The influence of electrode material upon prebreakdown current in viscous dielectric oil

Silicone fluids are now being used as an insulator. Watson,² Hakim,³ and Hosticka⁴ have carried out some investigations on this liquid. There is still, however, a great deal that is unclear about the breakdown mechanism under high electric field. It is of importance to know the behavior of the current stress characteristic in the region below breakdown, since the breakdown itself in the liquid dielectric is the ultimate point of the conduction stage.¹ It is, therefore, worthwhile characterizing more fully their dielectric performance in this region of electric field. Watson² has studied the influence of electrode separation upon current conduction in silicone oil and has shown that a linear relationship between the slope of the log voltage vs. log current [d(ℓnV)/d(ℓnI)] ani gap separation exists. He has also shown this phenomenon to be consistent with a model involving Fowler-Nordheim emission which initiates a convective charge transport mechanism with the fluid flowing in a vortex cell enclosing a steady irrotational funnel flow through its core at the cathode. In his interpretation, negative charge will be trapped in the moving core of liquid dielectric, and the space charge is contained laterally by the tendency of the vortex ring to resist stretching by the outward component of the electrostatic Maxwell stress. He reports that the Maxwell stress from this space charge will increase and enlarge the vortex by raising the applied voltage. This suggests that the phenomenon may be influenced by the viscosity of the liquid. The effect of electrode materials was also considered from the viewpoint of their capability to inject charge under stress.