Showing papers on "Liquid dielectric published in 2002"

Bird resistant power line insulation

Abstract: A method and apparatus for insulating high voltage electric wires from birds and other animals. A dielectric material is distributed on electrical wires or on the electric wire support at a location proximate to the wire. Such material can also be distributed on the wire to resist electric shorts when a bird contacts the electric wire and support or other wire. The dielectric properties of the material can be selected to control the distribution technique and resulting thickness of the dielectric material. The dielectric material can be distributed before installation of the electric lines or can be retrofitted to existing installations.

Electric-field-induced ion evaporation from dielectric liquid.

Abstract: A direct proof of ion field evaporation from dielectric liquids is presented. The flux of sodium ions ejected from the surface of an electrospray of formamide is measured using time-of-flight and retarding potential techniques. The electric field at the emitting surface is varied through the electrospraying parameters. We find that the evaporated ion current is a very steep function of the electric field.

An electrokinetic model of drop deformation in an electric field

Abstract: An electrokinetic model is proposed to describe a slight drop deformation which is induced by a weak external electric field The fluids forming the system are considered Newtonian incompressible dielectric liquids containing free electric charge carriers According to the model, the charge carriers take part in migration, diffusion and convection transport and there is no solute adsorption at the interface Thermodynamic quasi-equilibrium at the interface is assumed for the charge carriers in the contacting liquids The interfacial thermodynamic equilibrium is described using a common distribution coefficient for all the carriers The problem is simplified by assuming equal diffusion coefficients for the different charge carriers within the same liquid An analytical expression is obtained for slight drop deformation which is proportional to the second power of the applied field strength magnitude The expression derived represents the drop deformation as a function of the parameters employed in previous theories (O’Konski & Thacher 1953; Allan & Mason 1962; Taylor 1966) as well as two additional parameters The additional parameters are the ratios of the drop radius to the Debye lengths of the outer and inner liquids, respectively The expression obtained for the drop deformation is valid for arbitrary values of these parameters According to the theory prediction, with an increase in the drop radius, the drop deformation monotonically changes from that obtained by O’Konski & Thacher (1953) and Allan & Mason (1962) for perfect dielectric liquids to that obtained by Taylor (1966) for leaky dielectric liquids Two simplified versions of the general expression are suggested to describe particular cases of a conducting drop in a perfect dielectric liquid and of a perfect dielectric drop in a conducting liquid

Dielectric fluid having defined chemical composition for use in electrical apparatus

Abstract: The present invention comprises a mixture of hydrocarbons having a well-defined chemical composition that is suitable for use as a dielectric coolant in electrical equipment in general, and specifically in transformers. The dielectric coolants of the present invention are particularly suited for use in sealed, non-vented transformers, and have improved performance characteristics, including decreased degradation of the paper insulating layers, as well as a greater degree of safety and environmental acceptability. The present dielectric coolants comprise relatively pure blends of compounds selected from the group consisting of aromatic hydrocarbons, polyalphaolefins, polyol esters, and natural vegetable oils, along with additives to improve pour point, increase stability and reduce oxidation rate.

Electrohydrodynamic stability: Taylor-Melcher theory for a liquid bridge suspended in a dielectric gas

