Learnings
Outcomes
ADAPT
A review of the literature on studies of furans in oil and esters revealed many studies on laboratory-based correlations with DP of paper but little systematic study of samples from transformers evidencing the need for more work in this area, particularly in ester filled plant.
The published correlation between 2-FAL levels and paper health was found to hold for paper aged under both synthetic and natural ester fluids, however significant modification to the protocols in ASTM D5837/IEC 61198 for the detection of furans in oils by HPLC were necessary to ensure good peak separation in ester fluids. Direct injection of samples was found to be the most optimal approach.
Prolonged storage of oil samples leads to a reduction of furan concentration, reinforcing the need for prompt analysis and calling into question some historic results in National Grid’s database. Conversely the presence of excess moisture and interestingly, corrosive sulphur, tends to increase furan levels.
Due to a combination of the increased polarity and inherently higher moisture content of ester oils compared to mineral oils, existing correlations between 2-FAL and paper health need revising for ester oil filled plant. In synthetic ester filled plant utilizing dry paper a doubling of the furan levels is anticipated and end of life criteria (currently 10 mg/L) need revising to 20 mg/L.
An alternative approach to measuring furans based on aqueous extraction combined with UV/Vis spectroscopy could provide the plant engineer with a fast and cost-effective alternative to HPLC for monitoring furan levels in the field.
A regression-based approach based on measuring all five furans as opposed to just 2-FAL in isolation shows significant promise at improving the accuracy of paper health prediction in high voltage plant.
A study on fourteen transformer oils extracted from the National Grid fleet shows the superiority of furan measurements over simple measurements of oil condition (ASTM D1500) for determining plant health.
TREND
The project has produced an updated transformer lifetime assessment based on an enlarged scrapped and in-service transformer dataset. Two different approaches (statistical approach and thermal ageing-based approach) have yielded similar lifetimes for the transformer fleet.
The project has clarified the relative value of several ageing and diagnostic markers. It indicated the DP vs 2-FAL relationship for NGET fleet has been reinforced as the most reliable fleet-level paper ageing marker, while methanol, other furanic compounds and C3 gases have been positioned as supplementary indicators that may support specific diagnostic questions.
A further outcome is the development of improved understanding of 2-FAL partitioning behaviour. By showing how temperature, moisture and operating history affect measured 2-FAL concentrations, the project provides a basis for more reliable interpretation of oil test results and improved thermal condition scoring. Overall, the project has moved transformer ageing assessment toward a more data-driven and mechanism-based framework for supporting refurbishment, replacement and end-of-life planning.
Recommendations for further work
Ageing at lower temperatures which are closer to ageing conditions in a transformer would be beneficial at confirming the observed trends but would take many months to years to carry out.
The complex interplay between corrosive sulphur, paper ageing and furan levels needs investigating at lower corrosive sulphur contents which are more applicable to transformers in the field.
Historical information pertaining to the fourteen supplied transformer fluids would be useful to cross check the predicted states of health based on 2-FAL measurements to actual asset age and service history. A similar sample set from ester filed plant would confirm the validity of furan measurements in predicting asset health, especially if historical data was also made available.
Lessons Learnt
ADAPT
An overview of plausible mechanistic pathways for the degradation of cellulose identified new molecular species that could potentially be used for tracking the deterioration of paper insulation. GCxGC-MS shows potential for identifying these in practice, especially those lacking a UV chromophore which would not be found by HPLC-UV.
Through continuous improvement to the protocols for the detection of furans in oil by HPLC in ASTM D5837/IEC 61198, direct sample injection of ester samples into the HPLC for quantification of furans was found to be necessary as esters are miscible with the solvent used for liquid-liquid extraction of furans from mineral oil. Optimisation of instrumentation, elution conditions and column now allows for excellent peak separation of the various furans in ester oils.
It is clear that furans correlate with DP reduction as reported in the literature, but secondary reactions of furans influence the levels found and can cause an underestimation of the depolymerisation of cellulose. Specifically, long term storage of samples can result in a reduction in furan levels whilst the presence of moisture and interestingly, corrosive sulphur, was noted to cause a significant increase in measured furan levels for paper in the same state of health.
