Control energy is the electrical energy or power required in a control area to balance unforeseen fluctuations in supply and demand that 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 50Hz. If fluctuations have to be balanced out, electricity can be fed into the grid as well as withdrawn from the grid by using balancing power. 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 different application scenarios. When a fluctuation occurs, the primary control first takes effect 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. For fluctuations in the grid frequency that occur over somewhat longer periods, the minute reserve takes effect.
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 50Hz, the storage unit feeds power into the grid. Depending on the level of the deviation, only part of the storage's power is called up. From a deviation to 49.8Hz the full marketed power of the storage is called up. When the grid frequency is increased above 50Hz, the large-scale battery storage system draws power from the grid. Here, too, the power called up depends on the level of the fluctuation. From a deviation to 50.2Hz onwards, the storage unit stores power at full capacity. After a successful prequalification process, large-scale battery storage units are automatically activated to compensate for the grid frequency.
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.
The tender for primary control power runs over 4-hour time slices. For these four-hour periods, 1450 MW of primary control reserve are tendered for the ENTSO-E network (Association of European Transmission System Operators). Within Germany, the figure is 650 MW. Currently, 450 MW of battery storage are prequalified for primary control power in Germany. Due to the further expansion of large battery storage facilities, the balancing power market will be dominated by these in the future.
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.