Robert J. Schwabe

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.

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.

The Application of Electric Current Computed Tomography to Defect Imaging in Metals

Electric Current Computed Tomography (ECCT) is a technique for producing images of the electrical resistivity profile within a body from measurements made on the body’s exterior. To make these measurements, an array of electrodes is attached to the surface of the body. Sets of current patterns are applied through these electrodes and the voltages needed to maintain these specified currents are measured and recorded. These applied currents and measured voltages are then used in a reconstruction algorithm to produce images that represent approximations to the electrical resistivity distribution in the interior of the body.

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.

Estimation of particulate charging and migration for pulsed precipitator applications

Abstract A simulation is described which permits the evaluation of dispersed particulate behaviour under the transient ion density and space-charge fields appropriate to the operation of electrostatic precipitators with pulsed energization. Field, field-assisted diffusion and pure diffusion charging mechanisms are used to describe the charging and a novel stochastic scheme is introduced to evaluate particle charge density distributions in both space and time when turbulent diffusion is important. Supporting experimental measurements are introduced to measure the particle charge distributions and to estimate the spatial changes in dust concentrations as a result of the migration processes within a precipitator.

Simulation and measurement of corona for electrostatic pulse powered precipitators

The use of pulse powering for electrostatic precipitators has shown several advantages but the underlying physics has been insufficiently understood to allow any optimization. A numerical model has been assembled which allows the controlling corona process to be simulated in both time and space to permit estimation of the ion densities and field distributions to be expected for a wide range of transient electrical parameters and gas properties. Supporting laboratory measurements have been undertaken under dc and pulse conditions both to verify the simulation and to provide corona spot density data not readily obtainable by other means.

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.

Streamer Propagation in OIls and its Relationship to Arrester Application

Using a high speed image converter camera together with a shadowgraphic technique, positive streamer velocities have been measured as a function of instantaneous voltage on impulses of varying shape and duration in transformer oil and Marcol 70. They have been found to propagate at a constant velocity which is independent of the instantaneous value of the voltage. The velocity appears to correlate with the rate of rise of voltage rather than the initation voltage itself. This result has implications for the use of arresters as protection for electrical equipment and these implications have been examined using breakdown measurements.

Partial discharge characterization of machine insulation by the transfer of reactive power

In many cases, the electrical degradation of the stator insulation of large machines is preceded by a deterioration in mechanical integrity. Forces on the conductors which result from the interaction of the load current with the leakage slot flux will compress the groundwall insulation. Consequently, delamination or erosion of the structure will result in increased relative movements and concomitant changes in the partial discharge signatures. This paper explores the possibility of detecting mechanical degradation by changing the import or export of reactive power to affect the moment of the application of peak force with respect to the electric stress. Preliminary measurements on hydrogenerators would appear to indicate that this technique does indeed cause some change in the discharge signatures obtained and may permit machines with mechanically deficient slot insulation to be separated from those with intact groundwall structures.