Evidence for transitions in the prebreakdown mechanism of liquid dielectrics

Bubbles in Insulating Liquids: A Short Review

“…The observation made in [29] that in highly divergent fields, breakdown is preceded by the formation of a region of very low refractive index at the cathode seems to support the thoughts in [28]. It has to be noted, however, that such a low refractive index region was attributed by some to a gas phase composed of microbubbles in the liquid [30][31][32][33] and by others to a plasma raising the temperature of the liquid [2].…”

“…Pressure effects disappear when the applied field rate of voltage rise is greater than 4 MV/cm/μsec, indicating that the bubble mechanism may not be operative beyond a certain value of rate rise [36]. A combination of the reported theories between [2] and [30][31][32][33] was tried in [37], where it was proposed that although the initial stage of breakdown in transformer oil is due to a gas bubble formed by vaporization of liquid by local heating in a strong field region of the electrode surface, during breakdown a plasma channel will be formed with a high initial resistance. Such initial resistance changes from a few ohms to a few hundreds milliohms due to Joule heating caused by the arc current flowing through the arc channel.…”

“…Liquid flow and its effect upon the dissolution of bubbles is a subject needing further work. It is already known that circulating an insulating liquid offers higher breakdown strength than a stationary oil [7,30,60]. It is, however, questionable whether such a liquid flow effect is due simply to the liquid flow velocity or whether other parameters, such as liquid viscosity, liquid density and surface tension play an equally important role.…”