Showing papers on "Liquid dielectric published in 2017"

Enhanced electric polarization and breakdown strength in the all-organic sandwich-structured poly(vinylidene fluoride)-based dielectric film for high energy density capacitor

Abstract: It is essential to develop the dielectric energy storage capacitor for the modern electrical and electronic equipment. Here, the all-organic sandwich-structured composite with superior breakdown strength and delayed saturation polarization is presented. Furthermore, the energy storage characteristics of the composite are enhanced by the poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) fiber and the redistribution of local electric field. The dielectric permittivity of composite increases to ∼16, and the discharged energy density is high to ∼8.7 J/cm3 at 360 kV/mm, and the breakdown strength is up to ∼408 kV/mm. The excellent performance of the composite broadens the application in the field of power electronics industry.

Surfactant and graphite powder–assisted electrical discharge machining of titanium alloy:

Abstract: Surfactant and graphite powder–assisted electrical discharge machining was proposed and experiments were performed on titanium alloy in this investigation. Analysis was carried out to observe changes in dielectric fluid behaviour, material removal rate, surface roughness, recast layer thickness, surface topography and energy-dispersive X-ray spectroscopy. It was found out that the addition of surfactant to dielectric fluid (electrical discharge machining oil + graphite powder) improved the material removal rate and surface roughness. It was noticed to have reduced the recast layer thickness and agglomeration of graphite and sediment particles. Biface material migrations between the electrode and the workpiece surface were identified, and migration behaviour was powerfully inhibited by the mixing of surfactant. Surfactant added into dielectric fluid played an important role in the discharge gap, which increased the conductivity, and suspended debris particles in dielectric fluid reduced the abnormal discha...

Electrowetting-actuated liquid metal for RF applications

Abstract: Electrowetting is well-established as a fluid manipulation technique in such areas as lab-on-a-chip, visible light optics, and displays, yet has seen far less implementation in the field of radio-frequency (RF) electronics and electromagnetics. This is primarily due to a lack of appropriate materials selection and control in these devices. Low loss RF conductive fluids such as room temperature liquid metals (i.e. Hg, EGaIn, Galinstan) are by far the leading choice of active material due to their superior electrical properties but require high actuating voltages due to their inherently high surface tensions (>400 mN m−1) which often lead to dielectric breakdown. While the toxicity of Hg encourages the pursuit of non-toxic alternatives such as gallium alloys, the native surface oxide formation often prohibits reliable device functionality. Additionally, traditional electrowetting architectures rely on lossy electrode materials which degrade RF transmission efficiencies and result in non-reversible material diffusion at the electrode/liquid metal contact. In this work, we report on approaches to utilize liquid metals in electrowetting on dielectric (EWOD) devices that resolve all of these challenges by judicious choice of novel electrode materials, dielectric fluid, and device architecture. A functional RF device, namely an electromagnetic polarizer, is demonstrated that can be activated on demand through EWOD and provides an average signal attenuation of 12.91 dB in the on state and 1.46 dB in the off state over the range of 8–9.2 GHz, with a switching speed of about 12 ms. These results can be further extended to other RF applications such as tunable antennas, transmission lines, and switchable metasurfaces.

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 behaviours 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.

Meta-atom microfluidic sensor for measurement of dielectric properties of liquids

Abstract: High sensitivity microwave frequency microfluidic sensing is gaining popularity in chemical and biosensing applications for evaluating the dielectric properties of liquid samples. Here, we show that a tiny microfluidic channel positioned in the gaps of a dual-gap meta-atom split-ring resonator can exploit the electric field sensitivity to predict the dielectric properties of liquid samples. Employing an empirical relation between resonant characteristics of the fabricated sensor and the complex permittivity of water-ethanol or water-methanol mixtures produces good congruence to standardized values from the literature. This microfluidic sensor offers a potential lab-on-chip solution for liquid dielectric characterization without external electrical connections.

