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Overhead line crossing 1

Overhead line crossing

An overhead line crossing is the crossing of an obstacle —such as a traffic route, a river, a valley or a strait —by an

overhead power line. The style of crossing depends on the local conditions and regulations at the time the power line

is constructed. Overhead line crossings can sometimes require extensive construction and can also have operational

issues. In such cases, those in charge of construction should consider whether a crossing of the obstacle would be

better accomplished by an underground or submarine cable.

Crossings of roads and railway lines

Overhead line crossings of roads, railway lines, and small- and medium-sized watercourses do not normally require

special construction. However, a former law in Germany for crossing dictated that there must be an anchor pylon on

each side of a crossing of the national railway, which can be seen on some old power lines. For overhead line

crossings of motorways the pylons must be rebuilt before they wear out, because these demand additional

maintenance. If local conditions are appropriate, an overhead line can be implemented by way of a valley bridge. For

example, the Koersch valley bridge near Esslingen, Germany carries the 110 kV, three-phase line of the EnBW AGwith 2 circuits. Because of the danger of short circuits from falling objects, undercrossings are typically avoided.

Crossings of overhead lines at state borders

380 kV-powerline crossing the national border

between Bavaria and Czech near Waidhaus. The

pylon in the foreground stands in Bavaria the

others are in Czech

There is frequently an anchor pylon on each side of the border,

particularly if the lines on either side of the border are operated by

different companies. This setup reduces maintenance work, which

would otherwise require direct coordination of workers on both sides

of the border, and avoids possible authority problems associated with

border crossings as much as possible.

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Overhead line crossing 2

Crossings of other overhead lines

The only crossing of two different HVDC

overhead powerlines in the Western hemisphere,

CU and Square Butte in North Dakota

At crossings of overhead lines by other overhead lines, the two lines

must be kept at the necessary safety distances between the lines and the

ground. As a rule, the line with the lower voltage passes under the line

with higher voltage. Construction workers try to plan these crossings in

such a way that their construction is as economical as possible. This is

usually done by leaving unchanged the line that is crossed, if possible.

Undercrossings of existing lines are often constructed in proximity to

the line's pylons, since this can often be accomplished without raising

the existing pylons and while keeping the necessary safety distances

between the ground and the other line.

In the course of undercrossings the pylon picture is frequently changed,

and because of its small height it is preferable to create an arrangement

with conductors in one level. Sometimes at such crossings there can be problems because of the maximum pylon

height allowed for flight safety reasons. If it is not possible at a given location for the pylons of the upper line to be

built at a necessary height, the line running below it will be rebuilt on smaller pylons or replaced with an

underground cable.

A unique undercrossing of two powerlines can be found north of Kincardine at Scotland at 56°5'17"N 3°43'11"W.

Here crosses the powerline Kincardine-Tealing two other lines. One of the two circuits of Kincardine-Tealing

powerline crosses these lines on two small pylons and the other circuit via an underground cable.

Special crossings from overhead lines of other overhead lines

There are some crossings between two overhead powerlines, which are unique, either as both lines are of special type

or the unique way of implemention

Coordinates Line 1 Line 2 Reason of unity

56°2′26″ N

3°53′20″ W

Longannet Power Station -

Glasgow,Carmyle

Longannet Power Station -

Glasgow,Bishopbridge

Powerline Longannet Power Station -

Glasgow,Bishopbridge crosses powerline

Longannet Power Station - Glasgow,Carmyle as

underground cable

56°5′17″ N

3°43′11″ W

Longannet Power Station -

Glasgow,Carmyle, Longannet Power

Station - Glasgow,Bishopbridge

Kincardine - Tealing one circuit of double-circuit line crosses two

powerlines as underground cable

47°02′48″ N

100°05′49″ W

CU (Coal Creek Power Station -

Rockford, Minnesota)

Square Butte (Center, North Dakota

- Adolph, Minnesota)

only crossing of 2 HVDC powerlines outside

China

48°44′1″ N

38°43′26″ E

HVDC Volgograd-Donbass

(Mikhailkovkaya - Volgograd)

