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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]

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]

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]

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]

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]

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 research investigation was performed to seek the bio-based alternative for the liquid insulator in transformers. The current investigation aims to examine the effect of oil rancidity on the dielectric strength of bio-based oils along with the other preliminary examinations like fire point, flash point, and viscosity. Sunflower oil portrayed the notable performance under all the tests that were conducted. The exposure to the conditioned atmosphere uncovered the presence of a huge variation in DBV of bio-based oil the affirming that the rancidity of oil degrades the insulating ability of the oil. The multi-objective optimization tool MOORA (Multi-Objective Optimization based on Ratio Analysis) was utilized to optimize and select the best oil sample. Upon considering the outcome of all the tests performed and MOORA, sunflower oil was ranked first. [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]

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 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]

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]

This work is focused towards identifying alternate insulating medium for replacing the mineral oil. With this aim, investigations are carried out with natural esters based vegetable oils which are considered as environmental friendly and biodegradable products. This work also extends to analyze the properties of vegetable oils with the antioxidants for enhanced insulating medium of transformer applications. The natural esters based vegetable oils such as honge oil, neem oil, mustard oil, punna oil and castor oil are considered as samples for the investigations. The butylated hydroxy toluene, butyalted hydroxy anisole and gallic acid are picked as the investigating antioxidants. Different particle volume fraction ranges from 0% to 1% of antioxidants are included for studying the performance of natural esters. The performances of natural esters without and with antioxidants are examined according to the international standards. From the investigation results, it is found that the natural esters based vegetable oils without and with antioxidants have the potential to replace the mineral oil as the insulating medium in the transformers. The mathematical functions for properties are developed based on the results obtained for various concentration of antioxidant. These functions are used to estimate the intermediate values of properties for different antioxidant concentrations. [ABSTRACT FROM AUTHOR]

In 130 years the power transformer has grown in many aspects; ratings, performance, technology, aesthetics and eco-friendliness. Today, the transformer industry is witnessing the era of shifting from petroleum based mineral oils that are depleting, environmentally unfavorable, and proven dangerous to natural esters (vegetable oil based) that are renewable, environmentally friendly, and non-hazardous. The market and regulatory pressures to reduce liability risk exposure of mineral-oil-filled distribution and power transformers are increasing. In addition, there are demands to improve equipment efficiency and adopt more “earth-friendly” options in our power systems. In the last two decades there has been a resurgence of usage of natural ester insulating liquids for their “GREEN” and Safety credentials. These renewable resources have good dielectric properties and are compatible for use in power transformers. This paper evaluates and relates the new natural ester oil with existing mineral oil technology in terms of environmental concern, fire safety, physio-chemical properties, thermal performance, and dielectric performance. It also covers key modifications in design, manufacturing and operation to help assure excellent functionality and long life. In brief, this paper provides transformer designers and users about the advantages and challenges of “green insulating liquid”. Due to the voluminous properties that are compared and discussed, this paper is presented in two parts: Part I discusses the key properties and standards for the two liquids. Part II emphasizes additional key properties and material compatibility related to electrical design. Based on evidence discussed in both parts, it is concluded that natural esters are acceptable for both new transformers and for retro filling existing units. [ABSTRACT FROM PUBLISHER]

To efficiently manage the increasing heat generation within the new generation power transformers, it is essential now to develop advanced dielectric nanofluids having high thermal conductivity. We report here the synthesis and characterization of stable high thermal conductivity Newtonian nanofluids using functionalized nanodiamonds in naphthenic transformer oil without compromising its viscosity and electrical insulating property. Functionalized nanodiamonds are dispersed in naphthenic oil, used as standard transformer oil, forming stable suspensions with high shelf life. The electrical resistivity, dissipation factor and viscosity of the base oil is maintained for the nano-oil containing small weight fraction of the filler (0.12 wt%), whereas the thermal conductivity was enhanced by 14.5%. [ABSTRACT FROM PUBLISHER]

Power transformers are utilized to convert high voltages normally used in electrical power transmission to lower voltages more suitable for consumers. A transformer consists essentially of a magnetic iron core with primary and secondary copper windings. The alternating current flowing in the primary winding induces a magnetic flux in the core, which in turn creates a current in the secondary windings. If there is a difference in the number of turns in the primary and secondary windings, the secondary voltage will be scaled up or down proportionally to the ratio of the turns. In this way, a high voltage can be transformed to a low voltage. However, the desire to convert increasingly greater electrical loads using smaller power transformers results in both higher electrical and thermal stresses. The materials utilized to insulate the different electrically conductive components from each other must be designed to withstand those stresses. The insulating media often consist of pressboard and an insulating liquid. The liquid performs a double duty as it not only insulates the conductive parts but also functions as a liquid coolant. Here we are primarily interested in the oil as an insulating medium. [ABSTRACT FROM PUBLISHER]

Streamer and breakdown behavior of transformer oil under impulse voltage are investigated with experimental observations and simulation. The optical observation by using Intensified Charge-coupled Device (ICCD) fast snapshot technique indicates that streamer channel branching is of a stochastic nature. The 2-D axially symmetric modeling based on hydrodynamic drift-diffusion model is employed to depict the streamer dynamics of transformer oil. The experimental and simulation results reveal that bushy negative streamer requires higher excitation than filamentary positive streamer. Positive streamer travels faster than negative streamer from experimental estimation. [ABSTRACT FROM PUBLISHER]

