A transmission tower (also known as an electricity pylon, hydro tower, or pylon) is a tall structure used to support an overhead power line. It is usually a lattice or tubular tower made of steel. In electrical grids, transmission towers carry high-voltage transmission lines that transport electric power from generating stations to electrical substations; while utility poles are used to support lower-voltage, electricity contactor relays, sub-station, sub-transmission lines or distribution lines that transport electricity from substations to electricity installed capacity to end customers.
There are four categories of transmission towers: (i) the suspension tower, (ii) the dead-end tower, (iii) the termination tower, and (iv) 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.
Terminology
Transmission tower is the term 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 hydro tower is used, because hydroelectricity is the primary source of electricity for the country.
History
The first sketch for a transmission tower was made by the Pittsburgh Bridge Company in 1894 for a two-circuit three-phase AC power line from Niagara to Buffalo. They chose to place the power line on wooden poles. Lewis Buckley Stillwell: Development of electrical power transmission. Electrical Age, vol. 32, no. 5, p. 281-293. New York, May 1904. In 1901, two large towers for the Carquinez Strait Powerline Crossing were erected. The oldest power line entirely placed on steel towers was between Zamora de Hidalgo and Guanajuato in 1903.Robert McF Doble: The Guanajuato transmission line. The Journal of Electricity, Power and Gas, vol. 14, no. 9, p. 337-358, col. 1-2. San Francisco, CA, United States, September 1904 Despite the age, some towers from this power line are still in service as of 2025, serving for medium-voltage distribution. Another power line on steel towers existed in 1904 in Italy.
High voltage AC transmission towers Three-phase electric power systems are used for high voltage (66- or 69-kV and above) and extra-high voltage (110- or 115-kV and above; most often 138- or 230-kV and above in contemporary systems) AC transmission lines. In some European countries, e.g. Germany, Spain or Czech Republic, smaller lattice towers are used for medium voltage (above 10 kV) transmission lines as well. The towers must be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or trusses (wooden structures are used in Australia, Canada, Germany, and Scandinavia in some cases) 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 the 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. For economic reasons, some transmission lines are designed for three (or four) circuits, but only two (or three) 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
High-voltage direct current (HVDC) 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 arresters 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 (grounding) 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 (or multiples of two, usually four, eight, or twelve). 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.
Hybrid lines
It is possible that on electricity pylons AC circuits of different frequency and phase-count, or AC and DC circuits are installed.
Before the realization of the Ultranet HVDC line, which uses in most parts towers equipped with AC circuits and the DC circuit of Ultranet, towers carrying AC and DC circuits were very rare. There existed only two short sections with such installation: one was the terminal of HVDC Volgograd-Donbass on Volga Hydroelectric Power Station, the other were two towers south of Stenkullen, which carried one circuit of HVDC Konti-Skan and one circuit of the three-phase AC line Stenkullen-Holmbakullen.
Towers carrying AC circuits and DC electrode lines are a bit more common:
The pylons of a section of the AC-powerline between Adalph Static Inverter Plant and Brookston carry also the electrode line of HVDC Square Butte from Adalph Static Inverter Plant to a point situated at 46.924659 N 92.697275 W as one ground conductor. The electrode line of HVDC CU at the converter station at Coal Creek Station uses on a short section the towers of two AC lines as support. The metallic return of HVDC Vancouver Island is partly installed on poles that carry also AC-circuits. The longest section where the metallic return is installed on pylons that carry also AC circuits is on the Canadian mainland between 49.040333N 123.07579 W and 49.060995 N 123.044664 W with a length of 4.59 kilometres. The overhead section of the electrode line of Pacific DC Intertie from Sylmar Converter Station to the grounding electrode in the Pacific Ocean near Will Rogers State Beach is also installed on AC pylons as two ground conductors. It runs from Sylmar East Converter Station to Southern California Edison Malibu Substation, where the overhead line section ends, which serves also as ground conductor.
Much more common are electricity pylons that carry as well circuits of the public AC grid and circuits of the AC grid of the national railway company. Such towers may exist in countries where the national railway company uses AC with a frequency which is different from the AC grid of the public grid. Especially in Switzerland and Germany such lines are not rare.
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 US, Ireland, Scandinavia and Canada. They stand on two legs with one cross arm, which gives them an H-shape. Up to 110 kV, they were often 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 tower. In the United Kingdom the second level is often (but not always) 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 (two or more wires per phase). It is unlikely for all of them to break at once, barring a catastrophic crash or storm.