The presence of impurity particles in insulating oil is one of the main reasons for the deterioration of insulation performance. In order to promote the application of natural ester and control the quality of natural ester oil

the effects of fiber and metal particles on the breakdown characteristics of natural ester under AC and DC voltages were studied. The two-parameter Weibull distribution model was used to analyze the breakdown data under AC and DC voltages

and the particle accumulation patterns in the process of oil breakdown were recorded. The results show that the AC and DC breakdown voltages of natural ester decrease linearly with the increase of the concentration of fibers or metal particles in natural ester. The DC breakdown voltage of natural ester decreases more significantly with the increase of the concentration of fibers or metal particles than with the increase of AC voltage. Natural ester containing copper particles exhibits a lower breakdown strength than oil containing fiber particles under AC or DC voltages. The natural ester containing particles shows a particle accumulation phenomenon in the breakdown process under DC preloading pattern. By comparing the breakdown strength of natural ester under AC and DC voltage

it is shown that the formation of bridging particles is the reason that the breakdown strength of natural ester under DC preloading decreases greatly compared with that in the absence of DC voltage.