Technical information on British high-voltage power lines

Contents

• Overview
• Circuit count
• Phasing
• Voltage and conductors
  • Conductor bundles
  • Conductor types
    • Imperial/metric equivalence
  • Limits and uprating
  • Earthwires
• Junctions
• Crossings
  • Power lines
  • Transport and telecoms
• Circuit termination
  • Sealing end compound
  • Sealing end platform
• Transposition

Overview

This page covers a few of the basic technical details on British “pylon wires”, specifically 132 to 400 kV power lines carried on lattice towers (commonly referred to as “electricity pylons”).

This page is not exhaustive and is not in any way authoritative. The information on this page is provided solely to offer a little bit of background knowledge to anyone curious about pylons. The information presented here is collated from a variety of sources and is not guaranteed to all be correct or accurate. Quite possibly the entire page is nonsense. Caveat lector!

Circuit count

Most power is transmitted and distributed in three phases. Three-phase power transmission does not require a return conductor: the current from each wire is returned via the other two. This is why power lines are found with multiples of three cables. Each three-phase circuit requires three cables. The 11 kV feeds to farms and villages on wood poles are generally single circuit, with three cables to a pole, on pin insulators sat on a metal crossbeam. The higher voltage 33 kV feeds, also on wood poles, are more commonly double-circuit with either two parallel sets of three-wire poles, or double poles with six wires.

Steel lattice transmission towers can be either single circuit or double circuit, although double circuit is by far the most common. Each of the three phases per circuit can be a single cable or a bundle of two, three or four cables separated by spacers. Although lower voltage towers may continue the practice of conductor cables only, higher voltage lines (in particular 132 kV and above) typically have a fourth or seventh cable at the top for earth protection. (In some locations there are two such cables at the top to make eight in total.)

Typical double-circuit towers have six arms, three per side (forming three crossarms), with one phase per arm, although other arrangements exist. Typical single-circuit towers have three arms, two on one side and one on the other, but there are also single-circuit lines using double-circuit towers with one side of each tower unused.

Phasing

The diagram above suggests that phase 1 of each circuit is on the top crossarm, phase 2 of each circuit is on the middle crossarm, and phase 3 on bottom crossarm. This can be true, and is known as untransposed phasing, but is often not the way that the circuits are set up. For most supergrid lines, one circuit has the phases arranged top-to-bottom and the other circuit arranged bottom-to-top. This transposed phasing arrangement helps cancel out the electromagnetic fields emanating from the line. According to National Grid’s Phasing page, “only 50% of double-circuit overhead lines currently have optimum phasing” as transposed phasing was not part of the original design for the National Grid; the figure rises to 90% for 275 and 400 kV as the supergrid was designed with transposed phasing in mind.

Voltage and conductors

“Pylon wires” in the UK operate at a number of voltages. Supergrid lines operate at 275 and 400 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 National Grid voltage from the 1930s until the creation of the Supergrid at 275 and later 400 kV. Some 132 kV lines run on wood or metal poles instead (for example the “Trident” wood pole type), while some 33 kV and 66 kV distribution lines run on lattice towers. It would appear that—in England at least—132 kV lines are demoted out of the National Grid; in [Development Near Lines] Appendix II, National Grid note: “Local distribution companies generally own and operate lines with a voltage of 132kV and below.”

Higher voltages require larger insulators and larger clearances. With traditional 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. For 132 kV, you are more likely to see nine or ten insulators in a suspension string. For 132 kV, you’ll see around 10–11 insulators in a suspension insulator string (on a suspension tower), and around 8–9 insulators in a tension insulator string (on an angle or terminal tower). For 400 kV (a little over 3 × 132 kV), you would expect three times as many, 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 gives 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 (cables) 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.

Conductor bundles

132 kV circuits normally have a single cable per phase. For increased load carrying capacity, each phase can be formed of a bundle of two, three or more cables. This is uncommon for 132 kV, but standard practice for 275 and 400 kV. Most 132 kV tower types only support single cables, but L7 is rated for twin cables at what appears to be the standard diameter of 0.175^□″. L7 can also take twin heavy duty 0.4^□″ supergrid cables, although under these conditions the maximum distance between towers is reduced from 1000 feet to 785 feet due to the increased cable weight.

