| Plant oil-based insulating oil(such as FR3(?)insulating oil)is a liquid dielectric with high flash point and easy biodegradation.In the oil-paper insulation system,it can effectively delay the aging of the insulating paper,significantly improve the overload capacity and safe operation level of oil-filled power equipment such as transformers.Plant oil-based insulating oil is particularly suitable for the places with high safety and environmental protection requirements such as hydropower,offshore wind power,modern cities,and locomotive traction.Plant oil-based insulating oil has been well applied in distribution transformers,and gradually applied in power transformers with higher voltage level.However,Plant oil-based insulating oils has low anti-aging ability,high viscosity,and low lightning impulse breakdown voltage,which seriously hinder its application in power transformers with higher voltage level.Therefore,the development of a new plant oil-based insulating oil with anti-aging,high ignition point and low viscosity has important academic significance and engineering application value for the application of safety and environmental protection transformers.In this paper,a new medium-chain fatty acid triester(MTT)with low viscosity,low pour point,high fire point,and strong aging resistance was successfully prepared using saturated medium-chain fatty acids derived from woody plants and trimethylolpropane.In the oil-paper insulation system,the anti-aging ability of MTT insulating oil was experimentally studied,and the anti-aging mechanism of the insulating oil was analyzed based on the ReaxFF(Reactive Force-Field)reaction force field.The experiments on the breakdown and streamers discharge of MTT insulating oil were carried out.The molecular electrical characteristics(ionization energy and electron affinity)of MTT insulating oil were calculated by density functional theory(DFT),and the relationship between molecular electrical characteristics and breakdown and streamer discharge properties was obtained.① MTT insulating oil was successfully prepared by using medium-chain saturated fatty acids and trimethylolpropane.The effects of catalyst type and dosage,reaction temperature and acid-alcohol molar ratio on the yield of MTT insulating oil were studied.A comparative study of the basic physical and chemical properties of MTT and FR3 insulating oils shows that the fire point of MTT insulating oils is as high as 298℃,and its viscosity is only about half of that of FR3 insulating oils at transformer operating temperature.In addition,the pour point of MTT insulating oil is as low as-45℃(significantly lower than that of FR3 oil(-21℃)).The dielectric constant of MTT oil is closer to that of insulating paper,and it is expected to achieve a higher level of oil-paper insulation coordination.According to the international OECD 301B standard,the 28-day degradation rate of MTT insulating oil is 82.2%,which means that this new type of insulating oil is an easily biodegradable oil.②The anti-aging properties of MTT insulating oil were investigated.Under the same aging conditions,we compared the changes of moisture,acid value,viscosity and AC breakdown of MTT and FR3 insulating oils.With compared to FR3 oil,MTT insulating oil has strong anti-aging ability and the decline of its insulation performance is relatively slower.The moisture and acid values of MTT oil during aging are significantly lower than the corresponding values of FR3 oil.The anti-aging mechanism of the insulating oil was analyzed based on the ReaxFF(Reactive Force-Field)reaction force field:(1)Due to the higher dissociation energy of the C-O bond in the molecule of MTT oil compared with that of FR3 oil,the activation energy of the first-order kinetics of MTT oil is about twice as high as that of FR3 oil.(2)MTT oil molecules have fewer active sites in the aging process,which is difficult to trigger accelerated aging.In addition,in the late stage of thermal oxygen reaction,due to the intermolecular dehydration condensation of polyhydroxy compounds produced by thermal oxygen reaction of triglyceride molecule in FR3 oil,the macromolecular polymerization product is formed.Therefore,the viscosity of FR3 insulating oil keeps rising during the aging process.MTT oil hardly undergoes intermolecular dehydration condensation reaction,so its viscosity remains almost unchanged during the aging process.③The experiments on the breakdown and flow discharge characteristics of MTT insulating oil were carried out.We found that the effect of moisture on AC breakdown voltage of MTT insulating oil is not obvious,and its AC breakdown voltage(73.2 kV)can meet the application requirements of the ultra-high voltage and large power transformers.Through the high-speed observation platform,the initiation,propagation and breakdown of MTT insulating oil under lightning impulse voltage were recorded.The results show that under positive polarity,the breakdown voltages of MTT oil,FR3 oil and mineral oil are equivalent,and the flow pattern of the streamer is thin and long.The negative lightning impulse breakdown voltages of MTT and FR3 insulating oils are similar,but both are lower than that of mineral oils.Meanwhile,the streamers of MTT and FR3 insulating oils tend to develop axially,while the streamers of mineral oils tend to develop radially.The calculation results based on the density functional theory(DFT)show that the ester groups in MTT and FR3 insulating oil will generate highly electronegative oxygen-containing free radicals during the discharge process.It changes the combined field towards the plane electrode which will facilitate the propagation of the streamer.④We studied the effect of molecular electrical properties of MTT insulating oil on its breakdown and streamer discharge behavior.Based on the density functional theory(DFT),the effects of typical functional groups(C=C,ester group and β-hydrogen)and molecular chain length on the ionization energy and electron affinity of molecules of MTT and FR3 insulating oil were studied.The results show that the ionization energy of MTT and FR3 insulating oil molecules mainly depends on C=C and molecular chain length,while the electron affinity of molecules mainly depends on ester groups.The calculated ionization energy distributions of molecules of MTT,FR3 insulating oil and mineral oil reveal the mechanism of the effect of molecular structure on the formation of fast streamers under lightning impulse voltage.The ester groups are the main factor caused lower lightning impulse breakdown voltage of MTT and FR3 insulating oil compared with that of mineral oil.Ester groups can attract more electron collisions and promote the evolution of the streamer from slow to fast mode. |
PDF Full Text Request