Corrosion Mechanisms for Electrical Fields Leading to the

Article pubs.acs.org/IECR

Corrosion Mechanisms for Electrical Fields Leading to the Acceleration of Copper Sulfide Deposition on Insulation Windings Lijun Yang,† Sihang Gao,*,† Bangfei Deng,‡ and Zhidong Cheng† †

State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing, China, 400044 ‡ State Grid Chongqing Electric Power Research Institute, Chongqing, China, 400015 S Supporting Information *

ABSTRACT: Numerous failures of high-voltage transformers and reactors are caused by copper sulfide formation in oilimmersed insulations. This study explored the effect of electrical fields on copper sulfide formation. Accelerated aging experiments were conducted for mineral oil that contains dibenzyl disulfide, which was aged along with insulation windings under different conditions, including single thermal aging and electrical−thermal aging. The corrosive sulfur deposits were quantified using SEM/EDX and ICP-AES. The properties of the insulation oils were also measured and analyzed. Corrosion mechanisms for electrical fields leading to the acceleration of copper sulfide deposition on the oil-immersed insulation were proposed.

1. INTRODUCTION Copper conductor has been widely used in electrical power equipment, such as oil-immersed transformers, high voltage direct current transmission (HVDC) devices, and shunt reactors, due to its high conductivity.1 However, copper corrosion still occurs in electrical power equipment, which results in corrosion-related failures.2 The main insulation system of oilimmersed equipment consists of liquid insulation (e.g., mineral oil/vegetable oil) and solid insulation (e.g., paper/pressboard); this system degrades because of thermal stress, electrical field stress, mechanical stress, and chemical stress.3 The insulation paper that wraps the copper conductor and pressboard mainly provides electrical insulation between the windings in a coil, and the insulation oil provides electrical insulation to the windings and cooling to the equipment.1 Over the past decades, numerous failures in oil-immersed transformers and shunt reactors induced by the presence of corrosive sulfur compounds have been related to copper sulfide (CuxS) formation on the insulation windings.4,5 Dibenzyl disulfide (DBDS) has been considered to be the main corrosive sulfur in mineral oil, which can directly react with copper to form copper sulfide that deposits on the copper surface and the inner and outer layers of the insulation paper.6,7 Copper sulfide, which is a gray−black solid with high conductivity, reduces the performance of oil− paper insulation.4 Therefore, the present study extensively performed a series of standalone thermal experiments stipulated by IEC 62535 to investigate the chemical process of copper sulfide formation.8 Three reports have discussed the main corrosion mechanisms of copper sulfide formation. The first corrosion mechanism, proposed by the ABB Company in the United States, asserts that mercaptan (R−SH) is the main corrosive sulfur that © 2017 American Chemical Society

can react with copper under an aerobic environment to generate copper sulfide.9 The second corrosion mechanism, proposed by Mitsubishi Company in Japan, states that DBDS is responsible for copper sulfide formation and that DBDS can react with copper to generate the DBDS−Cu compound, which decomposes into copper sulfide, dibenzyl sulfide (DBS), and bibenzyl (BiBZ) on insulation windings.10 The third corrosion mechanism, proposed by Facciotti, adopts a contact-based mechanism to explain the copper sulfide contamination on the insulation paper and its displacement; Facciotti also postulated on the transport of copper sulfide particles to the first-layer paper through the interstitial oil, in which oxygen was observed to interfere with the surface interaction between the copper sulfide and copper, thereby causing its displacement.11 Aside from the above-mentioned types of corrosive sulfur compounds that cause corrosion mechanisms (i.e., mercaptan/DBDS), the difference was reflected in the reaction whether oxygen was a necessity in copper corrosion to generate copper sulfide. However, low oxygen exists in the inner part of sealed electrical equipment, such as transformers. The existence of oxygen accelerates the deposition of copper sulfide. Moreover, malignant oil mainly contains DBDS. Previous studies have also revealed that copper sulfide formation is influenced by various factors, such as temperature, concentration of corrosive sulfur (i.e., DBDS), oxygen, 2,6-ditert-butyl-p-cresol (DBPC), dissolved copper in oil, and acids in oil.12−18 The above-mentioned corrosion mechanisms and Received: Revised: Accepted: Published: 9124

