Contents I. Introduction
Transformer is a vital electromechanical energy conversion device used in transmission and distribution systems [1], [2]. It is used for both high and low current applications [2]. In order to reduce large power loss and voltage drop in transmission, voltage is stepped up at sending end and stepped down at the receiving end with the help of transformers [3]. Hence, it is also important to monitor its condition on regular basis to assess its ageing in order to take resolution at proper time and thus, maximizing the returns on investment and reducing the operational cost [4]–[6]. Liquid dielectrics are used in transformers to provide insulation between the live and the grounded parts apart from carrying out the heat transfer to the atmosphere [7]. Breakdown voltage of transformer oil, also known as dielectric strength of transformer oil is an important parameter to determine the condition of the transformer oil. Breakdown voltage is mainly affected by many factors namely moisture, suspended particles, acidity and pressure [8]. Even if 0.01 % water is present in transformer oil, the electrical strength is reduced to 20% of its dry state value [9]. Moisture is a major factor for affecting the performance of transformer oil by increasing the probability of bubble formation, and thus decreasing the dielectric strength of transformer oil [10]. Water is generated in insulating oils due to air moisture if the transformer has open-breather, faulty vessel seal or exposure during repairs and, due to molecular breakdown of cellulose based solid insulating materials [10], [11]. Water content in the transformer oil affects the life expectancy and loading capacity of the transformer. The dielectric strength of clean and new transformer oil decreases exponentially with an increase in relative humidity. The water content can be evaluated either by using moisture sensors or in laboratory with the help of Karl-Fischer titration through equilibrium curves. Moisture sensors are mainly used in case of online monitoring and Karl-Fischer titration is used in offline monitoring of Transformer Oil [11]. The water solubility is the total amount of water that can be dissolved in the transformer oil for its saturation at a particular temperature. The solubility of water in transformer oil is not constant but increases as an exponential function with increase in temperature. Thus, the amount of water that can be dissolved to saturate the transformer oil also increases exponentially with temperature [11], [12]. The solubility can be calculated using (1) as referred in [12]. \begin{equation*} \text{Log} s =A - (B/T) \tag{1}\end{equation*}
