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C60 as flow electrification inhibitor in mineral insulating oil
Journal of Electrostatics 69 (2011) 195e199
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
C60 as flow electrification inhibitor in mineral insulating oil P. Aksamit, D. Zmarz1y* Faculty of Electrical Engineering, Automatic Control and Computer Science, Opole University of Technology, ul. Prószkowska 76, 45-758 Opole, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2009 Received in revised form 19 January 2011 Accepted 25 March 2011 Available online 9 April 2011
The paper presents the concept of using C60 fullerenes as streaming electrification process inhibitor in mineral insulating oil. For the research, 20 samples of oil were prepared; one pure and 19 containing from 1 mg/l to 512 mg/l of C60. The electrification current was measured using a wireless electrometer spinning disk system for rotational speeds from 0 rpm to 400 rpm. The research revealed the C60 ability to significantly reduce the constant component of streaming electrification current in mineral transformer oil. The C60 additive causes also the significant change in flow electrification mechanism as indicated by current versus flow velocity relations. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Flow electrification Insulating oil Inhibitor C60 Fullerene Nanotechnology
1. Introduction During last decades, dozens of transformer failures connected with streaming electrification were documented around the world [1e7]. As the demand for electric power in the world grows, it necessitates an increase of power and rated voltage of transformer units. To dissipate the heat generated in high-voltage transformers, a forced flow of cooling oil is used. When insulating oil flows through transformer windings, the static electricity is generated in the vicinity of the solid insulation surface. The high electric charge may lead to surface discharges. In effect, the solid insulation degrades gradually, which can eventually cause a full discharge and transformer failure. To reduce the flow electrification, the oil flow velocity in most power transformers does not exceed 1 m/s. However, lower oil flow requires higher capacity of the transformer units to achieve sufficient heat dissipation conditions. Suppression of flow electrification phenomena would allow increasing the flow of the cooling oil, thus improving the cooling conditions and reducing capacities of transformer units, without bringing a threat of transformers failures due to static electrification. As each high-voltage transformer failure brings significant economic
* Corresponding author. Tel.: þ48 77 400 0571; fax: þ48 77 400 0573. E-mail address: [email protected] (D. Zmarz1y). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.03.009
losses and is dangerous for the entire power system integrity, intensive research works on flow electrification phenomena have been conducted in many research centers in the world [8e15]. Up to this date, the most broadly researched insulation oil additive, used for flow electrification reduction and as a copper surface passivator is 1,2,3-benzotriazole (BTA) [16e19]. For several years, BTA was used as inhibitor in transformers in Japan and Australia [16,19]. There are however some uncertainties about longterm ageing and a negative impact of BTA on breakdown voltage of insulation oil [3, 20e24]. The ambiguous results obtained by different researchers were probably the reason why BTA has not became a universally adopted solution to flow electrification problem. The authors examine the proposition of using C60 fullerene as an inhibitor for flow electrification in transformer oil. During the research, fresh, mineral insulation oil was doped with different amounts of C60 fullerenes and examined for flow electrification. 2. Preparation of oil samples For the purpose of the research, 20 samples of fresh mineral insulation oil were prepared. One sample contained the pure oil. The other 19 samples were doped with different amounts of C60 fullerenes (Table 1). Each sample was of 1 l capacity. To prepare the C60 doped samples, specific amount of C60 fullerenes was weighted using the analytical balance and pour into 1 l of oil. The C60 doping was based
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Table 1 Concentration of C60 fullerenes in insulation oil samples. Probe number
C60 concentration (mg/l)
C60 mass share (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0 1 2 4 8 10 12 16 20 24 32 48 64 80 96 128 192 256 384 512
0.0 1.1 2.3 4.5 9.0 11.3 13.6 18.1 22.6 27.1 36.2 54.2 72.3 90.4 108.5 144.6 216.9 289.3 433.9 578.5
on natural solubility of fullerenes in mineral oil. The oil was kept at room temperature of about 22 C. Each sample was stirred once a day with a glass stirrer to equalize the C60 concentration over the liquid volume. The highest amount of fullerenes of 512 mg/l dissolved completely in 16 days. 3. Measurement setup Each sample was examined against flow electrification using the wireless rotating electrometer developed at Opole University of Technology [9,10]. Mechanical structure of the setup is presented in Fig. 1.