April 20, 2017 June 12, 2017 July 3, 2017 July 3, 2017 DOI: 10.1021/acs.iecr.7b01666 Ind. Eng. Chem. Res. 2017, 56, 9124−9134

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Industrial & Engineering Chemistry Research

2 mm according to the ASTM D1816 standard.20 One of the insulation windings was connected to a high voltage source, and the other was grounded. Ac and dc power supplies were used to provide electric stress to insulation windings. Both the ac and dc power supplies have only one high voltage output terminal. For the ac power supply, the electric potential of the high voltage output terminal is always higher than that of grounding; the voltage phase and the corresponding current alternately switched with a frequency of 50 Hz. The dc power supply included positive dc power supply and negative dc power supply. When the electric potential of the high voltage output terminal is higher than that of grounding, and the flow of current is always from high voltage terminal to grounding, it refers to positive dc power supply. When the electric potential of the high voltage output terminal is lower than that of grounding, and the flow of current is always from grounding to high voltage terminal, it refers to negative dc power supply. Two gas valves were placed on the tank cover to inject or release gas. 2.3. Experimental Procedure. To explore the effects of electrical fields on copper sulfide formation, the windings and noncorrosive insulation oil with 200 mg/kg DBDS were aged with and without the application of voltage. Two insulation windings were fixed in parallel with a distance of 2 mm in a sealed stainless steel tank, which was subjected to a 150 °C dimethyl silicon oil bath before aging for 72 h according to the IEC 62535 standard. The applied voltages were 0, 2, 4, and 8 kV ac/dc, and the winding samples had 0, 1, 2, and 4 kV/mm ac/dc-applied electric stress. Table 1 lists the experimental conditions. The experiment was conducted as follows: Step 1. The oil and winding samples were dried and degassed in a vacuum oven at 90 °C/50 Pa for 24 h. The windings were then immersed in oil at 40 °C/50 Pa for 48 h. Afterward, the windings were mounted onto the sealed stainless steel tank with a distance of 2 mm. Step 2. The experiment was conducted in two groups, as shown in Table 2. In group 1, the DBDS with a constant concentration of 200 mg/kg was dissolved in sealed nitrogenfilled tanks with noncorrosive oil. In group 2, noncorrosive oil was added in the sealed nitrogen-filled tanks. To investigate the effects of electrical fields on copper sulfide formation, ac and positive/negative dc (1, 2, 4 kV/mm) were applied to the winding samples. For comparison, no voltage (thermal aging alone) was applied to the winding samples in groups 1 and 2. Step 3. The tanks were heated to 150 °C for 72 h. After the aging experiment, the insulation oil and windings were removed from the aging tanks, and the characteristic parameters of insulation oil were measured and analyzed. The corrosion degree of the windings was also observed and analyzed. 2.4. Characterization. Table 2 shows the measurement items and methods for the insulation oil and windings. Three methods were applied to measure the corrosion degree of insulation winding. The first method was to visually observe the surface morphology of insulation winding and compare the contamination degree of samples. The second method was the use of SEM/EDX to observe the microscopic morphology of the copper strips. Thus, the corrosive particles on the copper strip were observed and the chemical compositions of these particles were determined. The last method was the use of ICP-AES to quantify the amount of copper deposited on the insulation paper and dissolved in the oil. The testing process is described as follows. The insulation paper was burned to

associated factors mainly consider the effect of single thermal aging in oil-immersed electrical equipment. However, as the main insulation used in oil-immersed electrical equipment, oil-impregnated paper insulation is simultaneously affected by thermal and electrical fields (ac and dc field), which directly deteriorate the insulation performance. Moreover, few studies have considered the effect of electrical fields on copper sulfide formation. The WG A2-40 assessment indicated that the important initiation step in the reaction of copper sulfide formation is the production of electrons and ions, in which electrical and magnetic fields may play an important role.19 Therefore, the role of electrical fields on copper sulfide deposition in oil-immersed insulation warrants further investigation. In this study, mineral oil that contains DBDS with a concentration of 200 mg/kg was aged along with insulation windings under different conditions, including single thermal aging (150 °C) and electrical−thermal aging (1 kV/mm, 2 kV/mm, 4 kV/mm ac/dc, 150 °C). The scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM/ EDX) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were used to evaluate the corrosion degree of insulation windings. The characteristic parameters of the insulation oil were measured and analyzed. Finally, a new corrosion mechanism for copper sulfide formation is proposed to explain the effect of electrical fields on the contamination of copper windings in oil-immersed insulation.

