Showing papers on "Liquid dielectric published in 1999"

Dynamical instability of thin liquid films between conducting media.

Abstract: It is shown that thin dielectric liquid films between electrically conducting media are dynamically unstable if the electronic work functions of the conductors are different. As a consequence, they are expected to dewet by a spinodal dewetting scenario, i.e., via amplification of surface waves with mode selection. The wave number of the fastest growing mode is discussed as a function of the strength of the electric and dispersive forces. In contrast to what seems to be widely believed, it turns out that electric forces dominate over dispersive forces in most experimental situations of the rather general type studied here.

Dielectric plug-loaded two-port transmission line measurement technique for dielectric property characterization of granular and liquid materials

Abstract: There are numerous dielectric property characterization techniques available in the microwave regime each with its own uniqueness, advantages and disadvantages. The two-port completely-filled waveguide (transmission line) technique is a robust measurement approach which is well suited for solid dielectric materials. In this case, the dielectric material can be relatively easily machined to fit inside the waveguide and the subsequent measurement of the scattering parameters of this two-port device renders the dielectric properties of the material filling the waveguide. However, this technique is not well suited for measuring the dielectric properties of granular and liquid materials. These materials are used in the production of various composites which are increasingly replacing the use of metals in many environments. If this technique is directly applied to these types of materials, several approximations either in the measurement apparatus or the formulation must be made. To overcome this problem, this paper describes a modification to this measurement technique utilizing two dielectric plugs which are used to house the granular or the liquid dielectric material. In this approach no approximation to the measurement apparatus is made while the presence of the plugs are fully accounted for in the derivations. Using this technique, the dielectric properties of cement powder, corn oil, antifreeze solution and tap water, constituting low- and high-loss dielectric materials (granular and liquid) were measured. In addition, the important issue of measurement uncertainty associated with this technique is also fully addressed. The issue of optimal choice of various measurement parameters is also discussed as it relates to the measurement uncertainty.

Electrostatic filter for dielectric fluid

Abstract: An electrostatic filter for removing contaminants from a dielectric fluid when the filter comprises a plurality of electrically conductive plates positioned in a housing and having filter media disposed between the plates so that during the operation of the filters, fluid flows axially through the media between the electrode plates and into a central recovery zone and is discharged from the filter A replaceable filter cartridge, a method for removing contaminants from a dielectric fluid using the filter, a filtration system for removing both water and other contaminants from a dielectric fluid using the filter in combination with a water-removal filter and a system for maintaining a supply of filter dielectric fluid for a dielectric fluid user on a continuous basis are disclosed

Fire resistant natural ester dielectric fluid and novel insulation system for its use

Abstract: Insulation systems for electrical power and distribution transformers are being re-evaluated based on total owning cost from both economic and risk exposure perspectives. A developed high fire point dielectric fluid, based on natural esters, has superior safety, environmental, and health properties compared to current dielectric fluids. An insulation system developed for this fluid addresses the lower inherent resistance of esters to oxidation. Accelerated transformer life testing compares transformers using standard mineral oil to those with the natural esters dielectric system. Results of these accelerated life tests show the ester fluid to be suitable for transformer use and suggest the possibility of extended insulation life compared to mineral oil systems.

Measuring voltage response: a non-destructive diagnostic test method of HV insulation

Abstract: In insulation materials several chemical and physical deterioration processes arise, such as thermal and electrical ageing, and moistening, due to operational and environmental stresses. As well as electrical strength, these deterioration processes influence the dielectric processes, the conduction and the polarisation of the insulation. The dielectric strength of insulation cannot be determined without destroying it, but the intensities of dielectric processes can be investigated by non-destructive diagnostic test methods. Consequently, by diagnostic tests the ageing state of the insulation, i.e. the probable decrease in its electrical strength, can be detected without damaging it. By using the voltage response method for diagnostic testing of electrical insulation its condition can be easily determined, i.e. the probable decrease of the dielectric strength and the reliability of the insulation can be indirectly inferred. By testing cables with impregnated paper insulation the two main deterioration processes, i.e. thermal ageing and moistening, can also be distinguished.

