## 2 Background Theory  
Power transformers are maintained based on the information available for the components and sub-components of the transformer. A risk index is used to prioritise maintenance work in a group of transformers [3; 29]. This risk index is a function of the probability of failure and the consequences of failure. The probability of transformer failure is a parameter that is associated with the health index (HI) and varies over time. The consequences of failure evaluate and define the results of a failure event and do not vary if the transformer is not relocated. In the calculation of the transformer's HI, more condition parameters are used than for any other substation asset. The condition parameters used to calculate the HI for a power transformer are shown in Figure 1.  
Based on the condition parameters shown in Figure 1, oil analyses (DGA, OQA and FFA) are the easiest to perform without de-energising the transformer and provide the most useful health information [2]. Figure 1: Locations of condition parameters used to calculate the HI of the transformer. The most commonly used method to identify and diagnose transformer oil faults is DGA. The combination of gas concentrations generated in the transformer oil is a result of the nature of the fault, as well as the temperature and energy at the fault location. Faults can be identified from the gas concentrations in the DGA results using the methods for interpreting the DGA results reported in [6; 7; 10]. The key gases that are generated depending on the potential fault are listed in Table 1.  
Acetylene is one of the combustible gases that can be produced when a fault occurs in a transformer [6; 7]. The transformer insulation faults associated with the presence of acetylene in DGA results are high and low energy discharges and high thermal faults (Table 1). As seen in Figure 2, very high temperatures (>800 \({}^{\circ}\)C) followed by very rapid cooling are needed for the acetylene concentration to increase, causing it to accumulate in the oil, which occurs during arcing [5]. Traces of acetylene can also be formed at temperatures below 800 \({}^{\circ}\)C, as shown in Figure 2.  
The presence of acetylene in DGA results may also be due to the filtration of OLTC gases. According to [6; 7], the C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\) ratio can be used to determine the transformer oil contamination from the OLTC gases. When this ratio is greater than two or three, this indicates contamination. As expected, the DGA results for transformers with oil contamination from the OLTC gases will show a higher acetylene concentration than normal. According to the guides for the interpretation of the gases generated in the transformer oil [6; 7], there are different acetylene concentration limits based on whether or not there is communication between the OLTC compartment and the main tank, or even whether or not OLTC is used, as seen in Table 2. Table 2 shows the acetylene concentration limits collected in the guides [6; 7]. The IEEE guide indicates the gas limit depending on the result of the O\({}_{2}\)/N\({}_{2}\) ratio, also showing the 90 th and 95 th percentile of the typical acetylene concentration. The IEC guide indicates the 90 th percentile of the typical acetylene concentration for cases where there is communication between oil compartments, called communicating OLTC, and when there is no communication between oil compartments or no OLTC is used. The acetylene concentration range when there is no communication between oil compartments or no OLTC is used is 1-20 ppm,