Technical information on British overhead power lines

Contents

• Overview
• Supports
• Carrying power
  • Voltages
  • Circuits and phases
• Underground, overground …
• Conductors
  • Materials
  • Conductor types
    • ACSR
    • AAAC
    • AACSR
    • HTLS
    • Gap-type
    • ACCR
  • Designations
  • Conductor sizes
  • Imperial/metric equivalence
• Factors and conditions
  • Sag
  • Ice and wind loading
• Limits and uprating

Overview

This page briefly covers a few technicalities of overhead power lines, based on British practice. The information is neither exhaustive nor authoritative. More controversial subjects such as the effects on property pricing claims of cancer risks and ostensibly harmful effects of electromagnetic fields are intentionally excluded.

For official information on electromagnetic fields and their potential risks, see National Grid’s electric and magnetic fields site.

Supports

Overhead power lines comprise wires known as conductors suspended from supports. Supports in the UK take various forms including lattice towers (“pylons”), lattice masts and poles, single wood and metal poles, twin wood poles and metal portal structures.

The support types are not voltage-specific. 400 kV is normally found on lattice towers (“pylons”), but there is also the “T-pylon” metal pole design; wood structures are not used. 275 kV is carried on lattice towers. 132 kV is most commonly carried on lattice towers, but many single-circuit lines use wood poles (such as the “Trident” type) and, occasionally, metal supports. There are also metal and composite poles for double-circuit 132 kV lines.

The 33 kV and 66 kV distribution lines of the national grid were originally carried on steel towers just as with the 132 kV transmission lines. However, in the years after World War II, they were typically built using wood pole supports. The use of wood poles found favour also with 132 kV single circuit.

11 kV lines and 400 V and 230 V connections are almost always on wood poles.

Carrying power

The passage of electricity through wires generates heat through electrical resistance (joule heating), proportional to the current carried and the conductivity of the conductor (the metal or alloy chosen and the cross-section of the conductor). This heat represents a loss of energy: electrical energy converted to heat and lost to the atmosphere. This is negligible for data signalling and domestic appliances but significant for power transmission.

Energy loss through Joule heating is reduced by the use of high voltages. Ignoring complexities, power equals current multiplied by voltage (P = IV), indicating that the same amount of power can be transmitted by either high voltage at low current or high current at low voltage. The voltage drop due to this energy loss, per Ohm’s law (V = IR) is proportional to the current being carried and the resistance of the conductor, which is proportional to its resistivity and the length of the power line. The lower the current, the lower the voltage drop. Further, the significance of this loss is much greater at lower voltages: a 10 V drop on a 100 V line is a 10% loss in voltage, while a 10 V drop on a 10 kV line is only a 0.1% voltage loss (just using some simple figures for clarity). (AC is more complicated than DC, but the same basic principles stand.)

Domestic appliances, heating and lighting run on low voltages (230 V in the UK, formerly 240 V), which is impractical for long-distance transmission. To resolve this, output from power stations is stepped up to high transmission voltages, transmitted to major substations at high voltage, then stepped down to progressively lower voltages for distribution to towns and villages. Transforming current between high and low voltages requires alternating current (see war of the currents), which in the UK cycles at a frequency of 50 Hz. The use of AC brings its own problems, as high power AC will induce current into surrounding objects, including the earthwire.

Even when running at voltages up to 400 kV, the conductors can still run hot. The standard maximum operating temperature for steel-cored aluminium conductors was set at 50 °C, although as noted later, line uprating allows increases in temperatures to 75 °C or more, at a cost of greater resistive losses. The operating temperature is likely to be at least in part a compromise: the less heat you want to waste, the thicker and heavier the conductors need to be, which increases their cost as well as increasing the required strength and thus bulk, complexity and cost of the overhead line supports. There are also absolute limits at which the heat degrades the metal used for the conductors.

The maximum operating temperature at the point of design for a power line sets a limit on how much power can be carried because this is factored into the sagging basis for the line. That is, if more current were to be carried, the conductors would sag too much from heat expansion and would breach ground clearance. Power lines have separate summer and winter limits: in summer, when the conductors already get hot from the sun, less Joule heating is allowed to stay within sag limits. In winter time, higher current can be carried, when the sun is weak and cold air conducts away the heat.

