Showing papers on "Liquid dielectric published in 1998"

Propagation and structure of streamers in liquid dielectrics

Abstract: Our purpose is to present a critical review of the current understanding of streamer propagation in dielectric liquids in order to help define the direction of future research. We show that the molecular structure has a significant effect on streamer propagation. The main parameter affecting propagation is the electronic affinity of the liquid molecules.

AC (60 Hz) and impulse breakdown strength of a colloidal fluid based on transformer oil and magnetite nanoparticles

Abstract: A new class of colloidal dielectric fluids was developed by modifying the mineral oil based ferrofluids to enhance their dielectric performance. At its optimum composition, the colloidal insulation has AC (60 Hz) breakdown strength close to that of the carrier oil, while its impulse voltage withstand in needle to sphere geometry is improved: for the needle positive, the impulse breakdown voltage increased up to 50% as compared to the dry and degassed mineral oil (Exxon Univolt 60) while for the needle negative the breakdown value remains close to that of the oil carrier, so that the two values are practically equal. The PD inception voltage (AC, 60 Hz, 500 V/sec rise, needle to sphere geometry) showed and increase of up to 30%. The new colloidal insulation showed little change in electrical resistivity and kinematic viscosity after accelerated aging at 185/spl deg/C.

Practical foundations of the RVM (recovery voltage method for oil/paper insulation diagnosis)

Abstract: The original research into the application of Recovery Voltage Measurement to diagnosis of oil-paper insulation is described in detail. The goal of this research was to thoroughly review the conventional diagnostics used on oil immersed paper insulation systems in large power transformers and, after fundamental laboratory measurements and on-site measurements of power transformers, to propose a method for the future dielectric testing of insulation in large transformers in service. The results of the basic research are further illuminated by examples drawn from the numerous tests since performed on actual test objects in the field, mostly large power transformers, which give insight into the strengths and limitations of the method. A few measurements on oil-paper insulated cables suggest a yet to be explored potential application to the diagnostic testing of cables.

Optimization of multi-wavelength interdigital dielectrometry instrumentation and algorithms

Abstract: Interdigital frequency-wavelength dielectrometry can be used to measure the dielectric permittivity and conductivity of insulating materials. The complex dielectric permittivity is directly related to other material properties, such as moisture content, temperature, concentration of impurities and additives, density, aging status, etc. The analysis of spatial and temporal variations of these properties lends valuable insights into physical phenomena which take place in electrical equipment, provides instrumentation for system monitoring and diagnostics, and can be used for optimization of design and performance of electrical apparatus. The optimization of various aspects of this technology is described in this paper. Improvement of performance is achieved through variation of geometrical design, materials, manufacturing processes, and electronic circuitry. Accumulated effects of non-ideal geometry of the experimental setup and the sensor itself are accounted for through empirical measurements, calibration, and use of finite-element calculations. Three distinct operating modes are developed: floating voltage with grounded backplane, floating voltage with guarded backplane, and short circuit current. Measurements reveal that the interfacial contact quality has a strong influence on the sensor's response. Gain/phase measurements over the frequency range 5 mHz to 10 kHz agree well with theoretical calculations on the interfacial contact quality. Full-frequency measurements for several liquid and solid dielectrics are shown to have a good match with theoretical predictions.

Low dielectric constant material for use as an insulation element in an electronic device

Abstract: A low dielectric constant material is provided for use as an insulation element in an electronic device, such as but not limited to an integrated circuit structure for example. Such a low dielectric constant material may be formed from an aqueous fluoropolymer microemulsion or microdispersion. The low dielectric constant material may be made porous, further lowering its dielectric constant. The low dielectric constant material may be deposited by a spin-coating process and patterned using reactive ion etching or other suitable techniques.