Abstract: A liquid bridge is a column of liquid, pinned at each end. Here we analyse the stability of a bridge pinned between planar electrodes held at different potentials and surrounded by a non-conducting, dielectric gas. In the absence of electric fields, surface tension destabilizes bridges with aspect ratios (length/diameter) greater than π. Here we describe how electrical forces counteract surface tension, using a linearized model. When the liquid is treated as an Ohmic conductor, the specific conductivity level is irrelevant and only the dielectric properties of the bridge and the surrounding gas are involved. Fourier series and a biharmonic, biorthogonal set of Papkovich–Fadle functions are used to formulate an eigenvalue problem. Numerical solutions disclose that the most unstable axisymmetric deformation is antisymmetric with respect to the bridge’s midplane. It is shown that whilst a bridge whose length exceeds its circumference may be unstable, a sufficiently strong axial field provides stability if the dielectric constant of the bridge exceeds that of the surrounding fluid. Conversely, a field destabilizes a bridge whose dielectric constant is lower than that of its surroundings, even when its aspect ratio is less than π. Bridge behaviour is sensitive to the presence of conduction along the surface and much higher fields are required for stability when surface transport is present. The theoretical results are compared with experimental work (Burcham & Saville 2000) that demonstrated how a field stabilizes an otherwise unstable configuration. According to the experiments, the bridge undergoes two asymmetric transitions (cylinder-to-amphora and pinch-off) as the field is reduced. Agreement between theory and experiment for the field strength at the pinch-off transition is excellent, but less so for the change from cylinder to amphora. Using surface conductivity as an adjustable parameter brings theory and experiment into agreement.

Investigation of effective interface potentials by electrowetting

Abstract: We study a situation where the complete wetting of a dielectric solid by a dielectric liquid, in a conducting surrounding, is altered by the application of an electrical potential difference. Calculations predict a dewetting transition towards pseudo-partial wetting. More generally, in a pseudo-partial wetting situation (pre-existing or electrically induced), the mesoscopic film coexisting with the macroscopic droplet is expected to be thinned by the electrostatic pressure in a way that is directly related to its effective interface potential. This points out a possibility to tune the thickness of a fluid film in a nanometric regime.

Tunable capacitors using fluid dielectrics

Abstract: Capacitors (10,20,40,50,70,80) having a fluid dielectric material that is transported or undergoes a phase change are disclosed. The dielectric medium change results in a change in the total dielectric constant of the material between the electrodes (12, 14, 72, 74, 81, 82), thus changing the capacitance of the capacitors. Transporting or phase changing the dielectric fluids into and out of a the electric field of the capacitor, changes the effective dielectric constant and the capacitance of the capacitor.

Solar energy converter using optical concentration through a liquid

Abstract: Solar cells (1010) operating in liquid (1016) have an increased operating efficiency resulting from two independent physical phenomena, an increase in output current from the solar cells (1010) simply by wetting the front and back of the solar cells (1010) using a mounting table (1002), and enhanced collection of light through refraction and inner reflection of light in the liquid (1016). Dielectric liquids (1016) are found to increase output power efficiency of n/p junction solar cells (1010) and also function as a medium for optical concentration. The properties of dielectric liquid (1016) permit the construction of a solar cell panel (1000) with a suitable geometry for functioning as an optical concentrator.

Self-similar solutions for conic cusps formation at the surface of dielectric liquids in electric field.

Abstract: The nonlinear dynamics is studied for the free surface of an ideal dielectric fluid in an electric field. Self-similar solutions of electrohydrodynamic equations describing the formation of surface conic cusps are revealed. The behavior of physical quantities (field strength, fluid velocity, and surface curvature) near the singularity is established. The threshold value of the fluid dielectric constant required for the proposed mechanism of a cusp development is found.

Method, process and apparatus for high pressure plasma catalytic treatment of dense fluids

Abstract: A process for purifying a dense dielectric fluid containing a contaminant comprising applying a plasma to a dense dielectric fluid containing a contaminant at a pressure, temperature, and for a time sufficient to oxidize the contaminant.

Electrical characterization of gas discharges using a numerical treatment. Application to dielectric barrier discharges

Abstract: In this article a numerical method for an electrical data treatment which is aimed to define the key electrical parameters monitoring the thermal and chemical properties of gas discharges, in particular dielectric barrier discharges is presented Numerous electrical parameters characterize the electrical discharges: pulses onset voltage, pulse number, pulse height, pulse width, power injected in the discharge cell, etc Our treatment using Matlab® allows us to get such data, as is shown in the present article for a point-to-plane dielectric barrier discharge Moreover it shows how a separation between the different components of the current signals remaining after the elimination of the capacitive component can be obtained Much attention is paid, in particular, to the impulsive components (streamers and Trichel pulses components) and on the nonimpulsive ones constituting what we call the pseudocontinuous components, generally ignored in the literature So four different categories of charges are consider

