Fort Augustus Substation 400kV Upgrade

SSEN Transmission operates, maintains, and improves the high voltage electricity transmission network in the north of Scotland.

Our network extends over a quarter of the UK’s land mass and some of its most challenging terrain. This area has a lot of renewable energy potential such as wind, solar, hydro and marine power.

We work with the National Grid Electricity Systems Operator so that electricity generated in the Highlands and Islands can reach homes and businesses across Britain.

SSEN Transmission is owned 75% by SSE plc and 25% by Ontario Teachers’ Pension Plan.

We are closely regulated by the independent energy regulator, Ofgem, for the work we do constructing, operating, and maintaining the transmission network in the north of Scotland.

The costs of constructing, operating, and maintaining the transmission network are shared between all those using the transmission system, including generation developers and electricity consumers.

Ofgem set how much money we invest, what return we make on this investment and how these costs are recovered from users of the electricity transmission system.

What is Pathway to 2030

The Pathway to 2030 is a series of projects to increase capacity of the transmission network in northern Scotland. It is part of a national effort to upgrade power lines across Great Britain to connect and transport renewable electricity, especially from offshore wind farms.

These projects contribute towards meeting climate goals and renewable targets, ensuring energy security and supporting Scottish and UK Government targets for a just transition to a net zero future.

Why is it needed?

1. Addressing Climate Change: The UK and Scottish Government have ambitious targets to combat climate change and guarantee a secure and reliable supply of energy. The UK is aiming for 50 gigawatts (GW)* of offshore wind-generated electricity by 2030. Our Pathway to 2030 projects have been identified as required to help achieve such targets by delivering the vital infrastructure required.
   
   *For background, A GW is a unit of measurement of electrical power. It is equal to one billion watts.
   
   
2. Promoting Energy Independence: In 2022, the UK Government set out a strategy to reduce dependence on volatile global gas markets, moving to local, sustainable electricity sources instead. Establishing the necessary infrastructure for this is critical.
   
   
3. Planning for future need: Experts with the National Grid Electricity System Operator carry out extensive analysis and research to predict the UK’s future energy needs. This information is then carefully considered to guide infrastructure upgrade decisions.
   
   
4. Approved by Ofgem: Britain's independent energy regulator, Ofgem, granted approval for these projects in December 2022 as part of its strategy for accelerated network upgrades.

More information explaining the need for these projects can be found here, for more information on the Government policies that underpin this need and how the need has been identified and assessed please read our information leaflet.

To deliver energy security and net zero, further additional investment in new low carbon electricity generation and the enabling electricity transmission network infrastructure will be required across Great Britian, including the north of Scotland.

In March 2024, the independent National Energy System Operator’s (NESO) published its ‘Beyond 2030’ report, which confirmed the need for several new, replacement and upgraded transmission infrastructure projects in the north of Scotland.  In December 2024, Ofgem approved the next phase of regulatory funding to take these projects through the development phase.

These additional investments will soon be subject to extensive public consultation and engagement to help inform their development, with early consultation and engagement expected to take place within 2025. Additional regional investments are also likely to be required to deliver the UK Government's Clean Power 2030 ambition.  Further details on these will be communicated once the need and scope for these investments has been established.

More details on what this means for projects in the north of Scotland are included on our website here

SSEN Transmission is responsible for where and how the high voltage transmission network operates in the north of Scotland. However, we don't determine where energy is generated – nor where it is needed.

We have a legal obligation to provide electricity generators with access to our network, so that the electricity they generate can be transported across GB to meet the energy demand of homes and businesses.

The north of Scotland is rich in renewable energy, especially wind, water, and marine sources meaning this region is vital for the UK and Scotland's climate goals. Our area covers a quarter of the UK landmass and will be crucial in the move towards a low carbon future.

Our approach to optioneering (routes for overhead lines or sites for substations) is underpinned by our statutory obligations, as set out in the Electricity Act Schedule 9, to ‘develop and maintain an efficient, coordinated and economical electricity transmission system’. Having regard to the “desirability of preserving the natural beauty, of conserving flora, fauna and geological and physiographical features of special interest and protecting sites, buildings and objects of architectural, historic or archaeological interest; and do what we reasonably can to mitigate any effect which the proposals would have on the natural beauty of the countryside or on any such flora, fauna, features, sites buildings or objects’.

