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This article describes the design and performance of a dielectric fluid cooling concept for automotive power electronics. The concept combines a low-thermal-resistance package (which eliminates metalized ceramic substrates) with a high-performance convective cooling strategy (slot jets impinging on finned surfaces). Modeling was first used to design the cooling system to maximize thermal performance and minimize pumping power. A prototype was then fabricated, and experiments were conducted to validate the model predictions using three fluids at various fluid flow rates (16.7 cm3/s [1 L/min] to 68.3 cm3/s [4.1 L/min]) and inlet temperatures (30 °C and 70 °C). The final design was compact (120-cm3 total volume, including heat exchanger and conceptual power modules) and cooled 12 devices (e.g., silicon carbide [SiC]). The validated model was then used to predict the junction-to-fluid thermal resistance and pumping power for various conditions, including −40 °C fluid temperature. The results predict thermal resistance values as low as 19 mm2·K/W is possible using the dielectric fluid cooling approach. The dielectric fluid cooling system is predicted to provide thermal resistance and pumping power values that are approximately 56% and 90% lower, respectively, compared to an automotive power electronics cooling system. Moreover, the dielectric fluids can be used for direct cooling of the bus bars, which can be an effective capacitor and gate driver cooling strategy and enable the use of new driveline fluids tailored for this application. [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]

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]

The following seven books are reviewed: Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems— Performance Analysis and Applications (Rao, U.M., et al; 2022); Microsupercapacitors (Kobashi, K. Kaszczyk, K.; 2022); Functional Materials— Electrical, Dielectric, Electromagnetic, Optical and Magnetic Applications, Vol. 4, 2nd Edition (Chung, D.D.L.; 2022); Energy Storage for Modern Power System Operations (Dhundhara, S. and Verma, Y.P.; 2021); Oxide Electronics (Ray, A.K.; 2021); Flow Measurement by Electromagnetic Induction (Zhang, X.-Z.; 2020); and Residual Current Devices— Selection, Operation, and Testing (Czapp, S.; 2022). [ABSTRACT FROM AUTHOR]

Natural esters have become of interest for the industry in recent years as dielectric liquids for power transformers, and many studies are focused on their dielectric and chemical properties and on their influence in the degradation of the solid insulation due to aging. However, very little is known about their impact on the evolution of the mechanical properties of the paper insulation, which are acknowledged to have a considerable influence in their overall performance and reliability during the operating life of power transformers. This work studies the effects of thermal aging with vegetable oil in some commercial components, which are commonly used in power transformers, such as an insulated continuously transposed conductor (CTC) and samples of thermally upgraded crepe insulation. The changes in the properties of the crepe paper insulation are characterized through the degree of polymerization and tensile testing. Failure initiation and propagation in the insulation of the CTC is analyzed macroscopically. The results are compared with those obtained when using mineral oil, showing that the use of vegetable oil has a protective effect over mechanical properties of the studied types of paper insulation. [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]

Temperature can change the charge injection and migration characteristics in the liquid, affecting the insulation performance. First, the influence of temperature on the permittivity and Kerr constant of the liquid dielectric is studied. Also, the results prove that the permittivity and Kerr constant of propylene carbonate (PC) decrease with the increase of temperature. Moreover, to explore the influence of temperature on the insulation performance of propylene carbonate, we build a temperature-controllable breakdown test platform under voltage impulse, and the breakdown voltage of PC increases slightly with the increase in temperature. Based on the electrooptic effect, an optical platform is built to measure the electric field under the switching overvoltage. In addition, the space charge distribution is calculated, and the results indicated that the temperature in the range of 10 °C–40 °C is negatively correlated with the net space charge injection. Finally, the temperature effect on the rate of change and mobility of the space charges is explored. [ABSTRACT FROM AUTHOR]

In this article, we show the feasibility of improving the performance of a high voltage and high repetition avalanche transistor pulse generator by applying a two-phase immersion cooling technique. The forced air cooling, single-phase immersion cooling, and two-phase immersion cooling techniques were applied to a 60-stage subnanosecond pulse generator. The experimental results indicate that the two-phase immersion cooling technique can effectively control the temperature rising of the avalanche transistor and, thus, reduce the time base jitter and voltage amplitude jitter. Five types of dielectric fluid with different boiling temperatures were comparatively studied. The FC-72 was finally adopted for the balanced performance on voltage amplitude and repetition rate. The two-phase immersion cooling technique could reduce the thermal resistance between the case and ambient and increase the maximum power from 330 to 876 mW. With this cooling method, the surface temperature of the transistor can be effectively controlled below 62 °C. The pulse generator could achieve outstanding performance with a voltage of 2350 V and a rise time of 180 ps. It can work stably at 200 kHz for more than 30 min, and the burst repetition rate could be 260 kHz within 1 min. This article offers new perspectives in the design of high repetitive avalanche transistorized subnanosecond generators. [ABSTRACT FROM AUTHOR]

