Showing papers on "Liquid dielectric published in 2005"

Electrically induced pattern formation in thin leaky dielectric films

Abstract: The stability of the interface between two thin leaky dielectric liquid layers bounded between two flat electrodes is considered. A coupled system of evolution equations is derived for the interfacial location and charge density using lubrication theory. This system is parametrized by the dielectric constants of the two fluids in addition to ratios of their conductivities, viscosities, and thicknesses. A linear stability analysis is conducted and the behavior of the system in the nonlinear regime is also examined. The system is destabilized by electrical stresses that are resisted by capillarity and modified by viscous dissipation. Our results suggest that decreasing the thickness ratio is destabilizing, giving rise to periodic structures of decreasing wavelength. Decreasing the viscosity ratio was also found to lead to the formation of sharp-edged structures whose vertical extent is virtually equal to the gap width between the electrodes. Similar structures were also determined upon increasing the ratio of the dielectric constants and electric conductivities.

On-line detection and location of partial discharges in medium-voltage power cables

Abstract: • A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.

Residual stress state and hardness depth in electric discharge machining: de-ionized water as dielectric liquid

Abstract: Procedures and results of experimental work to measure residual stresses and hardness depth in electric discharge machined surfaces are presented. Layer removal method is used to express the residual stress profile as a function of depth caused by a die sinking type EDM. Thin stressed layers are removed from machined samples by electrochemical machining. Corresponding deformations due to stress relaxation are recorded for each removal to determine the stress profile from elasticity theory. The relational dependence of the machining parameters with residual stresses is obtained and a semi-empirical model is proposed for plastic mold steel for de-ionized water as dielectric liquid. These stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value, around the yield strength, and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses.

Properties of ester liquid midel 7131 as an alternative liquid to mineral oil for transformers

Abstract: The combination of a solid and a liquid insulation is the most frequently used insulating system in high voltage apparatus, where components have to he insulated and loss heat has to be dissipated. The requirements on the liquid part of the insulating system are not only the electric and dielectric performance but also the performance regarding environmental requirements and dehydration capability as well as low in flammability. The use of ester liquid midel 7131, partly or totally replacing mineral oil, reduces the risk of environmental pollution, increases the lifetime of the component and reduces the fire risk. In distribution transformers midel 7131 has been used for several years with very good experience and therefore the manufacturer are encouraged to use midel 7131 also in power transformers even if the viscosity of the new liquid is higher than the viscosity of mineral oil. Due to larger dimensions of power transformers in comparison to distribution transformers special consideration is necessary. Some results concerning the electric and dielectric behaviour of midel 7131 is presented and pure midel 7131 as well as mixtures with mineral oil fulfil the requirements on the electrical performance of liquid insulating materials.

Microelectromechanical System-Based Evaporative Thermal Management of High Heat Flux Electronics

Abstract: We describe the development of embedded droplet impingement for integrated cooling of electronics (EDIFICE), which seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes over 100 W/cm 2 , employing latent heat of vaporization of dielectric fluids. Micromanufacturing and microelectromechanical systems are used as enabling technologies for developing innovative cooling schemes. Microspray nozzles are fabricated to produce 50-100 pm droplets coupled with surface texturing on the backside of the chip to promote droplet spreading and effective evaporation. We examine jet impingement cooling of EDIFICE with a dielectric coolant and the influence of fluid properties, microspray characteristics, and surface evaporation. The development of micronozzles and micro structured surface texturing is discussed. Results of a prototype testing of swiss-roll swirl nozzles with dielectric fluid HFE-7200 on a notebook PC are presented

Cooling arrangements for integrated electric motor-inverters

Abstract: In order to provide a modular arrangement, an inverter for an electric traction motor used to drive an automotive vehicle is positioned in proximity with the traction motor. The inverter is located within a compartment adjacent to one end of the electric traction motor and is cooled in a closed system by spraying a liquid coolant directly onto the inverter. The liquid coolant absorbs heat from the inverter and is cooled by a heat exchange arrangement comprising a reservoir with pipes carrying a second coolant from the radiator of the automotive vehicle. In a preferred embodiment, the coolant is collected from the inverter in an annular reservoir that is integral with the compartment containing the inverter. In accordance with one embodiment of the cooling arrangement, heat from the inverter vaporizes the liquid coolant by absorbing heat from the inverter during a phase change from a liquid to a vapor. The vaporized coolant is condensed by a circulating second coolant in pipes connected to the vehicle's radiator through a condenser that is preferably coaxial with the motor and the annular reservoir, which annular reservoir in the second embodiment collects overspray liquid coolant. In order to avoid degrading the inverter, the coolant is a dielectric fluid.

