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Synthetic esters have become more and more popular in last few decades, explaining the increasing number of units filled with this liquid year by year. They have been investigated under different aspects, both from the fundamental point of view and breakdown mechanisms, well as from the application point of view. However, their use in high voltage equipment is always a challenge and deeper knowledge of the various aspects that can be encountered in their exploitation is needed. The intent of this review paper is to present the recent research progress on synthetic ester liquid in relation to the selected issues, most important for ester development in the authors’ opinion. The described issues are the breakdown performance of synthetic esters, lightning impulse strength and pre-breakdown phenomena of synthetic esters, synthetic esters-based nanofluids, combined paper-synthetic ester based insulating systems, application of synthetic ester for retro-filling and drying of mineral oil-immersed transformers, DGA(dissolved gas analysis)-based diagnosis of synthetic esters filled transformers as well as static electrification of synthetic esters. The different sections are based both on the data available in the literature, but above all on the authors’ own experience from their research work on synthetic ester liquids for electrical application purposes.

In this paper, a comparison of positive streamer diffusion propagation is carried out in three configurations of oil transformers: mineral transformer oil, mineral oil with solid dielectric barriers, and a nanofluid. The results have been solved using a finite-element method with a two-dimensional (2D) axi-symmetric space dimension selected. Additionally, previous results from other research has been reviewed to compare the results obtained. As expected, it is confirmed that the nanoparticles improve the dielectric properties of the mineral oil. In addition, it is observed that the dielectric solid blocks the propagation of the streamer when it is submerged with a horizontal orientation, thus perpendicular to the applied electric field. The computer used, with four cores (each 3.4 GHz) and 16 GB of RAM, was not sufficient for performing the simulations of the models with great precision. However, with these first models, the tendency of the dielectric behavior of the oil was obtained for the three cases in which the streamer was acting through the transformer oil. The simulation of these models, in the future, in a supercomputer with a high performance in terms of RAM memory may allow us to predict, as an example, the best concentration of nanoparticles to retard the streamer inception. Finally, other dielectric issues will be predicted using these models, such as to analyze the advantages and drawbacks of the presence of dielectrics inside the oil transformer.

The increasing demand for electrical energy has led to the widespread use of high-voltage transformers, commonly utilizing mineral oil as an insulating fluid due to its availability and low cost. However, concerns over environmental impact and safety have prompted the exploration of more eco-friendly alternatives, such as natural ester oil and GTL oil. This study compares the insulating performance of mineral oil, natural ester oil, and GTL oil at 100°C over periods of 168, 504, and 840 hours. Key findings include significant differences in the performance of the oils under aging conditions. At 100°C, natural ester oil maintained the highest breakdown voltage (43.16 kV), while mineral oil and GTL oil had lower values (24.36 kV and 23.68 kV, respectively). Mineral oil showed a notable decrease in resistivity, from 2.9 to 2.0, while natural ester oil remained more stable. In terms of tensile strength, natural ester oil exhibited a stronger performance (6.04 kN/m), compared to mineral oil (4.46 kN/m) and GTL oil (3.22 kN/m). These results highlight the superior insulating properties of natural ester oil and its potential for improving transformer performance and sustainability.

The service life of liquid insulation is influenced by various operational factors, particularly aging. This study investigates the suitability of high oleic natural esters and their blends with mineral oil as insulating liquids. The properties and thermal aging process of these liquids are examined. Natural esters containing high oleic fatty acids, such as canola oil and sunflower oil, are individually studied, as well as their blends and naphthenic mineral oil. From the proposed samples, the one with the best properties is selected for subsequent investigation into aging behavior in the presence of kraft paper insulation. Both the mineral oil and the chosen blended oil are subjected to accelerated thermal aging in separate closed vessels for 50 days at a temperature of 150 °C. Properties, including breakdown voltage, viscosity, flashpoint, density, acidity, and moisture content, are assessed according to international standards to ensure the samples possess suitable characteristics. The impacts of aging are analyzed by observing selected oil sample properties, the presence of dissolved gas, and the degree of polymerization of the kraft paper. The results of the aging tests indicate that the blended oil outperforms the mineral oil, and overall performance fluctuations are essentially the same for both types of oil. The findings suggest that natural ester and mineral oil-based blended oils could potentially be reintroduced as liquid insulation in the future.

Power transformers are the main element of an electric power system. The service life of these devices depends to a large extent on the technical condition of their insulation system. Replacing mineral oils with natural or synthetic ester (retrofilling process) may increase the efficiency and operational safety of transformers, and also limit their adverse environmental impact. It is technically unfeasible to completely remove mineral oil from a transformer. Its small residues form a mixture with fluid ester, with different physicochemical and electric properties. Streaming electrification is one of the phenomena which, under unfavorable conditions, may damage the insulation system of a forced oil cooled transformer. It is necessary to run prophylactic tests for the ECT (electrostatic charging tendency) of insulating liquid mixtures from the point of view of transformer retrofilling, which is being used more often than before. The article presents the results of studies on selected physicochemical, and electrical properties, and the ECT of mixtures of fresh and aged Trafo EN mineral oil with Nycodiel 1255 synthetic ester. In this regard, the density, the kinematic viscosity, the conductivity, and the relative dielectric constant were measured. The molecular diffusion coefficient was determined using Adamczewski’s empirical dependency. The streaming electrification was tested in a rotating disc system. The impact of the rotation time, the diameter, and the disc’s rotation speed on the amount of the electrification current generated were analyzed. In addition, the co-relation between the electrification current and the composition of the mixture was determined using fresh and aged mineral oil. On the basis of the electrification model, the volume density of the qw charge was calculated, which is a parameter defining the ECT of insulating liquids. Based on the results, it was concluded that the synthetic ester is characterized by a higher susceptibility to electrification than the mineral oil. However, combining synthetic ester with a small amount (up to 20%) of fresh or aged mineral oil significantly reduces its ECT, which is beneficial from the point of view of retrofilling power transformers.

