# Degradation Assessment of In-Service Transformer Oil Based on Electrical and Chemical Properties
## 5 Conclusions  
Insulating oil samples from three in-service power transformers were collected annually for 4 years, and the impact of different variables on the degradation of the oil were studied. The electrical and chemical characteristics of the oil were analyzed and, based on the results, the characteristics that indicated deterioration tendencies with each service year were explained, and characteristics that can be used as indicators of insulating oil degradation were found. A summary of the study's findings is provided as follows:  
1. All three transformer oil samples showed good electrical characteristics after each service year. However, the DDF and breakdown voltage showed variation with service age for all three transformers. 2. Moisture content showed an increasing trend with each service year for all transformers. The DGA results revealed that the concentrations of carbon monoxide, oxygen, and nitrogen increased after each service year. 3. The LV side load variation in the transformer connected to the distribution side of the network decreased the breakdown voltage of the insulation oil, whereas the oil and winding temperatures increased. However, for the two power transformers connected to the transmission network, the breakdown voltage of the insulating oil increased with the increase in oil and winding temperatures. 4. The degradation of cellulose insulation in the transformers was revealed by the concentrations of carbon monoxide, carbon dioxide, methane, oxygen, and moisture content. The carbon dioxide to carbon monoxide and oxygen to nitrogen ratios identified the presence of electrical faults in the transformers, leading to cellulose insulation degradation. For the current study, all of the samples had characteristic values that met the acceptance values on the whole, and none of them were significantly degraded with increasing age. Owing to the fact that the in-service transformers considered in the current study were in the early years of commissioning, the findings of this study have significant implications for the operational plans, condition-based monitoring, predictive maintenance, and lifecycle management of transformers. As transformers age, the degradation rate of their insulation tends to increase, particularly in the later stages of their service life. Henceforth, the key indicators of degradation identified in this study, such as dissolved gas analysis \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Gas Ratios**} & \multicolumn{8}{c}{**Transformer and Service Years**} \\ \cline{2-13}  & \multicolumn{4}{c}{**T1**} & \multicolumn{4}{c}{**T2**} & \multicolumn{4}{c}{**T3**} \\ \cline{2-13}  & **2020-21** & **2021-22** & **2022-23** & **2023-24** & **2020-21** & **2021-22** & **2022-23** & **2023-24** & **2020-21** & **2021-22** & **2022-23** & **2023-24** \\ \hline \(\rm CO_{2}/CO\) & 1.193 & 1.336 & 0.167 & 0.281 & 0.876 & 1.445 & 0.225 & 0.183 & 1.271 & 1.252 & 1.383 & 0.892 \\ \(\rm O_{2}/N_{2}\) & 0.209 & 0.178 & 0.184 & 0.210 & 0.201 & 0.178 & 0.171 & 0.200 & 0.356 & 0.307 & 0.273 & 0.266 \\ \hline \end{tabular}
\end{table} 
Table 5: Gas ratios of in-service transformers for each service year. (DGA), breakdown voltage, moisture content, specific dissolved gases, and gas ratios, can help mitigate operational risks and provide a foundation for early fault detection. Furthermore, the differences observed between the power and distribution transformers underscore the impact of load and thermal conditions on the health of oil and insulation. This understanding allows for the development of tailored operational guidelines that can enhance the performance and reliability of the transformers. In the future, more controlled experimental studies will be conducted by accelerating the aging of oil samples and studying the impact of single variables on the characteristics of the oil. Conceptualization, E.M. and B.A. ; methodology, E.M., B.A. and M.I.A. ; validation, E.M., R.S.A. and R.U. ; formal analysis, E.M., B.A. and M.I.A.