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.

中文翻译：

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直流电压下变压器油中纤维素杂质的桥接放电机制

纤维素杂质严重威胁油纸板绝缘的绝缘性能。在本文中，我们研究了直流电压下变压器油中纤维素杂质的桥接和放电机制。结果表明，在非均匀直流电场作用下，纤维素杂质主要受到重力、浮力、介电泳力和拖曳力等力的作用，其中介电泳力主导杂质的架桥。通过研究有和没有纤维素桥的干（水分 8 ppm）和湿（水分 26 ppm）油样的泄漏电流，推断出油中纤维素和水共存导致纤维素-水桥的形成，这会增加油中的泄漏电流，造成热效应并使桥上的水气化。进一步分析表明，当气化水量超过渗透通道阈值时，纤维素-水桥转变为纤维素-气桥。最终的石油分解发生在这个气态桥上。