Aging Characteristics Of Transformer Oil-Paper Insulation Under Continuous Lightning Impulses

With the continued global climate change,thunderstorm weather was becoming more frequent and the probability of transformers being subjected to continuous lightning impulse was increasing.As the core insulation system of oil-immersed transformers,the damage degree of oil-paper insulation constantly deepens under the cumulative effect of lightning impulse voltage.With the continuous increase of service life,transformers affect the safe operation of transformers.In order to study the influence of multiple lightning impulse voltages on the oilpaper insulation of transformers,the experimental platform of lightning impulse was built to simulate the effect of lightning impulse voltage on oil-paper insulation.An automatic switching device for high-voltage relays in polarization/depolarization current testing was developed.The oil-paper insulation test sample was made,and a continuous lightning impulse test was carried out on the sample.Then,the polarization and depolarization current tests were performed on the samples with different number of lightning impulse.Subsequently,the micromorphology changes of insulation paper were observed by scanning electron microscopy.The tensile strength of each paper and the water mass fraction of insulating oil were measured.Finally,the effect of the insulating paper on the strain of insulation paper was analyzed through the above tests.The depolarization trap energy level density spectrum of oil-paper insulation samples was obtained based on the depolarization current value.The relationship between the trap level density and aging degree of oil-paper insulation was analyzed.The results indicate that,with the increase in the number of applied lightning impulses,the deterioration degree of the fiber structure increased and the stress maximum decreased.The moisture content of the insulating oil increased,and the color of the oil deepened.The polarization/ depolarization current and the depolarizing trap density curves of oil-paper insulation moved upward.The peak value of depolarizing trap density curve and its peak time increased.In the macroscopic view,the deterioration of the fiber structure of the insulation paper was reflected in the decrease of the stress peak value.Under the effect of lightning impulses,the peak of depolarizing trap density curve and its peak time could be used as the aging characteristic quantity of oil-paper insulation.