2. EXPERIMENT 2.1. Experimental Materials. Analytical grade reagents (AR) were used. Dibenzyl disulfide (AR, 99%) was purchased from China Huaxia Chemical Reagent Co. Ltd. The insulation oil used was Karamay mineral oil, which was provided by China Chongqing ChuanRun Petroleum Chemical Co. Ltd. Karamay mineral oil has favorable electrical properties and oxidation stability, with the antioxidant (2,6-di-tert-butyl-4-methylphenol) mass concentration of 0.3% and with maximum total sulfur content of 0.05%, no detectable DBDS, no metal passivator additives, and noncorrosive according to the standard corrosion test IEC 62535.8 The oil parameters (see Table S1) were in accord with the specification of IEC 60296. The insulation windings (Figure 1) were provided by China Chongqing ABB

Figure 1. Winding diagram.

Transformer Co. Ltd. The copper strips were generated from electrolytic tough pitch copper with a 9 mm width, 2.4 mm thickness, and 80 mm length. Each conductor was wrapped with four layers of 0.07 mm thick Kraft insulation paper, which has standard electrical insulation. 2.2. Experimental Setup. Figures 2 and 3 show the electrical−thermal aging setup. A dimethyl silicon oil bath was used to control the experimental temperature. Two insulation windings were fixed in parallel at the upper and lower electrodes in a sealed stainless steel tank with a distance of 9125

DOI: 10.1021/acs.iecr.7b01666 Ind. Eng. Chem. Res. 2017, 56, 9124−9134

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Figure 2. Configuration of the electrical−thermal aging setup.

Figure 3. Electrical−thermal aging setup.

Table 1. Experimental Conditions classification

DBDS concn

distance of windings

electrical fields

electric stress

thermal aging condition

0 group 1

200 mg/kg

ac 1 kV/mm 2 mm

group 2

150 °C, 72 h, IEC 62535

positive dc

0

2 kV/mm negative dc 4 kV/mm

acid solution. The solution volume was fixed to 25 mL polypropylene flasks with 4% nitric acid. The diluted solution was tested using ICP-AES. The same method was applied to measure the amount of copper dissolved in the insulation oil. The measurement wavelength was set to the resonance line of copper (i.e., 324.75 nm). Therefore, the corrosive degree of different samples was accurately compared using this method.

Table 2. Tested Parameters of the Oil−Paper Samples object insulation oil

winding

project dissipation factor (%) volume resistivity (Ω·m) acidity (mg KOH/g) cpper ions (mg/kg) surface morphology microscopic morphology of the winding copper deposited on the insulation paper (mg)

method ASTM D924-08 ASTM D2739-97 IEC 62021-1 ICP-AES visually observe SEM/EDX ICP-AES

3. EXPERIMENTAL RESULTS 3.1. Surface Morphology of the Insulation Windings and Insulation Oils. Table 3 shows the surface morphology of the copper strips and insulation papers on the HV electrode and the grounding side under ac electrical−thermal aging and

ashes in a muffle furnace. The ashes were cooled to room temperature and then dissolved into 10 mL of 1:10 hydrochloric 9126


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Table 3. Surface Morphology of the Copper Strips and Insulation Papers under Ac Electrical Field and Thermal Aging (0 kV/mm)

Table 4. Surface Morphology and Corrosion Degree of the Copper Strips and Insulation Papers under Dc Electrical Field and Thermal Aging (0 kV/mm)

thermal aging alone, respectively. Visible gray−black deposits were observed on the copper strips, which indicates the occurrence of corrosion on all the windings under thermal and ac electrical−thermal aging conditions. However, in comparison with thermal aging alone, the ac fields accelerated the corrosion degree of the copper strips on the HV electrode, which is practically consistent with that on the grounding side. With the increase of ac electric stress from 0 kV/mm to 4 kV/mm, the corrosion degrees of the copper strips on the HV electrode and the grounding side were intensified. For thermal aging alone, few deposits were observed on the insulation paper, regardless of the innermost paper or the other layers of the paper. Similarly, the corrosion degree of the innermost papers under the ac fields was more significant than that under thermal aging alone; meanwhile, scarcely any deposits were observed on the outer insulation papers. The corrosion degree of the insulation papers on the HV electrode was practically consistent with that on the grounding side. Thus, applying high electric stress on windings would lead to greater deposits on the insulation paper. Table 4 shows the surface morphology of the copper strips and insulation papers on the HV electrode and the grounding side under positive/negative dc electrical−thermal aging conditions, respectively. Visible gray−black deposits were observed on the copper strips and insulation papers under dc electrical fields. The corrosion degree of the insulation windings under dc fields was the most significant, followed by that under ac fields; the least remarkable corrosion was that for the insulation

