Comparative analysis of lightning breakdown voltage of natural ester liquids of different viscosities supported by light emission measurement

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.