# Degradation Assessment of In-Service Transformer Oil Based on Electrical and Chemical Properties
## 3 Results
### Chemical Characteristics  
The impact of aging on the moisture content of transformers T1, T2, and T3 is plotted in Figure 3. For transformer T1, an increasing trend in the moisture content was observed throughout the service years. However, for transformers T2 and T3, except for a decreasing trend in the moisture content values during the second service year, an increasing behavior was noticed with each service year that passed.  
Figure 3: Moisture content vs. service years for each transformer.  
Figure 2: Breakdown voltage vs. service years for each transformer.  
The DGA analysis specified nine (9) key gases for the analysis of the oil samples. The gases included acetylene (C\({}_{2}\)H\({}_{2}\)), ethylene (C\({}_{2}\)H\({}_{4}\)), ethane (C\({}_{2}\)H\({}_{6}\)), methane (CH\({}_{4}\)), hydrogen (H\({}_{2}\)), carbon monoxide (CO), carbon dioxide (CO\({}_{2}\)), oxygen (O\({}_{2}\)), and nitrogen (N\({}_{2}\)). Acetylene, ethylene, ethane, methane, and hydrogen are considered to be produced due to the decomposition of oil, whereas carbon monoxide, carbon dioxide, and oxygen are generated due to cellulose insulation degradation. The concentration of the gases for each transformer is plotted for the service years in Figure 4. For all transformers under study, similar behavior for acetylene, ethylene, ethane, oxygen, and nitrogen was observed, where the values of the gas concentration increased in the oil samples with each service year. However, the values of methane increased for T1, T2, and T3 after the first two years of service, but then decreased after the third service year for T1 and T3. However, for T2, the DGA analysis of the oil samples taken after the third service year showed no traces of methane. Nonetheless, a significant increase in methane concentration was noticed after the fourth service year.  
Figure 4: Concentration of nine gases measured after each service year plotted for transformers (**a**) T1, (**b**) T2, and (**c**) T3.  
For transformers T1 and T2, the carbon monoxide concentration increased after the first three years of service, which then decreased after the fourth year. However, for transformer T3, an increase in the concentration of carbon monoxide after the first and second service years was recorded, which then decreased after the third service year. However, after the fourth service year, the concentration of carbon monoxide increased again. In the case of carbon dioxide, its concentration increased for the first two service years for all transformers. However, the DGA results obtained after the third and fourth years showed a decrease in its concentration for transformers T2 and T3. For T1, the carbon dioxide concentration decreased in the third service year and then increased after the fourth year.  
The concentrations of oxygen and nitrogen increased during the service years for all transformers. The hydrogen gas concentration showed a distinct behavior from the rest of the gases. It was observed that the concentration of hydrogen increased in the first two service years for transformers T1 and T2, whereas for the subsequent years, no traces were found of it. For transformer T3, no traces of hydrogen were noticed throughout the service years.  
It is important to mention here that the values of the moisture content and gas concentration were well below the acceptance values, as was the case for the DDF and breakdown voltage.