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]

Wide-Band-Gap semiconductors have enabled considerable miniaturization of power devices, which requires, however, new thermal management solutions to handle the resulting high heat fluxes. Recently, embedded liquid cooling in GaN-on-Si devices was demonstrated as a promising solution by flowing a coolant through microchannels etched in the silicon substrate. However, its impact on power devices’ electrical characteristics, especially at high voltage, is yet to be investigated, which is crucial to assess the viability of the technology. Besides, previous demonstrations were limited to relatively low-power devices, while embedded liquid cooling for high-current and high-voltage (650 V) commercial GaN transistors would show the full potential of the technology. Here, we integrate embedded liquid cooling on 650 V, 50 $\text{m}\Omega $ GaN-on-Si commercial power devices. We demonstrate no negative impact on the device dc or switching performance due to the embedded liquid cooling, which proves the robustness and validity of the technology. Besides, liquid-cooled devices show more than $4\times $ higher current capability and much-improved ${R}_{\mathrm{ON}} \times \,\,{E}_{{\text {oss}}}$ figure-of-merit in a large output current range compared to forced-convection air-cooling, highlighting their potential for high-current applications. Finally, deionized water and a dielectric fluid (3M Novec 7200) are compared as coolants, revealing a trade-off between thermal performance and reliability during high-voltage operation. [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]

High-vapor-quality flow boiling on structured surfaces has the potential to enhance heat transfer performance and reduce temperature/pressure instability compared to conventional lower quality flow boiling on smooth surfaces. In this article, we discuss recent research progress on structured surface-enhanced flow boiling. We discuss lessons learned and focus on the challenges remaining. Although some degree of mechanistic understanding of the effect of surface structures on the flow boiling process has been gained, many important challenges remain to enable real utilization. Primarily, a need exists for a greater fundamental understanding of the processes if design tools are to be developed capable of guiding engineers to select and implement enhancements. The challenges discussed stem from a community-wide workshop focusing on high-exit-quality flow boiling, held in June 2020. Four key challenges were identified: 1) the need for better understanding of the mechanisms governing surface-structure-enhanced flow boiling and their effect on heat transfer coefficient, critical heat flux, pressure drop, and flow stability; 2) the need for rigorous quantification of surface durability and manufacturing scalability; 3) the need to understand effects of using working fluids other than water including refrigerants, supercritical fluids, and other dielectric fluids; and 4) the need to establish thermal resistance limits dependent on liquid film thickness. We end this article by providing conclusions detailing where we believe that the community should direct both fundamental and applied efforts in order to solve the identified challenges, which limits the implementation of high-vapor-quality flow boiling on surface structures. [ABSTRACT FROM AUTHOR]

Electrical properties of transformer oil with additives of carbon nano-tubes are studied at the extremely low concentrations of the nano-tubes. Electrical strength of the mixtures is more than an order of magnitude lower than that of pure oil. Strengthening of the large gap is registered after several breakdowns in contrast to the small gap. The formation of a carbon bridge between flat electrodes is registered optically. This bridge is destroyed by an electrical discharge in the chain of bubbles generated on the bridge. Experiments with helium bubbles floating up in the mixture of carbon nano-tubes with transformer oil are performed at the deficiency of initiating electrons. Additives of carbon nano-tubes stimulate the partial discharges in the bubbles. Also, at lower voltages, the development of intense micro-flows in the liquid dielectric is observed that can be the result of the injection of free electrons from the nano-tubes. [ABSTRACT FROM AUTHOR]

