Our ETYS analysis

National Electricity Transmission System (NETS)

As NESO, we are responsible for the system operation of the electricity transmission networks in England, Wales, Scotland and in the surrounding offshore waters. 
The National Electricity Transmission System (NETS) spans GB and is comprised of overhead lines as well as underground and subsea cables, at 132 kV, 275 kV and 400 kV voltage levels.
These are all linked together via substations across GB which connect separately owned generators, interconnectors, large demands, and distribution systems. Electricity Distribution comprises of network below 275 kV in England and Wales and below 132 kV in Scotland respectively. It transports electricity from transmission “highways” to the end consumer. 
GB has three onshore transmission owners that separately own the network, as shown in figure 1. These are: 

  1. Scottish & Southern Energy Networks Transmission - owning the network in the north of Scotland.
  2. Scottish Power Electricity Networks - owning the network in the south of Scotland.
  3. National Grid Electricity Transmission – owning the transmission network in England and Wales.

There are also 28 licenced offshore transmission owners (OFTOs) appointed through Ofgem’s competitive tendering process. They own the assets that connect operational offshore wind farms to the transmission or distribution network. 

Together with the transmission owners, we regularly assess and model network behaviour under different conditions to test its resilience. We assess where the transmission, network may be under strain for various reasons and this analysis accumulates into the ETYS. This ensures we portray an accurate representation of the current transmission capabilities and identify future requirements.

Three TO's - ETYS

Boundary analysis

Learn more about how we analyse the transmission network: 

What is a boundary? 

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. Figure 2 shows all GB network boundaries. 

Defining the boundaries has evolved over many years of planning and operating the transmission system. When significant changes occur, new boundaries may be defined and some boundaries either removed or amended, and we communicate any changes with our stakeholders.

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

The boundaries used by ETYS 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 have a 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 System Security and Quality of Supply Standard (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.

GB boundaries ETYS

Determining boundary capability

Boundary capability is the maximum power flow that can be securely transferred across the boundary while meeting GB’s Security and Quality of Supply Standards. 
The SQSS has fixed rules for planning and operating the Main Interconnected Transmission System (MITS). The MITS must be kept secure against credible events such as network faults and loss of generation, while making sure circuits are not overloaded, voltages and frequency are kept within SQSS planning limits. 
For each boundary, we work with the transmission owners to undertake power system analysis, determining the boundary capability. Limiting factors on transmission capacity include:  

  •  thermal circuit rating  
  • voltage constraints  
  • dynamic stability  

For the ETYS network assessment, contingencies are applied, and the most severe SQSS limitation is used to determine the network boundary capability. The base capability of each boundary can be seen in the Electricity Transmission Network Requirements section.

Operability overview

Operation of the NETS is becoming increasingly complex as the system is transitioning from predominantly synchronous plant to a higher proportion of inverter-based and intermittent resources. Our latest Operability Strategy Report , published in March, sets out the principal operability challenge, including the methodology used to assess voltage and stability requirements and how this methodology may evolve in the future. Going forward, we will publish operability requirements as part of CSNP process.