A Transmission tower (also known as a 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 15 to 55 m (49 to 180 ft), 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. 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. 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.
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
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