winding under thermal aging alone. The corrosion degree of windings intensified with the increase of dc electric stress. However, the corrosion degree of windings has differences between the HV electrode and the grounding side under dc electrical−thermal aging. The corrosion degree of the windings under the HV electrode was more significant than that under the grounding side when the windings were applied with positive dc fields. However, the corrosion degree of the windings under the grounding side was more notable than that under the HV electrode when the windings were applied with negative dc fields. The color of insulation oils for group 1 presented differences under three different aging conditions (see Table S2). One of the most significant manifestations of the aging oil is that the color gradually becomes deeper, which is due to the degradation and oxidation of the oil itself. During the aging process, a certain amount of aging products (acid, sludge, etc.) produced will deepen the color of the mineral oil, and the inevitable interaction of the copper winding with some aging products (acids, moisture, nitrogen compounds) and corrosive sulfur will result in an increase in the content of copper ions in oil. The oil color under electrical−thermal aging was deeper than that under thermal aging alone. The color of oil under dc electrical− thermal aging was the deepest, followed by the oil under ac electrical−thermal aging, and the least was the oil under thermal aging alone. For ac and dc electrical−thermal aging, applying high electric stress on the windings would lead to deeper oil color. For the appearance of insulation oils for group 9127


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Figure 4. SEM figures of insulation papers under different aging conditions: (a) thermal aging; (b) ac electrical−thermal aging at 4 kV/mm; (c) dc electrical−thermal aging at 4 kV/mm.

low oxygen condition.24,25 An increase in the amount of acidic substances not only can corrode metals but also can degrade the properties of the insulating oil.23−26 Although the degradation of DBDS also has a pro-oxidant effect, a larger amount of DBDS cannot perform as a secondary antioxidant to slow down the oxidation process under low oxygen conditions due to the corrosion reactions and the presence of acidic byproducts. 3.2. Microscopic Morphology Analysis of the Insulation Windings. Figure 4 shows the SEM figures of the insulation papers under three different aging conditions. A small amount of corrosive particles on the insulation paper under thermal aging alone was observed. More corrosive particles

2 under three different aging conditions, see Table S3. The color of the oil under dc electrical−thermal aging was the deepest, followed by the oil under ac electrical−thermal aging, and the least was the oil under thermal aging alone. Oil color becomes deeper with the increase of electric stress. In general, the electrical field accelerated the aging of oil. Moreover, the oil color for group 1 was deeper than that for group 2, which indicates that the existence of DBDS accelerated the aging of oil. In this experiment, the oxygen level is low and the temperature is high. Degradation of DBDS is dominantly affected by thermal, and DBDS in the insulating oil can be easily transformed into sulfoxide and sulfonic acid compounds under the 9128


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Industrial & Engineering Chemistry Research with different shapes and sizes were deposited on the insulation paper after applying a 4 kV/mm ac at 150 °C. Large areas of corrosion were apparent because of the large corrosive particles after applying a 4 kV/mm dc at 150 °C. The main component elements of the corrosive deposits, including carbon, oxygen, copper, and sulfur, were analyzed via EDX, as shown in Table 5. Table 5. Contents of Element on the Surface of Insulation Papers under Different Aging Conditions elements (atom %) aging condition

C

O

Cu

S

thermal aging alone ac electrical−thermal aging (4 kV/mm) dc electrical−thermal aging (4 kV/mm)

72.47 70.09 67.83

26.13 24.82 25.26

1.16 3.62 4.95

0.34 1.47 2.16

Figure 6. Comparison of copper deposited on the insulation paper under different aging conditions.

