Study On The Discharge-related Parameters Of Triglyceride Molecules And Vegetable Insulating Oils Based On Density Functional Theory

Vegetable insulating oil is a high-ignition point,environmentally friendly liquid dielectric with good insulation properties and is considered as a potential substitute for mineral insulating oil.At present,vegetable insulating oil has been well used in distribution transformers,and has gradually begun to be applied to large power transformers.The main component of vegetable insulating oil are triglyceride molecules with various degrees of unsaturation,which cause the microscopic mechanism of the generation and dissipation of charged particles in vegetable insulating oil to be significantly different from mineral insulating oil.The final performance is the difference in discharge characteristics of the two types of insulating oil.Therefore,it is necessary to systematically study the law of generation,dissipation and migration of charged particles in vegetable insulating oil,and establish the correlation of discharge-related parameters between the microscopic level of triglyceride molecules and the macroscopic level of vegetable insulating oil.The research results provide a theoretical basis for the analysis of the discharge phenomenon in vegetable insulating oil,and provide experimental data and theoretical support for the design and manufacture of power transformers.The author carries out the first-principles study of triglycerides according to the type of triglyceride molecule in vegetable insulating oil and establish the correlation of discharge-related parameters between the microscopic level of triglyceride molecules and the macroscopic level of vegetable insulating oil.Based on this,the trap characteristics of vegetable insulating oil and the streamer phenomenon in long oil gaps are studied.This article mainly includes the following contents:(1)The variation law and mechanism of the electron affinity of triglyceride with unsaturation are studied.The results show that triglyceride molecules mainly rely on ester groups and nearby atoms to trap electrons;with the increasing of triglyceride molecular unsaturation,the distribution of LUMO orbital components on the ester group and nearby atoms decreases,which leads to the increase of electron affinity.The relationship between the trap characteristics of vegetable insulating oil and the electron affinity of triglyceride is established.The trap depth of vegetable insulating oil is calculated and the maximum chemical trap depth is 0.16 eV.The trap depth of vegetable insulating oil is measured by thermal stimulation current method,and the rationality of the calculation results are verified.(2)The variation law and mechanism of the ionization potential of triglyceride with unsaturation are studied.The results show that the unsaturated triglycerides provide ionized electrons mainly through carbon atoms on C=C double bonds,and saturated triglycerides provide ionized electrons mainly through oxygen atoms on C=O double bonds;the ionization potential of triglycerides is related to both the position and the number of double bonds;the contribution of carbon atom on C=C double bond to the molecular HOMO orbital component increases with the increasing of triglyceride unsaturation,which leads to the decreasing of ionization potential;the magnitude of the decrease in ionization potential gradually decreases,and the ionization potential of the fully unsaturated triglyceride molecules is not much different.Based on the stereospecific analysis results and the change of ionization potential with unsaturation,the ionization potential distribution of vegetable insulating oil is studied.The results shows that most of the triglyceride molecules in vegetable insulating oil are low ionization potential.A long gap discharge test platform is built.The differences of streamer characteristics among vegetable insulating oil,mineral insulating oil,1-methylnaphthalene and mixed oil are tested and the ionization potential distribution of vegetable insulating oil is verified.Based on the difference of ionization potential distribution between vegetable insulating oil and mineral insulating oil,as well as the test results,the empirical formula of the breakdown voltage of vegetable insulating oil with the length of the oil gap is obtained.The influence mechanism of ionization potential distribution on streamer characteristics of insulating oil is analyzed.(3)The geometry and molecular orbital changes of triolein molecules under electric field are studied and the evolution mechanism of molecular orbitals is analyzed.The results show that the changes in the orbital composition cause the HOMO orbital energy of triolein to increase,resulting in a decrease in the ionization potential of the molecule.The HOMO orbital energies of typical molecules in insulating oils under different electric field are calculated and the difference between the molecular orbital evolution mechanism of typical molecular molecules is analyzed.The results show that,compared to aromatic and alkane molecules,triglyceride molecules are more susceptible to electric field.A method for calculating the ionization potential of triglyceride molecules under electric field based on orbital energy and scissors correction is proposed.The calculated results are in good agreement with the existing results.(4)By analyzing the physical process of discharge,a carrier generation model considering the distribution of ionization potential is established.Using the finite element simulation method,the sreamer characteristics in the two insulating oils and influencing factor are analyzed.The simulation results show that the difference in the ionization potential distribution between the vegetable oil and the mineral oil is the cause of the difference in the streamer velocity and shape of the two types of insulating oils.The simulation model and result can accurately describe the streamer development characteristics of vegetable insulating oil and mineral insulating oil.This work is an preliminary exploration of discharge mechanism in vegetable insulating oil.The results could offer the theory basis for the improvement of the performance of vegetable insulation oil,and provide theoretical support for the design and manufacture of vegetable insulation oil filled power equipment.