Fig. 1. Mechanical structure of wireless rotating electrometer.
The electrification current flowing between the container and the disk, throughout the liquid is measured using electrometer assembled on the spindle above the steel disk. A fixed, steel container with outer diameter of 130 mm and 135 mm of height (volume of 1.6 l) is closed with cap that encloses the electrometer. The cap has isolated bearing which makes the rotation of
Fig. 2. Block diagram of the measurement system.
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197
Fig. 3. Relation between constant component of electrification current and rotational speed of the disk for pure insulation oil.
Fig. 5. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 80 mg/l.
electrometer setup possible. On the bottom of the cap there is a grip holding the spindle with disk attached. On the top of the cap there is a coupling that connects the motor with the cap. The motor is a brushless stepper motor 57BYGH (4.2 A, 180 Ncm) driven by SMC139 driver with micro-stepping mode. The stepper motor is controlled by a PC using a Bluetooth connection. Inside the dielectric liquid a metal disk of 12 cm diameter is immersed. The immersion and rotation rate are digitally controlled by host computer. The block scheme of the system is presented in Fig. 2. The figure shows main elements of wireless rotating electrometer. The wireless rotating electrometer consists of current to voltage converter with digital controlled feedback, 24-bit analog-to-digital converter, microcontroller, Bluetooth transceiver and battery pack as a power supply. The signal flows from the container through I/U converter to A/D converter in analog form. Starting from the A/D converter e microcontroller interface, the signal is forwarded only digitally. The host computer uses Bluetooth interface for wireless digital radio communication with electrometer. The standard uncertainty in the measurement of current was determined to be 35 fA, when averaging 2500 samples.
4. Results
Fig. 4. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 64 mg/l.
Fig. 6. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 128 mg/l.
For each of 20 samples of oil, electrification current was registered at 64 different rotational velocities from 0 rpm to 400 rpm. At each velocity, 2500 samples of electrification current were registered with sampling frequency of 500 Hz. From each registered samples set, the mean value of electrification current was calculated, that was used in the further analysis. For all oil samples, relations between constant component of electrification current and the rotational speed of the disk were calculated. In pure oil, the electrification current has negative sign and its absolute value increases monotonically with rotational speed (Fig. 3). The relation follows the same scheme for all examined concentrations of C60 fullerenes, up to 64 mg/l (Fig. 4). The only difference is that the absolute values of current generated in higher concentration samples are lower. When C60 content in oil reaches 80 mg/l, two significant differences are observed (Fig. 5). The charge starts to flow in the opposite direction as in samples of lower C60 concentration. This indicates that C60 content of 80 mg/l reverses the polarity of the double layer at liquidesolid interface. The other difference is the
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P. Aksamit, D. Zmarzły / Journal of Electrostatics 69 (2011) 195e199
Fig. 7. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 256 mg/l.
Fig. 9. Influence of C60 concentration to constant component of electrification current in insulation oil for rotational speed of 24 rpm.
lack of monotonicity of the relation between current and flow velocity. The current increases with rotational speed until it reaches its maximal value of about 1 pA at 100 rpm. Increasing the rotational speed of the disk above 100 rpm causes the electrification current to decrease. Such behavior of streaming electrification current in liquid hydrocarbons was never mentioned in the literature before. It also cannot be explained by the existing double layer and flow electrification models. It appears that C60 revealed some unknown electrification mechanism. Its complexity is indicated by the analogous relations observed in samples containing even higher content of C60 fullerenes (Figs. 6e8). To determine the influence of C60 on flow electrification current in steady hydrodynamic conditions, the relations between the constant component of electrification current and the C60 content were determined. Figs. 9 and 10 present the relations for rotational velocities of 24 rpm and 400 rpm. For all rotational speeds analyzed the electrification current is reduced with increase of C60 content. For a specific C60 concentration, the current reaches its minimal value. Increasing the concentration above that level causes the significant increase of electrification current, much above the values observed in the sample of pure oil.
