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Extensive high resolution electro-optic Kerr field mapping measurements have been taken with transformer oil near 20°c using parallel plane electrodes and field strengths near 0.2 MV/m. Static field measurements have been extended to the transient period following the application of a step voltage. Significant space charge effects resulting in electrode fields being enhanced by a factor of up to 2...
Since earlier work (1–4) had shown that polarity effects were important, the present study was designed to measure pressure effects under non-uniform field conditions using each polarity. Part of the study was conducted at high magnification to observe the streamer inception voltage under elevated pressures.
We propose a novel method to measure the dielectric constant of insulating liquids using a laser beam. We use a research ellipsometer composed of a laser source, a polarizer, a compensator, a sample holder, an analyzer, and a photo-detector. The laser beam passes through the oil sample and accordingly undergoes polarization changes of its electric vector components; parallel and perpendicular to the...
Experiments show that electric field promotes to form a vapor locking in a gap with a hot spot and in turn the formation of locked vapor produces the serious reduction of breakdown voltage. Especially it is argued that the dielectric strengh of liquid helium under serious hot spot conditions decreases below that for saturated gas.
1- The conduction current in oil under AC and positive DC voltages increases in general with increasing field strength or oil temperature. 2- When the oil is subject to positive DC voltage the leakage current is too much smaller than that with AC voltage. 3- The breakdown voltage of oil decreases as the oil quality decreases. 4- The breakdown voltage is lower for positive polarity then for negative...
The progressive improvement in the purification techniques of dielectric liquids renders the measurement of their resistivity more and more difficult, sometimes impossible (especially at room temperature) with conventional AC bridges (SCHERING bridge, transformer ratio bridge) they are not sensitive enough to detect losses (tanδ) or conductance G of a cell of capacitance C filled with a liquid of...
The comprehensive and detailed analysis of the data for real dielectrics [1] in particular in some sandstones [2] gives the exponent time dependence for the depolarization current that has been referred to in literature [1] as the “Universal Response” phenomenon (URP). We consider that one of the main reasons which determines the URP is the structure heterogeinity caused by random fractal geometry...
The studies reported have been carried out on the dielectric behaviour of dilute solutions of R1(CH2)nR2, n ≧12 where R1 and R2 are highly dipolar groups. A recently developed chain doubling synthesis technique allows very low levels of homologous impurity to be achieved, hence providing essentially monodispersed material at large n. Association effects are described together with molecular configurational...
Shear viscosity η is a basic parameter of the liquid state strongly correlated to the dielectric relaxation time of polar compounds. Thermal variations η(T) are often explained by the “old” free-volume model which implies a critical temperature T0 at which the free volume is vanishing and the viscosity infinite. This point of view was recently criticized (1, 2). The case of supercooled water is still...
In a negative-needle to plane configuration in transformer oil and liquid nitrogen (LN2), current-voltage (I–V) characteristics and liquid velocities were measured. Well-known linear plots of V vs I1/2 were obtained in the space charge limitation (SCL) for I>10−9 A. The liquid motion in SCL is computed from both the Navier-Stokes equation and the continuity equation by the finite difference method...
A modular pulsed power system with a matched source impedance of five Ohms was set up to investigate the breakdown phenomena in pressurized water. The hydrostatic pressure applied to the test cell could be varied from 0.1 MPa to 10 MPa. Experiments have been carried out under well defined conditions: sample purity, applied voltage waveshape (ns-rise time, 100 ns duration), electrode conditions (stainless...
The occurrence and severity of different types of discharge along solid dielectric surfaces in gases or in insulating liquids has been investigated in the past by means of many different experimental arrangements. A system giving discharges in a very thin disc between two solid dielectric layers has been tested and the results compared with those given by classical systems.
The time lag distributions of electrical discharges in thin layers of a dielectric liquid have two components, a statistical and a formative one. We provide some more precise data on the formative contribution than have been published before. The measurements show that a power-law relationship holds between the formative time and the distance between the parallel electrodes, the electrode gap.
PCB's are persistent ecological poisons. Hence the search for alternative dielectric liquids for use in fire-resistant transformers. One approach has been to use non-flammable vaporizable fluids such as the fluoroocarbon C8F160 which boils at about 101°C at atmospheric pressure.
The aspects of propagation of a discharge on the surface of electrolytic aqueous solutions and of an insulating oil, when an impulse voltage is applied to a point- plane gap arranged at the interface between liquids and air, are investigated by a high-speed Schlieren method. Consequently, a polarity effect in shape and size of density changes, features and mechanism of propagation of density changes...
The behaviour of transformer oil and other fluids used for the cooling and insulation of power system equipment is significantly influenced by enforced motion. Not only can charges generated by streaming electrification1 accumulate to prejudice dielectric integrity, but the dielectric strength of the fluid is also altered per se2 by the actions of the flow in a complex, but predictable, manner. Both...
The energetic balance shows that vaporization is dominant for slow streamers but ionization could be a possible mechanism. For fast streamers, dissociation and ionization are much likely, vaporization being also present.
In a negative razor blade to plane configuration in transformer oil, current-voltage (I–V) characteristics, liquid velocities and streamlines were measured. Linear plots of V vs Il/3 were obtained in the space charge limitation (SCL). The liquid motion in SCL is computed from both the Navier-Stokes equation and the continuity equation for two-dimensions, by the finite difference method. The computational...
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