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The microstructures of a XLPE cable sample in service for one year (sample-A) and a fresh XLPE cable sample (sample-B), which were applied high electrical stress in order to initiate electrical tree, were observed by means of scan electronic microscope (SEM) and optic microscopy. It is found that the electrical trees in sample-A are longer in length and more in dimensionality than those of sample-B. Some special traces appear in SEM image of sample-A, which are thought to be additive crystallite separating from polymer under high electrical stress. Dielectric loss tangent (tanδ) of sample-A at 110i is higher 30% than that of sample-B. However phenomenon of insulation resistance of sample-A falling did not be observed in the test, which is different from the monitoring results of sample-A in service.