Surface integrity of Inconel 718 in high-speed electrical discharge machining milling using air dielectric

Abstract: This paper proposes a novel, efficient, and environmental friendly machining method named high-speed dry electrical discharge machining (EDM) milling. The material removal rate of this method is much higher than that of conventional EDM and is also higher than that of traditional mechanical machining method for materials that are “difficult to machine.” Nickel-based superalloy Inconel 718 is widely used in the aerospace field as its surface integrity exerts significant effects. In this study, the microscopic characteristics, such as surface roughness, re-solidified layer, micro-hardness, micro-crack, and micro-void, of the machined Inconel 718 in high-speed EDM milling using air dielectric were studied compared with those using liquid dielectric. The formation mechanism of the microscopic characteristics was revealed. Experimental results showed that the machined Inconel 718 in high-speed EDM milling using air dielectric exhibited smaller surface roughness, thinner thickness of re-solidified layer, and smaller and less micro-cracks compared with those using liquid dielectric. Theoretical and technical foundations were laid for the industrial application of this novel machining method.

A rational co-design approach to the creation of new dielectric polymers with high energy density

Abstract: In order to interface power sources with any electrical load, energy storage and conversion are important components. The ability to store electrical energy is essential in many applications. Dielectric materials with high electric energy density are essential components in modern power electronics such as pulsed power systems and switched mode power supplies. The current state-of-the-art film capacitor is biaxially oriented polypropylene that has a breakdown strength of >730 MV/m and low dielectric loss of <0.0002 at 1 kHz, but suffers a low dielectric constant of 2.2. Novel dielectric materials for film capacitors were synthesized with guidance from a high throughput hierarchal modeling scheme, which involved combinatorial exploration based on density functional theory computations followed by successive screening. As a result, several organic polymers consisting of polythioureas and polyimides have been identified, synthesized, and characterized along with a new class of organometallic polymers. A fundamental understanding of the correlation between materials' molecular and micro structures and their electrical characteristics in the presence of various intrinsic and extrinsic factors is essential to the basic research for the design of new dielectric materials.

Analysis of dry electrical discharge machining in different dielectric mediums

Abstract: Dry electrical discharge machining (EDM) is a modification of the oil EDM process in which the liquid dielectric is replaced by a gaseous dielectric. This study investigates the effects of differen...

Potential of Jatropha curcas oil as a dielectric fluid for power transformers

Abstract: A dielectric liquid provides two main functions in power transformers: an insulant and a cooling medium. Mineral oil, largely due to its availability and low cost, has predominantly been used in power transformers for more than 100 years [1]-[3]. However, mainly due to its low biodegradability, a spill contaminating soil and water can be detrimental to the environment [4]. This shortcoming has prompted researchers to focus on safety, biodegradability, and renewability of dielectric fluids. The dielectric fluids based on vegetable oils (natural esters) are an innovative alternative since they have shown comparable, and sometimes better, dielectric, physical, and chemical properties to mineral insulating oils [5]-[8].

Fundamental investigation of dry electrical discharge machining (DEDM) by optical emission spectroscopy and its numerical interpretation

Abstract: Dry electrical discharge machining (DEDM) has been developed as an alternative manufacturing process to the traditional EDM in liquid dielectric media. The absence of the liquid dielectric allows DEDM to be performed by simpler and environmentally friendlier machines. The erosion in DEDM mainly occurs due to the bombardment of the workpiece electrode surface by charged particles produced by micro electric discharges. Thus, the understanding of the fundamental properties of the micro plasma is necessary to explain the erosion mechanisms in this process. Optical emission spectroscopy of DEDM single discharges and its numerical interpretation by emission spectra simulation are developed in the present work. The hypothesis of plasmas in local thermal equilibrium (LTE) is developed, whereas the formation of an electron beam in non-LTE plasmas is also considered and briefly introduced. The simulations show that large amount of different ionic species is produced from the anode workpiece material, and the estimated electron temperature profile is peaking at the plasma centre. Moreover, hot anode spots formed on the workpiece surface due to the plasma-material interactions seem to be considerably smaller than the total plasma diameter and the respective eroded crater. These characteristics indicate that DEDM produces discharges similar to anode dominated vacuum arcs, which present properties very different from EDM discharges in liquid dielectric.

