## 1 Introduction  
Power transformers are considered to be the most important assets in power substations. Thus, their maintenance is important to ensure the reliability of the power transmission and distribution system [1; 2]. Different maintenance strategies are used for assets based on the condition of and available information about the components and sub-components of the substation equipment, according to [3]. The current trend in maintenance strategies is to maintain a predictive maintenance approach based on prognostics to predict the future asset degradation [3]. Dissolved gas analysis (DGA) is the most commonly used method for managing the maintenance and establishing the health status of power transformers, with much weight given to the results [4]. The DGA method measures the gas concentrations in the oil. The gases normally measured are hydrogen (H\({}_{2}\)), acetylene (C\({}_{2}\)H\({}_{2}\)), methane (CH\({}_{4}\)), ethylene (C\({}_{2}\)H\({}_{4}\)), ethane (C\({}_{2}\)H\({}_{6}\)), carbon monoxide (CO) and carbon dioxide (CO\({}_{2}\)). These gases are formed by the decomposition processes of insulation, which are caused by active faults. From the gas concentrations obtained in the DGA results, it is possible to identify the type of fault. Although DGA continues to be performed in the laboratory,there is a trend toward online DGA monitoring, because it makes it possible to detect or diagnose faults that occur in the power transformer [1]. The presence of acetylene in the DGA results may indicate arcing or high-temperature thermal faults in the transformer [5; 6; 7]. Although the presence of acetylene in the DGA results does not always indicate a transformer fault, in old transformers with an on-load tap-changer (OLTC), oil or gases can be filtered from the OLTC compartment to the main transformer tank. Gas leakage between oil compartments may influence the DGA results and the identification of transformer insulation faults [6; 7; 8; 9]. This case was shown in [10]: two faults were identified as low energy discharges (D1); later, it was determined by inspection of the equipment that it was oil contamination from the OLTC gases. The guides [6; 7] utilise the C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\) ratio to give an indication of the transformer oil contamination from the OLTC gases. According to [11], the C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\) ratio is used to determine the filtering of fault gases in the main tank from the OLTC compartment when the ratio is greater than or equal to two and, furthermore, when the C\({}_{2}\)H\({}_{2}\) concentration is greater than or equal to 30 ppm. When electrical discharges occur in the transformer oil, the amount of acetylene is usually less than the amount of hydrogen. Since the solubility of acetylene is greater than the solubility of hydrogen, when electrical discharge is generated in the OLTC compartment, acetylene diffuses faster out of the OLTC tank. This results in the acetylene concentration in the transformer oil being greater than the hydrogen concentration. Several studies [12; 13; 14] have used this ratio to define the transformer oil contamination from the OLTC gases in several samples. In the data processing for the calculation of the typical gas concentration, gas generation rates and fault identification are influenced by the lack of knowledge about the existence of communication with the OLTC [14]. The report uses the gas ratios included in [7] to identify faults and the C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\) ratio to indicate the oil contamination from the OLTC gases. According to [15], there are three steps to follow in the fault identification rules, including the application of the C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\) ratio. The first step is to decide if the values of the gas concentrations are enough to establish that there is an active fault in the transformer oil. The second step consists of applying the C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\) ratio to distinguish between contamination or not from the OLTC gases.