Tower design
Contents
History
See the tower design history page.
Height requirement
The height of each tower is determined by the sag of the conductor spans: how close the cables come to the ground at their lowest point in between the towers. INMR article Implementing a Compact 400 kV Line demonstrates how the total height of a German 400 kV tower can be reduced from 58 m to 30 m by reducing the sag and switching to a single tier; the following diagram shows the degree to which the towers can be shortened as a result of the described invention:
Compare how the green cables sag less: this means that they can be suspended from a shorter tower. The CompactLine design also places all the phases side by side instead of the two tiers of the “Danube” lattice towers. The “Danube” towers also have a much taller earthwire peak than British towers: at only two tiers, it is not only taller than British two-tier designs (L9 and L12) but also taller than our three-tier L6 towers which are around 50 m tall.
Metrication
The oldest-known steel lattice towers in the UK appear to be Milliken L134, designed in 1927. For decades, all towers were designed to imperial measurements. When the UK’s steel industry was metricated in the early 1970s, metric towers were required.
Towers comprise steel bars—in both L section and flat—and metal plates. These parts are manufactured by steel mills to industry-standard dimensions; tower fabricators cut these parts to size. Imperial steel stock was dimensioned in fractional inches; metric steel stock is in millimetres and does not correspond to prior imperial sizes. In order to use existing imperial tower designs with metric steel stock, adjustments are needed to the design to accommodate the millimetre-based stock. Jon Bruford described the two approaches to metrication of imperial designs as follows:
- Metricated towers “[use the] closest metric bar up from the imperial standard size”; that is, they use “replacement bars that are the same or bigger than the imperial.”
- Converted towers, “where the bars are sized according to the loads without reference to the imperial bar they are replacing.”
Evidence suggests that these are the wrong way around. Converted tower series use a “(c)” suffix, e.g. L7(c), L3(c). This is the older approach, used for quite a few tower types.
Metricated towers are a lot less common. The first “metricated” type may have been be L4(m), and this was a completely new design of tower suspected to have been designed to a metric version of the L4 specification. L6(c) was replaced by L6(m); the latter appears to have been a metric redesign of BICC L6. L8(c) was apparently replaced by L8(m), and L3(c) by L3(m), although both of these are still pending confirmation.
The suggestion is that metric conversion uses the next metric size up (of bar cross section, plate thickness etc).
Aesthetics
In the Scotsman article “Kinetic sculpture on the grid” (6th October 1994), Bruce Howie, chief engineer with ScottishPower’s Transmissions Division is quoted as saying:
The recent trend has been towards “swept up cross arms”, which are regarded as less obtrusive than the confident protagonists of Buchanan’s water colours.
Here he seems to be referring to the design of L4(m) and L12. Quite how recent “recent” means is unclear, as L4(m) dates back to the 1970s.