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The breakdown voltage of mineral insulation oil was systematically investigated under the influences of moisture, acidity, pressure and particles. The standards IEC156/95 (VDE0370/Part5/96 and ASTM D1816) specified the test conditions.
Moisture provides charge carriers, therefore a moisture saturation of 0 to 20 % decreases the breakdown voltage from 72 down to 61 kV.
Acids as aging products decrease the dielectric strength too for the same reason. Not the total acid number TAN, but the low molecular acids effect the major decrease. Since the breakdown process starts with a microscopic bubble, an increasing pressure increases the breakdown voltage too. The observed asymptotic behaviour seems to come to a maximal end value of the breakdown voltage at around 150 kV. Under-inflation supports bubble generation, decreasing the dielectric withstand strength.
Dry particles (cellulose fibres) decrease the breakdown voltage only at under-inflation, nearly no influence was observed at atmospheric pressure.
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