DC FIELD EFFECTS ON STREAMING ELECTRIFICATION IN INSULATING OILS

The results of an experimental investigation into the influence of imp urities on electrification in insulating liquids in a tube model under an external dc field effect is described. Both conduction and streami ng currents were measured as a function of temperature and flow rate u sing a paper-pressboard model with concentric cylindrical electrode ge ometry. Positive or negative dc voltage was applied to the inner elect rode. The streaming current was measured from a shielded test chamber and the conduction current was measured from the ground electrode. Ene rgization by positive dc voltage always enhanced the streaming electri fication; however, when the applied voltage was negative, the polarity of streaming current was field strength dependent. At low fields (< 0 .52 kV/mm), the streaming current with negative dc field I-sn(c) incre ased with increased field and with field strength > 0.52 kV/mm, polari ty reversal was observed. The field at which current crossed the zero value has been defined as the zero cross over point (ZCOP). Flow rate did not have any influence on the ZCOP; however, the additive concentr ation, moisture content and the temperature affected this ZCOP. In the presence of the corrosion inhibitor carbazole, the streaming current with negative dc field, I-sn attained zero at low fields. The conducti on currents corresponding to fields where streaming currents were zero were also relatively low. It is therefore possible that by applying a moderate dc field of appropriate polarity the streaming current can b e reduced to very low values by adjusting the carbazole concentration in the test liquid.