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This article presents the results of an experiment designed to study creeping discharges on a solid insulating surface immersed in palm kernel oil methyl ester (PKOME) compared with those produced from mineral oil (MO). The investigation takes place under standard negative lightning impulse voltage application, and for the three following solid insulating surfaces: hardened glass (HG), porcelain (PO), and simple glass (SG). The distributions of the radial discharge occupancy densities as a function of the thickness of the solid material and the insulating liquid are evaluated. A statistical study of the total and final length of the branches produced by the discharges is presented, as well as the current amplitudes and the charge produced by this phenomenon. A fractal analysis of the phenomenon is also presented. The experimental results show that the distribution of the average radial density of the discharge, the total and final lengths of the branches, and fractal dimension depend strongly on the physicochemical characteristics of the insulation system. [ABSTRACT FROM AUTHOR]

The lightning impulse breakdown strength of natural ester is lower than that of mineral oil. This article focuses on a discharge mechanism difference analysis of natural ester and mineral oil under a strong impulse electric field (EF). Based on density functional theory (DFT), the ionization energy, electron affinity (EA), and excitation energy of natural ester and mineral oil were calculated under different EF strengths. The microparameters related to the discharge of natural ester and mineral oil were compared, showing that the former has a lower ionization energy and a larger dipole moment. This quality makes natural ester more likely to ionize and produce electrons, which more easily promotes the formation of electron avalanches and streamer. Meanwhile, natural ester molecules are more prone to transition from the ground state to the excited state than mineral oil molecules, exacerbating the photon release and formation of discharges in natural ester. The greater conductivity of natural ester leads to its dispersion and the rapid development of discharge streamers. In addition, an electrostatic potential (ESP) analysis on molecular surfaces was used to predict the active sites of the oil molecular structure. This article provides help for understanding the relationship between the micromolecular structure and the macroscopic discharge characteristics for different types of oil molecules. [ABSTRACT FROM AUTHOR]

The latest 2011 edition of the IEEE C57.100 standard introduced a new sealed tube accelerated aging test (STAAT) procedure and methodology for evaluating the thermal index (TI) of liquid-immersed transformer insulation systems. The application of this standard becomes increasingly important, as new insulating materials (both solid and liquid) have been developed, and their thermal performance shall be qualified. The experimental work presented in this article investigated the effects of different test parameters on the aging test results and data interpretation. The new findings on the dependence of the aging process on the ratio and type of materials in the system, as well as on the selected aging criteria provide a better understanding of the accelerated aging and interpretation of the data along with practical recommendations for the improvement of the test procedure. In parallel, for the first time, the TI of the Transformerboard, as a component of the so-called industry-proven insulation system, was investigated and defined based on the standard industry test. The results confirmed that the Transformerboard’s thermal class is, at least, 120 °C; this validates an empirically based application of this material as a part of the 120 °C class insulation system in both power and distribution transformers. [ABSTRACT FROM AUTHOR]

Insulating liquids (transformer oils) are dielectrics used in a wide range of electrical equipment and provide a medium for both insulation and cooling. During equipment operation, liquids are subjected to electrical and thermal stresses. With continued use, they chemically degrade and produce degradation products and aging markers. In this study, models based on Fourier-transform infrared spectroscopic (FTIR) measurements of liquids are proposed for estimating insulating liquid properties (acidity, interfacial tension (IFT), and density) using only a single measurement combined with spectral data analysis. Estimation models based on artificial neural networks (ANN) and partial least squares (PLS) were developed through training and validation on approximately 850 samples of mineral insulating liquids. The proposed models provide an effective means for estimating the acidity, IFT, and density of mineral insulating liquids. The models provide estimation results comparable in reproducibility to standardized laboratory analyses, provide the means for a rapid and accurate assessment of the condition of the insulating liquid, as well as allow the design of dedicated sensors to perform these analyses online. [ABSTRACT FROM AUTHOR]

Liquid dielectrics filled with nanoparticles have drawn huge attentions due to enhanced dielectric and thermal properties. Despite of great achievements have been made, the charging mechanisms of nanoparticles suspended in liquid dielectrics are still open questions. It is previously argued that field charging with Maxwell–Wagner relaxation fails to explain the enhanced breakdown strength when nanoparticles have longer relaxation time constant than the timescale of streamer development in liquid dielectrics. However, it is proven in this article when the condition that the charging time constant $\tau _{\mathrm {pc}} \ll $ the charging time $t_{\mathrm {pc}}$ is fulfilled, nanoparticles can acquire significant number of charges even if $t_{\mathrm {pc}} \ll $ relaxation time constant $\tau _{\mathrm {MW}}$. It is also showed that the field charging models for nanoparticles suspended in gases and liquids share the same formulation and thus can be unified. In addition, diffusion charging is proposed to be one of the charging mechanisms for nanoparticles in liquid dielectrics, and Fuchs’s model is extended from aerosol science to liquid dielectrics. It is demonstrated the diffusion charging could be far more important than field charging in liquid dielectrics. This work may bridge the gaps of charging mechanisms and models between gaseous and liquid dielectrics. [ABSTRACT FROM AUTHOR]

This article presents the comparison of the dielectric properties of three different refrigerant fluids, HFE-7100, HFC-245fa, and HFO-1336mzz(E) (R-1336E Opteon ME), the latter having a global warming potential of two orders of magnitude lower than the other two. Refrigerant fluids are used, among others, in generator circuit breaker applications where good thermal and chemical properties must be coupled with excellent dielectric strength and low electric conductivity. This article describes in detail the applied measurement techniques and the results obtained, as well as comparisons to previous measurements and literature. The comparison includes dielectric breakdown in liquid and vapor phase, dc conductivity, real permittivity, and dissipation factor. [ABSTRACT FROM AUTHOR]