The temperature dependence of the dielectric strength of transformer oil and pressboard insulation at HVDC polarity reversal stress

Abstract: This work reports on measurements investigating the dielectric strength of insulating oil and Transformerboard, from very low (-20/spl deg/C) to increased temperature (+60/spl deg/C). Different insulation structure models-board puncture and creepage-were stressed with 1 hour DC step-by-step voltage increase of reversed polarity each step, until breakdown. The impact of adhesives and their orientation relative to the electric field was investigated. As a reference, the AC strength and the moisture content of the oil was measured. With the results from this study, important knowledge of the DC behavior of oil/cellulose insulation is gained. The findings constitute a profound basis for improved HVDC power transformer insulation technology.

The effect of molecular structure on prebreakdown phenomena in dielectric liquids under a nonuniform field

Abstract: In the present work, prebreakdown phenomena in dielectric liquids which have various molecular structure, especially those with chlorine or fluorine atoms and some double bonds included in their molecular formula, are observed in detail for positive and negative point polarities, under the application of an impulse voltage (1.1/225 µs) to the point electrode. As test liquids, cyclic hydrocarbon liquids such as cyclohexane, benzene, chlorocyclohexane, monochlorobenzene, hexafluorobenzene, and cyclohexene are used. Results obtained in these liquids are compared with those obtained in straight-chain hydrocarbon liquids. As a result, the propagation of negative streamers, especially in dielectric liquids with chlorine or fluorine atoms in their molecular formula, is promoted, whereas the propagation of positive streamers in dielectric liquids with some double bonds is promoted. The former is related to a large electron affinity, the latter is due to π electrons, for both cyclic and straight-chain hydrocarbon liquids. © 2002 Scripta Technica, Electr Eng Jpn, 139(1): 1–8, 2002; DOI 10.1002/eej.1140

Influence of additives and hydrostatic pressure on streamers initiation and dielectric strength of liquids

Abstract: This paper is aimed at the influence of additives and hydrostatic pressure on the initiation phase of streamers and the dielectric strength of liquids, in a point-plane electrode arrangement under dc voltages. We show that the increase of hydrostatic pressure and/or the addition of a certain concentration of an electronic scavenger compound as Iodobenzene or Carbon Tetrachloride increases the initiation voltage of streamers in liquids such as cyclohexane, silicon oil or tetra-ester. This results in an increase of the dielectric strength of these liquids. The fact that the streamer is accelerated in presence of such additives does not lead necessarily to the reduction of the dielectric strength of liquid.

Electrohydrodynamic Kelvin–Helmholtz instability of two superposed Rivlin–Ericksen viscoelastic dielectric fluid-particle mixtures in porous medium

Abstract: The electrohydrodynamic Kelvin–Helmholtz instability of the interface between two uniform superposed Rivlin–Ericksen viscoelastic dielectric fluid-particle mixtures in porous medium is investigated. The considered system is influenced by a uniform horizontal electric field. Perturbations transverse to the direction of streaming are found to be unaffected by either streaming and applied electric fields for the potentially stable configuration as long as perturbations in the direction of streaming are ignored, and the system remains stable in this case. For the potentially unstable configuration, the system is found to be stable or unstable for a certain wavenumber range depending on acceleration due to gravity, surface tension, and fluid velocities. For perturbations in all other directions, there exists instability for a certain wavenumber range, and the instability of the system can be delayed by the applied electric field. Numerical results show that Stokes' drag coefficient, electric field, kinematic viscosity have stabilizing effects; and mass density of the particles, porosity of the porous medium, kinematic viscoelasticity, fluid velocity, number density of the particles have destabilizing effects; while permeability of the medium has a dual role in the stability of the considered system, and the porosity of the porous medium has no effect on the stability for large wavenumbers. The limiting case of Rayleigh–Taylor instability is also discussed, and stability conditions are obtained.

