Assessing Transmission Access Restrictions and Curtailment Impacts

Assessing Transmission Access Restriction Curtailment Impacts on Your Site Production

In recent years the GB electricity transmission and distribution networks have seen unprecedented demand for new connections as developers of, and investors in, new electricity generation and energy storage look to plan and build out new projects. This in turn has seen dates for connections extend due to the need for transmission network reinforcement, with many projects receiving connection dates into the early, or even late, 2030s.

The National Energy System Operator (NESO), OFGEM, and Distribution System Operators (DSOs) are collaborating on various initiatives and plans that will accelerate connection timescales for customers.  Without delving into the detail of all plans and initiatives, this blog focuses on one initiative that accelerates timescales for some who have applied for transmission connection agreements, offering early connection by the NESO but with temporary or enduring capacity access restrictions.  

Accelerated Transmission Connection Agreements with Defined Access Restrictions

Until recently, developments with transmission connection offers - subject to transmission upgrade works - must wait until the upgrade works are complete before being connected and allowed to export or import electricity. This is on the basis that the connection is “Firm” and that qualifying access restrictions to the transmission network for power import or export would be compensated.  However, the NESO is now offering accelerated connections through connection offers with uncompensated temporary access restrictions while upgrade works are progressed.  In some cases, there may also be some enduring access restrictions post upgrade works.

Connection access restrictions are typically listed in Appendix D (or Appendix DD when specific to Accelerating Storage Restricted Connections) of the connection agreement offered by NESO, setting out if restrictions will impact site import, export or both. In this process, the constrained circuits have been identified using forecast impacts of the Transmission Entry Capacity (TEC) register applied to the expected National Electricity Transmission System (NETS) developments. Then a “cardinal point” analysis studies the worst-case network power flows i.e. a set of high stress operational periods such as Winter Maximum Demand is coupled with a view of how system demand would be met via generation. If network constraint (capacity overloads) is identified during this analysis process, then the relevant circuits are associated within Appendix D.  

These restricted access circuits listed in Appendix D or DD may be classed as not being “Interruption” or not being “Relevant Interruption ” and hence would be uncompensated access restrictions.  

Appendix D/DD typically includes a number of parts. Part 1 lists identified circuits on the NETS that are relevant to the access restrictions, usually with reference to a network diagram. 

Groups of relevant circuits can be set out in network development phases where the groups of relevant circuits vary in each phase reflecting changing transmission network topologies. Estimated dates for the start and finish of each network topology phase are indicated usually with a target date and a backstop date. Each phase and the associated network access restrictions and dates, all require to be considered when estimating the impact on site production.

Part 2 typically sets out the relevant outage conditions that will result in restricted site access to the transmission network, including reductions down to zero MW export limitation. Outage conditions are described for each circuit, typically indicating if outages relate to planned (maintenance upgrades), unplanned (faults), or both and whether related to the circuit availability in part or in whole.  

Part 2 will also indicate if restrictions relate purely to circuit outages or to circuit loading thresholds being breached. This last point is important when defining the method of assessing site transmission access restrictions for import or export. If circuit loading conditions are indicated, then the NESO  operational decisions to restrict a sites access is not just a case of assessing the likely relevant circuits going out of service.  It is also necessary to assess power flows on circuits listed with loading thresholds including the normal network intact topology (normal for that phase of the upgrade works which will be outlined in Part 1 of Appendix D/DD).  As a simplification, assuming circuit outages will always cause a breach of circuits with loading thresholds when assessing transmission access impacts on site production can be considered, however, this runs the risk of significantly over estimating curtailment impacts. This simplification of impact assessment may be acceptable in some cases depending on the level of accuracy required but, in some cases, may result in overly conservative site production impacts. 

What Does this Mean for Developers and Investors?

The NESO offer of an accelerated connection date may bring financial benefits to a development, but this should be considered against the impact of associated access restrictions. It may be preferable to delay site construction to align with completion of transmission system upgrades rather than to invest earlier in site construction to align with an accelerated connection date, albeit with associated transmission access restrictions. Understanding the financial implications of each option can be a key investment decision and essential for parties looking to attract investment. Hence, it is important to fully understand the accelerated connection offer access restrictions and what they may mean in terms of limited site network access, reduced energy production and how this relates to market generated revenues.

The starting point is to understand what the accelerated connection offer Appendix D/DD restrictions mean. Key questions include:

• Do restrictions impact site export, import or both?
• Are restrictions uncompensated
• What are the start and finish dates for restrictions including phased network topology changes and enduring restrictions?
•  Do restrictions apply to identified circuit outages only including planned and unplanned outages or are there circuits with loading thresholds to consider? 