The optimal concentration of C60 depends on flow velocity (Fig. 11). For lower velocities, the optimal concentration is lower. For rotational speeds above 150 rpm, the optimal concentration is between 100 mg/l and 200 mg/l. Fig. 12 presents the absolute value of electrification current in insulation oil as a function of both rotational speed and C60 concentration. The figure covers all combinations of C60 concentrations and rotational speeds analyzed during the research. The lowest rotational speeds, below 20 rpm, give electrification currents of very small values. Regardless of the fullerene concentration, the values remain at roughly the same level. Increasing the C60 concentration above about 200 mg/l, causes significant increase of electrification current, much exceeding the values observed for pure oil. For varying hydrodynamic conditions, the most optimal concentration of C60 fullerene in insulation oil is between 100 mg/l and 200 mg/l. To choose the most optimal C60 concentration in steady hydrodynamic conditions, its dependence on rotational speed should be taken into consideration. For example, for rotational speeds between 20 and 150 rpm, the range of most optimal fullerene concentrations lies between about 20 mg/l and 80 mg/l.
Fig. 8. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 512 mg/l.
Fig. 10. Influence of C60 concentration to constant component of electrification current in insulation oil for rotational speed of 400 rpm.
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the electrification current versus rotational speed relations for C60 contents higher than 64 mg/l indicates also that in this case, the charge buildup at the solideliquid interface probably follows other scheme than the popular double layer model. As the solid materials used in the experiment are different than in real transformers, results presented in the paper cannot be directly translated to practical application. It needs further investigation to recognize the possibility of practical application of obtained results. Two of the most important issues to examine are the influence of the long-term high-voltage exposure and the longterm oxidation stability.
References
Fig. 11. Influence of C60 concentration to constant component of electrification current in insulation oil for chosen rotational speeds.
Fig. 12. Constant component of electrification current as a function of rotational speed and C60 concentration.
5. Conclusion The addition of C60 fullerenes changes the flow electrification of transformer oil and the change varies with concentration. The present studies show that with a concentration of C60 around 100 mg/l there is a minimum in charge separation that is independent of velocity. The exact nature of the influence observed is not known and hard to determine for the research conditions in case. The mineral insulating oil, which was analyzed in the research is a complex liquid consisting of several tens of thousands different hydrocarbons. Additionally, the ionic species being the source of flow electrification phenomenon come mainly from undefined impurities dissociated in the liquid. The effects observed suggest, that C60 is able to react with ions responsible for flow electrification phenomena. As well as the change of volume charge density in the diffusive layer, it might also affect the charge exchange mechanisms at the liquidesolid interface. These might be the reason of C60 ability to reduce the electrification current in insulating oil. The complex characteristic of
[1] D. Allan, J. Diesendorf, S. Jones, J. Rungis, Insulation failures and dielectric diagnostics, practices and challenges in the Australian region, in: CIGRE, 15/21/33-04, 1996. [2] D. Crofts, The static electrification phenomena in power transformers, in: Annual Report, Conference on Electrical Insulation and Dielectric Phenomena, Claymont, Delaware, 1986, pp. 222e236. [3] D. Crofts, The static electrification phenomena in power transformer, IEEE Transactions on Dielectrics and Electrical Insulation 23 (1988) 137e146. [4] S. Gasworth, J. Melcher, M. Zahn, Flow-induced charge accumulation in thin insulating tubes, IEEE Transactions on Electrical Insulation 23 (1988) 103e115. [5] M. Jeda, K. Goto, H. Okubo, T. Miyamoto, H. Tsukioka, Y. Kohno, Suppression