Influence of the Atmosphere on the Reaction of Dibenzyl Disulfide with Copper in Mineral Insulation Oil

Transcription

1 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 19, No. 3; June Influence of the Atmosphere on the Reaction of Dibenzyl Disulfide with Copper in Mineral Insulation Oil Shuangzan Ren, Lisheng Zhong, Qinxue Yu, Xiaolong Cao and Shengtao Li State Key Laboratory of Electrical Insulation and Power Equipment, Xi an Jiaotong University, Xi an, Shaanxi, , China ABSTRACT 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 (Cu 2 S), 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 kj/mol by using the Arrhenius equation. Index Terms Temperature, oxygen, dibenzyl disulfide, copper, mineral insulation oil, activation energy. 1 INTRODUCTION IN the past decade, many failures of high-voltage transformers and reactors occurred because of copper sulfide (Cu 2 S) formation [1, 2], and the presence of dibenzyl disulfide (DBDS) in transformer oil has been considered as one of the main culprits [3, 4]. A number of investigations have shown that the deposition of Cu 2 S on the insulation paper would weaken the dielectric strength of such oil immersed insulation paper [5, 6]. This phenomenon has attracted considerable worldwide attention [7 19], and CIGRE WG A2-32 Copper sulfide in transformer insulation and WG A2-40 Copper sulfide long-term mitigation and risk assessment were established in 2005 and 2009, respectively. T. Amimoto et al [12] proved that the mechanism of Cu 2 S formation involved the reaction of DBDS in oil with Copper to form the DBDS-Cu complex firstly, followed by the decomposition of the complex into Cu 2 S, dibenzyl sulfide (DBS), and bibenzyl (BiBZ). DBS then reacts with Copper, simultaneously generating Cu 2 S and BiBZ. They also elucidated the factors affecting Cu 2 S deposition on insulation paper [13 15]. Once the temperature increases by 10 C (the aging test was conducted from 120 C to 140 C), the deposition rate of Cu 2 S on the insulation paper doubles [13], the weights of Cu deposited on the insulation paper increase because of the Manuscript received on 28 September 2011, in final form 26 December presence of oxygen in oil [14], and the dissolved Cu (DBDS-Cu complex) and the DBDS concentration also affect the Cu 2 S deposition on the insulation paper [14, 15]. Different suppression methods have been developed because of the risks brought by corrosive sulfur. Antioxidants such as 2,6-di-tert-butyl-p-cresol (DBPC) and passivators such as 1,2,3-benzotriazole (BTA) and Irgamet 39TM have proven effective in suppressing Cu 2 S formation in transformer oil [16 19]. Until now, most of the reported experiments were conducted on paper-wrapped Cu. However, available data are insufficient to elucidate the influence of temperature and oxygen on the reaction of DBDS with Cu. In addition, the activation energy of this reaction remains to be determined. In the current study, a mineral insulation oil containing 500 mg/l DBDS was aged under different thermal (105 C to 165 C) and atmospheric (N 2 and air) conditions to gain a better understanding of the mechanism of reaction of corrosive sulfur with Cu. Various DBDS concentrations have been detected in the mineral insulation oil at different thermal and atmospheric conditions using high-performance liquid chromatography (HPLC). Based on the results, the activation energy of the reaction of DBDS with Cu has been obtained using the Arrhenius equation, and the mechanism by which oxygen dissolves in oil affects the deposition of Cu 2 S on the Cu or insulation paper surface is discussed as well /12/$ IEEE

2 850 S. Ren et al.: Influence of the Atmosphere on the Reaction of Dibenzyl Disulfide with Copper in Mineral Insulation Oil 2 EXPERIMENTAL 2.1 SAMPLE PREPARATION A commercially available mineral insulation oil containing no DBDS or any other corrosive sulfur substance was used as the sample oil. The corrosivity of the mineral insulation oil was verified according to ASTM D 1275-B (bared Cu, 150 C, 48 h) and IEC (paper-wrapped Cu, 150 C, 72 h). A series of oil samples with DBDS concentrations ranging from 80 mg/l to 680 mg/l was prepared by dissolving DBDS in the mineral insulation oil. Standard oil samples containing 80, 200, 320, 440, and 680 mg/l DBDS were used as the calibration samples, and the oil sample with a DBDS concentration of 500 mg/l was used for the test. The Cu wire (d = 1 mm) was first cut into a 25 cm long piece and then polished using a 240-grit silicon carbide paper. The wire was subsequently immersed in acetone for 1 