Showing papers on "Liquid dielectric published in 2016"

State of the art in powder mixed dielectric for EDM applications

Abstract: Electrical discharge machining (EDM) is a non-conventional machining technique for removing material based on the thermal impact of a series of repetitive sparks occurring between the tool and workpiece in the presence of dielectric fluid. Since the machining characteristics are highly dependent on the dielectric’s performance, significant attention has been directed to modifying the hydrocarbon oil properties or introducing alternative dielectrics to achieve higher productivity. This article provides a review of dielectric modifications through adding powder to dielectric. Utilizing powder mixed dielectric in the process is called powder mixed EDM (PMEDM). In order to select an appropriate host dielectric for enhancing machining characteristics by adding powder, a brief background is initially provided on the performance of pure dielectrics and their selection criteria for PMEDM application follow by powder mixed dielectric thoroughly review. Research shows that PMEDM facilitates producing parts with predominantly high surface quality. Additionally, some studies indicate that appropriate powder selection increases machining efficiency in terms of material removal rate. Therefore, the role of powder addition in the discharge characteristics and its influence on machining output parameters are explained in detail. Furthermore, by considering the influence of the main thermo-physical properties and concentration of powder particles, the performance of various powder materials is discussed extensively. Since suitable powder selection depends on many factors, such as variations in EDM, machining scale and electrical and non-electrical parameter settings, a thorough comparative review of powder materials is presented to facilitate a deeper insight into powder selection parameters for future studies. Finally, PMEDM research trends, findings, gaps and industrialization difficulties are discussed extensively.

Lattice Boltzmann model for Coulomb-driven flows in dielectric liquids.

Abstract: In this paper, we developed a unified lattice Boltzmann model (LBM) to simulate electroconvection in a dielectric liquid induced by unipolar charge injection. Instead of solving the complex set of coupled Navier-Stokes equations, the charge conservation equation, and the Poisson equation of electric potential, three consistent lattice Boltzmann equations are formulated. Numerical results are presented for both strong and weak injection regimes, and different scenarios for the onset and evolution of instability, bifurcation, and chaos are tracked. All LBM results are found to be highly consistent with the analytical solutions and other numerical work.

Electrical Discharge Machining of Ti6Al4V in Hydroxyapatite Powder Mixed Dielectric Liquid

Abstract: Hydroxyapatite (HA) powder suspension in deionized water was used as a dielectric liquid during electrical discharge machining (EDM) of Ti6Al4V work material. The machined surfaces were evaluated by scanning electron microscopy (SEM), energy dispersive spectroscopy, and optical microscopy. The powder particles in the dielectric liquid extensively migrated and formed an HA-rich layer on the work material surface under specific machining conditions. The result was attributed to the generation of secondary discharges due to altered discharge conditions. The particles divided the primary discharge channel into several secondary ones. When the main discharge channel subdivided into several sub-discharges with comparable energy densities, the particles in the discharge region got stuck among them. Then, they moved toward the melted cavities and penetrated the surface at the end of the discharge duration. The results suggest the process as a practical alternative for producing biocompatible interfaces or coating...

Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers

Abstract: The condition of the internal cellulosic paper and oil insulation are of concern for the performance of power transformers. Over the years, a number of methods have been developed to diagnose and monitor the degradation/aging of the transformer internal insulation system. Some of this degradation/aging can be assessed from electrical responses. Currently there are a variety of electrical-based diagnostic techniques available for insulation condition monitoring of power transformers. In most cases, the electrical signals being monitored are due to mechanical or electric changes caused by physical changes in resistivity, inductance or capacitance, moisture, contamination or aging by-products in the insulation. This paper presents a description of commonly used and modern electrical-based diagnostic techniques along with their interpretation schemes.