Supergrid lines are generally twin, triple or quad conductor: bundles of two, three and four cables per phase. The ubiquitous L2 series accommodates twin 0.4^□″ cables at 275 or 400 kV, while the smaller L3 only accommodates twin 0.175^□″ and has clearances only for 275 kV. (Note that modern cables are metric, but the Imperial system of measurements was in use when these tower types were introduced.) L2 was followed by L6; 20% taller in height and 47% wider at the base, L6 has the required strength to take quad 0.4^□″ cables per phase, although many can be seen strung as triple. Later tower series appear to have gone in for thicker cable, using all aluminium construction to keep the weight down, and thus only twin bundles, although this needs to be verified from source material. The new T-pylons however are quad bundle again. The implication thus is that any line using conventional lattice towers and triple or quad conductor bundles will be L6. The converse is not true: twin conductor does not rule out L6, as any tower type can be downrated to a lower voltage and lower conductor count, as is seen in various places around the country. For twin conductor, the tower type must be identified visually.

Generally, the conductors in a bundle are held apart by spacers positioned at regular intervals along each span:

A close-up photo taken at Blaw Knox L6 D60 tower 4KG116 on Flickr depicts a triple-conductor spacer; note the distinctly triskelion design.

The usage of spacers in bundles is typically but not universally the case. The 132 kV L7 Harker-Penrith line is strung twin conductor without spacers from Harker Substation to somewhere around Newby West outside Carlisle (by Newby Cross the line has changed to single conductor). The outer conductor for each phase is suspended lower than the inner conductor from the suspension towers, while at the tension towers they are level in height and held apart by the usual spacer next to the insulators. This can been on L7 D suspension tower DS 21 at 54.905° W 2.992° W near Grinsdale and L7 D30 tension tower DS 10 at 54.934° N 2.991° W between Rockcliffe and Cargo. Flickr photo 132kV Pylon, Double-Cabled depicts one of the twin conductor L7 D towers on this line.

The number of sub-conductors is not always constant along a power line. A Flickr photo of Blaw Knox L6 D60 tower 4KG116 depicts the changeover point from twin-conductor to triple-conductor bundles, or vice versa depending on perspective.

Conductor types

Unexpected tree and animal types appear in various places. These terms refer to the cable types used for the conductors and earthwires. All such cable types are described as “conductors” even though they are also used for earthwires and the earthwire is not regarded as a conductor. The different conductor types used in the UK include:

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

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

HTLS

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

SCA

Steel-cored aluminium: an older term for ACSR

Copper and aluminium are used for their good electrical conductivity. Steel was traditionally used for the inner strands for its mechanical strength, but for lightness aluminium alloy typically substitutes for steel now.

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”. “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).

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”.

Some applicable conductor types are listed below. The diameters given are nominal only according to British Standards and the weights can vary slightly by manufacturer.

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 the conversion factor for conductors is not 645.16 but 1000! 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 simply 1000 times the imperial figure. How does this make sense? Apparently this is simply a matter of pure coincidence.

The imperial figures are said to 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.

To convert from imperial to metric, first multiply 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 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 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 appear not to be 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 will be carrying proportionally less of the current.

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 is pre-stressed to rail neutral temperature for installation, but this too 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.

Overhead power cables 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].

Earthwires

An earthwire is a wire typically positioned above the phase conductors to provide lightning protection. Earthwires are seldom used on wood poles (132 kV and below) but are used on most steel tower lines including those below 132 kV. The carrying capacity of the earthwire is normally a fraction of that of the phase conductors. With single-conductor 175 mm² ACSR (Lynx) towers (e.g. PL16, L4(m)) the earthwire used is Horse (70 mm² ACSR). L2 towers on twin Zebra (400 mm²) used single Lynx for the earthwire, while L6 towers with quad Zebra bundles used single Zebra for the earthwire. L2 lines re-conductored with AAAC use Keziah (160 mm² AACSR) for the earthwire instead.

