‘Undergrounding’ electrical transmission cables

On 29 February 2024, Lord Swire (Conservative) is scheduled to ask the following question for short debate:

Lord Swire to ask His Majesty’s Government what assessment they have made of the advantages of burying overhead electrical transmission lines.

1. What is the current situation?

1.1 Electricity transmission and distribution

Once generated, electricity is transported via the electricity grid on transmission and distribution networks. Separate networks serve Great Britain and the island of Ireland.^[1]

The transmission network in Great Britain, known as the National Grid, transports high-voltage electricity of 275kV and 400kV. Electricity for homes is then delivered through distribution networks, which must first lower the voltage of electricity to up to 132kV to make it safe for domestic use.

The National Grid describes this process as follows:

A good way to think about the difference between our transmission and distribution operations is to imagine Britain’s road system:

• The transmission network is like the motorways, carrying vehicles (electricity) at high speed (high voltage) across the country. This is the network of big pylons and overhead lines you see around the country.
• The distribution networks are the local roads, connecting motorways with communities to help vehicles complete their journey. These are the smaller pylons (and underground cables) carrying lower voltage lines.

Our transmission business carries high-voltage electricity around on the ‘motorways’ of the network, and our distribution operation uses the ‘local roads’ to deliver it to where it’s needed in the Midlands, south-west or south Wales. If you’re in any other area, one of a dozen or so other regional networks distributes electricity to you.

In addition, small-scale generators of electricity, such as solar parks or wind farms, are also connected directly to the distribution network. This is known as embedded generation. The National Grid estimates that embedded generation currently makes up 29% of electricity generation on the electricity network.^[2]

In England and Wales, the National Grid owns, builds and maintains the pylons, overhead lines (OHL) and underground cables that make up the electricity transmission network.^[3] Distribution network operators (DNOs) manage the poles, pylons and cables in a local area.^[4] In some places, such as the Midlands, these are also run by the National Grid, whereas elsewhere other operators manage the distribution network. For example UK Power Networks is the DNO across London, the south-east and the east of England.^[5]

While energy policy in Scotland is largely reserved to the UK government, Scottish ministers are responsible for approving applications to build, operate or modify onshore electricity generating stations with capacities exceeding 50 megawatts, as well as all applications to install overhead power lines, large oil and gas pipelines, and associated infrastructure.^[6] The transmission network in southern Scotland is owned and maintained by SP Energy Networks, and in northern Scotland by Scottish and Southern Electricity Networks.^[7]

Northern Ireland energy policy is devolved.^[8] This briefing therefore focuses on the use of underground electricity cables in England and Wales.

1.2 Underground cables

OHL are significantly more likely to be used than underground cables in the transmission of high-voltage electricity. Overhead electrical cables are held up by pylons, technically known as suspension, tension or transmission towers^[9]

The National Grid estimates that there are more than 22,000 transmission pylons across England and Wales. ^[10] The transmission system for high voltage electricity is made up of approximately 4,500 miles of OHL and over 900 miles of underground cables.^[11]

The use of underground cables is more common in the distribution of electricity. In the areas where the National Grid is a DNO (the Midlands, south-west and south Wales) the network is made up of 60,000 miles of overhead line and 83,900 miles of underground cables.^[12] Regional DNOs have differing levels of overhead and underground cables, varying by location and voltage.^[13]

2. What is undergrounding and when is it used?

Undergrounding is the process by which OHL are replaced with underground cables. The process is only used in certain circumstances.

The government has stated (see section 5 below) that OHL should be the strong starting presumption for electricity network infrastructure developments. This situation is reversed in nationally designated landscapes where the strong starting presumption should be that the applicant will underground relevant sections of the line.

2.1 Visual impact

The visual impact provision (VIP) project is a £500mn allocation by the energy regulator Ofgem to help reduce the visual impact of existing transmission lines in English and Welsh areas of outstanding natural beauty (AONBs) and national parks. An independent process was established by the National Grid, and approved by Ofgem, to consider mitigating the impact of OHL in a number of locations.^[14] Outlining the work of the VIP project, the National Grid stated:

We have assessed 571km of our electricity transmission line in AONBs and national parks to identify which sections have the highest visual impact on their surroundings.

