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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]

Natural ester dielectric fluids are being used as alternatives for mineral insulating oils in oil-filled power apparatuses. Although partial discharges (PDs) are recognised as one of the main causes of degradation of dielectrics used in power equipment, their characteristic voltages appear to be temperature-dependent. In this contribution, the pre-breakdown and breakdown behaviour of two natural ester fluids, extracted from canola oil (NE1) and soybean oil (NE2) is reported. To assess the effectiveness of the temperature dependence, −5, 0, and + 20°C have been identified as test temperatures. It was found that the different pre-breakdown and breakdown characteristic voltages increase with increasing temperature for both types of esters. The delay time is always longer for canola oil when compared to soybean oil regardless of the test temperature. The analysis of the average propagation velocity of NE1 and NE2 indicates that PDs propagate faster in NE2 than in NE1. The field strength at the tip of the needle electrode during the inception of the streamers leading to breakdown indicates that streamers in both oils are propagating in the slow mode. It is found that the temperature has a significant influence on the PDs' parameters for both fluids. [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]

The intent of this work is to evaluate the influence of cellulose insulation paper on gassing tendency of transformer oil under electrical discharge conditions. For comprehensive enumeration, insulation oil was thermally stressed at 115°C as per ASTM D 1934 with a controlled thermal aging history. The thermally aged oils were considered for gassing tendency evaluation as per ASTM D 6180 (5 hours, 10 kV AC) in the presence of cellulose. Different cellulose Kraft paper proportions including 0, 10, 20, and 30% of aged oil were introduced in to discharge cell. Different cellulose proportions and aging conditions were considered to understand the impact of cellulose while understanding the decomposition aspects of insulation. Diagnostic measurements with those including dissolved gas analysis, moisture, interfacial tension, turbidity, dissolved decay contents, acidity, and dissipation factor were performed before and after gassing tendency evaluation. It was found that, cellulose paper aids significantly in increasing dissolved gases and moisture in oil during electrical discharge. Importantly, diagnostic measurements have shown the ability of paper to absorb certain decomposition products during the discharge process. [ABSTRACT FROM AUTHOR]

Highlights • Explicit description of measuring methods entitled "Dilatometry analysis" and "X-ray diffraction analysis" to study the performance of transformer solid insulation. • Thousand linear curves and coefficient of thermal expansion of cellulose pressboard aged in various insulating fluids using dilatometry have been reported for the first time. • Crystal size and crystallinity of cellulose kraft papers aged in various insulating fluids have been reported to estimate the exact magnitude of degradation. Abstract Physical degradation assessment of solid insulation in transformers is a challenge to the condition monitoring engineers. Solid insulation degradation in transformer is accessed indirectly by characterizing insulation oil. The present article describes application of dilatometry and X-ray diffraction (XRD) analysis as direct methods to characterize the degradation of transformer solid insulation. Experimental aging on oil/paper insulation has been simulated in laboratory conditions at elevated temperatures. Thermal expansion and shrinkage behavior of cellulose pressboards aged in mineral oil, synthetic ester, and mixed oil have been reported using dilatometry analysis. Measurement of change in cellulose crystal size and relative crystallinity of cellulose kraft papers have been reported using XRD analysis. The detailed procedure and interpretation of the dilatometry and diffractograph signatures for accessing the degradation of solid insulation are explicitly discussed. It is inferred that, dilatometry and XRD analysis provide a direct physical degradation assessment of transformer solid insulation. [ABSTRACT FROM AUTHOR]

In this paper, magnetite (Fe3O4) nanoparticles based nanofluid has been synthesized and characterized for application in oil filled transformers. The intent of this work is to check the feasibility of magnetite based nanofluid through appropriate characterizations. Magnetite nanoparticles have been synthesized by chemical co-precipitation method and the corresponding nanoparticles have been characterized by Raman spectroscopy, X-Ray diffraction, UV-Vis spectroscopy and Scanning Electron Microscopy. Further, ferrofluid has been characterized at different concentrations of nanoparticles those including 10%, 20%, 30%, and 40% w/v % to report optimal concentration for improved dielectric and thermal performance of the nanofluid. Electrical measurements such as resistivity, permittivity and loss tangent at different concentrations of magnetite are reported. It is inferred that 20% concentration of magnetite is an optimal concentration with an improvement of 34% of BDV. Importantly, with the application of magnetite nanoparticles overall thermal and dielectric behavior of the insulation fluid will be improved while reducing the size and weight of the transformers. [ABSTRACT FROM AUTHOR]

This study reports experimental study of cellulose performance in mixture of mineral and synthetic ester (SE) oils. Solid insulation impregnated with mineral oil (MO) and mixture oil has been subjected to accelerated thermal stresses to understand the deterioration rate of cellulose. Diffusion of oil–moisture mixture into paper and corresponding variation in dielectric strength of paper has been examined. Compatibility of solid insulation with MO and mixture oil is analysed by thermal expansion coefficients, X-ray diffraction analysis and dissolved decay products. Furthermore, chemical functional groups present in aged oils are identified by Fourier transform infrared spectroscopy analysis. It is observed that the performance of cellulose insulation is improved with addition of SE to MO. This is achieved by higher water solubility of esters, which enables higher moisture consumption during hydrolysis and keeps cellulose dry. [ABSTRACT FROM AUTHOR]