of static electrification of insulating oil for large power transformers, IEEE Transactions on Electrical Insulation 23 (1988) 153e157. [6] A. Morin, M. Zahn, J. Melcher, Fluid electrification measurements in transformer pressboard-oil insulation, in: Sixth International Symposium on HV Engineering, Paper 13.13, 1989. [7] H. Wu, S. Jayaram, DC field effects on streaming electrification in insulating oils, IEEE Transactions on Dielectrics and Electrical Insulation 3 (1996) 499e506. [8] J. Ke˛ dzia, Measurement of the electrification of liquids in the rotating cylinder system, Journal of Electrostatics 20 (1988) 305e312. [9] D. Zmarz1y, J. Ke˛ dzia, A noise analyzer for monitoring static electrification current, Journal of Electrostatics 63 (2005) 409e422. [10] D. Zmarz1y, Streaming electrification measurements in the range of low velocities, Pomiary Automatyka Kontrola 55 (3) (2009) 213e216. [11] D. Zmarz1y, Streaming electrification current density distribution inside pipes assuming overcharged boundary layer, IEEE Transactions on Dielectrics and Electrical Insulation 16 (2) (2009) 372e376. [12] S. Lindgren, Progress in the control of static electrification in power transformers, Electra 133 (1992) 51e59. [13] R. Radwan, R. El-Dewieny, I. Metwally, Investigation of static electrification due to transformer oil flow in electric power apparatus, IEEE Transactions on Electrical Insulation 27 (1992) 278e286. [14] J. Wang, M. Haolong, Computation of streaming current in oil pipes, IEEE Transactions on Dielectrics and Electrical Insulation 16 (2009) 299e304. [15] A. Washabaugh, M. Zahn, A chemical reaction-based boundary condition for flow electrification, IEEE Transactions on Dielectrics and Electrical Insulation 4 (1997) 688e709. [16] E. Moreau, T. Paillat, G. Touchard, Flow electrification in high power transformers: BTA effect on pressboard degraded by electrical discharges, IEEE Transactions on Dielectrics and Electrical Insulation 10 (2003) 15e21. [17] A. Morin, M. Zahn, J. Melcher, D. Otten, An absolute charge sensor for fluid electrification measurements, IEEE Transactions on Electrical Insulation 26 (1991) 181e199. [18] A. Washabaugh, M. Zahn, Flow electrification measurements of transformer insulation using a Couette flow facility, IEEE Transactions on Dielectrics and Electrical Insulation 3 (1996) 161e181. [19] P. Wiklund, M. Levin, B. Pahlavanpour, Copper dissolution and metal passivators in insulating oil, IEEE Electrical Insulation Magazine 23 (2007) 6e14. [20] Effects of BTA on Transformer Materials. Mitsubishi Electric Corporation, 1985. [21] H. Miyao, E. Mori, S. Isaka, M. Tsuchie, M. Takamoto, S. Kobayashi, T. Kobayashi, T. Ono, H. Okubo, The electrostatic charging tendency of inservice oils and the evaluation method of leakage current from transformer windings, in: Symposium Proceedings e Transformer Reliability: Management of Static Electrification in Power Transformers. EPRI, 1999, pp. 34e47. [22] G. Praxl, K. Neuner, Static electrification at solid/liquid interfaces of power transformers, in: Symposium Proceedings e Transformer Reliability: Management of Static Electrification in Power Transformers. EPRI, 1999. [23] A. Washabaugh, D. Schlicker, M. Zahn, Flow electrification measurements of transformer insulation, in: Symposium Proceedings e Transformer Reliability: Management of Static Electrification in Power Transformers. EPRI, 1999. [24] A. Washabaugh, M. Zahn, A study of the effect of BTA on flow electrification using rotating cylindrical electrodes, EPRI TR-105019, 1995.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
C60 as flow electrification inhibitor in mineral insulating oil P. Aksamit, D. Zmarz1y* Faculty of Electrical Engineering, Automatic Control and Computer Science, Opole University of Technology, ul. Prószkowska 76, 45-758 Opole, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2009 Received in revised form 19 January 2011 Accepted 25 March 2011 Available online 9 April 2011