A novel method for the development of a reconfigurable cavity bandpass filter using fluid dielectric is proposed. Dielectric material can produce an effective permittivity ϵeff of the resonant mode when it is loaded into the cavity. Thus, a tube filled with fluid dielectric, e.g., distilled water, can achieve controlled and reversible ϵeff by adjusting the amount of water in the tube. The same manner of resonant frequency can be achieved as the resonant frequency is related to ϵeff, and then frequency tuning is realized. The fluid property can realize easier and faster tuning mechanism than conventional solid dielectric. As ϵeff is affected by the loaded dielectric parallel to the electric field, a triple-mode resonator with resonant modes TE101, TE011, and TM110, which have orthogonal electric fields, is investigated to realize tri-band reconfiguration. The ϵeff, as well as the resonant frequencies, corresponding to each mode can be individually controlled by adjusting their related water posts. Then, reconfigurable single-band and tri-band bandpass filters are designed. A reconfigurable tri-band cavity filter using a triple-mode cavity resonator and fluid dielectric with individual and continuous frequency tuning is reported for the first time. Finally, the reconfigurable tri-band filter is fabricated and measured to validate the concept. [ABSTRACT FROM AUTHOR]

This paper presents the potential of using treated used cooking oil as a potential dielectric liquid for high-voltage power transformers. The acidity of used cooking oil is higher than the prescribed limit of natural ester for power transformers. Hence, this paper proposes a combination of treatment methods to reduce the acidity of used cooking oil to less than 0.06 mg KOH/g. The treatment methods are: (1) alkaline refining, (2) bleaching and (3) synthetic silicate treatment. The physicochemical and electrical properties (acidity, color, density, flash point, viscosity, water content, breakdown voltage and dissipation factor) of the treated used cooking oil are measured according to standards. The results show that the acidity of the treated used cooking oil is 0.0182 mg KOH/g, which is 99.67% lower than the initial value of 5.4664 mg KOH/g. This acidity is achieved by implementing the following treatment methods: (1) alkaline refining using caustic soda (normality: 2.0N); (2) two cycles of bleaching with Fuller earth's adsorbent (concentration: 10 wt%) in each cycle; and (3) treatment with synthetic silicate absorbent (concentration: 15 wt%). In addition, combined treatment methods affect the color rating of the treated used cooking oil, where the color rating is L0.5. The breakdown voltage and dissipation factor at 90 °C of the treated used cooking oil are found to be 100 kV and 0.00688, respectively. The other physicochemical properties (density, viscosity and water content) of the treated used cooking oil fulfills the BS EN 62770 standard except for the flash point, with a value of 234 °C. [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]

A dual-band sensor based on a planar rectangular cavity loaded with pairs of improved planar resonator is used to measure the permittivity difference of liquids with a small volume ($2.5~\mu \text{l}$). The planar cavity is connected by two microstrip transmission lines, constituting a two-port device with two branches. A pair of interdigital capacitor (IDC)-based split ring resonator (SRR) is coupled to the middle part of each branch. The distance between the IDC–SRRs is large, and their coupling can be neglected. Thus, two resonance frequencies are produced (4.15 and 9.18 GHz). One frequency is generated from the resonance of the two IDC-based SRRs (4.15 GHz), whereas the other results from the coupling of the rectangular cavity and the two IDC-based SRRs (9.18 GHz). Frequency splitting is used to detect changes in the dielectric characteristics of the liquids. If the two branches (IDC-based SRRs) of the proposed sensor are placed on the same liquid, only two resonance frequencies are obtained. When two different liquids are loaded by the sensor, the resonance frequency will be split into four. A planar rectangular cavity is used instead of a splitter configuration to motivate the two IDC–SRRs. This novel design affords a dual-band miniaturized sensor. [ABSTRACT FROM AUTHOR]

A six-band microstrip sensor, based on a pair of electrically coupled resonators (ECRs), is proposed to perform multifrequency dielectric characterization of liquids. A pocket is milled through the substrate bottom of the sensor to contain the fluid sample, avoiding the need for a sample holder. The pocket has been designed to achieve a higher electric field magnitude in the sensing region and a mechanism to separate the resonant frequencies, which helps to ensure a high span between the first and the last resonant frequency. The microstrip sensor is implemented on a 1.905-mm-thick substrate with $\varepsilon ^{\prime }_{r} = 12.2$ and $\tan \delta = 0.0019$. The sensor’s six resonance frequencies span the range between 1 and 3.5 GHz. Experimental tests show that the sensor can measure a wide range of $\varepsilon ^{\prime }_{r}$ values between 2 and 79 resulting in close similarities to the dielectric constants published in previous works. [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]

