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Mains frequency


The mains frequency indicates the frequency of the change of polarity in the AC mains. This is measured in Hertz (Hz). In Germany and in Europe, there is a constant grid frequency of 50 Hertz. Deviations in the frequency provide information about the relationship between power generation and power demand.
For a functioning power grid, the grid frequency must always be kept stable. This requires, on the one hand, synchronization among all power generators and, on the other hand, a balance between supply and demand. In order to be able to compensate for fluctuations, a functioning control energy system is also required.
The monitoring of the grid frequency is important in order to detect voltage drops at an early stage and thus prevent power failures.

How is the mains frequency generated?

The network frequency results from the alternating voltage that prevails in the power networks. AC voltage is a voltage in which the polarity changes continuously and the current periodically changes its direction. Two poles with different charges create a voltage field, which is the basis for the current to flow. The change of direction causes sinusoidal voltage waves. The frequency is calculated from the frequency with which the alternating current changes direction per second. In the German power grid, which has a frequency of 50 Hertz, 50 voltage waves occur in one minute and the voltage changes polarity one hundred times.

Why is it important to keep the frequency stable?

For our European interconnected grid to function properly, the grid frequency must always be kept constant. To achieve this, there must be a continuous balance between the production and consumption of electrical power. If too much or too little power is fed into the grid, fluctuations in the frequency and deviations from its setpoint of 50 hertz can occur. Slight fluctuations are normal in practice, which is why a tolerance range between 49.8 and 50.2 Hertz has been defined.
However, too sharp a rise or fall in the mains frequency can result in numerous electrical devices being affected in their function and the generators of connected power plants being damaged. This is because the rotational speed of the rotors of all generators in the grid are set precisely to 50 Hertz.  

What happens in case of deviations of the mains frequency? How is the mains frequency controlled?

In order for the grid frequency to be kept stable, it is primarily necessary to have Control energy. For the activation are in Germany the Transmission system operator responsible for activation. If the frequency falls below 50 hertz, additional power must be fed into the grid, which is referred to as positive control energy. If there is too much electricity in the grid and the frequency rises above 50 Hertz, excess electricity is taken from the grid and this is referred to as negative control energy.
In order to prevent a grid collapse in the event of excessive fluctuations, power plants disconnect themselves from the grid automatically, step by step, from the tolerance range onwards. This happens both when the grid frequency is too low and when it is too high.
At a frequency lower than 47.5 hertz, all power plants are disconnected from the grid, resulting in a complete breakdown of the power supply and thus in nationwide blackouts.

How can battery storage be used to stabilize grid frequency?

Due to their fast response times and ability to provide both negative and positive control energy, battery storage systems are particularly well suited for the provision of primary and secondary control power. When the grid frequency drops below 50 Hertz, battery storage systems feed additional power into the grid. When the grid frequency rises above 50 hertz, they draw power from the grid. The power called up depends on the level of fluctuations.