Gaseous Cavities Research Articles

An investigation into electrical ageing of insulating polymers by detection of charge injection under alternative divergent field

Our aim is to understand the electrical ageing of insulating polymeric materials under a.c. voltage and in discharge-free situation, i.e. in the absence of any pre-existing gaseous cavities in the bulk of the dielectric. In this report, we use a needle electrode moulded into the insulant to simulate defects producing local field enhancement. An experimental technique for measuring the charge flow injected by the needle into the dielectric under a.c. voltage is described. The transition between the discharge-free electroluminescent state to micro-partial discharge state (early electrical tree propagation phase) is investigated with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. Special emphasis is given to synthetic insulation of power cables. Low-density polyethylene is used as a dielectric with moulded metallic needle electrode. Semi-conducting electrodes prepared from the shield of a cable are also used in this investigation. Statistics on times to tree inception show that the needle tip cannot be considered as a homogeneous injecting surface. The conclusion is double : (i) the onset field for massive charge transfer derived from the measured curvature radius of the needle is not the absolute value and (ii) the discussion of the results obtained by a needle test must include a Weibull statistical analysis with determination of the location parameter.

Study of partial discharges and gassing phenomena within gaseous cavities in insulating liquids

Gassing phenomena under an AC voltage in various dielectrics liquids subjected to partial discharges produced in different gases are discussed. The roles of the numerous experimental parameters which affect the results have been studied using a uniform field test cell. It is shown first that from the classical PD equivalent circuit, a more refined analysis permits a good description of the influence of parameters such as gas and solid thickness as well as the amplitude and frequency of the applied voltage on gas volume variations, and discharge characteristics. This allows one to get suitable measurement of the charge and energies involved in the gaseous discharges, from which one can deduce the energy required to absorb (or to create) a gas molecule. This gassing energy, which is independent of the experimental parameters, is characteristic of each gas/liquid pair and constitutes a valuable quantitative evaluation of the gassing properties. >

Electrical stress and inception voltage of discharges in gaseous cavities in an anisotropic dielectric material

The electric field stress and the inception voltage of discharges in a gaseous void present in an anisotropic dielectric material is developed from the solution of the Laplace equation. The relevant equations for the determination of the electric field and the inception voltage of discharges in voids are presented in a format that can be readily used. The electric field distribution within the void and the maximum field value are determined for two shapes of voids, rectangular and cylindrical cross sections, over wide ranges of anisotropy and sizes of voids. It has been found that the maximum electric field in the void increases with increasing width and decreases with increasing thickness of the void. The inception voltage of discharges is determined over a wide range of void dimensions. The minimum breakdown voltage of a slab of anisotropic dielectric material containing a void follows a U-shaped curve when plotted against the product of the length and width of the void.

Generalized van der Waals theory. XIII. Curved interfaces in simple fluids

Recent work has shown that a simple free energy density functional theory (the GvdW theory) can resolve the surface tension and interface width of gas/liquid interfaces in simple fluids to good accuracy. In a straightforward extension the analysis and calculations are here applied to liquid drops and gaseous cavities in simple fluids. The fluid particles are assumed to interact by a Lennard-Jones (12-6) potential and to be sufficiently heavy to allow the use of classical mechanics. The surface tension and profile width are obtained as functions of the radius R for drops and cavities at several different levels of approximation illustrating different contributing mechanisms. A significant decrease in the surface tension is found for both drops and cavities as the radii decrease to become comparable to the particle diameter.

Analysis of discharge damage and breakdown in gaseous cavities in insulating materials

Partial discharges in cavities lead to the deterioration and breakdown of insulating media. Ion and electron bombardment have been considered as independently accountable for failure of dielectrics by sustained partial discharges. Electron bombardment is the dominant cause of damage. The rate of degradation is accelerated by increasing the frequency, magnitude and time of applied voltage. The main contributing factor in discharge damage is the kinetic energy of the electrons. The effective life of the dielectric is inversely proportional to the average kinetic energy of the bombarding electrons. The mechanism of dielectric breakdown by partial discharges in cavities is explained primarily by electron and ion bombardment. The technique of filling the cavities with insulating gases at higher than atmospheric pressures, or with insulating liquids, will decrease the rate of deterioration and increase the effective life of the dielectric. The present analysis attempts to correlate and justify the experimental findings with theory.

Stable and Transient Acoustical Cavitation in Water and in Liquid Helium

The occurrence of stable and transient acoustical cavitation in liquid helium has been reported recently by Finch et al. and by Jarman and Taylor. It appears to be similar to acoustical cavitation in unfiltered gassy water. Solutions to the non-dissipative incompressible differential equations for the motion of gaseous and/or vaporous cavities in water and liquid helium have been obtained on an analog computer, and these will be related to experimental data. [Work supported by a research grant from The University of Western Australia and by the U. S. Office of Naval Research.]