HVDC Volgograd-Donbass

electrode line (Mikhailkovkaya -

Smile)

crossing of HVDC and its electrode line

60°28′46″ N

17°14′12″ E

Fenno-Skan 2 (Finnbole - Rauma) Tierp-Gävle only crossing of HVDC overhead line with single

phase AC line in the world

50°28′55″ N

9°40′52″ E

Flieden-Bebra Fulda-Mottgers crossing of 2 single phase AC power lines

51°01′59″ N

9°34′31″ E

Bebra-Borken Kirchheim-Körle crossing of 2 single phase AC power lines

48°56′40″ N

8°48′18″ E

Karlsruhe-Mühlacker Vaihingen-Graben/Neudorf crossing of 2 single phase AC power lines

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Overhead line crossing 3

Overhead line crossings of aerial tramways

Overhead lines should cross the route of an aerial tramway only above it, if at all.

The necessary protection distances from overhead lines to the ropes of an aerial tramway are subject to regulations

concerning the construction of aerial tramways and overhead lines. In the case of an undercrossing of an aerial

tramway, the maximum safety distances between the overhead line and the floor of the aerial tramway cab must be

followed absolutely.

In principle, over- and undercrossings of aerial tramways are completely regulated. However, frequently at the range

of the crossing section, special precautionary measures are taken. Thus, at overhead line crossings at which the

overhead line runs above the rope of the aerial tramway, two catch ropes are occasionally installed to prevent the

conductor from falling off the rope of the tramway in case a pylon or insulator were to break. Alternatively, auxiliary

cross-bars can be installed on the pylons of the overhead line under the conductors, which prevent the conductor

cables from falling in case of an insulator failure on the aerial tramway. Occasionally, the span field of the line over

the aerial ropeway can be scaffolded with a rigid construction along its whole length, or at least for the span which

crosses the aerial tramway.

At crossings at which the aerial tramway runs above the power line, the line is frequently installed on special mastsin the crossing range, which scaffold the line in the area of the aerial tramway crossing. Such a measure is not

necessary according to power line regulations, but it is often done because, in case of aerial tramway failure, it is

possible to rescue people from the tram without switching off the overhead line. Such constructions may be seen at

110 kV power line crossings of the Penkenbahn at Mayrhofen, the Patscherkofelbahn at Innsbruck and south of 

Zermatt.

Overhead line crossings of broad rivers and straits

The two masts of the high level crossing of the River Severn estuary

between England and Wales, with the Severn Bridge behind. These

masts are 1.6km (1 mile) apart: a further 1.2-km line crosses the

River Wye estuary to the left.

Overhead line crossings of broad rivers and of straits, if 

the terrain on both sides is relatively even, frequently

consist of four pylons: two particularly substantial

anchor pylons for bracing the conductors of the

crossing section, and two tall carrying masts to keep the

line high over the water. These pylons have broader

cross-bars and greater distances between the cross-bars

than the other pylons of the line, in order to prevent the

conductor cables from striking against each other

during strong winds. In contrast to normal pylons, the

two carrying masts at both ends of the crossing are

frequently equipped with flight safety lamps, and havestairways for easy access to the top.

Overhead line crossings of rivers and straits with spans

of over 2 km are frequently prohibitively expensive to

build and operate; because of the danger of 

wind-induced oscillatory movements of the conductor cables, it is necessary either to install very large leader

distances or to mount insulators between the conductors in the area of the span. Bundle conductors, which are used

for almost all extra-high voltage lines, are more susceptible to oscillations from wind forces than single conductors.

Therefore single conductors must be used for the crossing section, which means the crossing section of the power

line determines the maximum transmittable power.

