| Oil-immersed power transformer plays an important role in the power supply and distribution system,and its internal insulation system is mainly composed of insulating oil and insulating paper.During the operation of power transformers,the aging of oil-paper insulation caused by the long-term effect of electric and thermal stress is the main reason of transformer insulation failures.However,in recent years,insulation failures caused by sulfur corrosion have occurred frequently on short running transformers,and a certain amount of dibenzyl disulfide（DBDS）has been detected in fault transformer oils after accident investigation,which makes the problem of adding DBDS to transformer insulating oil concerned.The aging process of transformer oil-paper insulation caused by sulfur corrosion reaction of DBDS is completely different from that caused by electric and thermal stress with the absence of DBDS.Research on the determination methods for DBDS,relevant inhibitor additives and aging characteristic（furfural）in transformer oil can help to realize the content monitoring of DBDS addition in transformer oil,and can also be used to track and analyze the change of oil-paper insulation performance at the same time.Studying the impact of DBDS on the aging state of transformer oil-paper insulation,and exploring the usage problem of current aging characteristics in the presence of DBDS can more accurately estimate the residual life of transformer oil-paper insulation and prevent accidental insulation failures.The full thesis is divided into four parts.In the first part,the current research status on the causes of sulfur corrosion failure by DBDS in transformer oil and related sulfur corrosion reaction mechanisms were summarized.The harm of adding DBDS in transformer oil to the inner insulation system of transformer was clarified.In view of the seriousness of transformer insulation failure caused by DBDS sulfur corrosion,the significance of research on developing a method for simultaneous determination of DBDS,additives and aging characteristics in transformer oil,and the study on the effect of sulfur corrosion reaction by DBDS on aging state of transformer oil paper insulation were proposed.In the second part,the determination method of additives and aging characteristics in transformer oil was studied.And a method for the simultaneous determination of DBDS,antioxidant additive（2,6-di-tert-butylphenol DBPC）,metal passivator additive（benzotriazole BTA）and aging characteristic（furfural）in transformer oil by vortex assisted liquid-liquid microextraction combined with high performance liquid chromatography（HPLC）was developed.The factors influencing the extraction and determination were discussed.The optimized extraction conditions were as follows:300.0μL acetonitrile was used as the extractant;and the volume ratio of oil sample to diluent was 2:1;the extraction time was 5min;the separation was carried out by centrifugation at 4500 r·min-1 for 5 min.After extraction,the acetonitrile phase was vortex extracted by 50μL n-hexane to remove the trace oil matrix and then determined by HPLC with acetonitrile and water as mobile phase.The results showed a good linear relationship in the concentration range of 0.1000～600.0 mg·kg-1for DBDS and BTA,100.0～5000 mg·kg-1 and 0.1000～100.0 mg·kg-1 for DBPC and furfural respectively,and the linear correlation coefficient（r）were all over 0.9998.The limits of detection（LODs）（S/N?=?3）were in the range of 4.5～423.1μg·kg-1,and the enrichment factors of DBDS,BTA,DBPC and furfural were 8.1,11.8,4.8 and 9.3,respectively.The established method was used to the determination in different commercial transformer oils and the aging transformer oils,and the recoveries of target analytes were in the range of 89.2%～114.3%.In the third part,in view of the deficiency existing in the current national standard（GB/T）and the International Electrotechnical Commission Standard（IEC）in the determination of corrosive sulfide-DBDS,based on the research in Chapter Two,the quantitative determination method for DBDS in transformer oil by liquid-liquid microextraction combined with HPLC was established including internal standard method and external standard method,and compared with GB/T and IEC standards.The results showed that both the internal standard method and external standard method had good linearity for DBDS in the concentration range of 0.1000～600.0 mg·kg-1 in transformer oil,the correlation coefficients（r）were all over0.999,RSD was 2.1%and 1.7%,respectively.The limits of detection（LODs）（S/N?=?3）and the minimum value of linear range were all lower than 5 mg·kg-1,which was the detection threshold of DBDS specified in the current relevant standards.In the 95%confidence interval,there was no significant difference between the internal and external standard methods established in this work and those provided by GB/T and IEC.In the fourth part,the controlled experiment for simulated accelerated thermal aging was designed firstly to study the influence of sulfur corrosion reaction by DBDS on the aging of oil paper insulation.The corrosion degree of copper conductor in the aging process of transformer oil-paper insulation was analyzed.And the concentration of furfural and the acid value of transformer oil in the accelerated thermal aging process were tracked and measured,the polymerization degree of insulation paper was calculated according to the furfural concentration in oil.The results showed that the change patterns of furfural concentration in the two experimental groups that with DBDS addition and without DBDS addition were similar,which increased first and then decreased with the increase of time during the accelerated thermal aging process.However,the sulfur corrosion reaction of DBDS accelerated the aging of transformer insulating paper,resulted in the furfural concentration in the transformer oil containing DBDS higher than that without DBDS at the same aging reaction time.In terms of the results of this simulated thermal aging experiment,the addition of DBDS in transformer oil makes the insulation paper reach the aging state of deterioration 5years in advance.The change trends of the acid value of transformer oils in the two control experimental groups with DBDS addition and without DBDS addition,showed monotonous increase.Due to the sulfide compounds and copper ions produced by sulfur corrosion have a great influence on the acid value of transformer insulation oil,the acid value curves in two control groups showed some differences.Secondly,the accelerated thermal aging oil samples containing DBDS were determined by Gas chromatography-mass spectrometry（GC-MS）after separated by liquid-liquid extraction and concentrated by vacuum rotary evaporation,and the results are compared with those of unaged oil samples.Under the optimized conditions,seven kinds of sulfur corrosion products were detected in the aging oil samples with the presence of DBDS,including benzyl mercaptan,dibenzyl sulfide,dibenzyl sulfoxide,benzyl alcohol,benzaldehyde and benzoic acid.And according to the MS ion peak abundance of the related substances measured,the primary exploration about the relative content change of the products in the aging process was carried out.It is hoped to provide data for the future study of sulfur corrosion reaction and the exploration of characteristic substances used to evaluate the degree of reaction. |
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