Reduction of Surface Charge-Induced Electric Field Enhancement and Increase in AC Surface Flashover Voltage

Surfactant is painted partially on a cylindrical insulator having a "back-side" (Inner) electrode, to change the distribution of the accumulating surface charge (residual charge) generated by ac partial discharge. The surfactants used are Neogen T®, Catiogen L®, or Tween 20®; the insulating material is polyvinyl-chloride heat shrinkable tube. Ac flashover voltage in air (ACFOV) depends on the surfactant applied: ACFOV decreases in the order Neogen>Catiogen≃non-painted≃ Tween; ACFOV of the Neogen-painted insulator is about 1.4 times higher than that of the Catiogen-painted one at an electrode distance of 140 mm, though the painted part of the two specimens has the same conductivity. The measured distribution of the accumulating charge differs between the Neogen-painted and the other specimens, though the maximum charge density on the surface does not differ much. These experimental results cannot be explained sufficiently by the decrease in the static electric field that is due to the increase in surface conductivity of the painted part. The calculation of the electric field enhancement due to the accumulating charge shows that the field near the electrode edge with the Neogen is lower than that for the other specimens. The higher ACFOV of the Neogen-painted insulator may be caused, not by increasing the surface conductivity, but by the decrease in the field enhancement on account of the difference in the distribution of the accumulating charge.