The accurate measurement of complex liquid permittivity in a frequency range provides important information on liquid properties in comparison with single frequency permittivity investigations. The multifrequency microwave characterization technique based on a single quartz whispering-gallery-mode (WGM) resonator with a microfluidic chip is proposed, developed, and demonstrated. This technique allows the complex permittivity of small volumes of liquid to be measured at six resonant frequencies in the 30–40-GHz frequency range. The calibration is performed by simulating the measurement cell and by plotting the calibration nomogram charts for all six investigated frequencies. The novel approach is applied in studies of the complex permittivity of the L-lysine in water solutions. The results open up the possibilities to investigate the complex permittivity of biological liquids at several frequencies and to further develop the microwave dielectrometry of small liquid volumes in a certain frequency range using the quasi-optical nature of a single high- $Q$ WGM resonator. [ABSTRACT FROM AUTHOR]

In the last 20 years the use of ester fluids as an alternative to mineral oil for transformer insulation has been an active field of research and development. These liquids have a much lower environmental impact than mineral oils, besides reducing the transformer’s fire risk. Although the use of natural and synthetic esters is nowadays frequent for certain applications, as for distribution transformers in highly populated areas, railway transformers or off-shore windmill transformers, the experience on large and medium sized units is still reduced. One of the critical aspects that must be assessed to use esters as dielectric fluids for large size transformers is the dielectric design of the equipment. The permittivity of esters and ester-impregnated cellulose are different from those of mineral-oil-cellulose systems, what has an impact on the electric field distribution in the transformer. Additionally, in some cases, the dielectric strength of ester fluids differs from that of mineral oil. This paper presents a study about the dielectric design of ester-based power transformers. The insulation system of a 400/138 kV, 280 MVA transformer was modelled using a finite element tool and a comparison of the field distribution under 50 Hz AC power voltage of both insulation systems is provided. [ABSTRACT FROM AUTHOR]

Impulse waveforms have been widely used in the studies of streamer and breakdown phenomena in dielectric liquids. In this article, both experiments and analyses were carried out to investigate the effects of impulse tail-time on streamer and breakdown characteristics of a mineral oil and a synthetic ester liquid. A range of positive impulse waveforms with the same front-time of $0.8~ \boldsymbol {\mu } \textrm {s}$ but different tail-times from 8 to $3200~ \boldsymbol {\mu } \textrm {s}$ were considered. Needle-plane electrodes with a needle tip radius of 10 $\boldsymbol {\mu } \textrm {m} $ and a gap of 10 mm were used. It was found that reduction of tail-time leads to an increase of 50% impulse breakdown voltage, while the instant breakdown voltage and the time-to-breakdown remain constant. On this basis, an analytical method is developed to estimate the 50% impulse breakdown voltage under waveforms with various tail-times. The method is valid when streamers remain of second mode and in divergent fields. It was also observed that some “self-regulation” by branching explains the constancy of streamer propagation velocity when the applied voltage is increased. [ABSTRACT FROM AUTHOR]

Nowadays, new alternative of dielectric liquids with biodegradable characteristics has been considered to replace conventional mineral oil in both distribution and power transformer applications. However, there are some manufacturing processes that need to be modified to incorporate these ecological fluids, mainly in power transformers. Ester liquids, either natural or synthetic, show higher viscosity than mineral oil; therefore, a significant impact on the impregnation process of insulation materials is expected. In this work, the impregnation process of cellulose insulation is investigated by using three different liquids: natural ester (NE), synthetic ester (SE), and mineral oil. Laboratory tests were carried out using insulation blocks and pressboard samples to evaluate the influence of each fluid in the cellulose material. It is shown that the impregnation process with ester liquids is longer than mineral oil, at room temperature; however, it could be reduced by increasing the temperature of the ester liquid. [ABSTRACT FROM AUTHOR]

Due to the growing interest in environmental concerns, synthetic and natural esters have become the key focus of the picture in the last two decades as insulating fluids for high-voltage equipment. This is because, unlike mineral oil, ester liquids are biodegradable, nontoxic, and safe for the environment and human health. These fluids are derived from renewable sources and have high fire resistance. However, synthetic and natural ester fluids are still used in a rather limited number of electrical equipment for several reasons (high cost, high viscosity and density, increased tendency toward electrostatic charge, faster propagation of streamers in an inhomogeneous electric field, etc.). Besides, a huge amount of equipment is filled with mineral oils. The massive replacement of oil-filled equipment with natural or synthetic esters may be extremely expensive to utilities and transformer owners. In addition, in the event of partial or complete retrofilling of mineral oil with an ester fluid, the equipment may evidently contain a mixture of two insulating fluids, and therefore, demanding the scope for research on mixed insulating liquids. The intent of this article is to present a comprehensive review of the literature on the blend of mineral oil and other alternative dielectric fluids. The critical research progresses, highlights, and challenges related to mixed liquids along with significant tutorial elements as well as some analyses are discussed. This review should be useful for researchers, utilities, and transformer owners concerned with ester liquids and retrofilling aspects. [ABSTRACT FROM AUTHOR]

This article reports the static electrification of ester nanofluids for its use in transformers adopting the spinning disk model. The results show that the streaming current is higher for the nanofluid involving surfactant compared to its effect without surfactant. The streaming current observed is negative at the fluid–metal interface and positive at the fluid/pressboard interface. The streamline pattern along with surface velocity magnitude is simulated under different disk velocities using COMSOL and the convection current model is formulated for the identification of streaming current. The mass transfer rate with surfactant-based nanofluid shows a shift from transport regime to reaction limited regime at higher Reynolds number. Furthermore, the interfacial zone of nanofluids is discussed for the streaming behavior considering its influence with the addition of benzotriazole (BTA). [ABSTRACT FROM AUTHOR]

