As part of the National Energy System Operator’s (NESO) Mid-Term Stability procurement framework, Blake Clough Consulting recently supported a number of developers in preparing for and successfully delivering Y-1 stability assessments. These studies were a prerequisite for participating in NESO’s Y-1 Stability market where solutions that contribute inertia or system strength are procured ahead of real-time operations to support secure and resilient grid behaviour. Our portfolio of projects included detailed analysis of different types of grid-forming plants, specifically GBGF-I (Grid Forming, Inverter-based) and GBGF-S (Grid Forming, Synchronous-based) technologies. In addition to these, we also evaluated a hybrid plant configuration that combined grid-forming capabilities with grid-following inverters.
Purpose of the Y-1 Mid-Term Stability Service
The purpose of the Y-1 Mid-Term Stability service is to secure essential stability services such as inertia and short circuit level one year ahead of real-time operation. It aims to reduce reliance on costly, short-term interventions through the Balancing Mechanism by enabling NESO to competitively procure stable and proven sources of system support from non-synchronous and innovative technologies.
To ensure full compliance with the NESO requirements, we conducted a comprehensive suite of studies, collaborating closely with the equipment OEMs to validate model accuracy and performance.
EMT Simulations were performed in PSCAD, and to efficiently manage the numerous simulation cases required, we developed an internal Python-based scripting tool that automated case setup, execution, and data extraction. This allowed us to meet NESO’s documentation demands by generating clear plots and organized Excel datasets for all relevant parameters.
Throughout the process, we identified several potential non-compliance risks related to system response and flagged these for further investigation. By combining expert analysis and OEM insights we delivered a thorough and well-documented study package that meets the stringent regulatory standards.
Since multiple iterations were required due to ongoing feedback from the OEM to ensure compliance, the use of scripting proved especially valuable. It enabled rapid updates, made it possible to keep the project on track and deliver on time, even while working closely with others and managing changes along the way. We used the PSCAD Python API along with our own custom tools to automatically create and run hundreds of simulation cases in one go. Backed by our dedicated high-performance simulation hardware, we were able to efficiently schedule runs, process outputs, and extract critical metrics.
PSCAD Python API
- Enables the production of scripts to automate simulations
- This can be used to generate simulation scenarios and then build simulation sets to run scenarios in parallel
- 100s of scenarios can then be run at the press of a button
Our Internal Tools
- Remote simulation and scheduling tools to manage usage of our specialized hardware.
- In-house tools which interface with the python API to facilitate the creation of project specific scripts
- Tools for managing PSCAD output files (out) which can automate the creation of simulation plots and perform data analysis (e.g. provide the maximum and minimum voltage value at various busbars for each scenario).
What Was Tested: Meeting NESO’s Y-1 Technical Requirements
The technical feasibility studies comprised a series of rigorous simulations designed to verify compliance with NESO’s Mid-Term Stability Market requirements. These included:
Steady-state reactive power capability tests to define the minimum and maximum reactive power under a range of voltage and power conditions.
Frequency event simulations were conducted to assess the solution’s inertia response, ensuring the plant reacts within milliseconds to system frequency changes and delivers the declared inertia support.
Voltage angle event tests evaluated the ability to withstand sudden grid voltage phase angle jumps of up to 60°, confirming stable operation across varying active and reactive power levels. Fault ride-through simulations tested the plant’s resilience to various fault types, including three-phase and earth faults, verifying continuous operation without tripping and rapid recovery of active power within specified limits.
Together, these studies ensured that the solutions demonstrate stability and compliance under realistic grid disturbances.
Looking Ahead: Long-Term (Y-4) Stability Tenders
With the Y-1 submission complete, we now turn our attention to NESO’s Long-Term Stability tenders. The LT2029 tender represents NESO’s first formal Y-4 market-based procurement for stability, reactive power, and restoration services.
Stability
Seeks to procure:
- Inertia – nationally (up to 10 GVA·s).
- Short Circuit Level (SCL) – regional, nodal basis.
Each region has identified reference substations with defined SCL requirements, and only solutions connected to acceptable sites within these regions will be considered. The effectiveness of a solution’s SCL contribution depends on system impedance and proximity to the reference node.
Reactive Power
Under LT2029, NESO is procuring both:
- Dynamic Reactive Power
- Static Reactive Power
Each region in the UK has specific MVAr needs. For instance, the East Midlands has a requirement for +300 MVAr, while London and the West Midlands face similar needs at substations such as Amersham and Feckenham. NESO’s voltage stability requirements under LT2029 are location-specific, with each region assigned a reference substation based on network voltage sensitivity analysis. Providers must connect either at these reference substations or designated alternative substations, where effectiveness decreases with electrical distance. Effectiveness factors quantify how well a given connection supports the target voltage location.
Restoration
NESO has requirements for both the anchor (primary) generator service and the top-up service.
- Anchor (primary) providers will have the ability to self-start and meet the full set of technical requirements.
- Top-up service providers are not expected to have the ability to self-start but can meet some of the technical requirements to further assist restoration.
Providers may choose to participate in one or more of these services. Each of these three services has its own detailed technical specifications and assessment criteria. NESO requires all participating providers to submit a Technical Feasibility Study, demonstrating that their proposed solution is capable of delivering the intended service reliably and in line with NESO’s performance standards. ESO will assess these studies as part of its staged procurement process, following the defined LT2029 tender timeline.
Our experience with Mid-Term stability assessment of multiple projects places us in a strong position to support future Long-Term (Y-4) Stability bidders. Whether it’s model readiness, feasibility assessments, or full tender-stage analysis, we’re ready to help developers meet NESO’s evolving stability service expectations.
References:
01 LT2029 Instructions to Tenderers EOI V1_0
03 LT2029 Eligibility Criteria V1
06 LT2029 Restoration ESR Technical Specification V1
05 LT2029 Voltage Service Technical Specification V1
00. Mid-Term Stability Market 26_27 Invitation to Tender Letter
04. Mid-Term Stability Market 26_27 Technical Specification v2
06. Mid-Term Stability Market 26_27 Feasibility Study Requirements