As SP Energy Networks progresses with its ambitious £10.6 billion investment plan (RIIO-T3) to upgrade Central and Southern Scotland's electricity transmission network between 2026 and 2031, a new independent report has shed light on the costs and characteristics of various transmission technologies.

The report, conducted by Mott Macdonald for the Institution of Engineering Technology (IET), compares the costs, environmental impacts, carbon intensity, technology readiness, and delivery timeframes for a range of options crucial for the UK's journey towards Clean Power 2030 and Net Zero targets.

The UK is embarking on a significant overhaul of its electricity grid to facilitate the greater use of green electricity, aiming to provide consumers with a more secure and affordable energy supply in the long term. The Mott Macdonald report details options for enhancing the existing network and adding new capacity, providing a cost comparison of different technologies.

Key findings from the report indicate significant cost disparities:

• Underground cables are, on average, approximately 4.5 times more expensive than overhead lines. For example, the report estimates the build cost of a typical 15km long, 5000MW overhead line at nearly £40 million, while an equivalent underground cable would cost around £330 million. This figure could rise to £820 million if tunnelling is required.


• Offshore high voltage direct current (HVDC) subsea cables are around 5 times more expensive than overhead lines for point-to-point connections.

• An offshore HVDC network connecting multiple sites to the onshore grid is estimated to be 11 times more expensive than overhead lines.

Beyond cost, the report underscores the importance of evaluating each technology based on its specific context, considering environmental impact, engineering challenges, and local impacts. While underground cables have a lower visual impact, they may only be suitable for certain terrains and come with a much higher cost. Similarly, long high voltage AC cables present unique electrical engineering challenges. Subsea solutions, while potentially reducing the need for onshore infrastructure, still require coastal landing points and have environmental implications for marine environments.

The report also highlights potential supply chain bottlenecks, particularly in cable manufacturing, which could affect delivery times and necessitate the development of long-term relationships with providers to mitigate these risks.

Furthermore, the study provides examples of deploying key technologies to achieve specific capacity over defined distances, factoring in fixed costs and distance-dependent costs. This detailed analysis aims to provide stakeholders with a clearer understanding of the economic and practical implications of different transmission infrastructure choices as the UK undertakes its most significant grid upgrade in decades to meet its clean energy targets.

Professor Keith Bell, Chair of the Project Board for the IET Transmission Technologies report, emphasised the critical nature of this grid development for achieving Net Zero, stating, "Access to a cleaner, more affordable, secure supply of energy requires the biggest programme of electricity transmission development in two generations." David Reid, Global Practice Leader, Integrated Electricity Networks at Mott MacDonald and lead author of the report, added that the report offers "valuable insight into the different technology options available and their relative merits," which will be crucial for stakeholders in accelerating the necessary infrastructure development.