Qinxue Yu

Influence of the atmosphere on the reaction of dibenzyl disulfide with copper in mineral insulation oil

A mineral insulation oil containing 500 mg/l dibenzyl disulfide (DBDS) was aged under different thermal (105 °C to 165 °C) and atmospheric (nitrogen and air) conditions to further understand the reaction mechanism of corrosive sulfur with copper. The DBDS concentration of each oil sample was measured using high-performance liquid chromatography (HPLC), and the change in the color of each oil and copper specimen was subsequently observed. The following conclusions were drawn: copper sulfide (Cu2S), which is the decomposition product of the DBDS-Cu complex, can be formed under both nitrogen and air environments but is highly prone to deposition on the copper surface under nitrogen. However, the DBDS-Cu complex tends to peel off the copper surface in air, which leads to significantly higher concentration of dissolved copper in oil or to the deposition on the wrapped insulation paper surface. DBDS simultaneously reacts with oxygen and copper when the oil contains oxygen. In addition, the activation energy of the reaction of DBDS with copper was determined as approximately 98.61 kJ/mol by using the Arrhenius equation.

Electrostatic charging tendency and correlation analysis of mineral insulation oils under thermal aging

In order to investigate the electrostatic charging tendency (ECT) of transformer oil under thermal aging and understand its correlations to the other insulation parameters, a series of accelerated aging tests were carried out on two types of mineral insulation oils, A and B, both of which were mainly used in 500 kV power transformers in China. Oil samples were pre-processed by degassing and dehydrating, and then aged at 120°C for various periods up till to 675 h, meanwhile, different contents of oxygen and copper/pressboard/silicon steel sheet were also added into the oil samples for sensitive study. The results indicate that the ECT of oil A increases notably with aging time, the ECT of oil B however does not change so much during the whole aging period. Injection of oxygen in the samples enhances the ECT of oil A, such a promotion effect is minor for oil B. The correlations between the ECT and other parameters, such as tanδ, volume resistivity (ρv) and acid number were analyzed as well. There are linear correlations between ECT and tanδ, acid number and log (ρv) of oil A, of which ECT and tanδ has the highest correlation coefficient of 0.881. Regarding oil B, there is only a positive linear correlation between ECT and acid number.

A research summary of corrosive sulfur in mineral oils

At present, a number of failures of transformers and reactors have occurred due to the resultant formation of copper sulfide (Cu2S) in the windings, which has attracted wide attention. To date, corrosive sulfur in mineral oils has been regarded as the culprit, however, because of the complicated existence of sulfur element in mineral oils as well as other sources, the corrosive mechanism, restraining, detection method and how to minimize such problems are still the key and puzzled research points. In this paper, a comparatively detailed summary of research status about this phenomenon in recent time is present, and some useful research proposals are introduced as well.

The resistivity of oil and oil-impregnated pressboard varies with temperature and electric field strength

The mineral insulating oil and pressboard are important insulation dielectrics in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the oil and oil-impregnated pressboard. In this paper, the influences of the temperature, field strength and aging on the resistivity of oil and oil-impregnated pressboard have been investigated. Samples used in the experiments are new oil, aged oil, pressboard immersed in new oil and aged oil respectively. The measuring device of resistivity was full of oil and isolated from outside. All samples were measured at temperature range of 25-80 °C under the electric field strength range of 2-10 kV/mm for oil and 3-30 kV/mm for oil-impregnated pressboard. The results showed that the resistivity of oil-impregnated pressboard decreased along with the increase of field strength and temperature. For all oil specimens and pressboard immersed in aged oil, the resistivity decreasing rate (|∂1gρ / E∂|) along with electric field strength showed a downtrend with the increase of temperature; while, for pressboard immersed in new oil, the decreasing rate showed an uptrend with the increase of temperature. For oil, the resistivity reduction rate with aging (r) decreased along with the increase of field strength and temperature. For pressboard immersed in oil, the reduction rate with aging (r) showed an uptrend with the increase of temperature.

Manipulating ice crystallization of 0.9 wt. % NaCl aqueous solution by alternating current electric field

In this paper, we propose a physical method to manipulate the ice crystallization by applying an AC electric field during the liquid-solid transition of 0.9 wt. % NaCl aqueous solution. By using optical microscopic observation, we found that the ice grains are miniaturized in the presence of AC electric field during freezing process. Further study suggests that the grain size and the associated ice crystallization fraction are dependent on the field strength as well as frequency. And AC electric field can reduce the grain size and the crystallization fraction down to 35 μm and 71%, respectively, on the optimal condition (with field strength E = 100 kV/m, frequency f = 106 Hz). Moreover, the dielectric spectrum measurement of our sample close to transition temperature implies that such an optimal condition corresponds with a dielectric relaxation state, which might be the reason for the reduction of crystallized ice. Our finding may provide a potential way for cryoprotective application.

