This paper presents the results of the studies concerning propagation of the streamers in natural and synthetic ester under lightning impulse 1.2/50 μs at inception voltage. The results are presented in the form of comparison with the results obtained for mineral oil at the same conditions. For the different interelectrode gaps changed from 5 to 20 mm, the inception voltage and propagation velocity were measured and analyzed. Additionally, the oscillograms of light emitted during discharge development under inception voltage and shapes of discharge forms recorded photographically for all three dielectric liquids, were compared. The obtained results showed that at the inception voltage both the light courses and shapes of discharge forms are very similar in all three cases, depending on voltage polarity. Identical values are also observed in the case of propagation velocity of the streamers and in the case of inception voltages determined statistically using maximum likelihood method.

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