The electricity flows they are a-changin' - but the voltage remains the same

Extra-high voltage – guaranteed

The power grid must function safely and reliably. That goes without saying. But what does this actually mean? Absolutely crucial to guaranteeing this is a constant voltage at each of the various levels in the grid. This is something grid operators like Amprion guarantee.

What is the voltage in the AC network?

Do you remember your physics lessons? Electric current flows when electrons move in a conductor. A voltage is required to move the electrons. It is measured in volts. In Europe, the voltage at a power socket is usually 230 volts, whereas in the extra-high-voltage grid, it can be up to 380,000 volts, or 380 kilovolts (kV). Germany’s AC grid operates at four different voltage levels, which are connected to each other via transformers and for which different grid operators are responsible. Alternating current gets its name from the fact that the current and voltage change their direction in fixed time intervals, i. e. periodically. The voltage curve has the shape of a sine wave.

Why do we need a fixed voltage in the grid?

The grid operators promise their customers that they will maintain a fixed voltage at the grid connection at all times. More precisely: they promise that the maximum voltage values are always within a fixed range. Experts refer to this as a “voltage band”. Only if the voltage remains within this band do machinery and systems continue to function perfectly. If the voltage is too low, production processes in many companies are severely disrupted. If the voltage is too high, the machinery and systems can even become damaged. According to European regulations, the voltage in the 380-kV extra-high-voltage grid must not fall below 360 kV or exceed 420 kV in normal operation.

What influences the voltage in the grid?

Imagine the transmission grid in Germany as a massive bed sheet that is stretched across the country and needs to be kept at an even height. A high level of power consumption leads to the voltage level in the grid falling: consumers grab and hang on to the sheet and pull it down. In order to pull the cloth tight and back to its original height, grid oper­ators such as Amprion support the “voltage bed sheet”. They do this by making use of the stabilising effect of conventional large-scale power stations. However, these are gradually being pulled from the grid. Amprion is therefore increasingly deploying systems in the grid that are specifically designed to maintain the voltage level.

What is the impact of the expansion in renewables?

On the other hand, it can also happen that the “voltage bed sheet” threatens to fly up and take off. This can be the case, for example, when wind and solar farms generate so much power due to the weather and feed it into the distribution grids that the distribution grids are forced to transfer the renewable energy to the transmission grid. This is happening more and more frequently owing to the expansion of renewables. Amprion then has to stabilise the voltage at all grid nodes in order to cope with the strong fluctuations in generation levels. But – and this is the good news – every single wind turbine and solar system can also play its part towards maintaining the voltage. That said, the task of coordi­n­ating the feed-ins of millions of small generating plants is a complex undertaking and represents a major challenge for grid operators in the wake of the energy transition.

Text: Volker Göttsche