min, left in air for approximately 5 min, enwinded into a spiral pattern, and then placed in a glass vial containing 15 ml oil sample with 500 mg/l DBDS. The glass vial was immediately placed in a stainless steel vessel that can be vacuumed to as low as 20 Pa. 2.2 THERMAL AGING The thermal aging conditions of the oil samples were arranged under N 2 gas and air conditions, as shown in Figure 1. The procedure can be described as follows: the stainless steel vessel was first vacuumed to as low as 20 Pa, and N 2 or air under an atmospheric pressure was injected into the vessel, as the experiments requirement. Two specimens were prepared at each thermal and atmospheric condition, with two aging times. The thermal aging conditions and experimental arrangements are shown in Table 1. For the flash point of the sample oil is 150 C, the experiments were not conducted at 165 C under the air condition. Temperature ( C) Figure 1. Photographs of the aging vessel and vial. Table 1. Thermal aging conditions. in N 2 Aging time in Air h, 288h 192h, 288h h, 288h 192h, 288h h, 144h 72h, 144h h, 96h 72h, 96h h, 20h MEASUREMENT OF DBDS CONCENTRATION IN OIL The HPLC technique was attempted in the current study. Considering that the components of the mineral insulation oil are mainly alkenes, cycloparaffinic hydrocarbons, and a small amount of aromatic hydrocarbons, separating the DBDS peak from those of other components during the measurement was difficult. The solid phase extraction (SPE) method [12] was used to effectively reveal the DBDS peak. Figure 2 shows the results of the HPLC analysis. Figure 2 HPLC analysis result, the retention time of DBDS was confirmed to be at around 6.2 min. The oil extraction procedure used in the study is similar to that in reference [12]. In brief, a neutral alumina SPE column was first pre-washed with 1 ml hexane. The oil sample (1 ml) was then penetrated into the SPE column. After the oil sample was completely submerged, the column was washed thrice with 0.8 ml hexane, followed by 2.5 ml benzene. The extracts were collected in a glass vessel, which was subsequently placed into a constant-temperature (70 C) incubator until the benzene completely evaporated. Finally, 5 ml benzene was added into the glass vessel, which was then sealed. The remaining benzene in the glass vessel was analyzed via HPLC; the detection parameters are shown in Table 2. Table 2. Detection parameters for the HPLC analysis. Equipment type HP 1100 HPLC Silica gel Chromatographic column 150 mm 6 mm, 5 um Detector Ultraviolet detector & DAD (diode array detector) Detection wavelength λ sig = 215 nm Column temperature 10 C Mobile phase (normal phase) hexane Flow rate 1.2 ml/min Injection volume 5 μl All standard oil samples were measured twice using HPLC to calibrate the DBDS concentration of each aged oil sample; the average value was used to plot the calibration curve. The relative deviation is shown in Table 3. Table 3. Measurement results for the standard oil samples. DBDS Peak Area x x 1 2 concentration (mg/l) x x 1 2 x 1 x 2 100%

3 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 19, No. 3; June Table 3 shows that all relative deviation for the HPLC results (the DBDS concentration in the mineral insulation oil ranges from 80 mg/l to 680 mg/l) are below 1%, indicating that the repeatability of using HPLC to measure the DBDS concentration in oil is reliable. Moreover, the obtained DBDS concentrations show a very good linear correlation with the HPLC peak areas, with the correlation coefficient R reaching (Figure 3). (a) (b) Figure 3. Calibration curve for DBDS concentration in the standard oil samples. 3 RESULTS AND DISCUSSION 3.1 DIFFERENT COMPONENT ON THE COPPER SURFACE DURING THERMAL AGING The changes in color of the oil samples and Cu spiral wires are significantly different after the aging test at different periods. Nearly no color change is observed in oil under the N 2 condition. However, the color of the oil samples has become considerably darker under the air condition when the temperature is higher and the aging time is longer. Nevertheless, the changes in color of the Cu are opposite those of the oil samples. A comparison of the samples aged at 135 and 150 C for 72 h is shown in Table 4. Table 4. Influence of the temperature and atmosphere on the changes in color of the oil samples and copper surface (72 h). Sample in N 2 in Air 135 C 150 C 135 C 150 C Oil Cu 4a* 4b* 1a* 1b* * According to ASTM D130 /IP 154 Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were used to analyze the various components on the Cu