This paper gives information on the electrical and physiochemical characteristics of six different types of palm oil compared with traditional mineral oil. We found that natural processed crude palm oil (PO-C) had a higher resistance to AC breakdown voltage than other types of palm oil, including traditional mineral oil. The results of the positive lightning impulse voltage test for PO-C were still the highest compared to other types of palm oil, including traditional mineral oil, at 58.26%. The summarised dissipation factors of all tested crude palm oils were significantly higher than those of mineral oils, which will make the palm oil less insulating, especially in PO-A palm oil (36.197%), where the values were higher than those of other oils, while mineral oil has a slightly increased dispersion factor. For relative permittivity, all palm oils were compared, and it was found that PO-C had a lower relative permittivity than the other oils. In terms of physical and chemical properties, in the moisture content test on all oils, PO-C had the percentile with the highest moisture content decrease of 58.74%. In the case of testing the surface tension value, it was found that traditional mineral oil had the highest value (48.46 m/Nm) when compared to palm oil. On the other hand, the acidity in traditional mineral oil is the lowest (0.03 mg KOH/g) compared to all palm oils. Results from studies demonstrate the possibility of using natural processed crude palm oil, or PO-C, as a replacement for traditional mineral oil. This is consistent with the results of electrical properties that show PO-C is higher than other types of palm oil and includes traditional mineral oil.

Transformer oil does not only serve as an insulating liquid, but also in removing heat from the windings and cores. Mineral oil (MO) has been widely used in transformers for more than 150 years. Recently, researchers have attempted to search for alternative insulating oils due to the possibility that MO will run out in the future together with the concern on fire safety and environmental pollution. Among the potential oils is rice bran oil (RBO). This work presents the studies of the lightning impulse (LI) of RBO behavior under various electric fields, gap distances and testing methods. The electrical performances of LI tests show that RBO and Palm Oil (PO) have lower LI breakdown voltage than MO under both uniform and non-uniform electric fields. However, the difference in LI breakdown voltages between RBO, PO and MO are slightly small which is less than 20%. In addition, there is no significant effect in the various testing methods under both uniform field and non-uniform field where the percentages of difference are less than 12% and 8% respectively. The data of LI breakdown voltage were statistically analysed to predict the withstand voltage and 50% breakdown voltage of oil samples by using Weibull distribution. The Weibull distribution of MO, PO and RBO has well fit with the fitting line. Finally, the relationship between LI voltages under a non-uniform field with various parameters of PO and RBO was obtained and proposed. From this work, it can be concluded that RBO shows promising results to be considered as an alternative to MO in transformer applications.

Written by leading experts in the field, the first edition of this textbook was the first of its kind to address numerous potential applications such as the technology of high-voltage insulation in pulsed inhomogeneous fields, and applications related to cavitation development in liquid dielectrics, treatment of different materials and plasma medicine. This new expanded edition also addresses the development of the theory over the past few years and features extensive revisions and some expanded chapters. It is intended for a broad audience, from students to engineers and scientists, who are interested in current research questions in electrodynamics and hydrodynamics of liquid dielectrics. Part of IOP Series in Plasma Physics.

This paper presents the experimental results of a study, which has been carried out to analyze dielectric strength of virgin coconut oil in order to check the suitability as liquid insulation in power transformers.In this study dielectric strength response measurements after five times breakdown voltage test both mushroom electrode and spherical electrode slightly decreased, but the value of breakdown voltage is still high enough. While the measurement of the water content decrease is not stable, viscosity increase almost linear, the same thing also happened linear increase in measurement of flash point.

Power transformers are one of the critical components in power system. Insulation plays a promistic role in determining the lifetime of transformers. Insulation used in transformers is of solid and liquid insulation. Solid insulation like kraft paper, press board on degradation process generates furanic derivatives such as 5-methyl furan, 2-acetyl furan, 2-hydroxymethyl furan and 2-furaldehyde. Traditionally, the presence of furanic compounds in transformer oil is determined using high-sperformance liquid chromatography. An effort has been made to estimate the presence of furanic derivatives using artificial neural network (ANN) with the aid of ultra-violet–visible light absorbance property. In ANN system, Levenberg Marquardt back propagation optimisation technique is used to determine the furanic derivatives in transformer oil. Degree of polymerisation of insulation paper and percentage tensile strength of paper remaining has been examined.