Electro-aerodynamic instability of a thin dielectric liquid sheet sprayed with an air stream.

Abstract: The instability of a thin sheet of dielectric liquid moving in the same direction as an air stream in the presence of a uniform horizontal electric field is studied theoretically. It is found that aerodynamic-enhanced instability occurs if the Weber number is much less than a critical value related to the ratio of the air and liquid stream velocities, the electric field, and the dielectric constant values. The electric field is found to have a stabilizing effect, and there exists a critical Weber number above which instability is suppressed by the surface tension effect. The condition for disintegrating the sheet is obtained in terms of the electric field values, and some limiting cases are recovered.

Vegetable oil based dielectric fluid

Abstract: A dielectric fluid system for use in electrical equipment comprising a dielectric fluid comprising vegetable oil and an oxidation reducing composition encased in a housing composed of a polymeric material that is substantially permeable to oxygen. The oxidation reducing composition is placed inside the electrical equipment such that it is in contact with a headspace defined by the dielectric fluid. Devices generating and distributing electrical energy incorporating the above-described dielectric fluid system and method of retrofilling such devices also are provided.

An agriculturally based biodegradable dielectric fluid

Abstract: The development of a biodegradable dielectric fluid from agricultural sources is described. This material originated in a research prestudy and culminated in field implementation in the last year. Benefits include a more environmentally-friendly insulating fluid possessing a high flash and fire point and excellent dielectric capabilities. The fluid is suitable for use in electrical transformers and is being studied for use in other electrical apparatus such as cables, bushings, capacitors and tap changers where nonbiodegradable fluids are presently used. The properties of the fluid are compared to conventional mineral oil-based insulating fluids as well as other synthetic dielectric fluids. Design considerations for equipment containing this alternative fluid are also described. Life testing in the laboratory and field performance of units with this insulating fluid are discussed.

Particle-particle interactions in electrorheological fluids based on surface conducting particles

Abstract: We develop a simple, analytical conduction model for the case of electrorheological fluids based on surface conducting particles. By modeling two contacting spheres in a dielectric liquid by a distributed impedances network we derive analytical expressions for the potential and current at the spheres surface, and for the electric field and the current in the liquid phase. The knowledge of the electric field in the dielectric liquid allows us to calculate the interparticle interaction force as a function of the applied voltage. The theoretical interaction force is compared with experimental results obtained on insulating spheres coated with a thin conducting polyaniline film. We find a good agreement between the theory and experiment. The materials properties which govern the response of the system are outlined. In this regard, the product of the liquid conductivity by the sheet resistance of the surface coating appears as a key parameter. Some applications of this model for the practical design of electro...

Measurement of Nonsymmetrical Electric Field Vector Distribution in Nitrobenzene Using Electrooptic Method

Abstract: The three-dimensional nonuniform electric field vector distribution was measured in a nonsymmetrical electrode system using the Kerr electrooptic effect with the reconstruction technique modified by computed tomography (CT). The intensity of the light transmitted through a dielectric liquid in nonuniform field was detected at many points on one plane. It was found that the nonuniform electric field in the liquid can be measured three-dimensionally and visualized.

Discharges in combined transformer oil/paper insulation

Abstract: Discharge signatures and breakdown levels have been recorded for combined transformer oil/impregnated paper insulation subjected to AC high voltage. Different electrode geometries have been studied, varying from the point-plane gap to a semi-homogeneous sphere-plane configuration. At high moisture levels discharge activity was monitored at an average electric field below 1 kV/mm, with breakdown occurring 6 kV/mm. Characteristic discharge patterns were identified for corona, wedge, void and creeping discharges, but depend strongly on applied voltage, time and moisture content of the insulation.