The results show that the sulfur content on the insulation paper under dc electrical−thermal was the highest, followed by that under ac electrical−thermal aging, and the least was under thermal aging alone. The atomic percentage of Cu and S was approximately 2.3−3.4 for the three aging conditions. All the above results indicate that some amount of copper sulfide was deposited on the insulation windings because of the reaction of DBDS and Cu. Ac and dc fields accelerated the corrosion of insulation windings. 3.3. Effects of Electrical Fields on Copper Sulfide Formation. To quantitatively analyze and compare further the copper sulfide deposition on the insulation windings under three different aging conditions, ICP-AES was used in quantifying the amount of copper deposited on the insulation paper and dissolved in the oil. As shown in Figures 5 and 6, the amount of copper deposited on the innermost papers under ac and dc electrical−thermal aging indicates that applying ac and dc fields accelerated the amount of copper sulfide deposition on the insulation paper remarkably faster than thermal aging alone, especially dc fields. High electric stress resulted in the deposition of a large amount of deposits on the surface of the insulation paper. Moreover, the amount of copper deposition on the insulation paper on the HV electrode was almost the same on the grounding side under ac electrical−thermal aging. However, the amount of copper deposition has differences between the HV electrode and the grounding side under dc electrical− thermal aging. For positive dc fields, the amount of copper

deposition on the insulation paper under the HV electrode was more than that under the grounding side. For negative dc fields, the amount of copper deposition on the insulation paper under the grounding side was more than that under the HV electrode. The polarity of dc fields influenced the deposition of copper sulfide. 3.4. Effects of Electrical Fields on the Properties of Insulation Oils. The insulation oil was gradually aging during the accelerated aging experiment; some aging products (acid, copper ions, colloid, asphaltene, etc.) produced will accelerate the aging of oil and influence the properties of oil. The insulation oil is gradually oxidized to generate a series of oxidation products, in which the acidic substances are deleterious, such as naphthenic acid and hydroxyl acid. Various acids and acidic substances will increase the conductivity of oil and accelerate the aging of solid fiber. Thus, the acidity of the insulation oil is a direct parameter to reflect the aging degree of oil. Typically, the more acidic substances it contains, the higher the acid value of the oil is. The insulation oil can be also reflected by the dissipation factor and volume resistivity. When aging products (acid and neutral oxides) are increasingly soluble in the insulating oil, the dissipation factor will be significantly augmented, and volume resistivity will be significantly declined, indicating the deterioration of the insulating properties. Moreover, the increase of copper ions dissolved in oil can

Figure 5. Copper deposited on the insulation paper under different aging conditions: (a) ac electrical−thermal aging; (b) dc electrical−thermal aging. 9129


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Figure 7. Properties of insulation oil that contains DBDS: (a) dissipation factor; (b) volume resistivity; (c) acidity; (d) copper ions.

promote the aging of oil, which subsequently also increases the conductivity and acidity of oil. Figure 7a shows the dissipation factor of oil that contains DBDS under different aging conditions. Dc electrical−thermal aging had the highest dissipation factor of oil, followed by the ac electrical−thermal aging and thermal aging alone. For ac/dc electrical−thermal aging, a high electric stress indicated a high dissipation factor of oil, and the dissipation factors of oil for positive/negative dc had no differences. Figure 7b shows the volume resistivity of oil that contains DBDS. Thermal aging alone had the highest volume resistivity of oil, followed by the ac electrical−thermal aging and dc electrical−thermal aging. For ac/dc electrical−thermal aging, a high electric stress indicated a low volume resistivity of oil, and the volume resistivity of oil for positive/negative dc had no differences. Figure 7c shows the acidity of oil that contains DBDS. Dc electrical− thermal aging had the highest oil acidity, followed by the ac electrical−thermal aging and thermal aging alone. For ac/dc electrical−thermal aging, a high electric stress indicated a high oil acidity, and the oil acidity for positive/negative dc had no differences. Figure 7d shows the copper ions of oil that contains DBDS. Dc electrical−thermal aging had the highest copper ions of oil, followed by the ac electrical−thermal aging and thermal aging alone. For ac/dc electrical−thermal aging, a high electric stress indicated high copper ions of oil, and the copper ions of oil for positive/negative dc had no differences. All the above results indicated that ac and dc fields can