A critical review on the characteristics of alternating liquid dielectrics and feasibility study on pongamia pinnata oil as liquid dielectrics

Abstract: Green insulating fluids are playing a vital role in insulation design of the distribution transformers. Traditionally used mineral oil has lesser biodegradable and low fire resistant characteristics. Hence, it does not satisfy the new environmental regulation. Besides that, the availability of fossil fuels is also going to run out. These negative aspects motivate to search for an alternate insulating oil. This research has been proposed for feasibility study on non - edible pongamia pinnata oil as an alternate liquid dielectric which can be used in Distribution Transformers. The paper also reviews the alternate insulating oil, electrical, physical and chemical properties. Subsequently, Pongamia Pinnata Oil's electrical properties (Dielectric Strength, Dielectric Constant, Dielectric Dissipation Factor and Specific Resistance), chemical properties (Water content, Acidity) and physical properties (Viscosity, Flash Point, Interfacial Tension) have been estimated according ASTM and IEC for comparing the Pongaima Pinnata oil with conventional mineral oil. Alongside, solid insulating material deterioration, both in pongamia pinnata oil and mineral oil, using XRD and SEM was carried out. The experiments are encouraging. If the oil suited for these characteristics the value addition has been made from this oil which leads to waste land utilization and rural development.

Computation and analysis of dielectric losses in MV power electronic converter insulation

Abstract: The newly available Medium Voltage (MV) Silicon-Carbide (SiC) devices enable a great extension of the design space of MV inverters. This includes the utilization of unprecedented blocking voltages, higher switching frequencies, higher commutation speeds, and high temperature operation. However, all these factors considerably increase the insulation stress. This paper details the computation of dielectric losses, which are directly related to the insulation stress and can be used for the insulation design and diagnostic. After a review of the method used to compute dielectric losses, scalable analytical expressions are derived for the losses produced by PWM waveforms of DC-DC, DC-AC, and multilevel DC-AC inverters. Finally, a Medium-Frequency (MF) transformer is analyzed and the impacts of the insulation material and the operating temperature on the dielectric losses are discussed. It is found that the insulation losses can represent a significant share (17%) of the total transformer losses.

Tamper-proof electronic packages with two-phase dielectric fluid

Abstract: Tamper-proof electronic packages and fabrication methods are provided including an enclosure enclosing, at least in part, at least one electronic component within a secure volume, a two-phase dielectric fluid within the secure volume, and a tamper-respondent detector. The tamper-respondent detector monitors, at least in part, temperature and pressure of the two-phase dielectric fluid. In operation, the two-phase dielectric fluid deviates from an established saturation line of the two-phase dielectric fluid within the secure volume with an intrusion event into the secure volume, and the tamper-respondent detector detects, from the monitoring of the temperature and pressure of the two-phase dielectric fluid, the deviation from the established saturation line, and thereby occurrence of the intrusion event.

Electrocaloric cooler combining ceramic multi-layer capacitors and fluid

Abstract: In this paper, an electrocaloric (EC) cooler prototype made of 150 ceramic-based Multi-Layer Capacitors (MLCs) has been detailed. This cooler involves a column of dielectric fluid where heat exchange with the MLCs takes place. The maximum variation of temperature in the fluid column due to the EC effect reaches 0.13 K whereas the heat exchanged during one stroke is 0.28 J. Although this prototype requires improvements with respect to heat exchange, the basic principle of creating a temperature gradient in a column of fluid has been validated.

Health indexes for power transformers: a case study

Abstract: Proper operation of power transformers is critical to ensuring transmission and distribution of electrical power. Most transformers have an electrical insulation system based on oil and paper. The state of the insulation system is the major factor influencing the state of the transformer. During service the dielectric materials within the transformer deteriorate, and small concentrations of impurities such as water, carbon monoxide, carbon dioxide, and furan compounds accumulate in the oil. Since it is easy to obtain oil samples from power transformers, the information most commonly collected by transformer fleet managers relates to the physical and dielectric properties of the oil. These properties include dielectric strength, dissipation factor, color, interfacial tension, and concentrations of dissolved gases, furans, acids, and moisture. Using these properties it is possible to determine whether a transformer has developed certain specific faults, e.g., partial discharges, arcing, sparking, overheating, etc. On the other hand, various health indexes have been proposed to characterize the general condition of a transformer [1]-[3]. The factors taken into account in these indexes vary, and are given different statistical weightings depending on their influence on the general condition of the transformer. In this article we evaluate the condition of a fleet of operating power transformers, using two recently proposed health indexes, and compare the results.