Voltages

Power lines in the UK operate at a number of voltages. At the top end, Supergrid lines operate at 400 and 275 kV and are classed as “transmission” lines: power lines that transmit power around the country. Below this are the 132 kV lines, classed as “distribution” in England and transmission in Scotland. 132 kV was the original transmission voltage established by the national grid constructed between 1928 and 1933. The creation of the Supergrid in the 1950s introduced higher transmission voltages of 275 and later 400 kV, and in more recent years the 132 kV lines in England and Wales have been downgraded to distribution status and divested out of the National Grid to distribution network operators (DNOs). Below these voltages are the 66 kV, 33 kV, 11 kV, 400 V and 230 V distribution voltages.

Higher voltages require larger insulators and larger clearances. With traditional ceramic and glass insulators, at lower voltages, the number of insulator units in the string is a good guide to the voltage: a single insulator unit (or two) implies 11 kV and a string of three insulator units implies 33 kV. 66 kV is more likely to be five units (not six) and for 132 kV you are more likely to see nine or ten insulator units in a suspension string, depending on the requirements of the environmental conditions. (132 kV uses typically 10–11 units in a suspension insulator string and around 8–9 units in a tension insulator string.) For 400 kV (a little over 3 × 132 kV), you would expect three times as many insulator units, but this is not the case. The Balfour Beatty L6 towers in Sundon for example have 21 insulator units per string on both suspension and tension towers. L2 towers vary in the region of around 14–20, with 400 kV towers appearing to have around 18–20.

National Grid’s Terminology – an introduction page previously gave an idea of the amount of power that each voltage of line supports. These figures are indicative only, as the power rating varies by conductor type.

In [Development Near Lines] Appendix II, National Grid give the following figures:

The increase in voltage is only one reason for the huge increase in capacity of a 400 kV line over a 132 kV line. 400 kV lines use not only higher cross-section conductors but also multiple conductors per phase, known as “bundles”. These heavier conductors and higher conductor counts require stronger towers to support the additional weight. The higher voltages require taller towers to provide greater clearance between the conductors and everything that they pass over (buildings, trees and the ground), and greater clearance between the phases. The end result is that a 400 kV tower is up to twice the height of a 132 kV tower. 132 kV towers come in at around 26 to 27 metres in height. 275 kV L3 towers are around 37 to 38 metres tall. 275/400 kV L2 towers are over 41 m tall and the L8 replacement type is 46 m tall, while L6 400 kV towers are around 50 m tall. L12, which superseded L6, is shorter at 46 m.

Circuits and phases

Most power around the world is transmitted and distributed in three phase alternating current. A three-phase circuit is in effect three separate circuits run in parallel with their AC cycles 120° out of phase with each other (hence the name). Most loads are connected to only a single phase, but heavy duty industrial equipment (especially high-power motors in machinery) may be connected to all three phases. The load drawn from each of the three phases should be roughly equal.

Each phase is a single “live” wire with no corresponding neutral wire: in three-phase power, the current from each phase is returned collectively via the other two phases. This is why power lines are typically found with multiples of three wires. Most overhead lines are three-phase but wood power lines can also be two-conductor single phase.

Power lines are typically single circuit or double circuit. A single-circuit line is a single three-phase circuit (three wires). The 11 kV feeds to farms and villages on wood poles are single-circuit. Many of the original (1928–1933) 132 kV national grid transmission lines were single-circuit, such as those found in the East Midlands and Yorkshire.

Double-circuit lines take the form of two side-by-side three-phase circuits. For 33 kV this may be two lines running side-by-side or, more commonly, both circuits suspended from twin poles. For 132 kV and above, double circuit is typical in much of the UK; both circuits share the same towers, one circuit on each side, for a total of six conductors. A double-circuit line provides resilience in that one circuit can be switched off for maintenance (such as tower painting, conductor renewal and refurbishment of fittings) while the other is left active.

The three phases that make up a single circuit are described as “red”, “blue” and “yellow”.

Underground, overground …

A perennial question, especially with regards tower lines, is why are power lines not all placed underground? After all, unlike in some other countries, most power distribution even within small villages is underground, with overhead lines used only for isolated farms and houses. (There are places in the UK where houses are fed from pole lines, for example the Painter Brothers poles in Sussex.)

