Understanding Curtailment Impacts from Grid Supply Point (GSP) Technical Limits

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 Electricity System Operator (ESO) 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, we wanted to focus on one initiative to help accelerate those who have applied for distribution connection agreements but are held up by the need to progress transmission network upgrades.

GSP Technical Limits

Until recently developments with distribution connection offers that are subject to transmission works progression have had to wait until transmission upgrades are complete before being connected and allowed to export, and in some cases import, electricity. With the introduction of Technical Limits however, the ESO has considered if distribution sites can connect on the basis of a non-firm arrangement associated with the transmission network constraints. While non-firm or flexible connections (which includes concepts such as Curtailable Connection agreements), allow the DSOs to curtail site export or import to manage the distribution network within operational limits, this concept has not, until now, been extended to include transmission network operational limits[1]. The ESO has now introduced the application of Technical Limits at the Grid Supply Point (GSP) which manages power flow across the interface of distribution and transmission network ownership and operational boundary.

So, what do GSP Technical Limits mean for those looking to connect to the distribution network? Without dwelling on how the ESO has determined the specific GSP Technical Limits (i.e. power flow import/export restriction) to be applied at selected GSP substations, where these are applied, DSOs are able to offer an alternative connection agreement which allow customers to connect prior to completion of transmission upgrade works. However, the condition of this early connection is the need for import or export curtailment in scenarios where the DSO must manage power flows through the GSP within the Technical Limits set by the ESO. Curtailment under Technical Limit agreements will persist until the progression of transmission works associated with the customer’s original connection offer are complete.

What Does this Mean for Developers and Investors?

When a GSP Technical Limits connection is proposed, a developer/investor is faced with a decision to wait for the transmission progression works to be completed and plan the build out accordingly, or to accept an earlier connection date with GSP Technical Limits, build out accordingly and experience a period of curtailment associated with the GSP Technical Limits whilst awaiting transmission reinforcement.

From our experience at Smarter Grid Solutions (SGS), we note that in many cases, GSP Technical Limits do not sit in isolation, and a key question for many developers and their investors is to understand how GSP Technical Limits add to the level of curtailment expected from constraints on the local distribution network (where constraint estimates are often provided to the developer with their original connection offer). Over the last 15 years, SGS has supported many developers in carrying out curtailment assessments that apply real world data input assumptions and scenarios rather than the more conservative network planning based assumptions applied by DSOs. This has assisted developers and their investors to establish a more informed view of likely curtailment impacts from DSO Active Network Management (ANM) control, inter-trip schemes and Load Management Schemes (LMS).

What are DSOs Doing to Assist?

In some cases we are seeing DSOs provide revised curtailment assessments that study the curtailment impact of the GSP Technical Limits and other distribution network constraints to assist developers/investors. As is the case in other areas of the electricity sector, we have not seen consistency in the provision of, or format of, Technical Limit curtailment reports across the DSOs. This can be a source of confusion for developers and the SGS consultancy team has received many clarification queries about these reports.

The team of consultants at SGS has noted that the DSO curtailment studies vary in the detail provided. They all provide an estimated level of annual curtailment for the site in question, albeit the actual metrics for curtailment vary across DSOs as well. At least one DSO considers a revised study scenario with a level of attrition in those sites higher in the Last In First Off (LIFO) priority queue. Some curtailment reports provide details of all sites higher in the priority queue and others do not, only stating the LIFO position. Some DSOs do, or are planning to, provide additional information in their public facing or developer facing data portals. This includes updated LIFO stack information to assist developers in carrying out further assessments of curtailment.

Curtailment assessments will change over time due to cancellation of sites higher in the LIFO priority queue and as network upgrades are completed. The variation across study approaches, curtailment metrics, and scenarios studied are often additional sources of confusion for developers, and one that SGS consultants have been approached to support.

What is SGS Doing to Assist?

The consultancy team at SGS is supporting developers who are now reviewing GSP Technical Limit offers, by performing independent scenario studies of the curtailment due to GSP Technical Limits.

