# Degradation Assessment of In-Service Transformer Oil Based on Electrical and Chemical Properties
## 1 Introduction  
Power transformers are critical components in electrical transmission and distribution networks, ensuring efficient and stable energy delivery. Transformer oil, commonly a mineral-based insulating fluid and natural ester, serves multiple functions, including cooling, insulation, and arc suppression. However, transformer oil is subject to aging, a complex process influenced by thermal, electrical, and chemical stresses inherent to prolonged operation. This aging process degrades the oil's insulating properties and accelerates the breakdown of cellulose-based insulation, eventually compromising transformer reliability and potentially leading to costly failures [1, 2]. Thus, the study of transformer oil aging is crucial for understanding the mechanisms that affect transformer health over time. Transformer oil degradation is driven by thermal, electrical, and chemical stressors, each contributing to molecular breakdown and structural alteration in both the oil and cellulose insulation materials. Thermal stresses arise from high operating temperatures that accelerate oxidation processes, resulting in acid formation and an increase in the oil's viscosity, which ultimately diminishes its insulating capacity [3, 4, 5]. Electrical stressors, particularly partial discharge (PD) and arcing, generate localized high temperatures that promote the breakdown of oil molecules and produce fault gases, such as hydrogen (H\({}_{2}\)), methane (CH\({}_{4}\)), acetylene (C\({}_{2}\)H\({}_{2}\)), and ethylene (C\({}_{2}\)H\({}_{4}\)). These gases dissolve within theoil, and their concentrations are analyzed to assess fault severity and type [6; 7; 8]. The concentration of hydrogen and hydrocarbon gases, such as methane, ethane, ethylene, and acetylene, are markers of electrical faults, including PD, arcing, and overheating in oil [8; 9]. Breakdown of oil due to chemical reactions within the oil is further accelerated by contaminants, including moisture, oxygen, and metal particles, which catalyze oxidation processes. The presence of moisture is particularly detrimental as it not only accelerates the oil's degradation but also promotes cellulose decomposition in paper insulation, leading to a release of carbon monoxide (CO) and carbon dioxide (CO\({}_{2}\)) [10]. Also, the presence of carbon monoxide and carbon dioxide is associated with the degradation of cellulose insulation due to thermal and oxidative stresses [11; 12]. Studies by [PERSON] and [PERSON] (2005) have demonstrated that increases in CO and CO\({}_{2}\) levels correspond with accelerated cellulose aging, particularly at elevated temperatures [13]. However, the studies conducted by [PERSON] et al. (2020) and [PERSON] and [PERSON] (2010) emphasize that carbon monoxide and carbon dioxide ratios serve as reliable indicators of thermal faults in the solid insulation materials of transformers [7; 14]. Furthermore, oxygen dissolved in the oil participates in oxidation reactions, forming acidic byproducts that degrade both the oil and cellulose insulation in a self-perpetuating cycle [15]. The aging of transformer oil exhibits both electrical and chemical alterations that can signal the onset of insulation failure. Electrically, aging reduces the breakdown voltage of the oil as impurities and dissolved gases weaken its dielectric strength [16]. Meanwhile, dielectric strength is a key metric for transformer health, as a reduction in this property can lead to dielectric breakdown under normal operating voltages. Thermally stressed oil also shows an increase in dielectric dissipation factor (DDF), also known as dielectric loss, a parameter that reflects energy dissipation and provides insight into the extent of thermal degradation [17]. Meanwhile, moisture and oxygen play critical roles in exacerbating both electrical and chemical degradation. [PERSON] et al. (2019) showed that the insulating properties of transformer oil strengthen with the decrease in moisture content [18].