Stochastic model of breakdown initiation in dielectric liquids

Abstract: An electric strength of perfluorodibutyl ether and transformer oil in small gaps between the hemispherical electrodes was investigated experimentally under AC voltage of linearly increasing amplitude. The macroscopic stochastic approach was proposed that describes breakdown initiation by certain function μ(E). This function is the probability density of streamer inception on a small element of electrode area in a short interval of time and depends on local electric field. This macroscopic approach allows one to obtain the analytical expressions for breakdown initiation probability on various parameters of experiment for flat, cylindrical and hemispherical electrodes under constant DC test-voltage, DC and AC ramp test-voltages. Several methods were developed to reconstruct the function μ(E) using experimental data on series of breakdown voltages for case of breakdowns in small gaps between electrodes. The values of function μ(E) for transformer oil and perfluorodibutyl ether were determined in the range of electric field from 0.3 to 0.9 MV cm−1. It was shown that perfluorodibutyl ether is a prospective dielectric liquid, especially for use in environmentally appropriate technologies. Stochastic computer simulations of breakdown inception were carried out. The series of breakdown voltages and pitting on surface of hemispherical electrode were obtained using stochastic model.

Electrohydrodynamic flow of a dielectric liquid due to autonomous injection of charge by a needle electrode

Abstract: An analysis is presented of the creeping axisymmetric flow induced by Coulomb’s force in a dielectric liquid subject to autonomous unipolar injection of electric charge through a paraboloidal metallic electrode. A stationary flow and a I∝V3 current-voltage characteristic are found when the constant voltage applied between the injecting electrode and another, collecting electrode is well above the injection threshold but below a certain critical value. For larger voltages the flow and the electric current become time dependent, owing to the instability of a charged layer that surrounds the injecting electrode. Qualitative order of magnitude estimates are presented for both regimes.

Insulating properties of some liquids after an electrical arc

Abstract: The insulating properties of a liquid are commonly studied before a breakdown occurs. In HV industrial apparatus such as high energy plastic metallized and liquid impregnated films capacitors, some local breakdowns, named self-healing, occur and the ability of the liquid to remain insulating after electrical degradation is of interest. To understand the influence of the liquid on the behavior of such capacitors, the post-arc behavior of the liquid alone has to be studied in a range of energy (/spl sim/mJ) close to the self-healing process. The present work deals with the study of the ability of different liquids to restore in the millisecond range their insulating property after an electrical arc. Using an experimental apparatus based on the repetitive charging with a dc HV generator and discharging through an arc of a resistor-capacity system, different kind of liquids were tested. The liquids we used were alcane, esters, alkyl benzene and polyaromatic. We showed as expected that liquids with the smallest ratio of carbon versus hydrogen in their molecular formula present the best restoration property after an electrical arc, property that is crucial for liquids impregnating metallized capacitors.

Percolative model of electric breakdown in liquid dielectrics

Abstract: A percolative approach, useful in modeling liquid dielectric breakdown, is presented. The dielectric is treated as a network of resistors having random values and breakdown characteristics based on a specified statistical distribution. The method is quite general, and lends itself to the inclusion of internal fluctuations and localized heating. It should also be applicable to mixtures and facilitate the inclusion of air bubbles. Despite its simplicity, the model successfully characterizes fractal structure in dielectric breakdown. In particular, the fractal dimension for a two-dimensional (2-D) lattice as given by the exponent of a power law, agrees with the theoretical value. The dependence of critical external voltage on the internal disorder is also investigated. It is shown that the overall breakdown process consists of the successive breakdown of individual elements to finally form a percolation cluster. The clusters have a typical dendrite structure. Also, in keeping with qualitative expectations, it is shown that V/sub br/ decreases with the disorder and increases with resistor variance.

