Showing papers on "Liquid dielectric published in 2012"

Dielectric constant of ionic solutions: a field-theory approach.

Abstract: We study the variation of the dielectric response of a dielectric liquid (e.g. water) when a salt is added to the solution. Employing field-theoretical methods, we expand the Gibbs free energy to first order in a loop expansion and calculate self-consistently the dielectric constant. We predict analytically the dielectric decrement which depends on the ionic strength in a complex way. Furthermore, a qualitative description of the hydration shell is found and is characterized by a single length scale. Our prediction fits rather well a large range of concentrations for different salts using only one fit parameter related to the size of ions and dipoles.

Effect of semiconductive nanoparticles on insulating performances of transformer oil

Abstract: In this paper, TiO2 semiconductive nanoparticles with a large relaxation time constant is added into transformer oil to form semiconductive nanofluids (SNFs), with the aim of enhancing insulating characteristics. ac, dc and lightning impulse breakdown voltage and partial discharge (PD) characteristics of oil samples before and after modification were measured according to ASTM standard methods. It was found that SNFs have ac, dc and lightning impulse breakdown voltage up to 1.2 times compared with pure oil. Meanwhile, the partial discharge resistance of SNFs was also dramatically improved. Charge trap and transportation characteristics of both samples have been measured by thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA). It was found that electron shallow trap density and charge decay rate are greatly increased in semiconductive nanoparticles modified transformer oil. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in dielectric liquids.

Analysis of dielectric response measurements for condition assessment of oil-paper transformer insulation

Abstract: This paper describes the unique characteristics of the oil-paper insulation system utilized in power transformers, their influence on the dielectric response and means for proper modeling of these characteristics. Dielectric response measurement between windings of a power transformer is used to estimate the water content of the solid insulation. By measuring the dielectric response over a wide frequency range different properties of the insulation materials can be determined. These properties can be modeled in a laboratory so that measured response can be compared to modeled response with known properties. This comparison enables the estimation of properties of the materials such as water content and conductivity. The influences of temperature, aging and insulation geometry are described. Case studies are presented which demonstrate the validity of the test method. The influence of frequency range, temperature, aging byproducts, oil conductivity and insulation material type on the accuracy of the moisture estimation is discussed.

Two-dimensional numerical analysis of electroconvection in a dielectric liquid subjected to strong unipolar injection

Abstract: Two-dimensional numerical simulations are carried out to examine the problem of transient electroconvection stability of dielectric liquids subjected to unipolar injection. The entire set of electrohydrodynamics equations associated with the electroconvective phenomena that occur in a layer of a dielectric liquid between two parallel electrodes subjected to a potential difference are solved numerically. We first validate the numerical simulation by comparing our linear stability electroconvection criteria with those obtained by other authors with a stability approach. In this paper, we restrict the study to the strong injection case, which corresponds to values of the non-dimensional injection parameter C greater than or equal to 10. The numerical solution of the electroconvective problem is then presented for rigid lateral boundary conditions. A detailed analysis of the scenario that occurs for different characteristic values of the stability parameter T is provided. The flow structure and its behaviour highlight the existence of different regimes, from laminar to chaotic. The development of charged plumes has been observed in particular. We compute the electrical Nusselt number for different values of the stability parameter and ion mobility. The electrical Nusselt number saturates with increasing T, a fact that it is in agreement with available experimental data. Finally, a spectral analysis is conducted for different aspect ratios of the computational domain. The spectral analysis gives an insight into the physical origin of the velocity and current oscillations.

Study of dielectric behavior of ester transformer liquids under ac voltage

Abstract: As replacement of mineral oil, the application of ester liquids in large power transformers has become a hot topic in recent years. Understanding esters' dielectric behaviors under ac voltage is of vital importance for the insulation design of power transformers. This paper presents experimental studies on the ac dielectric strengths of synthetic ester and natural ester in quasi-uniform electric field, using mineral oil as a benchmark. The results show that esters have similar breakdown voltages to mineral oil when filtered and dehydrated. However, their breakdown voltages are significantly higher than mineral oil with the presence of particle and water contaminations. Furthermore, the breakdown voltages of esters are reduced more sharply than that of mineral oil with the increase of electrodes' Effective Stressed Area (ESA). Based on the experimental results, an estimation of the withstand voltages of transformer liquids in practical qualities was made when considering the particle effect, water effect and electrode area effect, and it was found that the 1-mm-gap withstand voltages of esters are higher than that of mineral oil.