Per [Nicholls] (1945):

A single earth wire only was used on the original lines, but since 1938 double earth wires have been erected for 1 mile at each end of each line to provide better protection for the substation apparatus against direct lightning strokes. At first the double earth wires were placed inside the vertical plane through the top conductors and above them, but in 1938 the first line was built with these wires outside the top conductors (see Fig. 5).

The phrases “since 1938” and “in 1938” are verbatim, even though you would expect the years to differ. The oldest known 132 kV tower type to offer double earthwire CE PL3, the drawings for which date to 1936–37. Figure 5 in [Nicholls] depicts (without naming either type) SEE PL7 DD2 and CE PL3/PL4 DD2 as examples of the two double earthwire arrangements, in reverse chronological order. The most recent type to offer double earthwire as standard appears to be PL16, which was superseded by L4(m) in the 1970s. Double earthwire was also used with some SL types and at Lynemouth on adapted L7 towers.

There are however 132 kV steel tower lines with no earthwire. Route XCW in Scotland uses Blaw Knox K1420 flat formation angle towers and an unidentified model of line tower, with no earthwire. There are also lines in Scotland with Eve 0.125 towers with no earthwire.

The amount of protection afforded by the earthwire is called “earthwire shade”. 1920s and early 1930s designs provided very little earthwire shade on the angle towers; the chief difference between the CE PL3 and CE PL4 types is that the angle towers designed for CE PL4 were upgraded to approximately the same earthwire shade as the line towers.

45° is a typical shade angle, as used by both L7 and L4(m), illustrated on L7(c) D below.

Junctions

Details on junctions are scarce. Broadly speaking, junctions take three different physical forms. The simplest is where a double-circuit line splits into two single-circuit lines or, conversely, two single-circuit lines merge into a double-circuit line (earthwires and middle phases only shown for simplicity):

The PL16 D60 Junction and D90 Junction towers were designed for this purpose. L2 and L3 DJ also work in this manner. Note that the two circuits are not connected at the junction: the double circuit portion requires only a single set of supports (towers) and a single wayleave, instead of running two single-circuit lines side-by-side.

A double circuit line can also split into two double circuit lines:

Note that there are still no interconnections between circuits at the junction: one circuit effectively bypasses the junction entirely. The traditional design of these junctions seems to have been a triangle of three towers, but dedicated junction towers (L2 and L3 DJX, and DJT in later tower suites) took over this role.

There are also double-circuit junctions where the teed-off line is directly connected to the original circuits. This requires the circuit on one side of the tower to duck under the other line, achieved for example with a flat-formation gantry:

The diagram below depicts the triangular junction at Maidstone, possibly the last surviving junction of its kind in the UK:

Each of the junction towers appears to be a D60. Dedicated double circuit junction towers did not exist until L2 and L3 in the 1950s and first appeared at 132 kV with L7 in the 1960s.

The functionality of junctions, and the names of each type of junction, remain a mystery.

Crossings

Power lines

Power lines sometimes need to cross each other. There is more than one way to achieve this. One method is to simply keep one set of wires higher up than the other, which can be seen in example below from Redbourn in Hertfordshire. The two L2 towers are placed very close to each other to minimise sag and maintain clearance. The 132 kV line running between the PL16 towers—which are only around 60% of the height of a standard height L2 tower—simply passes underneath the 400 kV line. The L2 towers are extended a little in height and the sag of the 132 kV lines keeps the two lines separated.

A more conventional approach is a diamond crossing. Here, one of the lines is split off onto two low-height gantries placed either side of a tower from the other line. The gantries carry the conductors around the outside of the other line’s tower, where they join together at the next tower. This can be seen in Flickr photograph L3 pylons | Near Atworth, Wiltshire: here, the dual-voltage 132 kV/33 kV Bath-Norrington AG line on L3 towers crosses the 132 kV Y line on PL16 towers at 51.385° N 2.196° W. The 132 kV line also passes over two 33 kV lines in the vicinity.