Differentiation Between Electric Breakdowns and Dielectric Breakdown in Thin Silicon Oxides

Abstract: In several models of dielectric breakdown, nondestructive electric breakdowns precede destructive thermal dielectric breakdown. Both processes in oxides between 5 nm and 80 nm thick have been studied. The two breakdown phenomena have been differentiated, and the electric breakdowns have been separated from the dielectric breakdown. During constant voltage stressings, prior to dielectric breakdown, transient voltage spikes were measured and spots formed on the surface of the wafers due to electric breakdowns. Similar transient spikes occurred when measuring ramped breakdown voltages. It was found that the time dependent dielectric breakdown (TDDB) distributions measured on a series of identical oxides at the same voltages depended on the resistance and capacitance of the measurement test equipment due to the thermal nature of the dielectric breakdown. The TDDB distributions were shifted to shorter times if (i) the impedance of the test equipment was lowered and/or (ii) the capacitance of the test equipment was raised. The implications of this work are discussed in terms of electric and dielectric breakdown models and practical circuit and device operation.

On-site diagnosis of power transformers by means of relaxation current measurements

Abstract: The purpose of this paper is to show how the relaxation currents of a power transformer can be calculated, when the geometrical configuration of the main insulation and the dielectric properties of its components, oil and pressboard, are known. To model this insulation a simple equivalent circuit is presented, which is derived from basic relations of linear dielectric theory. The dielectric characteristics of oil and pressboard necessary for modelling are determined from dielectric response measurements on samples. Therefore some results of relaxation current measurements of pressboard samples in dependence on moisture content and temperature are presented. Finally, some examples of on-site measurements of relaxation currents on hitherto unused power transformers are shown which are used here to demonstrate how the moisture content of pressboard of these transformers can be quantified from evaluation of their relaxation currents.

Mechanisms causing nonlinear dielectric response of water treed XLPE cables

Abstract: The nonlinear dielectric response of water tree degraded XLPE power cable insulation has been found to be an important additional diagnostic criterion. It has been experimentally shown that in case of long water trees the dielectric response increase more than proportional with increasing test voltage above a certain voltage level. In this paper we present possible explanations for the formation of nonlinear dielectric response based upon the mechanical damage theory of water treeing. In order to verify the proposed hypothesis, we have performed electric field computations, water content and microscopy treeing analysis.

Stability analysis of a Taylor–Couette flow of insulating fluid subjected to radial unipolar injection of charge

Abstract: The linear stability of a dielectric fluid filling a cylindrical capacitor is analyzed. The outer or the inner cylinders forming the capacitor may rotate with fixed angular velocities, thus generating a Taylor-Couette flow inside the fluid. A weak, moderate or strong injection of charge is assumed to occur at the anode when a high voltage is applied to the electrodes. The physical mechanisms associated to the instability of the steady flow are discussed. Special attention is paid to the case when the mobility of ions is similar to or smaller than the hydrodynamic mobility of the fluid.

Viscoelasticity of a dielectric fluid in nonuniform electric fields generated by electrodes with flocked fabrics

Abstract: The rheological behavior of a dielectric fluid is studied in nonuniform electric fields which are generated by an electrode covered with flocked fabrics. Although no electrorheological (ER) effects are observed in uniform fields between metal electrodes with smooth surfaces, striking increases in viscosity and elastic response are induced by the electrode with flocked fabrics. The presence of flocked fabrics does not have a significant effect on the fluid rheology without electric fields. The ER behavior and current density are influenced by the fiber length even at a constant field strength. When a very small amount of fine particles is introduced in the electrified fluid without shear, we can see the rapid and large-scale motion of particles between the tips of fibers and plate electrode. In high DC fields, the Coulomb force acting on a free charge often gives rise to the secondary motion of fluid. The local motion of fluid in high electric fields is refereed to as electrohydrodynamic (EHD) convection. The additional energy may be required to change the periodic patterns of EHD convection by forced shear. Therefore, the ER effect demonstrated by the modification of electrode with flocked fabrics can be attributed to a combined effect of EHD convection and external shear.