Deformation, breakup and motion of a perfect dielectric drop in a quadrupole electric field

Abstract: A detailed nonlinear analysis of the deformation and breakup of a perfect dielectric (PD) drop, suspended in another perfect dielectric fluid, in the presence of a quadrupole electric field is presented using analytical (asymptotic) and numerical (boundary integral) methods. The quadrupole field is the simplest kind of an axisymmetric non-uniform electric field. A drop, when placed at the center of such a field, does not translate, thus allowing systematic investigation of the effect of non-uniformity of the electric field. The deformation of a drop under a quadrupole field for PD-PD systems exhibits several novel features as compared to that of a drop under a uniform electric field. The first order analysis predicts oblate deformation for a PD-PD system when the dielectric constant of the suspending medium is larger than that of the drop (Q = ei/ee < 1). This is in sharp contrast to uniform electric fields where oblate shapes are observed only in leaky dielectric systems. Prolate shapes are observed for ...

Enhancement of electrical insulation performance in power equipment based on dielectric material properties

Abstract: Research and development results of enhancement techniques of electrical insulation performance for higher electric field application in power transmission/substation equipment, such as transformers, switchgears and cables, are described, especially based on the view point of dielectric materials. Firstly, the electric field analysis, field optimization and field measurement techniques are introduced to discuss higher electric field stress applications in power equipment. Secondly, material types, including gases, liquids, solids, vacuum and their composite systems are discussed to make power equipment with higher insulation performance, lower losses, lower environmental impact and higher reliability. In the process of development, a highly sophisticated new approach to clarify the physical mechanisms of partial discharges was developed and applied. By the introduction and applications of the above mentioned new electrical insulation techniques based on dielectric materials, concepts of future power equipment with higher electric field stress are proposed. This paper is based on the Whitehead Memorial Lecture given at the IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) 2011 in Cancun, Mexico.

Experimental evaluation on dielectric and thermal characteristics of nano filler added transformer oil

Abstract: Liquid dielectrics play a major role in the insulation system used in high voltage equipments. The most commonly used liquid dielectric is transformer oil which serves the dual purpose as insulation and a coolant. It also acts as insulation between the conductingparts and as an arc quenching medium. Moreover the breakdown strength of the paper and the pressboard insulation depends on the properties of the oil in which it is impregnated. Therefore investigations have been attracted towards the development of nano fluids with better thermal and dielectric properties. In this work, ceramic nano particles are chosen because of their electrical insulating property. Because of higher relative permittivity Zirconia(ZrO2) and Titania(TiO2) nano particles are used here. The dielectric and thermal properties are measured for different concentrations of nanofillers. AC and Impulse breakdown voltage measurements were performed as per IS 6792:1972 and IS 11697:1986 respectively. The specific resistivity and the dielectric dissipation factor measurement were carried with reference to IS 6103:1971 and IS 6262:1971. The kinematic viscosity and flash point are measured as per IS 1448 Part-25:1970 and IS 1448 Part-21:1970 respectively.

How Suspended Particles Affect Surface Morphology in Powder Mixed Electrical Discharge Machining (PMEDM)

Abstract: The effect of SiC powder mixing into the dielectric liquid on surface topology and structure of interstitial free (IF) steel has been studied. Four process parameters, namely pulse duration, pulse current, concentration of powder, and type of dielectric liquid material, have been selected as varying parameters. Surface modifications due to suspended SiC particles were identified by using optical, scanning electron microscopy, and X-ray diffraction analysis. It was observed that suspended particles around discharge column accelerated and gained sufficient velocity to penetrate to the molten pool before solidification by means of electrophoresis and negative pressure induced after cessation of a discharge, which leads a surface embedded with added fine particles under that prevent formation of penetrating cracks during machining. However, surface cracks that formed due to high transformational stresses developed during solidification were found to be unaffected by means of mechanical action of the particles.