The paper presents the concept of using C60 fullerenes as streaming electrification process inhibitor in mineral insulating oil. For the research, 20 samples of oil were prepared; one pure and 19 containing from 1 mg/l to 512 mg/l of C60. The electrification current was measured using a wireless electrometer spinning disk system for rotational speeds from 0 rpm to 400 rpm. The research revealed the C60 ability to significantly reduce the constant component of streaming electrification current in mineral transformer oil. The C60 additive causes also the significant change in flow electrification mechanism as indicated by current versus flow velocity relations. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Flow electrification Insulating oil Inhibitor C60 Fullerene Nanotechnology
1. Introduction During last decades, dozens of transformer failures connected with streaming electrification were documented around the world [1e7]. As the demand for electric power in the world grows, it necessitates an increase of power and rated voltage of transformer units. To dissipate the heat generated in high-voltage transformers, a forced flow of cooling oil is used. When insulating oil flows through transformer windings, the static electricity is generated in the vicinity of the solid insulation surface. The high electric charge may lead to surface discharges. In effect, the solid insulation degrades gradually, which can eventually cause a full discharge and transformer failure. To reduce the flow electrification, the oil flow velocity in most power transformers does not exceed 1 m/s. However, lower oil flow requires higher capacity of the transformer units to achieve sufficient heat dissipation conditions. Suppression of flow electrification phenomena would allow increasing the flow of the cooling oil, thus improving the cooling conditions and reducing capacities of transformer units, without bringing a threat of transformers failures due to static electrification. As each high-voltage transformer failure brings significant economic
* Corresponding author. Tel.: þ48 77 400 0571; fax: þ48 77 400 0573. E-mail address: [email protected] (D. Zmarz1y). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.03.009
losses and is dangerous for the entire power system integrity, intensive research works on flow electrification phenomena have been conducted in many research centers in the world [8e15]. Up to this date, the most broadly researched insulation oil additive, used for flow electrification reduction and as a copper surface passivator is 1,2,3-benzotriazole (BTA) [16e19]. For several years, BTA was used as inhibitor in transformers in Japan and Australia [16,19]. There are however some uncertainties about longterm ageing and a negative impact of BTA on breakdown voltage of insulation oil [3, 20e24]. The ambiguous results obtained by different researchers were probably the reason why BTA has not became a universally adopted solution to flow electrification problem. The authors examine the proposition of using C60 fullerene as an inhibitor for flow electrification in transformer oil. During the research, fresh, mineral insulation oil was doped with different amounts of C60 fullerenes and examined for flow electrification. 2. Preparation of oil samples For the purpose of the research, 20 samples of fresh mineral insulation oil were prepared. One sample contained the pure oil. The other 19 samples were doped with different amounts of C60 fullerenes (Table 1). Each sample was of 1 l capacity. To prepare the C60 doped samples, specific amount of C60 fullerenes was weighted using the analytical balance and pour into 1 l of oil. The C60 doping was based
196
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Table 1 Concentration of C60 fullerenes in insulation oil samples. Probe number
C60 concentration (mg/l)
C60 mass share (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0 1 2 4 8 10 12 16 20 24 32 48 64 80 96 128 192 256 384 512
0.0 1.1 2.3 4.5 9.0 11.3 13.6 18.1 22.6 27.1 36.2 54.2 72.3 90.4 108.5 144.6 216.9 289.3 433.9 578.5
on natural solubility of fullerenes in mineral oil. The oil was kept at room temperature of about 22 C. Each sample was stirred once a day with a glass stirrer to equalize the C60 concentration over the liquid volume. The highest amount of fullerenes of 512 mg/l dissolved completely in 16 days. 3. Measurement setup Each sample was examined against flow electrification using the wireless rotating electrometer developed at Opole University of Technology [9,10]. Mechanical structure of the setup is presented in Fig. 1.