A correction to established correlations between 2-FAL levels and DP for mineral oil filled plant needs to be applied for synthetic ester filled transformers to account for changes in the hydrophobicity of the oil and moisture content.
As an alternative to HPLC-UV analysis of furans on a routine basis, UV/Visible spectroscopy combined with aqueous extraction may be a more rapid option and was demonstrated as a viable technique over a number of aged oil/paper systems composed of esters as well as mineral oil.
Dissemination
Three papers are anticipated from the project – one conference paper which has already been submitted to the 2026 International Conference on Dielectrics and two journal papers with anticipated submission in the summer of 2026,
Ian L. Hosier, Paul L. Lewin, Thomas Andritsch, Edward H. Jackman, Nikita S. Rank, Gordon Wilson, Ruth Hooton, “An Analysis of the Applicability of Furan Measurements to Determine Paper State of Health in Ester oil/Paper Insulation Systems”.
Ian L. Hosier, Paul L. Lewin, Gordon Wilson, Ruth Hooton, "An assessment of UV/Vis spectroscopy as a method of determining paper condition in oil filled high voltage transformers".
Sally Bloodworth, Nikita Rank, Ian L. Hosier, Paul L. Lewin, Gordon Wilson, Ruth Hooton, Richard Brown "Multivariate regression analysis of furans as a tool to determine paper condition in oil filled high voltage transformers".
TREND
Classical statistical models, compared with more modern artificial intelligence (AI) modelling, perform well with the level of data available for understanding transformer lifetime. AI will have a place, but the quantity of data required for training is not currently available.
While the kinetic based modelling may contain various uncertainties using a published standard thermal model and hot-spot factors derived through reversed engineering combined with temperature and water content dependent Arrhenius equation, the thermal lives of 106 National Grid in-service transformers have been estimated. As a result, the thermal life expectancy of the population is derived as 84 years. It has been identified that the modelled transformer’s load, winding-to oil gradient, and top-oil temperature rise are three possible error sources in the hotspot factor derivation and hence, the thermal model.
A key lesson from the project is that larger and more representative datasets are essential for improving confidence in transformer ageing assessments. While the expanded datasets have strengthened the analysis, the work also shows that caution is required when extrapolating results from selected manufacturers, design families or age groups to the wider transformer fleet. Future projects should therefore prioritise early identification of data gaps and ensure that sampling strategies are aligned with the diversity of the asset population.
The work has also shown the importance of combining statistical analysis with physical and chemical understanding. Diagnostic markers such as methanol, furans and C3 gases can provide useful information, but their interpretation depends strongly on context, including transformer design, operating conditions, oil intervention history and measurement availability. Future projects should avoid relying on individual indicators in isolation and should instead use multiple complementary data sources supported by understandings related to physical-chemical mechanisms.
Another important lesson is that operational history must be considered when interpreting condition monitoring data. The studies on 2-FAL partitioning show that measured concentrations can fluctuate due to temperature, moisture and loading history, not solely depending on insulation ageing. Future projects should therefore seek to integrate oil test results with loading, temperature and maintenance records so that genuine degradation trends can be distinguished from short-term or treatment-related variability.
Dissemination
Two conference papers were published based on the work in TREND:
S. Saurabh, S. Y. Matharage, Z.D. Wang, G. Wilson, R. Hooton, “Development of a Steady-State 2-Furfural production and partitioning model” 2025 International Symposium on High Voltage Engineering (ISH), Nagano, Japan, 2025.
S. Saurabh, S. Y. Matharage, Z.D Wang and G. Wilson, "Studies on the Partitioning of Furfural Between Liquid and Paper Insulation," 2025 IEEE International Conference on Dielectric Liquids (ICDL), Lodz, Poland, 2025.
One oral and one poster presentations were delivered at the 18th UHVnet colloquium in May 2026.