accelerate the aging of insulation oil that contains DBDS, especially dc fields. Figure 8 shows the dissipation factor, volume resistivity, acidity, and copper ions of the noncorrosive oil under different aging conditions. Dc electrical−thermal aging had the highest dissipation factor (acidity and copper ions of oil), followed by the ac electrical−thermal aging and thermal aging alone. Thermal aging alone had the highest volume resistivity of oil, followed by the ac electrical−thermal aging and dc electrical− thermal aging. For ac/dc electrical−thermal aging, high electric stress indicated a more significant aging of oil. All the above parameters of noncorrosive oil for positive/negative dc had no differences. More DBDS contributed to the acceleration of the aging of insulation oil; thus, a high DBDS content in oil would result in a more significant aging degree of insulation oil.23−26 Figures 7 and 8 show that the performance of oil that contains DBDS was worse than that of noncorrosive oil. However, the results in Figure 8 also indicate that the electrical field accelerated the aging of noncorrosive insulation oil; especially, dc fields considerably increased the dissolved copper in the insulation oil, which promoted oxidation and accelerated the aging of oil.

4. DISCUSSION AND ANALYSIS Copper sulfur corrosion of transformers refers to the process of copper deterioration and copper sulfide formation, which is the disintegration of copper starting at its surface through a 9130


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Figure 8. Properties of insulation oil excluding DBDS: (a) dissipation factor; (b) volume resistivity; (c) acidity; (d) copper ions.

chemical or electrochemical action.21 The copper sulfide is deposited on the surface of a copper strip and insulation paper due to the reaction of DBDS and copper ions. For thermal aging alone, the chemical corrosion of insulation windings is a process that involves copper dissolution in the insulation oil, and the dissolved copper can combine with DBDS to form copper sulfide. As the main insulation used in oil-immersed transformers, oil-impregnated paper insulation is simultaneously affected by thermal aging and electrical fields, which directly deteriorate the insulation performance. Thus, the corrosion processes are also affected by electrical fields in oil-immersed insulation, which is usually accompanied by natural electrochemical corrosion. For electrochemical corrosion to occur, an anode, a cathode, an electrolyte, and a circuit connecting the anode and cathode are required to form an electrolytic cell, which is composed of HV/low voltage (LV) insulation windings and insulation oil in oil-immersed insulation.21 Electrochemical corrosion will occur when the electrical fields are applied on the insulation windings, which leads to the generation of a large number of tiny electrolytic cells on the copper surface (Figure 9). Based on the anodic reaction (oxidation action) of the corrosion process in electrolytic cell, the copper surface loses electrons and produces copper ions dissolved in the oil. The anode is the region of high potential, whereas the cathode is the region of low potential. A larger electric potential difference between the anode and cathode

Figure 9. Schematic of the electrons escaping under the effect of electrical field.

indicates a higher rate of copper dissolution, which results in the generation of more copper particles near the anode. Based on the cathodic reaction (reduction reaction) in the electrolytic cell, the ions obtain electrons. Copper dissolution occurs at the anode, whereas the electrons gather in the cathode. The anode and cathode, which can be located close to each other on the copper surface or far apart depending on the circumstances, should balance their charges, as expressed in the following formulas: anode: Cu − ne− → Cu n +

(1)

cathode: 2H+ + 2e− → H 2

(2)

−

where e is the electron and n is the valence of the cations. Acidic substances are generated by the degradation of oil− paper insulation, which causes the presence of hydrogen ions in the insulation oil. Some reactions also occur on the hydrogen ions that obtained electrons. 9131


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Figure 10. Process mechanism of sulfur corrosion under the effects of electrical fields.

From the collected experimental data and electrochemical theory, this study proposed the corrosion mechanisms for electrical fields, which lead to the acceleration of copper sulfide deposition on the insulation windings due to the reaction of DBDS and copper (Figure 10). DBDS is responsible for copper sulfide formation, as stated by the Mitsubishi Company in Japan; however, in the present study, DBDS was only applied to the condition of thermal aging alone. This corrosion process became more significant when the oil−paper insulation suffered from electrical fields simultaneously, which resulted in a large amount of tiny electrolytic cells forming on the surface of the insulation windings. Copper ions were accelerated to dissolve in the insulation oil. The properties of insulation oil could be influenced by the copper ions, which resulted in hydrogen peroxide decomposition and free radical formation; this process would promote the oxidation process of the insulation oil.22 The reaction formula is given as follows: +

•

2+

−

(3)

ROOH + M2 + → ROO• + M+ + H+

(4)

ROOH + M → RO + M

+ OH

Figure 11. Schematic of the corrosion on windings under the effect of positive dc.

