Posted: July 29th, 2012 by Peter Botsoe
Comments (3)

Turning corners with tension

At National Grid we’re not focussing solely on developing Bystrup’s iconic suspension T-pylon design for possible future use on transmission routes. We’re working on developing a family of T-pylon options. These will include tension models, required to turn corners and reduce the impact of cascade line failure; and terminal pylons to terminate routes at substations.

In a perfect world electricity transmission lines would run as straight as possible, but natural barriers, such as hills, rivers and roads, have to be circumvented or crossed and land rights issues can often require a route to turn a corner.

This places a lot of lateral strain on a pylon, to the side where the line turns, and so the suspension design needs to be supplemented so pylons can resist being pulled to one side. Similarly, a pylon at the end of a route has to be stronger than those which form the rest of the route because of the extra force it must cope with – rather like an anchorman in a tug of war team.

This extra strength from tension pylons can also be deployed throughout a transmission route, typically at every tenth pylon, to prevent a line or structural failure spreading along the route. It is similar to how a domino toppling champion will place a physical barrier every few hundred dominos to prevent an accident causing the rest of the carefully arranged dominos to topple over.

Just as with today’s lattice pylon family, the extra strength required will mean that the wires will not be able to be suspended vertically from insulators, but will instead need to be held in place more securely by horizontal insulators tied to the pylon itself – hence the term, tension pylon.

Due to the lines being tied to the structure itself by insulators, we have to provide a path for the electricity to continue to flow. So, we use ‘jumper loops’, which are short sections of electrical wire connected to the main (live and earth) wires just before they tie to the insulators, terminating the line to the cross arm. The jumper loops are designed to ensure the live wire does not touch the earthed structure. If there is a risk of this occurring, we apply measures such as using rigid insulators installed vertically (’post insulators’) or adding weights to the jumpers (’jumper weights’) to reduce movement with the wind.

So far we’re evaluating four different tension pylon designs and you can compare them against today’s lattice tower which, at 50m tall, stands 16m above the height of these potential T-pylon alternatives.

For all the pylons described below, the earth wire is crucial and is held above the three lines carrying the current because it protects them from a lightning strike. If the earth wire is hit, a system known as Delay Auto Reclose (DAR) will cause the line to falter for a few seconds and then normal service is resumed provided the surge has gone. A strike on a live wire is far more likely to cause a failure in supply and so, in a way, the earth wire is our sacrificial lamb held above the three lines carrying the current.

The Flying T

Flying T

Flying T

The Flying T bears the closest resemblance to the T-pylon design, adding a second cross arm to offer extra support to the metallic ‘diamond earings’ carrying all four wires. Unlike the standard T-pylon, however, these are not insulator units but are made of solid metal to provide the integral strength for the line to turn a corner and to manage situations in which two conductors could fail.

Jumper loops could conceivably touch or get close to the metallic diamond earring and so ‘pilot’ insulator rods would need to be used to prevent this happening.

An option being actively considered for the Flying T would be to remove the lower cross arm and raise the diamond earings so their centre is level with the top cross arm.

Double T

Double T

Double T

The Double T doesn’t require pilot insulators because the design principle is more similar to today’s lattice family. With no diamond shape raising the risk of contact with the metallic body, jumper loops simply fit underneath the cross arms.

In this design the top cross arm would carry two phases (lines) while the bottom would carry a single phase. The earth wire would be held in place above the top cross arm through raised metallic supports.  We are running computer simulations as part of investigation into how we can reduce the length of the raised metallic supports without impacting on the reliability of the line.  This is to address some of the feedback received to date suggesting that this would improve the aesthetics of this pylon and also reduce the weight, height cost etc.

Triple T

Triple T

Triple T

The main difference from the Double T and Flying T options is the way in which the earth wire is supported. On the Triple T the raised arms seen in the Double T are replaced by a solid cross arm.

Double Diamond

Double Diamond

Double Diamond

The double diamond is being considered mainly as our aforementioned anchorman in a tug of war team at the end of a transmission line. As well as being a strong pylon, we can intersperse it throughout a route to ensure failures spread no further.