The insulation system in a transformer degrades over time and it needs periodic monitoring for uninterrupted operation of the power system network. Moisture content (MC), acid number, and dissolved gases are the important parameters that establish the aging and degradation status of the transformer liquid insulation. This work focuses on using high-frequency signals nonintrusively for estimating the degradation of the transformer liquid insulation by considering the MC and acid content. A novel nonintrusive and nondestructive testing method using an ${S}$ -band pyramidal horn antenna is introduced in this work to estimate the degradation of the liquid insulation with respect to aging duration. The scattering parameter ${S}_{{11}}$ of the liquid insulation is first characterized and measured in an anechoic chamber to avoid electromagnetic interference (EMI). These tests are performed initially on laboratory-aged mineral oil (MO) samples and a prediction curve of $\vert {S}_{{11}}\vert $ as a function of moisture and acid number is obtained. $\vert {S}_{{11}}\vert $ of the in-service transformer oil samples is then compared with the prediction curve and the degradation status is predicted. It is found that the chosen frequency band is able to differentiate the aging status of MO accurately. This technique can be used for establishing the degradation status of any type of liquid insulation. A miniature version of the proposed technique can be used for portable and faster condition assessment of insulating oils. [ABSTRACT FROM AUTHOR]

The article concerns a method of drying cellulose insulation of distribution transformers with the use of synthetic ester. This method uses the high solubility of water in the ester compared to other dielectric liquids. The method is based on the striving of the oil-paper insulation system for a state of moisture equilibrium. Water migrates from the cellulose insulation of the high moisture content to the synthetic ester which is subjected to continuous drying. The research was carried out on a complex laboratory model reflecting the transformer insulation system. In this model, the drying of the ester was carried out using a molecular sieve with an appropriately selected adsorber weight. To check the drying efficiency of the cellulose insulation, the water content in the pressboard strips taken from the model, both before and after the drying process, was determined. The water content was measured using the Karl Fischer titration method. The research showed the high efficiency of the proposed method of drying. With the ester moisture in the range of 110-130 ppm and the insulation system temperature of about 70 °C, the loss of water in the samples dried for 7 days was over one percentage point. The obtained test results constitute the basis for the validation of the method on distribution transformers.

The use of solubility coefficients for the measurement and evaluation of gas-in-oil analysis is critically discussed. A procedure for the relative calibration of headspace equipment for non-mineral insulating liquids, based on a known mineral oil calibration, is proposed. [ABSTRACT FROM AUTHOR]

Standards relevant to silicone liquids used in cable sealing ends from the standpoint of theoretical and empirical research are discussed. This article aims to identify and suggest reforms based on often-unnoticed observations. [ABSTRACT FROM AUTHOR]

Natural esters have the capability to enhance the life expectancy of the insulating cellulose paper used in oil-filled transformers. This work presents the aging behavior of an insulating paper when it is combined with a new natural ester (based on soybean oil) and compared with an industry-proven system according to the latest version of the standard IEEE Std. C57.100 (2011). The temperatures for the candidate system (natural ester + cellulose paper) are 160, 175, and 190 °C. Experimental results show that the average mechanical tensile retention from the industry-proven system aging system is 44.2%, which is used as reference to evaluate the candidate system. According to this investigation, the insulation system with the new natural ester liquid shows an improvement in the behavior of the aging performance compared to the mineral oil system. The Arrhenius life plots demonstrate that the system of natural ester-thermally upgraded Kraft paper shows a life expectancy higher than for the industry-proven system. Therefore, this alternative could be considered either to increase the lifetime of a transformer or to improve its overload capacity during operation. [ABSTRACT FROM AUTHOR]

The electric field uniformity imposes a significant effect on the breakdown mechanisms of dielectric liquids. In general, breakdown is dominated by streamer initiation under uniform field, while it is dominated by streamer propagation under non-uniform field. This paper presents a study on identifying and quantifying the transition field factor boundary, represented by the field enhancement factor ƒ, between the two dominating breakdown mechanisms. The methods for deducing the transition field factor boundary are introduced. The first method from our previous study was developed by measuring and processing both the streamer initiation and breakdown voltages. In this paper, a new method is developed by only processing the breakdown voltage data. Based on the breakdown results reproduced from the literature, the transition field factor boundaries are derived for the mineral oil under both positive and negative lightning impulses. It is found that the negative transition field factor boundary is lower than the positive one. A four-region model is proposed to describe the effect of impulse polarity on breakdown mechanisms of dielectric liquids and to interpret the transition of breakdown mechanisms with respect to the field uniformity. [ABSTRACT FROM AUTHOR]

The article elucidates the necessity for testing of material compatibility and the consequences when this is not done correctly. Recommendations for compatibility testing procedures on the basis of long-term experience with practical examples of their implementation are described. [ABSTRACT FROM AUTHOR]

A new compact cable design was investigated using only liquid nitrogen (LN2), general used as coolant of superconductors, as sole electric insulation. First, it was analyzed which conductor arrangement is better suited in terms of maximum electric field strength to withstand the applied voltage of 123 kV. It was found that a three-phase single conductor arrangement with a maximum electric field strength of 10.5 kV/mm is more suitable than a triaxial arrangement with a maximum field strength of 16.5 kV/mm. Furthermore, a corresponding supporting insulator was developed to provide the highest possible mechanical and electrical withstand. A major focus was on selecting the most appropriate material and its permittivity in combination with the surrounding LN2 (ϵr = 1.43) to reduce the electric field strength to its possible minimum. The best relative permittivity for the supporting insulator is in the range between 1.4 and 3.4. [ABSTRACT FROM AUTHOR]