Fig. 1. Mechanical structure of wireless rotating electrometer.
The electrification current flowing between the container and the disk, throughout the liquid is measured using electrometer assembled on the spindle above the steel disk. A fixed, steel container with outer diameter of 130 mm and 135 mm of height (volume of 1.6 l) is closed with cap that encloses the electrometer. The cap has isolated bearing which makes the rotation of
Fig. 2. Block diagram of the measurement system.
P. Aksamit, D. Zmarzły / Journal of Electrostatics 69 (2011) 195e199
197
Fig. 3. Relation between constant component of electrification current and rotational speed of the disk for pure insulation oil.
Fig. 5. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 80 mg/l.
electrometer setup possible. On the bottom of the cap there is a grip holding the spindle with disk attached. On the top of the cap there is a coupling that connects the motor with the cap. The motor is a brushless stepper motor 57BYGH (4.2 A, 180 Ncm) driven by SMC139 driver with micro-stepping mode. The stepper motor is controlled by a PC using a Bluetooth connection. Inside the dielectric liquid a metal disk of 12 cm diameter is immersed. The immersion and rotation rate are digitally controlled by host computer. The block scheme of the system is presented in Fig. 2. The figure shows main elements of wireless rotating electrometer. The wireless rotating electrometer consists of current to voltage converter with digital controlled feedback, 24-bit analog-to-digital converter, microcontroller, Bluetooth transceiver and battery pack as a power supply. The signal flows from the container through I/U converter to A/D converter in analog form. Starting from the A/D converter e microcontroller interface, the signal is forwarded only digitally. The host computer uses Bluetooth interface for wireless digital radio communication with electrometer. The standard uncertainty in the measurement of current was determined to be 35 fA, when averaging 2500 samples.
4. Results
Fig. 4. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 64 mg/l.
Fig. 6. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 128 mg/l.
For each of 20 samples of oil, electrification current was registered at 64 different rotational velocities from 0 rpm to 400 rpm. At each velocity, 2500 samples of electrification current were registered with sampling frequency of 500 Hz. From each registered samples set, the mean value of electrification current was calculated, that was used in the further analysis. For all oil samples, relations between constant component of electrification current and the rotational speed of the disk were calculated. In pure oil, the electrification current has negative sign and its absolute value increases monotonically with rotational speed (Fig. 3). The relation follows the same scheme for all examined concentrations of C60 fullerenes, up to 64 mg/l (Fig. 4). The only difference is that the absolute values of current generated in higher concentration samples are lower. When C60 content in oil reaches 80 mg/l, two significant differences are observed (Fig. 5). The charge starts to flow in the opposite direction as in samples of lower C60 concentration. This indicates that C60 content of 80 mg/l reverses the polarity of the double layer at liquidesolid interface. The other difference is the
198
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Fig. 7. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 256 mg/l.
Fig. 9. Influence of C60 concentration to constant component of electrification current in insulation oil for rotational speed of 24 rpm.
lack of monotonicity of the relation between current and flow velocity. The current increases with rotational speed until it reaches its maximal value of about 1 pA at 100 rpm. Increasing the rotational speed of the disk above 100 rpm causes the electrification current to decrease. Such behavior of streaming electrification current in liquid hydrocarbons was never mentioned in the literature before. It also cannot be explained by the existing double layer and flow electrification models. It appears that C60 revealed some unknown electrification mechanism. Its complexity is indicated by the analogous relations observed in samples containing even higher content of C60 fullerenes (Figs. 6e8). To determine the influence of C60 on flow electrification current in steady hydrodynamic conditions, the relations between the constant component of electrification current and the C60 content were determined. Figs. 9 and 10 present the relations for rotational velocities of 24 rpm and 400 rpm. For all rotational speeds analyzed the electrification current is reduced with increase of C60 content. For a specific C60 concentration, the current reaches its minimal value. Increasing the concentration above that level causes the significant increase of electrification current, much above the values observed in the sample of pure oil.
