Comparison of lightning breakdown voltage of natural ester liquids of different viscosities

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.