An isothermal microcalorimeter has been used to study the effect of solid copper in hexadecane, vegetable oil (ester) and mineral oil. The experiments have shown a catalytic effect of copper in oxidation of hydrocarbons and also the anti-oxidative effect of dibenzyl disulphide when combined with copper addition. The catalytic effect of copper is found in hydrocarbon as base oil and hexadecane, but not in experiments using inhibited oil and ester as the dielectric liquid. [ABSTRACT FROM AUTHOR]

This paper presents a comparative evaluation of the aging characteristics between a high oleic natural ester dielectric liquid (HONE) and mineral oil based on an accelerated single-temperature thermal aging experiment at 150°C for a period of 3000 hours. The important physical (e.g., viscosity, moisture content, acidity) and dielectric (e.g., power factor, alternating current (AC) dielectric breakdown) properties of both the oils were measured and analyzed at different aging intervals. As expected, results show differences in aging behaviors between mineral oil and the natural ester liquid. The relative moisture content in HONE had stable and lower values during the aging duration as compared to the case of mineral oil, where the relative moisture content increased with the aging duration. Further, unlike the observations in mineral oil, the power factor (PF) and acidity of HONE increased drastically with increasing aging times. The most relevant observation made from this study was that in spite of this significant increase in the PF and acidity of HONE with aging, its AC dielectric breakdown voltage was quite stable, which is not observed in the case of mineral oil. This indicates that for natural ester liquids, higher values of PF (or dielectric dissipation factor) and/or acidity due to aging may not necessarily give a true picture on the dielectric performance of the ester. It is therefore important to note here that the threshold limits of some of the diagnostic parameters which are already established for mineral oil do not apply for ester liquids. [ABSTRACT FROM AUTHOR]

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

Electrostatic field simulations have been performed to investigate the effect of barium strontium titanate (BST) nanoparticle suspensions on electric fields within synthetic oil dielectrics. We predict that by physically integrating nanoparticles of high dielectric constants into the breakdown regime, the self-break jitter in rep-rate oil switches might be reduced. The simulations show that the nanoparticle suspensions generate nonlinear and random electric field enhancements within the oil dielectric and also on the electrode surfaces. The BST nanoparticles have been modeled as perfect spheres which have an approximate dielectric constant of 2000. The oil in the simulation was given a dielectric constant of 2.33 and the electrodes are modeled as perfect electrical conductors with no field enhancements. A comparison is made between simulated electric fields on the surface of the cathode with increasing nanoparticle concentration, radius, and distance from cathode. Effects on capacitance with increasing nanoparticle concentration and radii are also investigated. All electrostatic simulations were performed with CST EM Studio. Preliminary experimental electric field breakdown data are included to validate simulation results. [ABSTRACT FROM PUBLISHER]

The impulse dielectric behavior of insulating liquids is of significant interest for researchers and engineers working in the field of design, construction, and operation of pulsed power systems. Analysis of the literature data on transformer oils shows that potentially there are several different physical processes that could be responsible for dielectric breakdown by submicrosecond and microsecond impulses. While for short submicrosecond impulses ionization (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, the thermal effects associated with Joule heating start to play an important role. This paper provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high-voltage (HV) impulses with duration sufficient to cause local heating, evaporation, and formation of prebreakdown gas bubbles. The proposed model is based on the assumption that dielectric breakdown is developed through percolation channels of gas bubbles, and the criterion of formation of these percolation chains is obtained. To test the developed model, the breakdown field-time characteristics have been calculated for the liquid with chemical composition close to that of transformer oils but with known thermodynamic characteristics (n -hexane). Its dielectric strength has been obtained as a function of externally applied pressure and temperature. The analytical results show good agreement when compared with the experimental data available in the literature. [ABSTRACT FROM PUBLISHER]

The temperature dependent dielectric spectroscopy in frequency domain is compared to physicochemical and electrical tests that are periodically applied on highly insulating transformer oils. Complex permittivity and tans data are obtained as a function of frequency and operation temperatures, to demonstrate the polarization phenomena and the induced loss intensification by the thermally stimulated currents. The acquired dielectric data of oil samples collected from 201 operating High Voltage (HV) power transformers, were correlated to their corresponding physicochemical ones in order to demonstrate the future potential employment of the proposed measuring technique in the field of high voltage engineering, towards reliability monitoring of oil filled electrical equipment (OFEE). [ABSTRACT FROM AUTHOR]

The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]???[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption. [ABSTRACT FROM AUTHOR]

This paper deals with the hot spot temperature of a transformer when filled with mineral oil, and later on, with natural ester fluid. 14 optical fiber sensors were installed in the 154 kV, 15 MVA transformer filled with mineral oil, with the hot spot temperature measured during the heat run tests. The transformer was retrofilled with natural ester fluid, after which the hot spot temperatures were measured again. For the rated power, the hottest spot temperature was 105.9 °C when the transformer was filled with mineral oil and 129.4 °C when it was filled with natural ester fluid. The hot spot temperature increased by 23.5 °C in the transformer filled with natural ester fluid. This paper applies the hot spot temperature measured using optical fiber sensors to the life curve in natural ester by McShane et al. The life curve of the thermally upgraded kraft paper in natural ester fluid was compared with that in mineral oil, as suggested by the IEEE guide. The life of the transformer filled with natural ester fluid was similar to that filled with mineral oil. Therefore, transformers filled with natural ester fluid do not require increase of cooling pump capacity and surface area of the radiator or reduction of the load to obtain the same life as that of the mineral oil. [ABSTRACT FROM PUBLISHER]

In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/μs) to 1 MV/cm (dV/dt 350 kV/μs) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials ? average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 μs) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry. [ABSTRACT FROM AUTHOR]