Showing papers on "Liquid dielectric published in 1997"

Experimental investigation of scaling laws for electrospraying: Dielectric constant effect

Abstract: Experiments were performed to investigate the effect of liquid dielectric constant on existing scaling laws for the electrospraying process. The variations of the droplet size and the emitted current were measured as a function of the dielectric constant for the electrospray operating in the cone-jet mode. Eight different solvents with dielectric constants, κ, ranging from 12.5 to 182 were tested. The residue particle size distributions were measured using a TSI scanning mobility particle sizer, (SMPS). The produced liquid droplet sizes were then calculated from the known solution concentrations. The results show that: (1) For the produced droplet size, Dd , experimental data are in agreement with the scaling law (G(κ) = 1.66κ −1/6) proposed by Ganan-Calvo et al. (1994) for solvents with high dielectric constants. The derivation in low dielectric constant cases may be that the assumption of Ganan-Calvo (1994) on the characteristic length may not hold for the present system; (2) for the emitted cu...

Performance characteristics of dielectrics in the presence of space charge

Abstract: The short and long-term dielectric behavior of a number of representative electrical insulating systems is compared in the presence of space charge. Dielectric materials, used both in the communications and power application areas, are considered. In this overview, particular attention is given to thin inorganic films, organic solid-liquid and solid polymer systems as regards to the manner in which space charge affects their dielectric loss, voltage breakdown, treeing and electrical aging characteristics.

Electrohydrodynamic plumes in point-plane geometry

Abstract: We examine the flow organization induced by charge injection from a needle of small radius of curvature into an insulating dielectric liquid. Experimental results are presented for the electrical current as a function of the point-plane distance and of the applied voltage. Schlieren visualization reveals the existence of very thin plumes which are slightly vacillating. An approximate analysis of the laminar plume is developed which takes into account the finite but very thin axial region where charge is confined. A classical type of asymptotic treatment results in a set of ordinary differential equations, provided the current and the field distribution are known. A new expression for the order of magnitude of the liquid velocity is proposed which leads to estimates of the radius a of the charge core and the typical radial scale /spl delta/ of the velocity profile. For a gap spacing d, these estimates are of the order of 10/sup -3/ d and 10/sup -2/ d which confirm the thinness of such charge plumes.

Dielectric fluid motors

Abstract: On the application of high electric fields to a dielectric fluid, a convective motion of the fluid is induced. By controlling the fluid motion in nonuniform dc fields, a new type of fluid motor is developed. An angular velocity of more than 15 s−1 (150 rpm) can be achieved at a dc voltage of 5 kV for a motor with a rotor radius of 10 mm. The efficiency of energy transformation from electric to kinetic energy is about 4%. Since magnetic fields and switching circuits are not required, the advantage of the fluid motor will be enhanced by size reduction. The dielectric fluid motor is attractive as a source of mechanical energy in a micromachine.

Measurement and modeling of dielectric response of composite oil/paper insulation

Abstract: Moisture and aging strongly influence the dielectric properties of oil/cellulose insulation systems. Dielectric measurements can therefore be used for assessing the condition of the oil/paper insulation. In this paper, we report on measurements and modeling of recovery voltages and polarization and depolarization current measurements on composite oil/paper insulation. A numerical model allows us to model the response of the composite system from knowledge of the dielectric properties of the constituents (oil and paper). In particular we demonstrate the consistency between recovery voltages and polarization and depolarization current measurements and show the influence of material properties on dielectric response.

Breakdown mechanism of liquid nitrogen viewed from area and volume effects

Abstract: We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN/sub 2/) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN/sub 2/ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN/sub 2/. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN/sub 2/. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN/sub 2/ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus.

Kerr electro-optic field measurement and charge dynamics in transformer-oil/solid composite insulation systems

Abstract: We measured the electric field in transformer oil/solid composite insulation system under dc voltage application using a Kerr electro-optic measurement system. When a pressboard was inserted at the center between two parallel-plane electrodes, the electric field at the mid-point between the pressboard and the cathode decreased with time. On the other hand, the electric field at the mid-point between the pressboard and the anode increased initially with time, reached a maximum value and then fell down. We measured also the electric field before and after polarity reversal of the dc voltage. The results revealed that the electric field after polarity reversal reached a magnitude 2.7/spl times/ as much as the average dc applied field. We interpreted the above results on the time transition of the electric field in terms of a charge dynamic model, considering the difference in charge accumulation speed on the pressboard between positive and negative charges. This model was verified to be applicable to the time transition of the electric field in oil/solid composite system and BTA added oil/solid composite system as well.