As previously noted, power lines run hot. Above ground, conductors are able to lose heat through heat radiation and conduction. This is one argument in favour of bare conductors over insulated conductors: insulating conductors traps in heat and limits their capacity. (A discussion on Facebook between various lineworkers indicated the general difficulty working with insulated 11 kV lines, in particular earthing lines during maintenance).

Underground cables must be insulated and therefore trap heat. As a consequence, Joule heating must be kept lower. To maintain the same current rating, higher diameter conductors are needed, which is more costly.

Underground power lines are not placed in a tunnel. They are laid using a cut-and-cover technique: dig a trench, lower the cables into the trench, then fill the trench in. This process is more disruptive to land usage, and leaves an underground obstacle that must not be ploughed through or dug up. During construction a wide working area either side of the trench is required, damaging a much greater land area than when erecting towers at intervals of 300 metres or more. Initial damage notwithstanding, this is less of an issue for national parks (where there is considerable pressure to bury power lines), but a significant concern for farmland and towns. Buried cables in towns commonly follow roads, because this is the only route possible. Overhead lines require a path to be cut through trees, but so would burying a power line along the same route.

Unlike cables buried under soil, which are easy to dig back up (at the considerable frustration of the farmer), faults with cables buried in built-up areas require roads to be closed and dug up.

Bare cables have another advantage. Insulation on cables upsets the power factor through capacitance, limiting the practical length of underground cables. Power factor correction stations are required when an underground power line gets too long, just as how DC (direct current) lines require expensive converter stations. Overhead AC (alternating current) power has its practical and economic advantages, just as it has its disadvantages.

Overhead power lines are most notably vulnerable to adverse weather conditions. Wood pole lines are more vulnerable; steel towers generally hold up better, but YouTube video “Classic Video of Galloping Conductors” depicts the tower damage to an L6 tower line due to adverse weather in January 1986. Storms do occasionally cause towers to collapse. Considerable engineering effort is required to deal with the effects of wind, snow and ice on overhead lines. Ice and snow build-up increases sag, and melting snow in particular can cause the conductor to leap upwards and flash over or strike other conductors.

Overhead line supports are also at risk of collisions from road and farm vehicles. Self-supporting towers (all UK tower types, ignoring Painter Brothers portals) have an advantage over guy-supported structures in having their total footprint more visible, but this doesn’t stop collisions from occurring.

There is no easy answer so long as the premise involves centralised generation of power for long-distance transmission and distribution, rather than local generation. Underground cables are more expensive to construct and more difficult to maintain. The power companies are never going to voluntarily cover these extra costs, and the public is never going to want to cover them either.

Conductors

Overhead power lines use stranded conductors. Originally these strands were made from copper. This was later replaced with steel-cored aluminium: steel inner strands for tensile strength and aluminium outer strands for a good compromise over lightness (much lighter than copper), conductivity (less than copper) and cost (cheaper than copper). Illustrated below are four of the UK’s common conductor types:

The gaps between the strands are typically filled with protective grease up to a specific layer (steel core, penultimate layer, entire conductor including entire surface, and entire conductor except the outer surface of the outer layer), not shown here.

There are two other historic types of note: cadmium copper and steel-cored copper. W J Nicholls noted of 132 kV lines (1945):

The conductor originally chosen for these lines (0·175 in² copper equivalent s.c.a., consisting of 30 aluminium and 7 steel strands) has proved generally satisfactory, and is still used, although when aluminium was required for more urgent purposes two alternatives of the same copper-equivalent sectional area were used. These were 19/·118 cadmium-copper and steel-cored copper, consisting of 30/·087 copper strands on a steel core of 7/·075 strands wrapped with tape and bitumen. Both these conductors are carried by towers designed for s.c.a. conductor. As experience with both these conductors has extended for only 2 or 3 years, judgment on them in comparison with s.c.a. must be postponed.

Based on what he wrote, these appear to have been wartime measures.

Materials

Details are somewhat scarce. Some common types used in the UK are:

Details of the respective aluminium alloys so far have resisted discovery. AL3, AL4 and AL5 are apparently all aluminium-magnesium-silicon alloys.

The same conductor type can be found as both AL3 and AL5. AL5 is the replacement alloy. For example, Poplar AAAC is both 303-AL3 and 303-AL5, with the latter being the replacement grade.