Research on streaming electrification of insulation mineral oil

As we know, streaming electrification due to oil flow is an important phenomenon in power transformers particularly on those high rated voltage ones, and the study around it is still a challenge. In this paper, the streaming electrification characteristic of insulation mineral oil from Nynas was studied by using an improved mini-static tester to measure the electrostatic charging tendency (ECT) of oils with different aging conditions. The oil samples were aged under 120°C in different periods up to 1350 hours, and together with different contents of oxygen and a certain amount of copper and pressboard. A comparatively better linear correlation is shown between ECT and volume resistivity (ρv), dissipation factor (tanδ) and water content, The tanδ and water content are increasing gradually with the aging time prolonged, while ρv and carboxyl decrease slowly, the acid number does not present evident fluctuation during the whole aging periods.

Experimental research on the streaming electrification of transformer oil under aging

A series of accelerated aging tests of high voltage transformer oil from Karamay of China were completed. All the oil samples were processed by dehydration and degasification firstly, and added with different contents of oxygen and copper, then aged under 120degC in different periods up to 675 hours, which is equal to 30 years on service of transformer oil. Improved mini-static tester was adopted to measure the electrostatic charging tendency (ECT), meanwhile, other traditional parameters, such as dissipation factor, volume resistivity, acid number, were also measured according to standard methods. The results indicate that ECT increases with aging time obviously, addition of copper advances the process of accelerated aging and leads to saturated period occurred in the late part of the aging (450 h-675 h). It can be found from mathematic analysis that correlation between ECT and dissipation factor is positive and the highest. These results are valuable for the life evaluation and condition monitoring of large power transformers.

DC electrical breakdown dependence on the radial position of specimens within HVDc XLPE cable insulation

Because of temperature gradients and inhomogeneous cooling rates after extrusion, the performances of HVDC XLPE cable insulation at radial position are not uniformly distributed. DC breakdown strength measurements of HVDC XLPE cable insulation specimens in inner insulation (near conductor shield), middle insulation and outer insulation (near insulation shield) have been carried out and show that DC breakdown strength of specimens in middle insulation is relatively lower than that of in inner and outer insulation at 30, 50 and 70 °C. At 90 °C, there is no significant difference of DC breakdown strength of specimens at radial position. Thus the micro performances of XLPE specimens at different radial position have been studied by FTIR, DSC, SEM and XRD. FTIR measurements show that cross-linked by-products (benzyl alcohol and acetophenone) content in middle insulation is higher; DSC measurements shows that crystallinity of specimens in middle insulation is slightly higher; SEM measurements show that the grains of specimens in middle insulation are unevenly distributed and their sizes have obvious difference; the results of XRD measurements show a higher content of by-products, a higher crystallinity and a larger crystalline interplanar spacing (a larger grain size) of specimens in middle insulation, which are consistent with FTIR, DSC and SEM analysis. Therefore, cross-linked by-products, crystallinity and crystal morphology together make DC breakdown strength of specimens in middle insulation lower.

Research on some related mechanisms of corrosive sulfur problems in mineral oils

Since a number of failures have happened on transformers and reactors due to the corrosive sulfur problem, especially for Dibenzyle-disulfide (DBDS) in mineral insulation oils, many researchers have carried out a series of studies on such issue. In this paper, some related mechanisms of corrosive sulfur problems in mineral insulation oils are introduced in detail. Such as the mechanism of DBDS how to change into copper sulfide (Cu2S), and which results are proved and checked by gas chromatography equipped with mass spectrometry (GC-MS), when there is copper sulfide deposited, what the surface color and microstructure of copper and insulation paper would be change are shown via scanning electron microscope (SEM), and the mechanism of Cu2S how to influence the dielectric properties of oil immersed insulation pressboard, together with the changing tendency of dissipation factor, permittivity and breakdown strength etc are introduced. Besides, the mechanism that how oxygen in oil to influence the Cu2S migrating from the copper surface into oil or insulation paper is given, and the activation energy of DBDS reacting with copper is obtained by Arrhenius equation, which is about 43.73 kJ/mol in air condition and 98.61 kJ/mol in nitrogen condition, at last, the mechanisms that how to inhibit such corrosive phenomena in mineral insulation oil via adding some passivators are introduced as well.

The effect of temperature and field strength on the resistivity of transformer oil

Safety operation of total ultra-HVDC transmission system could be directly affected by the operation reliability of converter transformer which is one of the most important equipments in the system. The transformer oil is the main liquid dielectric medium in the converter transformer. Thus, it is significant to study the dielectric properties of the transformer oil. In this paper, the dc resistivity of transformer oil under different conditions has been systematically investigated, and then the temperature and field strength were the test parameter variables. The variation range of temperature was 25~80°C, and under each temperature point the field strength changed ranging from 0.5V/mm to 10kV/mm. Meanwhile, 15 min is selected as the duration of testing resistivity. In this paper, the authors obtained the systematical data points and analysis curves of resistivity under different temperature and field strength, and obtained a empirical equation which describe the relationship between resistivity, temperature and field strength. It enhanced the understanding of the resistivity for transformer oil and provided the theoretical and test basis to expand the engineering application of this transformer oil.