Influence of solid insulating phase on streaming electrification of transformer oil

Abstract: Due to increased demand of electrical power, the trend of the capacity and voltage rating of power transformers increases with reliable design; especially in large oil-cooled power transformers. Consequently, the use of polymer blended and polymer coated insulations in such transformers are the most recent advances in modern technology. The consequences of the type of solid insulating material, in contact with the streaming oil, on the ECT (electrostatic charging tendency) of oil are the principal aim of this paper. ECT of transformer oil is experimentally investigated using different types of oils and solid insulting materials. Two laboratory analog cycles are used; namely, a closed and an open cycle. In the closed cycle, the investigated factors are the type and polarity of the applied voltage (/spl plusmn/10 kV and 10 kV/sub rms/ at 50 Hz), solid insulating material (polymeric and non-polymeric insulation), and oil type, temperature (20 to 70/spl deg/C) and velocity (1.5 to 3.25 m/s). Good agreement is achieved between the measured data under ac and dc voltages of both the conduction and streaming currents, and the derived formulas. The currents increase exponentially with oil temperature, velocity and applied voltage. All solid dielectrics used show quantitative as well as qualitative changes in oil electrification especially at low temperatures. In the open cycle, both the rotating cylinder speed (430 rpm) and the oil volume are kept constant, and the oil temperature is raised to 120/spl deg/C for different types of oils and solid dielectrics. ECT depends on the oil type and is activated with temperature. Cellulose-based dielectrics give the highest ECT and coating of these dielectrics with epoxy resin enhances the ECT. Different suppression characteristic is obtained using Theophylline in different fresh oils.

Dielectric response, breakdown strength and water tree content of medium voltage XLPE cables

Abstract: Dielectric response, breakdown strength and water tree content of medium voltage XLPE cables

Electrorheology of dilute suspensions induced by hydrodynamic instability

Abstract: The viscosity behavior in electric fields was measured for dilute suspensions of p-[perfluoro(2-isopropyl-1,3-dimethyl-1-butenyl)oxy]benzoic acid particles (PFNA) in silicone oils. The application of electric fields causes a viscosity increase in a wide range of shear rates. Since the electrorheological (ER) effect is much stronger at low shear rates, the flow becomes shear-thinning. However, contrary to conventional ER suspensions which are reversibly converted between Newtonian fluids and Bingham solids, the PFNA suspensions are fluids even in electric fields. When the particle concentration is increased to 5 wt.%, the ER effect reaches saturation. Further increase does not contribute to additional viscosity enhancement. These results cannot be explained through the chain formation mechanism established for conventional systems. After the ER experiments, the bob surface of the rheometer is covered with several stripes of aggregated particles. Although the strength of electric and shear fields is constant in the rheometer, the periodic structure may be formed in the flow of electrified suspensions. When a dielectric liquid is subjected to high electric fields, the secondary motion of liquid can be induced by the Coulomb force acting on free charge. The electrohydrodynamic (EHD) convection is responsible for the periodic distribution of particles concentration. The ER effect of PFNA suspensions may be generated by a combined effect of EHD convection and external shear.

Radio frequency filter and apparatus and method for cooling a heat source using a radio frequency filter

Abstract: The filter includes a housing (13) defining a cavity (14). The housing has a fluid inlet orifice (22) and a fluid ou let orifice (24) therein. At least one resonator (16), which is sized to receive and pass a radio frequency signal, is disposed in the cavity. A dielectric fluid (18) fills the cavity. The fluid inlet orifice is configured to supply a first quantity of the dielectric fluid to the cavity and the fluid outlet orifice is configured to remove a second quantity of the dielectric fluid from the cavity, so that the dielectric fluid is continuously replaced.

A 2-dimensional Kerr-effect technique for electric field distribution in liquid dielectrics

Abstract: Based on the electro-optical Kerr effect, an attempt is made to develop a measurement technique for the two-dimensional distribution of the electrical field in dielectric liquids. For rapid, two-dimensional measurements, it is convenient to introduce a computer-controlled electronic camera for optical detection. However, because of the two-dimensional non-uniformity resulting from the system components, the measurement principle needs a two-dimensional analysis. To simplify the mathematical analysis, square pulsed modulations are employed. The influence of the system non-uniformity is eliminated by use of both optical and electrical modulations. To improve the system sensitivity, an optimum system setup is analyzed. To decrease the effect of the noise component resulting from the EHD (electrohydrodynamic) motion of the dielectric liquid, a diagnostic image lock-in technique is proposed. The validity of the two-dimensional measurement principle and the effectiveness of the diagnostic image lock-in processing are examined by the measurement of the electrical field distribution in a nitrobenzene specimen between two parallel plate electrodes.