For negative dc electrical−thermal aging, the grounding side, the HV electrode, and insulation oil served as the anode, electrode, and electrolyte, respectively, thereby forming an electrolytic cell on the winding surface. At the anode, more copper ions were generated due to the action of dc fields. The corrosion degree of the grounding side and the aging of oil intensified. At the cathode, dc fields did not promote the escape of electrons from the HV electrode; however, the corrosion degree of the HV electrode intensified due to the aging of oil. The corrosion degree of the grounding side was more significant than that of the HV electrode (Figure 12).

where R represents the alkyl group and M represents copper. More copper ions promoted oil oxidation and accelerated the aging of oil. The aging of insulation oil resulted in the increase of the conductivity and the acidity of oil, such as carboxylic acid and hydroxyl acid. Specifically, carboxylic acid has high polarity, copper is polarized positively, and sulfur is polarized negatively. The carboxyl radicals in oil that contained carboxylic acids could accelerate the bond of copper and DBDS to promote corrosion in the process.15 Thus, such increase in the aging of oil sequentially accelerated the corrosion rate of the windings. For positive dc electrical−thermal aging, a stable dc field was formed between the insulation windings when high dc voltage was applied on the windings. The HV electrode, the grounding side, and insulation oil served as the anode, cathode, and electrolyte, respectively, thereby forming an electrolytic cell on the winding surface. The outer electrons of the copper atom may gain enough energy to escape at the anode under the action of dc fields. More copper ions were generated near the anode, which promoted the increase of copper sulfide formation. Although dc fields did not promote the escape of electrons from the grounding side, the increase of copper ions resulted in the aging of oil, which accelerated the corrosion rate. The corrosion degree of the grounding side also intensified but was inferior to the corrosion degree of the HV electrode (Figure 11).

Figure 12. Schematic of the corrosion on windings under the effect of negative dc. 9132



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For ac electrical−thermal aging, two insulation windings alternately switched as the anode or cathode with a frequency of 50 Hz; the time of the two electrodes serving as the anode or cathode was equal. Thus, the increase of copper ions between the HV electrode and the grounding electrode had no differences due to the action of the ac fields. The corrosion process between the two electrodes had approached each other, and the corrosion degree of the windings under ac electrical−thermal aging was more significant than that under thermal aging alone due to the occurrence of electrochemical corrosion. Given that sufficient energy was gained, the outer electrons of the copper atom moved from steady state to excited state and then escaped from the copper surface. However, escape of electrons from the copper surface is a process of energy accumulation. One of the two windings served as the anode, which was only 0.01 s under the ac field. A sufficient number of electrons may not escape from the copper surface in 0.01 s, which considerably reduced the possibility of a sufficient number of electrons to escape and resulted in less copper sulfide formation in comparison with dc field (Figure 13).

AUTHOR INFORMATION

Corresponding Author

*Tel: 86-15683416796. E-mail: [email protected]. ORCID

Sihang Gao: 0000-0003-1754-0364 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the National Science Foundation of China (51277187) and Fund Project of State Grid Corporation of China (J2017034).

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REFERENCES

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Figure 13. Schematic of the corrosion on windings under the effect of ac.

5. CONCLUSION Based on the experimental data collected and electrochemical theory, corrosion mechanisms for electrical fields, which lead to the acceleration of copper sulfide deposition on the insulation windings due to the reaction of DBDS and copper, are proposed. Ac and dc fields were found to promote the amount of deposited copper sulfide on the insulation windings and the copper dissolution in oil, especially dc fields. A high electric stress resulted in high amount of deposited copper sulfide on the windings. A high amount of dissolved copper could promote the aging of oil, and increasing the aging degree of oil could also accelerate the corrosion rate. Moreover, the corrosion degree of the insulation windings under dc fields had a polarity effect characteristic. For positive dc fields, the corrosion degree of HV windings was more significant; whereas for negative dc fields, the corrosion degree of LV windings was more significant.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01666. Typical parameters of the mineral oil used in the experiment and appearance of insulation oils for groups 1 and 2 under different aging conditions (PDF) 9133


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Corrosion Mechanisms for Electrical Fields Leading to the

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