Typically, developers have received from the DSO a Technical Limits curtailment assessment which shows a large increase in curtailment compared with their original connection offer and our team of consultants proposes curtailment assessment of scenarios based on more realistic study assumptions. These representative assumptions can include agreeing export profiles for Photovoltaic (PV), Wind, Gas, Battery Energy Storage Systems (BESS) and other generation technologies and also varying import profiles for BESS sites. Also, study scenarios with agreed potential attrition in the LIFO priority queue above their site or considering the relaxing of GSP Technical Limits as network upgrade works are completed.

DNOs acceptance timescales are often not allowing sufficient time for a full load flow model build that can be linked to models of the ANM Curtailable Connections implementation systems that manage network constraints. The compromise to reduce timescales and costs is to focus solely on the curtailment impacts from the new GSP Technical Limits in isolation. While this ignores the wider distribution network and associated ANM constraint management impacts, it does capture curtailment impacts from the ANM control based on the GSP Technical Limits and associated ANM priority queue of sites under control. This provides developers/investors with answers in a much shorter timescale and greatly reduced cost compared with a load flow model build and study. Once a developer has decided on their GSP Technical Limits connection offer, SGS can provide a load flow model build and assessment of the ANM curtailment impact of all identified distribution network constraints including the GSP Technical Limits.

GSP Technical Limit Curtailment Assessment Approach

Focusing on site curtailment impact solely from ANM applied GSP Technical Limits allows the application of streamlined and efficient modelling techniques. SGS has been delivering constraint and curtailment studies to DSOs and developers for over 15 years and we are able to quickly configure our curtailment simulation models to most effectively reflect the network conditions and provide representative time-series modelling of network and demand/generator behaviours. However, there are other factors that should be considered. This includes the ANM priority order for curtailing managed sites to maintain GSP power flows within the Technical Limits.

The curtailment model requires to represent managed sites in the ANM dispatch priority queue and their ANM observable relationship with the GSP constraint point. For example, if an ANM control scheme sends an instruction to a generator to reduce export by 5MW it is important to consider how much of that 5MW reduction is observed at the GSP with other demand and generator profiles remaining constant at that moment. For a GSP supplied distribution network that runs in isolation from other GSP supplied distribution networks under normal operating conditions, the ANM observable relationship between managed sites and the GSP constraint point is straightforward to identify. However, where one or more GSP networks operate in parallel then the ANM observable relationship is less straightforward to identify and requires more advanced modelling approaches (either directly via load-flow simulation or simplified study informed via load-flow simulation). This is a key factor in modelling ANM behaviour and ignorance of such conditions can lead to significant error in curtailment estimates.

At SGS we have developed a number of methods and approximations to estimate the observable relationship between sites in the ANM priority queue and each GSP in a group operated in parallel. The method applied is matched to the complexity of the GSP group and the interconnection routes between the GSPs in the group.

Another challenge with complex GSP groups is how the GSP ANM scheme is configured. Where there are two GSPs in parallel, does each GSP have a separate ANM LIFO stack, do some generators appear in each LIFO stack or do both GSPs share a common LIFO stack? Understanding the ANM configuration allows improved modelling of the GSP Technical Limit curtailment impacts on a customer project or site. SGS has been a supplier of ANM software systems since the first innovation projects more than 10 years ago and is a market leading provider of advanced ANM systems to DSOs in the UK. We therefore have a deep understanding of ANM system configuration and operations which is reflected in our ANM curtailment modelling.

Let’s Discuss

SGS is deeply engaged across the electricity sector from developers and investors in renewable energy systems to DSOs and Transmission Owners and operators and has, for many years, been supplier of software systems for asset control and of specialist advisory services such as curtailment modelling. We endeavour to 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 so that we may be informative and useful to other stakeholders participating in the energy system.

We are always keen to hear feedback on our shared thoughts and ideas for future articles from the developer community and others so please leave a comment or contact us 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