Showing papers on "Liquid dielectric published in 2007"

Mechanical vs. electrical failure mechanisms in high voltage, high energy density multilayer ceramic capacitors

Abstract: Causes of breakdown, both mechanical and electrical, in high voltage, high energy density, BaTiO3 capacitors were studied. The flexural strength of the capacitors was 96 MPa. Failure was due to surface defects or pores close to the surfaces of the samples. The dielectric breakdown strength of the samples was 181 kV/cm. The causes of breakdown were either electrode end effects or pores between the dielectric and electrode layers. Weibull statistics were used to determine if there was a correlation between mechanical failure and dielectric breakdown. A strong correlation between the two types of failure was not found in the study, in contrast to earlier studies of single dielectric layer capacitor materials.

Tunability of Liquid Dielectric Resonator Antennas

Abstract: Dielectric resonator antennas (DRAs) have in the past been used most often at frequencies in the GHz portion of the spectra. This letter presents a novel 50-MHz DRA that uses water as the dielectric. For purposes of keeping the antenna tuned to a specific frequency, ande for using the antenna at different frequencies, this antenna can utilize a pump to alter the level of water in the DRA, thus, altering its resonant frequency. An electronic technique for retuning the feed probe is also presented where varactor diodes are biased to provide a good impedance match across a range of approximately 50 to 100 MHz. These antennas may provide a compact solution at lower frequencies especially where a directional pattern may be required. The liquid dielectric not only provides frequency tuning but simple deployment and a reducible radar cross section.

Stochastic models of partial discharge activity in solid and liquid dielectrics

Abstract: A new model that can reproduce main stochastic features of partial discharge (PD) activity at AC and DC voltages was proposed. The type of PD activity because of microdischarges in small cavities present in dielectric materials was considered. Three different criteria were used to simulate an initiation of partial discharge inside voids. The simplest criterion of threshold type was used also to describe a decay of plasma in voids and subsequent decrease in conductivity to zero. After AC voltage was applied to solid dielectric, the narrow peaks of current in external circuit were observed in our simulations. Every peak corresponds to a moment of PD in a void. The behaviour of cavities in dielectric liquid under DC voltage was also simulated. In this case, PD activity is possible even under DC voltage because of both elongation of microbubbles present in a liquid and diffusion of charge carriers from the surface of a bubble into a liquid.

Vegetable oil dielectric fluid composition

Abstract: An electrical device having therein a dielectric fluid composition, wherein the dielectric fluid composition includes at least one refined, bleached and deodorized vegetable oil and at least one antioxidant, wherein the dielectric fluid composition has a pour point of less than about −20° C. as measured according to either of ASTM D97 or ASTM D5950.

Impedance spectroscopy of an electrolytic cell limited by ohmic electrodes

Abstract: We investigate the influence of the ohmic character of the electrodes on the impedance spectroscopy of a cell of a dielectric liquid containing ions. According to our calculations, the finite conductivity of the electrodes is responsible for an increasing of the real part of the electrical impedance in the low frequency range. It follows that the anomalous increasing of the resistance of the cell, experimentally observed by several groups, could be related also to the exchange of charge at the electrodes. Our analysis is based on a generalization of the Butler-Wolmer equation, for small values of the external voltage.

A Comparative Study of the Dielectric Strength of Ester Impregnated Cellulose for Use in Large Power Transformers

Abstract: It is essential to ensure that the dielectric strength of an oil-cellulose insulation structure is not reduced when using esters to replace mineral oil in power transformers. One advantage proposed to using esters is that their higher dielectric constants, when compared to mineral oil, provide a better match to the dielectric constant of the impregnated cellulose. This results in the electric stress being reduced in the oil and increased in the cellulose, beneficial as the oil tends to have the lower dielectric strength. This paper reports the ac and impulse dielectric strengths of ester impregnated paper and pressboard. It was found that the ac withstand voltages of cellulose impregnated with ester are at least equal, or better, than cellulose impregnated with mineral oil. The lightning impulse dielectric strengths of cellulose impregnated with ester were found to be comparable to mineral oil. An analysis is given on the breakdown process of cellulose, as cellulose can fail due to either a direct puncture or discharges created in the oil wedge.

Dielectric properties of rapeseed oil paper insulation

Abstract: This paper presents the basic physical, chemical and electrical properties of a type of rapeseed oil based dielectric liquid. Dielectric properties including relative dielectric permittivity, dissipation factor, and breakdown voltage of the rapeseed oil paper insulation are presented and also compared with those parameters of mineral transformer oil paper insulation. Moreover, four types of small oil gaps are designed for experiments. These four types of oil gaps include a simple oil gap, an oil gap with a paper-covered high voltage electrode, and two oil gaps with one and two layers of papers, respectively, between the high voltage and grounding electrodes. Both the refined rapeseed oil and the mineral transformer oil are used for the small oil gap experiments. Breakdown properties of the small oil gaps are measured. The analysis results of test data show the differences in dielectric properties between the rapeseed oil paper insulation and the mineral transformer oil paper insulation.

