Copper compound deposition for decreasing electrical insulation in aged oil-filled cables

Copper compounds, such as copper sulfide, are aggregated and deposited at specific locations on insulating paper in aged oil-filled cables via oil flow and dielectrophoresis. Thus, the depositions produce various black shapes such as straight lines along oil gap, spots, and tree-like shapes. Herein, the deposition of copper compounds and the effect on electrical insulation were investigated for each deposited shape using Fourier-transform infrared spectroscopy, Raman spectroscopy, and energy dispersive X-ray spectrometry. The results indicate that copper compounds deposit on all shapes, whereas copper sulfide deposits only on some portions of the shapes. Moreover, the electric charging degradation sites show low insulation performance because of having a starting point of partial discharge. In other words, an analysis of aged cables revealed that this deposition phenomenon was a type of oil-filled cable degradation.