Showing papers on "Liquid dielectric published in 2014"

Recent progress in nanofluids based on transformer oil: preparation and electrical insulation properties

Abstract: Given the adoption of ultrahigh-voltage ac and dc power transmission, it is desirable to develop a new kind of oil-paper insulation system, with higher dielectric strength and smaller volume compared with conventional insulation systems, in order to reduce the volume and mass of high-voltage power transformers and improve their long-term operational reliability [1], [2]. Methods of improving the dielectric properties of the oil-paper insulation have therefore been widely investigated [3], [4]. The most influential factors affecting the performance of oil-paper insulation systems are the low dielectric strength of transformer oil and its degradation caused by water and other contaminants [5]. The dielectric strength of oil-paper insulation may be increased by filtering the oil in order to remove water and other contaminants [6]. However, further increase in the dielectric strength of the oil is required in order to achieve reduction of transformer volume and mass.

Wireless Sensing of Complex Dielectric Permittivity of Liquids Based on the RFID

Abstract: A wireless sensing system for the evaluation of the complex dielectric permittivity of solvent liquids is presented. Two sensing tags are proposed for testing of the samples. The first tag is based on a cavity resonator and the second makes use of the epsilon-near-zero effect. Both circuits are designed over the planar substrate-integrated-waveguide (SIW) technology, and operate at 4 GHz. A quartz capillary tube is used for the liquid measurements where only a small amount of sample volume is required. With the addition of planar antennas at the input and output of the sensors, the complete system is implemented for wireless sensing of the materials following the RF identification scheme. The cavity perturbation technique for SIW structures is applied for the dielectric liquid characterization. The proposed sensing tags and system have high potentials for low-cost wireless measurements and real-time monitoring applications.

Comparative aging characteristics between a high oleic natural ester dielectric liquid and mineral oil

Abstract: This paper presents a comparative evaluation of the aging characteristics between a high oleic natural ester dielectric liquid (HONE) and mineral oil based on an accelerated single-temperature thermal aging experiment at 150°C for a period of 3000 hours. The important physical (e.g., viscosity, moisture content, acidity) and dielectric (e.g., power factor, alternating current (AC) dielectric breakdown) properties of both the oils were measured and analyzed at different aging intervals. As expected, results show differences in aging behaviors between mineral oil and the natural ester liquid. The relative moisture content in HONE had stable and lower values during the aging duration as compared to the case of mineral oil, where the relative moisture content increased with the aging duration. Further, unlike the observations in mineral oil, the power factor (PF) and acidity of HONE increased drastically with increasing aging times. The most relevant observation made from this study was that in spite of this significant increase in the PF and acidity of HONE with aging, its AC dielectric breakdown voltage was quite stable, which is not observed in the case of mineral oil. This indicates that for natural ester liquids, higher values of PF (or dielectric dissipation factor) and/or acidity due to aging may not necessarily give a true picture on the dielectric performance of the ester. It is therefore important to note here that the threshold limits of some of the diagnostic parameters which are already established for mineral oil do not apply for ester liquids.

Experimental Investigation on Surface Characteristics in Powder-Mixed Electrodischarge Machining of AA6061/10%SiC Composite

Abstract: In this study, machining of AA6061/10%SiC composite fabricated by mechanical stir casting process has been experimentally investigated using electrodischarge machining (EDM) process with tungsten-powder-mixed dielectric fluid (PMEDM). Peak current, pulse on-time, pulse off-time, and gap voltage are selected as machining parameters. Mathematical relation has been established for response quality characteristics like surface roughness with and without tungsten powder dielectric fluid using Central Composite Rotatable Design (CCRD) through response surface methodology (RSM). Surface characteristics like microhardness, surface topography, and white recast layer have been evaluated through scanning electron microscope (SEM), electrodispersive X-ray spectroscopy (EDS), and micro-Vicker hardness. Results have also been compared with simple EDM of AA6061/10%SiC composite without powder-mixed dielectric fluid. The existence of tungsten powder in dielectric fluid increases surface quality owing to decrease in numbe...

