ETYS and the Network Planning Process

To identify the future transmission requirements of the National Electricity Transmission System (NETS), there are several inputs that are fed into the planning process and at various stages.

The process starts with the Future Energy Scenarios (FES). These are a credible range of scenarios for how energy will be produced and consumed up to 2050. These scenarios form the foundation of our studies and analysis, and we use them to determine the peak demand and generation capacity regionally.

We determine the winter-peak network flows of the GB NETS by dispatching the generation from the FES to balance with peak demand. Network behaviour is simulated according to the NETS SQSS planning conditions to determine network conditions such as circuit loading and voltage levels.

We adjust boundary power transfers until the limit of network capability is found within the SQSS limitations.

Finally, looking at the capability of the different network boundaries and the expected future boundary flows, we identify points on the transmission network where more transfer capacity is needed to continue to deliver electricity reliably.

The Network Options Assessment (NOA) follows the ETYS.

Once we know what the network requirements are, we invite stakeholders to propose solutions that will be needed to meet these requirements. These proposals are then assessed through our NOA process, where the most economic and efficient solution is given a recommendation to proceed, and others told to hold or stop.

In NOA 2020/21, covering the next 10 years, we recommend investing £183 million this year to develop 41 asset-based projects worth £13.9 billion and four ESO-led commercial solutions that could provide an additional £2.1bn in consumer benefit.

Boundary analysis

Learn more about our analysis:

National Electricity Transmission System (NETS)

As the ESO, we are responsible for the operation of the transmission networks in England, Wales, Scotland and offshore. 

The NETS is mainly made up of 400kV, 275kV and 132kV assets connecting separately owned generators, interconnectors, large demands and distribution systems.

Here, ‘transmission’ generally means assets at 132kV or above in Scotland or offshore, but in some cases includes other lower voltage assets. 

In England and Wales, it relates mainly to assets at 275kV and above.  There are three onshore transmission owners (TOs) in Great Britain:

The offshore transmission systems are also separately owned.

There are 22 licenced offshore transmission owners (OFTOs) appointed through Ofgem’s competitive tendering process.

They connect operational offshore wind farms given Crown Estate seabed leases in allocation rounds.

The TOs and ESO work together to reflect real world changes in network modelling to accurately assess network behaviour under differing conditions.

Together with the transmission owners, the ESO works to make sure the assumptions made in the analysis are acceptable and any changes in their networks are reflected correctly in the network models.

This ensures the ETYS portrays an accurate representation of the current transmission capabilities and identifies future requirements.


A boundary splits the system into two parts, crossing critical circuit paths that carry power between the areas where power flow limitations may be encountered.

When we assess future requirements, we bear in mind that we have many signed contracts for new generation to connect to the NETS.

In addition, the development of interconnectors connecting Great Britain to mainland Europe will have a big impact on future transmission requirements. 

We do not know precisely how much new generation there will be, and where it will connect, or when existing generation will shut down.

Our FES helps identify credible ranges of future NETS requirements and present capability.

This is done using the ‘system boundary concept’, which calculates boundary capabilities and the future requirements for bulk power transfer.

The transmission network is designed to provide enough capacity to send power from areas of generation to areas of demand. 

Limiting factors on transmission capacity include: 

  • Thermal circuit rating 
  • Voltage constraints 
  • Dynamic stability 

From our network assessment, the lowest limit determines the network boundary capability.

The base capability of each boundary can be seen in the Electricity Transmission Network section.

This will be used in the NOA 2021/22 to assess reinforcement options to address potential future boundary needs.

Defining the NETS boundaries has evolved over many years of planning and operating the transmission system.

Click here to download the map

When significant changes occur, new boundaries may be defined and some boundaries either removed or amended (we are transparent about any changes).

We do not study all boundaries, specifically those where no significant changes in the FES generation and demand data have been identified in previous years. We assume the same capability as the previous year for these boundaries.

Determining the present capability and future requirements of the NETS boundaries

The boundaries used by ETYS and NOA can be split into two different types:

Local boundaries

Small areas of the NETS with a high concentration of generation. These small power export areas can give high probability of overloading the local transmission network due to too much generation operating simultaneously. 

Wider boundaries 

Large areas containing significant amounts of both generation and demand. The SQSS boundary scaling methodologies assess the capability of the wider boundaries.

These consider both the geographical and technological effects of generation, allowing for a consistent capability and requirements assessment.

We have continuously developed the transmission network to provide sufficient capacity to transport power efficiently and economically across the country.

The NETS SQSS defines the methodology to assess boundary planning requirements, based on:

The security criterionThe economy criterion

The boundary transfer requirements needed to satisfy demand without relying on intermittent generators or imports from interconnectors.

The methodology for determining the security needs and capability is set out in SQSS Appendices C and D.

The boundary transfer requirements when demand is met with high output from intermittent and low-carbon generators and imports from interconnectors.

This ensures capacity is adequate to transmit power from highly variable generation without any network constraint.

The methodology for determining the economy needs and capability are found in SQSS Appendices E and F.

Discover more about ETYS

ETYS and the Network Planning Process

Learn about how we prepare the ETYS and how it fits with our network planning process.

Electricity transmission network requirements

You can discover more about the various regions of the transmission system and their capabilities.

Way forward

Read how we are going to improve the ETYS and how you can get involved.

ETYS documents

Get to the appendices, glossary and other helpful contact information.