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.

The application of artificial intelligence algorithms for transformer incipient fault classification using dissolved gas analysis (DGA) is an interesting engineering approach. However, there are various factors that affect the performance of artificial intelligence algorithms. This article presents the influence of the data balancing approach on transformer DGA fault classification with the machine learning (ML) approach. In this work, a total of 4580 DGA samples from in-service transformers are considered for training various ML models. The main challenge for the DGA problem lies in the availability of the normal degradation transformer data and its uniformity corresponding to different faults is almost impossible. This is because DGA is not an exact science, but an empirical approach subjects to variability. Thus, it is a usual practice to apply data sampling techniques that largely influence the efficiency of the algorithms. The present work reports the impact of the data balancing schemes on the performance of the fault classification and demonstrates that a careful choice of the data sampling method and ML algorithm is essential for DGA problems. To demonstrate the global scale ability of the propose model, the model is tested on the IEC TC ten data (field inspection data), while these data are not exposed to the machine during the learning stage. The ability of the ADASYN method in significant enhancement of the global scale capability of AI-based transformer DGA fault classification is reported. This approach will be helpful for condition monitoring engineers in transformer insulation diagnosis in implementing the monitoring modules for large transformer fleets and understanding the insulation oil behavior over years.

Synthetic ester oil (SEO) is promising for bolstering sustainable development and enhancing operational reliability of transformers at low-temperature. The intention of this study is to acquire understanding of differential low-temperature AC breakdown characteristics exhibited by SEO and mineral oil (MO), while delving into the microscopic influencing factors. The results unveil that the breakdown voltage between -20°C and 20°C illustrates a "V"-shaped trend, with SEO consistently higher than MO. SEO presents its maximum breakdown voltage at -20°C and its minimum at 0°C, whereas MO demonstrates its maximum breakdown voltage at 20°C and its minimum at -10°C. Molecular dynamics (MD) and quantum mechanics (QM) calculations reveal that hydrogen bond and interaction energy associated with the state of water, along with the fraction of free volume, mean square displacement, and diffusion coefficient associated with particle transport property, collectively exert considerable influence on the breakdown voltage. Compared to MO, SEO exhibits a higher number of hydrogen bonds and interaction energies, while displaying lower fraction of free volumes, mean square displacements, and diffusion coefficients. Furthermore, the presence of electron trap, in conjunction with these combined factors, leads to a substantially higher breakdown voltage of SEO (31.4 kV) than that of MO at sub-zero temperatures. The wider energy gap of MO compared to SEO leads to a slightly higher breakdown voltage for MO (19.9 kV) compared to SEO at above-zero temperatures. This study provides experimental data and theoretical guidance for the promotion and stable operation of SEO-immersed transformers in UHVDC systems deployed in cold regions.

This paper describes experimental studies on streamer propagation in synthetic ester and mineral oil under lightning impulse voltage. These studies are concentrated around the small electrode gaps (15 and 20 mm) of the point-plane electrode arrangement. The spatial shapes of the developing streamers, oscillograms of light emitted and propagation velocities were analyzed and compared. Two streamer propagation modes were observed during the studies performed. Slow 2nd mode streamers developed below the so-called acceleration voltage while fast 3rd mode streamers developed at acceleration voltage and above. Independently of the liquid used the registered streamers shapes corresponding to a given voltage polarity were identical. Concerning the light oscillograms, higher frequencies of light pulses were observed in the case of synthetic ester. A significant difference between both the considered liquids was however noticed in the threshold value of the testing voltage at which the 3rd mode streamers appeared. Fast positive streamers started to develop in synthetic ester at lower testing voltage (of about 26 kV for 20 mm gap and 20 kV for 15 mm gap) than in the case of mineral oil. For negative polarity of lightning impulse, within the applied testing voltages, 3rd mode streamers were observed only in synthetic ester. For both electrode gaps acceleration voltage for negative polarity was 2.2 of estimated inception voltage. This allowed forming the conclusion that synthetic esters have a lower ability of preventing the development of fast and energetic streamers, even at small (15 and 20 mm) electrode gaps.

The present work is focused on the oxidation aspects (focused on gelling) of natural esters and its influence on the performance of transformer insulation systems. Even though natural esters are not suitable for breathing units, the scenario considered simulates a defect in the sealing system and thus bringing the liquid in direct contact with atmospheric air. Commercially available natural ester liquids are thermally degraded under open beaker conditions in the presence of cellulose with a controlled aging history. The accelerated aging conditions are maintained until the formation of sol and gel is evident in the bulk of the liquid. Later, the insulating liquids and insulating papers are analyzed to understand the behavioral impact of sol and gel on the insulation. The discussions in this article are supported by the physicochemical and electrical characterizations of the degraded liquids and papers. The characterizations include Fourier transform infrared spectroscopy (FTIR), ac breakdown voltage (BDV) failure probability rate, X-ray diffraction (XRD), degree of polymerization (DP), and viscosity. It is found that before the formation of gelling, sol is witnessed in the bulk of the liquid. The presence of the sol highly influences the viscosity of the liquid and aids in rapid acceleration of the oxidation process. It is understood that that the gelling phenomena in natural esters do not have a noticeable impact on the liquid BDV failure rate and paper degradation.

This article presents the results of the studies on lightning properties of two synthetic ester liquids and mineral oil. The measurements concern the standard lightning impulse voltage at both polarities and a point-to-sphere electrode system representing highly divergent electric field distribution. The studies were based on the IEC 60897 Standard wherein the standard recommendations were extended to include light emission registration using a photomultiplier technique. Comparisons of the lightning impulse breakdown voltages (LIBV), times to breakdown (TtB) and light waveforms were made. The results concerning LIBV and TtB showed that at negative polarity both the esters have almost the same lightning characteristics which are slightly below the characteristics of mineral oil. At positive polarity the results indicated an improvement. Regarding the light emission, the mineral oil tested seemed to be more resistant against the appearance of fast light pulses indicating the intense stepwise propagating discharges.