# Degradation Assessment of In-Service Transformer Oil Based on Electrical and Chemical Properties
## 4 Discussion
### Relationship Between Different Characteristics
#### 4.1.3 Impact of Load Variation, Oil Temperature, and Winding Temperature  
To enhance the understanding of the variation in DDF, breakdown voltage, and moisture content with service years, other variables of the transformers, i.e., LV side load and oil and winding temperatures were taken into account, as the operating temperature influences the transformer insulation. The results of the electrical and chemical characteristics along with the recorded values of LV side currents and oil and winding temperatures are summarized in Table 3 for each transformer.  
The correlation matrices were generated considering the LV side load, oil temperature, winding temperature, DDF, breakdown voltage, and moisture content for each transformer (Figure 8). It was observed that for transformers T1 and T2, the correlation of LV side load with oil temperature and winding temperature was either weak or negatively moderate. This was because T1 and T2 are connected to the transmission side of the network, i.e., at 220 kV; hence, the load variation had little effect on the oil and winding temperatures. Similar behavior was also observed for the values of the correlation coefficients of LV side load, DDF, and moisture content.  
For transformer T3, a positive moderate correlation was established for LV side load, oil temperature, and winding temperature. This impact of the load variation on the temperatures was due to the fact that transformer T3 is connected to the distribution section of the network, where the impact of the load variation is high.  
Figure 7: Moisture content and breakdown voltage correlations for each transformer.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Transances**} & \multirow{2}{*}{\
\begin{tabular}{c} **Service** \\ **Yeurs** \\ **Loal (A)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Oil Temp.** \\ **(A)** \\ \end{tabular} } & \multicolumn{6}{c}{**Parameters**} \\ \cline{4-17}  
From the correlation matrix, a prominent impact of oil and winding temperatures on the moisture content of T1, T2, and T3 was observed. With the increase in temperature, the moisture content decreased. This correlation of the oil and winding temperatures and moisture is because the moisture in transformers exists in both the oil and the cellulose insulation, with the latter holding the majority due to its hygroscopic nature. As the temperature of the oil and windings increases, moisture migrates from the cellulose insulation to the oil as a result of the shifting equilibrium [39, 40]. However, due to the elevated temperatures, combined with moisture removal systems, oil circulation, and degassing processes, the moisture content is effectively removed. For T1 and T2, the oil and winding temperatures had a weak correlation with the DDF, whereas a strong positive correlation between the breakdown voltage and oil and winding temperatures was noticed for T1, showing that the  
Figure 8: Correlation matrix for transformers (**a**) T1, (**b**) T2, and (**c**) T3. The terms LV Side Load, Oil Temp, Winding Temp, DDF, BDV, and MC refer to the low-voltage side load (A), oil temperature (\({}^{\circ}\)C), winding Temperature (\({}^{\circ}\)C), dielectric dissipation factor at 90 \({}^{\circ}\)C, breakdown voltage, and moisture content, respectively.  
breakdown voltage increased as both temperatures increased. This behavior was possible considering that the moisture and any foreign objects at these temperatures are dissolved in oil, whose impact is limited by the increase in the temperatures. The increase in oil and winding temperatures decreased the breakdown voltage of T3, while the impact of the moisture content was also noticed.