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The ubiquitous design of electricity pylons is changing for the first time in almost a century and the new T-shape appeals to both the energy sector and the public.
Energising cables carried by the world’s first T-shaped electricity pylons is only months away. Power will be switched on following their installation on the first 48 of 116 T-pylons which will form part of National Grid’s £900M Hinkley Connection Project.
It will be a pivotal moment for the UK’s utilities sector, as for almost a century it has solely relied on lattice pylons to support overhead electricity lines.
The T-pylon, designed by Danish firm Bystrup, won an international competition organised by the Royal Institute of British Architects, what was then the Department of Energy & Climate Change and National Grid in 2011.
New pylon design
The competition sought a new pylon design which would help meet demand for new electricity infrastructure and reduce visual impact on the surroundings.
After successful testing in the UK and abroad, National Grid commissioned the construction and installation of 116 pylons for its Hinkley Connection Project. The project involves the creation of a 57km long, high-voltage electricity connection between Bridgwater and Seabank near Avonmouth in the South West of England.
This will connect 6M homes and businesses with EDF’s 3.26GW Hinkley Point C nuclear power station – expected to be online in 2026 – and the existing Hinkley Point B.
It will also link to other sources of low carbon and renewable energy from the South West peninsula.
“In order for us to do that, we need to undertake some reconfiguration and installation of assets on our network,” says National Grid senior project manager Steven Haskayne.
As part of the project, 48.5km of 400kV overhead lines and 8.5km of underground cables are being installed and two substations are being constructed. In addition, 65.3km of existing 132kV overhead line and 249 pylons owned by Western Power Distribution will be removed.
Balfour Beatty was awarded the contract to construct all of the new pylons; supply, install and commission the new overhead line; and remove the 132kV overhead line.
Lower visual impact
During the planning process, National Grid consulted the communities affected by the project. Haskayne says that the company received 11,000 pieces of feedback and found a clear demand for electricity infrastructure that causes low visual impact.
National Grid decided to remove the existing pylons in the Mendip Hills Area of Outstanding Natural Beauty and replace them with underground cables. Once the pylon removal finishes, this area will be pylon-free for the first time since the 1960s.
In terms of the above-ground infrastructure, the feedback showed a strong preference for T-pylons instead of the traditional lattice design. This is unsurprising given that the T-pylons’ comparatively smaller size will reduce visual impact and free up more land for landowner use.
The competition sought a new pylon design which would help reduce visual impact on the surroundings
The T-pylons are 35m tall while equivalent 400kV lattice ones are 48m high. They also have a comparatively smaller footprint – 10m2 in a single location compared to lattice pylons’ 70m2 spread over the four legs. The variations are a result of stark differences in the structural and foundation designs.
T-pylons comprise a tubular steel shaft and two crossarms, which gives the structures their eponymous shape. Lattice pylons have six crossarms and four legs.
Haskayne says that the key difference between the T-pylon design and a traditional lattice tower is the formation in which it supports the cables. Traditional towers usually carry their cables in a vertical arrangement, suspended from either side of the tower on separate crossarms.
The diamond shaped configuration meant that the pylons could be 30% shorter than conventionally designed ones
The T-pylon suspends its conductors from just two points from which they are arranged in a diamond formation. This maintains the necessary clearance between the cables themselves, and between the conductors and the ground.
Using tubular steel shafts and the novel diamond arrangement helps create a structure which is 30% shorter than a lattice tower, says Balfour Beatty operations director for the Hinkley Connection Project Liam Cave.
According to National Grid, the main body of a T-pylon has more steel than an equivalent lattice pylon and is more expensive to produce.
Testing and installation
Full-scale structural testing of the T-pylons was undertaken at a dedicated transmission tower test station in Spain.
Cave says climatic loads were replicated using tensioned winch bonds. He adds that full-scale electrical tests were carried out on the insulator arrays between the tips of the “diamonds”.
The T-pylons were tested in the UK as well. Six of them were constructed at National Grid’s testing and training centre at Eakring in Nottinghamshire in 2015. “That helped us look at the design and practicalities of constructing and working in and around these structures,” says Haskayne.
Modular off site construction reduces installation time from an erection and wiring perspective
T-pylon structures arrive on site in three parts – the central node and two crossarms. They are put together with approximately 120, M48 bolt assemblies. The cables are transported to site on 2.5m high drums, weighing up to 7.5t.
“Despite the tubular shaft design being significantly heavier than the lattice steel equivalent [76.4t compared to 29.7t], modular off site construction reduces installation time from an erection and wiring perspective leading to programme efficiencies and less time spent on grantors’ land,” says Cave.
Haskayne says it takes a maximum of five days to erect a T-pylon, compared to more than two weeks for lattice ones.
Stringing – the process of installing cables on the pylons – is also different. The design of lattice pylons allows the structures to be climbed and for work to take place from the crossarms. But this is not possible with T-pylons so work at height must be done using mobile elevated work platforms.
Progress and future plans
National Grid anticipates that the 48 T-pylons between Bridgwater and Loxton in Somerset will be energised in October.
Access and foundation works are being carried out on the northern section of the route between Sandford and Portbury. Erection of the remaining 68 T-pylons will start in the third quarter of the year.
The 116 structures, will be an average of 360m apart and will cover around 80% of the 48.5km overhead line route. Haskayne explains that lattice pylons were selected for the rest of the overhead line route because “from an engineering perspective they can make tighter turning angles”, an essential feature for the geometry of the installations in those areas. All works are due for completion in late 2024.
Haskayne says that National Grid has no plans to replace other lattice pylons with the new design. A decision about this “will depend on the requirements, characteristics and feedback related to individual connection projects”.
Community benefits
Pupils at East Huntspill Academy placed a time capsule in one of the pylons
Aside from the lower visual impact of the T-pylons, communities along the Hinkley Connection Project route are benefiting from several skills creation, education and environmental mitigation programmes run by National Grid.
It has awarded £870,000 in grants to local projects, through its science, technology engineering and maths (STEM) education fund and adult skills programme. Through the adult skills programme, local unemployed people are given access to recognised construction qualifications which has helped some return to employment.
The utility company wants this project to trigger interest in STEM among local young people. School children are invited to visit the T-pylon sites, learn about this technology. Some can places time capsules inside the hollow stem of 20 pylons.
In late April, Year 6 pupils from East Huntspill Academy in Somerset, placed a time capsule inside the T-pylon close to their village. The material put inside the capsule – put together by the academy’s students ranging in age from 4 to 11 years old – included: self-portraits; ideas about how the school might change in the future and how energy may be used in the future. Parent letters; photographs; newsletters; maths equipment were also included along with a Covid test and mask to explain the unprecedented experience of living through a global pandemic.
National Grid also wants to ensure that the area covered by the Hinkley Connection project will be greener. It has recently planted its 2,000th tree on the project, and it is on track to deliver 5,306 more trees to the local area than it initially removed to enable construction. In the 2021/22 winter planting season, 1,257 trees, 39,527m2 of woodland and 9,960 linear metres of hedges were planted.
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I saw the competition for the design of these at the time, and I assumed that sustainability would be one of the prime criteria. However, I see that there is more than double the material (steel) in them. Is this mitigated elsewhere? What about foundations – how do they compare?
It’s difficult to beat a space lattice structure for material efficiency. You can put material where it’s needed and the connections are cheap (assuming smallish bolts instead of the M48’s for the T type.
What price aesthetics?