Showing papers on "Liquid dielectric published in 1989"

Theoretical models of the electrical discharge machining process. I. A simple cathode erosion model

Abstract: A simple cathode erosion model for the electrical discharge machining (EDM) process is presented. This point heat‐source model differs from previous conduction models in that it accepts power rather than temperature as the boundary condition at the plasma/cathode interface. Optimum pulse times are predicted to within an average of 16% over a two‐decade range after the model is tuned to a single experimental point. A constant fraction of the total power supplied to the gap is transferred to the cathode over a wide range of currents. A universal, dimensionless model is then presented which identifies the key parameters of optimum pulse time factor (g) and erodibility (j) in terms of the thermophysical properties of the cathode material. Compton’s original energy balance for gas discharges is amended for EDM conditions. Here it is believed that the high density of the liquid dielectric causes plasmas of higher energy intensity and pressure than those for gas discharges. These differences of macroscopic diele...

Stabilization of dielectric liquid bridges by electric fields in the absence of gravity

Abstract: The stability of liquid bridges in zero gravity conditions under the influence of an a.c. electric field tangential to the interface is examined in this paper. For the theoretical study, a static analysis was carried out to find the bifurcation surfaces as a function of the three relevant non-dimensional parameters: Λ, the slenderness or ratio of height to diameter of the cylindrical bridge; β0, the ratio of dielectric constants of the two fluids used and Ξ, a non-dimensional quantity proportional to the applied voltage. Stable and unstable regions of Λ−βo−Ξ space were distinguished. Results indicate a strong stabilizing effect for higher values of β0. The experimental study, using silicone and ricinus oil to approximate zero gravity conditions fully confirmed quantitatively the theoretical results.

Electrode for a corona discharge apparatus

Abstract: An electrode for a corona discharge apparatus is comprised of a conductor, which can be a stainless steel tube or filament, a metal bar or a rotatable roll, and which is covered by a dielectric, which can be an organic elastomer having a high dielectric constant or quartz or ceramic. The dielectric may be coated with a dielectric adhesive that is loaded with a granulated inorganic dielectric. In most applications a film resistor is inserted between the conductor and the dielectric to reduce any voltage spikes that can result if the corona discharge excites a resonant circuit.

Electrohydrodynamic traveling-wave pumping of homogeneous semi-insulating liquids

Abstract: Charge separation near a liquid/solid interface is explored by the electrohydrodynamic pumping of Freon TF, transformer oil, and silicone oil using high-voltage traveling waves. Liquid head displacement measurements show strong dependences on peak voltage amplitude (1.0-2.5 kV), frequency (0.1-20 Hz), liquid viscosity (0.5-20 cS), and liquid conductivity (10/sup -13/-10/sup -9/ Omega /sup -1/ m/sup -1/). Reverse pumping, opposite to the traveling wave direction, occurs when the ratio of the voltage to the frequency is small but switches to forward pumping when the ratio is large. The largest measured head was approximately 7 cm, which can drive an average flow velocity of 18 cm/s. A charge transport analysis coupled to viscosity-dominated flow is used to describe the pumping process. The time-averaged coulombic force on the migrating charge can result in pumping in either direction. In the regime where ion migration dominates charge relaxation, a universal curve for the pumping dependence on traveling-wave voltage and frequency is deduced experimentally and predicted by the model. Measurements of average volume charge density show that undoped liquids have a positive net charge density, whereas additives used to increase the conductivity lead to bipolar injection and an augmentation of the injection as well as the conductivity. >

Moisture sensing in transformer oil using thin-film microdielectrometry

Abstract: In-situ moisture sensing in transformer oil by measuring the change in dielectric properties of 1- to 7- mu m-thick layers of plasma-deposited bromobenzene coated on interdigitated 50- mu m-wavelength electrodes of a microdielectrometer chip is demonstrated. Gain-phase measurements in the frequency range of 0.0005 to 10000 Hz, normalized to the frequency of peak phase shift, are found to be universal. Evidence is presented that the coatings act as semipermeable membranes serving as barriers to the transformer oil, but absorbing moisture in proportion to its absolute content in the oil. Sensitivity to moisture from less than 5 to about 50 p.p.m. is found with a transient response of less than a few minutes. Although the method is well suited to following the dynamics of water in paper-oil systems over periods of days or perhaps weeks, a long-term loss of sensitivity must be overcome if the sensor is to be used without recalibration for extended periods of time. Its use for discerning abnormal behavior in a pilot transformer monitoring system is described. >

