# Degradation Assessment of In-Service Transformer Oil Based on Electrical and Chemical Properties
## 1 Introduction
Furthermore, the authors in [19; 20] have shown that the aging rate of paper insulation increases with a high level of moisture content. These studies have shown the detrimental effect of moisture content on both oil and paper insulation, as moisture not only accelerates hydrolysis but also diminishes the oil's dielectric strength, making transformers more susceptible to electrical failures. Consequently, controlling oxygen and moisture ingress is essential for extending transformer life. While different stressors contribute to the degradation of the oil in general, the degradation behavior of the two widely used types of transformer oils, i.e., mineral oil and natural esters, differ significantly. Natural esters have a higher ability to absorb moisture as compared to mineral oil, hence affecting the moisture dynamics inside the transformer [21]. However, natural esters have better aging resistance and thermal conductivity than mineral oil [22; 23]. Nonetheless, due to the presence of more carbon-carbon double bonds in natural esters, they are more vulnerable to oxidation as compared to oxidation [24]. In terms of their electrical characteristics, natural esters as compared to mineral oils maintain higher breakdown voltage [7; 25]. Mineral oil has been the preferred option for transformer oil in spite of the fact that natural esters provide several advantages This preference is primarily due to the lower cost, better thermal performance, and long-term reliability across various operating conditions of mineral oil [21; 26]. For in-service transformer oils, irrespective of the type, as degradation is multifactorial, it is necessary to continuously monitor them to assess both the electrical and chemical integrity of the oil and insulation materials over time, which requires diagnostic tools capable of identifying degradation trends early, before severe faults develop. Dissolved gas analysis (DGA) is the most prevalent diagnostic method, providing insights into the types and concentrations of fault gases generated during oil aging [27]. By analyzing the ratios of gases, such as hydrogen to methane or acetylene to ethylene, DGA allows for the categorization of fault types. The Dornenburg and Rogers ratio methods are established approaches within DGA, used to differentiate between PD, arcing, and overheating faults [12; 28; 29]. These methods, however, have limitations, particularly when gas levels are below diagnostic thresholds or when multiple fault types occur simultaneously. While significant research has been conducted on transformer oil aging, a comprehensive study that correlates both electrical and chemical characteristics over extended service periods remains limited [30; 31; 32]. Much of the existing literature focuses either on DGA interpretation or on isolated chemical and electrical properties, often without longitudinal analysis [33; 34; 35]. Additionally, studies rarely account for the combined effects of moisture, dissolved gases, electrical properties, and thermal stress in real-world operational conditions. Therefore, this study aims to provide a systematic analysis of both electrical and chemical indicators of aging in oil samples collected from in-service transformers over a period of service. This research analyzes the dielectric dissipation factor (DDF) at 90 \({}^{\circ}\)C, breakdown voltage, moisture content, and dissolved gas concentrations to offer insights into the degradation process in operational transformers installed in a 220 kV substation in Pakistan. By examining these parameters in tandem, this study seeks to identify early aging markers and contribute to the development of refined diagnostic methods, enabling more effective condition monitoring and predictive maintenance strategies. The paper is organized as follows. Section 2 outlines the characteristics of transformers under consideration and test methods adapted for oil degradation analysis. Section 3 discusses the results of the measurement, while the discussion is carried out in Section 4.