The ETYS is the ESO’s view of how the future transmission system requirements will be met. Currently, this focuses mainly on bulk power transfer requirements, particularly during winter-peak conditions. 

As operation of the National Electricity Transmission System (NETS) becomes increasingly complex, we need to expand our view of system needs across the year. This will require the use of innovative analysis techniques and the development of new tools. We are looking to bring together our work on year-round thermal analysis, our development of new tools for voltage and stability, and learning from the success of our NOA pathfinders to inform how we can communicate a wider set of system needs in ETYS.  

As part of the ongoing Network Planning Review, we are exploring how we can further enhance our communication of system needs in ETYS. This includes thermal year-round needs and voltage and stability needs, at a high-level, in line with our ongoing tools development. We summarise below the progress to-date and how you can get involved to share your feedback. 

We would like to hear your views on how we can improve the ETYS, we have set up a short survey to gather feedback. 

Take our improving ETYS survey

We have continued to develop our year-round thermal tool, POUYA (Power Uncertainty Year-round Analyser). It allows us to capture the power transfer limitations on the GB NETS not only during the winter but also across the other seasons in a year, driven by the seasonal characteristics of low carbon energy resources and circuit ratings as well as network outages for construction/maintenance. 

Where are we now? 

In our March 2022 report on the POUYA tool we shared an update on the tool’s development and some initial results demonstrating POUYA’s ability to identify boundary limitations under different seasons. 

As we take further steps in the development of year-round thermal analysis, we are now exploring a range of ideas on how we can communicate year-round thermal needs to support the development of our ‘business as usual’ process. We see the communication of year-round needs as serving the following purposes: 

  • Providing insight to our stakeholders into limitations or network needs across other seasons. 
  • Identifying additional scenarios that should be considered for detailed analysis. 
  • Identifying opportunities for targeted network or commercial solutions. 

This year, we are presenting a few examples of how we can communicate year-round thermal system needs. We would like to hear your thoughts on what additional information you would find helpful. Our new and improved version of the POUYA tool is still undergoing development and validation, therefore the examples presented here are based on our previous POUYA analysis using generation, demand, and network backgrounds from year 1 ETYS 2021. 

The examples we showcase provide insights on: 

  • Seasonal variations of thermal overloading. 
  • A wider view of limiting assets and their potential impacts on constraints. 
  • Multiple contingencies and their likelihood to cause network constraints. 
  • Year-round boundary transfers (overloaded/not overloaded). 

Ongoing development 

Certain capabilities of the POUYA tool are still under development in preparation for future ETYS publications. These additional capabilities include: 

  • Enabling our year-round system needs assessments to consider scheduled outages of transmission assets, more closely aligning our network planning studies with operational considerations. The results presented here have been conducted on an intact transmission network. 
  • Indicative constraint costs for individual overload occurrences and contingency scenarios. 
  • Developing data insight to identify underlying drivers for network limitations and identify scenario trends. 

We will continue to develop this process over the next year and transition these practices into our ‘business as usual’ processes. We will be exploring how to integrate the year-round system needs assessment into our existing options assessment process, to further improve our network investment decision making process. 

System needs on a geographic map

The image below shows a geographical representation of the transmission assets which experienced overloading across all seasons for the B6 contingencies. We indicate the constraint hotspots using a heatmap key with the impact of the constraint representing a measure of the likelihood and severity of the overloading on that asset. In the above example, Harker SGTs were identified as the constraint with the ‘highest impact’ across the four seasons. This limiting asset was the same as identified in our deterministic winter peak ETYS 2021 studies. A geographic map can provide readers with a visual representation of constraints bottlenecks that could help with the development of targeted network solutions.

POUYA constraint hotspots for the B6 boundary across the year

Download more examples with individual season information 


Relative frequency of circuit/asset overload

The image below shows the relative frequency of overloads experienced by individual transmission assets (the percentage of overloads occurring on that asset as a percentage of the total overload occurrences over the whole year or individual season). Using this we can identify and communicate not only the most severe limiting asset but also identify other assets which are limiting boundary capability across the range of snapshots.