Electrohydrodynamic instability of the interface between two fluids confined in a channel

Abstract: The stability of the interface between two dielectric fluids confined between parallel plates subjected to a normal electric field in the zero Reynolds number limit is studied analytically using linear and weakly nonlinear analyses, and numerically using a thin-layer approximation for long waves and the boundary element technique for waves with wavelength comparable to the fluid thickness. Both the perfect dielectric and leaky dielectric models are studied. The perfect dielectric model is applicable for nonconducting fluids, whereas the leaky dielectric fluid model is applicable to fluids where the time scale for charge relaxation, $\epsilon \epsilon_o/\sigma$, is small compared to the fluid time scale $(\mu R/\Gamma)$, where $\epsilon_o$ is the dielectric permittivity of the free space,\epsilon and \sigma are the dielectric constant and the conductivity of the fluid,\mu and \Gamma are the fluid viscosity and surface tension, and R is the characteristic length scale. The linear stability analysis shows that the interface becomes unstable when the applied potential exceeds a critical value, and the critical potential depends on the ratio of dielectric constants, electrical conductivities, thicknesses of the two fluids, and surface tension. The critical potential is found to be lower for leaky dielectrics than for perfect dielectrics. The weakly nonlinear analysis shows that the bifurcation is supercritical in a small range of ratio of dielectric constants when the wavelength is comparable to the film thickness, and subcritical for all other values of dielectric constant ratio in the long-wave limit. The thin-film and boundary integral calculations are in agreement with the weakly nonlinear analysis, and the boundary integral calculation indicates the presence of a secondary subcritical bifurcation at a potential slightly larger than the critical potential when the instability is supercritical. When a mean shear flow is applied to the fluids, the critical potential for the instability increases, and the flow tends to alter the nature of the bifurcation from subcritical to supercritical.

Electric field analysis of water drop corona

Abstract: Water drops on surface of polymer insulation materials may cause corona because they can enhance the electric field nearby. Water drop corona plays an important negative role in the long-term performance of composite insulators. In this paper, based on a model with two parallel electrodes water drop corona is studied. The relationship between initial corona voltage and different sets of water drops is obtained from experiments. Then several factors, which may affect the electric field analysis of water drop corona, are studied by numerical calculation on computer. Calculation is mainly on a 2-D model. However, a 3-D model is also used to study the effect of water drops distributed in space.

The electroosmotic droplet switch: Countering capillarity with electrokinetics

Abstract: Electroosmosis, originating in the double-layer of a small liquid-filled pore (size R) and driven by a voltage V, is shown to be effective in pumping against the capillary pressure of a larger liquid droplet (size B) provided the dimensionless parameter σR2/e|ζ|VB is small enough. Here σ is surface tension of the droplet liquid/gas interface, e is the liquid dielectric constant, and ζ is the zeta potential of the solid/liquid pair. As droplet size diminishes, the voltage required to pump eletroosmotically scales as V ∼ R2/B. Accordingly, the voltage needed to pump against smaller higher-pressure droplets can actually decrease provided the pump poresize scales down with droplet size appropriately. The technological implication of this favorable scaling is that electromechanical transducers made of moving droplets, so-called “droplet transducers,” become feasible. To illustrate, we demonstrate a switch whose bistable energy landscape derives from the surface energy of a droplet–droplet system and whose triggering derives from the electroosmosis effect. The switch is an electromechanical transducer characterized by individual addressability, fast switching time with low voltage, and no moving solid parts. We report experimental results for millimeter-scale droplets to verify key predictions of a mathematical model of the switch. With millimeter-size water droplets and micrometer-size pores, 5 V can yield switching times of 1 s. Switching time scales as B3/VR2. Two possible “grab-and-release” applications of arrays of switches are described. One mimics the controlled adhesion of an insect, the palm beetle; the other uses wettability to move a particle along a trajectory.

