The active sulphur compounds in transformer oil would react with copper wires and produce cuprous sulphides, which then attach themselves to the surface of copper wires and insulating paper, thus reducing the electrical performance of oil–paper insulation and causing insulation failure. This article aims to make a comprehensive summary on the field of sulphur corrosion of oil–paper insulation in the mineral oil transformer, including the corrosion mechanism of sulphide on copper sheets, the deterioration mechanism of sulphide on oil–paper, the factors influencing sulphur corrosion reaction, the method of characterising the degree of sulphur corrosion, the protection technology of sulphur corrosion and the field operation measures for sulphur corrosion. Moreover, four key problems that need to be further solved were sorted out: the synergistic deterioration mechanism of sulphide and its transformation mechanism, the micro‐process and forming path of sulphur corrosion reaction, the characteristic parameters and quantitative characterisation methods of sulphur corrosion degree, and the effective protection technology of sulphide corrosion. To solve these problems, efforts will be made for deeper understanding of the sulphur corrosion phenomenon and its deterioration mechanism based on theoretical background, the assessment methods and protection technology, endeavouring to put forward solid theoretical basis and feasible technical means for preventing sulphur corrosion insulation failure.

Although vegetable oil is the best substitute for mineral oil, it exhibits numerous disadvantages, including a high dielectric loss factor, low volume resistivity, and high viscosity. Nanoparticle modification may improve the electrical properties of vegetable oil considerably. In this study, vegetable oil infused with Fe3O4, TiO2, and Al2O3 nanoparticles was investigated. Thermal aging experiments were performed at 100 °C, 130 °C, and 150 °C. The aged vegetable insulating oil was periodically sampled and tested. The results revealed that the nanoparticles with a mass fraction of 0.001% exhibited the best modification effect. The electrical properties of the vegetable insulating oil modified with magnetic nanoparticles were pronounced, whereas the modification effect of nonmagnetic nanoparticles was mild. The general trend of the modification effect of the nanoparticles on vegetable oil became obvious with the increase in the temperature. At the same content, the modification effect of nano-Fe3O4 was superior to that of nano-TiO2 and nano-Al2O3. The results of this study provide experimental and theoretical support for the mechanism of long-term stability of vegetable insulating oil with nanoparticles and can be used to improve the long-term stability of vegetable insulating oil.

The image of secondary arc, which is a common phenomenon in ultrahigh voltage power system, contains a wealthy of information. To determine its geometrical characteristics, the secondary arc image is systematically processed involving gray scale, noise filtering, and image segmentation. To overcome the discontinuous and false edge caused by conventional operators, an improved algorithm is employed; the unrelated edges are fully removed, and the interrupted endpoints are dilated. Based on the results, the component and evolution of secondary arc are discussed; the correlation of the arc luminance and arc current is revealed. Occasionally, the arc roots and arc column have irregular motion behavior. Using the edge-detection technique, the detailed jumping location and velocity of anode arc root are tracked; the life expectancy and stability of coexistence of multicathode arc roots are determined; the morphology of short circuit of arc column is ascertained. Furthermore, the secondary arc dimension is estimated in this paper, including the arc radius and arc length. A number of formulas have been presented to predict the dimension. The obtained results are more accuracy than previous works.

The vegetable oil transformer has excellent fire resistance, environmentally friendly performances and electrical properties, which is the development direction of oil-filled transformer in the future. At present, the fault diagnosis methods are limited to pure electrical fault or thermal fault, also the diagnosis of combined electro-thermal fault on RAPO vegetable oil needs to be developed. In this paper, an electro-thermal fault experiment platform was established. The gas generation characteristics of RAPO vegetable oil under pure electrical faults, pure thermal faults and combined electric-thermal faults were obtained. Results show that the characteristic gases under pure electrical fault are methane and acetylene, while under pure thermal fault are ethane. Ethane is the characteristic gas of electro-thermal faults except combined high energy breakdown and low temperature thermal fault, of which the characteristic gas is acetylene. Carbon dioxide quickly reached the saturation solubility at the corresponding temperature in the early stage of the above faults, and its content remained stable until the end of the experiment. Therefore, carbon dioxide could not be used as the characteristic gas to diagnose faults. Under different electro-thermal fault conditions, the gas production trend and gas production quantity of hydrogen and carbon monoxide are generally similar, but still different to some extent. Then, the fault diagnosis method of RAPO vegetable oil was established based on the characteristic gas and the ratio criterion. The above study can provide reference for establishing the fault diagnosis method of RAPO vegetable oil and promoting the practical application of vegetable oil transformer.

In the era of low carbon and environmental protection, sustainable development ideas win support among the people. Vegetable oils have excellent environmental performance compared with traditional mineral insulating oils. The aging characteristics of insulating paper in vegetable oil have been discussed in experiments, but the deterioration mechanism at the microscopic level is still lacking. In this paper, the microscopic deterioration mechanism of insulating paper in vegetable oil and mineral oil is investigated by molecular dynamics. In addition, the intermolecular properties of insulating paper under different insulating oils are compared. It is found that the cellulose cleavage in insulating paper is divided into three ways: glycosidic bond breaking, pyran ring opening and dehydroxylation. The number of hydrogen bonds, binding energy and electrostatic energy between insulating paper and vegetable oil are higher than those of mineral oil. It proves that there is a stronger interaction between insulating paper and vegetable oil. Finally, the change rules of characteristic products CO, H ₂ O, H ₂ and formic acid are counted. It is found that vegetable oil can delay the deterioration and prolong the life of insulating paper compared to mineral oil. The above study reveals the microscopic influence law of vegetable oil and mineral oil on the degradation of insulating paper. It provides theoretical support for the large-scale application of vegetable oil in oil-immersed transformers.

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.