Comparative study of streamer phenomena in mineral, synthetic and natural ester oils under lightning impulse voltage

This paper presents the experimental results of a comparative study of mineral oils, synthetic ester oil and natural ester oils (vegetable oils) issued from different origins through the analyse of the initiation threshold voltage of streamers, their pattern and their stopping length (final length) Lf as well as the associated current and electrical charge, in a point - plane electrode arrangement submitted to a lightning impulse voltage (1.2/50 μs). It´s shown that the streamers are filamentary in the different tested oils whatever the polarity of voltage. Lf is longer when the point is positive than when it´s negative whatever the oil. It´s is generally longer in ester oils (natural and synthetic) than in some mineral oils. When the point is negative, Lf can be ten times higher in ester oils than in mineral ones. Basing on the stopping length of streamers, the positive polarity appears as the most unfavorable one for industrial applications and the dielectric strength of mineral oils we tested are generally better (in the considered experimental conditions) than that of ester oils.