Compared to AL2 and AL3 (of negligible difference), Western Power Distribution noted in SD8A/3 (February 2020) that “AL5 is of significantly lower resistivity resulting in higher ratings for a given size of conductor.” Some actual figures, per [NSP/004/030] (Northern Powergrid), are:

Conductor types

Power lines have at least three uses for wires: conductors (which supply the power), earthwires (which keep the towers at the same potential and shield the conductors from lightning strikes) and pilot wires that carry information. There are many different types of wire depending on age and function; they are all described as “conductors” regardless of usage, as the earthwire of one line type might use the same conductor type as the phase conductors of another line type.

AAAC

All-Aluminium Alloy Conductor; each type is named after a type of tree

ACCC

Aluminium Conductor Composite Core; a type of HTLS conductor; each type is named after a city

ACCR

Aluminium Conductor Composite Reinforced: a 3M proprietary type of HTLS conductor; each type is named after a type of bird

ACSR

Aluminium Conductor Steel Reinforced; each type is named after a type of mammal

AACSR

Aluminium Alloy Conductor Steel Reinforced; as ACSR but with aluminium alloy instead of aluminium

ACAR

Aluminium Conductor Alloy Reinforced

GAP

Gap-type ACSR

A type of HTLS conductor that has a layer of thermal-resistant grease (“gap”) between the aluminium alloy outer layer and the steel core, allowing the the aluminium layer to slide in relation to the core

GTACSR

Gap-type ACSR, thermal resistant

GZTACSR

Gap-type ACSR, super thermal resistant

HDBC

Hard-Drawn Bare Copper

HTLS

High Temperature Low Sag, not a type in itself but a category of types including ACCC, ACCR and G(Z)TACSR

OPPC

OPGW

Optical Phase Conductor and Optical Ground Wire conductors support optical fibres within a tube that replaces one of the core strands.

SCA

Steel-cored aluminium: an older term for ACSR, used up to the 1970s

Copper and aluminium are used for their good electrical conductivity. Aluminium conductors were initially not strong enough to support their own weight and required a steel core, but aluminium alloys were developed that allowed the entire conductor to be aluminium alloy.

ACSR

ACSR (Aluminium Conductor Steel Reinforced) was the original conductor type used on the national grid and remained standard for decades. Compared to hard-drawn bare copper it is cheaper and lighter but carries less current. ACSR was originally referred to as SCA (Steel-Cored Aluminium).

ACSR is still a current type in some locations. The North-South Interconnector, between Northern Ireland and the Republic of Ireland, is intended to use 600 mm² ACSR as Ireland still uses ACSR.

AAAC

AAAC (All Aluminium Alloy Conductor) replaces the steel core strands with aluminum alloy, improving the conductivity. The considerably lighter weight also means that a higher diameter of conductor can be utilised, increasing the capacity of a line. However, while the higher-diameter conductor weighs no more than the previous ACSR type, the increase in diameter results in higher ice and wind loading, itself putting a greater strain on the towers. Consequently, high temperature low sag (HTLS) conductors may be preferred, operating the line at a much higher temperature (up to 210 °C) to provide an increase in ampacity without an increase in conductor size.

Different grades of AAAC exist. Page 55 of [NSP/004/030] gives the details for AL3 and AL5 grades. The names (Poplar, Upas and Rubas [sic]), dimensions, weights and rated strengths are identical between the two. The only difference is a marginal reduction in the rated DC resistance (in ohms per kilometer): the AL5 grades have around 96% of the rated DC resistance of the AL3 grades. The AL3 figures are given “for information only” as “all replacement conductors shall utilise AL5 alloy”.

AACSR

AACSR (Aluminium Alloy Conductor Steel Reinforced) is as ACSR but with the aluminium strands replaced with aluminium alloy strands. The use of aluminium alloy gives greater strength and reduced sag over ACSR, and greater strength than AAAC. The only AACSR type known to be used in the UK is Keziah (183-AL5/43-ST1A), the AACSR equivalent to Lynx (183-AL1/43-ST1A). Keziah is used for earthwires. Confusingly, Lynx is nominally 175 mm² aluminium area, superceding the imperial designation of 0.175^□″ copper-equivalent SCA. Keziah is nominally 160 mm² aluminium area even though it has the same 183.4 mm² actual aluminium area as Lynx.

