Double-circuit transmission line

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A double-circuit transmission line is a transmission line where two circuits are carried on a tower line.

For a three-phase system, this implies that each tower supports and insulates six conductors. Single phase AC-powerlines as used for traction current have 4 conductors, on HVDC systems one uses almost only 2 conductors, as at such systems ground is often used as common second pole for both systems. However schemes with 4 conductors are also common, at which the grounded pole is fixed on small insulators in order to prevent electrochemical corrosion on the pylon.

[edit] Construction

For a lattice tower structure, these typically have a pyramidal base, then a vertical section, with three levels of two crossarms extending to each side. The insulators are attached to the crossarms.

The most common form has the middle pair of crossarms a bit longer than the other two pairs, although sometimes they're the same size. It is also common that the length of crossbars reduces with height and in rare cases grow with height.

There exist also other forms, which are especially in Europe used: towers with a single crossbar and 6 conductors and towers with two levels of two crossbars extending to each side. In most cases carry hereby the lowest crossbar 4 and the upmost one 2 conductors.

For systems with 4 and 2 conductors almost pylons with a single crossbar, carrying all conductors in a single level are used. At HVDC schemes where the grounded pole is mounted on the pylons it is often mounted on a small crossbar above the high voltage conductors serving as ground conductor. However, it must be fixed with insulators equipped with lightning arresters on the towers, otherwise electrochemical corrosion would occur.

A steel pole structure has a single (very thick) pole with six crossarms attached.

In some countries like Germany most powerlines with voltages above 100 kV are implemented as double, quadruple or in rare cases even hexuple powerline as rights of way are rare. Not always all conductors were installed after the erection of the pylons. It is common that some circuits are installed later. A disadvantage of double circuit transmission lines is that maintenance works can get more difficult, as either work in close proximity of high voltage or switch-off of 2 circuits is required and that in case of a failure both systems can be affected.