The gases generated by electrical and thermal faults are similar in the case of mineral oil, synthetic ester, and natural ester. However, there are considerable differences in the amount of the generated gases and their solubility in these liquids, what should be taken into account during power transformer diagnostics by means of dissolved gas analysis (DGA) method. The aim of the research was to compare the gases generated as a result of thermal faults (250°C, 350°C, and 450°C) in mineral oil and both esters. The following gases were analyzed: hydrogen, methane, ethane, ethylene, acetylene, propane, propylene, carbon oxide, and carbon dioxide. A qualitative and quantitative analysis of these gases was carried out by means of gas chromatography. A large share of propane in the sum of the gases generated in the esters, mainly in the synthetic ester, was found. Propane and propylene in most of the gas interpretation approaches are omitted. The authors proved that in the case of synthetic ester, using propane as the key gas for thermal faults detection is justified. The obtained results can be used to improve the interpretation of the DGA results, in the case of ester liquids used in power transformers.

This paper presents an experimental study focusing on the quantitative analyses of spatial characteristics of Mach-cone shaped shockwaves generated by streamers in a mineral oil and a synthetic ester in a fixed 12 mm needle-plane gap under positive lightning impulse. The influences of voltage level and liquid type are investigated. Results showed that Mach-cone shaped shockwaves are always observed ahead of actively propagating streamer channels. In the mineral oil, instantaneous streamer velocities undergo initial bursts before gradually decreasing and stabilizing at around 2 km/s. Shockwave pressures decrease from 6–8 GPa upon the streamer initiation to around 4 GPa as streamer propagates to a longer distance. Compared to the mineral oil, the initial bursts of instantaneous streamer velocities and shockwave pressures are lower in the synthetic ester. Investigated voltage levels from initiation to near breakdown do not impose significant effects on the observed shockwave parameters. In addition, the appearance of Mach-cone shaped shockwave is found to be an effective indicator of actively propagating streamer channel which can be used as a tool to quantify streamer main branches. [ABSTRACT FROM AUTHOR]

This manuscript critically reviews recent research on electrical insulation materials and systems used in power electronics devices and focuses on electrical treeing in silicone gel, PD modeling, and mitigation methods. For mitigation methods, electric field grading techniques, such as 1) various geometrical techniques, and 2) applying nonlinear dielectrics are discussed. Alternatives for silicone gel, such as liquid dielectrics, are also highlighted. The drawbacks of reported research and technical gaps are identified. In particular, we show that the investigations carried out to date are in their infancy regarding the working conditions targeted for next-generation WBG power devices. This review will provide a useful framework and point of reference for future research. [ABSTRACT FROM AUTHOR]

This article reports and discusses the results of impulse breakdown study of different insulating liquids under highly divergent electric field conditions. Samples of a natural ester (Envirotemp FR3), a synthetic ester (MIDEL 7131), and a naphthenic mineral oil (Shell Diala S4 ZX) at different levels of relative humidity were exposed to HV impulses with a nominal 7- $\mu \text{s}$ rise time and 150-kV peak voltage of both positive and negative polarity. A strong dependence of the breakdown voltage and time to breakdown of the investigated dielectric liquids was observed with respect to the polarity of the applied HV impulses. It was shown that the FR3 natural ester liquid has a higher dielectric strength when exposed to positive impulse than when under negative impulse stress. The opposite breakdown behavior was observed for the synthetic ester MIDEL 7131 and the naphthenic oil Shell Diala S4 ZX which exhibited lower breakdown voltage under positive energization as compared with the negative energization. The breakdown voltage and prebreakdown time obtained in the present tests of the naphthenic oil, Shell Diala S4 ZX, lie between that of the ester liquids irrespective of impulse polarity. It has been established that no statistically significant variations exist in the breakdown parameters (breakdown voltage and time to breakdown) of the studied dielectric liquids as their relative humidity is increased under either impulse polarity. The obtained results will help in coordination of practical applications of low environmental impact dielectric fluids in power and pulsed power systems and components. [ABSTRACT FROM AUTHOR]

Liquid dielectrics are widely used in high-voltage transformers owing to their excellent insulation properties. Space charge injected from electrodes can damage power systems. It is believed that space charge dynamics are related to the electrodes surface closely. In this study, surface modifications of copper and stainless-steel electrodes deposited by SiO2 and TiO2 nanostructured films with radio frequency magnetron sputtering were investigated. The films were characterized. The effects of the films on the breakdown voltage and space charge dynamics characteristics of the propylene carbonate were studied. These results indicate that when a TiO2 nanostructured film was deposited on the stainless-steel electrode, the breakdown voltage of the propylene carbonate increased by 7.3%, and when SiO2 and TiO2 films were deposited on the copper electrode, the increases were 7.4% and 10.2%, respectively. Furthermore, the amount of charge injection of the stainless-steel electrode was lower than that of the copper electrode, and the nanostructured films markedly suppressed the space charge. Both the TiO2 and SiO2 nanostructured films sputtered on the electrode surface changed the barrier height of the solid–liquid interface and reduced the depth and amount of charge injection. Compared with a sputtered SiO2 film, the TiO2 film had a better space charge suppression effect. [ABSTRACT FROM AUTHOR]

It is generally recognized that dissolved gas analysis (DGA) is a powerful tool in the diagnosis of insulating liquid filled power equipment. There exist numerous evaluating schemes for the mineral oil filled equipment, nevertheless, the experiences show that a fine tuning and revision of the schemes is required for equipment filled with synthetic esters. [ABSTRACT FROM AUTHOR]