Diagnosis of medium voltage XLPE cables by high voltage dielectric spectroscopy

Abstract: This paper summarize results from work on diagnosis of medium voltage (MV) XLPE cables with respect to water treeing performed at KTH, Stockholm The work, performed both in laboratory and in the field, has shown that high voltage dielectric spectroscopy is a good tool for diagnosis of MV XLPE cables and that it is able to differentiate between different stages of water tree ageing

Effect of Drop Deformation on Heat Transfer to a Drop Suspended in an Electrical Field

Abstract: Heat transfer to a drop of a dielectric fluid suspended in another dielectric fluid in the presence of an electric field is investigated. We have analyzed the effect of drop deformation on the heat transport to the drop. The deformed drop shape is assumed to be a spheroid and is prescribed in terms of the ratio of drop major and minor diameter, Results are obtained for both prolate and oblate shapes with a range of diameter ratio b/a from 2.0 to 0.5. The internal problem where the bulk of the resistance to the heat transport is in the drop. as well as the external problem where the bulk of the resistance is in the continuous phase, are considered. The electrical field and the induced stresses are obtained analytically. The resulting flow field and the temperature distribution are determined numerically. Results indicate that the drop shape significantly affects the flow field and the heat transport to the drop. For the external problem, the steady-state Nusselt number increases with Peclet number for all drop deformations. For a fixed Peclet number, the Nusselt number increases with decreasing b/a. A simple correlation is proposed to evaluate the effect of drop deformation on the steady-state Nusselt number

Detecting underground pipes

Abstract: Pressure waves are generated in a pipe (10) containing a dielectric fluid, by providing at least one electrode (11) adjacent to the pipe (10) and means (14) for applying a varying electric signal to the electrode (11) to generate a varying electric field in that pipe (10) The electric field is of a magnitude sufficient to create dipoles in the molecules of the dielectric fluid, and pressure waves are created by the creation and relaxation of these dipoles A detector (20, 21, 22) detects the pressure waves in the pipe (10) at a point remote from the electrode (11), so enabling the pipe (10) to be detected underground

Positive discharge development in insulating oil: optical observation and simulation

Abstract: Results of optical investigations of positive discharge channel development in transformer oil under divergent field configuration and impulse voltages to 650 kV are presented. The propagation of two different discharge structures (bush-like, tree-like) and the transition between these structures were observed. The conditions of their development and boundary of the bush-to-tree transition were determined. Anomalous behavior of the breakdown time dependence on applied voltage, consisting of a sharp drop in time at a critical voltage, was found. The dielectric breakdown model modified by introducing the varying channel conduction is presented. In the model discharge channel propagation is described by stochastic development of current and field instabilities. The model describes adequately the main spatial and temporal features of the various discharge structures observed in the experiments. The current and field characteristics of the simulated discharge channels which belong to different structures are compared. The bush-to-tree transition and the anomalous dependence of the breakdown time on voltage were explained by current instability in bush-structure channels.

Nonlinear EHD stability of the interfacial waves of two superposed dielectric fluids

Abstract: The slow modulation of the interfacial capillary–gravity waves of two superposed dielectric fluids with uniform depths and solid horizontal boundaries, under the influence of a normal electric field and in the absence of surface charges at their interface, is investigated by using the multiple-time scales method. It is found that the complex amplitude of quasi-monochromatic traveling waves can be described by a nonlinear Schrodinger equation in a frame of reference moving with the group velocity. The stability characteristics of a uniform wave train are examined analytically and numerically on the basis of the nonlinear Schrodinger equation, and some limiting cases are recovered. Three cases appear, depending on whether the depth of the lower fluid is equal to, greater than, or less than the depth of the upper fluid. The effect of the normal electric field is determined for the three stability regions of the pure hydrodynamic case. It is found that the normal electric field has a destabilizing influence in the first stability region and a stabilizing effect in the second and third stability regions. Moreover, one new unstable region or two new stable and unstable regions appear, all of which increase when the electric field increases. On the other hand, the complex amplitude of quasi-monochromatic standing waves near the cutoff wavenumber is governed by a similar type of nonlinear Schrodinger equation in which the roles of time and space are interchanged. This equation makes it possible to estimate the nonlinear effect on the cutoff wavenumber.

