Primary control power & secondary control power
Primary control power
AC networks in Europe have a nominal frequency of 50 Hz. The responsible transmission system operators (TSOs) must guarantee this normal frequency within a narrow bandwidth at all times. Frequency stability is ensured by balancing power generation and consumption. If there are deviations between generation and consumption, a frequency drift occurs and the grid frequency changes. By providing primary control power (PRL), this frequency drift is stopped and the grid frequency is thus stabilized. The PRL must be fully activatable within 30 seconds and available for at least 15 minutes.
In Germany, there are four transmission system operators, each of which ensures a balance between electricity feed-in and withdrawal and the provision of balancing energy in their grid areas and cooperates in the grid control network. The costs for this are refinanced via the network charges. This association was extended to include Denmark, Belgium, the Netherlands, Switzerland, France and Austria.
How primary control power (PRL) works
The activation of the primary reserve is frequency-dependent. The primary reserve provider independently measures the grid frequency at the point of generation or consumption and responds immediately to the change in grid frequency. The increase or reduction of the mains power must be proportional to the mains frequency as soon as it falls below or exceeds the range between 49.99 Hertz and 50.01 Hertz. Here, the primary reserve must be fully available within 30 seconds and maintained for up to 15 minutes. In addition, primary reserve power must be provided symmetrically. In addition to large-scale power plants, battery storage systems can also provide PRL.
Secondary control power
In connection with PRL, the term secondary control reserve (SRL) is also often used. Transmission system operators use SRL to regulate the frequency back to 50 Hz in case of deviations. The SRL is subordinate to the primary reserve (PRL) and replaces it. The primary control power initially stabilizes the grid frequency to a frequency that deviates from the nominal frequency. The secondary control power then ensures that the frequency returns to the nominal frequency. Secondary control power must be available in full for 15 minutes within 5 minutes. As a positive SRL, it can supply additional power to the power grid in the event of under-supply, or as a negative SRL, it can draw power from the power grid in the event of over-supply.