This article deals with the application of synthetic ester for drying cellulose insulation of distribution transformers. The most important advantages and disadvantages of the commonly used methods of drying transformer insulation on site are discussed. The emphasis is placed on the analysis and research of the most important factors affecting the drying efficiency of cellulose insulation using a synthetic ester, such as: insulation temperature, water content in ester, weight ratio of cellulose materials to ester, thickness of pressboard, and water content in solid insulation. The choice of values range of the individual factors which are used during tests on drying efficiency in laboratory conditions are justified. It is found on the basis of the obtained test results that to ensure the satisfactory drying effect an average insulation temperature should be at least of 70°C and the water content in the ester should not exceed 150 ppm. The results give the opportunity to estimate the drying efficiency depending on the conditions of the process. Moreover, the results are used to develop the procedure of drying of distribution transformers by means of synthetic ester. [ABSTRACT FROM AUTHOR]

The weak nonlinear dynamics of the free surface of a dielectric liquid in an electric field directed tangentially to the unperturbed boundary is investigated numerically. Within the framework of the strong field model, when the effects of capillarity and gravity are not taken into account, it is shown that nonlinear surface waves have a tendency to break. The collapse of the surface waves, the curvature of the boundary and the gradient of the local electric field undergo infinite discontinuity on the surface of the liquid (the angles of the boundary inclination remain small). The characteristic collapse time of a surface wave traveling in a given direction is calculated versus the dielectric constant of the liquid. It is shown that the time of the singularity formation increases infinitely at small and high values of the dielectric constant. The first case corresponds to the transition of the system to the neutral stability regime (the jump in electrostatic pressure at the boundary turns into zero). At high values of the dielectric constant of the liquid, the collapse time also increases. This effect is associated with the realization of a special regime of fluid motion, in which the propagation of nonlinear surface waves of an arbitrary configuration occurs without distortions. For the liquids with relative permittivity close to five, the wave breaking time reaches a minimum, i.e., the collapse of the surface waves for such liquids occurs most intensively. [ABSTRACT FROM AUTHOR]

The internal pressures in the insulation of two 5-m long pieces of a state-of-the art mass-impregnated non-draining subsea cable have been measured under load current cycling at ambient temperatures ranging from 3 to 37 °C. Thermal expansion of the mass gives in some cases rather extreme internal pressures (>30 bar) and radial pressure gradients (>1.5 bar/mm) in the insulation. This leads to an outwardly directed flow of mass during loading. After a load turn-off, the pressure drops rapidly. Values below 100 mbar were measured in the inner parts of the insulation. The backflow of mass proceeds much slower because the pressure gradient now is smaller. Such a radial redistribution of the mass and the low pressures are assumed to influence the dielectric properties of the insulation as the risk of creating harmful shrinkage cavities is believed to increase. For all the investigated ambient temperature levels the internal pressure in the insulation became around 1 bar or less when isothermal conditions were reached after the load turn-offs. This demonstrates that thermal expansion and contraction of the mass are not the sole mechanisms determining the internal pressures in such cables. Other, still unidentified phenomena are also contributing. [ABSTRACT FROM AUTHOR]

Several authors have suggested that one of the main differences between conventional insulating liquids and nanodielectric fluids is the behavior of the water in the nanodielectric fluid. It has been reported that the impact of water on the dielectric strength is reduced when nanoparticles are dispersed in the fluid, and that nanodielectric fluids present higher breakdown voltage values than the base liquids under similar moisture content. The second part of this two-part paper analyzes the effect of the presence of water on the AC breakdown voltage of several nanodielectric fluids prepared from mineral oil and Fe 3 O 4 nanoparticles in different concentrations. The fluids were conditioned to a range of moisture contents and tested under different conditions. An exhaustive measuring campaign was carried out that was completed with a non-parametric and a parametric statistical analysis of the results. As will be shown in the paper, an increase on the solubility of nanodielectric fluids has been observed which is related with the enhancement of the breakdown voltage of these liquids. The evolution of the breakdown voltage was analyzed both for fluids with similar absolute moisture contents and also for fluids with similar relative saturation conditions. [ABSTRACT FROM AUTHOR]

During the last 10–15 years, there has been an increasingly growing number of reports on the so-called stray gassing of mineral insulating oils. Some contributions suggested that the problem may be connected with introducing new types of insulating oils [1] and mainly associated with evolution of hydrogen, as also confirmed by transformer oil producers [2]. Oil passivation has also been reported to contribute to stray gassing, being used as a remedy for sulfur corrosion problems [3]. We exclude from further consideration gassing due to material incompatibilities, e.g., outgassing of paints, cross-linked polyethylene (XLPE) or other materials, as it remains the transformer manufacturer's duty to develop test methods for avoiding this problem. [ABSTRACT FROM AUTHOR]

Level sensors find use in various applications for liquid-level monitoring in tanks, containers, and vessels. In this paper, an impedance spectroscopy-based sensing platform has been reported, employing a co-axial sensing probe. The supporting electronics was developed using the MSP 432 microcontroller unit (MCU) from Texas Instruments (TI) and a newly designed Impedance Analyzer-Analog Front End (IA-AFE), also developed at TI. Different signal processing and data processing methods were employed for coaxial probe impedance modeling and their results discussed. The sensor can be used for conductive as well as non-conductive liquids, including liquids with low dielectric constant. Successful measurements of liquid levels were shown for water (${\epsilon \sim 80}$) as well as vegetable oil (${\epsilon \sim 3}$). The developed sensing platform can measure water level with the accuracy of about 1.2 mm across a 400-mm-long sensing probe while exhibiting a maximum error under 0.6 mm for oil over 150-mm range. The proposed level sensing system provides an accurate and economical solution for level sensing, which is cheaper than the commercially available level sensors while outperforming the accuracy compared with the present state of the art, to the best of our knowledge. [ABSTRACT FROM AUTHOR]