surface of the aging samples under different conditions. The results are shown in Figure 4 and Table 5, respectively. (c) (d) Figure 4. SEM images of the Cu surface (72 h), (a):135 C, in N 2 ; (b): 150 C, in N 2 ; (c): 135 C, in air; (d): 150 C, in air. Table 5. Elemental composition of the Cu surface (72 h). Element in N 2 in Air (atom %) 135 C 150 C 135 C 150 C C O S Cu Total Several dense granules are deposited on the Cu surface when the Cu wire is aged under N 2 (Figure 4). However, numerous cavities have appeared on the Cu surface when the Cu wire is aged under the air condition. The results in Table 5 also demonstrate that the sulfur content on the Cu surface aged under N 2 is much higher than that under the air condition. By contrast, the amount of oxygen deposited on the Cu surface under the N 2 atmosphere is lower than that under air. Based on the mechanism reported by T. Amimoto et al [12], the amount of sulfur deposited on the Cu surface should come from Cu 2 S, which is the product of the reaction of DBDS with Cu. Given that oxygen and sulfur are from the same main group, the oxygen deposited on the Cu surface should come from cuprous oxide (Cu 2 O). The Cu content in the oil samples was also measured using atomic absorption spectrometry (AAS-SOLAAR M6) to verify the phenomena shown in Figure 4. The procedure and detection parameters are as follows: 10 ml mixed acid (perchloric acid and concentrated nitric acid, 1:4 by volume) was added to 1 g oil sample, and the mixture was digested under heating five times. The digested sample was then diluted to 25 ml with deionized water. The wavelength of Cu is nm, with a band pass of 1 nm, and the flame type is air-c 2 H 2 at a flow rate of 1.1 l/min. The results are shown in Figure 5.

4 852 S. Ren et al.: Influence of the Atmosphere on the Reaction of Dibenzyl Disulfide with Copper in Mineral Insulation Oil 3.2 INFLUENCE OF THE ATMOSPHERE ON THE DBDS CONCENTRATION IN OIL As previously mentioned, the DBDS concentration of each aged oil sample was measured using HPLC. Figure 7 shows the decreasing rate (v) of each oil sample as calculated using equation (1), as follows: CDBDS ( ) CInitial CAged v (1) t t where ΔC is the concentration change of DBDS concentration in mg/l, and t is the thermal aging time in h. Figure 5. Concentration of Cu in the 105 and 120 C oil samples for 192 h, 135 and 150 C samples for 72 h and 165 C sample is in 20 h. Figure 5 shows that the Cu concentration in oil under air atmospheric condition is much higher than that under N 2, especially at higher temperatures. The cavities on the Cu surface (Figure 4) are due to the removal of the Cu atoms or compounds and their dissolution in oil. In another condition, if there is insulation paper covered on copper surface, more copper sulfide would be deposited on insulation paper [14]. A model that illustrates the influence of oxygen on Cu 2 S deposition on the Cu or insulation paper surface was developed based on the aforementioned situations (Figure 6). The DBDS-Cu complex forms and is deposited on the Cu surface at certain conditions. In the presence of oxygen, the DBDS-Cu complex easily dissolves in oil or is adsorbed in the insulation paper; thus, the Cu concentration in oil or Cu 2 S deposited on the insulation paper surface under the air condition is higher than that under the N 2 condition. Such action also results in the formation of cavities on the Cu surface (Figure 4). However, when there is no oxygen in oil, the DBDS-Cu complex would decompose into Cu 2 S on copper surface directly, because the oxygen dissolved in oil would react with copper and form Cu 2 O simultaneously, so there is more sulfur element on copper surface in nitrogen condition and more oxygen element on copper surface in air condition (Table 5). Based on such model, it also seems that the lower concentration oxygen in transformer oil, the less risk would happen even if there is corrosive sulfur in oil, because less Cu 2 S would be deposited on insulation paper surface even if it has formed. Figure 6. Model showing the influence of oxygen on Cu 2 S deposited on Cu surface or insulation paper surface. Figure 7. Variation in the DBDS concentration in oil under different atmospheric condition. Figure 7 shows that the decreasing DBDS concentration exhibits exponential growth with increasing temperature regardless of the atmosphere. The tendency of the rate to decrease under both the N 2 and air conditions is similar. v N2 is slightly lower than v Air before the cross point (at approximately 130 C) and is higher than v Air