Predischarge Current Measurements in Vacuum Gaps Bridged with Plexiglas Insulators

Predischarge currents have been observed for Plexiglas® insulators subjected to 60 Hz ac voltages. The predischarges currents were continuous and appeared at voltages much less than the breakdown voltage and were similar in shape and magnitude for both voltage polarities. The predischarge current peaks did not align with the excitation voltage peaks. The current peaks always appeared prior to the voltage peaks. Fowler-Nordheim (FN) plots were made to determine if the observed predischarge currents were due to field emission, as in a plane vacuum gap. The predischarge currents for cylindrical insulators of lengths 5 mm and 12.7 mm did not follow FN theory. The currents were, therefore, postulated not to be totally due to field emission currents. This implies that the predischarge currents observed were due to some interaction with the insulator. Predischarge currents for various triple junction constructions were measured. The triple junctions were constructed using a simple butt joint and with recessed electrodes, to shield the triple junction. No continuous predischarge current was observed for the recessed triple junction arrangement. Different surface treatments were studied for the Plexiglas specimens: an extruded surface, a polished surface, and a rough surface. Continuous currents were measured for the extruded and polished surfaces, while only current bursts were seen for the rough surface. The current inception and the breakdown voltages were found to depend on the surface condition. Preliminary studies were conducted with 30, 45, and 60° Plexiglas frustrums. The predischarge current followed FN behavior in all cases.