The optimal concentration of C60 depends on flow velocity (Fig. 11). For lower velocities, the optimal concentration is lower. For rotational speeds above 150 rpm, the optimal concentration is between 100 mg/l and 200 mg/l. Fig. 12 presents the absolute value of electrification current in insulation oil as a function of both rotational speed and C60 concentration. The figure covers all combinations of C60 concentrations and rotational speeds analyzed during the research. The lowest rotational speeds, below 20 rpm, give electrification currents of very small values. Regardless of the fullerene concentration, the values remain at roughly the same level. Increasing the C60 concentration above about 200 mg/l, causes significant increase of electrification current, much exceeding the values observed for pure oil. For varying hydrodynamic conditions, the most optimal concentration of C60 fullerene in insulation oil is between 100 mg/l and 200 mg/l. To choose the most optimal C60 concentration in steady hydrodynamic conditions, its dependence on rotational speed should be taken into consideration. For example, for rotational speeds between 20 and 150 rpm, the range of most optimal fullerene concentrations lies between about 20 mg/l and 80 mg/l.
Fig. 8. Relation between constant component of electrification current and rotational speed of the disk for C60 concentration of 512 mg/l.
Fig. 10. Influence of C60 concentration to constant component of electrification current in insulation oil for rotational speed of 400 rpm.
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199
the electrification current versus rotational speed relations for C60 contents higher than 64 mg/l indicates also that in this case, the charge buildup at the solideliquid interface probably follows other scheme than the popular double layer model. As the solid materials used in the experiment are different than in real transformers, results presented in the paper cannot be directly translated to practical application. It needs further investigation to recognize the possibility of practical application of obtained results. Two of the most important issues to examine are the influence of the long-term high-voltage exposure and the longterm oxidation stability.
References
Fig. 11. Influence of C60 concentration to constant component of electrification current in insulation oil for chosen rotational speeds.
Fig. 12. Constant component of electrification current as a function of rotational speed and C60 concentration.
5. Conclusion The addition of C60 fullerenes changes the flow electrification of transformer oil and the change varies with concentration. The present studies show that with a concentration of C60 around 100 mg/l there is a minimum in charge separation that is independent of velocity. The exact nature of the influence observed is not known and hard to determine for the research conditions in case. The mineral insulating oil, which was analyzed in the research is a complex liquid consisting of several tens of thousands different hydrocarbons. Additionally, the ionic species being the source of flow electrification phenomenon come mainly from undefined impurities dissociated in the liquid. The effects observed suggest, that C60 is able to react with ions responsible for flow electrification phenomena. As well as the change of volume charge density in the diffusive layer, it might also affect the charge exchange mechanisms at the liquidesolid interface. These might be the reason of C60 ability to reduce the electrification current in insulating oil. The complex characteristic of