Electrical aging tests on epoxy insulators in GIS

Abstract: Due to the relatively small insulator dimensions in gas-insulated switchgear (GIS), the electrical stressing is high by comparison to that of insulators for conventional switchgear. Aging tests on a large number of full-sized insulators in several vessels in an outdoor test station have been carried out to determine the stress limits. The tests, with voltages of several hundred kilovolts, last for several years. The aim is to work out conditions (by extrapolation) for a trouble-free switchgear operating life of at least 50 years. The most important test objects are postinsulators weighing 2 kg each. It has been confirmed that partial discharges have a major effect on the failure probability. The life curve obtained from the test results of six batches of partial-discharge-free insulators is compared with the required reliability data. >

Prebreakdown current pulses in n-hexane and other dielectric fluids

Abstract: A point-plane electrode geometry was used to apply negative DC stresses to n-hexane, trimethylpentane, and transformer oil. The resulting prebreakdown currents and light were observed with a state-of-the-art, low-noise amplifier and a high-speed digitizer. Currents appear as bursts of fast (on the order of 100 ns) negative pulses with progressively increasing amplitudes, and a wide bandwidth (at least 10 MHz) is required to resolve individual pulses. Light pulses occur coincidentally with current pulses, but are narrower. The effect of adding particles to the sample or increasing the applied high voltage is primarily to increase the burst rate, with little or no effect on other pulse characteristics such as shape, amplitude, or rate of increase. These observations suggest a model describing the source of the current pulses as a series of discharges inside an expanding low-density region. >

Implications from the influence of electrohydrodynamic motion on breakdown with respect to the discharge mechanism in dielectric liquids

Abstract: Electrical discharge measurements with constant voltage pulses in a thin dielectric liquid layer between plane-parallel electrodes provide information about the dielectric strength as a statistical distribution of the time lags to breakdown. Under a large variety of circumstances, the time-lag distributions are composed of two main parts, termed the fast and the normal components. The nature of these components has been identified. The initial fast component lasts as long as the electrical charges, present or created in the liquid, have not yet been able to induce electrohydrodynamic motion. When the point of instability is reached, the probability of breakdown drops by a large amount as evidenced by the normal component of the time-lag distribution. The ions in the liquid, which are responsible for the electrohydrodynamic instability, also condition the electric-field distribution. The peak value of the local field strength is likely to determine the breakdown characteristics. >

Dielectric phenomena in semicrystalline polymers

Abstract: For the application of polymers in insulation systems it is essential to understand the dielectric phenomena leading to distinct electrical properties such as conductivity, permittivity, or electrical strength. It is generally accepted that space-charge formation plays an important role in these complex interactions, causing distortion of the electric field determined by the electrode configuration. An assessment of the electrical behavior of partially crystalline polymers requires the knowledge of the DC and AC properties and the interrelations with space-charge processes as well as the dependencies of the electric properties from structural conditions and their relaxations. Sophisticated experimental techniques such as polarization and depolarization current measurements, on-line measurement of charge carrier motions in AC fields, and highly sensitive and time-resolved partial discharge inception measurements provide necessary data for the understanding of dielectric phenomena and permit first approaches in analytical and numerical modeling. >

Contact effects on silicon surface flashover studies

Abstract: We studied the breakdown of n/sup +/nn/sup +/ silicon devices in vacuum and various dielectric fluids. The breakdown voltage exhibited a dependence on the abruptness of the high-low junction and on the dielectric constant of the surrounding dielectric liquid.

Immersion-type electrical apparatus with increased fire protection

Abstract: The immersion-type electrical apparatus described offers increased safety in the event of an external fire. The apparatus comprises means associated with the tank in order to restrict the transmission of external heat energy to the liquid dielectric, said means being in the form of a reflective coating on the tank, such as a reflecting metal paint or zinc deposit, and if applicable, an added reflective wall (17), surrounding the transformer tank. Application: all electrical equipment of the immersion-type in which the tank contains a dielectric liquid, and in particular high-voltage/low-voltage public network transformers.

