Study On Determination Methods Of Degradation Products In The Aging Process Of Transformer Oil-paper Insulation

Power transformer is one of the important components of the grid,is the hub device of the grid security.If the transformer fails,the economy will suffer huge losses.The insulation failure is the leading cause of transformer accidents,and the aging of oil-paper insulation is the main cause of transformer insulation fault.The research of aging characteristic parameters of transformer oil-paper insulation is the key technology to develop the intelligent diagnostic method of the power transformer aging degree,and the establishment of relevant analytical methods is the basis of the study of the aging characteristic parameters.In this paper,determination methods of degradation products in the aging process of transformer oil-paper insulation were studied,and would be helpful to study on degradation products which are treated as new aging characteristic parameters of transformer oil-paper insulation.Four parts are included in this article.At the first part,the application status on the current aging characteristic parameters of the transformer oil-paper insulation and the research progress on the new aging characteristic parameters were reviewed.It described the reason of the generation of copper ions in aging transformer oil,and the interaction between copper ions and acidic substance generated from the aging transformer oil-paper insulation,and the effect of copper ions and acidic substance to accelerated aging of oil-paper insulation.Based on that transformer insulation material can be degraded into low molecular weight carboxylic acids and in view of increase in transformer oil acid value during the aging process,the aim of the study which low molecular weight carboxylic acids as new aging characteristic parameters is put forward.At the second part,determination method of copper ion in the oil sample of transformer oil-paper insulation aging was studied.Analysis method was developed for determination of copper ions content in transformer oil by the reversed phase liquid-liquid centrifugal extraction of large phase ratio integrating with flame atomic absorption spectrometry（AAS）.Preparation of simulative oil samples,types of extractant solvent,amounts of the extractant,extraction time and centrifugal rotational speed were investigated.The optimum extraction conditions were as follows:1-methylimidazole as cosolvent to prepare simulative oil samples,1%solution as extractant,amount of the extractant:5m L;extraction time:1min;centrifugal rotational speed:5500rpm,centrifugal time:10min.The quantitative analysis of copper ions in the transformer oil samples was performed by FAAS.The calibration curves was linear in the range of 5～1000μg·L^-1 with a correlation coefficient（r）of 0.998 and a detection limit of 1.6μg·L^-1.The relative standard deviation（RSD,n=6）was 0.7%,The recoveries of copper ions in aging transformer oil samples ranged from 100.0%to 105.1%.At the third part,first of all,determination the formic acid and acetic acid simultaneously by the high performance liquid chromatography（HPLC）was discussed.The method was established for the determination of formic acid and acetic acid in transformer oil by reversed phase liquid-liquid microextraction（RLLME）coupling with HPLC.The effect of some important parameters,such as p H value of the extractant,amount of the extractant,amount of the diluent（n-hexane）,extraction time,centrifugal rotational speed and centrifugal time were investigated to optimize the extraction conditions.The optimum experimental conditions were as follows:extraction solvent:neutral ultrapure water;amount of the extractant:150μL;ratio of dilution:Voil:Vn-hexane=2:1;extraction time:5min;centrifugal rotational speed:4500rpm;centrifugal time:5min.The quantitative analysis of formic acid and acetic acid in the transformer oil samples was performed by HPLC.The calibration curves was linear in the range of 0.05～50.0mg·L^-1 and of 0.10～20.0 mg·L^-1 with the correlation coefficient（r）of 0.999 and of 0.998,respectively,and the detection limit of 0.02 mg·L^-1and of 0.04 mg·L^-1,respectively.The relative standard deviation（RSD,n=6）was 1.33%and 1.75%,respectively,and the enrichment factor of formic acid and acetic acid was 10.5 and 13.9,respectively.The recoveries of formic acid and acetic acid in aging transformer oil samples ranged from 91.8%～97.0%and90.2%～96.3%,respectively.On the basis of the above-mentioned HPLC,the qualitative analysis of formic acid and acetic acid in aging transformer oil was further carried out by using gas chromatograph-mass spectrometer（GC-MS）.Formic acid and acetic acid were methylesterificated with H2SO4–CH3OH solution,the condition of esterification and GC were studied.The optimum experimental conditions were as follows:2m L of 5%H2SO4–CH3OH was derivatization reagent:2m L,the oil temperature of derivatization was 80℃,the time of derivatization was 60min,and the sample was extracted with hexane for 1 min after derivatization.Under the optimal condition,the GC-MS qualitative detection of acetic acid in aging transformer oil was carried out for the first time.Compared to the unaged oil,the generation of acetic acid was detected in the transformer oil-paper insulation aging degradation process.Next,the qualitative analysis of furoic acid in aging transformer oil was carried out by using HPLC-MS.Furoic acid in the oil samples of aging transformer was detected in two different mass spectra conditions,it could provide useful data for research on the mechanism of furfural content falling instead of rising at the later stage of transformer oil paper insulation aging.In the end,the content of copper ions,formic acid,acetic acid,furoic acid,furfural and the acid value of transformer oil were determined with different aging time.The results were as follow:the content of copper ions increased with the increase of the acidic substance content in transformer oil.The amount of formic acid,acetic acid and furoic acid increased monotonically with the increase of aging time,however,the content of furfural increased firstly and then decreased,it means that formic acid,acetic acid and furoic acid are better than furfural to indicate the aging of oil-paper insulation.This study has provided detection method and useful data for study on formic acid,acetic acid or furoic acid as the aging characteristic parameters of transformer oil-paper insulation.At the last part,main results were summarized,and some suggestions were given for the future work.