Transmission tower
A transmission tower is a tall structure, usually a lattice or tubular tower made of steel, that is used to support an overhead power line. In electrical grids, transmission towers carry high-voltage transmission lines that transport bulk electric power from generating stations to electrical substations, from which electricity is delivered to end consumers; moreover, utility poles are used to support lower-voltage sub-transmission and distribution lines that transport electricity from substations to electricity customers.
There are four categories of transmission towers: the suspension tower, the dead-end tower, the termination tower, and the transposition tower.
The heights of transmission towers typically range from, although when longer spans are needed, such as for crossing water, taller towers are sometimes used. More transmission towers are needed to mitigate climate change, and as a result, transmission towers became politically important in the 2020s.
Terminology
Transmission tower is the name for the structure used in the industry in the United States and some other English-speaking countries. In Europe and the U.K., the terms electricity pylon and pylon derive from the basic shape of the structure, an obelisk with a tapered top. In Canada, the term hydrotower is used, because hydroelectricity is the principal source of electricity for the country.High voltage AC transmission towers
systems are used for high voltage and extra-high voltage AC transmission lines. In some European countries, e.g. Germany, Spain or Czech Republic, smaller lattice towers are used for medium voltage transmission lines too. The towers must be designed to carry three conductors. The towers are usually steel lattices or trusses and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or EPDM rubber material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.Typically, one or two ground wires, also called "guard" wires, are placed on top to intercept lightning and harmlessly divert it to ground.
Towers for high- and extra-high voltage are usually designed to carry two or more electric circuits. If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction. Indeed, for economic reasons, some transmission lines are designed for three circuits, but only two circuits are initially installed.
Some high voltage circuits are often erected on the same tower as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for railway electrification.
High voltage DC transmission towers
transmission lines are either monopolar or bipolar systems. With bipolar systems, a conductor arrangement with one conductor on each side of the tower is used. On some schemes, the ground conductor is used as electrode line or ground return. In this case, it had to be installed with insulators equipped with surge arrestors on the pylons in order to prevent electrochemical corrosion of the pylons. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, often conductors on both sides of the tower are installed for mechanical reasons. Until the second pole is needed, it is either used as electrode line or joined in parallel with the pole in use. In the latter case, the line from the converter station to the earthing electrode is built as underground cable, as overhead line on a separate right of way or by using the ground conductors.Electrode line towers are used in some HVDC schemes to carry the power line from the converter station to the grounding electrode. They are similar to structures used for lines with voltages of 10–30 kV, but normally carry only one or two conductors.
AC transmission towers may be converted to full or mixed HVDC use, to increase power transmission levels at a lower cost than building a new transmission line.
Railway traction line towers
Towers used for single-phase AC railway traction lines are similar in construction to those towers used for 110 kV three-phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors. These are usually arranged on one level, whereby each circuit occupies one half of the cross arm. For four traction circuits, the arrangement of the conductors is in two levels and for six electric circuits, the arrangement of the conductors is in three levels.Tower designs
Transmission towers must withstand various external forces, including wind, ice, and seismic activity, while supporting the weight of heavy conductors.Shape
Different shapes of transmission towers are typical for different countries. The shape also depends on voltage and number of circuits.One circuit
Delta pylons are the most common design for single circuit lines, because of their stability. They have a V-shaped body with a horizontal arm on the top, which forms an inverted delta. Larger Delta towers usually use two guard cables.Portal pylons are widely used in the USA, Ireland, Scandinavia and Canada. They stand on two legs with one cross arm, which gives them an H-shape. Up to 110 kV they often were made from wood, but higher voltage lines use steel pylons.
Smaller single circuit pylons may have two small cross arms on one side and one on the other.
Two circuits
One level pylons only have one cross arm carrying 3 cables on each side. Sometimes they have an additional cross arm for the protection cables. They are frequently used close to airports due to their reduced height.Danube pylons or Donaumasten got their name from a line built in 1927 next to the Danube river. They are the most common design in central European countries like Germany or Poland. They have two cross arms, the upper arm carries one and the lower arm carries two cables on each side. Sometimes they have an additional cross arm for the protection cables.
Ton shaped towers are the most common design, they have 3 horizontal levels with one cable very close to the pylon on each side. In the United Kingdom the second level is often wider than the other ones while in the United States all cross arms have the same width.
T-pylons
In 2021 the first T-pylon, a new tubular T-shaped design, was installed in United Kingdom for a new power line to Hinkley Point C nuclear power station, carrying two high voltage 400 kV power lines. The design features electricity cables strung below a cross-arm atop a single pole which reduces the visual impact on the environment compared to lattice pylons. These 36 T-pylons were the first major UK redesign since 1927, designed by Danish company Bystrup, winner of a 2011 competition from more than 250 entries held by the Royal Institute of British Architects and Her Majesty's Government.Y-pylons
Y-pylons are a newer concept for electrical transmission towers. They usually have a guy-wire or support beam to help support the "Y" shape in the tower.Four circuits
Christmas-tree-shaped towers for 4 or even 6 circuits are common in Germany and have 3 cross arms where the highest arm has each one cable, the second has two cables and the third has three cables on each side. The cables on the third arm usually carry circuits for lower high voltage.Branch pylons
Special designed pylons are necessary to introduce branching lines, e.g. to connect nearby substations.Support structures
Towers may be self-supporting and capable of resisting all forces due to conductor loads, unbalanced conductors, wind and ice in any direction. Such towers often have approximately square bases and usually four points of contact with the ground.A semi-flexible tower is designed so that it can use overhead grounding wires to transfer mechanical load to adjacent structures, if a phase conductor breaks and the structure is subject to unbalanced loads. This type is useful at extra-high voltages, where phase conductors are bundled. It is unlikely for all of them to break at once, barring a catastrophic crash or storm.
A guyed mast has a very small footprint and relies on guy wires in tension to support the structure and any unbalanced tension load from the conductors. A guyed tower can be made in a V shape, which saves weight and cost.
Materials
Tubular steel
Poles made of tubular steel generally are assembled at the factory and placed on the right-of-way afterward. Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer the use of monopolar steel or concrete towers over lattice steel for new power lines and tower replacements.In Germany steel tube pylons are also established predominantly for medium voltage lines, in addition, for high voltage transmission lines or two electric circuits for operating voltages by up to 110 kV. Steel tube pylons are also frequently used for 380 kV lines in France, and for 500 kV lines in the United States.
Lattice
A lattice tower is a framework construction made of steel or aluminium sections. Lattice towers are used for power lines of all voltages, and are the most common type for high-voltage transmission lines. Lattice towers are usually made of galvanized steel. Aluminium is used for reduced weight, such as in mountainous areas where structures are placed by helicopter. Aluminium is also used in environments that would be corrosive to steel. The extra material cost of aluminium towers will be offset by lower installation cost. Design of aluminium lattice towers is similar to that for steel, but must take into account aluminium's lower Young's modulus.A lattice tower is usually assembled at the location where it is to be erected. This makes very tall towers possible, up to . Assembly of lattice steel towers can be done using a crane. Lattice steel towers are generally made of angle-profiled steel beams. For very tall towers, trusses are often used.