The basic components of power lines are towers (pylons, poles), conductors and insulation. Towers are constructed so as to enable the routing of service conductors by which electricity is transmitted, with adequate insulation distances from objects in the vicinity of the lines and from the tower structures themselves. The dimensions and type of transmission towers depend on many factors, including the rated voltage of the power line, the location of the structure and the role it is to play in the line. All types of transmission towers are manufactured according to standards that guarantee safety of use and durability.
The different types of transmission towers are designed according to the voltage at which the line operates. The higher the voltage, the greater the distances to be maintained between the wires in the power line and the ground and the tower structures themselves. This results in higher voltage line towers being taller and wider than the ones for lower voltages.
Eltel Networks Engineering SA project: WT335 series of towers, which includes, among other things, the above shown pass-through towers in the 2x400 kV/1x220 kV line between Łagisza and Rokitnica/Tucznawa, used for power evacuation from the Łagisza power plant. Two 400 kV circuits and one 220 kV circuit are suspended on these transmission towers. As you can see from this example, the individual circuit do not have to operate at the same voltage.
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Transmission towers are also differentiated according to the role they are to play in the line. Several criteria are considered here, the most general of which includes two types of support structure – suspension (pass-through) and section support.
The basic type of such structures are suspension towers. They are usually used on straight sections of overhead lines where the route does not change its course by more than 2 degrees. In this location, the suspension towers are used to support the cables and therefore mainly carry vertical loads, but not the higher forces caused by cable tension. The use of suspension structures at the positions where the line changes direction results in loads in them with the resultant force from the tension of the cables in the direction of the bisector of the break angle. Such loads can be significant and increase strongly as the angle increases. Such a positioning of the structure requires its appropriate design and it is then customarily referred to as an angle tower.
The loads on suspension towers are strongly related to the length of the span (distance between poles in a straight line) that the tower separates, which is why different types of suspension towers are designed for structural optimisation, depending on the span length (each type assumes a maximum length of span between which the tower can be set).
In practice, many types of suspension and dead-end (section) structures are used on a given line to form what is known as a series of towers. The basic types of transmission towers used on the line are the suspension and dead-end towers mentioned above. In addition to these, there are other, less common, types of towers.
Eltel Networks Engineering SA design- transmission tower ED24 W3S3 M6/K/R on 110 kV line Boguszów - Podzamcze/Boguszów - Marciszów.
The design of the transmission towers is largely determined by the location of the projected line. Depending on its location, the climatic loads to be taken into account are determined on the basis of the relevant standards. These loads determine not only the required strength of the transmission towers, but also their sizes. It is necessary to take into account the behaviour of the wires under wind and/or ice loads.
The type of the terrain across which the line runs is also important. Where lines run through woodland, additional types of structure are usually designed to reduce the necessary felling. In terms of operating conditions, they are mostly equivalent to the basic towers, but have a different geometry - the aim of these towers is to either reduce the width of the line (so-called forest poles) or to run the wires over the forest (so-called over-forest poles). Similarly, for 110 kV lines, so-called slim towers are often used in urban areas. They are also equivalent to basic towers, but their footprint is significantly smaller. At the cost of greater construction weight, this reduces the area occupied.
Transmission tower on 110 kV line Pelplin - Malbork. Design - Eltel Networks Engineering SA, execution - Eltel Networks Energetyka SA.
The transmission towers shall be designed in accordance with the PN-EN 50341-1:2013-03 standard with the PN-EN 50341-2-22:2016-04 appendix. Polskie Sieci Energetyczne and distributors have also developed their own technical specifications setting out additional requirements. In addition, account must be taken of the specific factors at the site which are often significant for the structures to be designed. In practice, whereas in the case of 400 kV lines it is not uncommon for structures to be individually designed for a specific site, for 110 kV lines catalogue towers are usually used. They are developed to be used on lines designed for standard parameters. Eltel Networks Engineering SA has also developed such structures, has them in its offer and uses them on its projects. These tower designs are also available for sale.
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Author: Dominik Brudniak
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