Beatrice Lang
Those tall electric lines are called transmission lines or power lines. They are used to transmit electrical energy over long distances. They are usually mounted on steel towers or wooden poles and carry high voltages of electricity. The voltage level of a power line depends on the length of the transmission path and the power required by the recipients of the electricity. In some countries, they are also referred to as pylons, although this term is more correctly used to refer to the suspension, tension, or transmission towers that support the lines.
| Characteristics | Values |
|---|---|
| Name | Overhead power lines, pylons (in the UK), suspension/tension/transmission towers |
| Use | Transmit electrical energy along large distances |
| Composition | One or more conductors (commonly multiples of three) suspended by towers or poles |
| Tower Composition | Wood, steel, aluminium, concrete, reinforced plastics |
| Conductor Composition | Aluminium, steel, composite materials (carbon and glass fiber), copper wires |
| Voltage | Low voltage (LV) – less than 1000 Volts, Medium voltage (MV) – 1000 Volts (1 kV) to 69 kV, High voltage (HV) – 115 kV to 138 kV, Extra high voltage (EHV) – 345 kV to about 800 kV, Ultra high voltage (UHV) – higher than 800 kV |
| Insulators | Pin-type, suspension type, strain insulator, made of wet-process porcelain, toughened glass, or glass-reinforced polymer |
| Alternatives | Underground cables, gas-insulated transmission lines (GILs) |
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What You'll Learn
These tall electric lines are called overhead power lines
The voltage level of a power line is determined by the length of the transmission path and the power requirements of the recipients. Overhead power lines can carry high voltages of up to 380,000 volts (380 kV). High-voltage power transmission is more efficient than low-voltage transmission as it results in less energy loss.
To optimise the size of the conductor in a power line, Kelvin's Law states that the cost of the energy wasted in the conductor should equal the annual interest paid on the portion of the line construction cost attributed to the size of the conductors. However, this optimisation problem is complicated by factors such as varying annual load, installation costs, and the discrete sizes of cables available.
Overhead power lines are generally the lowest-cost method for transmitting large amounts of electrical energy due to the good cooling, insulation, and optical inspection provided by the surrounding air. However, in some cases, underground cables are used for power transmission, especially in new and high-density areas. Underground cables can be more challenging due to the smaller distance between conductors, requiring individual protection with insulation.
In some countries, such as the UK, the towers supporting overhead power lines are commonly called pylons. Efforts are being made to minimise the visual impact of these structures in certain landscapes, with projects involving the removal or relocation of pylons and the construction of electricity tunnels underground.
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Overhead power lines are supported by towers or poles
Overhead power lines are used to transmit electricity over large distances. They are often referred to as "pylons" in the UK, but the more correct term is suspension, tension, or transmission towers. These towers or poles support one or more conductors, usually in multiples of three, and are typically made of wood, steel, aluminium, concrete, or reinforced plastics.
The design of these structures is important for their functionality and longevity. For instance, the sag of the conductor (the vertical distance between its highest and lowest points) depends on temperature and additional load, such as ice cover. Towers must also be designed to withstand lightning strikes, which is achieved through the use of surge arresters or shield wires.
The voltage level of a power line is determined by the length of the transmission path and the power requirements of the recipients. Overhead power lines can carry voltages ranging from low (less than 1000 volts) to ultra-high voltage (higher than 800 kV). The voltage is stepped up for efficient long-distance transmission and then reduced for local distribution, as high-voltage electricity is not suitable for everyday appliances.
In some cases, underground cables are used instead of overhead power lines, especially in new and high-density areas. Underground cables are more commonly used for low-voltage domestic and commercial connections, but they are also being used for extra-high-voltage transmission over long distances.
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Overhead power lines carry high voltages
Those tall electric lines are called overhead power lines. They are used to transmit electrical energy over large distances. Overhead power lines carry voltages ranging from low (less than 1000 volts) to ultra-high voltage (higher than 800 kV). The voltage level of a power line is determined by the length of the transmission path and the power required by the recipients. For example, electricity arrives at our homes at 230 volts, but much higher voltages are used to transport it over long distances.