Experimental investigation on effects of dielectric mediums in near-dry electric discharge machining

Abstract: A dielectric fluid plays a significant role on the machining efficiency of Electric discharge machining (EDM) Two phase (liquid-air) dielectric medium was utilized in near-dry (EDM) It is an environmentally friendly process The present article reports the effect of different dielectric mediums on responses of near-dry EDM Dielectric mediums used for experimentation were water-air, EDM oil-air and glycerin-air mixtures The process parameters selected for experimentation were current, duty factor, flushing pressure and lift Material removal mechanism of near-dry EDM is investigated using imaging techniques Dielectric fluid with higher viscosity and higher Prandtl number provided better results than fluids with lower viscosity The results reveal that the Material removal rate (MRR) with glycerin-air dielectric medium is about three-times higher than other dielectric medium used with negligible recast layer The tool wear rate is negligible in near-dry EDM in relation to all the dielectric mediums

Optimal parameter prediction of oxygen-mist near-dry wire-cut EDM

Abstract: Wire-cut electrical discharge machining (WEDM) is one of the important non-traditional machining processes to cut hard and high strength materials. It was observed from the literature that some environmental pollutants had been emitted owing to thermal decomposition of the liquid dielectric mediums used in WEDM. In this near-dry WEDM process, oxygen-mist is used as a dielectric medium which encourages the eco-friendly cutting process owing to minimal usage of liquid-based dielectric medium. In this paper, the experiments have been performed using the compressed oxygen gas mixed with minimum quantity of demineralised water as a dielectric medium. The design of experiments has been performed using Taguchi's L27 orthogonal array. The spark-current, pulse-on-time, oxygen-mist inlet pressure and mixing flow rate are selected as input parameters, and material removal rate and surface roughness are considered as response characteristics. After the experimentation, the regression analysis has been employed to develop the best mathematical models for the multi-objective optimisation purpose. Multi-objective artificial bee colony (MOABC) algorithm is introduced to predict the optimal set of input and output parameters using non-dominated Pareto-optimal-front solutions.

Effect of silicone oxide nano particles on dielectric characteristics of natural ester

Abstract: Transformer efficiency purely depends on integral functions of solid and liquid dielectric. A good dielectric should possess the following properties like; high breakdown strength, high thermal stability, chemical inertness, environmental friendly and good heat transfer capacity. Natural esters such as Corn Oil (CRO) and Coconut Oil (CO) are chosen based on the fatty acid compositions and considered to be the next generation heat transfer fluids. The SiO 2 nanoparticles was attributed with good abrasion resistance, electrical insulation and high thermal stability: dispersed to the CRO and CO in various proportions. The properties like breakdown voltage, dissipation factor, dielectric constant, volume resistivity, viscosity, flash point, fire point, acidity and iodine values were measured according to IEC and ASTM standards. Astonishingly at elevated temperature the altered natural esters shows excellent insulating properties.

Highly sensitive terahertz dielectric sensor for small-volume liquid samples

Abstract: We present a resonator-based sensor for the measurement of the refractive index of dielectric liquid samples. The proposed sensor operates on the basis of an electromagnetic resonance between a thin metallic grating and a reflecting ground plane. The fluid whose refractive index is to be measured fills the region between the metallic grating and the ground plane and causes a considerable shift in the resonance frequency (>500 GHz/RIU). The sensor has a relatively simple structure; therefore, it can be manufactured economically on industrial scales.

Modeling of conductive particle motion in viscous medium affected by an electric field considering particle-electrode interactions and microdischarge phenomenon

Abstract: Up and down motion of a spherical conductive particle in dielectric viscous fluid driven by a DC electric field between two parallel electrodes was investigated. A nonlinear differential equation, governing the particle dynamics, was derived, based on Newton’s second law of mechanics, and solved numerically. All the pertaining dimensionless groups were extracted. In contrast to similar previous works, hydrodynamic interaction between the particle and the electrodes, as well as image electric forces, has been taken into account. Furthermore, the influence of the microdischarge produced between the electrodes and the approaching particle on the particle dynamics has been included in the model. The model results were compared with experimental data available in the literature, as well as with some additional experimental data obtained through the present study showing very good agreement. The results indicate that the wall hydrodynamic effect and the dielectric liquid ionic conductivity are very dominant factors determining the particle trajectory. A lower bound is derived for the charge transferred to the particle while rebounding from an electrode. It is found that the time and length scales of the post-microdischarge motion of the particle can be as small as microsecond and micrometer, respectively. The model is able to predict the so called settling/dwelling time phenomenon for the first time.