Electrical discharge machine apparatus with improved dielectric flushing

Abstract: An electrical discharge machining system for machining a workpiece. The system comprises a wire guide support for positioning and guiding a wire electrode into a desired position relative to the workpiece and a source of a liquid dielectric fluid. The supply also provides means for directing the dielectric fluid from the source to a machining site between the electrode and the workpiece, and for atomizing the dielectric fluid in the machining site.

Condition Monitoring Of Transformer Insulation By Polarisation And Depolarisation Current Measurements

Abstract: The measurement and evaluation of the 'dielectric response' is one possible way of diagnosing a transformer's insulation condition. Moisture and ageing strongly influence the dielectric properties of oil/cellulose insulation systems. In our recent research project, dielectric measurement was used for assessing the condition of oil/paper insulation. The Polarisation and Depolarisation Current (PDC) analysis is a non-destructive dielectric testing method for determining the conductivity and moisture content of insulation materials in a transformer. On the basis of this analysis one can decide upon further actions. This paper presents a description of the PDC technique with the physical and mathematical background and some results of dielectric response measurements of several transformers. Data analysis and interpretation of the field test results are also presented in this paper.

Fluid ejection systems and methods with secondary dielectric fluid

Abstract: A fluid ejection system according to this invention operates on the principle of electrostatic or magnetic attraction. In various exemplary embodiments, the fluid ejection system includes a sealed diaphragm arrangement having at least one diaphragm portion and a diaphragm chamber defined at least partially by the at least one diaphragm portion, a nozzle hole located over the at least one diaphragm portion, an ejection chamber defined between the nozzle hole and the least one diaphragm portion and a secondary dielectric fluid reservoir containing a secondary dielectric fluid. The ejection chamber receives a primary fluid to be ejected. The secondary dielectric fluid reservoir is in fluid communication with the diaphragm chamber to supply the secondary dielectric fluid to the diaphragm chamber. In various exemplary embodiments, the secondary dielectric fluid is a liquid, a substantially incompressible fluid, and/or a high performance dielectric fluid having a dielectric constant greater than 1.

Dielectric frequency response measurement as a tool for troubleshooting insulation power factor problems

Abstract: This paper presents a new method of evaluating the dielectric frequency response (DFR) test for troubleshooting and identifying the cause of high or unusual insulation power factor test results. The method involves preparing a dielectric frequency response signature (DFRS) from the DFR test on a transformer. The signature of the measured response is then compared with a modeled response of a transformer with a "normal" insulation structure and a library of signatures of known defects. Based on this comparison, it is possible to diagnose power factor defects in the insulation structure of the transformer under test.

Inorganic fillers for corona endurance enhancement of selected polymers

Abstract: In high voltage applications, polymer insulation can be exposed to very high electrical field stress, resulting in long term exposure to corona. The electrical field stress may be much below dielectric breakdown threshold. Eventually the exposure to corona can lead to failure of the high voltage component. Nanometer sized inorganic fillers are increasingly used as reinforcing materials for mechanical or thermal property improvement of polymers. Improvements in mechanical modulus or heat deflection temperature are often realized. These fillers may also increase some electrical properties such as corona endurance in polymers. Polyhedral oligomeric silsesquioxane (POSS) loaded polymers are tested for corona endurance. POSS is a synthesized silicon based cage structure with an intermediate oxidation between SiO and SiO/sub 2/. Results suggest at least a five times improvement in ac corona. lifetime of selected POSS-polymer composites.

Time Series Measurements of Electric Field Vector Mapping in a Dielectric Liquid.

Abstract: We have been investigating the determination of three-dimensional electric field distributions using an original method. The method combines the reconstruction technique modified by the computed tomography technique and the Kerr electrooptic method. We have already reported on measurements of symmetrical and nonsymmetrical electric field vector distributions. By developing a simultaneous three-directional optical measurement system of parallel processing, we were able to carry out time series measurements of the electric field distribution in ms order. We report on the measurement of the electric field vector distribution changing with time on a circular area in a sphere-plane electrode system.