Study on Sulfide Distribution in the Operating Oil of Power Transformers and Its Effect on the Oil Quality

Abstract

Corrosive sulfides in transformer oil could react with copper wire to produce cuprous sulfide, causing insulation failure. At present, both the quantitative measurement method and distribution of sulfur components in operating oil are not clear yet. In this paper, the existing types and contents of sulfides in oil samples with different alkyl groups and different voltage levels were investigated. With quantitative testing methods, the distribution of sulfur composition in the operating oil was analyzed. Results showed that the thiophene sulfide in transformer oil existed mainly in the form of benzothiophene with an unsaturation of 6 and dibenzothiophene with an unsaturation of 9. The content of monosulfide sulfide with unsaturation of 3 or 6 was the highest. The disulfide existed basically in the form of Dibenzyl disulfide (DBDS). The influence of sulfides on the oil quality were analyzed on this basis. Results showed that the existence of sulfides would increase the moisture content in oil. The absorbed moisture could cause the decrease of the breakdown voltage and rise of the dielectric loss. The above study could provide some engineering practice for understanding the sulphide distribution in transformer oils and further prevent the sulfur corrosion faults.

Reviews on sulphur corrosion phenomenon of the oil–paper insulating system in mineral oil transformer

“…The micro-process with micro-molecular simulation methods can study on the sulphur corrosion reaction process from the molecular and atomic levels. Then it is able to establish a method to assess the degree of sulphur corrosion, and then give effective measures to prevent sulphur corrosion accidents in field operation: [9], including total sulphur content determination, elemental sulphur determination, hydrogen sulfide content determination, mercaptan sulphur content determination, DBDS content determination, thiophene sulphur and thioether sulphur content. For transformers that have been added with passivators, since the early Cu 2 S precipitation cannot be eliminated, the operating department is suggested to regularly monitor the transformer oil chromatography.…”

“…Then it is able to establish a method to assess the degree of sulphur corrosion, and then give effective measures to prevent sulphur corrosion accidents in field operation: Strengthen the monitoring of insulation oil products : for new insulating oil, it is recommended to conduct corrosive sulphur test according to ASTM D 1275 B [72] (degassing and heating at 150°C for 48 h) to eliminate the sulphur corrosion of insulating oil. For transformer oil in operation, a variety of standard and laboratory methods are used to regularly test different sulfide types and contents in transformer oil [9], including total sulphur content determination, elemental sulphur determination, hydrogen sulfide content determination, mercaptan sulphur content determination, DBDS content determination, thiophene sulphur and thioether sulphur content. For transformers that have been added with passivators, since the early Cu 2 S precipitation cannot be eliminated, the operating department is suggested to regularly monitor the transformer oil chromatography.…”

“…Normally, the ordinary insulating oil contains sulphur of 0.001-0.5% which may appear in elemental form or in part of some molecular structures of hydrocarbons. The occurrence states of sulphide include thiophenes, disulphides and polysulphides, such as thioethers, thiols and elemental sulphur [6][7][8], in which the disulphides appear to be the most common corrosive sulphides currently recognised [9]. The highly active sulphides present in transformer oil can react with copper wires and produce copper sulphides.…”

Influence of contact layer on the sulphur corrosion of copper conductors in power transformers

“…Copper conductors react with corrosive sulphur in mineral insulating oil, particularly the dibenzyl disulphide (DBDS), to form conductive copper sulphide (Cu 2 S) on the surface of the copper and the insulating paper. Cu 2 S deposited on windings weaken the strength of the oil-paper insulation and even lead to serious insulation failures [1][2][3][4]. Figure 1 shows an inter-turn breakdown case in a 500 kV transformer caused by sulphur corrosion of the copper conductor, whose insulation strength was weakened by the corrosion deposition on the insulating paper.…”

“…Insulating oil must undergo a non-corrosive test to reduce the risk of sulphur corrosion in the power equipment. To date, several testing standards have been proposed including DIN 51353, ISO 5662, ASTM D1275A, ASTM D1275B and IEC - 1 62535-2008. The corrosiveness of an insulating oil is graded according to the test method ASTM D 130/TP 154 by an accelerated corrosion test.…”