Thixotropic dielectric fluid for capacitors

Abstract: A thixotropic dielectric fluid that includes vegetable oil, transformer oil, and a clay material. The thixotropic dielectric fluid is particularly suited for use in a capacitor.

Conduction Models in Dielectric Liquids

Abstract: Conduction in gases and dielectric liquids (DL) has been reviewed many times. Comprehensive treatments on conduction in gases can be found in the works of Thompson and Thompson [1], and as for DL in the review by Gallagher [2], Adamczevski [3] and Felici [4]. In this section we shall devote our discussion to some of the major controversial issues concerning the natural conductivity due to special ionizing admixtures. The latter has important significance to Electrohydrodynamics applications [5, 6, 7]. Then we shall consider conduction models in dielectric liquids.

Guided waves propagating in a piezoelectric plate immersed in a conductive fluid

Abstract: In this research we report theoretical model and experimental measurements for the leaky Lamb wave propagation behaviors in a X-cut lithium niobate immersed in a conductive fluid. Starting with wave equations, a partial wave analysis procedure is taken for the modeling of plane wave reflection coefficient, which characterizes the dispersion relation of piezoelectric plate immersed in a dielectric fluid. Fluid conductivity is modeled by modeling the fluid as lossy dielectric media. Bulk wave ultrasonic transducers with V-shape arrangement are used in this particular experiment, running in pitch-catch manor. Close comparison with fairly well agreement between theory and measurements is obtained.

Barrier effect on the dielectric strength of oil gaps under DC voltage

Abstract: This paper deals with the influence of insulating barriers on the dielectric strength of two electrode arrangements - point-to-plane and point-to-point under dc voltage. It is shown that the dielectric strength of such insulating structures is the improved when the barrier is inserted near the sharp electrode. The effectiveness of the barrier depends on the geometry and the physical nature of the barrier. The presence of an air-pocket in the insulating structure or a hole within the barrier significantly reduce the dielectric strength of the system.

A method for manufacturing an electric device having an insulation system impregnated with a dielectric fluid

Abstract: A method for manufacture of an electric device having at least one conductor and a porous, fibrous and/or laminated electrically insulating dielectric system comprising a solid electrically insulating part impregnated with a dielectric fluid and a porous, fibrous and/or laminated body for use in an electric device produced with the method. The method comprises the steps of: providing a conductor and a porous, fibrous and/or laminated structure of a solid electrically insulating material associated with each other, pretreating the porous, fibrous and/or laminated body with a gelling additive such that the body comprises the gelling additive which imparts a high viscosity and elasticity to the fluid at conditions for which the device is designed to operate under, impregnating the pretreated body comprising the gelling additive with the dielectric fluid, wherein the gelling additive upon impregnation is brought into contact with the dielectric fluid and the gelling additive, at least in part, is dissolved by the fluid such that following the impregnation the dielectric fluid is gelled.

Electric switch for enhancing electric current flow by quantum tunneling effect

Abstract: A tilt switch includes a casing in which a shorting element, such as a ball, is freely movable. An inside surface of the casing forms one fixed electrode surface, and an element projecting into the casing forms a second fixed electrode surface. The shorting element rolls along the first fixed electrode surface toward and away from the second fixed electrode surface. The chamber contains a dielectric liquid which is wettable to the first and second fixed electrode surfaces, and to the surface of the shorting element. The liquid is either a non-polar liquid or a weakly charged polar liquid. Surface tension acting on the liquid causes the surface of the shorting element to be pulled into close intimate relationship with the fixed electrode surfaces, sufficiently for an electric current to be enhanced by a quantum tunneling effect.

Detection of partial discharge in solid and liquid insulation with an electric field sensor

Abstract: Partial discharges in high voltage insulation systems are often a precursor to dielectric breakdown. Detection of PD is thus an effective method to evaluate dielectric faults in the initial stage. Using an electric field sensor to detect PD was originally successfully developed to assess the condition of GIS. In this paper this technique is applied on solid insulation system and oil-impregnated system. PD signals are measured as individual pulses waveforms through a wide bandwidth measurement system by using an electric field sensor. Phase-resolved patterns obtained by the conventional PD detection method also are illustrated. The spectrum analysis shows electromagnetic waves with frequencies up to 1 GHz are emitted from partial discharge in the solid and liquid insulation system.

