Electrical strength of paper-oil insulation subjected to composite voltages

The results of a composite voltage, ac with lightning impulse (LI) tests of a paper-oil insulation model are presented. It was demonstrated that the presence of the background ac voltage leads to a decrease in LI breakdown voltage. This is particularly conspicuous at opposite polarities. The dispersion of LI breakdown voltages decreases save for the case when the LI is superimposed on the opposite polarities. The dispersion of LI breakdown voltages decreases save for the case, when the LI is superimposed on the opposite polarity peak value of the ac sinusoid. In this critical case the combined breakdown voltage (ac+LI) may be even lower than the ac one