A method for adding layers to a pwb which yields high levels of copper to dielectric adhesion

Abstract: A permanent curable photosensitive liquid dielectric polymer composition is disclosed for making copper printed circuit boards having at least one photosensitive dielectric polymer layer containing vias and circuitry thereon wherein the polymer layer has enhanced copper to dielectric adhesion. The dielectric composition preferably comprises an acrylated epoxy and a metal adhesion effective amount of amorphous silica. Printed circuit boards made using the photodefinable dielectric polymer and a method for making the printed circuit boards are also disclosed. A twice performed permanganate etching process for texturing the dielectric surface is also preferably used to further enhance the copper to dielectric adhesion.

Dynamic breakdown characteristics of liquid helium induced by a quench of superconducting wire and coil

Abstract: For practical insulation design of superconducting power apparatus, it is necessary to take into account an inherent phenomenon known as quench, the transition from the superconducting to the normal state. We investigated quench-induced dynamic breakdown and prebreakdown characteristics of liquid helium (LHe). Experimental results revealed that the quench of the superconductor drastically reduced the breakdown voltage of LHe in the thermal bubble disturbance. Moreover, dynamic breakdown characteristics in a quasi-uniform field using a superconducting coil were investigated. The results revealed that the breakdown was induced in the quench-initiated region where the largest thermal energy was generated.

Protection system for a three-phase distribution transformer insulated in a liquid dielectric

Abstract: A protection system for a three-phase distribution transformer immersed in a liquid dielectric contained in a tank, at least two of the three phases being equipped on the high voltage side of the transformer with respective current-limiting fuses. On each of the two phases equipped with current-limiting fuses, a protective micro-fuse is also connected in series with the current-limiting fuses and which operates faster than the current-limiting fuse. In addition, each micro-fuse is associated with a striker which, in turn, is connected to a mechanical trigger system which causes a three-phase short-circuiting device connected across the three phases to close in the event of a micro-fuse operating. Furthermore, the system includes at least one fault detector detecting one of pressure in the tank or the level of the dielectric. In particular, the three-phase short-circuiting device is situated on the high voltage side of the transformer between the current-limiting fuses and the high voltage windings.

The Effect of an Electric Field on Heat and Mass Transfer for Dielectric Crystallization

Abstract: This paper describes the effect of an electric field on heat and mass transfer for dielectric crystallization. A theoretical analysis was carried out to obtain the heat and mass transfer for crystal growth and the electric force effect on heat and mass transfer. Basic expressions for the force and for the relation of temperature and electric field were derived. Meanwhile, the effect of an electric field on crystallization is predicted. Experimental work has also been carried out by applying zero and 15kV d.c., in turn, to a dielectric sample melt. The results show good agreement with theoretical predictions.

Parametric instability of dielectric, slightly viscous liquid jets under ac electric fields

Abstract: The ac electric fields longitudinally tangential to a dielectric liquid column are able to excite capillary waves on the cylindrical surface. The phenomenon is theoretically predicted using a Lagrangian formulation of the dynamics of small amplitude perturbations of the free surface. As in the dc case, dielectric forces tend to stabilize those perturbations affected by the capillary instability, but if the field is periodic parametric resonance takes place for perturbations of shorter wavelength, otherwise stable with dc fields. A detailed description of the stability spectrum is included and the role of field intensity and frequency in the parametric resonance phenomenon is discussed. Non axisymmetric modes are also excited but they are always dominated by the axisymmetric one. The effect of a small viscosity is to diminish or even suppress unstable regions. The field necessary to observe parametric resonance is calculated.

Time domain PD-detection vs. dielectric spectroscopy

Abstract: A theoretically developed relationship between partial discharges and the response from a system for dielectric spectroscopy was experimentally confirmed. The losses caused by the discharges were highest at test voltages with low frequencies. At 0.1 Hz, tan/spl delta/ tip-up at discharge inception was very difficult to observe. A very clear tip-up could be seen by detection of the third harmonic in the current through the test object. This is much more sensitive to the detection of internal partial discharges than traditional loss measurements, in particular when using low frequency test voltages.

Electrohydrodynamic Taylor-Helmholtz instability of a liquid with constant shear

Abstract: The linear electrohydrodynamic Taylor-Helmholtz instability between a dielectric liquid of constant shear and a vacuum, when there is a constant vorticity in the liquid and under the effect of a constant electric field acting tangentially to the interface between the two media is studied. The interfacial stability conditions are obtained for constant shear, wavenumber (or wavelength) and electric field values. It is found that each of the constant shear, surface tension and the tangential electric field has a stabilizing effect. In the absence of the electric field, the corresponding pure hydrodynamical case is recovered and corrected. The case when the applied electric field is acting normally to the interface between the two media is also investigated for the cases when there are and there are not surface charges present at the interface, and stability conditions are obtained for both cases. It is found that the shear has a stabilizing effect, while the electric field has always a destabilizing influence, and the instability is more effective in the second case than in the first one.

