Balfour Beatty features in New Civil Engineer discussing the Accelerated Strategic Transmission Investment framework

North Scotland’s energy network was not designed to meet 21st century generation requirements. But it is now down to SSEN Transmission and its contractors to upgrade it by the end of 2030.

Like the rest of the UK and many other countries, Scotland is racing to upgrade its electricity transmission network so that it can meet targets for decarbonisation.

It has an ambition to connect 11GW of offshore wind power capacity by 2030. This requires significant investment and workforce expansion, as well as collaboration and planning.

While National Grid is delivering its Great Grid Upgrade and Scottish Power is working on its Action 2030 plan, SSEN Transmission has responsibility for upgrading the network in northern Scotland.

It is doing this under its Pathway to 2030 programme. This features four subsea high voltage direct current links, 550km of new 400kV overhead line, hundreds of kilometres more of refurbished overhead line, eight new substations and four new converter stations.

“The network in the north of Scotland was never designed for this level of generation,” says SSEN Transmission lead engineering manager Scott Gardiner.

“It was always to export electricity to rural communities, but now we want to take huge capacity from the rural communities and bring it back to the network, which is why, especially in the north of Scotland, we’re seeing such a massive reinforcement in the network needed.”

SSEN decided on the best locations for upgrades through its system studies, which are done in-house and which are ongoing. These show the best places for new connections.

“Then we’re looking at what sort of uplift is going to be required,” explains SSEN development portfolio manager Russell Stewart.

“What is the uplifting capacity needed? What options can deliver that? Then you get permutations put forward by our system planners.”

“You want to have sufficient nodes [substations or converter stations] so that the distance between the point of connection isn’t excessive, but you don’t want to have too many because each is quite expensive,” says SSEN Transmission head of project engineering Simon Fraser.

Optimal design means placing new nodes in locations within a few kilometres of where SSEN already has transmission infrastructure, enabling the new infrastructure to be easily tied into the existing network.

“That allows us to create a more resilient and integrated network,” says Fraser. “It’s a benefit to have a well-integrated and interoperable network rather than just a point-topoint energy transfer solution.”

The desired schemes were then submitted to the electricity systems operator (ESO) along with expected costs and dates for delivery. The ESO factored it into its central volume allocation along with the requirements from other transmission companies.

It determined a holistic network design which mapped the most economic, efficient and coordinated way to deliver the schemes to bring the required supply to where it is needed.

This has given SSEN a much clearer work requirement than it’s had before. “[Previously] it was subject to whether or not a customer was definitely going to [progress a project] or not, whereas the change with Pathway to 2030 is that [regulator] Ofgem and the government have given us this clear commitment,” Fraser says.

SSEN has said that the total cost of its projects for Pathway to 2030 will be £20bn, which is a figure that Stewart believes is robust thanks to input from contractors across the projects, who have been involved from an early stage.

However, there are always possible changes in the market that could affect the procurement or construction cost.

“It’s pretty competitive out there, so any major changes in the next six months could have an impact on us,” says Stewart.

“We’re not the only transmission operator pushing for 2030 and it’s not just a UK thing – we’ve seen TenneT, which runs the Dutch and German grids, taking up a lot of capacity on the subsea side.”

Pathway to 2030 involves four HVDC subsea links

Early contractor involvement

With around 1,500 steel towers to build, 9,000km of conductor to be installed and 12 major new substations and converter stations bigger than any SSEN has built before, it has had to establish its supply chain early.

“We opened up the frameworks again for substations and overhead lines to bring both our existing well-established cohort in, but also new contractors,” says Stewart.

“Through the framework they’ve been allocated projects that reflect their experience, their capacity and their ability to deliver.”

The appointed contractors include Balfour Beatty, Bam-Hitachi JV, Hitachi Energy, Murphy, Nexans, Siemens-Bam JV, Sumitomo and Van Oord.

“We’re bringing their expertise in at the early stages to make sure they’re happy with what they’re going to build and there are no surprises for them,” says Stewart.

Balfour Beatty has been contracted for around £3bn of the works for Pathway to 2030 including 285km of new build overhead line, 55km of overhead line upgrades, three major substations and two converter stations.

SSEN established the preferred routes but it engaged contractors shortly after it did so to provide input on tower locations, access plans, substation layouts and more before submitting the planning applications.

“The benefit with early contractor involvement is you get constructability built in,” says Balfour Beatty head of business development and preconstruction for Scotland Gordon Dinning.

In a traditional model the contractor receives the designs after a tender and negotiation process and only at this relatively late stage can have input into the design, which might highlight unforeseen issues.

This can then start a new round of back and forth as kinks are ironed out. “That can easily add 18 months to two years to the programme,” Dinning says. “SSEN was wise enough to know that it had to get the contractors on board early, especially with the volume of work across the piece.”

Early contractor involvement also facilitates greater clarity on the details of where the various projects interact.

“Each project has interdependency on other projects around it so there’s that interface risk that’s very complicated,” Dinning says.

As Balfour Beatty has been awarded interlinked projects, it takes on the programme and interface design risk associated with these projects, he explains.

He says that Balfour Beatty is in dialogue with SSEN every day. “As issues arise, we work together and do our best to understand where people are coming from and try and build solutions,” he says. “The collaboration has gone a lot better than I expected because usually there’s a client-contractor relationship, but this is about being one team.”

This carries over to relationships between the various contractors on the Pathway to 2030 portfolio.

Dinning reports that at workshops where the various contractors have gathered, the usual competitiveness has given way to conversations about optimising approaches.