A new class of variable capacitance generators based on the dielectric fluid transducer

Abstract: This paper introduces the novel concept of dielectric fluid transducer (DFT), which is an electrostatic variable capacitance transducer made by compliant electrodes, solid dielectrics and a dielectric fluid with variable volume and/or shape. The DFT can be employed in actuator mode and generator mode. In this work, DFTs are studied as electromechanical generators able to convert oscillating mechanical energy into direct current electricity. Beside illustrating the working principle of dielectric fluid generators (DFGs), we introduce different architectural implementations and provide considerations on limitations and best practices for their design. Additionally, the proposed concept is demonstrated in a preliminary experimental test campaign conducted on a first DFG prototype. During experimental tests a maximum energy per cycle of and maximum power of has been converted, with a conversion efficiency up to 30%. These figures correspond to converted energy densities of with respect to the displaced dielectric fluid and with respect to the mass of the solid dielectric. This promising performance can be largely improved through the optimization of device topology and dimensions, as well as by the adoption of more performing conductive and dielectric materials.

Dielectric liquid lens with chevron-patterned electrode

Abstract: A dielectric liquid lens driven using a chevron-patterned electrode is prepared. The bending angle of the electrode is 90°. This newly designed electrode can effectively and symmetrically deform the shape of the liquid lens, which in turn leads to a variable focus. For a 3-mm-diameter lens, a driving voltage changed from 0 to 52 V can vary its focal length from ~19.3 to ~4.9 mm. Using a 40-Vrms pulse voltage to impact the lens, the dynamic response time is ~6.7 s. Using a step voltage to impact the liquid lens, the response time can be largely reduced. Multiple lenses or a microlens array can be driven at the same time. Due to in-plane actuation, the driving voltage is insensitive to the size of the liquid lens.

Non-thermal plasma ethanol reforming in bubbles immersed in liquids

Abstract: Ethanol reforming in non-thermal plasma generated in atmospheric-pressure argon bubbles immersed in liquid ethanol/water solution is studied using a self-consistent multi-species fluid model. The influence of the dielectric constant of the liquid on the plasma dynamics and its effect on the generation of active species is analyzed. Several modes of discharge are obtained for large liquid dielectric constant. In these modes, we obtain either an axial streamer or a combination of two simultaneous streamers propagating along the bubble axis and near the liquid wall. The influence of these modes on the production of active species is also studied. The main reactions responsible for the generation of molecular hydrogen and light hydrocarbon species are analyzed. A possible mechanism of hydrogen generation in liquid phase is discussed.

Breakdown voltage for surface dielectric barrier discharge ignition in atmospheric air

Abstract: Breakdown voltage for surface dielectric barrier discharge ignition in atmospheric air has been measured for different parameters of the dielectric barrier, exposed electrode mounting, and supply voltage frequency. The analytical model of the discharge onset has been developed. It reveals that the discharge breakdown voltage is proportional to the square root of the dielectric barrier thickness and weakly depends on dielectric relative permittivity. The trajectory of discharge breakdown is managed mainly by gas properties and its density; it does not depend on dielectric barrier parameters and the breakdown voltage value. The analytical solution is in good agreement with the obtained experimental data and indicates the validity of the 2D approach for the breakdown process in the case of discharge between the strip and flat electrodes separated by a dielectric barrier.

Discharge characteristics and mechanism of conductive metal particles in function of the oil flow under AC voltages

Abstract: To obtain the characteristics and mechanism of partial discharge caused by metal particles in function of the oil flow under AC voltages, the author established a mechanical model of metal particles motion in dielectric liquid. The motion trajectory and distribution regularities of metal particles in flowing transformer oil were simulated and calculated. The discharge phenomenon caused by metal particles was explained based on 4 discharge types and discharge mechanism was investigated. Additionally, Experiments were carried out to research the variation of Partial discharge inception voltage (PDIV) with velocity and temperature caused by metal particles. It was observed that discharge amplitude increased within a little scale and discharge number decreased significantly with the increase of velocity. Rise time increased with the increase of velocity while duration time decreased. It was proved that insulating property of transformer oil which was contaminated with metal particles could be improved by increasing the velocity of transformer oil.

