The onshore transmission network in Scotland is owned by SSEN Transmission and SP Transmission. 

The Scottish NETS is divided into 7 boundaries:

  • B0 – Upper North SSEN Transmission
  • B1a – North West SSEN Transmission
  • B2 – North to South SSEN Transmission
  • B3b – Kintyre and Argyll SSEN Transmission
  • B4 – SSEN Transmission to SP Transmission boundary (shared by SSEN Transmission and SP Transmission)
  • B5 – North to South SP Transmission
  • B6 – SP Transmission to NGET (shared by SP Transmission and National Grid Electricity Transmission)

The map below shows the general pattern of power flow directions expected to occur most of the time in the years to come up to 2032, i.e. power will generally flow from north to south. The arrows in the diagram illustrate power flow directions and are approximately scaled relative to the winter peak flows.

Regional drivers 

Due to Scotland’s abundant availability of natural resources, both on and offshore, they are experiencing a large growth in renewable generation capacity, however this is often in areas where the current electricity network is limited.

Over the next ten years, this rapid growth in renewable generation capacity, mainly from offshore wind will contribute significantly to higher power transfer requirements in Scotland. This will increase the network reinforcement needs in some areas.  

Across all the scenarios in the FES, the fossil fuel generation capacity in Scotland reaches almost zero between 2030-2035. By 2030, all scenarios show an increase in wind generation capacity between 11-25 GW. The development of further wind generation in Scotland is key to reaching the UK & Scottish Governments’ targets for 50GW of offshore wind and 20GW of onshore wind by 2030 respectively. The ScotWind leasing rounds awarded rights to 20 offshore wind projects with capacities totalling 27.6GW, and this acceleration will be key to reaching the Scottish Government’s target to reach net zero emissions by 2045. 

With the closure of Hunterston B nuclear power station in January 2022, Torness power station is the last remaining nuclear plant in Scotland and is itself scheduled for closure by 2028. There will be no significant fossil fuel generation remaining in Scotland following the closure of Peterhead CCGT, though there are plans to possibly develop a 910MW carbon capture and storage equipped plant at the same site in the future. 

The reduction in synchronous generation could lead to system operability challenges with reduced short circuit levels and inertia. This potentially leads to increasingly dynamic network behaviour depending on factors such as weather conditions and price of electricity. To overcome this challenge, the NOA Stability Pathfinder Phase 2 successfully procured cost-effective services which will meet our immediate and future short-circuit level and inertia needs in the Scottish region. Contracts were awarded in mid-2022. 

With gross demand in Scotland not expected to exceed 6GW by 2030 (with an average embedded generation output of 1.7GW), generation capacity in Scotland far exceeds demand. Scotland will be expected to export power into England most of the time. 

With such high capacities of renewable generation connected in Scotland, under prolonged periods of low wind it is also credible that Scotland may need to import power from England due to the low synchronous generation capacity in the region. 

The anticipated increase in renewable generation in Scotland is increasing power transfer across the Scottish boundaries. On a local basis, with the anticipated generation development in the north of Scotland, including generation developments on the Western Isles, Orkney and the Shetland Islands, there may be limitations on power transfer from generation in the remote Scottish NETS locations to the main transmission routes (B0, B1a). The recent decision on Accelerating Strategic Transmission Investments (ASTI) by Ofgem included a 1.8GW Western Isles transmission link which will alleviate some of these limitations and facilitate the transfer of renewable generation to the mainland. 

As generation within these areas increases over time, due to the high volume of new renewable generation seeking connection, boundary transfers across the Scottish NETS boundaries (B0, B1a, B2, B3b, B4 and B5 and B6) increase. There are significant constraints observed across the B6 boundary (Anglo-Scottish border) which the ESO is looking to address via a commercial solution, the B6 Constraint Management Intertrip Service (CMIS). This has contracted with generators (totalling 2GW in capacity) in the region to provide a more economical method of managing constraints than actions through the balancing mechanism. 

Significant reinforcements are currently planned for the Scotland region, with some being actively accelerated following Ofgem’s decision on the Accelerated Strategic Transmission Investment (ASTI) framework, in order to facilitate the connection of large volumes of offshore wind from the HND and the upcoming HNDFUE. 

The need for any new network reinforcement to address potential capability issues will be evaluated in the upcoming HNDFUE and tCSNP2 following input from Transmission Owners and other interested stakeholders. Following the evaluation, the preferred reinforcements for the Scotland region will be recommended. 

Boundary regions

Click on the regions below to expand the boundary and understand its capability and challenges.

