Flow electrification of transformer oil effects of mixed fields

Abstract

Static electrification in transformer oil is investigated experimentally using a laboratory synthetic closed cycle, where the oil is pumped in a coaxial electrode arrangement. The electrode system is electrically energized radially with dc and/or ac, and the electrostatic charging tendency (ECT) of the oil is quantified by measuring the streaming current. The results indicate that for the same voltage ratio ( K = V / ( V + V )I, increasing the ac or the of dc voltage component leads to higher ECT of oil, although the ECT under only ac field is much lower than that under dc field. The unenergized streaming current is inherently affected by the electrode material and configuration; where electrodes having a lower work function give higher positive ECT of oil at high temperature. The energized streaming current increases with oil temperature, oil velocity and electric field; where negative dc voltage application to the outer electrode gives both higher conduction and streaming currents. While the conduction current decreases with oil velocity, increasing the frequency of mixed ac voltage has no significant effect on streaming current. Moreover, the effect of combined radial electric (ac or dc) and axial magnetic (ac or dc) fields on ECT of oil is investigated, and the results reveal that the ECT is enhanced by the magnetic field while the radial conduction current decreases. Derived formulas for the streaming current for unenergized and ac energized cases are also presented together with expressions of the volume and surface charge densities for the coaxial electrode system.

Investigation of electrification and breakdown strength about transformer oil/pressboard

“…Streaming electrification experiment research and theory calculation have been executed on the total dimension model of the transformer. However, the static electrification processes are complex and depend on the properties of the oil/pressboard material interface [10,11]. Adawy et al [12] presented an analysis for the physicochemical reaction in the case of dielectric liquid containing additives or impurities partially dissociated into positive and negative ions in contact with a conductive solid material.…”

Behaviour of electrical double layer under oil flow and voltage application inside a capacitive sensor

“…Equation (5) shows the dependence of the streaming current on both the mean flow velocity and the value of chemical reaction coefficient. In addition, it can be used for the determination of space-charge density associated with flow electrification experiments for the fully developed EDL.…”

“…The heat exchanger (3, 4) is attached to a thermostatic device, which controls the oil temperature between 5 and 90°C. Oil temperature is measured at the inlet of the sensor (5). The streamed charges by the liquid flow are relaxed in the streaming vessel (6), where the streaming current (I s ) is measured.…”

“…There is a little dependence on the applied field wave shape. It can be seen that the relative steady-state streaming current decreases with the increase in the frequency because of the vibration of the charge carriers which makes the diffusion of these ions in the flowing oil difficult [5,7]. In addition, the time needed for the physiochemical reaction is a key parameter in determining the streaming current.…”

“…This phenomenon in large forced-oil-cooled power transformers has been implicated in many catastrophic field failures worldwide for more than five decades [2][3][4][5][6][7][8][9][10][11]. In these transformers, Kraft paper wound coils and pressboard washers acquire a negative direct current (DC) polarity, while the flowing oil becomes positively charged [12].…”