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Abstract: Natural esters are beginning to be utilized as transformer oil, so certain problems happening during their operation need to be solved urgently. To explore the effect of pollution on the insulation performance of oil, in this paper we investigate the motion characteristics of cellulose particles and conductivity in a mineral oil and a natural ester. The influence of oil aging, DC electric filed strength, and moisture content of the cellulose particles is considered. It is found hat with increasing electric field strength, aging of the oil, and moisture of the particles, the motion of cellulose particles becomes more obvious. The bridging velocity becomes higher and stable bridges grow more compact. The saturation current in the contaminated oil increases exponentially with the electric field strength. The saturation current in the contaminated natural ester is smaller than that in the contaminated mineral oil under the same electric field and particle concentration, especially when the oil is aged. For the mineral oil, the ratio of the conductivity current in contaminated oil to that in clean oil is higher under a lower electric field. However, the situation is opposite in the case of the natural ester. Fewer cellulose bridges and lower saturation current imply that the natural ester has a better insulation property and resistance to pollution. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Synthetic ester oil (SEO) is becoming a replacement for mineral oil in transformers due to its biodegradability and for its better insulation properties. Silica nanoparticles dispersed in SEO can increase its corona inception voltage (CIV) and breakdown voltage compared to pure SEO. Physico-chemical studies were carried out to understand the characteristics of nanoparticles and nanofluids. Methodical experimental studies were carried out to understand the corona discharge activity in SEO based nanofluid. A ultra-high frequency (UHF) sensor was used to identify corona activity and it is concluded that the UHF signal radiated from corona activity in SEO and nanoparticle dispersed SEO have a bandwidth in the range of 700 MHz to 2 GHz. A comparative study on the CIV of SEO and nanofluid under harmonic AC voltages has been carried out, which indicates a reduction in the CIV as the total harmonic distortion of the supply voltage increases. Phase resolved partial discharge analysis indicates that the magnitude of discharges formed in nanofluid is much lower than in the SEO. Comparative statistical analysis are made with the breakdown voltages of the synthetic ester oil and nanofluid. A comparative analysis on the energy of the UHF signal due to the corona discharges is performed. [ABSTRACT FROM AUTHOR]

The paper presents research results of different properties of mixtures of the synthetic ester with mineral oil and synthetic ester with natural ester. For different concentrations of both mixtures, the following properties were investigated: flash point (open and closed cup), fire point (open cup), net calorific value, breakdown voltage, relative permittivity, dissipation factor, conductivity and dissipation factor. In order to compare results, prepared mixtures and base liquids were carefully arranged under uniform climatic conditions. This meant that the prepared samples had the same relative humidity. The obtained results were compared with the results of the base liquids. In addition, results of the investigation were analyzed in terms of fire safety and changes of electrical properties, in the context of the use of synthetic ester/mineral oil and synthetic ester/natural ester mixtures as insulation liquids in power transformers. [ABSTRACT FROM AUTHOR]

This paper presents experimental results of a comparative study of the characteristics of streamers propagating in olive oil and breakdown voltage with mineral oil, rapeseed oil (natural ester) and tetra-ester and methyl oleate (synthetic ester liquids). The comparison focuses more particularly on the pattern, stopping length, associated current and electrical charge of streamers propagating in a point-plane electrodes geometry under (1.2/50 μs) lightning impulse (LI) voltage as well as breakdown voltages under AC and LI voltages. A statistical analysis of breakdown voltages is also achieved. It is shown that the streamer characteristics (streamers shape, stopping length, associated current and electrical charge) of olive oil in small gaps are comparable to those of mineral oil. Breakdown voltages under AC and LI voltages are found to be higher in olive oil than in mineral oil. Consequently, olive oil can constitute a potential substitute for mineral oil in in high voltage power transformers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents experimental studies of streamer propagation and breakdown in a mineral oil and a synthetic ester liquid under both positive and negative DC voltages. A point-plane electrode configuration with a point tip radius of 10 μm was used. Characteristics of streamer including length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm using a multi-channel ultra-high speed camera. Under positive polarity, the streamers in both liquids propagate in the 2nd mode; the stopping length of the synthetic ester is longer than that of the mineral oil at the same applied voltage level. However there is no obvious streamer propagation (less than 10% of the gap distance) under negative polarity even when the applied voltage approaches to the breakdown voltage; the streamers in both liquids stay in the 1st mode. Breakdown tests at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. 50% breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities. [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]

Metal particles, such as copper and iron, not only reduce the breakdown voltage but also accelerate the oxidation of insulating oils. The effects of metal particles on the dielectric properties of mineral and natural ester insulating oils were investigated in this work. The experimental results indicate that the AC and impulse breakdown voltages of the insulating oil decrease with the increase of the size and the quantity of the metal particles. The positive breakdown voltages V b+ of insulating oils decrease moderately in the beginning, while the particles quantity increases to a certain extent, the V b+ decreases significantly. In comparison, the metal particles always reduce the negative impulse breakdown voltage V b− significantly. The effect of the copper particles on the insulating oils is slightly greater than that of the iron particles due to their higher conductivity. The permittivity of contaminated insulating oils increases quasi-linearly with the concentration of copper particles. Improved models considering the polarization of the surface oxidation layer of copper particles were proposed, and the calculated data were in good agreement with the measured permittivity of the contaminated insulating oil. [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]