Molecular Design And Properties Of Low Dielectric Constant Cellulose Insulation With Network Structure By Citric Acid Crosslinking

The oil-paper composite insulation system is a vital component in oil-immersed power transformers,and its performance directly affects the safe operation of transformers.On account of a lot of hydroxyl groups in cellulose structure,the structure has strongly polar,which can potentially result in a high dielectric constant and dielectric loss.After oil immersion treatment,its permittivity is approximately twice the permittivity of oil.According to the two-dielectric series model,the electric field strength of the oil duct is two times the strength of cellulose paper under an AC voltage.However,the electric breakdown strength of mineral oil is much lower than that of cellulose paper.Small oil ducts become the weakest of the entire insulation system.Cellulose insulation with low dielectric constant and loss can uniform the distribution of the electric field and increases the reliability of transformers.In almost all major studies,the dielectric constant and loss were reduced at the expense of the mechanical properties of paper.Synthetic fiber has not been widely popularized in domestic owing to its high cost and technological monopoly.Additionally,the correlation mechanism between the molecular structure of cellulose insulation and its properties is vague,causing low efficiency and blindness of modification researches.Thus,scientific methods were needed to prepare modified cellulose insulation with low dielectric constant and loss,and it is important for China’s power equipment and manufacture industry of insulation materials.Firstly,we explore the parameters for characterizing the static dielectric constant and dielectric loss of cellulose system via molecular dynamics simulation.The simulation results and experimental values of three types of cellulose insulation were compared to verify the rationality of the proposed method.Then,a network structure for cellulose insulation was proposed for decreasing its dielectric constant and loss,meanwhile maintaining its other properties.Using the parameter system,the crosslinking agent for forming network structure in cellulose was selected,the effects of its concentration and crosslinking degree on the dielectric properties,mechanical properties and thermal stability of modified cellulose system were explored.According to the design scheme,we prepared a new insulation paper,which was consistent with the crosslinking degree and molecular structure of design scheme.The preparation method can directly treat the formed insulation paper rather than cellulose pulp.The electrical properties and operation stability of the modified cellulose insulation after oil immersion were studied.The main results are as follows:（1）We explored the method for calculating the static dielectric constantε_s and parameters for characterizing the dielectric loss of cellulose system.To calculateε_s of fiber system,the fluctuation method was adopted.Three key parameters for the accuracy of the method were determined:the permittivity of the surrounding mediumε_RF→∞,the lower simulation time was 15 ns,and the reaction field approximation was adopted to calculate the dipole-dipole interaction instead of true interaction.To improve the efficiency of predicting the dielectric loss of fiber system,mean square displacement and polarizability were proposed to characterize the dielectric loss of fiber system.The polarizability is the ratio of the total dipole moment to the volume of the model.Hydrogen bond,free volume fraction and polarizability were proposed to reveal the mechanism of dielectric loss.The rationality of the proposed method was verified via comparing the results of simulation and experiment of three types of cellulose insulation.（2）A network structure for cellulose system was proposed to reduce the polarity.Using the parameter system,the crosslinking agent for forming network structure in cellulose was selected,the effects of its concentration and crosslinking degree on the dielectric properties,mechanical properties and thermal stability of modified cellulose system were explored.Firstly,among three crosslinking agents,citric acid（CA）is preferred as the crosslinking agent for the preparation of cellulose insulation with network structure.The effect of concentration and degree of crosslinking agent on the basic properties of CA-treated cellulose insulation was explored.When CA was completely crosslinked,the polarizability and static permittivity of the system decreased as the concentration increased.The network structure makes it harder for cellulose chains and side groups to rotate,leading to the reduction of dielectric loss.If CA was excessive,the unreacted carboxyl groups could not only have a negative effect on the polarizability,static permittivity and dielectric loss,but also thermal stability and mechanical properties.（3）CA-treated cellulose insulation with network structure was prepared in the laboratory.The effects of CA concentration and curing temperature on the dielectric properties,mechanical properties,breakdown properties and volume resistivity of CA-treated cellulose insulation were explored.The key preparation process parameters curing temperature and crosslinker concentration were optimized.Compared with the unmodified insulation paper,the dielectric constant and loss of the solid part of CA-treated cellulose insulation were reduced by 15.39%and 29.78%respectively;The tensile strength,elongation at break and AC breakdown field strength increased by9.43%,5.29%and 4.76%respectively;In addition to volume resistivity,citric acid modification had no negative effect on other properties of insulation paper,such as oil absorption and swelling,water absorption and dehydration.The existence of network structure in CA crosslinked insulation paper was confirmed via X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FTIR）and dissolution experiments.（4）The electrical properties and accelerated thermal aging tests of CA crosslinked cellulose insulation and mineral oil were carried out.The results shown that the modified composite system has excellent electrical insulation properties.The CA modification has little effect on the long-term operation compatibility of new composite system.In the system with the width 1-5 mm for oil gap,the AC breakdown voltage of new composite system was higher than that of unmodified composite system;When the applied voltage is 34 k V,the discharge repetition rate and discharge energy of the modified composite system were reduced by 49.00%and 46.47%respectively,and the electrical life index was increased by 37.94%,compared with the unmodified composite system with the gap of 1 mm.During accelerated thermal aging process at130℃,CA crosslinked cellulose insulation with network structure has slightly higher tensile strength,elongation at break and AC short-time breakdown strength.The thermal life of two type of cellulose insulation was similar.In the aging process,the modification of cellulose insulation has little effect on the properties of mineral insulation oil.