Having established a clear understanding of the access restrictions, the question then becomes one of how best to assess the impact of these restrictions on site energy production. There are a number of approaches that can be considered:

1. Isolated Outage Study: A high level approach which estimates the outage numbers and durations for each relevant circuit listed in Appendix D/DD, aggregates the resulting site impact based on an assumption of full site import/export curtailment for each outage across the period or periods of access restriction. Forecast outages for each relevant circuit can be estimated and achieved using historical outage data (requested from the NESO) for each circuit, alongside future planned outage data that may be available.  
   
   This study approach may be sufficient to provide an initial indication of whether to accept an accelerated connection offer or not. It may however lack accuracy in approximating the impact of constraint on site production, particularly where circuits are impacted by Appendix D/DD loading thresholds. As a result of this accuracy limitation, SGS does not adopt this approach, rather employing one of the following approaches, depending on the type of access restrictions listed in Appendix D/DD.
   
   
2. Probabilistic Outage Study: A more detailed approach, suited to restricted access cases with outage impacts, is to adopt probabilistic methods. This is an area that SGS has many years of experience in, studying the frequency, timing and duration of both planned and unplanned outages. This extensive experience is derived from studies assessing the impact of inter-trip schemes triggered by circuit outages, or in some cases, by power flow restrictions such as Load Management Schemes (LMS). Typically, SGS probabilistic studies consider a number of scenarios to help understand the relevance of unplanned, short duration planned (maintenance) and long duration (upgrade work) outages and combinations of these.  
   
   The probabilistic method applies Monte-Carlo stochastic simulation to derive sample-based approximations of annual outages. The Monte Carlo method derives a sample set of annual outages, derived by sampling from a probabilistic model of outage incidence and frequency. This approach derives an average year of curtailment across the sample set (typically thousands of study years are simulated). 
   
   In addition to approximation of an ‘average’ annual impact of restricted access, such an approach can provide curtailment approximation for other confidence levels, e.g. P90 where curtailment will be less than X% in 90% of the sample years or P10 where curtailment will be less than Y% in 10% of the sample years. This approach can be combined with results from time-step load flow analysis  to align impacts with load related triggers such as those associated with LMS. This suits cases where site network access restrictions are power flow related as well as circuit outage related.
   
   
3. Full Transmission Access Restriction Study: Applied to cases where Appendix D/DD  lists access restrictions under conditions of both circuit outage and remaining circuits exceeding power flow thresholds. In this case for representative, accurate, study a combination of probabilistic and network load flow analyses must be employed. 
   
   The full transmission access restriction study is delivered in two stages:
   
   •  Stage 1: Constraint Assessment. Time-series assessment of network conditions under defined transmission topologies (modelling intact and Appendix D/DD specified system outages). This identifies periods in the year where transmission system constraints may arise and thus trigger NESO disconnection/curtailment of the site. 
   
   • Stage 2: Probabilistic Operational Assessment. This assessment collects historical datasets about the outages
   that are detailed in Appendix D/DD and performs a probabilistic assessment to derive scenarios reflecting likelihood and duration of outages. This stage combines the probability of outage with the Stage 1 constraint-assessment to achieve an operational estimation of site unavailability and impact on site production. 

This two-stage approach requires suitable modelling of the GB transmission network which is complex and is described later in this blog.  

The Full Transmission Access Restriction Study is a more complex and time consuming approach, however this is essential in cases where the NESO has stipulated circuits with power flow restrictions and where developers/investors require a robust and more detailed approach to study.  

The above methods are sufficient in deriving estimates of transmission access restrictions in terms of curtailed MWh energy. For developers and their investors, the key question is how will this energy restriction translate to a financial revenue impact? Time-series breakdown of site curtailment can be aligned with scenario-based market prices to estimate the financial impacts of curtailment on an hourly, monthly or annual granularity. In our experience at SGS, we have supported both developer-delivery of this financial analysis, or through agreement of forecast market data for use then directly performed financial approximation. 

Combined Circuit Outage and Power Flow Limit Access Restrictions: Importance of “Full Transmission“ Study

Successful delivery of the Full Transmission Access Restriction Study method requires appropriate modelling of the GB transmission network and estimation of how generation will be dispatched across the network in the future.  
While it is a relatively straightforward to create a simulation model for smaller regions of network (e.g. distribution networks, corners of the transmission system), the process of creating and maintaining a simulation model of the full GB transmission system is more complex. Improvements in both network parameter datasets and future development plans have eliminated areas of uncertainty that previously restricted the ability to build a working and representative transmission network model.