This article presents a new type of hybrid antenna that combines the multiresonant modes produced by a liquid dielectric resonator antenna (DRA) and a magnetoelectric (ME) dipole. Such a combination could not be easily realized by using conventional solid dielectrics due to fabrication and air-gap problems. An aperture-fed ME dipole for 4.1–5.3 GHz is firstly designed using a standard structure and a relatively small dimension. Then, a hybrid antenna is built by loading the ME-dipole with an aperture-fed cylindrical liquid DRA. Without increasing the antenna size and/or modifying the ME-dipole structure significantly, the hybrid antenna has realized a much wider bandwidth from 2.45 to 5.3 GHz after loading the liquid material. Moreover, it is found that the resonances of the ME-dipole are not simply shifted to lower frequencies after using the dielectric loading, it has strategically combined the multiple resonances of the electric dipole, magnetic dipole, DRA, and feeding slot. The mode combination principle and design guideline have been presented. As an example, a prototype of the proposed hybrid antenna has achieved a 73.5% fractional bandwidth with an electrical size of 0.66 × 0.66 × 0.16 λ03 at the center frequency (3.88 GHz), over 5 dBi (up to 7 dBi) broadside gain, total efficiency >80%, and beamwidth around 80°–120° across the frequency band. The proposed hybrid antenna has advantages in terms of wide bandwidth, smaller size, and simple structure compared with other ME-dipoles and traditional wideband antennas. The idea of this antenna design could be extended to other ME-dipole and DRA structures. [ABSTRACT FROM AUTHOR]

The surface discharge of solid dielectric in liquid nitrogen has become one of the key problems in the development of high-temperature superconducting power devices. It is necessary to study the mechanism of surface discharge of the solid dielectric in liquid nitrogen. This article chose polytetrafluoroethylene and DW-3 epoxy resin as specimens, tested the surface discharge voltages of two kinds of specimens with different surface roughness under lightening impulse voltages in liquid nitrogen, and discussed the effect mechanism. The experimental results showed that the surface discharge voltage and discharge time of both materials in liquid nitrogen increased as the surface roughness increasing under lightning impulses. The surface roughness affects the surface discharge voltage of the solid dielectric in liquid nitrogen under lightning impulse, mainly in the charge left on the grinding surfaces of solid dielectrics, the heights of projections, and the creepage distance on the dielectric surfaces. The main discharge process is in the gasification path, and the effect of solid dielectric on surface discharge in liquid nitrogen is smaller. These reasons make the surface discharge in the liquid nitrogen less affected by the surface roughness. The results further explore the characteristics of surface discharge in liquid nitrogen and could provide methodological guidance to improve the surface discharge voltages in liquid nitrogen. [ABSTRACT FROM AUTHOR]

For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel type of tunable gravity tailored metamaterial absorber (TGTMA) which can obtain a broadband absorption. Both seawater and oil are filled in the grooves inside the glass structures, which are the resonators of an absorber. The proposed absorber can be regulated by the gravity field, which is a brand new one compared with the previous designs. When the operating state of such a TGTMA is adjusted from one to another, a marvelous alteration of absorption can be achieved. For TM wave, the absorption band runs from 5.6 to 7.91 GHz which is over 90%, while the absorption keeps larger than 90% from 4.32 to 5.83 GHz when this absorber is rotated 180° in xy plane. The larger incident angle is the better performance of this TGTMA can be achieved. Further, the simulation results of the structure parameters are discussed. The surface current distributions, the power flows, and the power loss densities are also given to expound the physical mechanism of the proposed absorber. Importantly, compared with most of current tailoring approaches of the absorbers, the proposed method is more convenient.

This article presents a compact, cost-effective, and contactless fractal modified electromagnetic bandgap structure (EBG)-based microwave sensing platform for dielectric characterization of liquids by analyzing the variation in the reflection coefficient of an antenna. The reported design is composed of a triangular-shaped antenna (0.323λo × 0.323λo) placed over a 3 × 3 array of Cesaro fractal-based EBG plane (0.67λo × 0.67λo) operating at 2.45 GHz. A significant enhancement of the E-field in the sensing region has been achieved with the incorporation of Cesaro fractals in the EBG plane which results in increased sensitivity and compactness. To validate its performance, absolute solutions of butan-1-ol, methanol, and water are loaded, and a maximum measured sensitivity of 0.875% and a maximum quality factor of 90.05 are achieved. Moreover, a maximum rms error in retrieved values of dielectric constant and loss tangent of liquid under test is found to be 1.092% and 0.813%, respectively. Our demonstrated EBG-based sensor has a compact footprint with good precision, affordability, and ease of operation in detecting liquids for microwave sensing applications. [ABSTRACT FROM AUTHOR]