Analysis, design and experimental results of a high-frequency power supply for spark erosion

Abstract: The LC/sub s/C/sub p/ resonant converter finds a new application in an electrical discharge machining (EDM) power supply, which is designed for the purpose of developing small size EDM systems. The switching frequency is tuned to the natural resonant frequency where the converter tends to act as a current source. In this way, three effects are achieved: 1) the necessary over-voltage is generated, first to ionize the dielectric and then to establish the electric discharge, 2) a constant current is supplied during the machining of the workpiece, providing the circuit with inherent protection under short circuit conditions, and 3) overall stability is guaranteed despite the equivalent negative resistance of the dielectric breakdown. The proposed control achieves an optimum and stable operation using tap water as dielectric fluid preventing the generation of undesired impulses and keeping the distance between the electrode and the workpiece within the optimum stable range. The EDM power supply has been validated to perform operations in a nuclear power plant application.

A comparative study on the surface integrity of plastic mold steel due to electric discharge machining

Abstract: The violent nature of the electric discharge machining (EDM) process leads to a unique structure on the surface of a machined part. In this study, the influence of electrode material and type of dielectric liquid on the surface integrity of plastic mold steel samples is investigated. The results have shown that regardless of the tool electrode and the dielectric liquid, the white layer is formed on machined surfaces. This layer is composed of cementite (Fe3C) and martensite distributed in retained austenite matrix forming dendritic structures, due to rapid solidification of the molten metal, if carbon-based dielectric liquid is used. The intensity of cracking increases at high pulse durations and low pulse currents. Cracks on the EDM surfaces have been found to follow the pitting arrangements with closed loops and to cross perpendicularly with radial cracks and continue to propagate when another discharge takes place in the neighborhood. The amount of retained austenite phase and the intensity of microcracks have found to be much less in the white layer of the samples machined in de-ionized water dielectric liquid. The number of globule appendages attached to the surface increased when a carbon-based tool electrode material or a dielectric liquid was used during machining.

Microwave characteristics of liquid-crystal tunable capacitors

Abstract: This letter investigates the microwave characteristics of the liquid crystal tunable capacitors for the first time. With the dielectric anisotropy properties, the liquid crystal capacitors present very different characteristics compared to the semiconductor or MEMS tunable capacitors. A quality factor of 310 with a control voltage of 5 V was achieved at 4 GHz. A tuning range of 25.3% for the control voltages from 0 to 5 V was obtained at 5 GHz. The results demonstrate the potential applications of liquid crystals as dielectric materials for capacitors with high quality factors and wide tuning ranges at high frequencies, particularly suitable for the future flexible electronics with transparent substrates.

Method and system using liquid dielectric for electrostatic power generation

Abstract: A system and method for generating power. The system has a first electrode member comprising a first region and a second electrode member comprising a second region. Preferably, the second electrode member is coupled to the first electrode member. An electret is coupled between the first electrode member and the second electrode member. The system has a spatial region provided between the first region of the first electrode member and the second region of the second electrode member. A volume of fluid (e.g., liquid, liquid and solids, gas and liquid, solids and gases) is provided between the first region and the second region and is adapted to move between the first region and the second region to cause a change in an electric field characteristic within a portion of the spatial region by the movement of at least a portion of the fluid within the portion of the spatial region to generate a change in voltage potential between the first electrode and the second electrode.

Measurement of Dry Rubber Content in Latex Using Microwave Technique

Abstract: This paper deals with an experimental study on the dielectric properties of natural rubber as a function of dry rubber content. The cost of rubber is directly dependant on the dry rubber content and therefore the study gains its importance. It shows that the dielectric properties of Hevea latex are mainly due to the orientation of loosely bound water molecules .The experiment was done at room temperature using a dielectric liquid cell used to hold the sample of natural rubber under study. The study comes to the inference that there is an inversely proportional relationship between the dry rubber content and its dielectric constant. i.e as the moisture content increases, the dielectric constant also increases. Keywords,dielectric constant ,latex, dry rubber