Transport and telecoms

Historically, crossings between power lines and roads, railways, canals and telephone wires were treated with significant caution: there was concern surrounding the risk of energised wires falling from towers. Duplex insulator sets were used to double the insulator breaking tension. Earthing arms were added to the towers either side of the crossing to catch fallen wires. In some instances, pairs of special GPO crossing towers were adopted, bearing a cradle guard: a wire mesh suspended over the crossing to catch fallen conductors.

Circuit termination

Sealing end compound

A sealing end compound is a fenced-off enclosure where the route changes from overhead lines to underground cables. The overhead lines terminate on a special tower, with downleads connecting the overhead cables to the underground cables. See Sealing-end compounds on EMFS.info for more details. Illustrated below is the St Albans sealing end compound near the entrance to Nightingale Lane off Highfield Park Drive (51.737° N 0.303° W). This spur connects Cell Barnes substation in St Albans to the Elstree–Rye House 132 kV line at Coursers Farm Anaerobic Digestion Plant between Tyttenhanger and Colney Heath, using JL Eve L16 towers. The line reaches St Albans on towers, and changes to underground cables until the substation. The photographs were taken on 1st May 2021.

Sealing end platform

These are the “baskets”, the enclosed raised platforms placed alongside towers where the downleads from the tower connect to underground cables. This approach is possibly intended to reduce the land usage requirement compared to a sealing end compound, but some sealing end platforms are nonetheless enclosed within a compound. Typically there is a separate pillar for each phase, and two separate assemblies, one per circuit (each one carries three phases).

Transposition

Transposition exchanges the vertical positions of the phases on the line. For example, moving the top phase to the middle crossarms, middle phase to the bottom crossarms and bottom phase to the top crossarms. On double circuit lines, each transposition takes place on a transposition tower; these towers are based on the respective D30 type and are designated DX. The diagram below depicts a single transposition, showing one side of a double-circuit line:

Per [Nicholls]:

Originally all 132-kV lines were transposed en route, generally three times so that the phasing was the same at each end following a complete transposition in the run of the line.

This is illustrated in the diagram below:

Single circuit lines are transposed by way of a line tower erected in reverse (facing the opposite direction), with the transposition formed from the two adjacent spans. The CEB L132 specification called for type SX single circuit transposition towers, but no such tower is known, and transposition of any kind (single or double circuit) is not known from any of the tower types built to that specification (for example PL16). Single circuit transposition can be done on a single tower—such as this overseas example—but is is not known within the UK.

Transposition in this manner is no longer used in the UK. Where transposition towers still exist, most are now wired as simple section towers. Transposition still exists (as of 2021) on the “baby PL16” 33 kV line between Willoughby-on-the-Wolds and Hawton (Newark-on-Trent). Tower PTH27 (MEE PL1, Dewsbury) also retains the transposition wiring on one side.

Details on why transposition was abandoned vary. Flash Bristow quoted an explanation from Ian McAulay on her Electricity Pylon Frequently Asked Questions page as follows:

Early transmission practice in the UK did use transposition towers, but they were found to be unnecessary and the transpositions were generally eliminated when reconductoring lines that included them. Where they remain, transposition towers are just used as ordinary tension towers, but the extended top and bottom crossarms are characteristic.

[Nicholls] explains the difficulties with single-circuit transpositions. Because the upper phase conductor switches from one side of the power line to the other below the earthwire, that phase conductor could flash over onto the earthwire upon the release of snow loading. Nicholls stated that, prior to World War II, “with the approval of the Post Office, these single-circuit transpositions were removed …”, which is a curious claim as single-circuit transpositions still exist both in England (for example Bedford) and in Scotland. No explanation as such is given as to why double circuit transpositions were removed; the article only makes the following comment:

On new lines transpositions en route are omitted with the approval of the Post Office, so that the question now seldom arises. Sufficiently good electrostatic balance for the Grid as a whole is provided by transposing complete lines at substations.

For details on the technicalities of power transposition, please refer to another reference source such as the transposition and transposition tower pages on Wikipedia.