A novel robust approach for permittivity measurements of liquid materials in a wide frequency range is proposed. A high-sensitivity microwave-fluidic sensor is developed enabling direct determination of liquid samples’ permittivity from the measured sensor’s propagation constant. Importantly, neither sensor calibration using liquid standards nor sensor’s geometry (except length) information is required, which is very advantageous for industrial applications. This is achieved through the sensor’s geometry, which is a two-wire transmission line (TWL) encapsulated in a 3-D printed container, and thus, the liquid-under-test fully encloses the propagating electromagnetic waves. The measurement setup comprises the sensor cascaded between two microstrip-to-TWL transitions for which a simplified de-embedding procedure is proposed using only a THRU standard. For demonstration, a setup was developed with transitions operating within 1–10 GHz and tested up to 18 GHz by measurement of a set of alcohols to produce useful data within the transition bandwidth, which can be extended at the cost of increased uncertainty by postprocessing. Finally, the measurement error study has shown that a relatively low uncertainty and error with respect to the reference is obtained, verifying the usability of the developed measurement technique. [ABSTRACT FROM AUTHOR]

A fluidically reconfigurable millimeter-wave (mm-wave)-lensed antenna is reported that covers the band from 32 to 35 GHz. The antenna is a modified antipodal Vivaldi antenna element with a fluidically loaded dielectric lens to facilitate single-element beam-steering. The fluid used in this work is ethyl acetate (C4H8O2) with $\boldsymbol {\epsilon _{r} = 5.0 + j1.3}$ , $\boldsymbol {\tan \delta = 0.26}$ at 33.5 GHz. The antenna element and lens are manufactured using a substrate with $\boldsymbol {\epsilon _{r}=3.55}$ and $\boldsymbol {\tan \delta = 0.0027}$. Experimental results show that the antenna can produce up to 25° of beam-steering at 32 GHz and up to 20° of beam-steering at 35 GHz. The maximum realized gain of the antenna is 8.6 dBi and the minimum realized gain is 5.6 dBi over the band. [ABSTRACT FROM AUTHOR]

This paper reports on a method combining the use of finite element simulations and external measurements to provide preliminary condition diagnosis of oil-filled cable sealing ends (CSEs) without requiring downtime. Good agreement has been obtained between the predictions from the electric field and thermal simulations and the measurements on a 132 kV oil-filled CSE. The electrostatic computation combined with electric field measurements can provide information regarding the electric field distribution inside the CSE and help in identifying potential issues with the CSE design, the materials or the cable termination process. The thermal computation combined with thermal imaging can reveal potential problems, such as high resistance connection to the busbar, and provide information regarding the cooling efficiency of the liquid dielectric. The method presented can provide the starting point for prioritizing maintenance operations on CSEs. [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 paper deals with the influence of microscopic morphological disturbances on the surface stability of streamer developing in a dielectric liquid. A dynamic model based on the stresses acting at the streamer channel surface is established, and the influence of different modes of microscopic morphological disturbances on the surface stability of streamer are analyzed. The model shows that disturbances on the streamer body can cause pinch-off when the applied voltage is low. At higher voltage, the short-wavelength disturbance is easier to cause unstable of streamer, such as branching, shrinkage, expansion and pinch-off, than long-wavelength disturbance. The symmetric disturbance grows faster than asymmetric disturbance. The streamer head is also sensitive to disturbances and becomes unstable. Depending on the form of the disturbance, the perturbation on streamer head may cause streamer branching and direction change. [ABSTRACT FROM AUTHOR]

The permittivity response from 0.5 to 40 GHz of ethyl acetate (C4H8O2), ethylene glycol (CH2OH)2, castor oil (C57H104O9), liquid paraffin, and other fluids commonly used for microwave microfluidic antennas and circuits is reported. The response of selected polymer solids, such as polydimethylsiloxane (PDMS), acrylonitrile butadiene styrene (ABS), acrylic, nylon, and others, used in microfluidics and 3-D printing of microwave structures was also studied from 0.5 to 20 GHz. Permittivity measurements were taken with a coaxial probe. Using a nonlinear least-squares fitting routine, the Cole–Cole dielectric relaxation model parameters are found for each material under test, thereby providing an analytic expression for their permittivity response. [ABSTRACT FROM AUTHOR]