Intense nonlinear magnetic dipole radiation at optical frequencies: molecular scattering in a dielectric liquid.

Abstract: We report white-light generation and intense linear scattering from magnetic dipoles established by the time-varying magnetic flux of an incident light field in a dielectric medium. Large magnetic response is very unexpected at optical frequencies, and should lead to the discovery of new magneto-optical phenomena and the realization of low-loss homogeneous optical media with negative refractive indices.

Dielectric properties measurements of transformer oil, paper and pressboard with the effect of moisture and ageing

Abstract: Dissipation factor (tan delta) and capacitance measurement at power frequency (50 Hz) has been commonly used to diagnose the insulation integrity and condition of transformers. The main drawback of this technique is that the measurement results obtained represent the status of the "complete" transformer insulation systems - a combination of the bulk of oil, paper insulation and pressboard. The contribution of each individual insulation material to the end result is not well known, and the degradation status of the whole insulation structure and each material are difficult to separate. In order to improve comprehension of the measurement results of this technique, the basic relationship between tan delta and each insulating material has to be established first. Hence, in this paper, experimental investigation has been done by studying the dielectric properties changes (tan delta and dielectric constant, epsivtau) of mineral oil, cellulose paper and pressboard with the affecting parameters, namely moisture and ageing. A simulation model of the combined transformer insulation has been used to correlate the tan delta of each insulating material to the measured combined value of a transformer.

Electrical charge transport and energy conversion with fluid flow during electrohydrodynamic conduction pumping

Abstract: The electrohydrodynamic (EHD) conduction pumping takes advantage of the electrical Coulomb force exerted on dielectric liquid by externally applied electric field(s). The conduction term here represents a mechanism for electric current flow in which charged carriers are produced not by injection from electrodes or induction from electric fields, but by dissociation of neutral electrolytic species within the dielectric liquid. The EHD conduction pumping can be applied to drive both isothermal liquid and two-phase fluids without the degradation of the working fluid electric properties. Such nonmechanical and low-power-consumption pumping mechanism can be utilized for active flow generation/control under both terrestrial and microgravity conditions. So far, the majority of conducted studies has been focused mainly on the experimental realization of the EHD conduction pumping phenomenon and the computational fluid dynamics simulation verification. More fundamental studies, such as theoretical analysis with convection terms included, generalized nondimensional modeling, and pumping efficiency prediction, are required for a complete understanding of this new EHD pumping phenomenon. An asymptotic nondimensional theoretical model for the EHD conduction pumping has been presented in this paper, with the fluid convection taken into account. The theoretical analysis provided here reveals the effects of flow convection on the EHD conduction pumping and the associated energy transport/conversion during the pumping process. Based on the asymptotic model, the pumping efficiency of the EHD conduction pumping is analytically derived and compared with the experimental data. Such results help clarify the capabilities and limitations corresponding to the nature of the EHD conduction pumping.

Understanding sub-20 nm breakdown behavior of liquid dielectrics.

Abstract: Nanoscale confinement of dielectric molecules is expected to influence their breakdown mechanism in applications such as nanoprobe based machining, molecular electronics, and other related technologies. This Letter presents the first experimental study of the breakdown of nonpolar, nonthiolated liquid dielectrics in the nanometer regime and develops a field emission assisted avalanche based approach to model such behavior. The studies show that dielectric breakdown in the sub-20 nm regime is independent of the cathode materials and is dominated by the electron emission and atomic cluster migration due to the "sub-20 nm scale confinement of the liquid dielectric."

Duplex electrical discharge machining

Abstract: An electrical discharge machine (22) includes first and second coaxial electrodes (30,32) and corresponding carriages (34,36). A liquid dielectric (44) is channeled between the electrodes (30,32) and a workpiece (12) which are suitably electrically powered for machining a duplex hole (14) in the workpiece (12).

Progression of positive corona on cylindrical insulating surfaces. I. Influence of dielectric material

Abstract: Experiments are described on the propagation of corona discharges over the surfaces of cylindrical PTFE and porcelain insulators under positive impulse voltages. A rod-plane electrode arrangement has been used to which a single value of peak voltage, with the same impulse profile, has been applied throughout. Quantities measured were inception times and voltages, corona current and light output at selected axial regions of the corona. Still photographs of the corona appearance have been taken. Comparison has been made between the surface corona and that in air alone. Interpretation is based on knowledge of the electric field profiles, as modified by the insulators and the possible effects of the surface on the discharge. These are discussed in terms of the presence or absence of charge attachment, the effects of the surface on electron production and the effect of the permittivity of the materials

