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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.

This article presents the results of the studies on lightning impulse breakdown voltage measurements performed for two commercial natural ester liquids of different viscosities. These viscosities measured at 100°C were 8.0 mm2/s and 4.6 mm2/s respectively. The dielectric test results presented in this paper concern the measurements performed using the recommendations of the IEC 897 standard where point-to-sphere electrode system is dictated. In addition to measuring the breakdown voltage other experimental techniques were applied such as time to breakdown estimation, light emission registration using the photomultiplier and static photograph collection. The results showed that the differences between the liquids tested concern only positive polarity of lightning impulse voltage. In the case of negative polarity from the standard based calculations and statistical analysis using Weibull distribution the breakdown voltages may be treated as a very similar. In turn the other discharge characteristics such as light waveforms, static photographs and propagation velocities did not showed any differences. In the case of positive lightning breakdown voltage the results were assessed only on the basis of the average values calculated because a deep statistical analysis was not possible due to the small scattering of data. These results indicated on slightly better lightning properties of the natural ester of higher viscosity. Confirmation of this conclusion shall constitute the light waveforms registered where the discharges developing in the low viscosity natural ester are characterized by higher intensity than corresponding discharges developing in the “classical” natural ester. However the static photos did not indicate any differences between the esters tested.

This paper concerns the studies on streamer propagation in mineral oil, synthetic ester and natural ester at standard lightning impulse inception voltage in small point-plane gaps. The inception voltages of both polarities were measured using step method and then analysed statistically. Simultaneously, streamer characteristics such as shape, emitted light and propagation velocity were compared between the liquids tested. The obtained results showed that the inception voltages were close to each other for all the three liquids tested. Concerning streamer characteristics, it was seen that the spatial shapes of the streamers were similar taking into consideration the given voltage polarity. The same may be said about the propagation velocities, indicating that the velocities of the positive streamers were twice as high as the negative ones. In the case of registered time courses of emitted light, the frequencies of the light pulses corresponding to the propagation of the streamers in both esters were a little bit higher than the pulses emitted by the streamers, which developed in mineral oil. In general, in all three liquids the streamers at inception voltages developed as so-called `stopping length' streamers.

This article presents the results of the studies on positive streamers behavior in an insulation system of non-uniform electric field distribution where point-to-sphere electrode system was divided on two parts by placing the pressboard barrier in the middle of oil gap. The studies were performed for two synthetic esters as per IEC 61099, natural ester as per 62770, blended low viscosity ester and mineral oil as per IEC 60296. The experimental techniques used in the measurements were both electrical and optical ones. The results showed that independently of the liquids tested lightning strength of the insulation system increased circa 3 times in relation to the system without pressboard barrier. In turn in the waveforms of light emitted by the streamers propagating in esters no differences were observed between them. However in comparison to the mineral oil duration of streamer propagation is longer in esters and intensity of emitted light is higher. In general the results indicated quite similar properties of the streamers developing in all the liquids tested in the condition of the experiment.

This article presents the results of the studies on negative streamer propagation in a point-to-sphere electrode system with a pressboard barrier placed between them. The proposed electrode system gave the opportunity to assess the influence of the insulating barrier on streamer development in the conditions close to the actual transformer insulating system where the liquid gap is typically divided in parts by using pressboard barriers. The studies were performed for five commercial dielectric liquids. Among them two were biodegradable synthetic esters and two were biodegradable natural esters. Mineral oil, as the fifth liquid, was used for comparison. The measurements were based on electrical and optical experimental techniques. From the results obtained it may be concluded that, independently of the liquids tested, the electrical strength of the insulating system considered was increased by about 50%. In the case of streamer development assessed using photomultiplier-based light registration it is not possible to indicate clearly which of the liquids tested is better under the conditions of the experiment. In all cases streamers always developed slowly (2nd mode) at all voltage levels applied during the studies. In turn, the intensity of the discharge processes, comparing the same voltage levels, was mostly higher when streamers developed in ester liquids, however, the differences noticed were minimal.

