Bridging and discharge mechanisms of cellulose impurities in transformer oil under DC voltage

Cellulose impurities seriously threaten the insulating performance of oil-pressboard insulation. In this paper, we investigate the bridging and discharge mechanisms of cellulose impurities in transformer oil under DC voltage. The results show that under the non-uniform DC electric field, the cellulose impurities are mainly subjected to forces including gravity, buoyancy, dielectrophoretic force and drag force, among which the dielectrophoretic force dominates the bridging of the impurities. By studying the leakage current of dry (moisture 8 ppm) and wet (moisture 26 ppm) oil samples with and without cellulose bridges, deductions are made that the coexistence of cellulose and water in oil leads to the formation of a cellulose-water bridge, which increases the leakage current within the oil, causing a heating effect and gasifying the water on the bridge. Further analysis indicates that when the amount of gasified water exceeds the permeation channel threshold, the cellulose-water bridge transforms into a cellulose-gas bridge. Final oil breakdown happens along this gaseous bridge.