For each time step across the study period, e.g. half hourly over a study year, simulating network load flow requires modelling of NESO/market dispatch of generation to meet demand while accounting for various factors including:

• Network Topology: The transmission state inclusive of all upgrade work completed within the analysis period.
• Thermal Constraints: The operational limits applied to Overhead Lines and Cables associated to pre-fault limits or boundary constraint limits, whichever is lower.
• Stability Constraints: The operational inertia requirement the network must maintain to satisfy present or future SQSS  requirement.
• Voltage Constraints: The operational voltage statutory limits to satisfy SQSS requirements.
• Wind and Solar Decisions: The profiles applied to wind and solar sites to determine the range of their potential yearly export per time step. Example: set to a historic yield.
• Dispatch Decisions: The profiles applied to market-based generators, normally set to Transmission Entry Capacity (TEC) and price graded against other tech types.  Example: nuclear (low cost), gas (medium cost), coal/oil (High Cost).
• TEC Register: The connections considered as part of the analysis period, where the effective connection date is used to consider new generators coming online within the study period. A method can then be applied for attrition in the TEC register to account for generators that will not connect according to the stated date.
• Grid Supply Point (GSP) Distributed Network Operator (DNO) Behaviour: The demand at the GSPs inclusive of erosion from embedded generation.
• Interconnectors: The behaviour of external markets. Normally set to historic or where no history is identified, a set of rules can be applied. The historic year chosen to align with Solar and Wind historic performance to achieve a link between market conditions internally driving external market behaviours. 

In our simulation of transmission system operation in each half-hour, SGS implements an optimisation known to be used by the NESO, the Security Constrained Economic Dispatch (SCED) optimisation. This approach ensures system demand is met by dispatching generators in merit order (derived from generation cost), whilst ensuring that the dispatch schedule does not cause stability issues or steady-state thermal overloads. 

The Constraint Assessment performs half-hourly study of network conditions and identifies periods where overload may occur on assets close to the generation site, essentially identifying the time-periods where the generator site under study may be liable for disconnection via NESO operator action.

Through delivery of this complex method to customer transmission studies over the last few years, the SGS team continue to refine the methodology, updating study assumptions and network model datasets as new information is shared.  

What is SGS Doing to Assist?

SGS has been supporting clients with the first tranche of NESO accelerated transmission connections to assess the impacts of access restrictions in the connection offers. We have supported a number of clients and over 10 site connections which have all employed the “Full Transmission” study approach combining power flow and probabilistic outage techniques. All sites have been large transmission connected Battery Energy Storage System (BESS) sites, but SGS understands that the NESO is looking to make further accelerated connection offers to a much larger group of connection applicants in Q1 2025, which will likely see a range of technologies involved.

In addition to the transmission-constrained study, an emerging area of study for the SGS team is modelling to optimise distribution connected Battery Energy Storage Systems (BESS) import/export where there are network capacity access restrictions due to Active Network Management (ANM) constraint management actions. This could be actions to manage capacity at the GSP Super Grid Transformers (SGTs) or other distribution network constraints. Where conventional curtailment study provides an approximation of total constraint to the total import/export envelope of the BESS site, the optimisation study delivers an additional step, approximating the true impact of curtailment, identifying overlap between curtailed periods and those when the BESS site is looking to maximise revenues in markets. An additional step can then quantify this impact, not just in MWh Energy terms, but as a financial impact on revenues compared to an unconstrained baseline equivalent. 

SGS’s Research and Development team continue to consider new innovation related to our DERMS software products that have applications for our consulting services.  Assessing the impact of network access restrictions and optimising BESS import/export dispatching to maximise BESS market revenues is one example.

Let’s Discuss

 SGS is deeply engaged across the electricity sector, providing analysis and advisory support to stakeholders from developers and investors in renewable energy systems to DSOs and Transmission Owners/Operators. For many years we have supplied software systems for energy asset control and delivered specialist advisory services such as constraint and curtailment modelling.  We follow industry changes relating to the connection and management of diverse generation and storage technologies.  We are keen to capture those experiences and our knowledge to ensure our services are informative and valuable to other stakeholders participating in the energy system.

We are always keen to hear feedback on our shared thoughts and welcome ideas for future articles from wider community, so please leave a comment if you have any observations or thoughts on this topic. Similarly, we are keen to hear from developers and investors that may require services such as those described in this blog – please get in touch if you would like to speak to one of our experienced grid consultants.