Liquid dielectric

About: Liquid dielectric is a research topic. Over the lifetime, 3702 publications have been published within this topic receiving 45150 citations.

Oil-immersed transformer gas analysis

Abstract: A sampling and testing device for diagnosing a condition of a dielectric fluid in an electrical apparatus, eg, an oilimmersed transformer, by sampling dielectric liquid from the apparatus and extracting dissolved gases from the sampled dielectric fluid and subjecting the extracted gases to analysis for composition so as to infer the condition of the dielectric liquid therefrom The device employs a cylinder disposed as part of the electrical apparatus, said cylinder having an inlet valve to draw dielectric liquid samples by vacuum Gases that may have been generated and dissolved in the sampled dielectric liquid because of local heating or electrical arcs and faults within the electrical apparatus, are liberated by creating partial vacuum in the cylinder, in a closed condition thereof Gases so extracted are conveyed through a controllable gas outlet valve on the cylinder into a gas analyzer for analysis of gas-composition so as to infer the condition of the liquid dielectric therefrom Dielectric liquid sampled from the electrical apparatus is returned thereto in a closed circuit Said cylinder comprises a piston including a flange portion which is expediently provided with a plurality of apertures connecting opposite sidefaces of the flange portion, whereby, during an outward stroke of the piston, dielectric liquid is ejected out of the apertures forcibly thereby facilitating degasification of the dielectric sample within the cylinder

Pre-breakdown cavitation development in the dielectric fluid in the inhomogeneous, pulsed electric fields

Abstract: We consider the development of pre-breakdown cavitation nanopores appearing in the dielectric fluid under the influence of the electrostrictive stresses in the inhomogeneous pulsed electric field. It is shown that three characteristic regions can be distinguished near the needle electrode. In the first region, where the electric field gradient is greatest, the cavitation nanopores, occurring during the voltage nanosecond pulse, may grow to the size at which an electron accelerated by the field inside the pores can acquire enough energy for excitation and ionization of the liquid on the opposite pore wall, i.e., the breakdown conditions are satisfied. In the second region, the negative pressure caused by the electrostriction is large enough for the cavitation initiation (which can be registered by optical methods), but, during the voltage pulse, the pores do not reach the size at which the potential difference across their borders becomes sufficient for ionization or excitation of water molecules. And, in the third, the development of cavitation is impossible, due to an insufficient level of the negative pressure: in this area, the spontaneously occurring micropores do not grow and collapse under the influence of surface tension forces. This paper discusses the expansion dynamics of the cavitation pores and their most probable shape.

The Law of Corona and the Dielectric Strength of Air-IV The Mechanism of Corona Formation and Loss

Abstract: The mechanisms of corona and corona loss have been studied with the cathode-ray oscillograph. High voltage power of the order of 0.1 watt can be measured with an accuracy of 1 per cent with this instrument. The measurements show that the loss follows the quadratic law above the visual critical voltage. On polished wires there is no loss until the visual critical voltage is reached. The loss then starts quite suddenly and takes a finite value on the quadratic curve. On cables and imperfect conductors there is a loss below the visual critical voltage on brushes at local ``rough'' spots. The loss due to these irregularities can be represented by the probability law. This is quite in accord with former work. In practise it is important not to mutilate the conductors in stringing. The really important factor in design is the irregularity factor, m e , for weathered conductors. No line should be operated with a corona loss under fair weather conditions. It is not necessary from the economic stand point since large diameters can be obtained with special types of conductors. The visual critical corona voltage can be calculated with great accuracy. As the applied a-c. voltage is increased above the visual critical value, the instantaneous critical voltage becomes lower and lower until finally corona starts at the zero point of the wave. This occurs when the applied voltage is twice the visual critical voltage. At still higher voltage, corona starts below zero or on the falling wave.

Electrohydrodynamic flow of a dielectric liquid around a blade electrode

Abstract: Injection of charge into a dielectric liquid, and a Coulomb force that sets the liquid into motion, may be obtained by applying a dc voltage to a blade-shaped, metallic electrode immersed in the liquid. An analysis of this motion and its influence on the transport of electric charge is carried out for a simple charge injection law. It is shown that the liquid motion, the electric field, and the charge distribution in a region around the electrode tip of size of the order of the electrode curvature radius determine the injected current as a function of the far electric potential seen by this region. The current increases exponentially with the potential when the contribution of the space charge to the electric field is negligible and algebraically when it is dominant, and presents a range of multiplicity in between. When the inertia of the liquid matters, the region around the electrode tip is also the origin of an electrohydrodynamic plume. An oscillatory current regime is found in which the space charge in the interelectrode space rearranges into many discrete lumps that, under constant voltage bias and small current, induce oscillations of the electric field at the injecting electrode and thus fire new lumps. An order of magnitude analysis and numerical computations for this regime give results in line with known experimental data. In conjunction with the hydrodynamic instability of the plume, this pulse firing mechanism is seen to lead to more complex, nonperiodic oscillations.

Duplex electrical discharge machining

Abstract: An electrical discharge machine (22) includes first and second coaxial electrodes (30,32) and corresponding carriages (34,36). A liquid dielectric (44) is channeled between the electrodes (30,32) and a workpiece (12) which are suitably electrically powered for machining a duplex hole (14) in the workpiece (12).