Study On Analytical Methods For Carboxylic Acids Degraded By Thermal Aging Of Transformer Oil Paper Insulation

The power transformer is an important equipment to ensure the safe and stable operation of the power system,and its insulation failure is mainly caused by the aging of the oil-paper insulation system inside the transformer.The assessment of the running state of transformer oil-paper insulation insulation is conducive to minimizing unplanned outages and avoiding failures.During the aging process of the transformer oil-paper insulation,the acid value of the oil increases monotonously,so the study of acid values of the oil for evaluating the aging state of transformer oil-paper insulation and the effect of carboxylic acids in the oil on the aging of oil-paper insulation should be taken into account.To study analytical methods for the carboxylic acids produced by degradation of aged transformer oils can not only provide scientific data for discovering new aging characteristics to evaluate the aging state of transformer oil-paper insulation,but also help to explore the aging mechanism of transformer oil-paper insulation.In this paper,the analytical methods for the carboxylic acids which may be generated by degradation of the transformer oil-paper insulation during the heat aging process were developed.The full text is divided into four parts.At the first part,the current status on the methods for assessing the aging state of power transformer oil-paper insulation system and the relationship between acid value and aging degree domestic and overseas were summarized.The law of the total amount of acidic substances degraded by oil-paper insulation aging and the effect of carboxylic acid substances on the aging degradation rate of oil-paper insulation are clarified.Based on the phenomenon of acid value increasing during transformer aging,combining with the reaction characteristics of internal insulating oil and paper materials of transformers that can be oxidized and degraded into short chain carboxylic acid,naphthenic acid and long carbon chain carboxylic acid,the significance of research on the analytical methods for the carboxylic acids,which generated by the aging of transformer oil-paper insulation system,were proposed.In the second part,in view of the problems existing in the actual use of furfural as aging characteristics,the determination of furfural in transformer oil and its possible degradation products including furfuric acid,maleic acid and fumaric acid by HPLC at the same time was studied,and a method for the simultaneous determination of furfural,furoic acid,maleic acid and fumaric acid in transformer oil by centrifugation liquid-liquid micro-extraction combined with HPLC was developed.The influences of extractant selection,pH value of extractant,amount of diluent,vortex time,centrifugation speed and time were studied,and the optimal extraction conditions were as followed:the extractant was ultrapure water without pH-adjusted,the amount of diluent used was V _oil:V_n-hexane=2:1,the vortex extraction time was 7 min,and the centrifugal rotation speed and time were 4000 r·min^-1 and 5 min,respectively.Under the optimal conditions,the sample was extracted and the aqueous phase was detected by HPLC directly.Both the furfural and maleic acid in the transformer oil showed good linearity in the concentration range of 0.01¹00.0 mg·L^-1,and the linearity of both furoic acid and fumarc acid in the concentration range of 0.01¹0.0 mg·L-1 was good,the linear correlation coefficient?r?of four target analytes were all over 0.999,and the detection limits were in the range of 1.0⁴.6?g·L^-1,the enrichment multiples of furfural,furoic acid,maleic acid and fumaric acid were 4.6,25.1,15.6 and 17.5,respectively.The recoveries of furfural,furoic acid and maleic acid in the aged samples ranged from 82.1%to110.6%,however,fumaric acid was not detected.Based on the retention time to determine whether the target analytes exists,liquid chromatography-mass spectrometry was used to qualitatively analysis of furoic acid and butylenedioic acid in aged transformer oil samples.Finally,the furfural,furoic acid and maleic acid in aging oil samples with different aging time were tracked and measured by the established LLME-HPLC.According to the experimental results,the reason why the content of furfural,which is an aging characteristic,didn't increase at the later stage of insulation paper aging was analyzed,and the degradation history of furfural was put forward.In the third part,the qualitative analysis methods for derived products obtained from methyl sulphate derivatization and silanization were developed for the possible short-chain carboxylic acid in aging transformer oil by GC-MS.Firstly,using methanol as the extractant,the analytical effects of esterification directly and esterification after extraction on derivatization of target short-chain carboxylic acids in standard oils were explored.The results of GC analysis showed that when the ratio of oil to extractant V _oil:V _extractant=20:1,the target analytes obtained after the extraction of the short-chain carboxylic acid in the standard oil sample and then derivatization by methyl sulfate were approximately 2 to 4 times the derivatization directly.GC-MS analysis for the methylated derivatives of the aged oil samples showed that acetic acid,pentanoic acid,heptanoic acid,succinic acid and benzoic acid in the aged samples were determined.Secondly,using acetonitrile as the extractant and N,O-bis?trimethylsilane?trifluoroacetamide?BSTFA?as the silanization reagent,the standard oil samples were extracted and derivatized.The chromatographic conditions and derivatization conditions were studied.The results show that the derivatization effect is optimal when 200?L acetonitrile was used as the extraction agent,the ratio of oil to extractant V _oil:V _acetonitrile=50:1,the content of derivatization agent BSTFA was 100?L,and the reaction temperature and time were 60°C and 60 min,respectively.Under these conditions,the carboxylic acids in the blank oil and aged oil samples were extracted and derivatized respectively,and analyzed by GC-MS.The results showed that formic acid,acetic acid,propionic acid,2-methylpropionic acid,glycolic acid,2-methylbutyric acid,n-valeric acid,3-methylbutyric acid,pent-4-enoic acid,n-hexanoic acid,3-hydroxybutyric acid,2-hydroxybutyric acid,furoic acid and benzoic acid was existed in aged oil samples.In the fourth part,the qualitative analysis methods for GC-MS using methyl alcohol/sulfuric acid as derivatization reagent were studied for naphthenic acid and long chain carboxylic acids that may exist in aging transformer oil.Firstly,sodium hydroxide/methanol was used as the extractant to explore the neutralization extraction condition of naphthenic acid and long-chain carboxylic acid in transformer oil and the derivatization conditions for the direct methyl esterification by concentrated sulfuric acid.The optimal reaction conditions were as follows:neutralization extracting agent sodium hydroxide/methanol volume was 2.0mL?10.0 mL oil sample?,concentrated sulfuric acid amount was 150?L,derivatization reaction temperature was 80°C,and reaction time was 60 min.Secondly,the extraction conditions of the derivatized products were optimized.1.5 mL of n-hexane was used as the extraction agent,the extraction time was 2 min,and the extraction phase could be directly analyzed.Finally,GC-MS analysis of naphthenic acid and long carbon chain carboxylic acids in the aged oil samples under optimized conditions showed the presence of stearic acid,behenic acid,13-methyl-pentadecanoic acid,?E?-5,6-dimethylammonio-5-enedioic acid,2-hydroxy-undecenoic acid,oleicacid,erucicacidand terephthalic acid,cyclohexanecarboxylic acid was not detected.