Using input from technical experts, local stakeholders and communities, the stakeholder advisory group recommended priority schemes where replacing the overhead line with an underground connection could have a transformative effect on the landscape.^[15]

It highlighted work that the National Grid had undertaken in the Cotswolds, Dorset, Snowdonia and North Wessex Downs.

In addition, the National Grid runs a landscape enhancement initiative (LEI) as part of the VIP project. The LEI is a grant scheme that seeks to reduce the impact of the National Grid’s existing electricity transmission lines in England and Wales. Outlining the initiative, the National Grid stated:

Between 2015 and 2021, the LEI provided almost £4mn for localised visual improvement projects. Over the next five years it aims to build on this success with funds in the region of £12mn available to eligible national landscapes (formerly areas of outstanding natural beauty) and national parks in England and Wales.

The LEI has been championed by the VIP project’s stakeholder advisory group, which includes organisations such as Natural England, Historic England, National Parks England and CPRE [formerly the Campaign to Protect Rural England] plus their Welsh counterparts, along with the National Trust, National Landscapes Association, the Ramblers and the Landscape Institute.^[16]

2.2 London power tunnels project

In 2016 the government was asked about the proportion of new transmission cables that had been undergrounded in the last 10 years and answered:

In England and Wales, the only major new electricity transmission lines built in the last 10 years by National Grid are all in the London power tunnels project (LPTP). This involved 32km of tunnels and 64 circuit kms installed from 2011 to February 2016. Otherwise, major projects have been asset replacement projects. These have generally been like-for-like, in that overhead line components have replaced older overhead line components, and underground cables have replaced underground cables.^[17]

The LPTP has run in two phases. Phase one started in February 2011 and was a seven-year, £1bn infrastructure project to rewire north London. The National Grid described the project, which saw the construction of 32km of underground tunnels, as “the first major investment in the electricity transmission system in London since the 1960s”. In phase one 192km of 400kV cable was installed in the tunnels, along with 30km of 132kV cable. The project, which was mentioned in the government’s answer to the written question above, was completed in 2018.^[18]

More recently, in spring 2020, further work was undertaken as part of phase two of the LPTP. This is also a seven-year project and aims to be complete and fully operational by 2027. This phase has focused on rewiring south London. The project will see 32.5km of 3m diameter tunnels being constructed deep below the road network between Wimbledon and Crayford, to carry high voltage electricity cables. The National Grid notes:

Most of the electricity supply in south London is currently transmitted through underground cables, traditionally found just below the road surface. Work to maintain them is carried out at street level and can be disruptive. By housing new electricity cables in deep underground tunnels, there will be a number of benefits:

• less disruption during construction as the majority of works will take place deep underground
• future repairs and maintenance work can be carried out without disrupting traffic, residents and businesses
• additional cables can be installed in the tunnels to meet future demand^[19]

3. What are some of the issues with undergrounding?

In 2015 the National Grid undertook a study examining the technical merits and challenges associated with the undergrounding of high voltage electricity lines, compared with installing OHL. It concluded:

[…] there are a number of issues that make the undergrounding option more technically challenging and expensive. However, despite the costs and technical challenges, there are circumstances in which underground cables are a more preferable option than overhead cables.^[20]

3.1 Technical considerations

The report explained some of the technical considerations to be taken into account. For example, the differences in the type of electrical insultation used for an underground cable when compared to an OHL. This can lead to electrical cables being generally larger than OHL to allow for the dissipation of heat and insultation to physically protect the cable from damage and ensure it is not a danger to people while in service. The report noted:

To match overhead line thermal performance for a 400kV double circuit, as many as 12 separate cables in four separate trenches may be needed, resulting in a work area up to 65m wide.^[21]

The potential disruption of undergrounding an overhead line, which would require digging a trench along the length of the route for the cable, was also noted. The report also drew attention to the possible environmental impacts of undergrounding cables, noting the potential costs such as threats to sensitive habitats and damage to archaeological heritage that undergrounding cables might cause.