Cavitation in liquid dielectric under nanosecond high-voltage impulse

Abstract: We consider the appearance of a cavitation near a needle-shaped electrode under nanosecond high-voltage impulse. It was shown that the gas–liquid boundary moves in an opposite direction to the needle-shaped electrode under the influence of electric field on this boundary. It leads to an increase of the cavity volume. The comparison with experiment shows that the dependence of dielectric permeability on the electric field strength does not dramatically change the result of the numerical simulation whereas accounting for the liquid compressibility and the choice of the high-voltage electrode size are important. It was shown that the existence of the caviation bubbles do not always lead to electrical breakdown in dielectric liquid through the gas phase.

Influence of Seeding Particle Type on Velocity Measurements in Silicone Oil Under High Voltage

Abstract: For several years, our research group has developed electrohydrodynamic (EHD) actuators able to set dielectric liquid in motion. More precisely, these actuators are designed to produce electroconvective flows up to 1 m/s. These flows are jets (impinging jet, wall jet, etc.) that can be used in various applications such as mixing, flow control, cooling systems, etc. The main advantages of these actuators are direct conversion of electric energy into inertial energy (without moving parts), low cost, and easy miniaturization. EHD pumps could be used in microfluidic applications. Nevertheless, the fundamental physics of these flows remains complex, and in order to have a better understanding of these electroconvective flows, we need a more reliable analysis method. The best method to date is particle image velocimetry. However, in order to show liquid movement, this method requires seeding particles, which can be electrically charged and so on can be influenced by the electric field produced by the electrodes. In this work, we study the behavior of three types of seeding particles under high electric field in order to estimate the error induced by the electric field on the measurement of the liquid velocity.

Thermal model for embedded two-phase liquid cooled microprocessor

Abstract: Chip embedded two phase evaporative cooling is an enabling technology to provide intra-chip cooling of high power chips and interlayer cooling for 3D chip stacks. Utilizing an interconnect-compatible dielectric fluid provides a cooling solution compatible with chip to chip interconnects for future high power 3D chip stacks. However, lack of high fidelity and computationally manageable conjugate thermal models limits the development of this technology. To address that, a thermal model for fast and accurate prediction of thermal and electrical behavior of an embedded two-phase liquid cooled micro-processor module is described in this paper. This model consists of a state-of-the-art conjugate heat transfer model for two-phase flow boiling through chip embedded micron-scale channels and a physics-based empirically tuned electrical model of the microprocessor. Extensive model validation using data from several experiments was performed to quantify the accuracy of this model under different operating conditions (including various chip operating frequencies and coolant mass flow rates). Results showed that this model can predict the electrical behavior as well as two-phase flow and heat transfer characteristics with very good accuracy. Overall, the chip junction temperature predictions were within two degrees of the experimental data and the temperature-dependent chip power predictions were within 10%.

Nanostructured insulation for high torque density electric propulsion motors

Abstract: It has been identified in ONR Next Generation Integrated Power System (NGIPS) Roadmap that fundamental research in dielectric insulation research enables payload efficiency and affordable high power density of integrated electric propulsion motors. The objective of this study is to revolutionize electrical insulation in the manufacturing of NGIPS motors for military electric propulsion with game-changing torque density and payload efficiency through the development of nanostructured insulation with significant improvements in both electrical and thermal performance. This paper presents the progress of nanostructured insulation innovation, high field characterization, performance and insulation integrity validation under high voltage, high frequency multi-stresses. The thermal, dielectric and voltage endurance properties of novel nanocomposite insulation based on 2D-nanostructured platelet fillers were investigated. It was demonstrated that nanostructured insulation could offer significant improvement over conventional insulation system in electrical, dielectric, thermal and mechanical properties. A Design of Experiment was employed to study the effects of various 2D fillers and their interplay, and more importantly to identify the optimal nanostructured formulation with high thermal conductivity of >0.8 W/mK, low dielectric constant of less than 5, low dissipation factor of less than 3% at 150°C and high breakdown strength of >1000V/mil. Furthermore, disk samples with optimal formulation from Design of Experiment were fabricated for voltage endurance tests in accordance with IEC 60343 standard for evaluation of their long-term endurance life.