Electrical breakdown in dielectric liquids-a short overview

Abstract: Power transformers are utilized to convert high voltages normally used in electrical power transmission to lower voltages more suitable for consumers. A transformer consists essentially of a magnetic iron core with primary and secondary copper windings. The alternating current flowing in the primary winding induces a magnetic flux in the core, which in turn creates a current in the secondary windings. If there is a difference in the number of turns in the primary and secondary windings, the secondary voltage will be scaled up or down proportionally to the ratio of the turns. In this way, a high voltage can be transformed to a low voltage. However, the desire to convert increasingly greater electrical loads using smaller power transformers results in both higher electrical and thermal stresses. The materials utilized to insulate the different electrically conductive components from each other must be designed to withstand those stresses. The insulating media often consist of pressboard and an insulating liquid. The liquid performs a double duty as it not only insulates the conductive parts but also functions as a liquid coolant. Here we are primarily interested in the oil as an insulating medium.

Dielectric properties of natural ester, synthetic ester midel 7131 and mineral oil diala D

Abstract: The insulating liquids used in industrial applications are typically mineral oils. In recent years however, significant attention has been paid to alternative insulating fluids, including synthetic and natural ester liquids. In order to expand their practical applications, it is important to have detailed information on their dielectric properties. In this present paper, the dielectric properties of synthetic ester, Midel 7131; mineral oil, Shell Diala D; and vegetable (rapeseed) oil have been investigated. It has been shown that Midel 7131 has a higher ac breakdown voltage (27.6 kV) as compared with Diala D oil (26.4 kV) and rapeseed oil (24.6 kV). However, the breakdown voltage of the Diala D oil has the smallest standard deviation (7%) amongst the tested liquids (13% for Midel 7131 and 11% for rapeseed oil). Statistical analysis of the breakdown voltages has been conducted and it has been shown that the ac breakdown voltages can be described by a normal distribution. dc I-V characteristics have been measured and the space charge saturation regime has been observed for all three liquids starting from ~9 kV for positive energisation and ~10 kV for negative energisation in the point-plane topology. Apparent mobilities of the charge carriers in the tested liquids have been obtained using I1/2-V curves; these mobilities can be used for calculation of the space charge influenced distribution of the electric field in liquid insulators stressed with dc voltage. Such analysis can be important for design and exploitation of HVDC power systems.

Analysis of partial discharge characteristics of natural esters as dielectric fluid for electric power apparatus applications

Abstract: Petroleum based mineral oils are generally used for electric power apparatus insulation and cooling applications. Since the conventionally used mineral oils are not biodegradable and difficult to decompose, there is a need for the development of alternate vegetable based insulation oils for such applications. Formation of partial discharges plays a major role in determining the life time of liquid insulation. Reports on partial discharge characteristics of thermal aged natural ester fluids for high voltage applications are scanty. The major aim of this research work is to understand the partial discharge characteristics of natural esters as a dielectric fluid for power transformer and other electric power apparatus insulation applications. Laboratory experiments are performed as per IEC (International Electro technical Commission) test procedures in order to understand the breakdown voltage and partial discharge characteristics of vegetable oils such as corn oil and palm oil under both virgin and thermal aged conditions. This paper presents a comparative assessment of the PD characteristics of thermal aged natural ester fluids with respect to mineral oil. Time and frequency domain analysis of PD pulse at both needle-plane and rod-plane electrode configurations are studied. Statistical analysis of PD pattern is also carried out. This analysis collects the typical PD patterns from natural ester fluids in order to find its suitability for power transformer and electric power apparatus applications.

The Electrical Breakdown of Thin Dielectric Elastomers: Thermal Effects

Abstract: Dielectric elastomers are being developed for use in actuators, sensors and generators to be used in various applications, such as artificial eye lids, pressure sensors and human motion energy generators. In order to obtain maximum efficiency, the devices are operated at high electrical fields. This increases the likelihood for electrical breakdown significantly. Hence, for many applications the performance of the dielectric elastomers is limited by this risk of failure, which is triggered by several factors. Amongst others thermal effects may strongly influence the electrical breakdown strength. In this study, we model the electrothermal breakdown in thin PDMS based dielectric elastomers in order to evaluate the thermal mechanisms behind the electrical failures. The objective is to predict the operation range of PDMS based dielectric elastomers with respect to the temperature at given electric field. We performed numerical analysis with a quasi-steady state approximation to predict thermal runaway of dielectric elastomer films. We also studied experimentally the effect of temperature on dielectric properties of different PDMS dielectric elastomers. Different films with different percentages of silica and permittivity enhancing filler were selected for the measurements. From the modeling based on the fitting of experimental data, it is found that the electrothermal breakdown of the materials is strongly influenced by the increase in both dielectric permittivity and conductivity.

