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Iron particle effect on the breakdown strength of insulating oil at AC and DC voltages

The metallic particles originating from the process of installation and operation of converter transformers can affect the insulating performance of transformer oil when subjected to electric field containing DC component and may cause discharge accidents. Therefore, the influence of particles on the breakdown strength of insulating oil at DC voltage and the mechanism behind the phenomena has gained much attention and needs to be further investigated. The breakdown voltage test of insulating oil contaminated with iron particles of different sizes under AC and DC electric field was carried out according to ASTM D877 and the influence of iron particles on breakdown voltage was obtained. With the change of iron particle concentration and particle size, the change regulation of breakdown voltage of insulating oil under AC and DC electric fields was obtained. In the meanwhile, this paper analyzed the electric field distribution of insulating oil containing different particle sizes of iron particles under AC and DC electric fields with COMSOL simulation software. The causes of the experimental phenomena are analyzed with the simulation results. The simulation results show that the maximum field strength and non-uniformity factor inside insulating oil will increase with the increase of particle concentration. Besides, the experimental results show that the small impurity bridge is more likely to form with the change of electric field, especially under the DC electric field, thus increasing the probability of breakdown.