Apparatus and method for monitoring and purifying dielectric fluids

Abstract: The present invention monitors and removes contaminants from dielectric fluids by utilizing an apparatus having a chamber and tube configuration. The chamber encloses a plurality of tubes which include porous hollow fibers with an inert polymer coating. Conduits route the dielectric fluid to and from operative equipment. An atmosphere control device creates an atmospheric condition within the chamber that causes contaminants within the dielectric fluid to migrate through the tubes into the chamber, thereby producing refined dielectric fluid. The method of the present invention removes contaminants from a dielectric fluid. The dielectric fluid is routed from operative equipment and passes through tubes enclosed within a chamber. An atmospheric condition is created within the chamber to cause contaminants within the dielectric fluid to migrate into the chamber. A refined dielectric fluid is produced which is directed back to the operative equipment. The contaminants removed from the dielectric fluid may further be analyzed, monitored, have any moisture therein removed, or be vented to atmosphere.

Electrohydrodynamically induced dielectric liquid flow through pure conduction in point/plane geometry-theory

Abstract: Mildly polar liquids generally exhibit an ohmic behavior when subjected to electric fields of limited values. The resulting conduction is then associated with heterocharge layers of finite thickness in the vicinity of the electrodes. In this paper, a simple conduction model is briefly presented which is based on the processes of dissociation of a neutral electrolytic species and recombination of the generated ions. This model is then applied to parallel plane electrode geometry to describe the build-up of the heterocharge layers and to obtain an approximate solution. Then, the case of point/plane configuration is considered where the effect of Coulomb force is different in the two layers next to the electrodes. A net motion toward the point electrode is predicted to occur. Taking into account the variation law of the harmonic field along the field lines, an approximate analysis is presented and an expression for the axial component of the total force exerted on the liquid is provided.

Identification of fundamental forms of partial discharges based on the results of frequency analysis of their acoustic emission

Abstract: The paper presented the spectral analysis of acoustic emission pulses generated by a sort of discharges on indeterminate-potential particles moving in liquid dielectric materials. Amplitude and power density spectra for the pulses was computed, followed by the derivation of their descriptors characterizing the discharges in a synthetic way. Some interesting properties of the discharges ware extracted from the descriptors, contributing to possible use of the method in diagnostics of insulating systems in electric power facilities.

Coulomb-driven flow of a dielectric liquid subject to charge injection by a sharp electrode

Abstract: Injection of charge by a sharp electrode into a surrounding dielectric liquid leads to Coulomb forces that set the liquid into motion. An analysis is presented of this motion in a small region around the edge of the electrode, which determines the injected current as a function of the far electric potential seen by this region. By using an injection law appropriate for nonpolar liquids, the analysis predicts an electric current that increases first exponentially and then as the power 73 of the harmonic part of the electric potential, sometimes with a range of multiplicity in between.

On the breakup of slender liquid bridges: Experiments and a 1-D numerical analysis

Abstract: Slender axisymmetric dielectric liquid bridges are made stable by the action of an axial electric field. In this paper, the subsequent dynamics of a slender liquid bridge after turning off the electric field is considered. The evolution in time of the bridge profile is investigated both theoretically and experimentally. A one-dimensional model is used to simulate the dynamic response of the system. Experiments are performed applying an axial electric field to a liquid bridge of 1 mm of diameter, and turning-off the electric field. The evolution of the liquid bridge is recorded using a video camera, and the digitized images are analysed. Good agreement between computations and experiments is found.