As a result, our optioneering process seeks to balance technical and cost considerations with environmental considerations. To select a proposed option which is economically viable, technically feasible, minimises impacts on important resources or features of the environment and reduces disturbance to those living in it, working in it, visiting it or using it for recreational purposes. The option must also be capable of being granted consent by either the Scottish Government (in the case of overhead line projects) or local planning authorities (in the case of substation projects).

To do this we consider specific topic areas within the environmental, technical and cost categories and assess each in terms of the potential for the option to be constrained. We allocate a Red/Amber/Green (RAG) rating to each topic to highlight where potential issues may be present and use this to help compare the different options. The appraisal seeks to compare the wider implications of each option on those topics (both individually and combined) and reach a reasoned conclusion, on balance across all topics, as to the ‘preferred option’ to take forward.

Any weighting of certain criteria as being of more importance than other criteria would depend on project specific considerations. For example, if a particular technical or environmental constraint has been identified as a priority consideration through stakeholder engagement (or engineering specification) and is unable to be overcome, this will inevitably carry greater weight in the comparison of options which may otherwise contain constraint that can be easily designed out or mitigated.

We follow this comparative approach through stages of refinement. For overhead lines this starts with wider corridors and progress to assessment of routes and alignments. For substations, we start with a long list of sites, which is refined through further assessment to identify one site. Consultation with stakeholders is essential throughout this process so that their views can be taken account of in our considerations prior to progressing to the next stage.

The culmination of this process identifies a ‘proposed option’ that will be taken forward to detailed design, environmental assessment and consent applications.

Consultations began at an early stage of project development, long before any decisions on power line routes or substation locations were made.

Tower locations were not shown because this information is not yet known. Early feedback is crucial, so potential route options were discussed with key stakeholders.

Our goal at this initial stage is to select routes approximately 1km wide that minimise physical or environmental obstacles and disturbances, while also being practical and cost-effective. We presented these routes to the public, statutory and non-statutory organisations to explain our current assessment and thoughts and listen to feedback.

Stakeholder feedback guides our projects from the start. All feedback is considered, and where possible and feasible, adjustments can be made, in which case we will ensure any changes are presented at future consultation events.

Feedback importance: SSEN Transmission values feedback from stakeholders and local communities and considers it throughout the project development process.

Whilst community feedback is not our only consideration, we wish to develop all projects sensitively and to reduce impacts on communities as much as possible. Community feedback provides an essential insight into local issues that helps to refine the design, routes, and substation locations.

What we can do: Following consideration of all feedback, we consider what opportunities there are to modify our project's design, route, and substation locations.

What we can't do: We don't decide on the overall need for the Pathway to 2030 projects; that's National Energy System Operator and Ofgem's role. Therefore, we can't consult on the core necessity of these 2030 plans.

In November 2020, SSE became the first company to produce a Just Transition Strategy, introducing 20 key principles to ensure fairness as we move away from carbon-heavy activities and embrace climate-friendly changes.

The Pathway to 2030 projects align with these principles, promoting job opportunities, local suppliers, and preserving cultural heritage. Early in project development, SSEN Transmission made sure to engage with communities for feedback and insight.

To read more about our principals click here

There will be no further consultation prior to the submission of the consent applications for the proposed new 400kV OHL’s.  We have undertaken public consultations at the corridor, route and alignment stages of the project development. This includes completing two best practice consultation events as part of the pre-application process for Section 37 applications, please see Electricity Act 1989 - sections 36 and 37: applications guidance - gov.scot.

In March 2025 we concluded the alignment stage consultation for the proposed new 400kV overhead line (OHL) projects, where we presented the Proposed Alignments we will submit as part of a Section 37 applications for consent. The Proposed Alignments have been refined from the various options that we have investigated during the development of the project.

Our alignment proposals presented at these consultations were the result of extensive engagement and project design. We plan to submit our Section 37 applications to the Scottish Government’s Energy Consents Unit (ECU) in Spring 2025. Once an application for consent has been submitted, there will be an opportunity for the public to make formal representations to the ECU before it takes a decision.

For our substation proposals, most consultations concluded in May and June 2024, with applications for full planning permission, under the Town and Country Planning (Scotland) Act 1997, now submitted to the relevant local authorities for consideration. In accordance with the Town and Country Planning (Environmental Impact Assessment) (Scotland) Regulations 2017, applications were accompanied by an Environmental Impact Assessment (EIA) Reports, details of which can be viewed on the relevant project web pages. 

