| The safety of power equipment is the first defense system for the safety of power grid,especially as the hub equipment of power system,the insulation running condition and health level of large oil-immersed power transformer are directly related to the safety and stability of power grid.Under the short-time emergency load condition,the main insulation of the transformer is faced with rapid temperature rise,which leads to the rapid precipitation of moisture in the oil-paper insulation and even the formation of bubbles.The presence of bubbles will reduce the electrical strength of transformer oil insulation,may lead to partial discharge in transformer oil insulation,and even lead to the breakdown of oil paper insulation due to the accumulation of bubbles.In this paper,an equivalent model of oil-paper insulation microstructure was established based on the capillary bundle model.Based on the ideal gas state equation,the evaporation process of water and the boundary pressure conditions of bubbles were taken as the fixed solution conditions,and the bubble generation model of oil paper insulation was established.The numerical results show that the bubble radius does not increase at a uniform rate during the heating process.The bubbles grow slowly in the early stage of warming.When the temperature reaches a certain threshold,the bubbles begin to expand violently and escape quickly.In addition,with the increase of water content,its influence on ITBE(Initial Temperature of Bubble Effect)decreases gradually,and with the increase of interfiber porosity,its influence on ITBE also decreases gradually.However,both of them were negatively correlated with ITBE.Secondly,the experiment of bubble effect caused by heat in oil-paper insulation was designed and carried out.In this part,according to the requirements of the heat induced bubble temperature rise test,a set of test platform was designed and built,including a box body,heating device and measuring device.Through the temperature rise test of winding model,it is concluded that the "stratification phenomenon" between insulating paper layers occurs ahead of the bubble effect,which means that the bubbles have gathered in the insulating paper layers before escaping.The more the number of layers of insulating paper is,the more intense the bubble will be when the sample initially escapes.At the same temperature rise rate,the more the number of insulating layers,the lower the initial temperature of bubble escape.Oil immersed board bubble experiment using the above test platform,a rapid temperature rise test was carried out to observe the bubble effect of vegetable oil impregnated board and its aging samples.Rapid temperature rise at 2k W,2.5k W and3 k W heating power respectively.After accelerated thermal aging at 130℃,the samples were adjusted to the same moisture content in the insulating paper through their respective oil paper moisture balance curves,and then the rapid temperature rise test was conducted to study the influence of thermal aging of cellulose on the bubble effect of oil paper insulation.The test results show that the microbubbles in the oilimpregnated cardboard also gather in the cardboard first,and only after reaching a certain pressure will they form a channel and concentrate to escape.The high temperature rise rate means a more intense escape and a lower ITBE.SEM results show that the influence of thermal aging on oil-impregnated paperboard fiber is that the number and diameter of interfiber pores and capillary tubes decrease with aging time,which inhibits the escape of bubbles and increases the initial escape temperature of bubble effect. |
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