The graphs show a distribution of power flows for each of our Future Energy Scenarios, in addition to the boundary power transfer capability and NETS SQSS requirements for the next twenty years. ​

Each scenario has different generation and demand so produces different boundary power flow expectations. From applying the methodology in the NETS SQSS for wider boundary planning requirements (as discussed in ETYS and future Network Planning Process), we determine for each scenario:

  • The economy criteria - solid coloured line

  • Security criteria - dashed coloured line

  • Current boundary capability - solid black line

Due to the NOA being published after the ETYS, the boundary capability line (red line) is prepared from the NOA 21/22 Refresh optimal path released in July 2022. This is the best information available at the time of publication and will change annually and over time as the network, generation, demand and more importantly the NOA optimal path changes. More information about the NOA methodology can be found on the NOA methodology page. The 50%, 90%, Economy Required Transfer (RT) and Security RT are calculated from the 2022 FES and ETYS processes. Where the NOA transfer capability is not available, there is a black line that provides the current ETYS 2022 transfer capability.

Note: Boundary capability line is affected by the generation and demand profiles within each FES background. Therefore, the graphs are provided for indicative purposes only and cannot be directly compared. ​

The calculations of the annual boundary flow are based on unconstrained market operation, meaning network restrictions are not applied. This way, the minimum cost generation output profile can be found. We can see where the expected future growing needs could be by looking at the power flows in comparison with boundary capability.​

On each graph, the two shaded areas provide confidence as to what the power flows would be across each boundary:

  • The darker region shows 50% of the annual power flows

  • The lighter region shows 90% of the annual power flows​

From the regions, we can show how often the power flows expected in the region split by the boundary are within its capability (red line). If the capability of the boundary is lower than the two regions over the next 20 years, there might be a need for reinforcements to increase the capability. However, if the line is above the shaded regions, it shows that there should be sufficient capability here and that potentially no reinforcements are needed from a free market power flow perspective until the shaded regions exceed the capability (red line).

 

 

Boundary B0 separates the area north of Beauly, comprising the north of the Highlands, Caithness, Sutherland and Orkney. 

Boundary flows and base capability

The capability line (in red) is based on the recommendations from the NOA 2021/22 optimal path which uses the 2021 FES and ETYS data as inputs. The 50%, 90% Economy RT and Security RT lines are based on FES 2022. The ETYS and NOA methodologies for this boundary are different and can result in different transfer capabilities.

The current boundary capability is limited to 1.15GW due to a thermal constraint on the Beauly - Shin 132kV circuit.

The power transfer through B0 is increasing due to the substantial growth of renewable generation north of the boundary. This generation is primarily centred around both onshore and offshore wind. There is also the prospect of new marine generation resource in the Pentland Firth and Orkney waters in the longer term.

 

Boundary B2 cuts across the Scottish mainland from the east coast between Aberdeen and Dundee to near Oban on the west coast crossing the main north-south routes from the north of Scotland.

Boundary flows and base capability

 

The capability line (in red) is based on the recommendations from the NOA 2021/22 optimal path which uses the 2021 FES and ETYS data as inputs. The 50%, 90% Economy RT and Security RT lines are based on FES 2022. The ETYS and NOA methodologies for this boundary are different and can result in different transfer capabilities.

The current boundary capability is currently limited to 2.6GW due to a thermal constraint on the Fetteresso - Kincardine 275kV circuit.

The potential future boundary transfers for boundary B2 are increasing at a significant rate because of the high volume of renewable generation to be connected to the north of the boundary. This increased generation capacity will drive increasing power flows down the east coast 275kV circuits.​

The increase in the required transfer capability for this boundary across all generation scenarios indicates the strong potential need to reinforce the transmission system.​

The generation behind boundary B2 includes both onshore and offshore wind, with the potential for additional pumped storage plant to be located in the Fort Augustus area. The thermal generation at Peterhead lies between boundaries B1a and B2, as do several offshore windfarms and the proposed future North Connect interconnector with Norway.​

Boundary B3b encompasses the Argyll and Kintyre peninsula, and boundary assessments are used to show limitations on the generation power flow out of the peninsula.

Boundary flows and base capability

 

The current boundary capability is limited to 0.44GW due to a thermal constraint on the Inveraray - Sloy 132kV circuit.

