Breakdown Characteristics of Natural and Synthetic Ester Liquids When Containing Varying Levels of Moisture

Ester insulating liquids provide a number of benefits when compared with conventional naphthenic oils. To increase acceptance of esters as a long term replacement for mineral oils in high voltage/pulsed power systems, it is of paramount importance that their dielectric properties are well known. This paper examines the breakdown voltage, and time to breakdown, of MIDEL 7131 synthetic ester, the natural ester Envirotemp FR3 and a mineral oil: Shell Diala S4 ZX, at varying levels of moisture saturation, when exposed to HV impulses of both positive and negative polarity. No statistically significant difference in the breakdown voltage of the ester liquids with increasing relative humidity for either polarity was observed. Noticeable polarity dependence in breakdown voltage values was observed with Envirotemp FR3 liquid, which possesses the highest breakdown strength under positive impulses, but the lowest under negative impulses. Inversely, MIDEL 7131 exhibits the lowest breakdown voltage for positive impulses, and the highest breakdown voltage under negative impulsive energisation.