Dinning foresees that the dialogue between projects will be even more important at the end of the decade when it comes to the energisation sequence. “You can’t switch them all on on the same day,” he says.

Modern infrastructure

Central to the upgrade of the network for Pathway to 2030 are stronger towers. Its current strongest overhead infrastructure on the network is the Beauly-Denny overhead line.

“That carries two conductors, but to get maximum capacity out of the new overhead lines that will now be three conductors,” says Gardiner. “So the load from the conductors alone has increased by 50%.”

Climate and location also require stronger towers. “In the north of Scotland, the wind and ice loading is higher, the altitude we’re putting them is higher than we have at the Beauly-Denny line,” says Gardiner.

Balfour Beatty has designed the new towers, which will become standardised for the whole project. They are taller than existing 49m tall lattice towers, standing at an average of 57m, but height is dependent on topography.

They also feature bigger steel sections and larger bracings compared to traditional lattice steel towers so they can achieve the required strength. The contractor is now manufacturing test towers.

“We are booking a test site where we’ll assemble the tower and try to pull it over,” Dinning explains. “That’s going to happen in Europe because there is not a test site in the UK suitable for the size of towers.”

Once the test tower has undergone resilience testing, which is expected to take place in spring 2025, they will go into mass production.

“We’re issuing the design to the other contractors and they will procure the towers they need for their sections of the work,” Dinning says.

Balfour Beatty was the contractor on the Hinkley Connection project which used innovative T-Pylons (NCE, June 2022) rather than more common lattice steel towers. These were considered by SSEN but were dismissed due to “a number of challenges when it comes to operability” explains Gardiner.

“They’re great if it’s on long flat ground and you can drive up to it to get access, but the majority of our network is going to be in the Highlands of Scotland, which is not easy to get to,” he adds. “Plus the capacity we need is higher than the T-Pylons can get.”

Fraser adds that visual impact was also a consideration. “While it’s quite subjective, the feedback from our visual specialists was that, with the landscape backdrop we have for our routes, a steel lattice fits in more effectively than a big white column,” he says.

The familiarity of lattice towers was also a factor. “We are always looking at potential future opportunities to revisit the fundamentals around overhead line design, but with the pace and scale of the installation required, a more known product allows us to reach that capacity,” says Fraser.

Undergrounding the power lines was also a consideration as the dislike of overhead infrastructure was “fed back considerably through the consultation process” according to Stewart.

However, there were again significant factors preventing that. “Recent studies have found that underground cables are at least five times more expensive than the equivalent overhead line,” says Stewart.

There is also concern about the performance of underground power lines. “They don’t perform in the same manner that an overhead line would, in that the voltage starts to move outside of its normal parameters much quicker,” Stewart explains.

“That effectively necessitates the need for additional substations, probably at a frequency of less than 10km, to stabilise the voltage.”

Constructing the trench would also be a major undertaking. “It’d be in the region of 40m to 45m wide, because we have to make sure each of the individual circuits is separated from each other as they’re underground and can’t cool down as effectively,” Stewart says.

“That means a 45m wide corridor that you have to excavate and whatever’s in front of you has to go.

“In fairness, you have to take down forestry for overhead lines, but you can be a bit more sympathetic.”

Underground transmission also presents operational challenges. “If there’s a fault on an overhead line you can generally fix that in a couple of days to a week,” says Gardiner. “If you have a fault in an underground cable, that can take months to locate and effectively repair it and the circuit is out for the duration of that fault.”

Stronger towers are key to the upgrade of the network; moving from two conductors to three increases the towers’ load by 50%.

Rapid construction

Apart from modifying the tower design, SSEN is looking to modify construction approaches to enable rapid delivery of Pathway to 2030.

“We do a lot of projects and I think we’re pretty good at it, but if we’re going to deliver it by 2030 we need to start doing things differently,” Gardiner says.

“That’s really been the focus in the engineering sphere and working with the supply chain – speeding up the programme, but safely.”

Modern methods of construction are a major consideration. SSEN is investigating the use of prefabricated foundations for the towers and modular buildings for the substations.

“Anything that can be built off site and then brought on site we see as a way of speeding up construction,” Gardiner says. “It improves the quality of the end product as well.”

Standardisation will be used where possible. “There’s a lot we can standardise in substations such as the bay arrangements in the control buildings,” says Gardiner.

“There will always be elements that are bespoke because you don’t know where your circuits are coming in, but we are looking to standardise whatever we can because it helps speed up the design stage and helps the supply chain.”

Gardiner says that the contractors “have been really open to sharing their ideas for innovation and standardisation”.

“I think it reflects that it’s not as much of a competitive environment as it was before,” he adds. “They realise that to deliver by 2030 we need to work together on it.”

Beyond 2030

With a project of this magnitude, it is imperative that a positive legacy remains. On top of delivering a resilient and green network, SSEN has committed to building 200 new homes, delivering biodiversity net gain with a broader range of native species and setting up a £10M community benefit fund.

However, Stewart sees the skills and workforce expansion as the main legacy of Pathway to 2030.

“There is a whole load of schemes coming on the back of what we’re doing, so there’s an opportunity continuing into the 2030s for that workforce to keep on operating and building transmission infrastructure, because 11GW doesn’t solve the problem, there’s still more to come,” he says.

“So we’re setting ourselves up for a prosperous 2030s, potentially 2040s as well. There’s solid engineering-based jobs that will rumble on for a couple of decades and could make the best parts of people’s careers. That’s a huge part for us to play.”