IEEE Transactions on Electrical Insulation Vol. EI-19 No.6, December 1984 PRE-BREAKDOWN AND BREAKDOWN PHENOMENA ALONG PMMA SURFACES IN VACUUM AND NITROGEN GAS STRESSED BY 60 Hz VOLTAGES J. Lewis, T. S. Sudarshan, J. E. Thompson, D. Lee and R. A. Dougal University of South Carolina College of Engineering Columbia, South Carolina ABSTRACT Predischarge current and breakdown measurements of solid Plexiglas® (PMMA) insulators in vacuum and nitro- gen gas gaps, excited by 60 Hz ac voltages, are reported. Results show that for bridged vacuum gaps, the predis- charge current is in the form of pulses. In addition to electron emission from the cathode, secondary emission from the insulator surface is postulated to play a significant role in determining the characteristics of the current pulses. It is reported that for bridged nitrogen filled gaps no partial discharges prior to breakdown were noted, except for the case of introduced artificial voids. Insulating spacers in the form of circular cylinders and conical frustrums of different angles have been studied. The breakdown characteristics are found to depend strongly on the shape of the insu- lating spacer and also on the surface condition. For any given gas pressure, the breakdown strength was found to decrease with decreasing angle of the cone frustrum at its base. A rough insulator surface ex- hibited considerable discharge track resistance compared to a smooth surface. For cylindrical spacers, the pre- discharge and breakdown characteristics are found to be strongly influenced by the shape and position of the introduced voids. The effect of gas pressure on the pre-discharge and breakdown characteristics are reported. INTRODUCTION There has been considerable interest in the pre- breakdown phenomena of bridged vacuum and compressed gas gaps. The insulating spacer is often the weakest link in any HV system. Previous work has attempted to link the breakdown phenomena with surface charg- ing of the insulator when subject to high stresses [1,2,3]. In vacuum it is postulated that the elec- trons emitted from the cathode strike the insulator with energies corresponding to a high secondary electron yield, leaving the insulator surface charged positively. The work reported here is an attempt to detect and study the characteristics of the predischarge activ- ity in bridged vacuum and compressed gas gaps. In previous work it was noted that the predischarge currents did not follow a Fowler-Nordheim plot which is characteristic of an unbridged vacuum gap [3]. This implies that the predischarge currents are not due to field emission alone, but rather are due to a combination of field emission and secondary emis- sion. In this work, further effort is made to under- stand the reasons for the departure from a Fowler- Nordheim type of prebreakdown emission current for a bridged gap subject to high voltage stresses. This is accomplished by measuring the predischarge current using a fast-response (5 pis) bridge circuit. The results obtained with the bridge circuit are compared with those obtained using the slower response compen- sation circuit (250 ps) used in the previous work [3]. In addition, the predischarge current measure- ments are extended to compressed nitrogen gas bridged gaps. The effect of various insulator and electrode parameters on the predischarge and breakdown charac- teristics of bridged gaps, are studied. These char- acteristics are expected to provide a better under- standing of the vacuum and compressed gas bridged gaps. 0018-9367/84/1200-0512$01.00 ® 1984 IEEE 512