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2. Network development inputs
ETYS 2020

2. Network development inputs

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.

Click on the cards below to see the various stages before the network capabilities can be determined.

1. Future Energy Scenarios

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We use the scenarios in the FES to determine the peak demand and generation capacity in various regions within the different scenarios.

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2. Apply dispatch criteria to set the background

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Dispatch the generation and interconnectors  from the FES to balance with the peak demand (from step 1) to determine network power flows.


3. Apply the NETS SQSS planning criteria

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Simulate network behaviour using the FES dispatched network model and NETS SQSS planning conditions.


4. Prepare the Network Model

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Apply the dispatch background to complete models of the GB NETS so circuit loading and conditions can be simulated.


5. Determine network capability

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Adjust boundary power transfers using the model until the limit of network capability is found within the SQSS limitations.


Future Energy Scenarios

Future Energy Scenarios (FES) represent a range of different, credible ways to decarbonise our energy system as we strive towards the 2050 target. 

To help us plan for an uncertain future, we develop a range of scenarios reflecting a number of credible energy futures, which helps us to better understand the uncertainties facing the energy industry.

This year there are four scenarios aligned to the following axes:

  • speed of decarbonisation – combines policy, economics, and consumer attitudes.
  • level of societal change - allows us to explore different solutions for decarbonisation of heat (e.g. electrification vs low carbon gas) alongside changes in consumer engagement, levels of energy efficiency and a ‘supply-led vs demand-led’ approach.

All scenarios show progress towards decarbonisation from today.

The scenarios in the centre of the matrix meet the net zero target in 2050 as legislated by the UK government, and those on the right and left represent the credible range of decarbonisation progress by meeting the target early and missing the target respectively. 

Scenarios close to the bottom of the axis involve lower levels of energy efficiency improvements, less change of heating technology (including continued use of the gas network) and lower levels of consumer engagement in flexibility services.

Scenarios closer to the top of the axis involve greater impact on consumers, with greater changes in heating systems and insulation and more consumer appetite for participation in provision of flexibility to help manage peak demand and intermittent generation. 

In the ETYS, we use the generation and demand based on the assumptions in the scenarios to develop a range of credible power flows on the network.

This is fed into our network model as the background to analyse the capability of the NETS.

Learn more about FES 

National Electricity Transmission System (NETS)

As the ESO, we are responsible for managing the system 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.

The ‘transmission’ classification applies generally to 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 in GB:

Electricity transmission map

The offshore transmission systems are also separately owned.

There are seventeen licenced offshore transmission owners (OFTOs) that have been appointed through the competitive tendering process administered by Ofgem.

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

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

This is done to make sure that the ETYS portrays an accurate representation of the current transmission capabilities and identifies any 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 need to 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 the rest of 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.

We use our FES to help us decide on credible ranges of future NETS requirements and present capability.

This is done using the "system boundary concept". It helps us to calculate the NETS’s boundary capabilities and the future transmission requirements of bulk power transfer capability.

The transmission network is designed to make sure there is enough transmission 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 the network assessment, the lowest known limitation is used to determine the network boundary capability.
The base capability of each boundary can been seen here.

This will be used in the NOA 2020/21, to help us assess the reinforcement options that will address the potential future NETS boundary needs.

Defining the NETS boundaries has taken many years of experience in planning and operating the transmission system.

Click here to download the map

When significant transmission system changes occur, new boundaries may be defined and some existing boundaries either removed or amended (we'll be transparent about any changes).

Some boundaries are also reviewed but not studied because of no significant changes in the FES generation and demand data of the area from the previous years. The same capability as the previous year is assumed 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

Encompass small areas of the NETS with high concentration of generation. These small power export areas can give high probability of overloading the local transmission network due to coincidental generation operation. 

Wider boundaries 

Split the NETS into large areas containing significant amounts of both generation and demand. The SQSS boundary scaling methodologies are used to assess the network capability of the wider boundaries.

These methodologies consider both the geographical and technological effects of generation. This allows for a fair and consistent capability and requirements assessment of the NETS.

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

The security criterionThe economy criterion

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

(The methodology for determining the security needs and capability are as per the SQSS Appendices C and D). 

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

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

The methodology for determining the economy needs and capability are as per SQSS Appendices E and F.

Over the years, we have continuously developed the transmission network to ensure there is sufficient capacity to transport power efficiently and economically across the country.

Jump to another section of the ETYS

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1. What is the ETYS?

Find out about the ETYS and how this fits with our entire planning process

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2. Network Development Inputs

Here is more information on how we prepare the ETYS

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3. Electricity Transmission System

You can find out about the various regions of the transmission system and their capabilities

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4. Year-round probabilistic analysis

You will find how we are applying year round conditions to assess the capability of the system

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5. Way forward

Find out about how we are going to improve the ETYS and how you can get involved

6. Further Information and appendices

Get to the appendices, glossary and other helpful contact information

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