Dielectric charging research status and prospect

Abstract: Dielectric charging phenomenon, caused by the interaction effect between space environment and spacecraft material, has a severe threat to the normal operation of spacecraft. Especially, the problem becomes more serious with the improvement of spacecraft operation voltage. An overview of dielectric charging research status, exsiting problems and prospect are reviewed. Firstly, dielectric charging phenomenon, hazards, research history and new challenges are introduced. There have five development stages: the initial attention, the focus of surface dielectric charging, the focus of deep dielectric charging, the rapid development of dielectric charging and new challenges and demands. The development demands of spacecraft dielectric in China are analysed. There have several insulation positions, which are easy to occur fault in practice. They include surface flashover on insulation material bewteen solar cells of solar array, surface flashover on insulation material between electrodes of solay array drive assembly, the insulation material in cables and pulse discharge of circuit board. Secondly, the specific research status and exsiting problems of surface dielectric charging and deep dielectric charging are summarized from the following aspects: dielectric charging mechanism, dielectric charging mitigation and numerical simulation. Next, the top issues about dielectric charging experiments and material characteristics are introduced. They include in-situ surface potential decay measurement and characteristics of dielectric radiated by electrons, in-situ space charge measurement and material characteristics under electron radiation, surface flashover characteristics of dielectric radiated by electrons in vacuum. Two kinds of space charge in-situ measurement, short-circuit PEA (Pulsed Electro-Acoustic) and open PEA are introduced. At present, there do not have the mature in-situ space charge measurement setup under electron radiation in China, the difficult points in design and debugging are analysed. It is interesting of the surface flashover of dielectric under electron radiation. The latest experimental results indicated that the flashover voltage shows a distinctly different variation tendency under the two cases (after electron radiation and during electron radiation). The first case of after electron radiation, flashover voltage becomes higher than that without electron radiation. Because of the effect of deposited electrons in dielectric surface layer and the changes of trap distribution inside material. The trap distributions of polyimide after electron radiation are calculated based on surface potential decay measurement and Simmons theory. The results show that the shallow trap level increases slightly with the increase of electron energy (3–11 keV), ranging from 0.81–0.85 eV, while the deep trap level remains unchanged about 0.94 eV; the second case of during electron radiation, flashover voltage exhibited a dramatic drop from 26.9 kV (energy: 5 keV) to 10.5 kV (energy: 30 keV), due to the combined effect of incident kinetic electrons flowing to dielectric surface and deposited electrons in dielectric surface layer. Finally, the key scientific issues in the filed of space dielectric charging are put forward and prospected based on the present research status.

The effects of the dielectric liquid temperature on the hole geometries drilled by electro erosion

Abstract: In this study, electrical discharge hole drilling processes was applied to DIN 1.2379 tool steel by using single-channel brass electrode and dielectric liquid which possess various level of temperature. In the study, the machining parameters were kept constant and the effect of the dielectric liquid temperature on the performance parameters such as material removal rate, electrode wear rate, white layer thickness, surface roughness, overcut in diameter and taper at the tip was examined. The results show that the decrease of dielectric liquid temperature improves performance parameters. The material removal rate was increased nearly 30%, surface roughness value Ra is improved from 4.8 to 2.7 μm, white layer thickness was decreased and the hole geometry is improved in terms of hole diameter and taper.

Fault development upon water ingress in damaged low voltage underground power cables with polymer insulation

Abstract: Developments in the electricity sector, related to integration of sustainable energy, not only affects the medium voltage but also the low voltage infrastructure. Condition monitoring of Low Voltage (LV) underground power cables becomes of increasing interest to safeguard reliable power delivery. Since the start of phasing out oil-paper insulated cables, polymeric insulation is most widely applied. As phenomena associated with high electric field stresses are not expected, little is known about the degradation mechanism of LV cables with polymeric insulation. External damage is known to be a major cause of defects, which directly causes a failure or initiates degradation resulting in a failure later on. To study the degradation mechanisms, short cable sections with artificial damage have been subjected to periodic water exposure in the laboratory. This shows that dry-band arcing is a major mode of degradation, with accumulation of pollutants due to chemical decomposition. Several factors such as water conductivity, presence of oxygen, soil type and insulation material have been investigated. Strong dependencies upon these parameters are observed. However, it may be hard to quantify the degradation process in practical situations. The presence of a water film that can sustain dry-band arcing will occur only in certain situations and for limited amount of time. However, the accumulation of these events, mainly caused by periods of precipitation, will eventually lead to breakdown at the damaged spot.