Liquid crystal polymers (LCPs) are a class of thermoplastic polymers widely used for packaging radio frequency (RF), microwave, and low millimeter (mm)-wave integrated circuits operating at frequencies as high as 77 GHz. Because of their relatively low cost and attractive electrical, thermal, and mechanical properties, LCPs are of potential interest as a packaging material for advanced circuits operating at high mm-wave frequencies, ~100–300 GHz. To potentially use LCPs at these frequencies will require a quantitative knowledge of the materials’ dielectric properties across a broad frequency band. Here, we present nondestructive measurements of the dielectric constant, Dk, and dissipation factor, Df, on six commercially made LCP materials now used for packaging RF/microwave circuits. Measurements using phase-sensitive transmission from 140 to 220 GHz show values of Dk ranging between 2.0 and 4.5, with four materials essentially nondispersive but two showing significant dispersion in this band. Df values were in the range 0.01–0.06, substantially higher than that has been reported for many LCP materials at RF/microwave frequencies. The results are useful to model accurately the performance of packaged mm-wave circuits. [ABSTRACT FROM AUTHOR]

The installation of distribution transformers in environmentally sensitive areas is a growing concern of electric utilities, and everywhere it sees a strong local resistance. Special and costly measures must be taken to address this concern, for example, by reducing the risk of transformer oils being inadvertently released into the environment. To solve this problem, promising natural esters based on edible oil-seeds were studied in the last few decades to ascertain their suitability based on industry-recognized standards. Some common natural esters available today in the transformer oil market are mostly produced from seeds of soybean, rapeseed, sunflower, and others [1–5]. However, due to the inherent disadvantage of high pour point, coconut oil has been sidelined for many years. Recent research on the coconut oil indicates that this can be used as a dielectric liquid at a distribution level transformer in a tropical region. Unlike main seed oil, the use of coconut oil in a transformer has not yet started despite its superior dielectric and physicochemical properties. It is a totally biodegradable and edible vegetable oil. Prototype transformers filled with coconut oil are in operation in Sri Lanka since 2001 where it has been studied satisfactorily by the researchers [6]. The superior oxidation stability, non-toxicity, and good dielectric properties of coconut oil are previously well known but no detailed comparison has been published on the electrical, thermal, and physicochemical properties of coconut oil, commercial natural esters, and mineral oil. This paper aims to present a comparison of the relative advantages, limitations, and scope of coconut oil for use in a transformer. [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]

In view of the contemporary scenario and importance of alternative liquid dielectrics, IEEE DEIS Technical Committee on Liquid Dielectrics laid emphasis on pre-breakdown phenomena of ester dielectric fluids. The present article reports the background knowledge on the breakdown theories and behavior of streamers in insulating oils. The major influencing properties of the streamers aroused in liquid insulating systems are discussed keeping in view of the ester based dielectric fluids. Importantly, modelling of streamers, testing approaches, and review of pre-breakdown phenomena in ester dielectric fluids has been presented. [ABSTRACT FROM AUTHOR]

In this article, two synthetic esters along with a natural ester are investigated to monitor the generation of colloidal and dissolved decay particles under thermal aging. The thermal aging of the oils is carried out as per a modified ASTM D1934 in the presence of cellulose and copper at elevated temperatures. Later, the colloidal and dissolved decay particles are monitored as per ASTM D1698 (centrifuge treatment) and ASTM D7150 (Fuller's earth filtration), respectively. The aged and treated dielectric liquids are characterized for the number of decay particles, oil absorbance curves, the concentration of dissolved decay content, and turbidity as per ASTM standards. The evolution of colloidal and dissolved decay contents is individually reported for ester fluids and mineral oils. The degradation in mineral oils has been witnessed by both dissolved and colloidal particles. The generation of colloidal particles following the degradation of ester fluids is almost negligible, whereas dissolved decay content is noticed with thermal degradation time. Importantly, the obtained results depict the ability of esters to dissolve more sludge. This is due to the high (oxidative) stability of ester leading to reduced radical decomposition products. Also, the temperature solvent nature of the ester group tends to dissolve sludge particles back to the liquid phase into the bulk liquid. [ABSTRACT FROM AUTHOR]

Pulsed electrical discharges in dielectric liquids are intensively studied due to the wide range of applications in which they are implicated. Despite the simplicity of the experimental manipulation of these discharges, the underlying fundamental physics is relatively complex. In this study, we use the electrical characteristics, voltage and current, of pulsed discharges in water (various applied voltage and pulse width conditions) to determine the equivalent electrical circuit of the plasma. Based on a Mayr-type model, the plasma resistance is time-dependent, but the inductance is not. Considering that plasma resistivity also depends on electron density and temperature (Spitzer formula), the temporal evolution of these two parameters was also determined. [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]