Process for impregnation of electrical capacitors

Abstract: The invention relates To a process for the impregnation of electrical capacitors which includes performing the impregnation of the coil winding with a dielectric fluid, eliminating the excess of the dielectric fluid, filling the casing containing the impregnated coil winding with a gelable composition and the dielectric fluid for impregnating the coil winding and gelling the gelable composition.

Design of de-ionised water cooling of power converters

Abstract: Power electronics converters need cooling devices. High thermal performances allows decrease of converter volume and cost, and have a major impact on long-term reliability. Semiconductors encapsulation structure have to combine two opposite requirements, namely low thermal resistance and electrical insulation capabilities. Among numbers of designs (ceramics, diamond, polymers etc.), dielectric fluid cooling has been used for high voltage applications (oil, freon and substitutes, deionised water). In de-ionised water cooling, due to water intrinsic resistivity, special care has to be taken with design and corrosion issues. This paper presents design rules and figures for medium voltage applications.

Method for carrying out the electrical breakdown of a gaseous dielectric in a highly non-uniform field

Abstract: The invention pertains to testing technique and designed to be used in the research n the dielectric rigidity of high-voltage equipment gaseous isolation. The technical result of the invention is the achievement of gas dielectric breakdown with relatively low voltage. A corona discharge is generated by supplying the initial voltage to the electrodes, defining the discharge gap so as to ensure a three dimensional load at the corona-producing electrode. Then, the total voltage, sufficient for complete breakdown, is supplied to the discharge gap with the uniformly distributed three-dimensional load and the breakdown of a gaseous dielectric takes place. The present method provides the decrease of corona discharge and formation of impulse pedestal with the help of additional dynamic discharger in the circuit, which electrodes are switched from the closed state to the open one.

Electrical apparatus with dielectric fluid blend of polyalphaolefins and polyol esters or triglycerides

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

Pressure effects on partial discharges in liquid dielectrics

Abstract: Liquids are routinely used for insulation in a wide range of high voltage applications. These liquids can be subjected to a variety of hydrostatic pressures during normal operating conditions. The work presented here examines the influence of pressure on the behaviour of partial discharges. Details of these events were recorded using simultaneous wide bandwidth current monitoring and high speed imaging of the associated partial discharge streamers that appeared in the liquid. It was determined that the events could be divided into two characteristic classes, with a behaviour that depended not only on the polarity of the voltage, but also on its magnitude. The two classes were found to have widely different pressure dependencies. One group was seen to be strongly pressure dependent, while the other was practically unaffected by pressure over the range examined. While it was found that the lowest inception voltages produced partial discharges that fell predominantly into the pressure sensitive group, it was the pressure insensitive events that ultimately led to breakdown.

An evaluation of the gassing tendency under electrothermal stress of some dielectric liquids

Abstract: It is known that, under electrical or thermal stress, dielectric liquid produces gas. The performance of oil cooled, insulated, or impregnated electrical equipment such as transformers, cables, or capacitors, is affected by the presence of these small gaseous bubbles, which can give rise to partial discharges and to eventual insulation breakdown. Therefore, the gassing properties of an insulating liquid, i.e. its tendency to absorb or evolve gases, have been recognised as a factor of major importance in characterising dielectric liquids. The transition from gas absorption to gas evolution can occur at different temperatures and electrical stresses for different liquids. The effects of temperature, electrical stress, and ageing on gassing tendency have been well studied. Efforts have also been made to correlate the aromatic content of oil with a gassing tendency. However, such studies are limited to mineral insulating transformer oil. In the present work, an effort has been made to study the gassing characteristics of some common dielectric fluids, including linear alkyl benzene (LAB), silicone oil, and capacitor oil, along with some naphthenic and paraffinic transformer oils.