Effect of electrode geometry on performance of an EHD thin-film evaporator

Abstract: This paper presents details of an optimization process of electrode geometry for an electrohydrodynamically (EHD) driven thin-film evaporator. The operation principle of the device is based on the action of the EHD force on the molecules of a dielectric liquid in a highly convergent electric field. The force starts at the end of a pair of electrodes, where the electric field changes from zero far from the electrodes to a finite value in between the electrodes. This force drives the liquid up into the spacing between the electrodes. The electrodes in this study were deposited thinly on a SiO/sub 2//Si wafer, so the liquid could be held within micrometers of thickness over the surface. Since the performance of the device in removing heat from the surface is a function of its pumping head and consequently its electrode geometry, the performances of different electrodes were evaluated by testing twelve sets of electrode pairs with different geometries. Then the optimum electrode design was incorporated into the design of a large size (32/spl times/32 mm/sup 2/) EHD thin-film evaporator. The device was fabricated, and its pumping and heat transfer performances were tested. A pumping head equal to the full height of the electrodes and a heat transfer coefficient of 1.9 W/cm/sup 2/./spl deg/C was achieved using HFE-7100 liquid.

Dielectric characteristics of HTS cables based on partial discharge measurement

Abstract: We have discussed dielectric characteristics of liquid nitrogen (LN/sub 2/)/polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Focusing on the partial discharge (PD) inception characteristics and mechanism, the volume effect and its saturation on PD inception electric field strength (PDIE) was evaluated at atmospheric and pressurized condition. PD inception characteristics under lightning impulse voltage application were also investigated using an electrical and optical PD measuring system, and compared with those under ac voltage application.

Electromechanical and electro-optical properties of nanowires

Abstract: In an external (static of high-frequency) electric field, image forces cause a torque moment to act on semiconducting or metallic nanowires. As a result, a system of nanowires which are suspended in a dielectric liquid acquire characteristics, typical of those of nematic liquid crystals, such as field-induced optical anisotropy. For freestanding nanowires prepared on a substrate, an applied electric field is shown to be capable of causing nanowire bending, which can potentially provide a means of nanoactuation. The bending of such nanowires in a high-frequency electric field is comprised of a superposition of static bending and induced mechanical oscillations, whose amplitude depends on the resonant mechanical vibration characteristics of the nanowires.

Optical and electrical investigations on creeping discharges over solid/liquid interfaces under impulse voltage

Abstract: This paper deals with the experimental characterization of discharges propagating on solid/liquid interfaces under impulse voltages using a point-plane electrode arrangement. It's shown that the nature and the thickness of the solid insulating significantly influence the characteristics of creepage discharge and especially the density of branches. This later decreases when the thickness of the solid insulator increases. This indicates the important role of the capacitive effect in the propagation mechanism. For a given thickness, the final length of these discharges increases quasi-linearly with the voltage. The current waveform of creepage discharges depends on the polarity of the voltage.

Polyolefin insulation degradation in electrical field below critical inception voltages

Abstract: The degradation mechanism of polymeric materials is poorly predictable, and problematic to measure in the network. Polyolefins are widely used in the electric insulation of eg cables and capacitors due to their good dielectric properties and most competitive material price. The target for this paper is to explain the basic phenomenon related to the degradation of polyolefin in electrical discharge. Especially the reasons are reviewed why the material has such a sudden break down after a very long period of conservative electrical fields. K e y w o r d s: polymeric materials, dielectric properties, degradation, electrical discharge, polyolefin

On the Nature of the Vorob'evs Effect in Pulse Breakdown of Solid Dielectrics

Abstract: The Vorob’evs effect consists in certain features of the discharge observed when a solid dielectric in contact with two rodlike electrodes is placed in a liquid dielectric medium and a voltage pulse with increasing front is applied to the electrodes. When the pulse front slope is small, the discharge develops in the liquid over the solid dielectric surface; whereas the discharge at a sufficiently large slope of the pulse front penetrates into the solid and produces its fracture with cleavage of the surface fragments. In order to explain this phenomenon, it is suggested that, at a sufficiently high voltage buildup rate, a displacement current that is related to the motion of the surface discharge plasma passes through a microprotrusion occurring on the electrode surface at the contact site and causes the electric explosion of this microprotrusion. The metal plasma jet generated as a result of this explosion penetrates into the solid dielectric and forms a discharge channel in depth of this material. The surface discharge plasma formed at a small slope of the voltage pulse front closes the electrode circuit, thus preventing the discharge penetration in depth of the solid.