[1] D. Allan, J. Diesendorf, S. Jones, J. Rungis, Insulation failures and dielectric diagnostics, practices and challenges in the Australian region, in: CIGRE, 15/21/33-04, 1996. [2] D. Crofts, The static electrification phenomena in power transformers, in: Annual Report, Conference on Electrical Insulation and Dielectric Phenomena, Claymont, Delaware, 1986, pp. 222e236. [3] D. Crofts, The static electrification phenomena in power transformer, IEEE Transactions on Dielectrics and Electrical Insulation 23 (1988) 137e146. [4] S. Gasworth, J. Melcher, M. Zahn, Flow-induced charge accumulation in thin insulating tubes, IEEE Transactions on Electrical Insulation 23 (1988) 103e115. [5] M. Jeda, K. Goto, H. Okubo, T. Miyamoto, H. Tsukioka, Y. Kohno, Suppression of static electrification of insulating oil for large power transformers, IEEE Transactions on Electrical Insulation 23 (1988) 153e157. [6] A. Morin, M. Zahn, J. Melcher, Fluid electrification measurements in transformer pressboard-oil insulation, in: Sixth International Symposium on HV Engineering, Paper 13.13, 1989. [7] H. Wu, S. Jayaram, DC field effects on streaming electrification in insulating oils, IEEE Transactions on Dielectrics and Electrical Insulation 3 (1996) 499e506. [8] J. Ke˛ dzia, Measurement of the electrification of liquids in the rotating cylinder system, Journal of Electrostatics 20 (1988) 305e312. [9] D. Zmarz1y, J. Ke˛ dzia, A noise analyzer for monitoring static electrification current, Journal of Electrostatics 63 (2005) 409e422. [10] D. Zmarz1y, Streaming electrification measurements in the range of low velocities, Pomiary Automatyka Kontrola 55 (3) (2009) 213e216. [11] D. Zmarz1y, Streaming electrification current density distribution inside pipes assuming overcharged boundary layer, IEEE Transactions on Dielectrics and Electrical Insulation 16 (2) (2009) 372e376. [12] S. Lindgren, Progress in the control of static electrification in power transformers, Electra 133 (1992) 51e59. [13] R. Radwan, R. El-Dewieny, I. Metwally, Investigation of static electrification due to transformer oil flow in electric power apparatus, IEEE Transactions on Electrical Insulation 27 (1992) 278e286. [14] J. Wang, M. Haolong, Computation of streaming current in oil pipes, IEEE Transactions on Dielectrics and Electrical Insulation 16 (2009) 299e304. [15] A. Washabaugh, M. Zahn, A chemical reaction-based boundary condition for flow electrification, IEEE Transactions on Dielectrics and Electrical Insulation 4 (1997) 688e709. [16] E. Moreau, T. Paillat, G. Touchard, Flow electrification in high power transformers: BTA effect on pressboard degraded by electrical discharges, IEEE Transactions on Dielectrics and Electrical Insulation 10 (2003) 15e21. [17] A. Morin, M. Zahn, J. Melcher, D. Otten, An absolute charge sensor for fluid electrification measurements, IEEE Transactions on Electrical Insulation 26 (1991) 181e199. [18] A. Washabaugh, M. Zahn, Flow electrification measurements of transformer insulation using a Couette flow facility, IEEE Transactions on Dielectrics and Electrical Insulation 3 (1996) 161e181. [19] P. Wiklund, M. Levin, B. Pahlavanpour, Copper dissolution and metal passivators in insulating oil, IEEE Electrical Insulation Magazine 23 (2007) 6e14. [20] Effects of BTA on Transformer Materials. Mitsubishi Electric Corporation, 1985. [21] H. Miyao, E. Mori, S. Isaka, M. Tsuchie, M. Takamoto, S. Kobayashi, T. Kobayashi, T. Ono, H. Okubo, The electrostatic charging tendency of inservice oils and the evaluation method of leakage current from transformer windings, in: Symposium Proceedings e Transformer Reliability: Management of Static Electrification in Power Transformers. EPRI, 1999, pp. 34e47. [22] G. Praxl, K. Neuner, Static electrification at solid/liquid interfaces of power transformers, in: Symposium Proceedings e Transformer Reliability: Management of Static Electrification in Power Transformers. EPRI, 1999. [23] A. Washabaugh, D. Schlicker, M. Zahn, Flow electrification measurements of transformer insulation, in: Symposium Proceedings e Transformer Reliability: Management of Static Electrification in Power Transformers. EPRI, 1999. [24] A. Washabaugh, M. Zahn, A study of the effect of BTA on flow electrification using rotating cylindrical electrodes, EPRI TR-105019, 1995.