Control energy is the electrical energy or power required in a control area to compensate for unforeseen fluctuations in supply and demand, which could otherwise jeopardize the stability of the power grid. In order to permanently stabilize the power grid, the German transmission system operators (TSOs) have the responsibility to keep the frequency of the German power grid constant at 50 Hz. If fluctuations have to be compensated for, electricity can be fed into the grid as well as withdrawn from the grid through the use of control energy. In order to keep the grid frequency stable at all times, a distinction is made between three types of control energy, which are suitable for various application scenarios. If there is a fluctuation, the primary control system first takes effect and balances the network frequency. This works automatically and without communication with power plant operators. If the primary control power is not sufficient to compensate for the fluctuation, the secondary control is activated. The minute reserve is effective for fluctuations in the network frequency that occur over somewhat longer periods of time.
Large-scale battery storage systems are some of the few systems that can provide both positive and negative control energy within milliseconds. When the grid frequency drops below 50 Hz, the storage system feeds electricity into the grid. Depending on the level of the variance, only a portion of the memory's power is used. Starting at a deviation of 49.8 Hz, the full marketed performance of the memory is called up. When the grid frequency is higher than 50 Hz, the large battery storage system absorbs power from the grid. Here, too, the power requested depends on the amount of fluctuation. From a deviation of 50.2 Hz, the memory stores at full power. After a successful prequalification process, large battery storage systems are automatically controlled to balance the grid frequency.
In order to participate in the PRL market, the systems must be prequalified by the responsible transmission system operator. To do this, the system passes through certain load profiles to ensure that it meets the control energy requirements. For example, the system completes the so-called double hump curve and must feed into the network twice in a positive direction and twice in a negative direction (PQ drive) in order to complete the prequalification process vis-à-vis the TSO.
The call for tenders for primary control power runs over 4 hours. For these four-hour periods, 1450 MW of primary control power for the ENTSO-E network (Association of European Transmission System Operators) will be tendered. Within Germany, it is 650 MW. 450 MW battery storage systems are currently prequalified for primary control power in Germany. As a result of the further expansion of large battery storage systems, they will dominate the control energy market in the future.
Storage power plants have the lowest marginal prices of all market participants, which is why the marginal costs/opportunity costs of storage power plants will form the market in the future. Severe price fluctuations offer flexible systems (especially storage power plants) alternative revenue paths and thus raise opportunity costs in PRL.
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