3.2 Financial costs

In 2012 the Institution of Engineering and Technology (IET) and engineering firm Parsons Brinckerhoff published the findings of an independent study into the costs of new electricity transmission infrastructure. The study concluded that excluding building costs, the cost of operation, maintenance and energy losses was broadly the same for OHL and underground cables. However, it also found that while capital building costs for undergrounding varied greatly, underground cables were always more expensive when compared to equivalent OHL.^[22]

However, the study also concluded:

No one technology can cover, or is appropriate in, every circumstance, and thus financial cost cannot be used as the only factor in the choice of one technology over another in a given application.

The government cited the figures produced in the report in November 2023, when it argued that while it was possible to bury power lines, “it is up to 10 times more expensive and that cost will fall on the bill payer”.^[23]

4. Calls for more undergrounding

The arguments in favour of undergrounding can be seen in responses to the government’s consultation on draft national policy statements for energy infrastructure, which ran between March and June 2023.^[24]

Responses to the government’s proposal that there should be starting presumption for OHL except in nationally designated landscapes were as follows:

The responses we received for question seven [starting presumption for overhead lines except in nationally designated landscapes] were very polarized between industry views and community views. The majority of industry respondents were in favour of the presumption citing the cost and time efficiency of overhead lines and the clarity that the policy brings for applicants and decision makers as the key reasons. Conversely the views expressed in the individual and community responses were largely against the starting presumption citing the visual and landscape impact as the most common reasons.

Of the 92 respondents who answered the open question, 17 were industry, 19 were environmental or advisory groups, 12 were local government, 15 were local action groups (including three sets of campaign responses) and 29 were individuals or members of the public.^[25]

The government summarised the concerns raised as follows:

• Environmental concerns, such as the impacts that OHL could have on local and migratory birds and the effect on ancient or valued woodland. Conversely, a few respondents, primarily environmental groups, commented that in some areas overgrounding could be more appropriate and less disruptive for the surrounding environment and that this starting presumption had the potential to significantly positively impact the environment.
• Scope of starting presumption. A few environmental groups specifically commented that the presumption of undergrounding only in nationally designated landscapes should be expanded to all biodiverse areas, including those adjacent to nationally designated areas. Similarly, a few local government respondents called for the presumption to be expanded to areas of local or historic value. They suggested these areas could equally benefit from the starting presumption and their lack of inclusion risked damaging the local character, biodiversity, and value of these areas.
• Visual impacts. Some local action groups, members of the public and local authorities expressed concern about the visual impact of this starting presumption. A few respondents disagreed with the starting presumption due to the adverse visual effects of OHL, describing them as ugly or a blot on the landscape.
• High cost of undergrounding. Some industry respondents, a few action groups and a few members of the public commented on the high cost of undergrounding, citing the time and cost efficiency of OHL compared to underground lines.

Respondents in one campaign, which the government classified as a campaign response given the similarity of words and phrases used, argued that the presumption in favour of OHL and pylons was outdated, unnecessary and harmful. In particular they argued:^[26]

• OHL are highly damaging to habitats and bird strikes into power lines are a major killer acknowledged in the national policy statements
• OHL are less resilient in extreme weather than underground cables or sub-sea grids
• OHL cause significant damage to landscape, archaeology and cultural heritage including the settings of AONB (even when the pylons are outside of the AONB), scheduled monuments and listed buildings
• forcing pylons upon communities without genuine alternatives is not fair

However, the Royal Society for the Protection of Birds (RSPB) noted in its response to the consultation that both forms of development of electricity infrastructure have environmental impacts:

Overhead lines and underground cables have differing impacts on ecological features, often depending on the habitat and/or species in question. For example, the process of constructing underground cables may disturb species, require the removal of hedgerows or have effects on the ability to manage wetland habitats through restricting future activity, whilst overhead lines may require ongoing control/cutting of vegetation beneath them, which can be particularly damaging if passing through woodlands. It is therefore important that routing is considered carefully as a means of minimising impacts as well as the type of cabling proposed.^[27]

The government published a response to the consultation in November 2023. This is discussed in section 5 below.

