| Mineral oil-paper insulation is the main insulation form for power transformers,which gradually ages under operating conditions,resulting in decline in its insulation and mechanical properties,thus safe operation of transformers is seriously threatened.Faults caused by insulation aging account for an important part of transformer failure,thus delaying the aging of oil-paper insulation is extremely important to ensure the safe and reliable operation of transformers.With the increase in old transformers operating for more than 15-20 years,scientifically and effectively extending their insulation service life and improving their reliability are major requirements for ensuring the safety of the power grid.Taking performance improvement of insulation oil as the starting point,research on a new insulation oil that can delay chain scission rate of cellulose insulation paper and replace mineral oil in in-service transformers directly,is an effective solution to prolong the insulation service life of old transformers.Although a large number of studies have shown that vegetable oils have the ability to significantly delay the chain scission rate of cellulose,their viscosity and dielectric loss are high,and their key parameters cannot satisfy mineral oil standard requirements,thus they cannot be directly filled in power transformers in-service.In this paper,mineral insulation oil is used as base oil,vegetable oil is used as anti-aging basic element,and adjustment methods of viscosity and dielectric loss are adopted to develop a mineral oil-vegetable oil mixed anti-aging transformer oil(hereinafter referred to as anti-aging oil),to achieve the goal that extending the insulation service life of transformers in-service.Firstly,a vegetable oil was selected as anti-aging basic element,and adjustment methods of various performance parameters such as viscosity,acid value and dielectric loss after mixing with mineral insulation oil had been studied,to determine the formulation of anti-aging oil;secondly,effect of anti-aging oil on the thermal aging of cellulose insulation paper was analyzed by experiments,and its anti-aging mechanism was furtherly analyzed;then,effect of temperature,moisture and insulation thickness on breakdown characteristics of the anti-aging oil-paper system was obtained,and difference was compared and analyzed when compared with the mineral oil-paper system;Finally,gas production characteristics of the anti-aging oil-paper system were studied,and gas production difference as well as its reason was analyzed in comparison to mineral oil-paper system,which provided theoretical and data support for the operation and maintenance of anti-aging oil transformers.The main results and innovations in this paper are listed as follows:(1)Developed a 3-element mixed anti-aging oil composed of mineral oil and two vegetable oils,which not only can delay the aging rate of insulation paper,but also meets the IEC 60296(2012)transformer oil standard,thus it can be directly used in oil immersed power transformers in-sevice and extend their insulation life.Taking mineral insulation oil as the base oil,soybean oil is preferably used as the anti-aging element based on the theoretical calculation results of molecular frontier orbits and polarization strength,combined with the test results of fiber chain scission rate,and PFAE is used as viscosity adjustment element,finally the preparation formula of 3-element mixed anti-aging oil is proposed:76 v.%mineral oil+19 v.%soybean oil+5 v.%PFAE+0.2wt.%T511+0.2 wt.%L06.(2)Test has proved the significant effect of anti-aging oil on delaying thermal aging of insulation paper,and the main delaying mechanism that anti-aging oil inhibites acid-catalyzed cellulose hydrolysis to reduce cellulose chain scission rate has been revealed.Accelerated thermal aging results at 130oC show that the molecular chain scission rate of insulation paper in anti-aging oil is only 52%of that in mineral oil.For aged mineral oil-paper system,oil-replacement with anti-aging oil decreases chain scission rate by 65%when compared with that oil-replacement with mineral oil.Molecular dynamics results have shown that,a large number of hydrogen bonds have formed between anti-aging oil and H3O+in the oil-paper system,and their interaction energy is increased,thus the formation of cellulose conjugate acid is hindered,and finally the acid catalysis of cellulose chain scission process is inhibited,thereby decreasing the fiber chain scission rate.(3)Effect of factors(temperature,moisture and insulation thickness)on power frequency breakdown characteristic of oil-paper insulation has been studied,and the results show that,when compared with mineral insulation oil,power frequency breakdown strength of anti-aging oil-paper system is less affected by water content and insulation thickness,and?anti-aging oil gap+oil-immersed pressboard"structure has a higher power frequency breakdown strength than"mineral oil gap+oil-immersed pressboard?.The anti-aging oil is able to maintain its power frequency breakdown voltage higher than 60k V within 10?50ppm water content,while the breakdown voltage of mineral oil decreased by about 19?39%when the water content increases from 10ppm to 30ppm.The breakdown strength of anti-aging oil-paper is less affected by the change of thickness,i.e.,the breakdown field strength decreases by 8.15%when the insulation thickness of oil-paper increases from 0.108mm to 0.432mm,while that decreases by 15.7%in mineral oil-paper system.In the structure of?oil gap(0.6?2.4mm)+oil-immersed pressboard(0.3mm)?,the breakdown voltage of anti-aging oil-paper system is increased by 11%?15%compared with mineral oil-paper system.The binding effect of vegetable oil on water molecules and high dielectric constant of anti-aging oil are important reasons for the above law.(4)Gas production characteristics of oil-paper insulation under aging,overheating fault and discharge fault have been comparative studied,the results show that the anti-aging oil-paper system produces more dissolved CO2,H2 and C2H6 under the condition of normal aging and overheating fault when compared with mineral oil-paper system,the formation reasons of the phenomenon have been discussed by molecular simulation and theoretical analysis,further,new combined gas features and identification method which are suitable for faulted anti-aging oil-paper system have been put forward.Aging results at 90oC showed that CO2 amount produced by the anti-aging oil was about 1.5 times that of the mineral oil.In the case of overheating at120oC?750oC,H2 content in the anti-aging oil was about 3 times that in mineral oil,and C2H6 content was also higher than that in mineral oil,especially C2H6 content was10 times that in mineral oil in the case of low temperature overheating at 120oC.Decarbonylation reaction,Diels-Alder reaction and peroxide reaction of unsaturated fatty acids are important reasons for anti-aging oil to produce more CO2,H2 and C2H6,respectively.Gas production characteristics of the two oil-paper systems are similar under partial discharge and breakdown fault.Based on new characteristic gas combinations(H2,C2H2 and C2H4),a triangle fault identification model for overheating faults and discharge faults has been constructed for anti-aging oil-paper system,which is able to distinguish different overheating faults,partial discharge faults,insulation oil or oil-immersed paper discharge faults. |
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