The volume and area effect in transformer oil

The breakdown voltage, both power frequency ac and standard lightning impulse, for transformer insulating oil was measured in especially designed test chamber, The test chamber used in these tests allows to perform investigations on full-scale parts of the high voltage power transformer insulation utilizing kraft paper and transformerboard saturated with oil. The results obtained for the breakdown voltage of mineral oil were influenced by the kind of electrode arrangement used for the measurements, oil preparation, as well as a type of applied voltage. The prebreakdown state under ac voltage was observed using partial discharge monitoring. The breakdown ac stress was calculated for a step-raised voltage and a ramp voltage. The breakdown voltage under lightning impulse was obtained using the up-and-down method. The investigation objectives were as follows: (i) to correlate the prebreakdown activity with breakdown stress, (ii) to verify the breakdown stress of investigated commercial oil against published data, (iii) to test the large-scale chamber before testing the insulation parts of power transformer.