5. Government policy

Government planning policy relating to electricity infrastructure can be found in two national policy statements (NPS), the ‘Overarching NPS for energy (EN-1)’ and ‘NPS for electricity networks infrastructure (EN-5)’. The National Grid describes these as the main documents that inform decision making on major infrastructure projects.^[28] The most recent statements were presented to Parliament on 22 November 2023 and came into force on 17 January 2024.^[29] This followed a consultation on draft policy statements which ran from 30 March to 23 June 2023.

EN5 confirms government policy regarding OHL and underground cables, noting that overhead lines should be “the strong starting presumption for electricity network developments in general”.^[30]However, the policy statement notes that this “presumption is reversed” when proposed developments cross part of a nationally designated landscape, such as a national park, the Broads, or an AONB.^[31] EN5 explains:

2.9.21 In these areas, and where harm to the landscape, visual amenity and natural beauty of these areas cannot feasibly be avoided by rerouting overhead lines, the strong starting presumption will be that the applicant should underground the relevant section of the line.

2.9.22 However, undergrounding will not be required where it is infeasible in engineering terms, or where the harm that it causes is not outweighed by its corresponding landscape, visual amenity and natural beauty benefits. Regardless of the option, the scheme through its design, delivery, and operation, should seek to further the statutory purposes of the designated landscape. These enhancements may go beyond the mitigation measures needed to minimise the adverse effects of the scheme.

2.9.23 Additionally, cases will arise where—though no part of the proposed development crosses a designated landscape—a high potential for widespread and significant adverse landscape and/or visual impacts along certain sections of its route may result in recommendations to use undergrounding for relevant segments of the line or alternatively consideration of using a route including subsea cabling.

2.9.24 In these cases, and taking account of the fact that the government has not laid down any further rule on the circumstances requiring use of underground or subsea cables, the secretary of state must weigh the feasibility, cost, and any harm of the undergrounding or subsea option against:

• the adverse implications of the overhead line proposal
• the cost and feasibility of re-routing overhead lines or mitigation proposals for the relevant line section
• the cost and feasibility of the reconfiguration, rationalisation, and/or use of underground or subsea cabling of proximate existing or proposed electricity networks infrastructure

2.9.25 In such cases the secretary of state should only grant development consent for underground or subsea sections of a proposed line over an overhead alternative if they are satisfied that the benefits accruing from the former proposal clearly outweigh any extra economic, social, or environmental impacts that it presents, the mitigation hierarchy has been followed, and that any technical obstacles associated with it are surmountable.^[32]

The policy statement includes details of the factors which the secretary of state should consider in this instance, namely:^[33]

• the landscape and visual baseline characteristics of the setting of the proposed route
• the additional cost of the proposed underground or subsea alternatives, including their significantly higher lifetime cost of repair and later uprating
• the potentially very disruptive effects of undergrounding on local communities, habitats, archaeological and heritage assets, marine environments, soil (including peat soils), hydrology, geology, and, for a substantial time after construction, landscape and visual amenity
• the potentially very disruptive effects of subsea cables on the seabed and the species that live in and on it
• the applicant’s commitment, to mitigate the potential detrimental effects of undergrounding works on any relevant agricultural land and soils (including peat soils)

On 22 November 2023, Secretary of State for Energy Security and Net Zero Claire Coutinho also announced “an ambitious programme to deliver transformation of the electricity network, ensuring the network can support our energy security and the transition to net zero”.^[34]

This included plans to halve the time it takes to build new transmission infrastructure and reduce the amount of time taken for viable projects to connect to the grid. One of the areas mentioned in the plan was a proposal to introduce “a community benefits package for communities who host transmission infrastructure, alongside a national communications campaign to improve public understanding of electricity infrastructure and its benefits”. The secretary of state said that the proposals would reduce consumer bills, support economic growth and drive jobs and investment in the UK.

6. Read more

• Department for Energy Security and Net Zero, ‘Electricity networks: Transmission acceleration action plan’, 22 November 2023
• House of Commons Library, ‘Introduction to the domestic energy market’, 23 October 2023
• Department for Energy Security and Net Zero, ‘Community benefits for electricity transmission network infrastructure’, 30 March 2023

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Cover image by Thomas Despeyroux on Unsplash.