Learnings

Outcomes

2021/22 

EPRI programs cover a variety of topics providing useful lessons that can be applied to our own operations, these may not always relate to the key objectives NGET has in selecting the programs.  In 2021/22, EPRI concluded a supplemental project into the use of synthetic esters in transformers. 

Regardless of the observations about the environmental impacts of synthetic ester, it is held to the same regulatory standards as for mineral oil and its superior environmental performance confers no special treatment when handling spills or accidental releases, i.e., they must be contained as if they have the same properties.  Infrastructure that is commonly installed for preventing release of mineral oil to the environment is not sufficient for doing the same for synthetic ester.  The work by EPRI confirmed through testing the expectation that larger oil/water separators should be fitted on substations where ester filled transformers are being installed.

2022/2023

Analysis of both online bushing power factor and temperature during laboratory testing has shown that oil insulated bushings in poor condition do not only produce an increase in absolute power factor, but also that the increases are influenced by temperature. This could support the diagnosis of in-service bushings where power factor results suggest a deterioration in condition.

A long-term study of the performance of natural and synthetic esters in conditions comparable to a transmission transformer provided a number of outcomes:

• Flash and fire points remained stable.
• Ageing of cellulose was comparable in both esters and slower than in mineral oil
• Ageing of the fluid was more advanced (colour, acid, power factor) in natural ester than in synthetic ester.
• Fault gases for synthetic esters are similar to mineral oil, although carbon dioxide and carbon monoxide behaviours are different. Low activity partial discharge (corona) did not generate significant levels of gases, but high activity partial discharge (sparking) did produce gases. Ratio limits were identified to help distinguish between faults.
• Performing cold start operations in sub-freezing conditions should not be hindered in a transformer filled with aged mineral oil, natural ester and synthetic ester fluids (as long as their temperature ranges for use are adhered to).

By example it was demonstrated that acoustic emission partial discharge monitoring may be used to enable continued operation of transformers in service as it allows for closer monitoring of key health parameters of transformers with known internal issues.

2023/2024

The low melting point alloy that NGET has successfully deployed for sealing SF6 leaks has been evaluated for transformer oil leaks.  The mould injection method used for SF6 appears to be less suitable than a spray method.  Further evaluation of this technique is being taken forward in NIA2_NGET0049 - Sprayed Metal for Effecting Leaking Transformer Repairs (SMELTeR).

Despite some limitations, robots and their payloads were found to provide useful data and imagery that could make them a beneficial tool for substation inspections.  NGET will continue to participate in more trials with EPRI.  Based on the outcome of the trials, NGET has selected the best performing robot for NIA2_NGET0060 - Robot, AI and Drone Enhanced Detection of Discharge (RAIDEDD).  

2024/2025

EPRI has conducted a detailed investigation of NGET’s reactor population including in service and scrapped assets.  Using statistical analytical techniques ageing markers have been correlated with likely paper condition and likelihood that the reactors are in a state requiring replacement.  The study suggests that reactors are sufficiently different from power transformers in their behaviour that incorporating them into the PTX system would be challenging.  However, it is likely that an analysis tool could be provided to provide confidence that a reactor is reaching end of life in an objective and systematic manner.

Utility response to climate change is an urgent and evolving field.  Climate READi was a three-year effort by EPRI to develop industry guides and standards to help guide utilities towards a consistent approach of evaluating risk and developing a response.  National Grid provided input to EPRI on our work in this space, and much of the outputs of our climate change adaptation work aligned with EPRI’s recommendations, providing credibility from an industry-recognized third-party source.  One specific example is using EPRI’s Wildfire Tool Evaluation to review risk assessment tools on the market to choose the best fit for National Grid’s wildfire risk assessment.

2025/2026

Alignment to Success Criteria

All success criteria detailed earlier in this report have been met, and further detailed below: 

Ageing mechanisms
Long-term experimental datasets (>100,000 hours) and field studies improved understanding of insulation ageing, including ester–cellulose interactions and degradation pathways. This provides clearer insight into how operational and environmental conditions influence asset life. 
Ageing markers
Enhanced diagnostic indicators were developed, including improved interpretation of DGA behaviour across mineral oil and alternative fluids, and multi-sensor monitoring approaches. These enable earlier and more reliable identification of emerging faults. 
End of life criteria
The programme strengthened the link between condition indicators and actual asset health through correlation of monitoring data, ageing studies, and analysis of in-service and retired assets. This supports a more evidence-based and less conservative determination of end-of-life thresholds. 
Investigation and comparison of data from different utilities
Industry-wide datasets and benchmarking across utilities were expanded, enabling comparison of asset performance, failure modes, and maintenance practices. This improves confidence in asset decision-making by grounding conclusions in a broader evidence base. 
Analysis of data collected during scrapping activity
Analysis of scrapped and in-service assets provided validation of diagnostic techniques by linking observed ageing markers to physical condition. This improves confidence that monitoring outputs reflect true asset degradation.
Application of new algorithms for data analysis
Advanced analytics were developed and scaled, including natural language processing, hazard and survival modelling, and risk-based asset analytics frameworks. These enable structured use of large datasets and support predictive, fleet-level decision-making.

The total NIA spend at project conclusion was £2,857,528, above the planned budget of £2,470,000. This is due to the scale of the project and the unanticipated internal stakeholder time required to manage the simultaneous workstreams. 

Lessons Learnt

Due to character limit previous years lessons learnt can be found in previously published progress reports 

2025/26

Monitoring improvements showed that unvalidated or poorly contextualised data can lead to incorrect diagnostics, reinforcing the need for stronger data assurance and combined use of multiple techniques. Robotics and UAV trials confirmed that autonomous operation is still constrained in live environments, particularly due to interference, navigation drift and payload limitations. Similarly, AI-based approaches demonstrated capability, but are highly dependent on the quality and scale of training data, with deployment challenges driven more by integration into existing processes than by model performance itself. For SF6 detection and alternative materials, results emphasised that performance is highly dependent on real-world conditions (e.g. geometry, proximity, material compatibility), and that system-level effects must be considered alongside component-level benefits.

Dissemination 

Dissemination opportunities in 2021/22 were limited as a result of Covid-19 restrictions.  

Knowledge transfer sessions were held in May 2022, January 2023, January/February 2024 and January 2025 which provided highlights of current research and an opportunity to discuss projects in detail in Warwick, UK to which other licensees were invited. A similar event will be arranged for January 2026, also in Warwick.  

A paper was presented at the inaugural Polaris International Conference and Exhibition in Glasgow in November 2022.  The paper “Thermal Performance of a Novel Transformer Fluid using a Model Transformer” will be uploaded to the ENA portal along with this report.  It was based on the outputs of work completed in 2021/2022.

A 3-D printed model of the modified membrane dryer for transformer oil was displayed at the Energy Innovation Summit in Liverpool in 2023.

A paper based on work in this project entitled “Environmental impacts and mitigation options for synthetic esters” co-authored by National Grid, EPRI, Consolidated Edison and New York Power Authority was presented at the International Conference of Doble Clients in Boston, MA, in March 2025 by Gordon Wilson (NGET).
Climate READi (Resilience and Adaptation Initiative) is supported jointly by this project and National Grid US.  The multi-utility project has a dedicated website at www.epri.com/READi which contains more detail about the project, publicly available downloads, and media links to show how the initiative is being disseminated through industry media sources.  The project is now complete and anyone can access tools and get an overview of the project and resources at the Climate READi Compass https://apps.epri.com/climate-readi-compass/en/