Essentially it would be our zero deviation option which, as the term suggests, would not be used for turning corners but to provide extra strength where it is required. As the wires still tie to the diamond earrings and jumpers are applied to maintain the electrical path, it is still considered to be a tension pylon.

Like the Flying T design, it would need pilot insulators to prevent jumper loops coming into contact with its earthed metallic frame. This shape is being reviewed to support larger angles of deviation.

Seeking views

This requirement for pilot insulators has led many of our colleagues at National Grid and delivery partners to state a preference for either of the more conventional-looking Double T and Triple T options over the Flying T which, arguably, bears the closest resemblance to the winning T-pylon design.

We’re keen to get the public’s view and so will be showing the scale models at our Annual General Meeting as well as conducting research with the general public to get a view on which they see as the most aesthetically pleasing. It’s vital we get public feedback so we combine our engineering knowledge with opinions from stakeholders as we progress the project to develop the T-pylon family. We have a questionnaire running on Survey Monkey which you can complete to give us feedback: Please note that we have not included the Double Diamond in the tension design survey. The survey will close on Aug 31.


  1. Reza Amirabgir /

    Hi dear,

    I think use of hollow section is brilliant idea but may be in different way and shape of pylons.

    Presumably Flying T or Double T types will be used for future heaviest conductor system i.e. a triple 700mm2 AAAC Araucaria, in accordance with TS3.04.20 and TS 3.04.02 and standard span of 366m with maximum sum of adjacent spans of 805. Expected total unbalance force due to brokenwire condition (2 No. off bundle) on crossarm strut tubular hollow section is circa 220kN in total.
    Applied bending moment on strut member will be around 1750 kN.m (for 8 m long arm).
    Required plastic modulus for this bending moment is about 5500 cm3 if steel 355 N/mm2 yield strength is used, hence the outside diameter hollow section required is about 800 mm with 16 mm thick.
    This is only for broken wire load, in additional to this self-weight and conductor dead load and secondary moment will need to be added. Probably 1m in diameter hollow section, only for crossarm which means the pylon probably need to be around 2.5m in diameter.
    Question is would this be an economical and environmentally friendly option?

    Reza Amirabgir

    • Peter Botsoe /

      Hi Reza, thanks for your post and apologies for my delayed response.  

      The T-pylon is being designed for triple Araucaria i.e. the largest and heaviest conductor currently available to National Grid.  This conductor system provides a rating greater than 5,000 A i.e. higher than our current substations. Our view of future conductor systems is that they are going to be based on composite core conductors which are lighter but capable of higher ratings with fewer wires.

      W.r.t. your calculation assumptions, they produce results similar to those we have; please refer to the article “New Pictures of the T-pylon” which includes structure dimensions.

      On the question you raise we continue to investigate this internally and externally to National Grid to minimise the costs of the T-pylon and environmental impact.  

      Tackling economics, firstly, the design work is ongoing and we have a solution on the drawing board that matches the capability of the L13 lattice but we are still seeking to optimise the design to make the structure even smaller although still taking account of phenomena such as EMF, conductor galloping etc…Secondly, until the design is fully reviewed with fabricators and a prototype developed, fabrication costs are estimated…Thirdly, the economics of the T-pylon are based on the Whole Life Value i.e. construction footprint, resourcing, erecting time, maintenance strategy and frequency etc…T-pylon takes ~25% of the time to construct a lattice and with possibly an equivalent amount (if not less) with maintenance as it is undertaken from “cherry pickers”.

      From an environmental perspective, we expect the smaller size of the T-pylon to result in a smaller construction footprint and a smaller permanent footprint (~3m in diameter depending on tension pylon angle) compared to the L13 which has a ~9.3m square base i.e. more land is returned to the grantor for farming etc.  We continue to explore maintenance access requirements e.g. eliminating the need for a permanent footprint and also seeking to avoid the need for painting by utilising alternative surface finishes.  We are working hard to ensure that our design is sustainable and continue to work closely with environmentalists to fully understand the impact of our proposed development.


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