Research on effecting mechanism of particles in powder-mixed EDM

Abstract: Using kerosene with suspended powder as the dielectric fluid,powder-mixed electrical discharge machining(EDM) can work steadily at very low pulse energy,and can provide satisfying results.The effect of the powder particles is believed important in the machining procedure.Considering the electric field aberration in the interelectrode gap caused by the particles,the influence of the particles on the discharge gap and the dielectric constant of the working fluid is calculated utilizing the ″Wagner Model″.The calculated results show that the discharge gap and the dielectric constant both increase due to the action of the particles,but the former increases much more than the latter does.So the equivalent capacitance decreases much more in powder-mixed EDM than in conventional EDM, which accounts for the improvement in working stability of powder-mixed EDM.Finally,experiments were conducted,and the results are consistent with the calculated value.

A high-frequency electrospray driven by gas volume charges

Abstract: High-frequency (>10kHz) ac electrospray is shown to eject volatile dielectric liquid drops by an entirely different mechanism from dc sprays. The steady dc Taylor conic tip is absent and continuous spraying of submicron drops is replaced by individual dynamic pinchoff events involving the entire drop. We attribute this spraying mechanism to a normal Maxwell force produced by an undispersed plasma cloud in front of the meniscus that produces a visible glow at the spherical tip. The volume charge within the cloud is formed by electron-induced gas ionization of the evaporated liquid and produces a large normal field that is much higher than the nominal applied field such that drop ejection occurs at a voltage (at high frequencies) that is as much as ten times lower than that for dc sprays. The ejection force is sensitive to the liquid properties (but not its electrolyte composition), the ac frequency and trace amounts of inert gases, which are believed to catalyze the ionization reactions. As electroneutral ...

Investigations on dielectric breakdown of ceramic materials

Abstract: In order to increase the knowledge of the physical mechanism involved during the dielectric breakdown of ceramic material, different experiments are performed Samples of Al/sub 2/O/sub 3/ with different thicknesses are tested until breakdown occurs After data analysis, electromechanical mechanism is claimed to be responsible of the observed behaviors Complementary measurements (electrical and optical) used to confirm this assumption are presented

Effect of powder mixed dielectric on machining performance in electric discharge machining (edm)

Abstract: In this study, the variations of machining performance outputs, namely, workpiece surface roughness, workpiece removal rate, electrode wear rate, relative wear, workpiece surface hardness, and workpiece surface microstructure were experimentally investigated with the varying machining parameters for metal powder mixed dielectric liquid in electrical discharge machining (EDM). The machining tests were conducted by using prismatic steel workpiece and copper electrodes with graphite and boric acid powders (H 3 BO 3 ) mixed kerosene dielectric at different powder concentrations and pulse time settings. The experiments have shown that the type and concentration of the powders mixed into the dielectric and the pulse time were effective on machining performance outputs in EDM.

Stochastic model of partial discharge activity in liquid and solid dielectrics

Abstract: A new model that can simulate the main stochastic features of partial discharge activity at AC and DC voltages was proposed. In our simulations the narrow peaks of current were observed at the moment of every microdischarge in voids in solid dielectric after AC voltage was applied. The behavior of single cavity in dielectric liquid under DC voltage was also simulated.

Kerr electro-optic measurement of electric field distribution in rapeseed ester oil and pressboard composite system

Abstract: In oil-immersed power transformers, an insulating oil/pressboard (PB) composite insulation system is generally applied. In order to make power transformers more compact, it is necessary to optimize the insulation design with considering the charge behavior in liquid dielectrics. In addition, environment-friendly insulating oil for transformers is being interested. In this paper, we directly measured the electric field in parallel-plane electrodes and rapeseed ester oil/PB composite insulation system under dc voltage application using Kerr electro-optic technique. From the results, we compared the electric field in rapeseed ester oil with that in mineral oil and discussed the charge behavior in composite system. Finally, we confirmed the basic applicability of rapeseed ester oil for transformers.

A Fundamental Characteristic and Image Analysis of Liquid Flow in an AW Type EHD Pump

Abstract: Non-intrusive two-phase fluid pumping based on an electrohydrodynamically (EHD) induced flow phenomenon with free liquid surface exposed to gas-phase corona discharges is experimentally investigated. Dielectric liquid flow generated near a corona discharge electrode progresses toward an inclined plate electrode, and then climbs up the surface against the gravitational force for an air-wave (AW) type EHD pump. The AW type EHD pump is operated on ionic wind field along the inclined plate electrode. The pumping performance of time-averaged liquid flow rate and the liquid-phase flow motion are characterized. The liquid flow characteristics related to a dimensionless parameter of corona discharge fields are presented.