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.

Most power transformers operating in a power system possess oil-paper insulation. A serious defect of this type of insulation, which is associated with long operation time, is an increase in the moisture content. Moisture introduces a number of threats to proper operation of the transformer, e.g., ignition of partial discharges (PDs). Due to the varying temperature of the insulation system during the unit’s normal operation, a dynamic change (migration of water) takes place, precipitating the oil-paper system from a state of hydrodynamic equilibrium. This causes the PDs to be variable in time, and they may intensify or extinguish. Studies on model objects have been conducted to determine the conditions (temperature, humidity, time) that will have an impact on the ignition and intensity of the observed phenomenon of PDs. The conclusions of this study will have a practical application in the evaluation of measurements conducted in the field, especially in relation to the registration of an online PD monitoring system.

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

This paper presents the effect of the impact of moisture in paper insulation used as insulation of transformer windings on its thermal conductivity. Various types of paper (cellulose and aramid) and impregnated (mineral oil, synthetic ester, and natural ester) were tested. The impact of paper and impregnated types on the changes in thermal conductivity of paper insulation caused by an increase in moisture were analyzed. A linear equation, describing the changes in thermal conductivity due to moisture, for various types of paper and impregnated, was developed. The results of measuring the thermal conductivity of paper insulation depending on the temperature are presented. The aim of the study is to develop an experimental database to better understand the heat transport inside transformers to assess aging and optimize their performance.

The article presents an analysis of thermal properties of a mixture of mineral oil and synthetic ester used in transformers. The oil/ester rate was as follows: 100%/0%, 95%/5%, 80%/20%, 50%/50%, 20%/80%, 5%/95%, and 0%/100% The measured thermal properties were: thermal conductivity, specific heat, viscosity, density, and the thermal expansion factor. On the basis of measurement results, the authors calculated the heat transfer factor α. This factor determines the ability of a liquid and it has the key influence in transformer cooling. The measurements were done for a wide temperature range from 25 to 80°C.

Increasing environmental demands influence the requirements for devices and materials used in the power industry. One example is a power transformer and an electro-insulating liquid used in it. In order to meet these requirements, electro-insulating liquids should be characterized by, inter alia, high biodegradability and good fire properties. One of such liquids is natural ester. However, its oxidation stability is low in comparison to mineral oil and demands improvement, which can be achieved by the addition of an antioxidant. The authors of this work used fullerene nanoparticles for that purpose. Pure natural ester samples were prepared, and samples with two concentrations of fullerene, 250 mg/L and 500 mg/L in natural ester. All these samples were aged in a thermal oxidation process. Thereafter, the aging properties of all the samples were compared to assess the oxidation stability of modified liquids. Moreover, the electrical properties of prepared insulating liquids were investigated to assess if fullerene deteriorates these properties after aging process. Based on the obtained results, it was proved that the aging process slowed down in the case of both fullerene concentrations in ester. The acid number of natural ester modified using fullerene was lower than in the case of pure liquid.