Key Words: Transformer paper, transformer oil, moisture equilibrium, vapor pressure, relative humidity he presence of moisture in a transformer deteriorates transformer insulation by decreasing both the electrical and mechanical strength. In general, the mechanical life of the insulation is reduced by half for each doubling in water content [l]; the rate of thermal deterioration of the paper is proportional to its water content [2]. Electrical discharges can occur in a high voltage region due to a disturbance of the moisture equilibrium causing a low partial discharge inception voltage and higher partial discharge intensity [ 3 ] . The migration of a small amount of moisture has been associated with flow electrification at paper/oil interfaces and is presumed to be due to charge accumulation on highly insulating interfacial dry zones [4,5]. Water in mineral oil transformers also brings the risk of bubble formation when desorption of water from the cellulose increases the local concentration of gases in the oil [6]. The importance of moisture presence in paper and oil systems has been recognized since the 1920s.It is useful to know the moisture partitioning curves between oil and paper under equilibrium conditions. When the transformer is in equilibrium operation, this provides a quick way of examining the moisture content in paper to predict future failure by measuring the moisture in oil. Over the years, many scientists have reported such a set of curves, but there has not been a comprehensive review and comparison for different curve sets. The research spans several decades and is an important resource for electric utilities and insulation and testing equipment manufacturers. This paper gives an overview of the classic moisture equilibrium curves and their history and provides useful information on the relationships among them and their validity.

Bubble evolution from overloading of transformers is a concern due to possible dielectric failure. Since overloading is inevitable for short periods during peak demand periods, it is necessary to specify the limits of overloading with respect to winding hot spot temperature which determines bubble evolution. Through extensive coil model testing in the early 1990s, a mathematical equation was formulated to compute bubble evolution temperatures under a variety of conditions: moisture in insulation, gas content of oil, and the pressure in the system. This paper describes the coil models, test results and the mathematical formula. Also presented is a tentative mechanism of bubble formation.

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A simple flow model was designed to simulate oil flow in the insulation ducts of power transformers. It consisted of an annular paper tube, relaxation tanks, pump and flow controls. Both laminar and turbulent flow modes wer& studied in the temperature range 27 to 70 C. Current development was monitored as a function of temperature and flow velocity. The temperature dependence was approximately the Arrhenius type; no current peak was observed as reported for transformers, in the temperature range studied. The current increased linearly with flow velocity at low flow rates, and increased as the square of velocity at high flow rates with a gradual transition in the intermediate range of flow. development was characterized by a sharp rise initially as the flow was turned on, levelling off to a maximum after several minutes. Partial discharge patterns were observed in the volt-time plot, increasing in frequency and temperature at elevated temperatures and flow velocities. A strict application of the test results to transformers is not possible due to unknown parameters.