Environmentally friendly power generator based on moving liquid dielectric and double layer effect

Abstract: An electrostatic power generator converts mechanical energy to electrical energy by utilising the principle of variable capacitance. This change in capacitance is usually achieved by varying the gap or overlap between two parallel metallic plates. This paper proposes a novel electrostatic micro power generator where the change in capacitance is achieved by the movement of an aqueous solution of NaCl. A significant change in capacitance is achieved due to the higher than air dielectric constant of water and the Helmholtz double layer capacitor formed by ion separation at the electrode interfaces. The proposed device has significant advantages over traditional electrostatic devices which include low bias voltage and low mechanical frequency of operation. This is critical if the proposed device is to have utility in harvesting power from the environment. A figure of merit exceeding 10000(108μW)/(mm2HzV2) which is two orders of magnitude greater than previous devices, is demonstrated for a prototype operating at a bias voltage of 1.2 V and a droplet frequency of 6 Hz. Concepts are presented for large scale power harvesting.

Compensating the effect of temperature variation on dielectric response of oil-paper insulation used in power transformers

Abstract: Being an offline measurement technique, Polarization Depolarization Current (PDC) data is conventionally measured after the oil-paper insulation of the equipment reaches thermal equilibrium. Depending on the condition of the insulation, PDC data measurement time typically ranges from several minutes to few hours. It is understood that atmospheric conditions controls the value of ambient temperature. During field measurement, these conditions may not remain constant throughout the PDC recording process. Variation in ambient temperature disturbs the thermal equilibrium that the equipment may have attained at the start of the measurement procedure. Such a condition invariably affects the recorded dielectric response data. However, methods that are available for analyzing relaxation current assume that temperature equilibrium is maintained throughout the measurement period. Results presented in this paper show that dielectric response data is indeed affected if the ambient temperature increases/decreases monotonically. A method is also proposed in this paper using which the effect of such temperature variation on the recorded relaxation current data can be eliminated. This also ensures correct interpretation of PDC data for condition assessment. Analysis presented shows that the value of the equilibrium temperature does not affect the performance of the proposed technique. The applicability of the proposed methodology is first tested on laboratory samples. Thereafter, it is applied to data collected from a real life power transformer.

Pre-breakdown mechanisms in dielectric liquids and predicting models

Abstract: This paper presents a synthetic review of the state of knowledge relative to the pre-breakdown phenomena in dielectric liquids and especially the initiation and propagation of streamer/discharge and their characteristic parameters. The main influencing parameters namely the structure, stopping length, velecity and propagation modes of streamers, current, charge and associated emitted light are developed. These parameters include especially the chemical composition of the liquid (pure or containing specific additives); the applied voltage; the electrode arrangement; the temperature and the hydrostatic pressure. The mechanisms involving in the inception and propagation of streamers are discussed. Recent models enabling to predict the type of streamers that can be initiated and their characteristics when propagating within liquid, are also presented and discussed. It is shown through the experiments and modeling that the pre-breakdown phenomena in dielectric liquids are governed by both electronic and gaseous mechanisms even if one of them is dominant in one case and the second in another.

The electrical characteristics of the dielectric barrier discharges

Abstract: The electrical characteristics of the dielectric barrier discharges have been studied in this paper under different operating conditions. The dielectric barrier discharges were formed inside two reactors composed of electrodes in the shape of two parallel plates. The dielectric layers inside these reactors were pasted on the surface of one electrode only in the first reactor and on the surfaces of the two electrodes in the second reactor. The reactor under study has been fed by atmospheric air that flowed inside it with a constant rate at the normal temperature and pressure, in parallel with applying a sinusoidal ac voltage between the electrodes of the reactor. The amount of the electric charge that flows from the reactors to the external circuit has been studied experimentally versus the ac peak voltage applied to them. An analytical model has been obtained for calculating the electrical characteristics of the dielectric barrier discharges that were formed inside the reactors during a complete cycle of the ac voltage. The results that were calculated by using this model have agreed well with the experimental results under the different operating conditions.

