## 3 Results and Discussion
### Test Analysis
The IR spectra of oil with fibers are shown in Figure 8. The absorption peaks a \(\sim\)f are main ingredient of new naphthenic transformer oil. Specific information of the absorption peaks is shown in Table 1. With increasing aging time, there was no change in the position of the main absorption peaks a \(\sim\)f, because the molecular structure of the major elements C and H in the oil does not change. However, the intensity of the absorption peaks changes during heat aging. In the early stages of aging, the intensity of the absorption peaks in Figure 8a,b decreases. This indicates that the characteristic groups of fiber-containing oil suffer certain damage. In the middle of aging, the intensity of the absorption peaks in Figure 8a,b increases, indicating that both TPEE and PTFE were pyrolyzed and the macromolecular chains broken. Some of the aging products, like methyl and methylene, were dissolved in oil. Moreover, the rate of dissolution is greater than that of oil aging, resulting in intensity increases of the absorption peak. Free H+ and hydrocarbon groups promote further increases in water content and acid value in the oil. In the later period of aging, the intensity of the absorption peak in Figure 8a continues to increase, indicating that the TPEE in the high-temperature oil continues to age. In Figure 8b, the decrease in intensity of the absorption peak shows that the degradation of the oil predominates and the aging rate of the PTFE slows down. To further verify the existence of different aging degrees in the optical fiber sheath, we used SEM at 200\(\times\) magnification to observe two kinds of sheath at days 0 and 24 during aging. The result is shown in Figure 4:0  
Figure 9a,b show the thermal aging of the TPEE sheath at days 0 and 24. High-temperature oil causes corrosion and deterioration of the TPEE, resulting in many longitudinal cracks on the surface. The original sheet texture has been blurred. Figure 9c,d show the thermal aging of PTFE sheath at days 0 and 24. The longitudinal texture of the PTFE is still apparent, but the surface structure has become uneven due to corrosion. From Figure 9a-d, the observed physical changes in the optical fiber sheath reflect changes in the insulating features of the oil samples containing fiber sheaths. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Absorption Peaks** & **Wave Number (cm\({}^{-1}\))** & **Adscription** \\ \hline a & 2952 & Asymmetric stretching vibration of methyl C-H \\ b & 2923 & Asymmetric stretching vibration of methylene C-H \\ c & 2868 & Stretching vibration of methyl C-H \\ d & 1461 & Bending vibration of methylene C-H \\ e & 1377 & Bending vibration of methyl C-H \\ f & 972 & Bending vibration of carbon ring C-C \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Attribution of main IR absorption peaks of oil. Figure 9: _Cont._  
Based on the above mechanistic analysis, it can be seen that the oil samples containing fibers were deeply aged in a sealed vacuum at 130 \({}^{\circ}\)C, whose water contents, acid values, and hydrocarbon groups accumulated during the thermal aging periods are higher than those of the pure oil. Therefore, the fiber-containing oil has worse electric performance. Compared with the research of [PERSON] and colleagues [27], this paper uses a similar aging test method to analyze the aging process of fiber sheath in transformer oil. According to the literature [27], after 200 hours of heat aging test, the cross-linked polyolefin and polyurethane sheath decomposed to a certain extent in the transformer oil, and the infrared spectrum of the PVC sheath did not change much before and after heat aging. Therefore, the cross-linked polyolefin sheath has the worst heat aging resistance in the transformer oil, the polyurethane sheath is the second, and the PVC sheath has the best performance.