## 3 Study Characteristics  
This study was based on 388 laboratory DGAs of 175 transformers with OLTCs performed over a period from the middle of 2017 to the middle of 2019 (two years). The transformers had age, voltage class and power rating ranges of 1-69 years, 25-400 kV and 3-450 MVA, respectively. Figure 6 shows the distribution of these transformers according to age, voltage class and power rating. Figure 4: Types of oil compartments used by different OLTC designs [32]. Figure 5: Different pathways for transformer oil contamination from the OLTC gases [8]. In addition, the different types of OLTCs that were installed in the power transformers are shown in Figure 6 according to the classification of Figure 3. Abbreviations that show the type of OLTC were generated from the concatenation of the letters in Figure 3: the first letter indicates the type of contacts used by the OLTC (acting (A) or vacuum contacts (V)); the second letter indicates the type of bridge (resistor (R) or reactor type (X)); the last letter shows the type of compartment of the diverter switch and the tap selector (different (S) or the same (C)). Most of the OLTC types for the transformers studied corresponded to the in-tank types of Figure 4a,b. The newer transformers have the double compartment installation type, as shown in Figure 4e. Figure 7 shows the acetylene concentrations measured in the DGAs as a function of the sampling date. The concentration range for transformers without an OLTC or communication with the main tank, as specified in the IEC guide [7], is shown in green. The concentration range for transformers with communicating OLTC, according to [7], is shown in red. Before applying the \(\mathrm{C_{2}H_{2}}/\mathrm{H_{2}}\) ratio to the data, it was necessary to check the reliability of the data in cases that presented an abnormal acetylene concentration, following the indications given in the IEEE guide [6]. After studying the data in which there was a large increase in the acetylene concentration, the results due to gas formation in the transformer were differentiated from those of a bad sample. With this revision, it was possible to discard a result showing 1443 ppm of acetylene (not included in Figure 7), and three DGA results were considered uncertain pending the new DGA results. Figure 6: Distribution of power transformers according to (**a**) age, (**b**) voltage class and (**c**) power rating. A, arcing; R, resistor; V, vacuum. Figure 7: Acetylene concentration measurements. Once the data due to a bad sample were removed from the dataset, the \(\rm C_{2}H_{2}/H_{2}\) ratio was applied. After applying the ratio, five results with errors were observed and needed investigation. These errors were due to a hydrogen concentration of 0 ppm. Table 4 lists the gas concentrations of the five transformers in which no ratio result was obtained. Based on the trend for the rest of the DGAs related to these five transformers, three transformers were discarded because of a lack of contamination from the OLTC, and Transformers 3 and 4 were defined as contaminated from the OLTC. Table 5 lists the DGA results for the transformers with \(\rm C_{2}H_{2}/H_{2}\) ratios greater than two. Comparing the DGA results listed in Table 5 with the rest of the DGA results related to the same transformer, it was observed that Transformers 12 and 13, despite having ratios greater than two, had a very low acetylene concentration in all of their DGA results. The acetylene concentrations of these transformers had ranges of 4-6 and 1-8 ppm, respectively. Thus, these transformers were defined as uncontaminated by the OLTC gases. Based on the comparison of the DGA results listed in Table 5 with the rest of the DGA results for each transformer, twelve transformers were defined as contaminated from the OLTC gases, in addition to Transformers 3 and 4, which were previously defined as contaminated.