The mains frequency indicates the frequency of the change in polarity in the AC mains. This is measured in Hertz (Hz). In Germany and Europe, there is a constant grid frequency of 50 Hertz. Deviations in the frequency provide information about the relationship between electricity generation and electricity demand.
The grid frequency must always be kept stable for a functioning electricity grid. This requires synchronization between all electricity generators on the one hand and a balance between supply and demand on the other. In order to be able to compensate for fluctuations, a functioning control energy system is also required.
Monitoring the grid frequency is important in order to detect voltage drops at an early stage and thus prevent power outages.
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.
In order 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 electricity is fed into the grid, the frequency may fluctuate and deviate from its target value of 50 hertz. Slight fluctuations are normal in practice, which is why a tolerance range between 49.8 and 50.2 hertz has been defined.
However, if the grid frequency increases or decreases too much, this can affect the functioning of numerous electrical devices and damage the generators of connected power plants. This is because the speed of the rotors of all generators in the grid are set to exactly 50 hertz.
In order to keep the grid frequency stable, balancing energy is required first and foremost. In Germany, the transmission system operators are responsible for activating this. If the frequency falls below 50 Hertz, additional feeds into the grid are necessary, this is referred to as positive balancing energy. If there is too much electricity in the grid and the frequency rises above 50 Hertz, excess electricity is withdrawn and this is referred to as negative balancing energy.
In order to prevent a grid collapse in the event of excessive fluctuations, power plants are automatically disconnected from the grid step by step from the tolerance range onwards. This happens both when the grid frequency is too low and too high.
From a frequency lower than 47.5 Hertz, all power plants are disconnected from the grid and there is a complete collapse of the power supply, resulting in widespread blackouts.
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.
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