E. Nagao

Highly sensitive detection method of dibenzyl disulfide and the elucidation of the mechanism

Highly sensitive detection method of dibenzyl disulfide (DBDS) in insulation oil was realized using solid phase extraction (SPE) with active alumina adsorbent followed by gas chromatography equipped with a mass spectrometry (GC-MS). The detection limit of DBDS is around 0.1 ppm. This method is also useful for detecting bibenzyl (BiBZ) and dibenzyl sulfide (DBS). The good relationship between the weight change of copper strip and BiBZ /DBS formation was obtained under heating with insulating oil containing DBDS and copper strip, so it was clarified that BiBZ and DBS are the byproducts of the copper sulfide generation with the chemical reaction between DBDS and copper. The chemical reaction formula between DBDS and copper was proposed based on the assumption that copper sulfide should be generated via Cu-DBDS complex. The temperature dependence of the weight change of copper strip before and after the heating in the insulating oil containing DBDS was quantitatively explained with the chemical reaction formula.

Identification of affecting factors of copper sulfide deposition on insulating paper in oil

Effects of oil brand, temperature and atmosphere conditions on copper sulfide deposition on insulating paper were investigated by thermal aging test. Copper sulfide deposition occurred in a certain oil brand and dibenzyl disulfide (DBDS) was found in the oil. There was an incubation period observed before copper sulfide deposition occurred. Copper sulfide deposition was accelerated by temperature. The deposition rate after the incubation period became twice when the heating temperature was increased by approximately 10degC. The incubation period and the deposition rate were the same at 140degC whether the thermal aging test was done in air or nitrogen atmosphere. On the other hand, the incubation period in air was shorter than that in nitrogen at 120degC. The deposition rate was not affected by atmosphere condition at 120degC. Intermediate substances due to copper dissolution into oil are introduced to explain the incubation period and the atmosphere dependence of incubation period observed at 120degC.

Duration and mechanism for suppressive effect of triazole-based passivators on copper-sulfide deposition on insulating paper

The suppressive effect of triazole-based passivators such as 1,2,3-benzotriazole (BTA) and Irgamet 39/sup TM/(CIBA Specialty, Basel, Switzerland) on the deposition of copper sulfide on insulating paper in transformers and reactors was investigated by heating test with bare and paper-wrapped copper plates. Commercially available mineral oil meeting the specification of IEC 60296 was used. This oil contained 136 ppm wt./wt. of dibenzyl disulfide (DBDS). The deposition of copper sulfide occurred after 144 h of heating at 140deg without these passivators. However, little deposition of copper sulfide occurred even after 432 h of heating when either of these passivators was added to the oil. The concentration of dissolved copper significantly decreased when either of these passivators was added to the oil, which reflects the formation of complex layers on the copper surface caused by the reaction between these passivators and copper. The complex layers were confirmed by time-of-flight secondary ion mass spectrometer (TOF-SIMS). The suppressive effect was derived from retarding copper from dissolving into the oil due to the reaction inhibition between DBDS and copper by the complex layers. The duration of these passivators can be determined by the durability of the complex layers.

Concentration dependence of corrosive sulfur on copper-sulfide deposition on insulating paper used for power transformer insulation

Copper-sulfide deposition on coil insulating paper used for power transformer insulation is investigated with the aim of obtaining the concentration dependence of corrosive sulfur. Several mineral insulating oils containing dibenzyl disulfide (DBDS), which is typical corrosive sulfur causing the copper sulfide deposition, are used as sample oils. Thermal aging tests are done at 140degC with an air atmosphere up to 1296 h with Kraft paper-wrapped and bare copper plates. The test results indicate that the deposition rate is proportional to the DBDS concentration and the incubation period before onset of the copper-sulfide deposition is inversely proportional to the DBDS concentration. These concentration dependences can be derived by considering equilibrium between three compounds such as dissolved DBDS-Cu complexes in the oil, adsorbed DBDS-Cu complexes and deposited copper-sulfide on the insulating paper.

Role of dissolved copper and oxygen on copper sulfide generation in insulating oil

We have developed a new heating test to investigate the influence of dissolved copper (DBDS-Cu complex) and oxygen on copper sulfide generation in alkyl benzene insulating oil with dibenzyl disulfide (DBDS) added. The weight of the deposited and dissolved copper was systematically studied in atmospheres of nitrogen and artificial air. The copper sulfide was deposited on the insulating paper using the heating test by separating the insulating paper from the copper strips in the insulating oil. It was found that dissolved copper (DBDS-Cu complex) has influence to the copper sulfide generation on the insulating paper. The weight of the copper deposited on the insulating paper was increased by the existence of oxygen. The acceleration effect on copper sulfide deposition on insulating paper was clarified for the first time by strictly controlling the experimental atmosphere. Moreover, the influence of the existence of oxygen was investigated individually on insulating oil and insulating paper. The oxidation of both alkyl benzene and insulating paper was found to accelerate copper sulfide generation.

