What are high-voltage power lines?

Electricity arrives to the sockets in our homes with a voltage of 230 volts (230 V), but much higher voltages are used to transport it to us. The overhead power lines for transporting electricity from power plants to towns and urban centres carry up to 380,000 volts (380 kV).

Transport

High-voltage power transmission is more efficient than low-voltage transmission because less energy is lost. Nevertheless, the voltage cannot be increased indefinitely.

The voltage level of a power line is determined based on the length of the transmission path and the power required by the recipients of the electricity.

Direct and alternating current

At the start of the 20th century, high-voltage grids only used alternating current (AC). Unlike direct current, AC changes direction 100 times a second in Western Europe, resulting in a frequency of 50 Hz.

Today, it is also possible to operate high-voltage grids with direct current, thereby avoiding the energy loss that occurs with alternating current. High-voltage direct current (HVDC) transmission is therefore a good alternative for long transport routes. In Western Europe, it is mainly used for power transmission via subsea cables.

Overhead lines and underground cables

380 kV Overhead line

Over land, electricity is transported predominantly using overhead lines but also using underground cables.

In the case of overhead lines, the electricity is transported by the conductors carried on the pylons. Unlike cables, the conductors are not surrounded by an insulating layer and are therefore hung out of the reach of humans.

In Germany, high-voltage and extra-high-voltage power lines are predominantly implemented as overhead lines. Regional transport often still takes place via low-voltage overhead lines, in which case the houses are generally supplied via roof poles.

The mast construction serves as the first clue in order to identify the voltage level for which an overhead line is used:

• High-voltage and extra-high-voltage lines are usually carried on high lattice steel masts.
• Low- and medium-voltage lines are more likely to be carried on smaller poles made of wood, concrete or tubular steel.

The second clue comes from narrowing down the voltage level by taking a closer look at the conductors:

• Extra-high-voltage power lines:
  220 kV lines use bundled conductors, often consisting of two or, in the case of 380 kV lines, three or four individual conductors carried closely to one another in parallel. Spacers are fitted between the individual conductors at regular intervals (as can be seen in the picture of the 380 kV overhead line, for example).
• High-voltage power lines:
  For a voltage of 110 kV, the use of conductor bundles is much less frequent.

Underground cables can contain one or more conductors, which are each individually protected from mutual contact by insulation. As a result, the conductors are at a much smaller distance from one another than in overhead lines. Underground cables are mainly used for domestic and commercial connections at low voltage. However, they are also increasingly being used as extra-high-voltage lines for transporting electricity over long distances.

Gas-insulated transmission lines (GILs) are also used. In Germany, these lines have so far only been used over very short distances, e.g. to transport electricity from the interior of switchgears to the outside. GILs consist of an inner aluminium conductor that is carried on post or disk insulators at regular intervals inside an aluminium tube. The pipe is filled with a gas for insulation.

More information

The Video "Stromleitungen und Strahlenschutz" (in German only) answers many questions related to voltage power lines and radiation protection.