| Oil-paper insulation is widely used as the insulation structure in large power transformers.The high electric field intensity and complex electric field distribution in the transformer put forward high requirements for the performance of oil-paper insulation.Large power transformers will withstand the effect of lightning impulse voltage during operation and the insulation performance of oil-paper directly determines whether the transformer can withstand the impact of lightning impulse.In the process of transformer manufacturing,operation and maintenance,its internal insulating oil will inevitably introduce metal particles.The existence of these metal particles will significantly reduce the performance of transformer oil-paper insulation.However,there is a lack of research on the influence of metal particles on the performance of oil-paper insulation under lightning impulse voltage.Given this,the research work on the impact of metal particles on the lightning impulse discharge characteristics of transformer insulation oil and its oil-paper insulation are carried out in this paper.The main contents are as follows:(1)The lightning impulse discharge test platform for oil-paper insulation was set up and improved.A preparation method of insulating oil containing fixed metal particle concentration is proposed.The appropriate breakdown strength test method of insulation oil was selected through experiments.The results show that the oil-paper insulation discharge test platform can effectively obtain the image of streamer discharge in insulating oil.The rising voltage method is beneficial to reduce the dispersion of the lightning impulse breakdown voltage data.(2)The effect of metal particles on the lightning impulse discharge characteristics of insulating oil was investigated.The effect of metal particles on the streamer development characteristics and breakdown characteristics of the insulating oil was obtained.The mechanism of metal particles on the breakdown process of insulating oil under lightning impulse voltage is analyzed with the development characteristics of streamer.The results show that the metal particles will promote the initiation and propagation of streamer in insulating oil.The stop length and velocity of streamer are positively correlated with the particle concentration,while the initial voltage and breakdown voltage are negatively correlated with the particle concentration.It is considered that the interaction between metal particles and streamer accelerates the development of streamer,which in turn reduces the breakdown voltage of the insulating oil.(3)The effect of metal particles on the lightning impulse discharge characteristics of oil-immersed insulating paper were investigated.The influence of metal particles on the lightning impulse discharge characteristics of two oil-impregnated insulating paper models was obtained.The differences of the streamer characteristics and breakdown characteristics between two oil-paper models are compared and analyzed.The results show that the insulation paper placed along the surface has less effect on promoting the development of streamer and reducing the breakdown voltage than metal particles.The insulation paper placed on the plane will block the formation of main discharge channel and eliminate the influence of metal particles.Therefore,the breakdown voltage of insulating oil is increased.The above work is of great significance to master the influence mechanism of metal particles on the breakdown characteristics of oil-paper insulation,to obtain the breakdown characteristics of oil-paper insulation contained with metallic particles,to guide the design of the internal insulation structure of the large power transformer and the formulation of solid particle standard in insulating oil. |
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