Electrohydrodynamic linear stability analysis of dielectric liquids subjected to unipolar injection in a rectangular enclosure with rigid sidewalls

Abstract: We investigate the linear stability threshold of a dielectric liquid subjected to unipolar injection in a two-dimensional rectangular enclosure with rigid boundaries. A finite element formulation transforms the set of linear partial differential equations that governs the system into a set of algebraic equations. The resulting system poses an eigenvalue problem. We calculate the linear stability threshold, as well as the velocity field and charge density distribution, as a function of the aspect ratio of the domain. The stability parameter as a function of the aspect ratio describes paths of symmetry-breaking bifurcation. The symmetry properties of the different linear modes determine whether these paths cross each other or not. The resulting structure has important consequences in the nonlinear behaviour of the system after the bifurcation points.

Micro-electro discharge machining of non-conductive zirconia ceramic: investigation of MRR and recast layer hardness

Abstract: Increasing material removal rate (MRR) and minimizing recast layer hardness are critical issues in machining non-conductive ceramic using micro-electro discharge machining (micro-EDM). This paper presents the analysis of MRR and recast layer hardness of zirconium oxide (ZrO2) due to micro-EDM using EDM-3 dielectric fluid and tungsten tool electrodes. The two main parts of this research are process development and the analysis of MRR and recast layer hardness. In process development, the appropriate use of assisting electrode (AE), polarity, flushing, feed rate, gap voltage, and tool electrode rotational speed are identified. The better machinability of ZrO2 was found to be with copper adhesive as AE, positive workpiece polarity, 3-μm/s feed rate, and workpiece submerged in dielectric fluid with one-way circulation. Empirical models are developed for the estimation of MRR and recast layer hardness. The optimum parameters for maximum MRR and minimum recast layer hardness are found to be at a 370-rpm rotational speed and at 80-V gap voltage.

Studies of nanosecond pulse surface ionization wave discharges over solid and liquid dielectric surfaces

Abstract: Surface ionization wave discharges generated by high-voltage nanosecond pulses, propagating over a planar quartz surface and over liquid surfaces (distilled water and 1-butanol) have been studied in a rectangular cross section test cell. The discharge was initiated using a custom-made, alternating polarity, high-voltage nanosecond pulse plasma generator, operated at a pulse repetition rate of 100?500?Hz, with a pulse peak voltage and current of 10?15?kV and 7?20?A, respectively, a pulse FWHM of ?100?ns, and a coupled pulse energy of 2?9?mJ/pulse. Wave speed was measured using a capacitive probe. ICCD camera images demonstrated that the ionization wave propagated predominantly over the quartz wall or over the liquid surface adjacent to the grounded waveguide placed along the bottom wall of the test cell. Under all experimental conditions tested, the surface plasma ?sheet? was diffuse and fairly uniform, both for positive and negative polarities. The parameters of ionization wave discharge propagating over distilled water and 1-butanol surfaces were close to those of the discharge over a quartz wall. No perturbation of the liquid surface by the discharge was detected. In most cases, the positive polarity surface ionization wave propagated at a higher speed and over a longer distance compared to the negative polarity wave. For all three sets of experiments (surface ionization wave discharge over quartz, water and 1-butanol), wave speed and travel distance decreased with pressure. Diffuse, highly reproducible surface ionization wave discharge was also observed over the liquid butanol?saturated butanol vapor interface, as well as over the distilled water?saturated water vapor interface, without buffer gas flow. No significant difference was detected between surface ionization discharges sustained using single-polarity (positive or negative), or alternating polarity high-voltage pulses. Plasma emission images yielded preliminary evidence of charge removal from the liquid surface between the pulses on a microsecond time scale. Products of the plasma chemical reaction that accumulated in the ionization wave discharge over the liquid butanol?saturated butanol vapor interface were detected ex situ, using FTIR absorption spectroscopy. Reaction products identified include CO, alkanes (CH4,C2H6, C3H8), alkynes (C2H2), aldehydes (CH2O) and lighter alcohols (CH3OH).

