Introduction
Transmission towers (electrical pylons) carry large amounts of high-voltage current over long distances. These structures typically stand 50 to 150 feet tall (16m to 45m), with the tallest towers being 1,247 feet (380m) tall. Transmission towers connect power plants to a series of substations, which allows one bulk power region of the grid to connect to another.
Transmission Towers
Higher voltages on power lines require space between each line and other objects, allowing people, vehicles, and other equipment to move freely beneath. The tower’s live conductors are supported by insulators, the length of which increases with the increasing voltage of the circuit. For this reason, transmission towers usually stand 50 feet to 150 feet high (16m to 45m), or higher if spanning waterways or other natural chasms.
Most tower structures are manufactured from steel, but some are manufactured from concrete, wood or even ductile iron. Wooden distribution poles, found in local neighbourhoods (unless using underground power lines), are generally about 40 feet (12m) tall. Transmission voltages are usually between 23,000 volts to 765,000 volts.
From a mechanical perspective, a tower’s conductors behave like wires whose sag between their points of support depends on the temperature and the pre-tensioning of the conductor. Tensile forces in the conductor have a great effect upon a tower’s design.
Transmission Tower Sag and Clearance
Transmission Tower Conductors
Transmission tower conductors are usually manufactured from steel-reinforced aluminium cable (ACSR-Aluminium Conductor Steel-Reinforced) and are almost always arranged in sets of three for three phase (3~) alternating current transmission; a fourth neutral cable may be used for transmission over short distances, but this is not common.
Conductors are grouped by phase. There could be one conductor line per group (three total), two conductor lines per group (six total), or more. Groups are installed in multiples of three i.e. 3, 6, 9, and maybe arranged in a triangular shape or parallel to each other.
Tower Conductor Bundle Configurations
Three-way grouping increases transmission efficiency. However, if you look at the top of a transmission tower, you may see one or two smaller, solitary wires. These wires have several names, overhead ground wire, static wire, or pilot wire, but all describe the same wire. An overhead ground wire (static wire / pilot wire) absorbs or deflects lightning strikes, conveying electricity safely to the ground. Under normal conditions, the overhead wire does not carry electricity (its voltage potential is 0).
Some overhead ground wires are grouped with fiber-optic cables that convey telecommunication data. Essentially made of glass, fiber-optic cables cannot conduct electricity and are not affected by lightning strikes.
Alternatively, you may notice fiber-optics running a few feet (<1m) below transmission conductors. Adding telecommunication lines increases the return on investment associated with building transmission networks. Fiber-optic lines may be operated by the utility or leased to cable or phone companies.
Workers Installing a Fiber-Optic Cable
Transmission Tower Structures
The structures commonly used on transmission lines are either lattice type or pole type. Lattice structures are usually composed of steel angle sections. Poles can be wood, steel, or concrete. Each structure type may be self-supporting or guyed (supported by cables).
Transmission Tower Structure
Pole type structures are generally used for voltages of 345-kV or less, while lattice steel structures are favoured for higher voltage levels. Wood pole structures can be economically used for relatively short transmission distances and lower voltages.
The configuration of a transmission line tower depends upon many factors, some are listed below:
- The number and type of conductors.
- The length of the insulator assembly.
- The minimum clearances to be maintained between conductors and the tower.
- The location of ground wire(s) with respect to the outermost conductor.
- The mid-span clearance required considering the dynamic behaviour of conductors and lightning protection of the line.
- The minimum clearance of the lowest conductor above ground level.
The factors governing the height of a tower are:
- Minimum permissible ground clearance (h1).
- Maximum sag (h2).
- Vertical spacing between the top and bottom conductors (h3).
- Vertical clearance between the ground wire and top conductor (h4).
The total height of tower is given by the sum of all four factors (h1+h2+h3+h4).
Transmission Tower Structure
Tower Configuration
Depending upon the requirements of the transmission system, various line configurations must be considered ranging from single circuit horizontal to multi-circuit vertical structures, with single or V strings in all phases, as well as any combination of these. Also, for very high voltages (500 kV and above), conductors are bundled to reduce corona emission and reduce line inductance.
The configuration of a transmission line tower depends upon many factors, some of the most important are listed below:
- Voltage.
- Number of circuits.
- Type of conductors.
- Type of insulators.
- Possible future addition of new circuits.
- Tracing of transmission line.
- Selection of tower sites.
- Selection of rigid points.
- Selection of conductor configuration.
- Selection of height for each tower.
Towers are classified according to their use, independent of the number of conductors they support. A tower must withstand mechanical loads from a range of directions e.g. straight, at an angle etc. To simplify tower design and ensure an overall economy in cost and maintenance, tower designs are generally confined to a few standard types.
Transmission Tower Types
There are several types of transmission tower and many variations, but they can be roughly grouped as:
- Suspension Towers – conductors are suspended between two towers using suspension insulators.
- Terminal Towers – conductors from a transmission line are connected to a substation or underground cable via a tower’s strain insulators.
