Preparation And Properties Of Low Dielectric Constant Insulating Paper Modified By BTCA

Oil-paper insulation system is the main form of internal insulation of power transformer.Cellulose is the first choice of insulating paper because of its good insulation and mechanical properties,economy and environmental protection.However,there is a mismatch of dielectric constant between cellulose insulating paper and insulating oil,which makes oil gap become a weak link in the internal insulation of oil paper.With the promotion and application of high voltage grades such as extra-high voltage and ultra-high voltage,higher and higher requirements are put forward for the insulation performance of oil paper system.Reducing the polarity and dielectric constant of fiber insulation through chemical and physical modification can effectively improve the distribution uniformity of AC electric field between oil and paper and reduce its dielectric loss,which has important engineering application value for improving the reliable operation of oil-paper insulation of power transformer.1,2,3,4-butane tetracarboxylic acid(BTCA)is widely used as a cellulose cross-linking agent.The carboxyl group on BTCA can react with the hydroxyl group on cellulose,and the unique tetracarboxylic group structure can cross link the cellulose chain together to form a stable network structure while consuming the polar group hydroxyl group,which can reduce the orientation polarization of cellulose under the action of electric field.On this basis,this paper explores the influence of BTCA modification on the dielectric properties of cellulose insulating paper,and tests and analyzes the electrical strength,mechanical strength and thermal stability of the modified paper.First of all,the preparation process of BTCA modified cellulose insulating paper was explored in the laboratory.By controlling the esterification reaction conditions between BTCA and cellulose,the effects of BTCA concentration,baking temperature and baking time on the dielectric constant and other electrical and mechanical properties of the paper were investigated.Then,the modified insulating paper was prepared under the condition of an optimized process to study the long-term thermal stability and the influence on the performance parameters of insulating oil in the process of accelerated thermal aging.Finally,the influence mechanism of BTCA modification on the dielectric properties,mechanical properties and thermal stability of insulating paper were analyzed with molecular dynamics calculation method.The main conclusions and results are as follows:(1)BTCA modification can effectively reduce the dielectric constant and dielectric loss of cellulose insulating paper,and has little effect on the electrical strength and mechanical properties.In this experiment,the modified insulating paper with BTCA concentration of 0.2mol/l,baking temperature of 180 ? and baking time of 2min has excellent performance.At this time,the dielectric constant and dielectric loss of the modified insulating paper are 21.6% and 27.5% lower than that of the ordinary insulating paper.The AC breakdown strength and tensile strength are 9.7% and 9.7%higher than that of ordinary insulating paper,and the decrease of elongation at break is not obvious.(2)Through FTIR characterization and molecular dynamics simulation results,the mechanism of BTCA modification on the dielectric and mechanical properties of cellulose insulating paper was revealed.BTCA cross-linking can reduce the polar group(-OH)on cellulose,decrease the movement of cellulose chain(MSD),and decrease the orientation polarization of cellulose molecules,thus reducing the dielectric constant and dielectric loss of insulating paper.Crosslinking increases the rigidity of cellulose molecules(tensile modulus,shear modulus and elastic modulus increase),but decreases the flexibility(Cauchy pressure and MSD decrease),so that the tensile strength and elongation at break of insulating paper increase.Some carboxyl groups in BTCA are not cross-linked.The greater the degree of cross-linking,the more obvious the effect on dielectric and mechanical properties(3)The effect of BTCA modification on the thermal stability of cellulosic insulating paper was not significant.The mechanism of BTCA modification was revealed by molecular dynamics simulation and FTIR characterization.The dielectric properties,breakdown strength,volume resistivity,tensile strength and other properties of BTCA modified paper are slightly better than those of ordinary paper in the process of 130 ? accelerated thermal aging,and the difference between the two properties reaches the maximum value in the middle of aging,but the difference is not significant as a whole.The modification of BTCA consumes part of the hydrophilic group hydroxyl,and the crosslinking restricts the thermal movement of the molecular chain(the glass transition temperature increases),which is conducive to delaying the thermal aging rate of insulating paper.