Earth rope
The earth wire is a grounded , electrically conductive cable, the above high voltage - transmission lines to protect against direct lightning strikes is tensioned. Colloquially, equipotential bonding lines for earthing conductive moving parts (e.g. control cabinet doors with the cabinet ) are also referred to as earth cables.
Overhead lines
The task of the earth wire is to prevent a direct lightning strike in conductor ropes by offering the lightning discharge a more attractive target as an interception device . According to international statistics, around 65% of all unplanned shutdowns of overhead lines can be attributed to the effects of lightning. The use of earth ropes, together with the earthing of the masts , is therefore an economically sensible measure to limit the shutdowns to an acceptable level. High-voltage lines with operating voltages above 50 kV are almost always equipped with an earth wire.
functionality
As with most lightning protection systems on buildings, the lightning protection effect is primarily based on the fact that corona discharges form directly above the earth wire due to the high edge field strength during atmospheric charges . These discharges lead, preferably at the tips and edges, to a partial ionization of the surrounding air due to tip discharge , which means that a subsequent lightning strike is more likely to strike the earth wire than the conductors of the overhead line. To increase the edge field strength desired here, earth ropes are therefore deliberately not designed as bundles , but always as a single rope with the smallest possible radius.
The ground wire also offers advantages when it hits a mast directly, as it distributes part of the lightning current to neighboring masts. The potential increase due to the earthing resistance of the mast and the ground is therefore lower, and there is less likelihood of dangerous step voltages occurring around the mast base and backward flashovers from the mast to the conductors.
Attachment
Usually the earth rope is attached to the top of the mast in order to reach the highest possible position. It is often not screwed directly to the mast, but rather mounted over a short insulator , which is normally bridged with a piece of rope. This makes it possible to influence the earth contact resistance.
If there are increased requirements for lightning protection, two parallel earth cables are laid instead of a single one in order to widen the protected area. Either a separate traverse is used for this, the so-called earth wire crossbar , or the top of the mast forks in a V-shape into two earth wire horns , which is often found on the lines of the north-west German power plants .
Many 110 and some 220 kV lines from the former Badenwerk carry the earth ropes directly on the outside of the top traverse. This type of suspension is also very common in the Netherlands .
Secondary functions
A fiber optic cable is often integrated in earth cables for the transmission of messages. On some lines that were laid by the then Energie -versorgung Schwaben (now EnBW ) until 1985 , such a telephone cable is laid in a garland on the ground wire. With external fiber optic cables, however, direct lightning strikes can cause thermal damage to the fiber optic cable.
Where flight warning balls are required to mark the overhead line as an obstacle to aviation , they are usually attached to the earth wire .
In some HVDC overhead lines, the earth wire also serves as the electrical connection between the converter and the earth electrode. In this case, the earth wire must be attached to the mast in an insulated manner and is only connected to it by a surge arrester , as otherwise current would flow permanently through the mast and lead to high electrochemical corrosion there.
Insulated earth wires can also be found on extra-high voltage lines in the former Soviet Union, where they are used to transmit carrier frequency signals.
Computational effects
When determining the K-factor , the earth ropes must be included. The zero impedance of an overhead line is significantly reduced through the use of earth cables.
Examples
Conductor cables and earth cables on two overhead lines
The masts of the Elbe crossing 2 carry two earth ropes on earth rope horns
Management of the Schluchseewerk with earth ropes on the ends of the upper traverse
HVDC Fenno-Skan with isolated earth ropes
Earthing cable of the railway earthing on a track with an electrical contact line
Railway overhead lines
In the railway sector , a special railway earth cable is attached to the outside of the catenary masts along the route . In this case, it is not primarily used as a lightning protection device, but as a safety precaution in the event that the earthing of a mast is damaged (e.g. during construction or maintenance work). Each overhead line mast is connected to its neighbors via the earth wire . In addition, this earth wire embedded in the ground improves the return flow of the current to the feeding substation , which normally occurs via the rail.
literature
- Klaus Heuck, Klaus-Dieter Dettmann, Detlef Schulz: Electrical energy supply . 7th edition. Vieweg + Teubner, Wiesbaden 2007, ISBN 978-3-8348-0217-0 .
- F. Kießling, P. Nefzger, U. Kaintzyk: Overhead lines: planning, calculation, execution. According to EN 50341 . 5th edition. Springer, Berlin, Heidelberg 2001, ISBN 978-3-540-42255-6 .
Individual evidence
- ↑ Valentin Crastan : Electrical energy supply 1 . 3. Edition. Springer, Berlin Heidelberg 2012, ISBN 978-3-642-22345-7 , p. 472 ff .