The standard frequency in Europe’s electricity grid is 50 Hz. In North America and parts of Japan, on the other hand, a standard frequency of 60 Hz is used. These frequencies must be kept as stable as possible for various reasons:
- Time measurement: there are still many clocks which go by the frequency in the electricity grid. If the frequency is higher, they go faster. If the frequency is lower, they go more slowly.
- Machines: large electrical machines can suffer damage if the frequency is too low or too high, or if it changes rapidly.
To ensure the frequency always remains at a stable level the balance between production and consumption of electrical power must always be right.
If the consumption of electrical power is lower than production, the frequency is higher; if consumption is higher than production, the frequency is lower. The reason for this is as follows: the electrical generators of an electricity grid rotate more readily and faster when consumption is low. Consequently, they rotate with a higher frequency. Conversely, the electrical generators rotate more laboriously and with a lower frequency when consumption is greater.
Everyday comparison with a bicycle
On a level road it is easy to maintain speed. On reaching a gradient, however, the rider needs to make more effort to achieve the same speed. Going downhill, the rider needs to apply the brakes to keep the same speed.
In the entire European network the electrical generators are set up in such a way that they automatically and immediately respond to a change in grid frequency. Depending on the level of consumption they increase or lower their capacity. This ensures that the frequency remains stable. This automatic adjustment can be compared with the cruise control in a car.
Grid time deviation
The grid time is a time measurement which is based on the standard grid frequency of 50 Hz in Europe. Fifty oscillations in alternating current equate to one second of grid time. Frequency fluctuations lead to deviations in grid time. If the frequency is lower than 50 Hz, the fifty oscillations last slightly longer. If, on the other hand, the frequency is higher than 50 Hz, the fifty oscillations are shorter. Since one second of grid time always constitutes precisely fifty oscillations, the grid seconds therefore last slightly shorter or longer depending on the frequency. The grid time deviation is calculated by comparing with UTC time (coordinated universal time), which is determined using highly precise atomic clocks.
This grid time deviation is constantly balanced out. If the time deviation is more than twenty seconds the frequency is corrected in the grid. In order to balance out the time deviation again the otherwise customary frequency of 50 Hz (Europe) is changed as follows:
- 49.990 Hz, if the grid time is running ahead of UTC time
- 50.010 Hz, if the grid time is lagging behind UTC time