J. C. Devins

Breakdown and prebreakdown phenomena in liquids

In this paper we present a comprehensive account of our results on streamer propagation in dielectric fluids in point‐plane geometries. Propagation velocities for both positive and negative streamers have been determined as a function of the following parameters: temperature, pressure, density, viscosity, composition, and conductivity. Effects of voltage and interelectrode spacing were examined. Current and light emission during streamer growth were measured. The relation between shock wave and streamer velocities was investigated. Small concentrations of low‐ionization potential additives markedly accelerated the positive streamers, while electron scavengers accelerated the negative streamers. Mechanisms to account for these observations are discussed.

Progress in the Field of Electric Breakdown in Dielectric Liquids

For some fifty years the mechanism of electric breakdown in liquid insulation has been a subject of great interest for both theoretical as well as practical reasons. Over the years a number of promising hypotheses of breakdown have been advanced and it has been necessary to modify, and sometimes reject, interpretations of the breakdown data as additional experimental evidence has been accumulated.

The physics of partial discharges in solid dielectrics

The work described in this paper was carried out some years ago. It was published as part of a U.S. Air Force report [1] and presented as a Conference Paper [2] at this Conference. Since the latter appeared only as an extended abstract and the former did not receive wide distribution it seemed appropriate to present it here, and in a more detailed paper to be published shortly [3].

The 1984 J. B. Whitehead Memorial Lecture the Physics of Partial Discharges in Solid Dielectrics

The physical processes occurring during partial discharges in voids in dielectrics have been studied utilizing fast oscilloscopic techniques to observe the buildup and decay of current as a function of time for single discharges. A key parameter is the overvoltage at breakdown, determined for time-dependent voltages by the rate of rise of voltage and the statistical time lag. Two distinct types of discharge have been identified, named ¿Townsendlike¿ and ¿streamerlike¿, and mechanisms responsible for their development have been proposed. The relationship between current-time pulse shapes as well as discharge magnitudes (charge per discharge) and various parameters such as overvoltage, void size, and dielectric thickness and permittivity have been explored. The conditions governing the transition between the two types of partial discharges have been examined quantitatively. The relationship between streamer propagation and Lichtenberg figure development has been studied. Measurements of statistical time lags for partial discharges have been made and significant paramters examined. The usefulness of these results in connection with future studies of chemical degradation of insulating materials are discussed.

Replacement Gases for SF6

Sparking potential measurements are reported on some thirty-five electronegative gases. Their value as replacement gases for SF6 has been analyzed in terms of their useful pressure range at lower operating temperatures, and has been empirically correlated with molecular structures. Figures of merit have been developed.

Streamer Propagation in Liquids and over Liquid-Solid Interfaces

The recent activity in the area of pre-breakdown streamer development has produced a considerable quantity of data characterizing the phenomena. It is not the aim of this paper to review this work in detail, much of which has been discussed in two recent reviews [1,2]. Rather we hope to point out from this work in non-uniform field geometries, some interesting observationswhich are striking and could offer insight into the mechanisms operative, yet at present remainunexplained.

Transient Behavior in Transformer Oils: Prebreakdown and Breakdown Phenomena

A quantitative correlation between streamer velocities and impulse breakdown voltages has been established for point-plane geometries. We have extended these studies to less divergent fields (rod-plane) where we have identified two regions for impulse breakdown in liquids, oneiationinitiation-Inintrolled and a second controlled by streamer propagation rate. Which controls breakdown is determined by electrode geometry (field divergence), liquid composition, and wave shape. The influence of these parameters has been determined and a general model for the prediction of impulse breakdown voltage is proposed.

Prebreakdown phenomena in sphere‐sphere electrode configurations in dielectric liquids

Negative streamer growth is observed in a highly refined mineral oil for sphere‐sphere electrode configurations. The shape of the streamers is somewhat different from that found in point‐plane geometries. The addition of electron trapping additives accelerates the negative streamer velocity as was found to be the case in point‐plane geometry. Breakdown occurs before the negative streamer crosses the gap and appears to originate at the anode. Very fast positive streamers are, in fact, observed.

Prebreakdown phenomena in liquids: electronic processes

Measurements of streamer velocities in a highly refined mineral oil are reported for negative and positive polarities using a point-to-plane geometry. The effects of several additives are investigated. The results indicate that whereas the negative streamers accelerate in the presence of electron-trapping additives, the velocity of the positive streamers increases in the presence of low ionization potential compounds.

The Influence of a Dc Bias on Streamers Produced by Step Voltages in Transformer Oil and over Solid-Liquid Interfaces

Using shadowgraphic techniques in conjunction with an Imacon camera, measurements of streamer velocities and minimum voltage required for streamer propagation (MVP) have been made with and without dc superimposed on voltage steps (1.2 ¿s rise time). This has been done in point-plane geometries and semi-uniform fields in a transformer oil alone as well as with pressboard barriers. Velocities and MVP have also been measured for streamers propagating on the surface between cylindrical electrodes perpendicular to insulating sheets having a ground plane on the opposite side. Again the influence of superimposed dc was assessed. The solid materials used were polymethylmethacrylate and pressboard. Measurements with the latter material were made using an optical multichannel analyzer to follow the light emission from the propagating streamer. In most cases both streamer velocities and MVP depend upon the arithmetic sum of the step and dc voltages. With pressboard, however, marked departures from the additivity rule were found for positive streamers. These results will be discussed in terms of space charge due to the dc potential.