The generation within boundary B3b includes both onshore wind and hydro generation, with the prospect of further wind generation resource and the potential for marine generation being connected in B3b in the future, triggering the requirement for future reinforcement of this network.​ 

B3b is not currently subject to NOA reinforcement options as current contracted enabling works for customer connections will increase the ability to export power from this region, effectively splitting the network in the South West and altering the boundary.​ 

Boundary B3b encompasses the Argyll and Kintyre peninsula, and boundary assessments are used to show limitations on the generation power flow out of the peninsula.

Boundary flows and base capability

 

The current boundary capability is limited to 0.44GW due to a thermal constraint on the Inveraray - Sloy 132kV circuit.

The generation within boundary B3b includes both onshore wind and hydro generation, with the prospect of further wind generation resource and the potential for marine generation being connected in B3b in the future, triggering the requirement for future reinforcement of this network.​ 

B3b is not currently subject to NOA reinforcement options as current contracted enabling works for customer connections will increase the ability to export power from this region, effectively splitting the network in the South West and altering the boundary.​ 

Boundary B4 separates the transmission network at the SP Transmission and SSEN Transmission interface running from the Firth of Tay in the east to the north of the Isle of Arran in the west.

B4 cuts across two 275kV double circuits, two 132kV double circuits, two 275/132kV auto-transformer circuits, two 220kV subsea cables between Crossaig and Hunterston substations, and a double circuit with one circuit at 400kV and the other at 275kV.

Boundary flows and base capability

The capability line (in red) is based on the recommendations from the NOA 2021/22 optimal path which uses the 2021 FES and ETYS data as inputs. The 50%, 90% Economy RT and Security RT lines are based on FES 2022. The ETYS and NOA methodologies for this boundary are different and can result in different transfer capabilities.

The current boundary capability is limited to 3.4GW due to a thermal constraint on the Westfield - Longannet 275kV circuit.

With increasing generation and potential interconnectors in the SSEN Transmission area for all scenarios, the required transfer across boundary B4 is expected to increase significantly over the ETYS period. The prospective generation behind boundary B4 includes around 2.7GW from Rounds 1–3 and Scottish territorial waters offshore wind located off the coast of Scotland.​ ​

In all scenarios in the FES, the power transfer through boundary B4 increases because of the significant volumes of generation connecting north of the boundary, including all generation above boundaries B0, B1a, B2 and B3b. This is primarily onshore and offshore wind generation, with the prospect of significant further offshore wind and new marine generation resource being connected in the longer term.​

 

Boundary B5 is internal to the SP Transmission system and runs from the Firth of Clyde in the west to the Firth of Forth in the east.

Boundary flows and base capability

The capability line (in red) is based on the recommendations from the NOA 2021/22 optimal path which uses the 2021 FES and ETYS data as inputs. The 50%, 90% Economy RT and Security RT lines are based on FES 2022. The ETYS and NOA methodologies for this boundary are different and can result in different transfer capabilities.

The current boundary capability is limited to 3.9GW due to athermal constraint on the Kincardine - Tealing 275kV circuit.

The generating station at Cruachan, together with the demand groups served from Windyhill, Lambhill, Bonnybridge, Mossmorran and Westfield 275kV substations are located to the north of boundary B5.​

In all the scenarios in the FES, the power transfer through boundary B5 increases because of the significant volumes of generation connecting north of the boundary, including all generation above boundaries B0, B1a, B2 and B4. This is primarily onshore and offshore wind generation.​

Boundary B6 separates the transmission network at the SP Transmission and National Grid Transmission interface running roughly along the border between Scotland and England.

Boundary flows and base capability

The capability line (in red) is based on the recommendations from the NOA 2021/22 optimal path which uses the 2021 FES and ETYS data as inputs. The 50%, 90% Economy RT and Security RT lines are based on FES 2022. The ETYS and NOA methodologies for this boundary are different and can result in different transfer capabilities.

The current boundary capability is limited to 6.3GW due to a thermal constraint on the Harker – Moffat 400kV circuit

Across all FES, there is an increase in the power transfer requirements from Scotland to England due to the connection of additional generation in Scotland, primarily onshore and offshore wind.​

With the FES including many wind farms in Scotland, the spread of boundary power flows is very wide due to the intermittent nature of wind generation. With low generation output in Scotland, it is credible to have power flowing from south to north feeding Scottish demand, particularly on closure of the remaining nuclear plants north of the boundary such as Hunterston B which was decommissioned in 2022. The magnitude of the south to north power flows is low compared to those in the opposite direction so network capability is sufficient to support those conditions.​

While the south to north transfer capability is enough to meet demand in Scotland, it is still necessary for conventional synchronous plant to remain in service in Scotland to maintain year-round secure system operation.​