Differentiation between electric breakdowns and dielectric breakdown in thin silicon oxides

Abstract: It has been known for some time that non-destructive electric breakdowns precede destructive thermal dielectric breakdown. We have been studying both processes in oxides between 5 nm and 80 nm in thickness. We have shown that the electric breakdowns can trigger dielectric breakdown under certain conditions. This triggering of dielectric breakdown causes TDDB distributions to be non-unique. The TDDB distributions could be shifted to shorter times if (a) the impedance of the test equipment was lowered and/or (b) the capacitance of the test equipment was raised. The implications of this work are discussed in terms of electric/dielectric breakdown models and practical circuit and device operation.

A dissociation-injection model for non-polar liquid conduction and wire-cylinder geometry

Abstract: The conduction of a nonpolar liquid filling a cylindrical capacitor has been investigated with a model that includes both the dissociation of ionic pairs in the bulk of the liquid and charge injection at the inner electrode. Three different cases, of increasing complexity, have been analyzed. Firstly, injection has been neglected and the dissociation rate has been taken as constant. Then, the effect of the electric field on the dissociation rate has been considered. And, finally, a field dependent injection of charge has been also included in the model. The current voltage characteristics corresponding to these three models have been evaluated for different ratios of the electrodes radii. Approximate analytical expressions are provided for the charge density and electric field distributions over a wide range of parameters of physical interest.

Bipolar electrical discharge machine which adjusts voltage polarity based on short circuit detection

Abstract: An electrical discharge machine for applying a pulsed voltage to a machining gap between an electrode and a workpiece disposed opposite thereto. An insulating dielectric fluid, such as oil or water mixed with a compound additive, is disposed in the gap. A positive polarity voltage and an opposite polarity voltage are applied via switches and respective impedances to the machining gap in order to generate a discharge for machining the workpiece. A pulse having a first polarity is applied to machine the workpiece and a pulse having a second polarity is applied when the first polarity pulse is not applied. The time of application of the second polarity pulse is increased depending upon the number of short-circuits detected between the electrode and the workpiece.

Electrooptic measurement of non-symmetrical electric field distribution in a dielectric liquid

Abstract: We carried out the measurement of the three-dimensional nonuniform electric field distribution in a non-symmetrical electrode system. An optical system was composed of a He-Ne laser, a polarizer, a quarter-wave plate, an experimental cell, an analyzer, and a photo-detector. When a negative high voltage pulse was applied to the electrodes system, the light intensity transmitted to the analyzer was measured by the photo-detector. The intensity measurement was carried out at many points in each direction from 0 to /spl pi/. From the obtained data, the electric field distribution was reconstructed with algebraic reconstruction technique (ART) in computed tomography (CT) technique. The measured distribution was interesting.

Study on Process Reaction Force Caused by Bubble Formation in EDM

Abstract: This paper describes the process reaction force which is generated in EDM by the explosive expansion of a bubble due to evaporation and dissociation of the dielectric liquid. The reaction force influences on the machining accuracy because the position of the tool electrode is displaced under the reaction force. Thus in this research, the reaction force is calculated by integration not only the pressure inside the bubble but also the pressure in the dielectric liquid distributed over the working surface. We compared the calculated results with the reaction force measured in a single discharge and obtained the relationship between the reaction force and the machining conditions. Then we also measured the reaction force in a series of pulse discharges and obtained the following results: (1) The pressure distribution in the dielectric liquid in the working gap accounts for the most part of the reaction force. (2) Hence the reaction force is almost proportional to the area of the working surface. (3) The reaction force in a series of pulse discharges decreases with the lapse of time as the working gap is filled with the generated bubbles in place of the dielectric liquid.

An electric device with a porous conductor insulation impregnated with a dielectric fluid exhibiting a rheologic transition point

Abstract: An electric device comprising a conductor and a porous conductor insulation impregnated with a dielectric fluid. Where the dielectric fluid comprises; a polymer; and a hydrocarbon-based fluid. The dielectric fluid being composed such that a part of the polymer molecule interacts with the hydrocarbon-based fluid or another part in the polymer molecule in such a way that the dielectric fluid at temperatures within a first low temperature range is in a highly viscous and elastic, essentially gelled, state; at elevated temperatures within a second higher temperature range, is in low viscosity easy flowing and essentially newtonian state; and that the viscosity of the dielectric fluid is, over a third limited temperature range, the transition range, changed between the low viscosity state and the highly viscous state. The transition range comprises temperatures between the first and the second temperature ranges.