Studies of biological solutions require high measurement accuracy, the ability to detect low changes in substance concentration, and small amounts of the liquid under test. A microwave complex permittivity measurement technique based on a high-quality-factor whispering-gallery-mode resonator with a microfluidic chip allows small amounts of dielectric liquids to be investigated with high accuracy. An existing technique based on a sapphire resonator does not provide for a low-concentration detection limit for substances with low molecular weight. Here, we present an advanced technique based on a quartz dielectric resonator. The detection limit obtained for glucose in water solution was found to be about one order of magnitude lower for the quartz resonator cell than for the sapphire-resonator-based measurement cell. The limit is about or lower than the concentration of glucose in human blood. This fact means that the technique can be used as the sensor for investigations of biological solutions in the microwave range. A study of glucose, lactalbumin, and bovine serum albumin was successfully performed in the $Ka$ -band using the developed sensor. [ABSTRACT FROM AUTHOR]

This paper presents an improved method for the measurement of the conductivity of a liquid. In the proffered technique, the contact-type potential terminals of a traditional four lead conductivity probe are replaced with non-contacting terminals thus preventing electrode corrosion and contamination affecting the conductivity measurement. An additional guard electrode introduced in the present design ensures that the current through the external/surrounding medium becomes negligible thus reducing the error due to current flow, external to the probe. A self-balancing signal conditioning circuit, specially designed to suit the probe, alleviates the new problems of the modified probe. Simulation results and the results obtained from a prototype built and tested establish the efficacy of the proposed technique. The worst case error was found to be ± 0.78% with the prototype probe. [ABSTRACT FROM AUTHOR]

Liquid nitrogen (LN2) is one of the most effective insulation medium for the realization of a new and environment friendly power switchgear system. This novel power switchgear will be a promising alternative to the gas insulated switchgear system. Preliminary study about arc phenomenon is very important for the future design of this novel approach. The fundamental research on high-density arcs was demonstrated using LN2 for the very first time and some important first-hand investigations were previously observed. These investigations were based on electrodes that were geometrically symmetrical. The thorough study of arc behavior in LN2, also demands the asymmetrical analysis be done in the same manner. These new results would further enrich our understanding of arcs in LN2 and would be helpful for the future design of liquid nitrogen insulated metal enclosed switchgear (LNIS). The objective of this research is to study the arcing behavior in LN2 using asymmetrical electrodes. The research investigates the arcing times, arc energies, LN2 consumption rates, temperature difference after arc occurrence, and surface erosion of the electrodes due to arcing. Experiments were done at four different dc voltage levels (i.e., 200–500, with a 100 V step) and arcs were intentionally created on asymmetrical electrodes with different electrode arrangements and materials, respectively. The geometrical arrangements for the experiments are sphere-plane and rod-plane, whereas the electrode materials are aluminum alloy (Al6061), pure copper (Cu), and stainless used steel (SUS) type, respectively. Arcing times, residual voltages, arc voltages, and arc currents were measured using different apparatuses. The results are quite in agreement with the previous investigations done by the first author. LN2 has the shortest arcing time in comparison to air insulation and sphere-plane quenches arcs quicker than rod-plane type electrodes in most of the cases. It was also observed that LN2 consumption due to arcing is quite lower even at heavy interrupting currents and overall difference in temperature due to arcing is approximately zero when the volume of LN2 is more than 100 L. [ABSTRACT FROM AUTHOR]

Electrospraying has found multiple applications in various fields of engineering, science, and everyday life. This paper presents the study of behavior of electrosprayed dielectric liquids, such as gasoline or diesel fuels, in high-pressure environments. Computer models were created and utilized to evaluate the use of electric fields in spatial and temporal manipulation of sprayed electrostatically charged droplets. The focus of the study was to investigate the feasibility of such concept in cylindrical geometries, where the target application of this technique is to improve fuel economy and reduce harmful exhaust emissions of internal combustion engines. [ABSTRACT FROM AUTHOR]

The paper presents the results of investigations on the thermal properties of cellulose and aramid paper impregnated with various liquid insulations. The liquid insulations investigated were mineral oil, synthetic ester, and natural ester. The heat transfer coefficient of the liquid was calculated on the basis of measurements of the thermal expansion, specific heat, density, thermal conductivity and kinematic viscosity of the liquid insulations. Temperatures in the paper and liquid insulations were calculated. The results allow to select the best paper and liquid insulations which provide the most effective cooling in transformers. [ABSTRACT FROM AUTHOR]

As vegetable insulating liquids have different chemical compositions from mineral oils, dissolved gas characteristics under thermal and electrical faults have differences; thus fault diagnostic methods based on dissolved gas analysis (DGA) used for mineral oil-filled transformers may not be suitable to vegetable oil-filled transformers. This paper presents experimental results of dissolved gases in Camellia insulating liquid under typical thermal and electrical faults in transformers. Thermal fault simulation experiments include three parts, which are low temperature, medium temperature and high temperature faults, and the electrical fault simulation experiments also include three parts, which are partial discharge, breakdown discharge and arc discharge. Experimental results show that apart from CO and CO 2 , H 2 and C 2 H 6 are the main characteristic gases under thermal faults, and the percentage of H 2 decreases while CH 4 , C 2 H 4 and C 2 H 6 increases with temperature. In addition, H 2 and CH 4 are the main characteristic gases under partial discharge faults; C2H2 is the main characteristic gas under breakdown faults, and C2H2 and C2H4 are the main characteristic gases under arc discharge faults. Furthermore, graphic fault diagnostic methods based on DGA of Camellia insulating liquid including Triangle and Pentagon diagnostic methods are proposed. Compared with the current diagnostic method, the proposed methods in this paper have higher diagnostic accuracy. [ABSTRACT FROM AUTHOR]