Environmental Impact Assessment Reports (EIARs) for the Pathway to 2030 projects will be prepared for the chosen options, in line with the relevant legislation.

Once completed, the EIARs will be accessible online via our project website, the Scottish Government’s Energy Consents Unit applications portal, and relevant Local Authority planning portals (depending on the consent being applied for).

Hard copies will also be available to view locally and these will be advertised at the time.

When developing proposed project alignments, we seek to minimise impacts on woodlands and forestry where possible, however given the numerous environmental and technical constraints, some impacts are unavoidable.

Where the proposed alignment passes through woodland and forestry, an Operational Corridor is identified and trees are removed within the Operational Corridor to ensure the safe operation of the overhead line. Detailed forestry assessments are carried out as part of the Environmental Impact Assessment (EIA) reports that are submitted with Section 37 consent applications to Scottish Ministers, and these assessments are used to inform the Operational Corridor design through woodland.   

The Operational Corridor width for a 400kV overhead line will typically be 45m either side of the centre line of the overhead line, but this width may vary depending on the type of woodland/forestry and local topography. In accordance with the Scottish Government’s Control of Woodland Removal Policy, we are committed to providing appropriate compensatory planting for any net loss of woodland.

We prioritise environmental protection in our infrastructure projects, strictly adhering to environmental policies and regulations.

We follow a mitigation hierarchy strategy of “avoid, minimise, mitigate and restore” to safeguard local, national and international designated environmentally protected areas.

As the first developer to consult upon and implement an award winning approach to deliver Biodiversity Net Gain (BNG) on all new sites, we’re committed to delivering a “greener grid”, focusing on habitat restoration and creating biodiversity growth as we invest in our network. We are committed to delivering 10% Biodiversity Net Gain on all sites gaining consent going forward. This ensures that we don’t just restore our natural habitats but actively improve them for the benefit of local communities, wildlife, flora and fauna.

During our assessments, comprehensive surveys identify potentially affected wildlife, guiding mitigation efforts. For example, bird surveys are ongoing, with winter surveys planned for 2023/24.

We also assess habitats and other species along our routes. Our consultation process to date has highlighted sensitive areas, and we continue to work with environmental experts and seek community feedback to refine our approach.

Scotland has protected areas that represent the very best of our landscapes, plants and animals, rocks, fossils, landforms and cultural heritage.  These protected areas are also known as designated sites.  The purpose of the designated site is to ensure that the features of special interest, for which the site is designated, remain in good health for all to enjoy, now and in the future.

Designated sites are usually identified by a site boundary and a definition explaining why the site has been designated.  Designated sites have different levels of protection, some are protected by international and national legislation or through national and local planning policy. Some sites can have more than one designation e.g. A Site of Special Scientific Interest (SSSI) may also be designated as a Special Protection Area (SPA).

Statutory Authorities such as Nature Scot, Historic Environment Scotland (HES) and Local Authorities can identify and designate a site for its special feature(s).

Examples of designated sites are:

Dalroy and Clava Landforms SSSI - This SSSI is renowned for its “high-level shell beds”. These fossil rich marine clays occur at around 150m above present sea-level and were first recorded in the 1800’s.  Clava is also noted as a particularly fine example of “flow tills”. The presence and composition of the till provides a key to understanding the location of glaciers during the last ice age as well as the direction in which they moved. SiteLink - Dalroy and Clava Landforms SSSI

Crathes Castle Garden & Designed Landscape (GDL) - Most famous for its outstanding gardens which were started in the early 17th century and were admired by Gertrude Jekyll in the 1930s. The wider 18th-century designed landscape of parkland and woodland makes an impressive setting for the A-listed castle. CRATHES CASTLE (GDL00119)

When planning our overhead line and substation projects, we thoroughly consider environmental, cultural, and built heritage factors in potential locations.

We make use of national archives and data sources as well as gathering data from Local Authorities and detailed site surveys to identify and assess the potential impact on archaeological sites, listed buildings, and other heritage assets.

Environmental Impact Assessment Reports (EIARs) detail these findings and recommend ways to lessen any potential adverse effects. We've received feedback about sensitive archaeological and cultural sites from a range of stakeholders, including local heritage and archaeological groups, all of which our environmental experts have considered.

