Technical information on British overhead power lines

Contents

• Overview
• Voltages
• Supports
• Circuits and phases
• Conductors
  • Conductor types
    • ACSR
    • AAAC
    • HTLS
    • Gap-type
    • ACCR
  • Conductor sizes
  • Imperial/metric equivalence
• Limits and uprating

Overview

This page covers some of the technical details on British overhead power lines. The information is neither exhaustive nor authoritative and is provided for public information only. More controversial subjects such as the effects on property pricing, claims of cancer risks, effects of electromagnetic fields and the cost of overhead versus underground lines are intentionally excluded.

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

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.

Longer distance power delivery is more efficient in the form of high voltage low current. High current causes energy losses from electrical resistance (energy wasted as heat, resulting in a voltage loss), so power is transmitted and distributed at high voltages. The output from power stations is stepped up to the high transmission voltages, transmitted to major substations, then stepped down to progressively lower voltages for distribution to towns and villages. Transforming current between high and low voltages requires alternating current (AC), which in the UK cycles at a frequency of 50 Hz. (For reference, see Ohm’s law and consider that electric power = voltage × current.)

Higher voltages require larger insulators and larger clearances. With traditional ceramic and glass insulators, at lower voltages, the number of insulators in the string is a good guide to the voltage: a single insulator (or two) implies 11 kV and a string of three insulators implies 33 kV. 66 kV is more likely to be five insulators (not six) and for 132 kV you are more likely to see nine or ten insulators in a suspension string, depending on the requirements of the environmental conditions. (132 kV uses typically 10–11 insulators in a suspension insulator string and around 8–9 insulators 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 insulators 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.

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, 400 V and 230 V lines are almost always wood pole lines.

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”.

Conductors

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.

Conductor types

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

All-Aluminium Alloy Conductor Steel Reinforced

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”.

HTLS

High-temperature low sag conductors can be run at a much 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.

Conductor sizes

Unexpected tree and animal types appear in various places. These terms refer to the different conductor sizes.

Imperial conductor types are defined by the cross-section area of the equivalent copper conductor. Metric conductor types are defined by the actual aluminium cross-section area, rounded to a memorable figure. A 0.175^□″ (0.175 square inch) steel-core aluminium conductor is so named as it has the same current-carrying capacity as a 0.175^□″ copper conductor; its actual cross-section area will be quite different. By comparison, a 300 mm² all-aluminium alloy conductor does indeed have approximately 300 mm² cross-section of aluminium alloy.

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.

Limits and uprating

Transmission and distribution line voltages are limited by the line dimensions: insulator string length, crossarm spacing and tower height. The limit on the current is a little different. Metal expands as it heats up, and this applies to pylon wires just as it does railway track or any other metal object. Various techniques exist 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 can occur in extreme weather conditions. Railway track originally used expansion joints attached with 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 can buckle or break apart.

Overhead power line conductors are not fitted with any such compensation mechanisms: they simply sag lower as they warm up, and pull tighter as they cool down. Sag is a key factor in the design of overhead lines: the lines must remain at a safe height above the ground even in extreme heat. The sag limit also sets the maximum current on a line: the higher the current, the more the wires heat from resistance, and therefore the more they sag. Further, the earthwire must not sag more than the top phase conductors, otherwise the phase conductors could flash over onto the earthwire.

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] and not covered in detail here. 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].