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The recent activity in the area of prebreakdown streamer development has produced a considerable quantity of data characterizing the phenomena. It is not the aim of this paper to review in detail this work, 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 observations which are striking and could...
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 µs 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...
Recently, we have correlated impulse strength of oils with measurements of prebreakdown streamer velocities.1 This paper extends such work to composite solid-liquid systems. The methods for measuring streamer velocities by shadowgraphy have been described previously.2 The creepage fixture utilizes two cylindrical electrodes (1.27 cm diameter) resting on the solid dielectric whose other surface is...
A simple model was proposed by two of the authors for tree initiation in polyethylene,1»2 in which the repeated process of charge injection and extraction between the electrode and the dielectric is responsible for tree initiation. It is made either directly between the two or indirectly via a certain gas phase which is considered to exist inevitably between the two. The former process is related...
A large number of measurements of electron mobilities in low dielectric constant insulating liquids have yielded values close to those to be expected for negative ions. Recent evidence suggests that electrons initially produced may possess considerably higher mobilities for a short period prior to “trapping” to give finally ion-like carriers. For example, Minday et al.1 obtain one or two orders of...
We recently presented a comparison of the economics of paper-oil insulated pipe-type cables with those of a conceptual polymer-fluid cable of similar design for use in the EHV range from 345 to 700 kV.1 In the present work we have extended our calculations to cover a number of other systems currently under consideration for obtaining larger blocks of power at lower cost. These include pressurized...
The need for lower dielectric losses in EHV cables has stimulated recent interest in the use of oil-impregnated polymer films in place of paper. To minimize swelling by the oil and resultant degradation of mechanical and electrical properties, polymers with relatively high glass transition temperatures such as polyphenylene oxide, polydian carbonate, and polysulphone have received most attention.1,2...
Perhaps the most familiar property of a dielectric material is the capacitance increase observed when a sheet of dielectric is inserted between the plates of an “air” capacitor. In fact, the ratio of dielectric to air-filled capacitance is often used to define the dielectric constant ε'. Defined in this way, its magnitude becomes a property of the material, independent of the measuring voltage or...
Interest in the dielectric behavior of polymers swollen in liquids has recently been intensified by commercial developments in the areas of capacitors and EHV cables. In such applications very low dielectric losses are required, and these often stem from uncontrolled amounts of unknown impurities in the system. In the work described here, we have attempted to examine the mechanisms and behavior of...
Pressures have been growing for increased power transmission via underground cables, in spite of costs which may be 10 to 20 times as great as for corresponding overhead systems. To reduce these costs, the trend has been toward higher power capability through higher operating voltages. This has led to a growing concern over increasing dielectric losses and to a more careful consideration of the replacement...
The behavior of electrons injected into amorphous hydrocarbon systems, such as many polymers and liquids, is of interest from a number of viewpoints. For the purposes of the present symposium it is perhaps sufficient to note that any electronic theory of breakdown of such materials requires a detailed knowledge of electron — molecule interactions and that presently this information is meager.
No major breakthroughs are reported by our authors this year, although the total volume of the literature continues to rise. Considerably increased activity has been observed in certain areas, but this is the result of discoveries in the past. Thus, the frequency doubling of coherent light in piezoelectric crystals, discovered in 1961, has added impetus to the already fast-growing laser field. The...
Usually investigations of the nature of electrical discharges in gas voids in solid dielectrics are made using ac voltages or at least voltages changing continuously with time. Owing to statistical time lags this implies that each discharge will occur at a slightly different applied voltage always somewhat larger than the minimum Townsend threshold value. We have recently obtained evidence suggesting...
We recently measured the first Townsend ionization coefficient, α in a series of n-alkyl chloride gases1, and found that the dependence of α upon molecular structure in these gases was similar to, but different in detail from, that indirectly inferred in the hydrocarbon gases by Devins and Crowe2 and Heylen and Lewis3 from sparking potential measurements. In order to determine whether these differences...
Studies of the current build-up as a function of time after the application of voltage continue to be most fruitful in elucidating breakdown mechanisms in gases. Loeb1, in a review article on formative time lags, emphasizes the differences between the initial threshold for onset of a current independent of the external supply of electrons (Townsend threshold) and later thresholds for transitions to...
A number of studies of the statistics of Townsend avalanche production were made in 1958. Frommhold (1), bas examined the conditions under which the theory of Wijsman, relating the number of avalanches to their size, is valid. Using counting techniques, he measured avalanche size distributions in oxygen, air and methane, and found the Wijsman statistics valid providing the field was not too close...
When gas voids in solid dielectrics break down under the application of an electric field, the current flow (as observed in the external circuit) is discontinuous, usually consisting of very short (ca. 10−7 sec) pulses. This general behavior is not unexpected, since the charge produced by breakdown of the gas cannot flow through the solid insulation, and therefore reduces the potential across the...
Two years ago at this Conference we presented a paper in which it was demonstrated that the sparking potentials of simple hydrocarbon gases could be related directly to certain physical properties of the molecules of which the gases were composed(1). In order to accomplish this it was first necessary to derive an expression relating the first Townsend coefficient, α, of a gas to the pressure, P, the...
A sufficiently high a.c. voltage impressed upon a sample of dielectric containing a void will result in discharges (corona) in the void, and thus to degradation of the dielectric. The discharge current is in the form of very rapid pulses, < 0.1 μsec in duration, and it is the ultimate purpose of this work to examine the factors controlling the number and size of these pulses and to correlate these...
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