We are committed to ensuring that all our access to land is subject to appropriate levels of biosecurity protocol in line with the relevant industry or Government biosecurity guidance, and to ensure that both practical and reasonable measures are adopted. 

All of our contractors have been instructed to implement stringent biosecurity measures on taking access to properties for survey and investigation works and then for construction works, as and when our projects reach this stage, to minimise the risk of contamination and the spread of animal and plant diseases, parasites and non-native species.  The guidance we provide contractors reflects a two-stage process.

• Stage 1:
  • Clean footwear, vehicles, plant, tools, and temporary access materials to remove soil and debris with brush and water.
  • Use facilities provided by landowners for cleaning footwear and machinery.

Stage 2:

• Clean footwear and machinery with suitable disinfectant if land is affected by disease or high risk of contamination.
• Clean between different areas within a property if requested by the landowner

Additional measures include soil sampling and testing for Potato Cyst Nematode and Clubroot where there are potato crops, transporting machinery via low-loader for thorough wash-down between properties and frequent internal team meetings to review and update biosecurity measures.

It is important that landowners take any opportunity to engage with our contractors and land managers on biosecurity concerns and inform them of any known disease on the property to allow for appropriate mitigation measures.

When planning routes for overhead lines and substations, we consider visual impacts and how this may affect the local scenery, visitor experience and communities.

For each project we develop, we conduct a Landscape and Visual Impact Assessment. This is one element of the Environmental Impact Assessment Reports that forms part of our application to the local authorities and Scottish Government. In this assessment, we consider visual impact from centres of population, popular spots, like walking paths and tourist sites, and where possible reduce any potential negative visual impacts.

When developing overhead lines and substations, we focus on having a minimal noise impact. Detailed noise assessments are conducted as part of our Environmental Impact Assessment, and include current noise levels, potential new noise as a result of our infrastructure, and mitigation measures where required, to ensure noise is within acceptable levels.

The impact of noise on people is heavily dependent on the nature of the noise, time of day and the noise receptor. As such, there is no single decibel level that can be applied. We engage with the relevant Local Authority’s Town and Country Planning Team to agree practical noise limits to ensure that there is no adverse impact on nearby residential properties.

Specifically, noise impacts for overhead lines are assessed having regard to external and internal noise impacts. The impact of external noise is determined by comparing the noise from the overhead line to the existing background noise levels combined with the noise sensitivity of the receptor. This is a complex assessment but dependent on context. An increase at the noise receptor of 10dB is likely to indicate a significant adverse impact and an increase of 5dB is likely to indicate an adverse impact. Internal noise resulting from external noise is calculated assuming a partially open window, that being in accordance with the guidance in BS 8233 (Guidance on sound insulation and noise reduction for buildings)

Construction noise has been assessed using BS 5228-1 (Code of practice for noise and vibration control on construction and open sites – Noise).

A detailed description of how noise impacts have been assessed is included in our submissions for consent under Section 37 of the Electricity Act 1989 to Scottish Ministers.

An aviation impact assessment, including engagement with key stakeholders, has determined there is no identified requirement for flashing lights on top of the towers.

In the UK, the lighting of obstacles is guided by two main documents:

• The Air Navigation Order (ANO) Article 222;

The Air Navigation Order 2016

• Civil Aviation Publication (CAP) 168

CAP 168: Licensing of Aerodromes | UK Civil Aviation Authority

In our early project development, we consider flood risks and drainage. We aim to avoid areas prone to flooding and conduct assessments when necessary.

Our team includes a variety of water and flooding experts who help design systems to manage water flow around our sites. Before construction, we will have a plan in place to protect both surface and groundwater and reduce potential impacts.

The Pathway to 2030 and clean power depends upon a safe, reliable and resilient electricity transmission system that connects homes and businesses with renewable generators. The north of Scotland, with its vast renewable energy resource, plays a critical role in the energy transition. Our ambition is for our network to have the capability to meet 20% of Great Britain’s demand for clean power by 2030. This means investing to grow the network, while working with our stakeholders to ensure the transition is fair and sustainable. We recognise that the unprecedented pace and scale of network expansion will have impacts on communities, on the natural environment, and on the climate.
 
We are committed to leaving a positive legacy, for people, the environment, the economy, and our wider world.
 