after the point. This result indicates that regardless of the presence of oxygen, the atmosphere seems to have little effect on the DBDS reaction with Cu in oil. Some oil samples with DBDS concentration 500 mg/l and without Cu wire added were used to validate the above mentioned elucidation. The oil samples were aged for 72 h at 120 C, 135 C and 150 C in N 2 and air conditions. The varieties of the DBDS concentrations in oil were measured, and the results are shown in Figure 8. The changes in the DBDS concentrations are small under the N 2 condition. However, the DBDS concentration gradually decreases with increasing temperature in the presence of oxygen (air condition), indicating that the DBDS in oil reacts with oxygen. According to reference [21], sulfur compounds in mineral insulation oil can be oxidized into sulfoxide and even to sulfonic acid, resulting in the decrease of the DBDS concentration in oil. The results shown in Figure 7 and Figure 8 indicate that although the DBDS concentration decreasing rate in oil is similar in air and nitrogen condition, the actual situation is very different. In nitrogen condition, the DBDS decreasing

5 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 19, No. 3; June is due to its reaction with Cu, however in air condition, in addition to reacting with copper, the DBDS concentration decreasing is mainly due to its reaction with oxygen. Table 6. Parameters used to calculate E a. Temperature in N 2 in Air C 1/T v (K -1 ) (mol l -1 h v -1 lnv ) (mol l -1 h -1 ) lnv e e e e e e e e e e e e e e The value of E a /R under N 2 and air conditions can be obtained by plotting the data in Figure 10, where E an2 and E aair are kj/mol and kj/mol respectively. Figure 8. Variation of DBDS concentration in oil samples without Cu added. 3.3 INFLUENCE OF THE ATMOSPHERE ON THE REACTION OF DBDS WITH COPPER Figure 9 shows the mechanism of Cu 2 S formation [12]. Figure 10. Activation energy in the reaction of DBDS with Cu/oxygen in oil. Figure 9 Mechanism of the reaction of DBDS with copper In a general chemical reaction, the effect of temperature on the chemical reaction rate is commonly expressed using the Arrhenius equation, as follows [20]: a / k Ae E RT (2) where k is an equilibrium constant of the reaction rate, R is the molar gas constant equal to J K -1 mol -1, E a is the activation energy of the reaction in kj/mol, and T is the absolute temperature in K. In general, the reaction rate v (Equation 1) and equilibrium constant k (Equation (2)) would meet Equation (4), where 2 v k [ Cu] [ DBDS] (3) The unit of v in equation (1) is mg l -1 h -1, whereas that in equation (3), should be converted into mol l -1 h -1. Combing equations (2) and (3), yields Ea / RT 2 v Ae [ Cu] [ DBDS] (4) Given that initial amount of Cu and the concentration of DBDS in oil are the same for each sample, so Equation (4) can be converted into the following Equation: ln v ln B Ea / RT (5) where 2 ln B ln A [ Cu] [ DBDS] (6) Based on Figure 7 and equation (5), the related parameters used to calculate E a are shown in Table 6. According to section 3.2, when it is in nitrogen condition, the DBDS concentration decreasing is due to its reaction with Cu, however in air condition, the DBDS in oil could react with Cu and oxygen simultaneously, so that might be why the activation energy obtained in N 2 is more than in air condition (98.61 kj/mol vs kj/mol). Besides, when it is in air condition, some other conclusions could also be drawn that at a lower temperature ( C), the DBDS decreasing might be mainly due to its reaction with oxygen, whereas at a higher temperature ( C), the DBDS decreasing is mainly due to its reaction with Cu (Figure 10, dashed points). 4 CONCLUSIONS A mineral insulation oil containing 500 mg/l DBDS with Cu was aged at different temperatures (105 C 165 C) and under different atmospheric conditions (N 2 and air). Some conclusions were drawn as follows: (1) Under the nitrogen condition, the Cu 2 S is prone to deposit on Cu surface, for there is much sulfur element deposited on Cu surface. On the other hand, under the air condition, the Cu complex tends to dissolve in oil, which leads to a significantly higher Cu concentration in oil compared with that under the N 2 condition, and many cavities would also appear on the copper surface. Based on these results, it seems that the lower concentration oxygen in transformer oil, the less risk would happen even if there is corrosive sulfur in oil, because less Cu 2 S would be deposited on insulation paper surface even if Cu 2 S has formed in a closed system.