Influence of inhibitor and oil components on copper sulfide deposition on kraft paper in oil-immersed insulation

Copper sulfide deposition on Kraft paper in oil-immersed transformers is found to be accelerated by di-tert-butyl p-cresol (DBPC), which is widely used as an antioxidant, and paraffinic hydrocarbons in their oils, which is verified in laboratory experiments. DBPC can prolong the lifetime of precursors of the copper sulfide deposition by reacting decomposition products of copper dibenzyl disulfide (Cu-DBDS) complex and by creating the Cu-DBPC complex. Because the lifetime of the Cu-DBPC complex is far longer than that of the Cu-DBDS complex, considerable amounts of copper sulfide are deposited on Kraft paper instead of being formed on the surface of copper. A good correlation is obtained between the concentration of paraffinic hydrocarbons and the amount of copper sulfide deposited on Kraft paper in the three commercial oils.

Effect of DBDS concentration and heating duration on copper sulfide formation in oil-immersed transformer insulation

Copper sulfide is found to be more likely formed on Kraft paper in oil-immersed transformers with an increase in either dibenzyl disulfide (DBDS) concentration or heating duration, which is verified by heating tests stipulated by IEC 62535. Two effects of DBDS concentration on the formation rates of copper sulfide were obtained. The formation rates of investigated oils are proportional to the DBDS concentration when the concentration is low. However, the amounts of copper on the Kraft papers are constant when the DBDS concentrations are more than certain values, which is considered to reflect the saturation of dissolving DBDS-Cu complex in the oil. These concentrations depend on oil brand. Copper sulfide begins to form after some duration of heating and the duration decreases with an increase in the DBDS concentration. Copper sulfide formed on the Kraft paper is a portion of the total copper reacted with DBDS by comparing the amount of copper formed on the Kraft paper with the total amount of reacted copper calculated from the amount of consumed DBDS.

Development of Quantitative Evaluation of Copper Sulfide Deposition on Insulating Paper

We have developed a new heating test to investigate the influence of dissolved copper and oxygen on the copper sulfide generation in alkyl benzene insulating oil with dibenzyl disulfide (DBDS) added. The weight of the deposited and dissolved copper was systematically studied in atmospheres of nitrogen and artificial air. The copper sulfide deposition on the insulating paper originated from the dissolved copper (copper complex compound) in the insulating oil. The weight of the copper deposited on the insulating paper was increased by the existence of oxygen. The acceleration effect on copper sulfide deposition on insulating paper was clarified for the first time by strictly controlling the experimental atmosphere. Moreover, the influence of the existence of oxygen was investigated individually on insulating oil and insulating paper. The oxidation of both alkyl benzene and insulating paper was found to accelerate copper sulfide generation.

Influences of oxygen and 2,6-di-tert-butyl-p-cresol on copper sulfide deposition on insulating paper in oil-immersed transformer insulation

Copper sulfide deposition on cellulosic insulating materials in oil-immersed transformers was investigated by heating tests stipulated by IEC 62535 with controlled atmospheres to which mineral insulating oils were exposed. Both oxygen in the atmosphere and 2,6-di-tert-butyl-p-cresol (DBPC) in the oil were found to accelerate the copper sulfide deposition on the insulating paper when the oil contains dibenzyl disulfide (DBDS). The weight of copper sulfide deposition on the insulating paper was found to increase when the oxygen concentration was increased up to 20 volume %. X-ray absorption near edge structure (XANES) analysis was applied to study the chemical bonding state of copper atoms in copper deposits on the insulating paper for the first time. It can be clarified the copper deposits on the insulating paper are mainly Cu2S and small amounts of CuO are contained.

Advanced monitoring system for gas density of GIS

Detection of SF6 gas leakage from a gas insulated switchgear (GIS) has recently become an important issue from an environmental standpoint in addition to the demands for the insulating reliability. This paper describes a state-of-the-art gas leakage detection system, which consists of a high-performance gas pressure sensor and a new algorithm improving accuracy of the leakage rate calculation. The gas pressure sensor has enough properties, resolution of 20 Pa and the stability of 0.004 % per year. Furthermore, in order to achieve high accuracy of leakage detection in the actual field, the new algorithm of the leakage rate calculation has been developed to cancel the interference due to solar radiation and weather. A yearlong test of the 84 kV GIS system demonstrated that the algorithm reduces fluctuations of obtained gas pressure to one fifth and enables to detect the leakage of 0.5 % per year, which is standardized maximum leakage by the new standard, IEC 62271-203.