Our Sustainability Strategy sets out our priorities for a transition that is fair and sustainable. Developed through an inclusive and evidence-based process, this Strategy informs our decision making and action up to and including 2030.
 
In 2024/25 alone, our existing network transmitted 17.1 TWh of renewable electricity, which displaced a total of 3.5 million tonnes of CO2e in one year. This is with 11GW of renewable generation capacity connected to our network.  We estimate that our reinforcements out to 2030 will allow the connection of at least another 11GW, with corresponding increases to the amount of renewable electricity transmitted and associated carbon emissions displaced.  In contrast, we estimate the total embodied carbon from our Pathway to 2030 construction programme to be around 2 million tonnes of CO2e. On this narrow basis, the carbon payback period for the network infrastructure would be less than a year. Read more here.

Following the publication of UK Government guidance, every transmission project with new substations or overhead line will provide a community benefit fund. Funding will consist of a strategic fund available to all communities across our network area and local funds for communities close to new infrastructure. If our planned projects go ahead, this funding is worth in excess of £100m to communities over the coming years.

Community Benefit Funds enable us to give back to the communities hosting our transmission network and to help fund projects that can leave a lasting, positive legacy in those areas. 

Additionally, our projects will boost the economy, supporting local jobs and businesses. Independent studies show our Pathway to 2030 programme could contribute over £6bn to the UK's economy, support 20,000 jobs across the UK, and benefit Scotland by around £2.5bn, supporting 9,000 jobs. 

The UK Government has confirmed that households living close to eligible new or significantly upgraded electricity transmission infrastructure will receive discounts on their electricity bills. The scheme is intended to ensure that communities hosting vital network infrastructure directly benefit from investment in the electricity grid.

Under the scheme, households located within 500 metres of eligible transmission infrastructure are expected to receive discounts of up to £2,500 over 10 years.

Pending final confirmation, the UK Government has announced the first projects expected to fall within the scope of the scheme, with payments anticipated to begin from 2027 for eligible households - once projects have secured the necessary consents and main construction works have started. Most eligible households are expected to receive the discount automatically through their electricity supplier.

More information about the scheme is available on the UK Government website: Households living near new pylons to save thousands on bills.

We value engaging with local communities and residents to gather feedback on our plans. While our projects can cause some uncertainty, we aim to be transparent and start these discussions early. We always welcome and encourage suggestions on improving our approach and work collaboratively with communities as our projects evolve.

We would like to reassure you that we develop, build and operate our infrastructure to meet all health and safety legislation, and guidance set by relevant bodies - including the UK Government, Scottish Government, the Health and Safety Executive and the industry regulator, Ofgem – including guidance associated with EMF exposure.

In respect of EMFs, we strictly follow the guidance set by the UK Government, which is informed by international guidance.   

As well as setting exposure limits that protect against known, established effects of EMFs; the UK Government’s guidance includes precautionary measures to protect against possible effects below the exposure limits.  

The UK Health Security Agency and Department of Health have a remit to review new research in this area and ensure that current guidelines and policies are reflective of that research.

There have been over four decades of research looking into whether EMFs associated with electricity transmission projects can cause health effects and there are no established effects below the exposure limits. When we design our overhead lines, substations and cables we do so to ensure they will not exceed those exposure limits, even when operating at 100% capacity, and we also ensure that precautionary measures are also applied to the design where required.

The guidance we follow, which remains subject to ongoing review as required, ensures that safety measures will be applied to our 400kV infrastructure protecting us all against EMF exposure, and keeping our network safe for the public.

More information is available in our EMF Leaflet.

We understand that there are concerns about the potential impact of our proposed developments on properties within the vicinity of SSEN Transmission’s proposed overhead line alignments and substations sites.

Throughout the development of projects SSEN Transmission has looked to mitigate impacts on residential properties as far as possible, and these impacts will be assessed as part of the Environmental Impact Assessments that accompany our applications for consent. Extensive surveys have been carried out at identified receptors, including selected residential properties so that we are able to model potential impacts on the wider area.

Concerns in relation to impacts on property are being noted by our team. However, as a regulated business, SSEN Transmission is obliged to follow a statutory legal framework under the Electricity Act 1989 and Land Compensation Act 1961. If you are entitled to compensation under the legal framework we will assess any claim on a case-by-case basis under the direction of this legal framework. If this is the case, we will recommend that you engage a professional adviser and SSEN Transmission will generally meet reasonably incurred professional fees in these circumstances. However, for the avoidance of doubt, we should advise that SSEN Transmission will not meet fees incurred in objecting to our proposed developments.