Electrical conduction and dielectric breakdown characteristics of alkyl ester dielectric fluids obtained from palm kernel oil

Abstract: Naturally occurring palm kernel oil (PKO) and its ester derivatives are being considered as sustainable alternatives to synthetic oils for use as dielectric fluids. This paper reports on their dielectric properties, which have been studied and compared to BS148 mineral oil. The low frequency complex dielectric response of the PKO and its derivatives are related to ionic conduction and electrode polarization phenomena. The purified PKO has an electrical conductivity of 3.04 × 10-12 Sm-1 at 30°C; this is 10 times lower than crude PKO but about 10 times greater than the BS148 oil. The bulk conductivity is thermally activated, activation energy = 0.47 eV, and influenced by viscosity. The ester derivatives have a higher conductivity than the PKO, which is related to ionic impurities introduced during processing. The breakdown field was measured in a bespoke cell enabling smaller volumes of oil (15 ml) than that used in ASTM D1816. The characteristic ac breakdown strength of purified PKO and is alkyl esters were found to be in the range, 41 to 43 kV/mm, which is significantly higher than the mineral oil (27 kV/mm). The results support the proposition that a dielectric fluid derived from palm kernel oil, once re-purified, may be a suitable replacement for mineral oil based fluids in HV electrical equipment.

Numerical simulation of the electro-convective onset and complex flows of dielectric liquid in an annulus

Abstract: We conducted a numerical study on the onset of electro-convection as well as the complex flow phenomena of dielectric liquid subjected to unipolar autonomous charge injection in the annular gap between two concentric circular cylindrical electrodes The Nernst-Planck equations governing the charge density transport, the Poisson equation for the electric potential and the Navier-Stokes equations for the fluid flow are solved numerically using the finite volume method The developed code is validated by comparing the critical stability parameter values for the onset of electro-convection with those obtained from the linear stability analysis We identify in a parameter space the stable hydrostatic state and the electro-convection state The electro-convection is again divided into three regimes: stationary, oscillatory and chaotic For inner cylinder radius ri ≥ 10, we observed an increase in the number of charged plumes and vortex pairs with stability parameter T before the electro-convection becomes chaotic For outer injection, although the onset of electroconvection starts at T higher than the inner injection, the onset of chaotic motion occurs at lower T

Flexible capacitive sensor encapsulating liquids as dielectric with a largely deformable polymer membrane

Abstract: Flexible and highly-sensitive capacitive sensors that are capable of detecting pressure distribution on curved surfaces are now in demand Using solid dielectric material could limit the sensors flexibility, while using air as the dielectric might compromise the sensors sensitivity Proposed is a distributed capacitive sensor that contains highly dielectric liquid, which increases the sensor sensitivity while maintaining their flexibility Since the liquid used in this work is incompressible, an escape reservoir is used that allows the liquid between the electrodes to flow into the reservoir when external pressure is applied and the gap between the electrode decreases In prior work, the escape reservoir was designed to be alongside the sensing area, which resulted in a large footprint A sensor has been designed that contains encapsulated highly dielectric liquid and allows the liquid to escape beneath the sensing area The footprint was successfully reduced by 75% The encapsulated liquid enhanced the maximum measurable pressure from 100 kPa to 800 kPa The amplification ratios of the sensitivity with DI water and glycerine increased 7 and 35 times respectively, as compared to the device without the liquid encapsulated

Nonlinear oscillations of gas bubbles submerged in water: implications for plasma breakdown