In this work mineral oil, natural ester, synthetic ester and a mixture of mineral oil and synthetic ester have been subjected to accelerated thermal stressing in presence of copper and cellulose. All these samples were subjected to 110°C, 150 °C, 170 °C and 200 °C thermal stressing for different test durations. Thermal degradation of the samples has been monitored continuously with UV Spectroscopy and visual inspections. Additionally interfacial tension of mineral oil and mixture oil has been monitored under the same test conditions to observe the miscible performance of the fluids. It has been found that the degree of degradation is less with synthetic esters than other samples. Interfacial tension and visual observations revealed less sludge formation in synthetic esters. Furthermore, aged insulation paper has been analyzed using X- ray diffraction technique to find the degradation rate of cellulose in various insulating fluids. It has been found that the rate of degradation is low in synthetic ester and performance of mixed oil is better than mineral oil. [ABSTRACT FROM AUTHOR]

Mineral oils have been used in transformers for cooling and insulation for many years. Despite their long-time success, mineral oils are facing critiques by the electrical power industry in terms of thermal performance?mineral oils have relatively low flashpoint and fire point and will burn in the event of a fault [1], [2], [3]; environmental performance?mineral oils are less biodegradable, toxic in nature, and hazardous in the soil and water [4]; and risk assessment?mineral oils are imperilling near hospitals, near high rise buildings, and in residential areas. [ABSTRACT FROM PUBLISHER]

Operational factors are known to affect the health of an in-service power transformer and to reduce the capabilities and readiness for energy transmission and distribution. Hence, it is important to understand the degradation rate and corresponding behavioral aspects of different insulating fluids under various fault conditions. In this article, the behavior of mineral oil and two environmentally friendly fluids (a synthetic and a natural ester) are reported under arcing, partial discharges, and thermal fault conditions. Arcing, partial discharges and thermal faults are simulated by 100 repeated breakdowns, top oil electrical discharge of 9 kV for five hours, and local hotspots respectively by using different laboratory-based setups. Some physicochemical properties along with the gassing tendency of fresh and aged insulating liquids are investigated after the different fault conditions. UV spectroscopy and turbidity measurements are used to report the degradation behavior and dissolved gas analysis is used to understand the gassing tendency. The changes in the degradation rate of oil under the influence of various faults and the corresponding dissolved gasses generated are analyzed. The fault gas generations are diagnosed by Duval's triangle and pentagon methods for mineral and non-mineral oils. It is inferred that; the gassing tendency of the dielectric fluids evolve with respect to the degradation rate and is dependent on the intensity and type of fault. [ABSTRACT FROM AUTHOR]

The intent of this work is to understand the influence of low energy discharge electric faults in mineral oil and synthetic esters on liquid degradation and gassing tendency at different aging conditions (based on acidity values). A low energy discharge electric fault has been created by continuous discharge of 9 kV for five hours on the liquid surface using a suitable laboratory setup. Liquid degradation is reported by adopting UV spectroscopy, turbidity, and particle counter measurements. The gassing tendency is understood by dissolved gas analysis using Duval's triangle and Duval's pentagon methods for mineral oil and non-mineral oils accordingly. It is observed that the influence of low energy discharges on liquid degradation is higher in mineral oils than synthetic esters. The fault gasses in mineral oil are involved with electrical and thermal faults accompanied by stray gassing whereas only partial discharge activity is noticed for synthetic esters. Importantly, the existence of low energy discharge faults like corona discharges will involve a generation of excess high molecular weight products as compared to low molecular weight products that are soluble in liquid volume. [ABSTRACT FROM AUTHOR]

• Thermal degradation of cellulose in ester and ester nanofluid. • Performance analysis based on breakdown voltage, thermal conductivity and tensile strength measurements. • Influence of the nanoparticle concentration on the degradation of cellulose insulation. Natural ester nanofluids offer improved dielectric and thermal properties than non-biodegradable mineral oil. In the present work, cellulose samples impregnated with unfilled and nanofilled mineral oil and ester are subjected to thermal aging at 130 °C for 168, 336, 504, and 1200 h under open beaker conditions. After 1200 h of thermal stress, the AC breakdown voltage, thermal conductivity, and tensile strength of natural ester impregnated cellulose is 36.13%, 0.46%, and 1.09 times higher than that of mineral oil impregnated cellulose samples, respectively. Also, the AC Breakdown voltage, thermal conductivity, and tensile strength of 0.02 wt% natural ester nanofluid impregnated cellulose is 37.81%, 1.7%, and 1.1 times higher than that of cellulose impregnated with 0.02 wt% mineral oil nanofluid, respectively. The present measurements demonstrate an improvement in the solid insulation performance with natural ester nanofluids, suggesting a possibility of extended service life of solid insulation in liquid filled high voltage transformers. [ABSTRACT FROM AUTHOR]