We aim to work cooperatively with stakeholders, including landowners, during the development of the projects.

While we might need to acquire land for substations or rights for overhead lines, our preference is to reach voluntary agreements. Using statutory powers is an absolute last resort.

If we do use them, we'll ensure fair compensation based on established industry standards.

A Notice of Intended Entry is a formal notice given to landowners and occupiers, based on the Electricity Act 1989.

It grants us and our contractors the right to enter land after 14 days to conduct surveys related to our project, including environmental assessments, engineering walkovers, and drone surveys.

It's essential to note that these letters are not linked to compulsory land purchase. If there are any concerns, recipients can contact us using the information provided in the letter.

We have statutory obligations to maintain a minimum ground clearance between conductors and other objects which are included in Regulation 17 of ESQCR 2002. Further guidance of standard clearances for the industry are set out in Energy Networks Association Technical Specification (ENA) TS 43-8.

The minimum clearance from the nearest line conductor to any object which is ordinarily accessible (including permanently mounted ladders and access platforms) or to any surface of a building is 5.3m. This includes temporary structures such as mobile and construction equipment. We provide guidance and information to third parties who intend to develop land near our electricity transmission assets, this includes overhead lines, pylons, substations, and underground cables. Find out more here.

The required technology for the new 400kV overhead lines which form part of the Pathway to 2030 have been determined to be a new double circuit 400kV HVAC (High Voltage Alternating Current) overhead line.

The overhead line would consist of steel lattice towers with an average height of approx. 57m which would support six conductor bundles on six cross arms and an earth wire between the peaks for lightning protection. The average distance between towers is expected to be 338m. Tower height and the distance between them will vary dependent on several factors such as altitude, climatic conditions and topography.

This is similar to our Beauly—Denny line, where 80% of its 600-plus towers are below 57m, ranging from 42m to 65m in height.

The cost of improving the electricity network is covered by GB electricity consumers. It's to balance this cost with environmental, technical, and societal factors.

In April 2025, the Institution of Engineering and Technology published independent research undertaken to determine the difference in cost between different technology options for electricity transmission infrastructure, comparing the typical cost of overhead lines, underground and subsea cables. The study found that, typically:

• Undergroundcables cost around 4.5x more than comparable overhead lines; and
• Subsea cables cost up to 11x more than comparable overhead lines.

We conduct a Cost Benefit Analysis for our projects, but the cheapest options aren't always chosen. Factors like minimising environmental impact or ensuring technical viability also play an important part in this.

SSEN Transmission's return on investment is determined by Ofgem's regulations, no matter what the technology used.

In their assessment of the upgrades required to the GB Transmission Network to meet the UK Government’s 50GW of offshore wind 2030 target, National Electricity Operator assessed a number of potential solutions comprising both onshore and offshore schemes. Their Pathway to 2030 Holistic Network Design (HND), which was assessed and recommended as a single integrated GB wide network plan, concluded that a combination of both onshore and offshore schemes is required to transport the volumes of generation necessary to meet 2030 targets to key centres of demand across the country.

Overhead lines can carry roughly three times more power than subsea cables, making them more efficient and cost effective for energy bill payers.

These onshore reinforcements will also strengthen network reliability and security of supply for homes and businesses across the north of Scotland.

Where a line exists, it doesn’t necessarily mean that it would be appropriate to build a new one next to it, there are many considerations as to why this may not be possible.

Sometimes, there is no space for new infrastructure due to existing constraints, including proximity to homes. However, in some areas, it is possible to place new lines near the old ones.

These aspects are considered in the optioneering phases of our projects for which we also seek stakeholder views.

The environmental, technical, and operational constraints associated with undergrounding at extra high voltages, particularly 400kV, make the option extremely challenging to deliver in many areas of Scotland.

Some of the challenges that contribute to this position:

1. Technical Limitations: No underground cables for these high voltages (275kV or 400kV) currently exist in our area. Underground cables need specific ground conditions and present challenges in maintenance and power restoration, especially if faults occur.
   
