## 3 Results and Discussion
### Test Analysis  
Through comparison of the main thermal aging characteristics of the oil in different aging stages, it was found that all types of built-in optical fibers could exacerbate the oil characteristics and affect several of them. In thermal aging, the main reason for the deterioration of insulating oil is the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Oil Category** & **Fitting Equation V = A+B \(\times\) exp(C \(\times\) t)** & **R2** \\ \hline oil containing TPEE fiber & \(V=-0.069\) + 0.039 \(\times\) exp(0.1113 \(\times\) t) & 0.93 \\ oil containing PTFE fiber & \(V=\) 2.553 - 2.577 \(\times\) exp(-0.0067 \(\times\) t) & 0.98 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fitting relations between the acid values and aging time. Figure 4: Water content of oil. Figure 5: Acid of oil curve. pyrolysis reaction, which generates organic acids and unstable small molecule hydrocarbon radicals. The hydrocarbon in acid is strongly hydrophilic group, which can increase the saturated solubility of water in oil [30]. Therefore, the water and acid values of all kinds of oil samples increase. The mineral oil degradation process can be expressed as follows. \[R\mathrm{H}+e^{*}\to R^{*}+\mathrm{H}^{*}\]  
\(e^{*}\) is the energy acting on the mineral oil molecule \(R\mathrm{H}\), \(R^{*}\) and \(\mathrm{H}^{*}\) are hydrocarbon radicals and hydrogen radicals, respectively. At the same time, the sheath of fiber is composed of organic polymeric materials. Thermal aging can cause depolymerization. Figure 6a illustrates the structure of the polyether TPEE used in this study, which consists of a highly crystalline hard segment of polybutylene terephthalalate (PBT) and an oligomer soft segment of butanediol. For TPEE, as the hydrogen at the chain-end of the hydrocarbonly group is more active than the hydrogen at the \(\beta\)-linked carbon atom of the macromolecular ester group, the degradation activation energy at the chain-end is less than at interchain. When the test is below 200 \({}^{\circ}\)C, the degradation at the chain ends is dominant. The degradation at the chain ends produce a large amount of carboxyl groups (Figure 6b) and it undergoes a polycondensation reaction with a hydrocarbon group at the TPEE macro terminal (Figure 6c). The water produced diffuses into the insulating oil. Moreover, PTFE (Figure 7a) undergoes mild depolymerization during thermal aging. Although the breakage of C-C and C-F in perfluorocarbons requires energy absorption of 346.94 kJ/mol and 484.88 kJ/mol respectively, the depolymerization of PTFE to generate TFE requires only 171.38 kJ/mol. Therefore, PTFE was mainly depolymerized into TFE in this test (Figure 7b). Due to the processing formula of PTFE, its pyrolysis product also contained a small amount of free water, amines and carbon dioxide. Figure 6: TPEE depolymerization reaction. (**a**) Chemical structure of the TPEE; (**b**) thermal degradation at chain ends; and (c) polycondensation. Based on the above analysis, it was shown that both TPEE and PTFE depolymerized in high-temperature transformer oils, producing water and small molecule hydrocarbon groups. The accumulation of small hydrocarbon groups promoted positive feedback of pyrolysis of the oil. The free hydrogen produced by oil pyrolysis increased the acidity of the oil, which in turn increased the solubility of the water produced by depolymerization of the sheath. However, different oil samples show a huge diversity in the characteristics of thermal aging, mainly due to the different degrees of aging for various fiber sheaths. To confirm the above inference, we tested the IR spectra of each oil sample. The results are shown in Figure 8 along with information of a-f absorption peaks shown in Table 3.  
Figure 8: The infrared spectrogram of oil sample containing optical fiber. (**a**) TPEE-containing oil; and (**b**) PTFE-containing oil. Figure 7: PTFE depolymerization reaction. (**a**) Chemical structure of the PTFE; and (**b**) thermal degradation.