6 854 S. Ren et al.: Influence of the Atmosphere on the Reaction of Dibenzyl Disulfide with Copper in Mineral Insulation Oil (2) Under the nitrogen condition, the DBDS in oil only reacts with Cu at certain temperatures. However, when it is aged under the air condition, apart from reacting with Cu, the DBDS would also react with oxygen simultaneously. (3) The reaction energy of the DBDS reaction with Cu in oil is determined as approximately kj/mol using the Arrhenius equation. ACKNOWLEDGMENT This work is supported by State Key Laboratory of Electrical Insulation and Power Equipment in Xi an Jiaotong University, China (EIPE10311), and the National Basic Research Program (973 Program) of China (2011CB209404). The authors also would like to acknowledge Mr. Riccardo Maina in Sea Marconi Corporation and Dr. Robert Jeanjean in RJ Consulting for their great suggestions, and the collaboration with Toshiba Corporation. REFERENCES [1] A. H. 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Tsuchie, et al., Influence of Diverse Compounds on Electrostatic Charging Tendency of Mineral Insulating Oil used for Power Transformer Insulation, IEEE Trans. Dielectr. Electr. Insul, Vol. 16, pp , Shuangzan Ren (S 08) was born in Hengshui City, Hebei Province, China, in He received the B.Sc. degree from Chang an University, Xi an, China, in He began his M.S. career from 2006 to 2008 in Xi an Jiaotong Universtiy, and studied for the Ph.D. degree in advance since then. His main research field is insulation faults diagnosis for power equipments and insulation oils. Lisheng Zhong was born in Sichuan, China, in He received the B.Eng., M.Sc. and Ph.D. degrees in electrical engineering, from Xi an Jiaotong University, respectively in 1983, 1986 and Currently, he is a professor at the State Key Laboratory of Electrical Insulation and Power Equipment in Xi an Jiaotong University. His research interests include dielectrics and electrical insulation. Qinxue Yu was born in Pucheng city of Shaanxi province, China, in He received the M.Sc. degree from Xi an Jiaotong University in 1989 and the Ph.D. degree from the same University in His interests focus on online monitoring and diagnosis technology of power equipments, and aging performance as well as life assessment of insulation materials. Xiaolong Cao (SM 98) was born in Xi an city of Shaanxi province, China, in He received the B.Sc. M.Sc. and Ph.D. degree in electrical engineering and cable technology from the Xi an Jiaotong University in 1968, 1983 and 1986, respectively and has been a professor since He is involved in electrical insulation measuring technology, dielectric properties of insulation materials; besides, he is also a CIGRE member of B1. Shengtao Li was born in Sichuan, China, in He received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from Xi an Jiaotong University in 1983, 1986, and 1990, respectively. Currently, he is a professor at the State Key Laboratory of Electrical Insulation and Power Equipment in Xi an Jiaotong University. His research interests include electronic ceramics and devices, insulating materials and insulation systems, and electrical treeing in polymers. Professor Li is an Associate Editor of the IEEE Transactions on Dielectrics and Electrical Insulation.