Nonlinear dynamics of the free boundary of an ideal dielectric liquid in a vertical electric field is studied in a confined axisymmetric geometry. We derive an amplitude equation which describes the behavior of a fluid surface in the weakly nonlinear approximation. It is demonstrated that, due to the effect of quadratic nonlinearities, the regime of excitation of the surface electrohydrodynamic instability is always hard. It is also shown that cubic nonlinearities suppress the instability development for liquids with relatively small permittivity. On the contrary, the nonlinearity begins to play a destabilizing role for high values of dielectric constant. As a consequence, the instability development takes an explosive character: the amplitude of boundary perturbations grows unlimitedly over a finite time. Using an analogy with the motion of an effective Newtonian particle in a certain potential, we formulate conditions for explosive buildup of perturbations. Differences in the behavior between axisymmetric perturbations of the free surface and the cases of hexagonal, square, and planar symmetries are discussed. [ABSTRACT FROM AUTHOR]

The focus of this study is on thermal bubbles which limits the application of phase-change cooling technology in high electric fields. In order to investigate the impact of the boil-induced bubble flow on the partial discharge (PD), a PD test chamber is set up which is modified to adjust the intensity of the bubble flow. PD signals and characteristics of the bubble flow are recorded under different heating conditions and electric fields. It is found that thermal bubbles have negligible impact on the PD characteristics under a non-uniform field, but the discharge frequency decreases as the boiling intensity increases. A deviation occurs in the trajectory of bubbles, which makes discharge in bubbles impossible. It is also found that the heat induced fluid affects the density of dielectric liquid and ionization coefficient of the discharge environment, which reduces PD frequency. [ABSTRACT FROM AUTHOR]

In the present study, a test chamber filled with perfluorohexane (FC-72) is set up to investigate partial discharge (PD) in phase-cooling bubbles. Under the non-uniform electric field with needle point-to-hemisphere electrodes, thermal bubbles are generated from the needle point. The difference between PD with and without the bubbles is analyzed to identify features of the discharge in the bubbles. It is observed that the bubbles reduce the PD inception and intensify the discharge. A model of bubble motion is established to investigate bubble behavior in an electric field. Based on the observation of the flow and streamer patterns, a mechanism of PD in bubble flow is established and confirmed by phase-distribution of PDs with bubble transit time. [ABSTRACT FROM AUTHOR]

In this article, a novel microwave microfluidic coupled-line sensor allowing for monitoring of proportional quantities and determination of complex permittivity of liquid dielectric samples through simple analytical formulas is proposed. The application of broadside coupling configuration enabled the sample to be placed in-between coupled strips, thus a high sensitivity can be obtained as the electric field for differential excitation is confined within the sample’s volume. Moreover, the use of 3-D printing technology allowed for the realization of an insert with the microfluidic channel and dedicated liquid inlet/outlet. The sensitivity analysis of such a sensor is provided along with the system calibration technique, which is straightforward because of a fixed and well-defined geometry of the sensor-sample setup. An exemplary, general-purpose sensor along with a cal-kit was realized in a stripline structure and manufactured using a combination of laminate and PolyJet 3-D printing technologies. The broadband measurement results are provided for a water–ethanol solution with variable solvent content up to 12 GHz and for various alcohols featuring frequency-dependent complex permittivity up to 5 GHz. The sensor’s performance is assessed and discussed. [ABSTRACT FROM AUTHOR]

A microwave microfluidic sensor for detecting binary liquid mixtures with a dielectric method at RF/microwave frequencies is presented in this article. The sensor is based on a split-ring resonator (SRR) that is implemented in a microstrip transmission line, with interdigitated electrodes (IDEs) being integrated into the ring for liquid detection. Based on the equivalent circuit of the IDE-SRR device and with a series of finite element simulations, the detection theory is developed, and the device design optimization is investigated. The validation measurements on water–isopropanol liquid mixtures with various concentrations show that the proposed IDE-SRR sensor has higher sensitivity than the previous standard SRR sensor. The IDE-SRR sensor is then used to detect two binary liquids, i.e., water–methanol mixtures and water–tetrahydrofuran mixtures. The measured effective permittivity results of the binary mixtures at RF/microwave frequency range are compared with the existing mixing models for binary dielectric mixtures at zero frequency. [ABSTRACT FROM AUTHOR]

