Effect of cross-field flow on conduction current and breakdown in transformer oil using point-to-plane electrodes and direct voltage

This paper reports some of the results obtained from extensive tests on the effect which enforced cross-field flow has on the variation of breakdown voltage with gap length (200-900 μm) and of conduction current with voltage for transformer oil using direct voltage and point-to-plane electrode geometry. Tests with both polarities of the point electrode were carried out on degassed oil, O2- and SF6- saturated oil and oil with 1-methylnaphthalene and dimethylaniline as additives. These additives were chosen in order to discriminate between electron trapping properties and ionization potential. The results are complementary to those previously reported by the authors on the effect of co-field flow on breakdown and conduction. Conduction and breakdown measurements carried out under enforced flow conditions provide direct evidence that there is a strong correlation between breakdown and conduction. The present results also indicate that cross-field flow is more effective than co-field and that the electron trapping properties of the additives used are more important than their ionization potential in their influence on both conduction and breakdown mechanisms