High-voltage power transmission is more efficient than low-voltage transmission because less energy is lost. Overhead power lines can carry up to 380,000 volts (380 kV) and even higher voltages in some cases. High-voltage direct current (HVDC) transmission is a good alternative for long-distance transport as it avoids the energy loss that occurs with alternating current.
The type of mast construction can give a clue about the voltage level of an overhead power line. High-voltage and extra-high-voltage lines are typically carried on high lattice steel masts, while low- and medium-voltage lines are usually on smaller poles made of wood, concrete, or tubular steel.
Insulators are critical in allowing the use of higher voltages. They support the conductors and must withstand both normal operating voltages and surges due to switching and lightning. At higher voltages, suspension-type insulators are commonly used, made of materials like porcelain, toughened glass, or glass-reinforced polymers.
Vegetation growth around high-voltage transmission lines is strictly regulated to prevent damage. This is why you may notice open spaces or rights of way where these lines run.
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Overhead power lines are more efficient than low-voltage lines
Those tall electric lines are called overhead power lines. They are used to transmit electrical energy over large distances. Overhead power lines are classified by the range of voltages into low voltage (LV), medium voltage (MV), high voltage (HV), extra-high voltage (EHV), and ultra-high voltage (UHV).
Secondly, the surrounding air provides good cooling for the conductors, reducing the need for additional cooling mechanisms. This natural cooling effect contributes to the efficiency of overhead power lines.
Additionally, the height and spacing of overhead power lines facilitate better insulation. The distance between conductors is larger in overhead lines compared to underground cables, which helps maintain effective insulation. This improved insulation further reduces energy loss.
Moreover, overhead power lines offer the advantage of optical inspection. The elevated position of the lines makes it possible to visually inspect the lines for any issues or maintenance needs. This ease of inspection contributes to their efficiency by facilitating proactive maintenance and reducing the likelihood of unexpected disruptions.
While larger conductors in overhead power lines have lower electrical resistance, resulting in reduced energy loss, they also come with higher costs. Kelvin's Law addresses the optimization challenge by considering the balance between the cost of energy wasted in the conductor and the construction costs associated with the size of the conductors.
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Overhead power lines can be unsightly, so some countries are moving them underground
Overhead power lines, also known as transmission lines, are structures used in electric power transmission and distribution to transmit electrical energy over long distances. They consist of one or more conductors suspended by towers or poles.
While overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy, they can be unsightly and are vulnerable to damage from vegetation and storms. As such, some countries are moving their power lines underground. This process is known as "hardening the grid" and can help to limit power outages and extend maintenance intervals.
In the United States, several utility companies are investing heavily in relocating power lines underground. For example, Dominion Energy and WEC Energy Group are currently undergrounding more than 6,000 miles (9,656.1 km) of power lines combined. PG&E has also set a goal to underground more than 10,000 miles (16,093.4 km) of overhead electrical lines in High Fire Threat Districts to further harden its system and help prevent wildfires.
In Arizona, property owners can propose Underground Conversion Service Areas to relocate overhead power lines underground in their neighborhoods. This process involves complying with statutory requirements and procedures, including working with SRP's Distribution New Business and the relevant Arizona Revised Statutes. Residents can also elect to have their power lines moved underground on an individual basis, but they may incur all the costs, which can be significant.
Undergrounding power lines offers several potential benefits, including improved aesthetics, reduced vulnerability to damage, and more reliable power transmission. However, it requires robust long-term investment and specialized equipment for installation over long distances.
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Frequently asked questions
Those tall electric lines are called overhead power lines or transmission lines.
Transmission lines carry electricity from power plants to towns and urban centres. They are usually mounted on steel towers.
Transmission lines can be classified based on the range of voltages: low voltage (LV), medium voltage (MV), high voltage (HV), extra-high voltage (EHV), and ultra-high voltage (UHV).
In the UK, these structures are called pylons, but they are more correctly referred to as suspension, tension, or transmission towers.
Yes, transmission lines can be installed underground, especially in new and high-density areas. Underground cables contain one or more conductors and are individually protected from mutual contact by insulation.
