is the price for control energy determined?

The primary control power prices are formed according to the merit order procedure. Merit order refers to the order in which power plants that produce electricity are deployed on the electricity trading market. In the merit order, power plants are arranged according to their marginal costs/opportunity costs. Marginal costs describe the variable costs of an additional unit produced and are therefore independent of the fixed costs of a power generation technology. For conventional power plants, these are quite precisely the costs of the energy carrier used. Opportunity costs are the revenues that could be generated by the plant in other electricity markets.
Power plants with higher marginal costs are awarded the contract until the demand is covered by the current 1450 MW. The most expensive power plant called up then determines the market price for all balancing power. In this way, operators are motivated to build plants with the lowest possible marginal costs in order to obtain a larger margin.

What are the marginal costs of storage power plants (e.g. large-scale battery storage)?

Storage power plants have the lowest marginal prices of all market participants, therefore the marginal costs/opportunity costs of storage power plants will shape the market in the future. Strong price fluctuations offer flexible plants (especially storage power plants) alternative revenue paths and raise the opportunity costs in the primary control power.

How will the coal and nuclear phase-out and the energy transition affect balancing energy?

The phase-out of nuclear energy (2022) and coal (2038 at the latest) will reduce the supply of prequalified capacity. At the same time, this increases the opportunity costs in the electricity market for remaining capacities, as these power plants in reserve are technically and legally unable to provide balancing power.
Both of these factors have a positive effect on the development of the contribution margins from primary control.