E. O. Forster

Observation of Prebreakdown and Breakdown Phenomena in Liquid Hydrocarbons II. Non-Uniform Field Conditions

The prebreakdown and breakdown processes have been recorded in n-hexane toluene and Marcol 70, both in a pure state and with selected impurities. The study was carried out using a point-plane geometry. A low ionization potential additive had only a small effect on the breakdown voltage or the streamer propagation speed but did significantly alter the shape of the prebreakdown streamer when the needle was positive, an anode. For a negative needle, a cathode, chemical impurities affected the breakdown voltage. The significance of these findings is discussed in detail.

High Speed Laser Schlieren Studies of Electrical Breakdown in Liquid Hydrocarbons

The events occurring just prior, during, and after electrical break-down in liquid hydrocarbons have been photographed using 15 ns laser pulses produced with a ruby laser, to illuminate the space between two parallel plate electrodes, and schlieren optics. In this manner it was possible to obtain experimental evidence of the density gradients produced by the movement of charge carriers during prebreakdown and the energy released during and after breakdown. Bysplitting the laser beam and delaying one half by 40 ns with respect to the other, the events occurring in the electrode gap could be studied as a function of time. Attempts to use the light emitted prior to breakdown as trigger signal for the laser were not successful. The initiation and propagation of breakdown are discussed in the light of the available photographic evidence.

The Dynamics of Electrical Breakdown in Liquid Hydrocarbons

The light emission associated with pre- and post-breakdown events has been studied in detail using a uv sensitive photomultiplier tube and an optical multi-channel analyzer. The information gathered in this manner suggests that the light emitted by partial discharges is of a similar origin as that coming from breakdown itself, i.e. due to gas phase reactions. The post-breakdown emission process is tentatively assigned to the reactions associated with the decay of the conducting plasma column.

Observation of prebreakdown and breakdown phenomena in liquid hydrocarbons

The prebreakdown and breakdown processes have been recorded in n-hexane toluene and Marcol 70, both in a pure state and with selected impurities. The study was carried out using a point-plane geometry. A low ionization potential additive had only a small effect on the breakdown voltage or the streamer propagation speed but did significantly alter the shape of the prebreakdown streamer when the needle was positive, an anode. For a negative needle, a cathode, chemical impurities affected the breakdown voltage. The significance of these findings is discussed in detail.

Prebreakdown Cathode Processes in Liquid Hydrocarbons

Measurements are presented of the initiation of prebreakdown streamers at a point cathode in liquid hydrocarbons. Using a computer implementation of the method of images, the electric field is computed for selected geometries to confirm that the field strengths in the vicinity of these streamers are probably high enough so that electron multiplication processes can occur. High magnification photographs of streamer initiation show that the initial streamer velocity is (2.8±0.4) x 104 cm/s in toluene.

In the search for universal features of electrical breakdown in solids, liquids and gases

This paper discusses the commonality of electrical breakdown phenomena in gases, liquids, and solids. It is shown experimentally that the change in density fluctuations occurring in the three phases is the strongest common influence in the development of electrical breakdown.

Research in the Dynamics of Electrical Breakdown in Liquid Dielectrics

The events leading to electrical breakdown of dielectric liquids have been the subject of many studies [1]. In recent years it has become evident that optical rather than electrical techniques should be used in this study since these events occur very rapidly [2-5]. In the investigation of these phenomena, the selection of the experimental conditions is important. Some investigators believe that their experimental equipment should simulate actual conditions occurring in industry. They select point-plane configurations [3-5] which produce highly divergent fields. Others select the simpler case of nearly uniform field conditions [2]. The next consideration is the distance between the electrodes, the gap, which is selected according to arbitrary prejudices. Often the available power supplies determine this selection. Therefore, the data coming from these investigations cover a variety of gap sizes, pulse forms, temperatures, and materials. This situation makes it extremely difficult to use the published data to derive some valid and useful model of electrical breakdown. For example, observations and conclusions based on point-plane and parallel plate electrode systems exhibit considerable disagreement even though both experiments use identical pulse shapes, liquids, gap spacing and ambient temperature [6]. Obviously, the nature of the electrical field has an influence on the process. The problem is further compounded by the use of different optical detection systems. Not every system offers the same resolution. Also, the time scales used for these observations differ greatly, ranging from nanoseconds to microseconds.

The dynamics of electrical breakdown in liquid hydrocarbons I.

The electrical breakdown process in highly purified and degassed liquid hydrocarbons has been studied, using high speed schlieren photography as well as photomultipliers and an optical multichannel analyzer. It was shown that the breakdown process appears to be initiated primarily on the cathode surface. The ensuing formation of tree-like streamers was found to be associated with light emission. This light appears to be of the same spectral composition as that given off during actual breakdown. Spectral analysis of the emitted light indicated the presence of atomic and molecular hydrogen, carbon molecules of the type C2 and C3, as well as metal atoms emanating from the electrode surface. The significance of these findings is discussed in detail.

Electrical Transport Processes in Heavy Hydrocarbon Fluids

Scarcely any information is available about electrical transport in heavy petroleum oils, which, in addition to having large and complicated hydrocarbon molecules, also contain heteroatoms (especially N, OH, and S) which should affect the transport processes. Petroleum conductivity is an important Parameter for a number of commercial particulate separations processes based on electrophoresis, but information on the mechanism is lacking. Furthermore, the heteroatoms will affect the orientation polarizability in ac electric fields; the presence of permanent dipoles may affect the viscoelastic properties of these oils. The response of heavy oils to applied electric fields proved to be quite similar to that of polar polymers. There is a glass transition, followed by a rapid but frequency-dependent increase in the dielectric permittivity. At higher temperatures , a true dc component is modelled as the translation of charges among the ir-orbitals of the polynuclear aromatics in the oil; no permanent electrochemical effects were observed. Although the oil does not possess a long-chain structure with identifiable repeat units like a polymer, the close resemblance of the dielectric data of the oil to that of polymers may well imply that the oil is not simply composed of an entirely random mixture of unaffiliated molecu1es.

Role of Atmospheric Gases in the Space Charge Polarization of Low-Density Polyethylene in a Divergent AC Field

Charge injection into lowdensity polyethylene by a divergent ac field at room temperature has been investigated using a high-resolution ther mally stimulated discharge current technique. It was observed that TSDC spectra obtained after charging with ac fields were dissimilar to those obtained with dc fields of similar magnitude and were influenced by sam ple history and by the nature of the gaseous charging environment. Thoroughly outgassed samples charged in a high vacuum showed little change in charge release characteristics for ac and dc charging fields. Evidence of additional large polarizations was found in samples charged with ac fields only in a gaseous environment. It is suggested that this is due to the trapping of electrons by ionized gas molecules adsorbed by the polymer. The presence of oxygen gas initiated irreversible changes in the polarization behavior of LDPE by interaction with an ac field.