Modelling and experimental research of the non-stationary processes of conduction and space charge accumulation in dielectric liquids

Abstract: The non-stationary processes of conduction and space charge accumulation in dielectric liquids are characterised by the current-time dependencies, by maximum in the curves of the voltage-current characteristics, recorded for different time intervals, by maxima in the curves of the reversal currents and by non-linear distribution of the electric field in the interelectrode spaces. The nature and purity of the dielectric liquids and conditions on the electrode-liquid contacts determine the features of all these dependencies, where the electrochemical reactions can occur. For description of the current-time characteristic the physical and mathematical model of conduction of dielectric liquids can be used. This model supposes that the dielectric liquid contains a small amount of electrolytic impurities and represents homogeneous medium with constant permittivity. Analysis of the numerical modelling shows that for time in an interval longer than the chemical relaxation time, the charging current does not change in time. Nevertheless, as it follows from experimental data, the current in dielectric liquid can be dropped for a long much longer than the chemical. This process is sufficiently complicated and it needs further investigation. For description of the current, as a function of time in the quasi-steady-state range the developed model based on the attenuated memory principle [2] is proposed.

Effect of liquid dielectrics on the efficiency of silicon solar cells

Abstract: The results of experimental studies of the change in the photoelectric characteristics of silicon solar cells produced as a result of depositing thin, liquid dielectric layers (glycerine, acetone, isopropyl alcohol, butanol, dioxane, deionized water) are presented. It is shown that the presence of these liquids reduces the forward and reverse currents, substantially raises the short-circuit currents and open-circuit voltage, and significantly increases the efficiency (by up to 40–60%). Possible physical models are proposed for this effect.

Dielectric breakdown in insulating gases : space charge effects and non-uniform fields

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Stability of a bubble in a dielectric liquid in an external electrostatic field

Abstract: Critical instability conditions are found for a gas bubble in a liquid dielectric in a uniform external electrostatic field E0. It is shown that they depend both on the magnitude of E0 and on the properties of the liquid, as well as on the gas pressure in the bubble. In a linear approximation with respect to the square of the eccentricity of an equilibrium spheroidal form, the equilibrium eccentricity of the bubble exceeds the equilibrium eccentricity of a drop in the field E0. The gas pressure in the bubble lowers the critical electric field E0 for development of an instability in the bubble.

EHD self-modulation of capillary-gravity waves on a liquid layer of uniform depth

Abstract: The electrohydrodynamic self-modulational of capillary-gravity waves on the surface of a dielectric fluid layer of finite depth subjected to a tangential electric field is investigated by using the method of multiple scales. A nonlinear Schrodinger equation for the complex amplitude of quasi-monochromatic travelling waves is derived. The stability characteristics of a wave train are examined on the basis of the nonlinear Schrodinger equation. It is demonstrated, for the pure hydrodynamical case, that the capillary-gravity waves are modulationally stable for the wavenumbers and the liquid depths belonging to three stable regions. The introduction of the electric field has a stabilizing effect for small values of the wavenumber k in the first region; it does not have a significant effect in the second region; and it has a destabilizing effect in the third region. Higher values of the electric field, generate two new regions of stability and a new unstable region, however related to the previous ones. Further increasing the electric field decreases the first new stable region, while the second new stable region decreases and the new unstable region increases. Therefore, the effect of the electric field is different for the different regions of stability, and this effect is more strong if the dielectric constant of the upper fluid is less than the one of the lower fluid.

Stochastic features of initiation of liquid dielectric breakdown at small area of positive electrode

Abstract: The regularities of breakdown initiation in dielectric liquids in uniform and quasi-uniform pulse electric fields are studied in terms of the function /spl mu/(E) introduced previously. This function means the probability density of a streamer inception at a small element of an electrode area in a short time internal. The dependence of the function /spl mu/(E) on an electric field is reconstructed from experimental data on a breakdown in n-hexane. The increase in the pulse electric strength of dielectric liquids in narrow gaps for hemispherical electrodes is analytically explained.

