Project Snapshot
Client: Major UK Battery Energy Storage Developers (Undisclosed)
Location: Central Scotland, UK
Services: Front-End Engineering Design (FEED), Grid Code Compliance (G5/5, G99)
Date: January 2026
What We Delivered
- Early validation of voltage, fault level, harmonic, and reactive power performance
- Clear, evidence-based assessment of G5/5 and demonstration of G99 compliance
- Reduced grid connection risk through structured FEED and proactive modelling
- More effective engagement with the Distribution Network Operator (DNO) with a clearer pathway toward connection approval
Project Overview
Battery Energy Storage projects are increasing in scale across the UK. At the same time, network operators are applying greater technical scrutiny to new connections. As a result, developers now need stronger early-stage evidence to support grid approval.
For this 200MW Battery Energy Storage System located in Central Scotland, our client required early technical certainty. They DNO required confidence that the proposed design would operate within voltage limits, manage fault level contribution, comply with harmonic requirements, and deliver reactive power capability in line with ENA Engineering Recommendation G99.
High-level system overview developed during FEED, defining the grid interface, voltage levels, and key electrical boundaries of the Battery Energy Storage System.
We were appointed to deliver a combined package of FEED and grid code compliance studies. From the outset, our objective was clear. We aimed to remove uncertainty early, reduce connection risk, and provide evidence that would stand up to detailed DNO review.
The Challenge
From our power system engineer’s initial review, we identified several technical areas that could introduce risk if left unaddressed:
For this project, the primary technical risks included:
- Voltage stability across charging, discharging, and standby operation
- Fault level contribution from inverter-based generation and its interaction with existing protection schemes
- Harmonic emissions and compliance with ENA Engineering Recommendation G5/5
- Demonstrating reactive power capability at the Connection Point under G99
If addressed late, these issues often lead to extended review cycles or design changes. Therefore, we focused on resolving them early in the project lifecycle.
EPS Approach
EPS structured the work to combine early-stage FEED development with detailed grid compliance modelling, ensuring that key design decisions were validated before progressing to later project phases.
By doing so, we identified constraints early and refined the design iteratively. This approach also gave the client a clear understanding of system behaviour before progressing to later stages.
Front-End Engineering Design (FEED)
Conceptual single-line diagram illustrating the proposed electrical topology, from the transmission connection point through to inverter interfaces, established during the FEED phase.
During the FEED phase, we established the project’s technical foundations. In particular, we defined how the Battery Energy Storage System would interact with the transmission network.
We developed a high-level electrical topology to define system boundaries, interfaces, and voltage levels. At the same time, we set out electrical design criteria covering operating voltage ranges, control modes, and loading scenarios for both import and export operation.
In addition, we reviewed preliminary transformer, inverter, and switchgear ratings. We assessed thermal capability, efficiency, and compliance with IEC and ENA standards. Where necessary, we challenged initial assumptions to ensure the design remained suitable for future network conditions.
As a result, we identified potential constraints early and reduced the likelihood of redesigns later in the project lifecycle.
Load Flow & Short Circuit Analysis
EPS carried out detailed load flow and short circuit simulations using DIgSILENT PowerFactory to assess system behaviour under a wide range of operating and contingency conditions.
Summary of fault level contributions at the point of connection, confirming compatibility with network fault withstand limits and protection coordination requirements.
The load flow analysis examined voltage performance at the site and grid interface during charging, discharging, and standby operation. Our engineers also assessed variations in import and expert levels. This confirmed that steady-state voltage remained within acceptable limits under realistic conditions.
Thermal loading assessments evaluated transformers, cables, and switchgear under normal operation, peak demand, and N-1 contingency scenarios. The studies confirmed that equipment ratings and thermal limits were maintained with appropriate design margins.
Short circuit analysis assessed maximum and minimum fault level contributions from the Point of Connection. This confirmed compatibility with fault withstand limits and supported early consideration of protection coordination.
G5/5 Harmonic Analysis
Harmonic performance is a critical consideration for large Battery Energy Storage projects, particularly where long high-voltage cable circuits and inverter-based technology interact with a constrained transmission network. For this reason, we undertook a detailed harmonic assessment in accordance with ENA Engineering Recommendation G5/5, with a specific focus on identifying potential risks at an early stage.
