## 3 Results and Discussion
### Comparative Spectral and Multimodal Analysis  
The wealth of results obtained from the different analytical techniques, breakdown voltage testing, frequency domain spectroscopy (FDS), and Fourier-transform infrared spectroscopy (FTIR), show strong consistency, revealing a marked interdependence between the chemical, electrical, and dielectric aspects of degradation. * Breakdown behavior: it was observed that the mineral oil Polaris GX leads to the greatest reduction in breakdown voltage, indicating accelerated degradation of the solid insulation. This loss in dielectric strength aligns closely with the early chemical deterioration of the paper that we observed via FTIR. * Dielectric properties (FDS): Through the analyses, the ranking of the oils based on key dielectric parameters (loss tangent, conductivity, and permittivity) follows the order of decreasing performance: NE 1215 > NE 1204 > Polaris GX. This hierarchy directly reflects the extent of aging experienced by each oil-paper system. * Chemical degradation (FTIR): The FTIR spectra corroborate these trends, with increasing levels of oxidation and hydrolysis observed from NE 1215 (most stable) to Polaris GX (most degraded), both in the oil and the impregnated paper. This convergence of findings highlights a fundamental principle: higher chemical stability of the oil-paper system (i.e., low oxidation and hydrolysis) corresponds to better preservation of dielectric properties and breakdown strength. In other words, chemical degradation serves as a reliable predictor of functional deterioration. Among the tested liquids, natural ester NE 1215 stands out as the best-performing solution, benefiting from a favorable synergy between chemical stability and dielectric properties. Based on the findings, it is believed these results position NE 1215 as a highly promising alternative to mineral oils for extending the service life of transformers under thermal stress (Table 6). The combined analysis of FDS and FTIR results revealed significant correlations between chemical and dielectric parameters. In particular, the increase in loss factor (tan \(\delta\)) and permittivity is associated with an increase in carbonyl groups (C=O) and a reduction in hydroxyl groups (O-H) in the FTIR spectra. This observation confirms that \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Oil** & \begin{tabular}{c} **Breakdown Voltage** \\ **Drop (S0\(\rightarrow\)S3)** \\ \end{tabular} & \begin{tabular}{c} **tan \(\delta\) Evolution** \\ **(FDS)** \\ \end{tabular} & \begin{tabular}{c} **Paper Oxidation** \\ **(ATR-FTIR)** \\ \end{tabular} & \begin{tabular}{c} **Oil Chemical** \\ **State (ATR-FTIR)** \\ \end{tabular} & \begin{tabular}{c} **Aging Rate** \\ \end{tabular} & \begin{tabular}{c} **Overall** \\ **Performance** \\ \end{tabular} \\ \hline Polaris & \(-\)16.6\% & \begin{tabular}{c} Significant rise \\ from S2 to S3 \\ \end{tabular} & \begin{tabular}{c} Marked from S1, \\ accentuated at S3 \\ \end{tabular} & \begin{tabular}{c} Significant \\ oxidation from S1 \\ \end{tabular} & Fast & Low \\ NE 1204 & \(-\)10.8\% & \begin{tabular}{c} Gradual \\ increase \\ \end{tabular} & \begin{tabular}{c} Moderate, mainly \\ from S2 \\ \end{tabular} & \begin{tabular}{c} Mild acid \\ formation \\ \end{tabular} & Moderate & Good \\ NE 1215 & \(-\)7.6\% & \begin{tabular}{c} Slight increase \\ only at S3 \\ \end{tabular} & \
\begin{tabular}{c} Low, visible only \\ at S3 \\ \end{tabular} &
\begin{tabular}{c} Stable, minimal \\ evolution \\ \end{tabular} & Slow & Very good \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary comparison of aging and breakdown effects on the oil–paper systems. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} Strong degradation \\ (dielectric and chemical) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary comparison of aging and breakdown effects on the oil–paper systems. chemical degradation, especially oil oxidation and cellulose hydrolysis, can predict the loss of dielectric performance.