Since the beginning of the 20th century, alternating current has been used instead of direct current for our electricity supply. The main advantage with alternating current is that it is possible using transformers to increase and decrease voltage in line with demand flexibly and without much loss. This means that electricity can be transported over long distances without losing as much energy.

The alternating current in the Continental Europe Synchronous Area has a frequency of 50 hertz (Hz). The frequency is the number of oscillations per second. Each oscillation has a positive and a negative half-wave with two zero-crossings. This means that a frequency of 50 Hz changes its polarity 100 times.

To keep a constant frequency of 50 Hz, Electricity generation and consumption always need to be in balance. Any imbalance will result in a change in the frequency. A decrease in generation will lead to a decrease in the frequency, and vice versa.

What happens when the system frequency does not stay at 50 Hz?

Any changes in the frequency must be kept to a minimum to avoid damage to electrical installations. Any change bigger than 0.2 Hz will trigger protection mechanisms that disconnect networks and plants from the grid.

The Transmission system operators (TSOs) responsible for keeping the balance in the grid can use different types of Balancing energy from frequency containment reserves (FCR), automatic frequency restoration reserves (aFRR) and manual frequency restoration reserves (mFRR) (see also Bundesnetzagentur - Regelenergie (in German)).

What exactly happens when there is a decrease in the system frequency?

The system frequency is kept at a constant 50 Hz using balancing energy. If there is a change in the frequency, the synchronous area has uniform rules saying which measures need to be taken when. If there is an unpredictable change in the frequency, the TSOs can use balancing services to balance generation and consumption. Further information on balancing services is available here. If the frequency were to fall to below the critical mark of 47.5 Hz, networks and plants would be disconnected and there would be an outage. This could happen for various reasons, including natural disasters, damage to lines or transformers, sudden changes in load, and temporary power plant failures. To avoid this happening, frequency containment reserves are activated whenever there is a change of just 0.02 Hz or more. These reserves are used to adjust generators' rotation speeds to counteract the change in the frequency and contain the fluctuations. The frequency containment reserves are provided within 30 seconds. However, they are replaced by automatic frequency restoration reserves within five minutes, as these are used to try to restore the system frequency to its nominal level. After 15 minutes, manual frequency restoration reserves are activated.