## 3 Study Characteristics
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Transformer** & **Sample** & \(\rm C_{2}H_{2}/H_{2}\) & **Age** & **Acetylene** & **Hydrogen** \\
**No.** & **Date** & **Ratio** & **(Years)** & **(ppm)** & **(ppm)** \\ \hline
1 & 25/04/2019 & – & 54 & 0 & 0 \\
2 & 24/05/2018 & – & 10 & 0 & 0 \\
3 & 13/06/2018 & – & 44 & 52 & 0 \\
4 & 27/05/2019 & – & 29 & 15 & 0 \\
5 & 25/09/2017 & – & 26 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Transformer data without the \(\rm C_{2}H_{2}/H_{2}\) ratio result. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Transformer** & **Sample** & \(\rm C_{2}H_{2}/H_{2}\) & **Age** & **Acetylene** & **Hydrogen** \\
**No.** & **Date** & **Ratio** & **(Years)** & **(ppm)** & **(ppm)** \\ \hline  
& 45 & 11 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Transformer data with the \(\rm C_{2}H_{2}/H_{2}\) ratio greater than 2. Table 6 lists the transformers with DGA results that exceeded 10 ppm of acetylene and C\({}_{2}\)/H\({}_{2}\) ratios smaller than two. The transformers established as contaminated in Table 5 are excluded from Table 6. As listed in Table 6, the acetylene concentrations of these transformers and their ages had ranges of 11-219 ppm and 18-50 years, respectively. As in the previous cases, the transformers in Table 6 were compared with the rest of the DGA samples corresponding to each one. Transformers 28 and 33 had acetylene concentration ranges of 7-23 and 8-11 ppm, respectively. Transformer 33 was discarded as contaminated because of its low acetylene concentration values and stability over time. Based on the acetylene concentration listed in Table 6, Transformer 28 could be contaminated. All of the DGA results except for that of one sample (23 ppm) indicated low and stable acetylene concentrations. Because of this and the fact that it was connected to the power transmission network, it was defined as uncontaminated. In this way, the transformer was controlled by applying the limits of Table 2 corresponding to a transformer without OLTC to its monitoring. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Transformer** & **Sample** & **C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\)** & **Age** & **Acetylene** & **Hydrogen** \\
**No. ** & **Date** & **Ratio** & **(Years)** & **(ppm)** & **(ppm)** \\ \hline
4 & 09/05/2018 & 0.55 & 29 & 11 & 20 \\
19 & 01/04/2019 & 0.88 & 48 & 219 & 248 \\
19 & 17/10/2018 & 0.92 & 48 & 144 & 157  
From Table 6, the old transformers (Nos. 19, 20, 21, 22, 23, 24, 25 and 27) in the 43-50 year range had hydrogen and acetylene concentrations with high values. The high hydrogen values were due to gas accumulation over time because hydrogen is produced in almost all incipient faults. The historical data for each transformer were reviewed, and no fault that indicated arcing in the insulation was found. Thus, it could be said that the high acetylene values were due to OLTC gas contamination. Transformers 19, 20, 21, 22, 23, 24, 25 and 27 were defined as contaminated. As in the previous case, the historical data for Transformers 26, 29, 30, 31 and 32 were reviewed, and no arcing fault was found in the transformer insulation. As can be seen in Table 6, this group of transformers had low acetylene concentrations, except for Transformer 26, and a relatively stable trend. Despite this, they were classified as contaminated from OLTC gases because no arcing fault was found in the historical data. In summary, twenty-six of the 175 transformers were defined as contaminated from the OLTC gases. Out of these, eighteen of the 26 transformers defined as contaminated were of the OLTC ARC type, and the remaining eight were of the OLTC ARS type. Figure 8 shows the distribution of these transformers according to age, voltage class and power rating. In addition, the different types of OLTCs installed in these power transformers are shown in Figure 8.  
It should be noted that the methodology followed in this section is an assumption of transformer oil contamination based on the study of the DGA samples.