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.

An adaptive liquid lens with radial interdigitated electrode

Abstract: A newly patterned electrode is designed for a dielectric liquid lens. The electrode has a radial interdigitated structure which can provide a symmetrical-nonuniform fringing field. This electric field can effectively deform the shape of a dielectric liquid droplet in radial direction, causing the focal length of the liquid droplet to change. For a lens using glycerol as the droplet and optical oil (SL-5267) as the surrounding medium, its focal length (f) can be tuned in the range of ???8.28????f???????4.4 mm, when the applied voltage is changed from 0 to 120 Vrms. In contrast to previous approaches, the liquid lens with radial interdigitated electrode has the advantages of scalable aperture size, wide variability of focal length, and good optical characteristics. Moreover, the driving voltage is insensitive to the size of the droplet.

A study on the dielectric properties of Palm Oil and Coconut Oil

Abstract: One of the important parameter for insulating fluid is the dielectric properties. With the introduction of vegetable oil as possible alternative fluid for transformers application, it is essential to ensure this type of oil could meet the dielectric properties requirement. In this paper, a study is carried out to investigate the dielectric properties of Palm Oil (PO) and Coconut Oil (CO). The type of PO used in this study is Refined, Bleached and Deodorized Palm Oil (RBDPO) Olein. Several parameters such as AC breakdown voltage, dielectric dissipation factor, relative permittivity and resistivity are examined for both as-received and dried samples. It was found that there are differences on the dielectric dissipation factor and resistivity between RBDPO and CO and the dielectric properties performances of all samples are improved after subjected to the drying procedure.

Effect of Dielectric Liquid on Characteristics of WC-Co Powder Synthesized Using EDM Process

Abstract: This research used electrical discharge machining (EDM) to synthesize nanocrystalline powder from scrap cemented tungsten carbide (WC-Co). The effect of dielectric liquid (deionized water, ethanol and kerosene) and process parameters on the dimensional and structural characteristics of debris was studied. The results show that, with respect to the amount of powder synthesized in dielectric medium, deionized water performs better than ethanol and kerosene. The results of X-ray diffraction show that the existing phases in the synthesized powder are affected by the type of dielectric liquid, and different chemical compositions of the dielectric liquids lead to diverse chemical content in the produced debris. The average size of the particles produced in deionized water is the smallest, whereas substructure of the particles generated in kerosene has the smallest mean value. Debris synthesized by this method is nanostructured and can be readily converted to nanoscale WC-Co powder via heat treatment. Overall, t...

Three-dimensional LBE simulations of a decay of liquid dielectrics with a solute gas into the system of gas-vapor channels under the action of strong electric fields

Abstract: The three-dimensional simulations of an anisotropic decay of binary mixtures of a dielectric liquid with solute gas in a strong electric field are carried out. The Lattice Boltzmann Equation method (LBE) is exploited for computer simulations of the evolution of such systems with the newly arising interfaces between vapor and liquid phases. The parallel implementation of the LBE algorithm is realized on a large number of cores in the GPU. For the GPU programming, the CUDA technology is used. It is important that new regions of the low-density phase appear as thin quasi-cylindrical gas-vapor channels oriented along the electric field. The gas-vapor channels expand because of the diffusion of the solute gas from the mixture, evaporation of liquid into the channels and also due to the coalescence of channels with each other. The critical values of electric field necessary for such decay of a binary mixture are considerably lower than the critical electric field for pure dielectric liquids. Hence, if we take into account a solute gas, the electric fields for which the anisotropic mechanism of streamer channels generation and growth is operated, become considerably lower. Thus, at a breakdown of dielectric liquids in a strong electric field, the anisotropic instability is possibly the key mechanism of the generation of a gas phase, inception of conducting streamer structures, their fast growth in the form of thin filamentary channels, as well as branching of streamer structures during propagation.

