Investigations on streamers phenomena in mineral, synthetic and natural ester oils under lightning impulse voltage

This paper presents the experimental results of a comparative study of mineral, synthetic and natural ester oils issued from different origins through the analyze of the initiation threshold voltage of streamers, their pattern and their stopping length (final length) L_f as well as the associated current and electrical charge, in a point - plane electrodes arrangement submitted to a standard 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. The stopping lengths L_f are longer and the conductivity higher when the point is positive than when it´s negative. L_f is generally longer in ester oils (natural and synthetic) than in some mineral oils. When the point is negative, L_f can be ten times higher in ester oils than in mineral ones. Different modes of propagation of streamers are also observed; the transition between these modes of propagation depends on the experimental conditions and the molecular structure of liquid. This transition is well pronounced with some mineral oils when the point is negative.