- Tension Towers – the tower can cater for the weight of the cables and axial loading (strain in a horizontal direction).
- Transposition Towers – the tower changes the position of the conductors on a transmission line relative to each other e.g. in position x, out position y.
There are too many tower variations to be discussed here, but some of the most common will now be discussed further.
Transmission Tower Types
Suspension Towers
Suspension towers (tangent towers) are used primarily on tangents but are often designed to withstand angles in the line only up to 2°, in addition to wind, ice, and broken conductor loads. If the transmission line traverses relatively flat featureless terrain, ninety percent of the line may be composed of this type of tower. Thus, the design of tangent tower provides the greatest opportunity for the structural engineer to minimize the total weight of steel required for the transmission system.
Tangent Transmission Tower Top View
Angle Towers
Angle towers, sometimes called ‘semi-anchor’ towers, must resist transverse loads induced at an angle (in addition to the usual wind, ice and broken conductor loads). Angle towers are heavier than suspension towers by necessity.
Angle Transmission Tower Top View
Angle towers are used when the line deviation exceeds an angle greater than 2°; they are classified as:
- Small angle towers (2-10° line deviation).
- Medium angle towers (10-30° line deviation).
- Large angle towers (30-60° line deviation).
Tension Tower / Strain Tower
Unlike suspension towers, tension towers use strain insulators to resist axial loading placed on the tower from the conductors (net tension acting on the tower).
Suspension and Strain Tower Side View
Dead-End Tower
Dead-end towers (anchor towers) support the weight of the connecting conductors and cater for the tension in the conductors; this type of tower also uses strain insulators. Dead-end towers are typically used at the end of a transmission line before the line passes to a substation or underground line. Dead end towers are often installed periodically between a series of suspension towers; this setup reduces the likelihood of a series of towers cascade failing (can occur when a conductor on the transmission line fails).
Transmission Tower Loads
The loads acting upon an electrical transmission tower are numerous and dynamic, some are listed below:
- Dead load of tower.
- Dead load from conductors and other equipment.
- Load from snow on conductors and equipment.
- Ice load on the tower itself
- Erection and maintenance loads.
- Wind load on the tower.
- Wind load on conductors and equipment.
- Loads from conductor tensile forces.
- Seismic activity loads (earthquakes etc.).
The major load acting on a transmission tower arises from the conductors, and that the conductors behave like chains able to resist only tensile forces. Consequently, the dead load from the conductors is calculated by using the so-called weight span, which may be considerably different from the wind span used in connection with the wind load calculation.
Weight and Wind Loads
The average span length is usually chosen to be between 300 and 450 metres. The occurrence of ice and snow etc. adds to the weight of the parts covered, and it increases their exposure to the effects of wind. Underestimation of these circumstances has frequently led to damage and collapse of transmission towers.
The size and distribution of ice and snow loads depend upon the climate and local conditions. The wind force is usually assumed to be acting on a horizontal plane. However, depending on local conditions, a sloping direction may have to be considered. Also, different wind directions (in the horizontal plane) must be taken into account for the conductors as well as for the tower itself. The maximum wind velocity does not occur simultaneously along the entire span, so coefficients are introduced into load calculations to account for this.
Tensile forces in the conductors act on the two faces of the tower in the line direction. If the forces are balanced, no longitudinal forces will act on a tower suspending a straight line. For angle towers, longitudinal forces result in a resultant force acting in the angle bisector plane. For terminal towers, the forces can cause heavy longitudinal resultant forces. As tensile forces vary with external loads, even suspension towers on a straight line are affected by longitudinal forces.
Additional Tower Purposes
Transmission towers often serve a dual or tri purpose. Weather data & communication collectors are often installed upon transmission towers. For example, you may have noticed the spinning cups of an anemometer measuring wind speed, or other meteorological equipment installed on a tower. Additionally, cell phone antennas may be attached to some transmission towers in strategic locations.
Early tower designers discovered that some large birds like to build nests on top of the towers. Unfortunately, birds can cause an outage if excreted waste lands on an insulator and causes a short circuit. To prevent these unintentional outages occurring and to maintain a positive relationship with the local wildlife, designers now include special nesting platforms for the birds.
Birds Nesting on Transmission Towers
3D Model
This 3D model shows a typical electrical transmission tower used for distributing voltage greater than 200 kV. The tower is designed to be structurally strong. Its structure is also designed to reduce the effect of high winds upon the tower. The tower’s bushings insulate the tower from the electrical cables (transmission line), thus ensuring the electrical potential of the tower remains at zero.
This 3D model shows all major components associated with a typical high voltage electric pylon; these include:
- Tower Top
- Beam
- Bushings
- Fork
- Crossarm
- Tower Window
- Overhead Ground Wire
- Conductor Bundle
- Tower Body (Waist, Leg, Diagonal Members)
- Tower Base (Foundation)
Additional Resources
https://en.wikipedia.org/wiki/Transmission_tower
http://www.hydroquebec.com/learning/transport/types-pylones.html
https://www.electrical4u.com/electrical-transmission-tower-types-and-design/