Technical information on British tower lines
Contents
- Overview
- Identities
- Circuits and phases
- Conductors
- Earthwires
- Junctions
- Crossings
- Circuit termination
- Transposition
Overview
This page covers a few of the basic technical details on British tower lines (“pylon lines”), specifically 33 to 400 kV power lines carried on lattice towers (commonly referred to as “electricity pylons”).
For details common to all forms of overhead power line (poles and towers), see the general overhead lines page.
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. Caveat lector!
Identities
Tower lines in the UK typically have a short alphabetic or alphanumeric identity, for example “V” or “PUA”. These identities are region-specific: the same identity can exist in different parts of the UK. This can be seen most notably by UK Power Networks lines north and south of the Thames, where many route identities are re-used between the two areas. Route identities beginning with “4” denote quad-conductor lines, e.g. 4YW, introduced in the 1960s on L6 towers. [Overhead Line Handbook] In some instances, the two circuits on a tower line may have separate identities; for example, the two 132 kV circuits to Lydney sharing the same towers are routes PBW and PRZ; these routes diverge at Arlington and presumably PBW and PRZ represent the two single-circuit lines that head in different directions from that point onwards.
Circuits and phases
On lattice towers lines, the phases can each be a single wire or they can be a bundle of two, three or four wires separated by spacers.
Higher voltage lines (typically 132 kV and above) normally have an extra wire at the top, the earthwire, which offers protection against lightning strikes.
Typical double-circuit towers have six crossarms, three per side, with one phase per crossarm, although other arrangements exist. Typical single-circuit towers have three crossarms, two on one side and one on the other.
Phases
The order of phases is not fixed. Red phase for example can be at the top, middle or bottom position on a tower. Colour tags are used to indicate the order of phases on a power line, although these are not normally fitted.
The order of phases may differ between each side of a tower. Where the order (e.g. red–blue–yellow) is the same on each side, this is known as untransposed phasing. 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.
Conductors
Details on conductors are given on the general overhead lines page. The following details are specific to tower lines.
Conductor bundles
132 kV circuits normally have a single conductor (wire) per phase. For increased load carrying capacity, each phase can be formed of a bundle of two, three or more conductors. This is uncommon for 132 kV, but standard practice for 275 and 400 kV. Most 132 kV tower types only support single conductors, but L7 is rated for twin conductors at what appears to be the standard diameter of 0.175□″. L7 can also take twin heavy duty 0.4□″ supergrid conductors, although under these conditions the maximum distance between towers is reduced from 1000 feet to 785 feet due to the increased conductor weight.
Supergrid lines are generally twin, triple or quad conductor: bundles of two, three and four conductors per phase. The ubiquitous L2 series accommodates twin 0.4□″ conductors at 275 or 400 kV, while the smaller L3 only accommodates twin 0.175□″ and has clearances only for 275 kV. (Note that modern conductors 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□″ conductors per phase, although many can be seen strung as triple. Typically, any line using triple or quad-conductor bundles will be on L6 towers, as later types prefer twin bundles.
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.
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 is 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 towers which do not support an earthwire. The two older light construction types in Scotland also have no earthwire (each type was later adapted into a new type that did support an 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.
SSEN are proposing to re-introduce transposition on the new Beauly–Peterhead 400 kV line with their new AS4 tower series. As noted in Beauly–Peterhead Section 37 Application Volume 2 (Main Report) Chapter 3 (Project Description) section 3.7.12 (page 3-11):
Due to the length of the OHL for the Proposed Development, voltage unbalance can occur between the different phases (conductor arrangements on either side or the tower). To reduce voltage unbalance transposition of the phases is required i.e. the conductors are moved to connect to different arms of the towers; this is illustrated in Plate 3.2. For the Proposed Development this will take place at two locations; Towers CB6-4A to CB6-4B and Towers CB14-24A to CB14-24B. To facilitate this, two terminal towers with extended top crossarms, as can be seen in Plate 3.3, will be positioned approximately 100 m apart; the location of transposition towers is presented on Figure 3.1: Site Layout.
For details on the technicalities of power transposition, please refer to another reference source such as the transposition and transposition tower pages on Wikipedia.