Effect of the electrode material on the breakdown voltage and space charge distribution of propylene carbonate under impulse voltage

Abstract: This paper reports three types of electrodematerials(copper,aluminum, and stainless steel) that are used to measure the impulse breakdown voltage of propylene carbonate. The breakdown voltage of propylene carbonate with these electrodematerials is different and is in decreasing order of stainless steel, copper, and aluminum. To explore how the electrodematerial affects the insulating properties of the liquid dielectric, the electric field distribution and space charge distribution of propylene carbonate under impulse voltage with the three electrodematerials are measured on the basis of a Kerr electro-optic test. The space charge injection ability is highest for aluminum, followed by copper, and then the stainless steel electrodes. Furthermore, the electric field distortion rate decreased in the order of the aluminum,copper, and then the stainless steel electrode. This paper explains that the difference in the electric field distortion rate between the three electrodematerials led to the difference in the impulse breakdown voltage of propylene carbonate.

Electrohydrodynamics of viscous drops in strong electric fields: Numerical simulations

Abstract: Weakly conducting dielectric liquid drops suspended in another dielectric liquid and subject to an applied uniform electric field exhibit a wide range of dynamical behaviors contingent on field strength and material properties. These phenomena are best described by the Melcher-Taylor leaky dielectric model, which hypothesizes charge accumulation on the drop-fluid interface and prescribes a balance between charge relaxation, the jump in Ohmic currents from the bulk and charge convection by the interfacial fluid flow. Most previous numerical simulations based on this model have either neglected interfacial charge convection or restricted themselves to axisymmetric drops. In this work, we develop a three-dimensional boundary element method for the complete leaky dielectric model to systematically study the deformation and dynamics of liquid drops in electric fields. The inclusion of charge convection in our simulations permits us to investigate drops in the Quincke regime, in which experiments have demonstrated a symmetry-breaking bifurcation leading to steady electrorotation. Our simulation results show excellent agreement with existing experimental data and small-deformation theories.

Near Axisymmetric Partial Wetting Using Interface-Localized Liquid Dielectrophoresis.

Abstract: The wetting of solid surfaces can be modified by altering the surface free energy balance between the solid, liquid, and vapour phases. Liquid dielectrophoresis (L-DEP) can produce wetting on normally non-wetting surfaces, without modification of the surface topography or chemistry. L-DEP is a bulk force acting on the dipoles of a dielectric liquid and is not normally considered to be a localized effect acting at the interface between the liquid and a solid or other fluid. However, if this force is induced by a non-uniform electric field across a solid-liquid interface, it can be used to enhance and control the wetting of a dielectric liquid. Recently, it was reported theoretically and experimentally that this approach can cause a droplet of oil to spread along parallel interdigitated electrodes thus forming a stripe of liquid. Here we show that by using spiral shaped electrodes actuated with four 90o successive phase shifted signals, a near axisymmetric spreading of droplets can be achieved. Experimental...

Tilt sensor based on three electrodes dielectric liquid capacitive sensor

Abstract: This paper presents a design, fabrication and characterization of a low-cost capacitive tilt sensor. The proposed sensor consists of a three-electrode capacitor, which contains two-phase of the air and liquid as the dielectric media. The three electrodes hold a plastic tube and the tube is positioned on a printed circuit board (PCB) which consists of a 127 kHz sine wave generator, a pre-amplifier, a rectifier and a low pass filter. The proposed sensor structure can measure tilt angles in the rage of 0° to 75°, where the linear relationship between the angle to be measured and the output signal was observed in the range of 0° to 50°. The sensitivity and resolution of the sensor are measured to be 40mV/degree and 0.5 degree, respectively.

Development of a bi-directional electrohydrodynamic pump: Parametric study with numerical simulation and flow visualization

Abstract: We propose a bi-directional electrohydrodynamic pump developed for transporting dielectric liquid, where the electrodes are symmetrically configured but the applied voltage is non-symmetric. The un...