HTLS

High-temperature low sag is a general class of unrelated conductor types that can be run at a higher temperature than older conductor types without an increase in sag. Conductors were originally rated for 50 °C operation, but depending on clearances and conductor type, 60 or even 75 °C operation is possible. HTLS conductors however allow operation at temperatures as high as 210 °C or more, presenting a substantial increase in capacity.

There are various approaches to high-temperature low sag including Gap-type, Aluminium Conductor Composite Core (ACCC) and 3M Aluminium Conductor Composite Reinforced (ACCR).

Gap-type

Gap-type HTLS conductors were intended as a direct replacement to ACSR on existing lines, especially L2. However, it is heavy and difficult to string, requiring tower reinforcements. These problems have led to the introduction of 3M’s ACCR types.

Gap-type ACSR can be GTACSR or GZTACSR. The “G” appears to denote “gap-type” structure. The “T” appears to denote thermal-resistant zirconium aluminium alloy, known as “TAl” (tango alpha lima). “ZT” appears to denote “Z-TAl”, super-thermal-resistant zirconium aluminium alloy. The one gap type known to be used in the UK is Matthew, 620 mm² GZTACSR, named after a National Grid linesman who passed away. TAl has a maximum operating temperature of 150 °C while Z-TAl can be run up to 210 °C; these are said to offer a 160% and 200% increase in capacity over ACSR of equivalent size. Approximately (due to the curvature) trapezoidal-shaped wire strands allow the conductor to provide more wire for a given diameter (bearing in mind that diameter affects wind and ice loading).

ACCR

ACCR (Aluminium Conductor Composite Reinforced) is a proprietary HTLS type from 3M, using a composite core formed from high purity aluminium reinforced with alumina fibres. This results in a conductor type that has double the ampacity without extra weight. 3M have certified ACCR up to 240 °C operation.

Designations

Conductor types have various designations depending on the country and standards bodies involved, as well as the point in history when the conductor was specified. Current British practice is to indicate the cross-section area of each metal along with the metal type. The same conductor type can have multiple designations at the same time, with varying levels of precision.

Steel-cored aluminium (SCA) conductors were specified as the cross-section area of the equivalent copper conductor. For tower lines, the most common conductor types were 0.07^□″, 0.175^□″ and 0.4^□″ SCA. A 0.175^□″ SCA conductor has the equivalent conductivity as a 0.175^□″ hard-drawn copper conductor; the actual cross section area (either of aluminium, or of both the aluminium and steel collectively) was greater, as aluminium and steel are both less conductive than copper.

Metric conductor types are defined by the actual aluminium cross-section area. A 300 mm² all-aluminium alloy conductor does indeed have approximately 300 mm² cross-section of aluminium alloy. There are various designations used for metric conductors:

• Aluminium cross-section area, rounded to a convenient size (typically a multiple of 10 or 25). Such designations do not specify the material grades. Multiple conductor types may match the same designation. For example, the designation “100 mm² ACSR” matches Hare (6 aluminium strands and 1 steel strand, all 4.72 mm diameter, with 14.16 mm overall diameter), Dog (6 aluminium and 7 steel strands, 4.72 and 1.57 mm diameters respectively, with 14.15 mm overall diameter) and Hyena (7 aluminium strands and 7 steel strands, of 4.39 and 1.93 mm diameters respectively, with 14.57 mm overall diameter). Each of Hare, Dog and Hyena is nominally 100 mm² of aluminium cross-section but their structures differ.
• A simple codename, such as “Lynx” (ACSR with 30 aluminium strands and 7 steel strands of 2.79 mm diameter) or “Araucaria” (AAAC with 61 aluminium alloy strands, all of 4.14 mm diameter). The type of codename (e.g. mammal, tree or bird) indicates the conductor family. ACSR uses mammal names and AAAC uses tree names. Northern Powergrid document NPS/001/007 places these names under the “Stranding Details” column (listed after the actual stranding details, e.g. “30+7/2.79mm Keziah”), separate from the “Conductor” column that gives only the nominal total aluminium cross-section area.
• A sequence of material areas separated by “/”, such as 183-AL1/43-ST1A for Lynx (183 mm² total of aluminium strands surrounding 43 mm² total of zinc-coated steel strands) or 821-AL5 (821 mm² total of AL5 aluminium alloy strands); the number of strands and their respective diameters is not considered relevant.