This article presents the results of experimental studies concerning the lightning characteristics of two different commercial synthetic ester liquids and mineral oil. The measurements were conducted at both negative and positive standard lightning impulse voltage using a point-to-sphere electrode system. The step method recommended by the IEC 897 Standard was extended to the light emission detection using a photomultiplier technique. On this basis comparisons were made using the point of view of lightning impulse breakdown voltages (LIBV) obtained, times to breakdown (TtB) measured and the light oscillograms registered. 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 while at positive polarity the results show an improvement. Concerning the light emission, the mineral oil tested behaves as more resistant against the appearance of fast light pulses that indicate the intense stepwise propagating discharges.

This article describes the measurement techniques used for the study of mechanisms of electrical discharge development in ester fluids under lightning impulse voltage. These techniques were applied in a laboratory experimental system which enabled the acquisition of a wide range of experimental data. An analysis of the data gives the possibility of assessing the processes responsible for electrical discharge propagation in different types of dielectric liquids. The photographic registration system provides photographs of developing discharges. This uses the shadowgraph method with an impulse laser as a flash lamp. The system of light emission registration enables collection of the time courses of light emitted by the developing discharge. Both systems operating together are synchronized using light guide communication. They are also unaffected by external disturbances such as network overvoltages and high electrical field stress. Preliminary results obtained on the basis of the described techniques, in the field of electrical discharge development in synthetic and natural esters, are presented in the article. These results confirm suitability of the methods used and give the possibility to formulate first conclusions.

This article presents the results of the experimental studies concerning the lightning impulse breakdown voltage estimation of two natural ester liquids of different viscosities. First ester was characterized by viscosity equal to 8.0 mm2/s while second had viscosity equal to 4.6 mm2/s (both measured at 100 °C). In the latter case this value was much closer to the typical viscosity of mineral oil. The dielectric test results presented in the paper concern the measurements performed using the IEC 897 standard where point-to-sphere electrode system is recommended. The lightning impulse voltage supplied to the electrode system was a negative standard lightning impulse 1.2/50 μs. A gap distance was 25 mm and a method of breakdown voltage estimation was a step method with 5 kV voltage increment. The measurements were supported by light emission registration using the photomultiplier technique. The results showed that the differences between the liquids tested were very small. The median of breakdown voltages was higher for the “classical” natural ester while location parameter of Weibull distribution was slightly higher for the low viscosity natural ester. Simultaneously, lack of the differences was noticed between the oscillograms of light registered for the given liquid.

This article presents the results of experimental studies concerning comparative measurements of electrical strength of pressboard specimens impregnated by different dielectric liquids: mineral oil, synthetic ester and natural ester. The electrode systems used in the experiment was system of parallel electrodes proposed in standard IEC 60243. The specimens had the thicknesses of 0.5 and 1 mm respectively. Impregnation was realized for all three liquids in the same way. The results were analyzed both on the basis medians calculated and using three-parameter Weibull distribution. In this latter special emphasis was put on location parameter Vo meaning the threshold value of voltage below which breakdown should not occur. Comparison of the results obtained from the point of view of the liquids used for impregnation showed that electrical strength calculated on the basis of median was very similar for both thicknesses. However for Weibull distribution based analysis higher electrical strength characterized specimens impregnated by mineral oil.

This article presents the results of the measurements of lightning breakdown voltage of three different commercial dielectric liquids of different chemical composition: two natural esters and one synthetic ester. The measurements were performed in point-to-sphere electrode system which is recommended by the IEC 897 standard. The voltage supplied was a positive standard lightning impulse 1.2/50 μs. A gap distance was 25 mm and a method of breakdown voltage estimation was a step method with 5 kV voltage increment. The results showed that the differences between the liquids tested exist. The average value of breakdown voltages from 20 measurements was the highest for one of the natural ester (marked as natural ester I) with simultaneous the highest value of time to breakdown. This means that in the testing conditions applied in the studies presented, in a non-uniform electrical field at positive standard lightning impulse voltage, this type of the ester is characterized by the best lightning strength.