We present a new concept to passively steer the antenna radiation beam to a fixed direction by utilizing the liquid fluidity behavior due to gravity. Completely different from existing technologies, the proposed antenna does not require any additional mechanical structures, phase shifting circuitry, liquid pumps, tunable elements, or their associated components. By properly designing the structure and resonant mode of the liquid dielectric resonator, the proposed passive antenna can automatically steer the radiation beam to the fixed and desired direction for communication upon physical reorientation of the antenna. An antenna prototype has been proposed with a fundamental resonant band of 1.4–1.8 GHz by using new liquid materials, while its passive beam-steering feature has been experimentally verified. This simple, elegant, and low-cost antenna has combined the gyroscopic correction, gravity, and beam steering into one for the first time, presenting significant implications for radio communications and radar systems that demand an adaptive antenna beam in certain directions. [ABSTRACT FROM AUTHOR]

The interest in developing new nanodielectric fluids suitable for electrotechnical applications has increased significantly in the last decade. Several authors have reported experiences on fluids manufactured using different base fluids and nanoparticles. Most of these studies are focused on the comparison of the thermal and dielectric properties of the nanofluids and the base liquids and most authors have found that the addition of nanoparticles to insulating liquids can lead to an improvement of their dielectric properties. This two-part paper analyzes the enhancement of the AC breakdown voltage of several nanodielectric fluids prepared by dispersing variable concentrations of Fe 3 O 4 nanoparticles in a mineral oil. In the first part of the study, the performance of fluids that had been previously dried and degasified were tested. In the second part of the study the impact of the presence of moisture in the dielectric strength of the liquids is analyzed trying to get insight into the physical mechanisms that justify the observed enhancements when nanoparticles are added to insulating liquids. [ABSTRACT FROM AUTHOR]

This article presents the implementation of a novel microfluidic sensor based on a resonant cavity. The cavity is based on the substrate integrated waveguide (SIW) technology and is fabricated by using stereolithographic 3-D printing, which guarantees fast prototyping and allows arbitrary geometry. The proposed sensor consists of a square cavity and an inner meander pipe provided with two openings, where the liquid under test is injected and extracted, and is fed by a coaxial probe, which represents the microwave port of the circuit. S-parameter measurement allows retrieving the electromagnetic properties of the liquid injected in the pipe. In particular, the loss tangent of the liquid is extracted from the variation of the cavity quality factor compared to air-filled pipe, after removing the actual losses of the structure. The dielectric permittivity of the liquid is retrieved from the shift of the cavity resonance frequency relative to the case of air-filled pipe, without any hypothesis of small losses. The performance of the sensor is discussed through the electromagnetic characterization of several fluids. [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]

This paper deals with flow electrification phenomenon for turbulent flows of liquids through pipes of a circular cross section. Two models are proposed. The first one is based on the comparison of the radial fluctuant velocity magnitude with the magnitude of the diffuse layer relaxation velocity: it assumes that in the region where the fluctuant velocity is greater than the relaxation velocity the diffuse layer is totally perturbed so that the charge is homogenized by the turbulence, whereas when the relaxation velocity is greater than the fluctuant velocity the diffuse layer is considered not perturbed at all. The charge transported by the flow is then calculated with this new charge profile. The second model consists in solving the system of the diffuse layer equations perturbed by the turbulence, taking into account the eddy diffusivity; then with the profile of the charge obtained in the cross section the charge transported is calculated. Finally, the predictions of the charge transported using each model are compared in a specific case and discussed. [ABSTRACT FROM AUTHOR]

A glucose concentration sensor for microliter-volume water–glucose solutions is presented. The proposed sensor is composed of an open-loop microstrip resonator with a dielectric liquid holder (5–25- $\mu \text{L}$ volume) glued onto the gap between the line ends. The resonator is coupled to two microstrip lines forming a two-port network whose S-parameter response provide information about the dielectric properties of the liquid under study. Three versions of the sensor at resonant frequencies between 2 and 7 GHz are presented. The sensors are assessed by measuring the complex permittivity of standard liquids reported in the scientific literature. Models of the sensors are presented, which properly match the experimental results. This paper presents an experimental study of the sensors as glucose concentration retrievers. The main novelty is the use of $Q$ factor and the maximum S21 of the resonance as sensing magnitudes. The dependence of these parameters on the glucose concentration of the solutions obeys almost linear relationships. Good sensitivities have been obtained for the proposed sensors, which are coherent with previous works by other authors. [ABSTRACT FROM AUTHOR]