Electric field induced sheeting and breakup of dielectric liquid jets

Abstract: We report experimental observations of the controlled deformation of a dielectric liquid jet subjected to a local high-voltage electrostatic field in the direction normal to the jet. The jet deforms to the shape of an elliptic cylinder upon application of a normal electrostatic field. As the applied electric field strength is increased, the elliptic cylindrical jet deforms permanently into a flat sheet, and eventually breaks-up into droplets. We interpret this observation—the stretch of the jet is in the normal direction to the applied electric field—qualitatively using the Taylor-Melcher leaky dielectric theory, and develop a simple scaling model that predicts the critical electric field strength for the jet-to-sheet transition. Our model shows a good agreement with experimental results, and has a form that is consistent with the classical drop deformation criterion in the Taylor-Melcher theory. Finally, we statistically analyze the resultant droplets from sheet breakup, and find that increasing the applied electric field strength improves droplet uniformity and reduces droplet size.

Pin–Plate Electrode System for Emulsification of a Higher Conductivity Leaky Dielectric Liquid into a Low Conductivity Medium

Abstract: In this experimental study we propose the use of highly nonuniform electric field to emulsify a leaky dielectric oil into another oil. Specifically, a pin–plate electrode system is used to emulsify castor oil into silicone oil. The method is suitable for the emulsification of a higher conductivity leaky dielectric oil dispersed in a lower conductivity medium and is suitable for an already existing emulsion, unlike electrospray methods. The process is stabilized by charging of the dispersed phase and the associated electrohydrodynamic flow. A balance of electrocoalescence and breakup leads to a stationary drop size distribution. A short process time indicates its suitability for continuous operation.

Temperature effects on dielectric liquid lenses

Abstract: The thermal stability of dielectric liquid lenses is studied by measuring the focal length at different temperatures. Two types of liquids lenses are investigated: Type-I (SL-5267/glycerol) and Type-II (glycerol/ BK7 matching liquid). A threshold-like behavior is found. Below the threshold temperature, the focal length is temperature insensitive. Above the threshold, the focal length changes exponentially with the temperature. Both refractive index and surface profile are responsible for the focal length change, although the former decreases linearly with the temperature. The threshold temperature of Type-I and Type-II liquid lens are 60°C and 40°C, respectively. Type-I lens shows a good temperature stability in a wide range. Moreover, the lens can recover to its original state even though it is operated at a high temperature.

Nonlinear dielectric effect in supercritical diethyl ether

Abstract: Nonlinear dielectric effect (NDE) describes changes of dielectric permittivity induced by a strong electric field in a liquid dielectric. The most classical finding related to this magnitude is the negative sign of NDE in liquid diethyl ether (DEE), recalled by Peter Debye in his Nobel Prize lecture. This article shows that the positive sign of NDE in DEE is also possible, in the supercritical domain. Moreover, NDE on approaching the gas-liquid critical point exhibits a unique critical effect described by the critical exponent ψ ≈ 0.4 close to critical temperature (T C) and ψ ≈ 0.6 remote from T C . This can be linked to the emergence of the mean-field behavior in the immediate vicinity of T C , contrary to the typical pattern observed for critical phenomena. The multi-frequency mode of NDE measurements made it possible to estimate the evolution of lifetime of critical fluctuations. The new way of data analysis made it possible to describe the critical effect without a knowledge of the non-critical background contribution in prior.

Electrohydrodynamic flow of dielectric liquid around a wire electrode - effect of truncation of onsager function

Abstract: Electrohydrodynamic flow of a dielectric liquid around a wire electrode sandwiched between two parallel flat-plate electrodes is studied both numerically and experimentally. The Poisson-Nernst-Planck equations for the ion transport and the Navier-Stokes equations for the fluid flow are solved numerically by using a commercial code. Charge generation in a dielectric liquid (dodecane + 0.5% wt Span 80) due to Onsager effect is considered in the ion transport equations. Numerical results well predict the experimental ones both qualitatively and quantitatively. At high electric field the numerical approach slightly over-predicts the flow velocity measured from the experiment, but the level of discrepancy can be further lowered by using a truncated series for the Onsager function.