Shaping of filamentary streamers by the ambient field

Abstract: We have simulated the fast streamer stage of liquid dielectric breakdown as stochastic growth of a branching fractal tree. Breakdown and threshold properties of the fluid are represented in the random filter procedure. A range of fractal densities, from sparse to bushy, is approximated by the choice of power-law (cube to linear). The choice of threshold (cutoff) voltage also significantly affects the growth form. These parameters combine with the shape and concentration of the electric field, to regulate the distribution and directedness of the local discharge growth pattern. A large grid (128 cubed) is used for the discretization. Diagonal growth paths to neighbor-vertices are included, increasing the choice of available directions for each discharge event.

Numerical investigation of insulation conductivity effect on electrical treeing

Abstract: A self-consistent model of electrical tree propagation and partial discharge activity within growing tree channels is developed. The electric field and the damage accumulation in the dielectric material surrounding the channels govern tree growth. The damage value is proportional to the amount of energy which is released in the channels due to partial discharges. The electric field distribution is determined by the charge placement within the insulator and electrode geometry. The charge distribution changes due to partial discharges and the conduction of the dielectric material. The numerical three-dimensional realisation of the model has been used to investigate the influence of the value of dielectric materials conductivity on electrical treeing under sinusoidal voltage in the needle-plane geometry. The temporal-spatial and phase-resolved characteristics of partial discharge activity have been studied for perfect and poor-performance insulators.

Rape-seed oil-a substitute for mineral oil?

Abstract: This paper presents the results obtained from the study on the basic characteristics of rape-seed oil considered to be a substitute of mineral oil in power transformers. The results show obviously that rape-seed oil is usable as an insulating liquid. Furthermore, results are presented concerning the breakdown behaviour in uniform (plane-plane geometry) and non-uniform (rod-plane geometry) fields under impulse conditions with gaps up to 40 mm and voltages up to 650 kV. A sufficient high 50Hz breakdown voltage is the most important precondition for a medium to be used as an insulator. The dielectric strength of rape-seed oil exceeds more than 50 kV/2,5 mm and consequently fulfils the requirements of the standards. The measurement of the permittivity and tan δ at 90°C yields to 3,18 and 0,015, resp. and is comparable to the values of standard mineral oil. However, there are still some problems to be overcome. Beside the insulating task the liquid dielectric has also a cooling function in power apparatus. Since the dynamic viscosity of rape-seed oil (about 70 mm 2 /s at 40°C) is higher than that of mineral oil, rape-seed oil has a lower ability for heat conduction. A further problem is the solidification of rape-seed oil below -5°C.

Dielectric response during curing of a resin-rich insulation system for rotating machines

Abstract: Pre-impregnated (resin-rich) mica tape based insulation is used as electrical insulation in rotating machines. The dielectric properties of this system change drastically during the curing of the resin. We report on measurements of capacitance and loss, in the frequency range of 10 mHz to 1 MHz, on insulation models during the curing process. In order to relate the degree of curing to the dielectric response and to understand more about the curing reaction, differential scanning calorimetry (DSC) measurements were made. The parameters in a simple thermal activated model for the degree of cross-linking were estimated from DSC data. From shifting the dielectric data measured at different temperatures it is concluded that there are different activation energies for the uncured and the ready cured samples. Two simple equivalent circuit models are proposed to describe the dielectric behaviour. The conclusion is that dielectric response measurements can be used to monitor the curing process and evaluate the final degree of curing.

Recovery voltage measurements on XLPE cables

Abstract: The dielectric response provides information on the physical and chemical structure of a material. For aging processes that affect physically and/or chemically insulating materials, the change in polarization phenomena can be exploited to monitor insulation degradation. One economic way for estimation of the dielectric response are recovery voltage measurements (RVM). Originally, recovery voltage measurements have been used to assess the depolymerization of the paper insulation in transformers. In this paper, it is discussed to what extent RVM can be applied to evaluate the condition of XLPE cables. An adoption of the common RVM test circuit is presented in order to ensure reproducible and meaningful results. Two models have been developed to explain the RVM spectra obtained from unaged cables and a cable containing water trees.