We assessed incremental harmonic emissions from the inverter systems alongside background harmonics and network resonance effects. While incremental harmonics from the BESS itself were found to be within acceptable limits, our wider assessment identified significant resonance between the proposed 400 kV cable connection and the wider transmission network.
When background harmonics and resonance effects were included, the resulting Total Harmonic Distortion at the Point of Connection was approximately 7.0%, exceeding the G5/5 planning level of 3.5% across a range of operating scenarios, including both import and export operation and load levels between 20% and 100%. In addition, multiple individual harmonic order limit violations were observed.
By identifying this non-compliance at the FEED stage, we were able to clearly quantify the associated risks, including the potential for overvoltage, increased insulation stress, and adverse impacts on surge arresters and other network equipment. Importantly, this work provided the client and the Distribution Network Operator with a transparent and defensible understanding of the harmonic behaviour of the system, rather than allowing these issues to emerge later during detailed design or commissioning.
This early insight enabled informed discussions around mitigation strategy, connection risk, and programme implications, reinforcing the value of rigorous harmonic assessment as part of early-stage FEED and grid compliance studies.
For further detail on harmonic compliance requirements and assessment methodology, explore our G5/5 Harmonic Assessment Guide.
G99 Reactive Power Capability
The G99 assessment focused on verifying that the BESS could deliver compliant reactive power performance and provide effective voltage support under a wide range of operating conditions.
P-Q Reactive Capability Requirement
“G99, Clause 13.5.6”
All Power Park Modules, shall be capable of satisfying the Reactive Power capability requirements at the Connection Point as defined in Figure 13.14 when operating below Registered Capacity.
EPS Simulation Results
Reactive power capability envelope confirming compliance with G99 requirements and demonstrating the system’s ability to provide voltage support across the full operating range.
Detailed inverter modelling confirmed that:
- The system could meet the required reactive power range in both import and export modes
- Reactive capability was maintained across the full operational envelope and under varying grid strengths
- Voltage support functions operated effectively during voltage rises, dips, and system disturbances
- Power factor requirements could be achieved consistently under steady-state and dynamic conditions
- The study confirmed that the installation would not only meet regulatory requirements but also contribute positively to local voltage control and wider system stability.
Learn more about Why Was the G99 Grid Code Introduced
Project Outcomes
By integrating FEED with grid compliance modelling, we delivered more than a set of compliant studies.
Specifically, our work:
- Reduced technical and programme risk through early issue identification
- Provided clear, evidence-based assurance ahead of DNO engagement
- Supported more efficient technical discussions with the network operator
- Clearer pathway toward informed connection discussions
Crucially, the client gained confidence in how the system would perform in day-to-day operation, not just on paper.
Why FEED and Grid Code Compliance Matter
Across the UK, many Battery Energy Storage projects experience delays. In most cases, this is not due to technology limitations. Instead, it results from insufficient early evidence around network interaction, harmonic performance, or reactive capability.
By combining FEED and grid compliance studies early, we help developers move forward with clarity. As a result, projects face less rework and smoother network engagement.
Conclusion
This 200 MW Battery Energy Storage project represents a significant addition to Scotland’s grid-scale storage pipeline, operating within an increasingly constrained and scrutinised transmission environment. Through structured Front-End Engineering Design and detailed grid code compliance assessment, we provided the client with a clear, evidence-based understanding of how the proposed system would interact with the wider network.
Rather than focusing solely on minimum compliance, our work prioritised early identification of technical constraints and performance risks. This included transparent assessment of voltage behaviour, fault level contribution, harmonic performance, and reactive power capability at the Connection Point. By addressing these considerations at the FEED stage, we enabled informed decision-making and avoided the risk of unforeseen issues emerging later in the project lifecycle.
The studies delivered a technically sound design basis, with clearly defined performance characteristics and future considerations, supporting constructive engagement with the Distribution Network Operator and realistic planning of next steps. As grid requirements continue to evolve, this approach reflects how we support Battery Energy Storage developers at EPS: by providing clarity early, quantifying risk honestly, and helping projects progress with confidence grounded in evidence rather than assumption.