The codenames (such as Zebra, Poplar etc) are country-specific. Confusingly, the same codename can be used in different countries with very different meanings. British Curlew and American Curlew for example are neither related nor the same type of conductor.

To avoid confusion is not uncommon for more than one designation to be used at once.

Conductor sizes

Some conductor types used in the UK are listed below. The cross section areas given are nominal only according to British Standards and the weights can vary slightly by manufacturer. Note that the usage below only covers tower lines. “Twin”, “triple” and “quad” refer to bundled conductors.

See NPS/001/007 – Technical Specification for Overhead Line Conductors from Northern Powergrid for additional details. More detailed and more complete lists of the various conductor types can be found under Common UK conductor specifications (external material) and the ACSR (British Standard) and AAAC (British Standard) pages from Witthinrich GmbH. The named conductor assignments are taken from [SWS Forum 08/06/2017] and [Uprating of Overhead Lines] Appendix D Table D1 (PDF page 205), both external material. [NSP/004/030] gives some of the conductor and earthwire types used with the various tower series albeit by cross section only, and this is ambiguous. The definition of Collybia is known only from [EuropeAid 114479].

Imperial/metric equivalence

There is a very curious equivalence between imperial and metric ACSR conductor figures. 1 square inch is equivalent to 645.16 square millimetres, yet the astute will notice that simple numeric conversion factor is not 645.16 but 1000! For example, 0.4 square inch ASCR is equivalent to 400 mm² ASCR (Zebra) and 0.175 square inch ACSR is equivalent to 175 mm² ACSR (Lynx): the metric figure is (numerically) simply 1000 times the imperial figure. How does this make sense? Apparently this is simply a matter of pure coincidence.

The imperial figures refer to the copper equivalent area: that is, the cross-sectional area of copper needed to achieve the same conductive capacity as the aluminium being used. The metric figures however refer to the actual aluminium area. By chance, the latter method results in a number approximately 1000 times greater.

Ian McAulay explained the conversion from imperial to metric figures. Firstly, convert the copper-equivalent area (used by imperial types) to actual aluminium area by multiplying the imperial figure by the ratio of the resistivities of the elements. The resistivity of aluminium at 20 °C is 26.5 nΩ⋅m; for copper the figure is 16.78 nΩ⋅m. For 0.4 in² copper-equivalent area, the actual aluminium area would be 0.4 in² × (26.5 nΩ⋅m ÷ 16.78 nΩ⋅m) = 0.632 in². This figure can now be directly converted from square inches to square millimetres, giving 407.6 mm². This gives us a nominal aluminium area of 400 mm². For Lynx, 0.175 in² × (26.5 ÷ 16.78) = 0.276 in², or 178.3 mm², for a nominal aluminium area of 175 mm². The steel core strands in the ACSR conductor are not counted; their conductivity will be something like 10–20% at most that of the aluminium (depending on the exact type of steel used), and because of the skin effect they will be carrying proportionally less current than their relative cross-section area.

The actual figures are a little different. Lynx is 183-AL1/43-STA1, 183 mm² aluminium area, versus 178.3 mm² equivalent and 175 mm² nominal. Strangely, Keziah—the AACSR version of Lynx—is nominally 160 mm² for the same aluminium area. Zebra is 429-AL1/56-STA1, 429 mm² aluminium, versus 407.6 mm² equivalent and 400 mm² nominal.

Factors and conditions

Sag

Despite the colloquially negative tone of the word, “sag” in overhead line terminology is a neutral term referring to the catenary curve formed by a suspended conductor or earthwire:

Sag is unavoidable. Lower sag requires higher conductor tension. Higher conductor tension risks the conductor breaking, and increases the harm caused by wind-induced vibration. The original 132 kV steel tower lines were strung too taut, resulting in vibration damage. Vibration dampers were fitted to combat this, and subsequent lines were strung to significantly lower tension (2560 lbf at 40 °F in still air instead of 3450 lbf). The increase in sag was addressed by increasing the height of the towers.