Complex phenomena study in dielectric fluid from gap during the W-EDM processing in ultrasonic field

Abstract: The current paper is studying several mechanisms for material removal at wire electrical discharge machining (W-EDM), with ultrasonic activated wire electrode. Vibrations with an ultrasonic frequency of the wire electrode have been transmitted to the working environment existent in the gap, influencing the performances of the W-EDM process. The study focuses on dielectric phenomena that go together with a singular electrical discharge initiated between electrodes; it is studied in different stages, as well: the priming of the electrical discharge, the electrical discharge evolution, the physicochemical effects into the gap, as well the material removal. We show that the ultrasonic energy transferred into the dielectric fluid from the gap intensifies the electro-erosive phenomena, beginning with the emergence of the gaseous phase and the initiation of the discharge channel and ending with the evacuation of erosion particles.

Effect of Powder Mixed Dielectric fluid on Surface Integrity by Electrical Discharge Machining of RENE 80

Abstract: This paper mainly focussed to investigate the formation of Recast Layer and surface integrity of RENE 80 Nickel Alloy during powder mixed electrical discharge machining (PMEDM) process. The experiments were conducted considering electrical parameters (pulse on time, peak current and pulse off time) at optimal combination by varying powders like aluminium, graphite and combination of aluminium and graphite powders and their concentrations. The experimental results reveal that reduction in Recast Layer Thickness (RLT) with addition of powders into dielectric fluid. The minimum recast layer thickness (RLT) (14.42 μm) was observed at aluminium powder concentration of 9 g/l added in to the dielectric fluid. It was observed from the surface topography using scanning electronic microscope (SEM) micrographs that, with increase of powder concentration into the dielectric medium, increasing in crater depth and diameter. Thick appendages were found around the periphery of crater may be due to resolidified molten metal. The presence of oxygen and carbon elements was observed on machined surface from the EDAX analysis.

Numerical simulation of circular Couette flow under a radial thermo-electric body force

Abstract: The transition from circular Couette flow of a dielectric fluid with a radial temperature gradient and an alternating electric voltage has been investigated by a direct numerical simulation. The inner cylinder is rotating while the outer one is fixed. The radial temperature gradient and the electric voltage acting on a dielectric fluid generate a radial dielectrophoretic (DEP) force which can induce thermal convection. The flow is controlled by the Taylor number Ta that measures the intensity of the centrifugal force and the electric Rayleigh number L that indicates the intensity of the DEP force. For each value of L, the instability threshold (Tac) is determined and compared with that predicted by the linear stability analysis. Nonlinear coefficients of the Landau equation are computed to reveal the nature of the transition: oscillatory axisymmetric modes occur via a subcritical transition, while steady axisymmetric and oscillatory non-axisymmetric modes occur via supercritical bifurcations. The momentum...

Thermal analysis of natural esters in a low-voltage disc-type winding of a power transformer

Abstract: This work compares the temperature distribution and hot-spot temperatures obtained in a disc-type winding of a power transformer when using as dielectric liquid a mineral oil or natural esters. The comparison is made with a reference case for mass flow rate and temperature in the inlet and uniform losses for the discs. A further comparison is performed by increasing the mass flow rate at the inlet form 0.78 kg/s to 0.9 and 1.0 kg/s, thus nine case studies are considered in this work. These cases have been analyzed via CFD techniques using the software COMSOL Multiphysics® with a 2D-axysymmetrical model using the Conjugate Heat Transfer module. For the analysis, the hotspot factor H is considered as an indicator of the cooling circuit efficiency since the losses are uniform. Results shows that for the base case, the hot-spot temperature obtained for the mineral oil is 9–11°C higher than the obtained with esters whereas for the increased mass flow rate, hot-spot temperature of mineral oil is equal or even lower than the obtained for natural esters. The analysis of the hot-spot factor shows the dependence of the cooling circuit efficiency on the kind of oil and inlet conditions.