The second most frequently overloaded assets other than the Harker SGTs identified was the Harker – Moffat 400kV single-circuit, this is the limiting asset identified for the B6 boundary in the ETYS 2022 studies (following the implementation of the HAEU reinforcement which changed the banking arrangement on two SGTs at Harker, alleviating some of the constraint on the SGTs at Harker 400kV substation).

Relative frequency of overloads observed for individual assets, considering B6 contingencies, across the year  

Download more examples with individual season information


Relative frequency of overloads by contingency

This representation helps identify some of the critical contingencies for a given boundary together with an indication of how overloading frequency differs across the seasons. As the generation background, demand profile, and asset thermal ratings change across the year, the frequency and severity of the overloads occurrences across the network will differ across the seasons. From our POUYA studies, in the summer and spring periods, lower network flows generally result in less frequent overload occurrences. Autumn and winter experience much greater volumes of thermal overloading at a similar frequency to one another.

Boundary flows across B6 are on average just 3.5% lower in autumn than winter, whereas asset thermal ratings can be the same or up to 20% lower in autumn than in winter.  This means that although the volume of flows in autumn may be lower than in winter, it is possible that a similar or higher frequency of overloading may be observed in autumn than winter and will largely depend on the seasonal thermal rating of individual assets.

Relative frequency of overloads observed following each contingency across B6, split by season 


Boundary Transfers (overloaded/not overloaded)

For each snapshot studied by POUYA the boundary transfer is determined. By considering whether any assets are overloaded under that snapshot we can define this boundary transfer as ‘overloaded’ or ‘not overloaded’.

 Expected frequency for different volumes of power transfer across B6 over the year, split by acceptable and unacceptable outcomes according to SQSS compliance limits 

Download more examples including individual season information

As shown in the above image the overloaded and not overloaded outcomes may overlap, and acceptable boundary transfers may occur at flows higher than the single snapshot winter-peak boundary capability.

For example, for the indicated 6.1GW winter-peak capability of B6, our test POUYA data identified 137 snapshots where this capability would be insufficient to meet boundary requirements. This means that for 137 hours (1.5%) of the year, additional boundary capability would be required. However, because this additional capability requirement is not persistent throughout the year, short-term operational activities (e.g., via the balancing mechanism) could satisfy this requirement in a more economical manner.

As indicated in the ‘Ongoing developments’ section above, we are still developing functionality to allow POUYA to calculate constraint costs associated with relieving specific overloads. As we expand our analysis to capture not only frequency and severity of overloads but also consider likely economic (constraint) impact, we hope to be able to further explore some risk-based approaches.

Our ambition is to communicate a long-term view of year-round voltage needs in ETYS with more specific procurement needs communicated through our reactive markets which are still under development. We are currently developing a new enduring process for voltage needs assessment which will evolve how we identify voltage management requirements, communicate them to industry, and procure services to fulfil these requirements. 

Where are we now? 

The ESO has undertaken a review of the processes for identifying voltage needs and how some of these needs could be integrated into the ETYS. Currently, voltage requirements across the NETS are addressed through two processes. Our ETYS/NOA process addresses low voltage issues which typically arise in a winter peak demand scenario, whilst high voltage issues have been addressed through our High Voltage NOA Pathfinder process which focuses on a summer minimum demand scenario. These processes provide investment signals and procurement (for NOA Pathfinders) of solutions to address high or low voltage issues however, we would like to make the approach to the identification and procurement of voltage needs more consistent. 

Furthermore, we have also been publishing a GB Voltage Screening Report every June to indicate areas that could face voltage issues in the future. Going forward we will integrate voltage screening results into ETYS and expand this to capture the high-level needs once our tools are further developed. 

Voltage enduring process  

To develop an enduring repeatable process, we have mapped out the key components of our future end-to-end voltage process and identified areas that require further development to allow us to achieve an enduring state.

Overview of our proposed enduring process for voltage analysis

Inputs: The voltage analysis process will take generation and demand profiles from the FES scenarios which will be fed into our market dispatch tool to produce a credible year-round economic dispatch that is applied to our network models. We are in the process of reviewing the methodology with the Transmission Owners and we are progressing actions to improve our MVAr demand forecasts.