Integrated optical coupling device using movable liquid

Abstract: Light energy is selectively coupled between two adjacent wave guide channels 2,3 co-extensive in a surface layer of a substrate by causing the overlying medium 8, to be either a dielectric liquid 9 or air This is achieved by moving the dielectric liquid selectively over and away from the wave guide channels 2,3, preferably using the electro-capillary effect between two electrodes 5,6.

High temperature studies of perfluoroalkoxy (PFA) film (for HV capacitors)

Abstract: Teflon perfluoroalkoxy (PFA) film was characterized electrically to explore the possibility of its use for high-temperature high-voltage capacitors for space applications. The properties measured at temperatures up to 250 degrees C included the AC breakdown voltage and the dielectric constant and dissipation factor at frequencies up to 100 kHz. To understand the breakdown mechanism taking place at these higher temperatures, the prebreakdown conduction currents were also monitored. The results show a drastic reduction in the dielectric strength, as well as an increase in dissipation factor and conduction current with increasing temperature. The results are explained in light of the breakdown models available. >

Device for protection of a submerged electrical apparatus by detecting leakages of dielectric liquid

Abstract: The protection device is installed on an electrical apparatus 1 containing at least one winding 2 immersed in a dielectric liquid 3 contained in a vessel 4, this apparatus 1 including current-limiting fuses 6 wired between the supply 7 and the winding 2 to be protected. This device comprises means 14, 15, 16 for causing an abrupt short-circuit between the earth 9 of the apparatus 1 and a point 11 situated downstream of the fuse 6 but upstream of the winding 2, these means being triggered when the level 5 of the dielectric 3 in the vessel 4 drops to a predetermined value 12. Use for the protection of an electrical apparatus such as a high-voltage submerged transformer by detecting leakages of dielectric liquid from the vessel.

Method and reaction for sustaining high energy reactions

Abstract: A liquid dielectric having a surface that is contacted by an end of an electrode so as to form a contact area between the electrode and dielectric liquid wherein said area is limited thereby providing a contact resistance that is large compared to the rest of the electrode and dielectric liquid. Current through the contact area causes local heating of the electrode at the contact and raises its temperature toward the melting point of the electrode. Heating causes an increase in resistance of the electrode region so that the area of contact can be increased further immersion of the electrode. The contact area becomes a high-energy region where high energy reactions can be sustained such as fusion and the erosion of the electrode to generate powder having a very fine particle size. When the intention is to generate powders, compounds may be dissolved in the liquid containing elements that combine with the eroding surface of the electrode to provide particles having a stoichimetric ration that depends on the concentration of the dissolved compound. In this manner, finely divided particles of Ba Ferrite are generated using an iron electrode and Barium Hydroxide dissolved in water.

Dielectric test with oscillating discharge of synthetic insulation cables

Abstract: The dielectric testing of insulating materials, particularly cables, is commonly performed by applying voltage on them, either AC or DC voltage. Here the authors describe an alternative to the DC test, namely the oscillating discharge test. They present the results of such a test on long MV cables with a spark gap type fault. The performance of this method is then assessed.

Effect of dissolved water on electrical conduction in liquid dielectrics

Abstract: The state of water in liquid dielectric was analyzed by Fourier transform infrared spectroscopy during a conduction test. The liquid dielectric used was di-octyl-phthalate (DOP) with different proportions of water. The free water in DOP is shown to contribute to the conduction. The states of the free water or weakly bound water are shown to change under stress. >

Electrodynamic Stability of a Self-Gravitating Cylinder

Abstract: The electrodynamic (in-) stability of a self-gravitating dielectric fluid cylinder ambient with dielectric medium is discussed. A general dispersion relation valid to all (axisymmetric and non-axisymmetric) modes of disturbance is derived. The model is gravitationally stable to all non-axisymmetric disturbance modes while to axisymmetric mode it is so if the longitudinal wavenumber normalized with respect to the radius of the fluid cylinder is equal to or greater than 1.0668 and vice versa. The electrodynamic force has always stabilizing effects to all modes of disturbance for all wavelengths whatever the electric field intensity strength. As the latter is increasing the gravitational instability is decreasing and above a certain value of the electric field intensity the gravitational instability is completely suppressed and stability sets in. These analytical results are confirmed numerically.

