Influence of solid insulating phase on streaming electrification of transformer oil

Due to increased demand of electrical power, the trend of the capacity and voltage rating of power transformers increases with reliable design; especially in large oil-cooled power transformers. Consequently, the use of polymer blended and polymer coated insulations in such transformers are the most recent advances in modern technology. The consequences of the type of solid insulating material, in contact with the streaming oil, on the ECT (electrostatic charging tendency) of oil are the principal aim of this paper. ECT of transformer oil is experimentally investigated using different types of oils and solid insulting materials. Two laboratory analog cycles are used; namely, a closed and an open cycle. In the closed cycle, the investigated factors are the type and polarity of the applied voltage (±10 kV and 10 kV_rms at 50 Hz), solid insulating material (polymeric and non-polymeric insulation), and oil type, temperature (20 to 70°C) and velocity (1.5 to 3.25 m/s). Good agreement is achieved between the measured data under ac and dc voltages of both the conduction and streaming currents, and the derived formulas. The currents increase exponentially with oil temperature, velocity and applied voltage. All solid dielectrics used show quantitative as well as qualitative changes in oil electrification especially at low temperatures. In the open cycle, both the rotating cylinder speed (430 rpm) and the oil volume are kept constant, and the oil temperature is raised to 120°C for different types of oils and solid dielectrics. ECT depends on the oil type and is activated with temperature. Cellulose-based dielectrics give the highest ECT and coating of these dielectrics with epoxy resin enhance the ECT. Different suppression characteristic is obtained using Theophylline in different fresh oils.