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Robot translation, needs major edit.
An overhead line crossing is the crossing of an obstacle, like a traffic route, a river, a valley or a strait by an overhead line. The execution of an overhead line crossing depends strongly on the local conditions and the regulations for powerline construction valid at its construction date. However overhead line crossings are in many comparable cases similarly implemented. Overhead line crossings can sometimes require extensive constructions and can also have operational disadvantages. In such cases it should be considered if an execution of the crossing of the obstacle is better done by a ground or by a submarine cable.
overhead line crossings of roads and railway lines
overhead line crossings of roads, railway lines, small and medium sized watercourses do normally not require special constructions. However it was in earlier times the law that for crossing a national railway on both sides of the lines there must be an anchor pylon, which one can see very beautiful at some old powerlines in Germany. For overhead line crossings of motorways the pylons must be often renewed prematurely, because for these increased statical requirements exist. If the local conditions let it appear appropriate, an overhead line can be crossed also by a valley bridge. So the Koersch valley bridge near Esslingen, Germany crosses a 110kV-three phase current line with 2 circuits of the EnBW AG for example. For obvious reasons (danger of short circuits by falling down things) one tries however to avoid such undercrossings, whose execution are completely described by the relevant regulations and require no special permissions.
overhead line crossings of state borders
in particular if the line sections on both sides of the borders are operated by different power companies there are frequently two anchor pylons on both sides of the border. One reduces thereby maintenance work, which would require a direct coordination of workers on both sides of the border, to an absolute minimum and avoids possible authority problems for the bordercrossing line section as well as possible.
crossings of other overhead lines
At crossings from overhead lines by other overhead lines the necessary security distances between the lines and to the ground must be kept. As a rule the line with the low voltage is there usually passed through under the line with higher voltage. In principle one tries to plan such crossings in such a way that their realization takes place as economical as possible. This is reached usually by leaving the line which is crossed unchanged, if possible. However one, if this line, that is to be crossed under the line, lowers a possibly existing ground conductor frequently within the crossing range on the height of the highest cross beam, in order to reduce the necessary height of the masts of the crossing line. Undercrossings of existing lines are often realized in the proximity of its pylons, since into this case frequently without heightening the existing pylons the necessary safety distances to the ground and to the other line are kept. In the course of undercrossings frequently the pylon picture is changed and because of its small height an arrangement of the conductors n one level is preferred. Sometimes at such crossings there can be problems because of the maximum pylon height given for reasons of flight safety. If it is not possible at a given location for the pylons for the crossing of the line running above to built pylons of necessary height, the line running which is crossed above will be rebuilt on smaller pylons or it is replaced by an underground cable.
overhead line crossings of wire ropeways
overhead lines should, if at all only cross the track of an earial tramway above the wire of the aerial tramway. In the proximity of Zermatt, Switzerland, an overhead line for 110kV is led to cross the course of an aerial under the rope of the aerial tramway in a protective scaffolding. The necessary protection distances from overhead lines to the ropes of an aerial tramway is to be taken from the relevant regulations concerning the building from aerial tramways and powerlines. In the case of undercrossing an aerial tramway the necessary safety distances between overhead line and soil of the aerial tramway cab must be ensured also in the most unfavorable case. In principle both over and undercrossings are completely regulated by the relevant regulations.
overhead line crossings of broad rivers and straits
overhead line crossings of broad rivers and of straits consist, if the terrain on both sides is relatively even, frequently of 4 pylons: two particulary substantial anchor pylons for bracing the conductors of the crossing section and two carrying masts of large height for the realization of the necessary line height over the water. These pylons are equipped with broader cross beams and greater distances between the cross beams than the other pylons of the line, in order to prevent striking the conductor cables to each other at strong winds. In contrast to normal pylons the two carrying masts at both ends of the crossing are frequently as flight safety obstacle equipped with flight safety lamps and equipped with stairways for an easy access to the top. Overhead line crossings of rivers and straits with spans over 2 kilometers are frequently uneconomic and under operational conditions very unfavorable, because of the danger of wind-induced oscillatory movements of the conductor cables very large leader distances must have to be installed or there have to be insulators mounted between the conductors in the area of the span. Because bundle-conductors which are used at almost all extra-high voltage are much easier erected to oscillations by wind influence than single conductors, bundle conductors cannot be used on the crossing section, which requires the usage of single conductors for the crossing section and making it so to the section of the powerline determining the maximum of transmittable power. A further problem follows from the circumstance that one cannot build the pylons at the two ends of the crossing section for economic and administrative reasons arbitrarily high, but on the other hand however because of ships crossing under the line a considerable minimum height of the powerline has to be kept, so there is often a high push tension in the conductors at long spans. This push tension can only rope with high steel portions stand, which have a worse electrical conductivity than the common overhead line conductors consisting of copper, Aldrey or aluminum-encased or steel, what also limits the amount of transmittable electrical power. For this reason for crossings with span-eidth of more then approximately 2 kilometre, it should be always examined if a realisation as underwater cable, which is for waterways of more then 5 kilometres width the only practicable solution, is the better way. Alternatively one could think still to build if necessary in the water which is to be crossed one or more pylons. One finds such crossings occasionally in North America. They are however only then more economical and practical than a cable laid in the water, when the water is not very deep and no large passage heights are needed for vessels. Also such constructions are permission-legally very problematic, because pylons standing in the water are possible dangerous obstacles for shipping especially in the fog. It can by the way quite occur that overhead line crossings of broad waters are replaced through in the water laid cable. So the overhead line crossing the Strait of Messina, which was with a span width from 3646 meters one of the longest overhead line crossings in the world and their crossing pylons with a height of 200 meters belonged to the highest pylons in the world, was replaced because of its small maximum transmittable electrical power by a submarine cable.
examples
overhead line crossing of valleys
overhead line crossings of valleys consist of two anchor pylons at both ends of the valley. If the topography of the valley is suitable, these do not require to be very high. At very far valleys it is favourable to use for each phase its own mpylon in order to achieve a sufficient distance of the conductors from each other. In these cases there is frequently a further anchor pylon behind the crossing used, in order to realize the angle change of the conductor cables behind these. The problems concerning large spans, mentioned in the last chapter, exist naturally also in these cases, but can, if a suitable topography does not require high crossing pylons be obtained very ecconimacally by using a seperate pylon for each conductor.
examples
Source: http://de.wikipedia.org/wiki/Freileitungskreuzung
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