Stationary coaxial electrified jet of a dielectric liquid surrounded by a conductive liquid

Abstract: The electrohydrodynamic problem describing the evolution of a coaxial jet of two immiscible Newtonian liquids injected through concentric orifices into a region of uniform electric field is formulated in the framework of the leaky dielectric model, and the dimensionless parameters governing the flow are identified. A simplified model is proposed combining a quasi-uni-directional approximation for the flow and the transport of electric charge with a fully numerical evaluation of the electric field. Results of this model are compared with boundary element solutions of the full governing equations for viscosity-dominated flows. The current/flow rate characteristic of a coaxial viscosity-dominated jet is computed and the ranges of flow rates in which a stationary axisymmetric jet is realized are discussed in the case of an outer liquid of finite electrical conductivity surrounding a dielectric liquid, in which the electric shear that stretches the jet is concentrated at its outer surface. The dependence of the flow on the applied electric field and the flow rates of the liquids, as well as the effects of the viscosity and electrical conductivity of the inner liquid, and of the surface tension of the inner surface, are discussed.

Electrohydrodynamic instability of a dielectric compressible liquid sheet streaming into an ambient stationary compressible gas

Abstract: The effect of compressibility of fluids on the linear electrohydrodynamic instability of a dielectric liquid sheet issued from a nozzle into an ambient dielectric stationary gas in the presence of a horizontal electric field is investigated. It is found that increasing the Mach number from subsonic to transonic causes the maximum growth rate and the dominant wavenumber of the disturbances to increase, and the increase is higher in the presence of the electric field. Liquid compressibility has been found to have a minimal effect on instability. At constant wavenumber and electric field values, the growth rate of disturbances increases as the gas Mach number tends to 1, and then begins to decrease with further increase in the gas Mach number. At small values of wavenumber, antisymmetrical disturbances grow faster than symmetrical ones, while the growth rate of both types of disturbances approach each other at large wavenumbers, which increases by increasing the electric field values. At small Weber numbers, antisymmetrical disturbances exhibit a higher maximum growth rate and a lower dominant wavenumber than symmetrical disturbances. However, the maximum growth rate and dominant wavenumber of the two types of disturbances are almost identical when both Weber number and electric field values become large. An increase in the gas to liquid density ratio enhances the instability, and this effect is enhanced for high electric field values. Surface tension and electric fields always oppose and increase the development of instability, respectively; and they have opposite effects for long wavelengths and high Weber numbers.

Schlieren optical visualization for transient EHD induced flow in a stratified dielectric liquid under gas-phase ac corona discharges

Abstract: A flow pattern characterization of electrohydrodynamically (EHD) induced flow phenomena of a stratified dielectric fluid situated in an ac corona discharge field is conducted by a Schlieren optical system. A high voltage application to a needle-plate electrode arrangement in gas-phase normally initiates a conductive type EHD gas flow. Although the EHD gas flow motion initiated from the corona discharge electrode has been well known as corona wind, no comprehensive study has been conducted for an EHD fluid flow motion of the stratified dielectric liquid that is exposed to the gas-phase ac corona discharge. The experimentally observed result clearly presents the liquid-phase EHD flow phenomenon induced from the gas-phase EHD flow via an interfacial momentum transfer. The flow phenomenon is also discussed in terms of the gas-phase EHD number under the reduced gas pressure (reduced interfacial momentum transfer) conditions.

Liquid transport generated by a flashing field-induced wettability ratchet

Abstract: The authors develop a model for ratchet-driven macroscopic transport of a continuous phase. The transport relies on a field-induced dewetting-spreading cycle of a liquid film based on a switchable, spatially asymmetric, periodic interaction of the free surface of the film and the solid substrate. The concept is exemplified using an evolution equation for a dielectric liquid film under an inhomogeneous voltage. The authors analyze the influence of the various phases of the ratchet cycle on the transport properties. Conditions for maximal transport and the efficiency of transport under load are discussed.

Study on Electrical Aging Lifetime of Vegetable Oil-paper Insulation

Abstract: As a new environmental friendly dielectric liquid,vegetable insulating oil has high fire point and flash point,almost fully biodegradable,excellent electric properties.Oil-paper structures are broadly used in electrical equipments because of its steady characteristics.For evaluating the lifetime of vegetable oil-paper model,the step-stress test is used in accelerated electrical stress aging test,and two parameter Weibull distribution is used to statistic analysis for the mean lifetime.This paper uses inverse power and exponential model to fit test data and calculate voltage endurance coefficients.As a contrast sample,the accelerated electrical stress aging test is also used to mineral oil-paper model.It is indicated that the relative permitivity of vegetable insulating oil is higher than that of mineral oil,so the electric field distribution in vegetable oil-paper insulation is more uniform than that in mineral oil-paper insulation.Therefore,the electrical aging behavior of vegetable oil-paper insulation is better than mineral oil-paper insulation.