The effect of deionisation time on the electrical discharge machining performance

Abstract: An experimental and numerical study to evaluate the time needed for dielectric deionisation between consecutive discharges as well as its effect on performance of electrical discharge machining was done. In the numerical study, the open voltage (gap width definition), current intensity, discharge duration and discharge interval were used among the large number of input parameters used experimentally in the process of electrical discharge machining. The performance parameters values, as well as the time needed for a complete deionisation of the dielectric liquid using a finite element model were calculated. In the numerical study, the time needed for the complete deionisation of the dielectric liquid was calculated using the condition that the temperature reached in the boundaries of the plasma channel with the electrodes has a value between 6,000 and 4,500 °C. In the experimental study, the researchers acquired the time needed for the complete deionisation of the dielectric liquid using small discharge duration and a discharge interval variable between the minimum acceptable by the control of the machine and the one that causes a significant decrease in the material removal rate. The numerical results are in agreement with the experimental data.

Comparison Between the DC and AC Breakdown Characteristics of Dielectric Sheets in Liquid Nitrogen

Abstract: With the emergence of superconductor technology, high-temperature superconducting (HTS) power devices are quickly being developed. HTS power devices are receiving considerable attention due to their high capacity and low loss. Compared with ac HTS power devices, dc superconducting power devices have the advantage of zero resistance. To aid in the development of superconducting power devices, the ac and dc dielectric breakdown characteristics of dielectric sheets in liquid nitrogen ( LN2) should be better understood. In this paper, results are reported for ac and dc breakdown tests conducted in air (room temperature) on six types of sheets, including polypropylene laminated paper (PPLP), Nomex (polyamide) T410 fiber paper, Kapton (polyimide) 100HN, Kapton 100CR, Kapton 150FN019, and Kapton 150FCR019, and in LN2 on seven types of sheets, including PPLP, Kapton 100HN, Kapton 100CR, Kapton 150FN019, Kapton 150FCR019, Nomex T410, and Nomex T418 fiber paper. The insulation dielectric properties in air (room temperature) were compared with the insulation dielectric properties at a low temperature. At room temperature, the dc breakdown strength at 63.2% probability is 1.12-1.72 times the ac breakdown strength at 63.2% probability, and at a low temperature, the 63.2% probability of the dc breakdown strength is 1.52-2.14 times the 63.2% probability of the ac breakdown strength. Compared with the dielectric sheets of Kapton 150FCR019, Kapton 100CR, Kapton 150FN019, Kapton 100HN, and PPLP, Nomex T410 sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared with air.

Cavitation nanopore in the dielectric fluid in the inhomogeneous, pulsed electric fields

Abstract: This paper discusses the nanopores emerging and developing in a liquid dielectric under the action of the ponderomotive electrostrictive forces in a nonuniform electric field. It is shown that the gradient of the electric field in the vicinity of the rupture (cavitation nanopore) substantially increases and determines whether the rupture grows or collapses. The cavitation rupture in the liquid (nanopore) tends to stretch along the lines of the original field. The mechanism of the breakdown associated with the generation of secondary ruptures in the vicinity of the poles of the nanopore is proposed. The estimations of the extension time for nanopore in water and oil (polar and nonpolar liquids, respectively) are presented. A new mechanism of nano- and subnanosecond breakdown in the insulating (transformer) oil that can be realized in the vicinity of water microdroplets in modern nanosecond high-voltage devices is considered

Corona discharges under various types of electrodes

Abstract: Partial discharge (PD) is the most unwanted phenomenon in high voltage insulation system. One of the wellknown types of PD in high voltage insulation is corona discharge. In this work, the characteristics of corona discharges from different types of electrodes (sharp, flat and sphere electrodes) were studied to enhance the understanding of corona discharges. The electrical field distribution for all types of the electrodes was also simulated to study the influence of the field on the PD characteristics. The results show that the characteristics of corona discharges and its electric field distribution are heavily influenced by the shape of the electrodes under high voltage stress.

Effect of the Mobility Parameter on the Oscillatory Electroconvection of Dielectric Liquids Subject to Strong Unipolar Charge Injection

Abstract: Two-dimensional numerical calculations were carried out for the isothermal electroconvection in a dielectric liquid induced by the unipolar charge injection. In the dimensionless form, the system is mainly governed by the injection strength number C, the electric Rayleigh number T, and a mobility number M. A strong injection case with C = 10 was considered. Along with the increase in driving parameter T, oscillatory electroconvection occurs. Critical values of T corresponding to the onset of such oscillatory convection were shown to be highly dependent on M and were accurately determined. Some precautions, including the algorithm for the charge density equation, were taken to reduce the numerical diffusion and to ensure the reliability of our findings.