Further, one cannot build pylons arbitrarily high at either end of the crossing section, and there is a usually a

considerable minimum height because of ships crossing under the line, so there is often a high mechanical tension in

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Overhead line crossing 4

the conductors at long spans. This tension requires conductors made largely of steel, which have a worse electrical

conductivity than the common overhead line conductors consisting of copper, Aldrey or aluminum-encased steel,

and also limits the amount of transmittable electrical power. For this reason, for crossings with a span width of more

than approximately 2 km, those in charge of construction should consider laying an underwater cable as the more

practicable solution.

Alternatively, it might be possible to erect one or more pylons in the water to be crossed. Such crossings can be seenoccasionally in North America. They are, however, only used when it is more economical and practical to do so than

to lay a cable underwater, such as when the water is not very deep and no large passage heights are needed for

vessels. Also, such construction can be very problematic as far as getting legal permission to build, because pylons

standing in the water are likely to be considered dangerous obstacles for ships, especially in foggy conditions.

In some cases on bridges small crossing a wider waterway pylons or crossbars for the conductors can be mounted.

Such a solution, which may lead to safety problems at bridge maintenance, was for example realized at the Danish

Storstrøm Bridge.

It is quite likely that overhead line crossings of broad waters can be replaced with underwater cables. The overhead

line crossing the Strait of Messina  — which, with a span of 3646 meters, was one of the longest overhead line

crossings in the world, with 200-meter pylons among the highest in the world — was replaced by a submarine cable,

because of its small maximum transmittable electrical power.

Overhead line crossing of valleys

Overhead line crossings of valleys consist of two anchor pylons, one at either end of the valley. If the topography of 

the valley is suitable, these do not need to be very high. In very wide valleys, it is better to use a pylon for each phase

in order to achieve sufficient distance between the conductors. 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 associated with large spans also exist in these cases, but these can be easily and economically ameliorated,

if the topography does not require high crossing pylons, by using a separate pylon for each conductor.

Structures

A crossing pylon is used for crossing over a body of water or a valley. Due to the long [span widthspan, crossing

pylons across rivers and sea straits are frequently taller than standard pylons. They may have marking lamps, and

unlike standard pylons, often have stairways for easy access to the top. In many cases, their height makes them ideal

for carrying radio antennas and transmitting equipment.

Crossing pylons for valleys, depending on the local topography, are not necessarily tall, but the distance between the

conducting cables must be sufficient to prevent high winds knocking the conductors into one another; these pylons

have wide crossbars to prevent this. For very long spans each phase has a separate pylon, particularly if the pylonsare short.

Special crossing pylons are often used where aerial tramways cross power lines. These pylons are designed with

integral scaffolding so that the tramway cars can be reached without touching a live power line. This enables

passengers to be rescued from the tramway if it fails without cutting the power from the power line. Such

installations can be found, for example, south of Zermatt, Switzerland; at the Patscherkofelbahn near Innsbruck,

Austria; and at the Penkenbahn in Mayrhofen, Austria.

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References

This article draws heavily on the corresponding article in the German-language Wikipedia[1]

.

References

[1] http://de. wikipedia. org

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Article Sources and Contributors 6

Article Sources and ContributorsOverhead line crossing Source: http://en.wikipedia.org/w/index.php?oldid=415569901 Contributors: Altenmann, AzaToth, Birdhurst, Brim, Chairman S., Chayka, Chick Bowen, Chris the

speller, Chuunen Baka, Folajimi, Gxti, LouScheffer, Onore Baka Sama, Pearle, Petri Krohn, Quale, Rodney Boyd, Schmiteye, Steinsky, Vegaswikian, Wtshymanski, Zonk43, 62 anonymous edits

Image Sources, Licenses and ContributorsImage:Powerline_Border_Crossing.JPG Source: http://en.wikipedia.org/w/index.php?title=File:Powerline_Border_Crossing.JPG License: Public Domain Contributors: User:Zonk43

Image:HVDC_Crossover_North-Dakota.jpg Source: http://en.wikipedia.org/w/index.php?title=File:HVDC_Crossover_North-Dakota.jpg License: GNU Free Documentation License

 Contributors: User:Wtshymanski

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