High voltage breakdown and pre-breakdown properties in rape-seed insulating oil

Abstract: Rape-seed oil is considered for electrical insulation at high voltage. In this work, breakdown and streamer propagation in rape seed oil are studied in point-plane gaps up to 20cm, under impulse voltage of both polarities. In both polarities, two main streamer types are observed: "slow" streamers with a velocity of about 2km/s, and much faster streamers up to 150km/s. Compared to mineral oil in the same conditions, fast streamers in rape-seed oil appear at much lower voltage. They are able to propagate over large distances within a short time at moderate voltage. From these results, it is concluded that the properties of streamers in rape-seed oil are less favorable for high voltage insulation than in mineral oil.

Numerical study for the electrified instability of viscoelastic cylindrical dielectric fluid film surrounded by a conducting gas

Abstract: The linear electrohydrodynamic cylindrical instability of annular Walters B ′ viscoelastic dielectric fluid layer surrounded by a conducting gas in the presence of radial electric field is investigated. The obtained dispersion relation is found to be complicated and cannot be treated theoretically easily. Two limiting cases of interest are investigated, when the inertia is dominant, and when both the kinematic viscosity and viscoelasticity are high, and the corresponding new stability conditions are obtained for both cases. We solve the eigenvalue problem numerically using the continuation method which gives better results than the classical non-linear solvers such as Newton and Secant methods. It is found that the applied radial electric field has a dual role on the stability of the considered system, depending of the chosen wavenumbers range. Both the kinematic viscoelasticity and liquid depth are found to have stabilizing effects, while both the kinematic viscosity and surface tension have destabilizing effects on the considered system. The stability or instability breaks down for critical wavenumber values at which the growth rate vanishes. The behaviors of both the maximum growth rate and the corresponding dominant wavenumber are discussed in detail corresponding to the effect of all physical parameters. Finally a comparison between the results obtained here for Walters B ′ viscoelastic fluids, and those obtained here too if the fluid is replaced by a Rivlin–Ericksen viscoelastic one is achieved. The limiting cases of absence of electric field and/or kinematic viscoelasticity are also investigated in detail.

Nematic–isotropic phase transition of binary liquid crystal mixtures

Abstract: We experimentally studied the nematic–isotropic phase transition of (a) binary mixtures consisting of nematic and racemic liquid crystals and (b) binary mixtures consisting of positive and negative dielectric liquid crystals. We observed that the phase transition temperature is very sensitive to the chemical structures of the constituent components. We also used Maier–Saupe theory to calculate the transition temperature of binary mixtures. By fitting the experimental data, we obtained the interaction coupling constant between the constituent components.

Electrophoretic display medium

Abstract: PROBLEM TO BE SOLVED: To provide an electrophoretic display for enabling a stable display over a long period of time. SOLUTION: An electrophoretic display medium is used in the electrophoretic display device or the like, and at least one set of coloring particles is contained in a dielectric fluid. The dielectric fluid is a silicone fluid. The display medium has an electric conductivity of about 10 -11 S/m to about 10 -15 S/m. COPYRIGHT: (C)2008,JPO&INPIT

Evaluation of Transformer Insulation by Frequency Domain Technique

Abstract: Measurements of Dielectric Response in Frequency domain is being employed for assessing the condition of the paper-oil insulation system in Transformers. The laboratory measurements were carried out on Models based on simple Transformer insulation system. The dependence of frequency, temperature, voltage and ageing of the insulation system on the spectral behaviour of dielectric responses are discussed. The paper also presents correlation and comparison of the laboratory results, to the measurements performed on in-service Power Transformers. The healthiness of the insulation system of the in-service Transformers were evaluated and recommendations were made to improve the condition of the insulation system to extend the service life of the Transformers.

Apparatus and method for electric spark peening of gas turbine components

Abstract: Peening provides compression of component (6, 46, 56) surfaces in order to create residual surface compressions to resist crack propagation in components such as aerofoils. Previously peening techniques have had problems with respect to achieving adequate treatment depths, speed of treatment and with respect to effectiveness. By the present method arrangement an electrical conductor (1, 41, 51) in the form of a wire is subject to electrical pulses to cause evaporation and subsequent breakdown with high power ultrasound (HPU) propagation in a volume of dielectric fluid towards a component and so peening. The electrical conductor (1, 41, 51) ensures that there is limited possibility of electrical discharge to the component (6, 46, 51) surface whilst the positioning of the wire (1, 41, 51) relative to the surface can be adjusted to achieve best effect particularly if reflector (5) devices are utilised to concentrate (HPU) pulse presentation to the component (6, 46, 56). Furthermore, the component (6, 46, 56) can be surface treated in order to provide protection from potentially damaging emissions from evaporation and electrical discharge to the wire (1, 41, 51).