| Epoxy resin is widely used in the construction of DC power system in China due to its excellent insulation characteristics and low cost.However,under the continuous stress of the DC electric field,charge accumulation centers are easily formed on the surface as well as in matrix of epoxy resin and cause electric field distortion,which may lead to a decline in the insulation performance of the material,and even result in severe accidents such as flashover or breakdown,threatening the stable operation of the DC power system.Thus,it is of great significance to study the technical method of improving the DC insulation characteristics in epoxy resin.In this paper,the comprehensive regulation of nanocomposite and gas-phase fluorination on the electrical properties of epoxy resin was probed,and on this basis the interaction mechanism between surface charge dynamic characteristics and DC flashover,as well as between space charge dynamic characteristics and DC breakdown are investigated.The main contents are as follows:(1)Epoxy resin/Al2O3 nanocomposites with nanofiller content of 0,1,3 and 5wt%were prepared,and each group of samples was subjected to 0,15,30,and 60 min by gas phase fluorination.The bonding structure and micro-morphology of fluorinated epoxy resin nanocomposite were studied.The results showed that the fluorination would introduce a fluorination layer with a thickness of about 1 to 2?m on the surface of the epoxy resin through substitution and addition reactions,and the content of nano-filler should not exceed 5wt%,otherwise it will cause serious agglomeration.In addition,fluorination could significantly increase the surface roughness of epoxy resin,but nanocomposite can suppress this trend,indicating that there is a certain competition between the two modification.(2)The effects of the nanocomposite and fluorination on the charge dynamic characteristics,trap distribution and flashover performance of epoxy resins were measured,and the interaction mechanism between the surface charge and flashover performance was investigated.The results showed that nano-Al2O3 composite could introduce a large number of deep traps on the surface of epoxy resin and reduce the surface potential decay rate,and also cause the DC flashover strength to decrease.The fluorination could reduce the surface trap energy level of epoxy by introducing chemical defects,which accelerates the surface potential decay trend and significantly improves the DC flashover strength.The surface charge scan results revealed that the surface charge before the flashover mainly accumulated near the top of electrode,and the nanocomposite would intensify the density of the charge accumulation center,which distorted the electric field and assisted the generation and development of streamer,thereby reducing the DC flashover voltage in epoxy nanocomposite.After the flashover,an equipotential discharge channel will appear on the surface of the sample.This channel has a relatively low charge density and a polarity opposite to the flashover experiment.(3)The effects of different nano-filler content and fluorination time on the dielectric properties,space charge distribution,and breakdown performance of epoxy resin were obtained,and the mechanism of space charge on the DC breakdown process was discussed.The results showed that nano-Al2O3 doping would increase the dielectric constant and dielectric loss of the epoxy,while the fluorination caused the dielectric constant to decrease but the dielectric loss to increase.Both nanocomposite and fluorination could inhibit the space charge injection behavior in epoxy resin.According to analysis,deep traps introduced by nanocomposite could capture a large number of homocharges in the epoxy surface to weaken the effective electric field at the interface,while fluorination could reduce the free volume on the epoxy surface,leading to the contraction of charge migration channels.These two phenomena cloud increase the interface barrier to shield the space charge injection,thereby weakening the bombardment and destruction of epoxy molecular structure during the migration or recombination of space charges,thus improving DC breakdown strength of epoxy resin. |
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