Abstract: Gas bubbles submerged in a dielectric liquid and driven by an electric field can undergo dramatic changes in both shape and volume. In certain cases, this deformation can enhance the distribution of the applied field inside the bubble as well as decrease the internal gas pressure. Both effects will tend to facilitate plasma formation in the gas volume. A practical realization of these two effects could have a broad impact on the viability of liquid plasma technologies, which tend to suffer from high voltage requirements. In this experiment, bubbles of diameter 0.4?0.7?mm are suspended in the node of a 26.4?kHz underwater acoustic standing wave and excited into nonlinear shape oscillations using ac electric fields with amplitudes of 5?15?kV?cm?1. Oscillations of the deformed bubble are photographed with a high-speed camera operating at 5130?frames?s?1 and the resulting images are decomposed into their axisymmetric spherical harmonic modes, , using an edge detection algorithm. Overall, the bubble motion is dominated by the first three even modes l?=?0, 2 and 4. Electrostatic simulations of the deformed bubble's internal electric field indicate that the applied field is enhanced by as much as a factor of 2.3 above the nominal applied field. Further simulation of both the pure l?=?2 and l?=?4 modes predicts that with additional deformation, the field enhancement factors could reach as much as 10?50.

Nanomaterials fabricated using spark erosion and other particle fabrication processes

Abstract: Methods, systems, and devices are disclosed for fabricating clean, oxidation-free nanoparticles of electrically conducting metals and alloys using spark erosion techniques. In one aspect, a method includes dispersing bulk pieces of an electrically conducting material in a dielectric fluid with mechanical vibrations within a container; generating an electric field using electrodes in the dielectric fluid using by an electric pulse, in which the electric field creates a plasma in a volume existing between the bulk pieces that locally heats the bulk pieces to form structures within the volume, the dielectric fluid quenching the structures to form nanoparticles, and filtering the nanoparticles through a screen including holes of a size allowing nanoparticles of the size or smaller to pass through the screen to a region in the container, in which the dielectric fluid inhibits oxidation of the surface of the nanoparticles.

Direct thermal to electrical energy conversion using 9.5/65/35 PLZT ceramics in the ergodic relaxor phase

Abstract: This paper reports on direct thermal to electrical energy conversion by performing the Olsen cycle on 9.5/65/35 lead lanthanum zirconate titanate (PLZT). The Olsen cycle consists of two isothermal and two isoelectric field processes in the electric displacement versus electric field diagram. It was performed by alternatively dipping the material in hot and cold dielectric fluid baths under specified electric fields. The effects of applied electric field, sample thickness, electrode material, operating temperature, and cycle frequency on the energy and power densities were investigated. A maximum energy density of 637 ± 20 J/L/cycle was achieved at 0.054 Hz with a 250-μm-thick sample featuring Pt electrodes and coated with a silicone conformal coating. The operating temperatures varied between 3°C and 140°C and the electric field was cycled between 0.2 and 6.0 MV/m. A maximum power density of 55 ± 8 W/L was obtained at 0.125 Hz under the same operating temperatures and electric fields. The dielectric strength of the material, and therefore the energy and power densities generated, increased when the sample thickness decreased from 500 to 250 μm. Furthermore, the electrode material was found to have no significant effect on the energy and power densities for samples subject to the same operating temperatures and electric fields. However, samples with electrode material possessing thermal expansion coefficients similar to that of PLZT were capable of withstanding larger temperature swings. Finally, a fatigue test showed that the power generation gradually degraded when the sample was subject to repeated thermoelectrical loading.

Dielectric frequency response measurements and dissipation factor temperature dependence

Abstract: The condition of the insulation is an essential aspect for the operational reliability of electrical power transformers, generators, cables and other high voltage equipment. Transformers with high moisture content can not without risk sustain higher loads. Bushings and cables with high moisture content at high temperature can explode due to “thermal runaway”.