   
2. Environmental Impact: Undergrounding can have lasting environmental effects, for example impacts on habitats and hydrology, and the area required for laying cables needs to be clear from significant constructions or vegetation for easy access during construction and repairs.
   
   
3. Terrain Concerns: The region's terrain often has slopes that are difficult to install and finding a suitable route for underground cables without challenges is extremely difficult.
   
   
4. Infrastructure Needs: For underground cables longer than 1-2km, additional substation infrastructure would be needed, enlarging the project's footprint.
   
   
5. Operational needs: Restoring power in the event of a cable fault can take significantly longer than for an overhead line. Faults on overhead electricity lines can typically take a few hours to a few days to repair and are generally easy to locate. Underground cable faults often require extensive works, specialist resource, tools and equipment to locate the fault, followed by significant civils work to expose the damage, replace the damaged section and then it can take up to a month to carry out the repairs. This presents significant risks to security of supply and network reliability. It also impacts on our ability to meet our licence obligations of maintaining an efficient transmission network.
   
   
6. Cost: Underground cables at 400kV are estimated to be between 5 and 10 times more expensive than overhead lines, and since these costs are reflected in consumer bills, it's a factor that needs to be considered.

Even if technically feasible, undergrounding over a significant length of or the entirety of a project would be unreasonable as it would be contrary to our licence obligations to be economic and efficient in respect of additional costs to the end consumer and also have additional risk to the electricity transmission network in the event of cable failure and consequent outages.

Given these constraints and our responsibility for an economical and efficient transmission network, overhead lines are our main choice for the onshore ASTI projects. Where there is a clear evidence base to justify undergrounding, this will be carefully considered.

The choice between underground cables and overhead lines is based on the technology used and the power requirements:

1. Technology Type: The Western Isles connection uses High Voltage Direct Current (HVDC) technology, suitable for connecting the Western Isles' power generation. This HVDC technology has low electrical losses and provides better control over the island's network conditions. It requires only two cables for a 1.8GW rating.
   
   
2. Power Requirements: The Pathway to 2030 projects, including routes like Spittal-Beauly, requires a 400kV onshore overhead line to transport much more power, 5GW. This setup permits the connection of much more generation. Achieving this rating with HVDC would necessitate at least three subsea cables.
   
   
3. Feasibility: Using underground cables for such high ratings isn't viable technically, economically, or environmentally due to the sheer number of cables needed.
   
   While underground cables work for the Western Isles connection, they aren't practical for the larger Pathway to 2030 projects.

While the T-Pylon has been developed for use in England and Wales, it is not currently deemed suitable for our projects in the north of Scotland for several reasons:

1. Weather Impact: Our region experiences more severe weather conditions, with increased impact from wind and ice. Using T-Pylons would mean shorter spans between structures, resulting in more structures, thus increasing the visual impact, land occupation, and construction traffic.
   
   
2. Material Lifespan: The composite material used for T-Pylon's diamond insulators might only last about 20 years. In contrast, the glass insulators on our lattice steel towers have a life expectancy of 40-50 years.
   
   
3. Transport & Delivery: The T-Pylon's large steel sections need a large number of low-loaders for delivery, significantly affecting local traffic. Their design could also force new lines to be closer to major roads, impacting our ability to route away from properties. Lattice steel towers are more compact for transport, minimising traffic disturbance.
   
   
4. Design Flexibility: T-Pylons can only turn up to 30° angles, while our lattice steel towers can turn up to 90°. This flexibility helps us avoid communities, viewpoints, and environmentally crucial sites. The limited direction change of T-Pylons would hinder our ability to minimise line impact.
   
   
5. Reliability & Repair: T-Pylons support an entire circuit on a single diamond so in the unlikely event of a failure the entire circuit could be lost, whereas lattice steel pylons use a double circuit. Repairs would take significantly longer, delaying power restoration.

Given these factors, we've determined that T-Pylons aren't currently suitable for our projects, as they might result in greater community and environmental impact and increased construction traffic.

The conductor types being utilised for the new overhead lines proposed between Spittal - Beauly, Beauly – Peterhead and Kintore – Tealing are conventional AAAC (All Aluminium Alloy Conductors). This technology is used extensively on our existing network and has a proven record of safe and reliable operation.  A conductor selection exercise was carried out considering electrical performance, EMF (Electric and Magnetic Fields) and noise impacts, mechanical performance, reliability and cost. The key advantages over the other conductors considered was proven reliability, the ability to reduce conductor noise and reduce electrical losses.

