Research On The Ethylation Of Vegetable Insulating Oil And Its Dielectric Performances

Vegetable insulating oil is a new type of environment-friendly liquid dielectric.It has the advantages of high ignition point and good biodegradability which has been widely used in distribution transformers.However,vegetable insulating oil generally has the problems of relatively high dielectric loss,large kinematic viscosity,and weak antioxidant capacity.In view of the above problems,the method of ethyl ester modification which are dependent on moleculer structure of vegetable insulating oil was studied.A multi-stage purification method of ethyl esterified vegetable insulating oil and antioxidant synergy technology were developed.The physico-chemical and dielectric properties of the fatty acid ethyl ester were tested and analyzed.The main research content of this paper is:（1）The kinetics of the esterification reaction of vegatable insulating oil and ethonal was studied,and the kinetic parameters and activation energy of the esterification reaction were obtained.The KOH/Al₂O₃ solid base catalyst which reduced the activation energy was prepared by co-precipitation method.The optimal conditions were molar ratio of ethonal to oil 8:1,reaction temperature 65°C,reaction time 30 min,solid base catalyst mass fraction 0.9%obtained by controlling variable method.Under these conditions,the conversion of fatty acid ethyl ester reached 87%and the kinematic viscosity decreased by 84.4%.（2）The process of refining fatty acid ethyl ester was studied,and the adsorbent compounding technology of fatty acid ethyl ester was developed.The effect of type,proportion and adsorption temperature of adsorbent on the dielectric properties of fatty acid ethyl ester was discovered.The results show that when the adsorbent content was5wt%,mineral sand:silica gel:molecular sieve=1:2:1,and the adsorption temperature was 60°C,the dielectric properties of fatty acid ethyl ester increase significantly.The breakdown voltage of the ethyl esterified vegetable insulating oil after refining and adsorption treatment was increased by 160.1%,the dielectric loss factor was reduced by95.4%,the volume resistivity was increased by 16 times.The dielectric constant was2.93,and the acid value was 0.05 mgKOH·g^-11 and the kinematic viscosity was 5.6mm²·s^-1.（3）In order to improve the antioxidant property of fatty acid ethyl ester,the antioxidants selected by the PDSC method were added in fatty acid ethyl ester.The synergistic effect of three antioxidant formulations was due to the steric effect of the phenolic antioxidants.The oxidation stability of oil samples after adding antioxidant was tested.The results showed that the fatty acid ethyl ester had good antioxidant performance when the content of antioxidants was TBHQ（0.4%）+VE（0.1%）.（4）The power frequency breakdown voltage,dielectric constant,dielectric dissipation factor and volume resistivity of each preparation step of fatty acid ethyl ester were tested and analyzed;the effects of temperature and frequency on different molecular structures were also studied.The breakdown voltage was 51 kV,the relative permittivity was 2.93,the dielectric loss factor was 2.29%,the volume resistivity was1.12×10¹⁰Ω·m and the viscosity was 5.6 mm²·s^-1.