Developing interface localized liquid dielectrophoresis for optical applications

Abstract: Electrowetting charges the solid-liquid interface t o change the contact area of a droplet of a conduct ing liquid. It is a powerful technique used to create variable focus li quid lenses, electronic paper and other devices, bu t it depends upon ions within the liquid. Liquid dielectrophoresis (L -DEP) is a bulk force acting on the dipoles through out a dielectric liquid and is not normally considered to be a local ized effect acting at the interface between the liq uid and a solid or other fluid. In this work, we show theoretically ho w non-uniform electric fields generated by interdig itated electrodes can effectively convert L-DEP into an interface loc alized form. We show that for droplets of sufficien t thickness, the change in the cosine of the contact angle is propor tional to the square of the applied voltage and so obeys a similar equation to that for electrowetting ‐ this we call dielectrowetting. However, a major difference to el ectrowetting is that the strength of the effect is controlled by the ele ctrode spacing and the liquid permittivity rather t han the properties of an insulator in a sandwich structure. Experimentally, we show that that this dielectrowetting equation ac curately describes the contact angle of a droplet of oil viewed across parallel interdigitated electrodes. Importantly, t he induced spreading can be complete, such that contact angle saturation does not occur. We then show that for thin films, L-DEP can shape the liquid-air interface creating a spatially perio dic wrinkle and that such a wrinkle can be used to create a voltage programmable phase diffraction grating.

Retraction control of an impacted dielectric droplet through electrical pressure

Abstract: When a deposited dielectric liquid interface is exposed to a unipolar ion injection in a gaseous medium, the liquid expands over the grounded substrate due to the squeezing force of electric pressure (S. R. Mahmoudi, K. Adamiak, G. S. P. Castle, Spreading of a dielectric droplet through an interfacial electric pressure, Proc. R. Soc., 2011, 467, 3257–3271). A new active method based on the concept is proposed to control the deposition of an impacted dielectric droplet. An electric pressure resulting from the electric surface charge produced by corona discharge squeezes the droplet interface towards the grounded substrate and generates a resistance against the droplet retraction. It is demonstrated that the electrical pressure effectively suppresses the droplet retraction at voltages above the corona discharge threshold.

Combined aerodynamic and electrostatic atomization of dielectric liquid jets

Abstract: The electrical and atomization performance of a plane–plane charge injection atomizer using a dielectric liquid, and operating at pump pressures ranging from 15 to 35 bar corresponding to injection velocities of up to 50 m/s, is explored via low current electrical measurements, spray imaging and phase Doppler anemometry. The work is aimed at understanding the contribution of electrostatic charging relevant to typical higher pressure fuel injection systems such as those employed in the aeronautical, automotive and marine sectors. Results show that mean-specific charge increases with injection velocity significantly. The effect of electrostatic charge is advantageous at the 15–35 bar range, and an arithmetic mean diameter D 10 as low as 0.2d is achievable in the spray core and lower still in the periphery where d is the orifice diameter. Using the data available from this higher pressure system and from previous high Reynolds number systems (Shrimpton and Yule Exp Fluids 26:460–469, 1999), the promotion of primary atomization has been analysed by examining the effect that charge has on liquid jet surface and liquid jet bulk instability. The results suggest that for the low charge density Q v ~ 2 C/m3 cases under consideration here, a significant increase in primary atomization is observed due to a combination of electrical and aerodynamic forces acting on the jet surface, attributed to the significantly higher jet Weber number (We j) when compared to low injection pressure cases. Analysis of Sauter mean diameter results shows that for jets with elevated specific charge density of the order Q v ~ 6 C/m3, the jet creates droplets that a conventional turbulent jet would, but with a significantly lower power requirement. This suggests that ‘turbulent’ primary atomization, the turbulence being induced by electrical forces, may be achieved under injection pressures that would produce laminar jets.

Encapsulation of electrodes in solid media for use in conjunction with fluid high voltage isolation

Abstract: An inductively-coupled plasma source for a focused charged particle beam system includes a conductive shield that provides improved electrical isolation and reduced capacitive RF coupling and a dielectric fluid that insulates and cools the plasma chamber. The conductive shield may be enclosed in a solid dielectric media. The dielectric fluid may be circulated by a pump or not circulated by a pump. A heat tube can be used to cool the dielectric fluid.