High tension Low Sag (HTLS) conductors are particularly well suited for re-conductoring projects, where existing transmission lines are upgraded by replacing the current conductors with newer, higher capacity alternatives. These projects do not require new towers or structures, making HLTS conductors a practical option due to their reduced sag under load.

Despite these advantages, HTLS conductors also present several significant limitations. Their unique composition introduces challenges in construction, operation and maintenance. These challenges become more pronounced in areas with undulating or uneven terrain; conditions that are present along large sections of our proposed new 400kv overhead lines. Such topography places additional mechanical stress on the conductors, which HTLS types are less capable of handling compared to conventional conductors.

If these challenges are not carefully managed, they can lead to reduced asset lifespans and an increased risk of unplanned outages. In addition, addressing these issues typically extends installation timelines.

Another important consideration is that HTLS conductors tend to exhibit higher electrical losses than conventional conductors (AAACs) – particularly when operating near capacity. These losses reduce the amount of electricity delivered to end users, with the cost of this inefficiency ultimately reflected in consumer energy bills.

Given these factors, HTLS conductors were not considered appropriate for our proposed Pathway to 2030 new overhead line projects but are being utilised for one of our related re-conductoring projects.

Limits of Deviation (LoD) comprise an area which defines the practical limits within which movement of the Overhead Line (OHL) towers, conductors and access tracks can be sited and construction can be undertaken within the terms of the Section 37 planning consent.

A vertical and horizontal LoD is typically applied to OHL projects.  The vertical LoD relates to the addition of a further section of a tower.  It also applies should the foundation design change or the location of the tower move to a higher elevation. Whilst the structure height won’t change these activities can result in the tower height increasing. 

The horizontal LoD is a distance either side of the OHL and access tracks.  This is typically 100 – 200 metres for OHLs and 50 metres for access tracks.  However, the distance applied on either side can vary and be project specific.

LoDs are required to provide the flexibility needed during construction to adapt to unforeseen circumstances. This helps ensure that the consented project design can still be implemented even if unexpected engineering or environmental challenges arise.

An example of where a LoD may be utilised is where poor ground conditions are identified that may make the planned location of infrastructure impractical or less suitable. In such cases, the infrastructure position, initially shown in the plans, can be adjusted to a new location within the LoD, allowing construction to proceed. Any movement of infrastructure within the LoD must be agreed by the project team and assessed against the Environmental Impact Assessment Report submitted with the Section 37 application, before the change in location can proceed.

All project documents, which include maps of the suggested routes for the overhead lines can be found on our project webpages. Contact details of your local Community Liaison Manager are also available.

The 3D models used at our consultation events are created by combining several datasets with the project's proposed infrastructure integrated into them. The base model consists of a 5m resolution Digital Terrain Model (DTM), overlaid with high quality 25cm aerial imagery and 1:25,000 scale Ordnance Survey mapping. It also includes detailed road layouts. On top of this, our consultants incorporate building shapes and tree locations using specialised national data.

Terrain

The model uses OS Terrain 5 data from Ordnance Survey to shape the group surface in the model.This dataset is created from high-resolution scans and simplified to a 5-metre resolution for use in 3D modelling. You can find more information about the dataset here.

Aerial Imagery

The model uses 25cm resolution aerial photos, also from Ordnance Survey. These images are collected on a rolling three-year cycle and matched to Terrain 5 data for accuracy. You can find more information about the dataset here.

Trees

Tree locations come from the National Tree Map, supplied by Bluesky. It shows  vegetation over 3 metres tall, classifying it as either conifer or broadleaf, and represents them using typical tree shapes. You can find more information about the dataset here.

Buildings

The model uses Level of Detail 2 building data from Bluesky. This includes accurately shaped pitched and flat roof shapes, based on high-resolution stereo aerial imagery. You can find more information about the dataset here.

All of this information is georeferenced using the British National Grid Projection. This ensures that every part of the model lines up correctly , making it accurate when integrating proposed infrastructure.

The 3D models are based on geospatial data at the time of modeling. While the data is regularly reviewed and updated, the model may not reflect recent changes, such as newly constructed buildings, recently removed structures or vegetation. changes (e.g. felled trees). This model is intended to give a general visual impression and may not capture every detail of the current environment.