## 2 Background Theory
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{5}{c}{**Gas Generated**} \\ \cline{3-6}
**Fault Type** & & **H\({}_{2}\)** & **CH\({}_{4}\)** & **C\({}_{2}\)H\({}_{6}\)** & **C\({}_{2}\)H\({}_{4}\)** & **C\({}_{2}\)H\({}_{2}\)** \\ \hline Thermal faults in oil (\(<\)300 \({}^{\circ}\)C) & T1 & \(\circ\) & \(\bullet\) & \(\bullet\) &. & \\ Thermal faults in oil (300–700 \({}^{\circ}\)C) & T2 & \(\circ\) & \(\circ\) & \(\circ\) & \(\bullet\) &. \\ Thermal faults in oil (\(>\)700 \({}^{\circ}\)C) & T3 & \(\circ\) & & & \(\bullet\) & \(\circ\) \\ Partial discharges & PD & \(\bullet\) & \(\circ\) & & &. \\ Low energy discharge—sparking & DI & \(\bullet\) & \(\circ\) & & &. \\ High energy discharge—arcing & D2 & \(\bullet\) & & & \(\circ\) & \(\bullet\) \\ \hline \hline \end{tabular}  
*: major concentration; \(\circ\): secondary concentration; \(\cdot\): trace concentration.  
\end{table} 
Table 1: Gas formation based on the type of failure [6; 7]. Figure 2: Gas generation based on temperature (reprinted with permission from CIGRE [5], © 2019). while the range is 60-280 ppm when there is communication between the OLTC compartment and the main tank. Acetylene is generated in the OLTC during tap changes as a result of arcing between the fixed and moving parts [30; 31]. Acetylene appears mainly in a non-vacuum-type OLTC, but acetylene traces can be produced in a vacuum-type OLTC [30; 31]. There are several OLTC classifications according to [30; 31; 32]. The classification proposed in [30] is one of the simplest and most commonly used. It is given in Figure 3.  
According to [30], the OLTC design influences the gases that can be generated due to normal operation depending on the gas generating components, as shown in Table 3.  
The types of oil compartments used for the different OLTC designs are listed in Figure 3, and the pathways through which OLTC gases can contaminate the transformer insulation are shown in Figures 4 and 5, respectively. The transformer oil contamination from the OLTC gases via the gaskets and oil compartment (which is not gas tight) mainly occurs when the OLTC compartment is inside the main tank, as shown in Figure 4a-c. Contamination via the common air space occurs when the transformer and OLTC share an oil expansion tank with a common air space. This type of connection \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**IEEE [6]**} & \multicolumn{2}{c}{**IEC [7]**} \\ \cline{2-5}  & \(\mathbf{O_{2}/N_{2}\ Ratio\leq\ 0.2}\) & \(\mathbf{O_{2}/N_{2}\ Ratio>0.2}\) & **No OLTC** & **Communicating OLTC** \\ \hline
90 th percentile & 1 & 2 & 2–20 & 60–280 \\
95 th percentile & 2 * & 7 & - & - \\ \hline \hline \end{tabular}  
*: This value is 4 ppm when the transformer age is \(>\)30 years.  
\end{table} 
Table 2: Acetylene concentration limits (ppm). Figure 3: OLTC classification (reprinted with permission from CIGRE [30], © 2010). \begin{table}
\begin{tabular}{c c} \hline \hline
**OLTC Components** & **Gas Sources** \\ \hline Arc-switching contacts & High energy discharge gases \\ \hline Commutation contacts, by-pass contacts & Low energy discharge gases \\ \hline Vacuum interrupters & No gases \\ \hline Transition resistors & Heating gases \(\leq\) 300 \({}^{\circ}\)C (normal operation) \\ Heating gases \(>\) 300 \({}^{\circ}\)C (overload, fault case) \\ \hline Transition reactance (preventive autotransformer, inside transformer tank) & No gases \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Gas source based on OLTC design (reprinted with permission from CIGRE [30], © 2010). usually applies to in-tank OLTC types because compartment-type OLTCs generally have a gas space under the tank cover [8; 33]. One of the most commonly used methods for identifying faults using an OLTC DGA is the Duval triangle 2 for OLTCs according to [5; 30; 31; 34]. The Duval triangle 2 makes it possible to distinguish between normal and abnormal gas formations.