In this paper the behaviour of the waves generated on insulating liquid-air interfaces and subjected to an electric field is experimentally studied. The experimental device is mainly composed of a tank filled with diesel oil and two parallel metallic electrodes; one of them immersed in the liquid and brought to a high potential and the other one placed above the liquid. The waves are created by a vibrator the frequency and amplitude of which are controlled. The electric charges present at the liquid/air interface come from injection and/or dissociation and/or conduction phenomena. A high speed camera records the shape of the waves and an associated image analysis system extracts from them all the information needed (frequency, amplitude, wavelength). This information is used to investigate the influence of the electric field and liquid conductivity on the wavelengths at different amplitudes and frequencies and to calculate the surface tension. The propagation of waves on electrically charged liquid/air interfaces is also studied. The measurements are carried out for different frequencies and amplitudes of the vibrator at various applied voltages with different electrical conductivities of the diesel oil. We found that the wavelength and the apparent surface tension decreased with the applied voltage. The amplitude of the vibrator practically has no influence on the wavelength and the surface tension. [ABSTRACT FROM AUTHOR]

Processes of propagation and interaction of nonlinear gravity-capillary waves on the free surface of a deep non-conducting liquid with high dielectric constant under the action of a tangential electric field are numerically simulated. The computational method is based on the time-dependent conformal transformation of the region occupied by the fluid into a half-plane. In the limit of a strong electric field, when the gravitational and capillary forces are negligibly small, there exists an exact analytical solution of the electro-hydrodynamic equations describing propagation without distortions of nonlinear surface waves along (or against) the electric field direction. In the situation where gravity and capillarity are taken into account, the results of numerical simulations indeed show that, for large external field, the waves traveling in a given direction tend to preserve their shape. In the limit of a strong electric field, the interaction of counter-propagating waves leads to the formation of regions, where the electrostatic and dynamic pressures undergo a discontinuity, and the curvature of the surface increases infinitely. In the case of a finite electric field, the wave interaction results in a radiation of massive cascade of small-scale capillary waves that causes the chaotic behavior of the system. The investigated mechanism of interaction between oppositely-traveling waves can enhance development of the capillary turbulence of the fluid surface. [ABSTRACT FROM AUTHOR]

Subsea DC transmission and distribution system is a promising technology for powering subsea oil and gas fields with high power, long distance and ultra-deepwater depth. In subsea DC transmission and distribution, wet-mate (WM) connectors are considered as a challenging component due to the complicated electrical field distribution in the insulation system and harsh undersea wet-mating environmental conditions. In order to design reliable liquid insulation systems for subsea connectors, it is essential that the behavior of the DC-stressed liquid insulation be fully characterized for thorough understanding. In this paper, liquid conduction phenomena in synthetic ester oil are studied using the current-voltage characteristics and electric field distribution mapping based on the Kerr electro-optic effect. The correlations between the current-voltage characteristics and DC field grading among different electrode gaps and oil types were analyzed. The experimental results suggest that for applied voltage less than the saturation voltage (V s), the conduction is dominated by ion dissociation in the bulk oil. Correspondingly, the liquid conduction is ohmic and with formation of heterocharges around the contacts giving symmetrical field distribution. At applied voltage higher than saturation voltage (V s), this bulk conduction will transit to extrinsic dominated conduction with carriers injected through liquid-electrode interfaces. Under such carrier injection, the corresponding field distribution becomes asymmetrical. In addition to the laboratory experiments, computer simulations with an ion drift-diffusion conduction model taking into account electrode injection were carried out to validate the experimental field distribution at different electrode gaps. The effect of electrode material on the electric field distribution in the oil is investigated and implications for wet-mate connector design principles are presented. [ABSTRACT FROM AUTHOR]

In many of the high voltage apparatus, liquid insulation is used as the coolant and isolating medium between conducting parts. Petroleum based mineral oil is utilized as one of the liquid insulation in most of the applications such as transformers, oil bushing and etc. Recently many research works are engaged in the development of alternate liquid insulation with vegetable oil based natural esters. Major concern with natural esters is their higher viscosity which influences in the cooling characteristics of liquid insulation. With the aim of reducing the viscosity, the edible natural esters are treated with ultrasonic waves of 100 W ultrasonic power at a fixed frequency of 30 kHz. This work is carried out with edible natural esters such as sunflower oil (SFO), palm oil (PO) and sesame oil (SO). Further analysis on the impact of ultrasonic wave for different exposure conditions, the natural esters are exposed to ultrasonic waves for different time period of 15 minutes to 60 minutes with different temperatures of 30, 60 and 90°C in ultrasonic wave treatment setup. This process results in the reduction of viscosity of natural esters after ultrasonic treatment. The influence of ultrasonic waves on viscosity is studied by measuring the values of viscosity of treated natural esters after 24 and 720 hours for analyzing the retention of reduction in viscosity after treatment. This investigation reveals that the ultrasonic waves have impact on the reduction of viscosity of natural esters. Also the outcome of the ultrasonic treatment is derived as the mathematical model to estimate the viscosity values for different processing temperature and exposure time. [ABSTRACT FROM AUTHOR]