Service experience with epoxy resin for both insulation and construction material and its application in a future design of insulation enclosed switchgear

Abstract: The authors show that epoxy resin is an excellent insulation material which leads, provided it is properly designed, to safe and reliable switchgear They then describe the use of epoxy resin as an insulation and construction medium, in a new design of substation switchgear for 12 and 24 kV A brief description is given of the overall design of the new installation Special attention is then paid to the characteristics of the insulation material, such as dielectric and mechanical requirements, temperature management and appropriate tests Finally, the area of application of the relevant switchgear is specified

Wire electric discharge machining device

Abstract: PURPOSE:To prevent a wire from being attracted toward a work as well as to solve a problem on machining instability speedily by retracting the work at high speed when the machining instability occurs, while spraying a dielectric fluid, under high pressure, in a direction of separating the wire from the work CONSTITUTION:When electro-discharge machining voltage is lowered because continuous discharge and a short occur in the point midway in operation, a numerical control system detects this fact and retracts a work 2 at high speed, while it opens solenoid valves V1-Vn selectively, and sprays a high pressure dielectric fluid out of a cylinder 4 to an opposite direction to a high-speed backward direction of the work 2, namely, a machining forward direction from high-pressure injection nozzles A1-Am, A1'-Am' selected With this spraying, the high-pressure dielectric fluid is led into a gap between the work 2 and a wire 1, and thereby the wire 1 is attracted to the side of the work 2 and separated from the work 2 without entailing any deflection in projecting form Thus, a problem on machining instability is solved speedily, making disconnection hard to occur, and erosion rate is thus improvable

The great dielectric phenomenon

Abstract: With the expansion of demand immediately after the First World War, serious problems arose in the design and insulation of high-voltage cables

Electrostatic capacity type inclination sensor

Abstract: PURPOSE:To secure stability by arranging electrode plates which are halved into right and left parts alternately opposite each other and providing undivided common electrode plates successively. CONSTITUTION:First electrodes and second electrodes made of the metallic plates which are made of brass or stainless steel, etc., and divided into the right and left parts and the undivided common electrodes 3 are arranged in a glass or plastic container 4 while insulated from one another. Dielectric liquid 5 such as silicone oil is charged in the container 4 up to a half as much as its capacity. Then, when the ambient temperature of the electrostatic capacity type inclination sensor varies abruptly, the 1st electrodes 1, 2nd electrodes 2, and common electrodes provided in the container 4 conduct external heat to the whole of the dielectric liquid 5 speedily to uniform the temperature of the whole of the liquid 5 speedily for adaption to the ambient temperature, thereby uniforming the temperature in the container 4. Consequently, its fluctuation state is terminated in a short time and convection phenomenon is eliminated to obtain a stable state.

Electrically insulating fluids

Abstract: An electrical capacitor which contains a dielectric fluid, said fluid consisting essentially of ditolylmethane.

An exact solution for electrically-driven convection between vertical electrodes

Abstract: A first-order model of electrically driven flows is described and applied to flows driven by Joule heating of a dielectric fluid. An exact solution for the reduced, first-order equations of electrohydrodynamics is developed for the case of a steady, fully developed flow between vertical, parallel, planar electrodes. The solution is shown to depend on four dimensionless groups: the Grashof and Reynolds numbers and two new quantities that represent the magnitude of charge-induced electric field perturbations over the magnitude of the field at an electrode surface and the ratio of the Joule heating rate to the thermal conduction rate, respectively. It is demonstrated that for small field perturbations the solution yields a parabolic velocity profile. The thermal and velocity profiles are only slightly asymmetrical for cases in which the electric field is strongly nonuniform due to space-charge perturbation of the field. >

Composition, its use as an electric insulator and process for manufacturing same

Abstract: of EP0365380The invention relates to a composition comprising at least one electrically insulating oil (A) and at least one product (B) containing at least six carbon atoms and carrying an aldehyde functional group. The Applicant has thus greatly increased the dielectric properties of benzyltoluene and its oligomers (A) by incorporating 3-ethoxy-4-hydroxybenzaldehyde therein. Application of this composition as a dielectric liquid in electrical transformers or capacitors and as a solvent for carbonless copying paper.