The amount of sag is proportional to the conductor’s operating temperature. The operating temperature is determined by both the current being carried as well as to the season and weather conditions. Metal expands with heat; the hotter the conductor, the more it sags. The sag must never breach the minimum ground clearance.

Various techniques exist in other industries and sectors to accommodate expansion due to heat: gridiron and invar pendulums for clocks, expansion joints and pre-stressed track for railway lines, and automatic tensioning equipment in railway overhead lines. As the sun warms the overhead wires on an electrified railway, heavy weights or powerful springs take up the slack to keep the wires taut; if this is not done, there is a danger of dewirement (overhead line damage), something that still occurs when temperatures exceed the capacity of these measures. Railway track originally used short lengths of rail connected by expansion joints (each section of rail connected across a short gap to the next one with a fishplates, responsible for the “clickety-clack” sound) but modern railways use welded track that is pre-stressed to rail neutral temperature for installation. Continuous welded rail also has its limits: if the weather is too hot or too cold, the steel rails will attempt to expand or contract beyond the design limitations and buckle or break apart.

Overhead power line conductors are not fitted with any such compensation mechanisms: the sag is simply allowed to vary by operating temperature. As such, the engineers designing a power line must factor sag into the calculations. Overhead power lines must remain at a safe height above the ground even in extreme heat. The sag limit sets the maximum current on a line, and this limit varies throughout the year according to the seasonal temperatures. In summer heat, the conductors already sag more, and thus the increase in sag from Joule heating must be reduced, constraining the current that the conductor can carry. On tower lines with an earthwire, the earthwire must not sag more than the top phase conductors, otherwise the phase conductors could flash over onto the earthwire.

Ice and wind loading

Ice loading refers to the build-up of ice around a conductor in cold conditions. This increases the weight of the conductor and thus the sag and the (mechanical) tension. Wind loading refers to the effects of wind on the conductor, which also increases the tension on the conductor.

Limits and uprating

Transmission voltages are limited by various factors including corona discharge losses and electromagnetic noise, such that the UK has so far not built transmission lines beyond 400 kV. For the size of the country and the distances involved between power generation sources and major load centres, this has seemed sufficient.

As global temperature rises and demands on the power infrastructure by an increasingly electrified society grow, the thermal limit of power lines can be reached. Solutions to this include additional lines, line replacement with higher-capacity towers (larger size for higher voltage and stronger to carry more and larger conductors), voltage increase through adapting existing towers for greater clearances and higher insulation and more tolerant conductors that better withstand heat (e.g. the introduction of invar alloy) or that have better conductivity for their weight. Improvements in conductor technology allows for lines to be uprated by exchanging older conductors with new ones.

Thermal limits and uprating are discussed extensively in [Uprating of Overhead Lines]. In particular, the following passage summarises uprating through reconductoring:

Re-conductoring is the most common and effective method of current uprating that requires minimal modifications of existing structure. Although this method is comparatively expensive than any other current uprating method, it is cheaper than building a new line [2.24, 2.26]. Replacement by a conductor with a slightly high cross sectional area (same conductor weight) or by High Temperature Low Sag (HTLS) conductors can provide significant current uprating without any structural modification. For example, the replacement of Aluminium Conductor Steel Reinforced (ACSR) by All Aluminium Alloy Conductor (AAAC) of same cross-sectional area can improve the thermal rating up to 40% [2.35]. In the UK, AAAC is extensively used to replace ACSR which allows an increase in maximum operating temperature from 50 °C to 75 °C, with a corresponding 25% increase in thermal rating [2.32, 2.36].

Multiple approaches have been taken for tower lines in the UK, each involving replacing the basic steel-cored aluminium (SCA or ACSR) conductors with more modern and sometimes more exotic types. AAAC (all-aluminium alloy) conductors replace the steel core with aluminium alloy, increasing conductivity. The lower weight of the alumimium alloy core means that a further increase in capacity can be obtained by increasing the diameter of the conductor such that the AAAC conductor weighs the same as the old steel-core conductor, but this increases the wind and ice loading (higher wind resistance and more surface area to carry ice). (There is a curious and unexplained contradiction regarding how power lines operating up to 75 °C or can ice over …)

Another approach is the use of high temperature low sag (HTLS) conductors. These are a broad set of different conductor designs that, through various means, allow high operating temperatures without the expected increase in sag.