In the recent decades, the natural esters have gained more attention as an alternate liquid insulation instead of the petroleum based mineral oil due to the development on power system networks and their environmental friendly nature. The failure free functioning of high voltage transformer mainly depends on the reliability of insulation used inside it. In this work, edible and non edible natural esters are studied and analyzed by using statistical distribution of the AC breakdown voltage in order to ensure their reliability. In this investigation work, sunflower oil (SFO), palm oil (PO) and sesame oil (SO) are studied under the category of edible natural esters, while honge oil (HO), neem oil (NEO) and punna oil (PAO) are analyzed under the non edible natural esters category. The applicability of investigated esters as liquid insulation is analyzed with distribution models like normal, Weibull and generalized extreme value (GEV) distributions. The statistical failure reliability analysis is carried out with the distribution functions such as survival rate, hazard rate and failure rate from the 100 distributed values of AC breakdown voltages of the investigating natural esters. The statistical analysis reveals the better characteristics for natural esters (edible and non edible) such as superior survival rate at higher value of electric stress, possibility of failure occurrence only at the wear out range and elevated statistical withstand voltage at various failure rates. Comparing both the category of natural esters, non edible natural esters have shown the superior characteristics than the edible natural esters. Consequently, it is evident that the natural esters are reliable and potential surrogate to the conventional liquid insulation for the application in high voltage transformers. [ABSTRACT FROM AUTHOR]

The use of natural ester–filled transformers started in 1996, with two industrial transformers installed at the Cooper Power System factory to provide power for two core annealing ovens. Investigations conducted according to the IEEE C57.100 accelerated transformer life test method (Lockie Test) [1] and the sealed tube accelerated insulating system material life test method [2]–[4] indicated a possible extension of transformer life, associated with reduction of the paper degradation rate when impregnated with natural ester liquids. [ABSTRACT FROM AUTHOR]

Printed electronics is a fast-growing emerging technology, which is expected to play a major role in smart sensors for the Internet of Things domain. While there has been considerable progress at the device level by new materials, on circuit level results are still rare. In this letter, we report on the design, simulation, fabrication, and measurement of an inkjet-printed latch, which is an essential building block for digital circuits. Our technology approach is based on electrolyte-gated transistors, which enables low-voltage operation and yields an outstanding electrical performance due to high mobility of the semiconducting, precursor-made In2O3 channel. The fabricated latch can operate reliably down to 0.6 V, where the power consumption is only 16 \mu \textW . The area requirement is about 7 mm2, and in terms of lifetime, no performance degradation was observed during an experimental period of 12 weeks. [ABSTRACT FROM PUBLISHER]

In this paper, we present a new “hybrid” method for on-wafer dielectric measurements of nanolitre fluid samples. The first part of the hybrid method uses a technique that extracts the complex relative permittivity of the material used to make microfluidic channels. The extraction is performed with an empty channel measurement, and thus requires no extra deembedding structures. The second part of the hybrid method involves an accurate extraction of the complex relative permittivity of dielectric fluids. The hybrid method uses three different extraction algorithms and calculates their Type B uncertainties within the NIST Microwave Uncertainty Framework. By choosing the calculation algorithm with the smallest uncertainty at each frequency, the hybrid method can achieve accurate measurements of the fluids’ permittivity over a broad bandwidth. One of the three algorithms is a new algorithm based on closed-form equations. A trace-based algorithm is also applied to fluids measurements, for the first time to our knowledge. Through the uncertainty analysis, we found out that these two algorithms should be favored over a traditional least-squares optimization-based algorithm at millimeter wave frequencies, due to their lower sensitivities to probe-placement errors. [ABSTRACT FROM AUTHOR]

Thermal and electrical degradation properties in low-viscosity (20 mm2/s, (20 cSt) at 25 °C) silicone liquid and insulation paper were investigated to improve reliability and to enable preventive maintenance technology in transformers. Heat-accelerated degradation in samples of 20 cSt silicone liquid and insulation paper in sealed glass tubes with copper, silicon steel, and pressboard was examined to replicate the environment of the transformer tank. After the examinations, the tensile strength of the paper was measured, and the quantity of decomposition compounds was also measured. It was concluded that diagnosing faults by analyzing decomposition compounds with silicone liquid should be possible in the same way that it is with conventional mineral oil. [ABSTRACT FROM AUTHOR]

The effects of liquids immersion and subsequent drying on the surface properties of filled high temperature vulcanized (HTV) silicone rubber have been reported in this series of paper. In this first paper, the surface hydrophobicity characterized by static contact angle and surface chemical properties characterized by a comprehensive analysis of Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD) and thermal desorption gas chromatography mass spectrometry (TD-GC/MS) have been studied. The liquids employed in this study were deionized water, NaCl solution and nitric acid solution, which represented the actual liquids that composite insulators could encounter in practice use. As for the NaCl solution, the lower the concentration, the greater its impact on the hydrophobicity of the silicone rubber (the deionized water can be considered as the NaCl solution of concentration of 0 mol/L). As for the nitric acid solution, the higher the concentration, the greater its impact on the hydrophobicity of silicone rubber. During the permeation process of deionized water and NaCl solution into HTV silicone rubber, some Alumina Tri-hydrate (ATH) filler and low molecular weight (LMW) could be dissolved into the liquids. Nitric acid solution not only dissolved the ATH filler, but also interacted with the ATH filler and degraded the polymer matrix. [ABSTRACT FROM AUTHOR]

Increased power densities drive researchers to enhance and improve the life of insulation system employed in transformers. In order to reduce the oxidation process and enhance the life of liquid insulation, Anti-Oxidant (AO) additives are usually added with Liquid Insulation (LI). The main aim of this research work is to understand the behaviour of LIs with AO additives under thermally aged condition. Here, Butylated Hydroxy Toluene (BHT) AO and natural esters like Sunflower oil (SFO) and Rapeseed oil (RSO) are used for analyses and then subjected to an accelerated ageing test. Important life determining properties of inhibited and uninhibited Mineral Oil (MO), SFO and RSO samples are examined and compared. From the experimental results, it is concluded that the degradation rate of inhibited samples are lesser than that of uninhibited samples. Also, inhibited SFO samples show similar or better performance compared to MO and RSO samples. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]