Tools and Analysis: Our current approach is manual and time consuming and will not be suited for analysis of multiple year-round snapshots. Voltage analysis tools have been tested as part of a proof of concept to help automate the analysis process for multiple year-round analysis. These tools should help identify both high or low voltage issues concurrently and communicate them on a geographic map. A full project is being taken forward to deliver a voltage optimiser tool by the end of 2024.

Outputs: We would like to provide more clarity on how we will communicate system needs in the future. Once our tools are delivered, we envision that the ETYS will provide a high-level long-term view of needs highlighting trends in system needs and areas requiring more detailed analysis that will be undertaken through our medium-term process. The medium-term will involve more detailed analysis to identify specific needs for which we need to run procurement events ensuring sufficient lead-time to address the need. These needs will be communicated through the future reactive markets, once established.

Our new process will ensure that we capture all voltage system needs and procure services to reduce possible voltage constraint issues, driving down constraint costs and ensuring that we can operate the system in a secure and reliable manner.

Our ambition is to communicate a long-term view of year-round stability needs in future ETYS publications with more specific procurement needs communicated through our stability market which is still under development. Further work is still ongoing to develop tools and techniques to allow year-round long-term stability analysis (via automation). While we develop our analytical capabilities, we will continue to communicate stability needs via our dedicated stability pathfinder page where we currently communicate stability needs when they are identified. Once our tools and processes mature, we will integrate a long-term view of stability needs within ETYS.  

Where are we now?

The identification of stability constraints is more complex, and work is still ongoing to further test and develop tools to allow us to undertake year-round stability assessments.

In spring 2022, we concluded our Network Innovation Allowance (NIA) project with energy consultants TNEI to build a machine learning (ML) tool for labelling stable and unstable system conditions. This innovation project demonstrated the capability for such a tool to be built but highlighted some key difficulties in implementing it with our current models, data and systems. The key challenges included:

  • Challenges establishing feasible solutions for the ETYS network model for year-round conditions.
  • Challenges with data quality relating to dynamic characteristics of generators.

The NIA project successfully demonstrated a proof of concept, but the above challenges prevented the project from completing its aims of training a tool to run on the full GB system. Further development and testing is ongoing, including enhancing our modelling data to allow us to test and identify key functionality and enhancement required to run year-round scenarios on our network models. We will continue to provide you with updates as we develop this work.

Over the past year, we have undertaken a review on whether system needs at the Transmission/Distribution (T/D) interface could be communicated within ETYS. We have worked with the Transmission Owners (TO) and Distribution Network Owners (DNO) as part of the Electricity Networks Association (ENA) Open Networks WS1B Product 5 on the Network Development Plan to review whether system needs at the T/D interface should be published and if so, where they could best be communicated.

Following publication of the DNO Network Development Plans (NDP), stakeholders have requested for more information to be published from a transmission interface perspective to help complement the Network Headroom reports. Stakeholders feel that because the NDP only indicates distribution constraints it could be potentially misleading regarding capacity constraints and an additional view on transmission would give a fuller picture.

TOs have shared some of the work they have done previously to indicate Grid Supply Point (GSP) capacities based on contracted connections, and the importance of not just indicating capacity at the GSP but also constraints on the wider network. Previous experience of trialling a range of approaches around communicating needs at the transmission interface has meant that TOs have opted not to publish any GSP ‘capacity’ type data based on feedback received from their stakeholders on its accuracy. Also, from a connections-perspective, the challenge with communicating ‘limits’ or ‘capacity’ is that the data becomes obsolete as soon as published because the connections process is live and always changing.

The TOs agreed to assess the feasibility of providing a snapshot of demand/generation of the earliest in-service date (EISDs) for all GSPs on an annual basis to DNOs for their NDPs (specifically to add to the Network Scenario Headroom Report tables). An output in March 2023 could potentially be used by DNOs to supplement their May 2023 Network Scenario Headroom Report tables.

ESO supports the proposed approach to communicate a view of T/D interface EISDs at the point of publication of DNO Network Scenario Headroom Reports. As this is a single snapshot view, it provides direct value when aligned to the DNO publication timelines as the ETYS publication is later in the year and would not provide the full context compared to publication alongside the DNO data sets. For more information on this, please see the Form of Statement of Network Development Plans - 2022 Update published by ENA Open networks in December 2022.