Detection and measurement of discharges in gaseous cavities in solid dielectrics under direct-voltage conditions

A technique for detecting and measuring discharges in gaseous cavities in solid dielectrics under direct-voltage conditions has been developed, whereby the voltage pulses arising from the discharges are amplified, shaped and recorded on magnetic tape. This tape is afterwards played back through a pulse scaler set to count above any desired level. The system is calibrated by feeding known voltages into the circuit from a pulse generator.An example of this use of a tape recorder is provided by an experiment on a cylindrical air-filled cavity, of known dimensions, in polythene. It is considered that the technique will be of value in investigating gaseous discharge phenomena in a wide range of insulating materials, and may also be applicable to complete insulating systems.

Detection and measurement of discharges in gaseous cavities in solid dielectrics under impulse voltage conditions

A technique for detecting and measuring internal discharges in solid dielectrics under impulse-voltage conditions has been developed. The test object forms one arm of a capacitance bridge which is fed with 1/50μs voltage impulses having peaks of up to 15kV. The bridge is connected through a pulse transformer and a bandpass filter to valve amplifiers and a cathode-ray oscillograph, on which are displayed the voltages arising from internal discharges. Calibration is effected by feeding known voltages into the circuit from a pulse generator.The technique has been used in an experiment on a cylindrical air-filled cavity of known dimensions in polythene. It is considered that it will be of value in studying gaseous-discharge phenomena and processes that can lead to the breakdown of a wide range of solid insulating materials.

Electric stresses in gaseous cavities in solid dielectrics: experiments with an electrolytic tank

Electric stresses in gaseous cavities in solid dielectrics: experiments with an electrolytic tank

Electric stresses in gaseous cavities in solid dieletrics

Gaseous cavities of two simple geometrical shapes are considered: (a) an elliptic cylindrical cavity with the minor axis of its elliptic cross-section parallel to the field, and (b) an oblate spheroidal cavity with its axis parallel to the field. Circular cylindrical and spherical cavities are treated as special cases.When a cavity of each of these forms is located in an infinite dielectric, which is subjected to an electric stress that is uniform at very large distances from the cavity, the electric stress in the cavity is also uniform, and the relation between the two stresses is well known. When, however, the dielectric is contained between two infinite plane parallel electrodes whose distance apart is not great compared with the dimensions of the cavity, the electric stress in the cavity is no longer uniform, and the problem does not yet appear to have been rigorously studied.In the paper, the electric stresses existing at points on the minor and major axes of the cross-section of the elliptic cylindrical cavity and on the axis and radius of the oblate spheroidal cavity have been calculated, in both alternating- and direct-voltage conditions, in terms of the stress in the solid dielectric, using a technique devised by Rayleigh for solving the Laplace equation. It is shown that the electric stress is greatest in the elliptic cylindrical cavity at the extremities of the major axis and in the oblate spheroidal cavity around its periphery. Formulas for the mean electric stress along the minor axis of the elliptic cylindrical cavity and the axis of the oblate spheroidal cavity are also derived.

Electric stresses at conducting surfaces located in the field between plane parallel electrodes

In the preceding paper, electric stresses in elliptic cylindrical and oblate spheroidal gaseous cavities located in a dielectric contained between infinite plane parallel electrodes were calculated, using a technique devised by Rayleigh for solving the Laplace equation. The same methods give the maximum electric stresses at conducting surfaces of similar geometrical shapes located in the field between plane parallel electrodes, only very slight modifications to the equations developed previously being necessary. The formulas that have been derived in the present paper can be applied to calculate, for example, the maximum stress between successive turns of a long conducting helical coil subjected to a uniform voltage gradient and the maximum stress at the surface of the ellipsoid in an ellipsoidal voltmeter.

XI.—On certain Vegetable and Mineral Formations in Calcareous Spar.

When occupied several years ago in studying the fluid and gaseous cavities in minerals, my attention was drawn to certain remarkable formations in perfectly transparent specimens of calcareous spar. These formations were of two kinds—the one apparently of vegetable origin, and the other obviously mineral.

1. On the Pressure Cavities in Topaz, Beryl, and Diamond, and their bearing on Geological Theories

In this paper the author gave a brief account of the various phenomena of fluid and gaseous cavities which he had discovered in diamond, topaz, beryl, and other minerals. He described—1. Cavities with two immiscible fluids, the most expansible of which has received the name of Brewstolyne, and the most dense that of Cryptolyne, from the American and French mineralogists.2. Cavities containing only one of these fluids.3. Cavities containing the two fluids, and also crystals of various primitive forms, some of which melt by heat and recrystallise in cooling.4. Cavities containing gas and vapour.