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Practical implications of transformer flow electrification studies
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ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 693-698
P r a c t i c a l I m p l i c a t i o n s of T r a n s f o r m e r F l o w E l e c t r i f i c a t i o n S t u d i e s J. A. Palmer ~ and J. K. Nelson b Colorado School of Mines, Division of Engineering, Golden, CO 80401 b Rensselaer Polytechnic Institute, Electric Power Engineering Dept., Troy, NY 12180
1
Introduction
In the past, the issue of streaming electrification has been very politically charged, with utilities, manufacturers and suppliers of materials all blaming each other for failures. In fact, all parties are partially responsible, and each may contribute to the eradication of this problem as a cause of failure. The need for further action is underscored by the continued occurrence of failures, as recently as last year. As has been shown in recent papers [1] [2], there are issues relating to all aspects of transformers, both in design, manufacturing, maintenance and operation which influence streaming electrification. Many of these factors will be addressed in this paper in the context of practical improvements to reduce the incidence of failure. The application of some experimental work to the full-scale transformer is justified because while the details may not scale uniformly, many of the features observed in the laboratory have been likewise observed in the full-scale transformer. For example, a short term transient in streaming electrification was observed in a small scale model (See Fig. 1)[3], as well as in actual transformer leakage current measurements [4]. Likewise, behavior consistent with a longer-term transient seen in the same model were observed in testing of another transformer(See Fig. 2).
2
T r a n s f o r m e r D e s i g n a n d M a n u f a c t u r i n g Issues
Of all the aspects of the experimental results obtained by previous researchers, only flow rate and AC electric fields, seem to universally enhance leakage and streaming currents in varying degrees. Clearly, in a transformer, issues of field strengths have been seriously considered by designers and efforts have been made to minimize irregularities in field distributions. It would certainly do no harm to consider electric field intensity in regions of high flow, but there are not likely to be significant gains in this aspect. On the other hand, altering the flow rate may provide significant gains against streaming electrification. Leakage currents have been shown to depend on flow rate by as much as a 0304-3886/97/$17.00 © ElsevierScience B.V. All rights reserved.
Pll S0304-3886(97)00117-4
694
J.A. Palmer, J..K. Nelson~Journal of Electrostatics 40&41 (1997) 693-698
:-i _ l Figure 1. Leakage current resulting from initiation of flow through an experimental model [2].
g
0=
~ -~' i
~
~ _30 .~o~--
~
- -
~
-40 I
2
3
4
6
8
7
8
9
10
11
12
13
14
15
16
17
18
19
20
"nm e~ou=)
Figure 2. Long term change in quasi-steady-state leakage current in an experimental model [2]. power of four in the steady state, and the short term transient was also strongly enhanced by the initiation of higher flows. Reduction of flow rate has already been used as a defense against streaming electrification [5]. Nevertheless, it is likely that further reductions may be possible in key areas such as in the lower plenum and near the entry to the flow ducts. In particular, the high velocity jet from the pump outlet may have strong effects on the streaming electrification. In one case, the flow f~om one pump into the end of the transformer caused static discharges, but those discharges were eliminated when a second pump drove a perpendicular stream [6]. Although the average flow rate was doubled, the interference between the pump jets appears to have significantly reduced the maximum local flow velocity. A full scale shell-form transformer that was tested with flow rates up to four times rated without any static discharges was unique in its unusually large lower plenum[7], which probably provided significant damping for the pump discharge jet, reducing the velocity of oil impacting the insulation structure. Whether by altering the pump outlet, enlarging the lower plenum, or some other method, damping should be provided in the lower plenum (without surfaces against which to generate more charge) to eliminate the presence of high speed jets or eddies caused by those jets. Another flow related issue that should be considered in design is the flow in channels. The abrupt opening into a channel as the basic geometry of the computational model presented in [1] is representative of the entry into the transformer cooling ducts, as well as the flow through the blocking structure. The problems of flow electrification in the ducts may be reduced by geometric changes of the spacers. The sharp leading edge of a spacer may produce an eddy (See Fig. 3), which significantly enhances the charge generation, as evidenced by the abundant tracking on and around the blocking structure of many of the failures.
J..A. Palmer, J..K. Nelson~Journal o f Electrostatics 40&41 (1997) 693-698
695
This enhancement may be reduced by eliminating sharp corners in the flow channels. A typical blocking structure for Y a shell4orm flow duct is shown 2.0 in Fig. 4 with the spacer corners 1-5 rounded. This would not totally ...................................... "~,, ",,j~i eliminate the flow eddies, but it 1 , 0 - - - would eliminate the highest flow and field gradients at the corners ==================== . . . . . . : : : : : ; : : 0.5 of the spacers, reducing the leakage currents and likelihood of par0.0 tial discharges. The same principle -2.4 0.0 2.4 -4.8 4.S applies at the entry to the ducts, X edges of the washers, etc. Figure 3. Streamlines for flow impacting the leading edge of a channel with Reynolds Number 565. DimenThe importance of several transsions are normalized by channel half height[2] former manufacturing issues is also brought out in the recent studies. The most critical issue with respect to manufacturing and streaming electrification is the selection of materials. Oil should be selected with minimal electrostatic charging tendency (ECT). If the charging tendency is not sufficiently low, filtering through an ion exchange filter or Fullers earth may be used to remove the excessive ions. The use of an additive such as benzotriazole (BTA) has been controversial, but merits further investigation.
=
The pressboard is also a domain in which further investigation is needed. The longer term transient has been hypothesized to be due to the formation of deposits on the surface of the cellulose, by the electrolysis action of streaming electrification, leading to an over all reduction in the surface charge density, and hence a reduction in streaming electrification activity. Based on this, further research should be undertaken into processes which will accelerate the formation of such insoluble deposits without entering streaming electrification regimes that would pose a danger to the cellulose. It is likely that greater care in the selection of the oil used for the initial impregnation of the paper and pressboard may also reduce charging.
3
Figure 4. Modified version of the shell-form blocking structure for reduced charging.
I m p a c t of Maintenance and Operational Procedures
Transformer design and manufacturing issues are Important, but are limited to new transformers. Maintenance and operational issues, on the other hand, apply to both new and
696
J..A. Palmer, JK. Nelson~Journal of Electrostatics 40&41 (1997) 693-698
existing transformers. The fact that many of the flow electrification related failures have occurred in a short time following installation or maintenance indicates that these areas merit some attention in prevention efforts. In view of the extreme sensitivity of streaming electrification to contamination, any time the sealed transformer is entered, every precaution should be taken to prevent contamination, including airborne contamination. After the transformer has been resealed, or if the transformer has been off line for an extended outage, the pumps should each be run individually, and then in pairs to ensure that any ionic contaminant that might become entrained in the flow is detected in a leakage current or ECT measurement and eliminated prior to bringing the transformer on line. The pumps should not all be energized simultaneously under any circumstances, as the evidence seen in [8] demonstrated that the short term transient yields considerably higher peaks when the initiated flow is higher. Furthermore, in light of the information on the long term transient, it is advisable that the oil be circulated at a low ftow rate for several days prior to increasing the flow to higher levels. An additional maintenance issue would be the implementation of an ECT monitoring program. While not a prevention technique per se, such a program would alert the utility to increases in electrostatic charging tendency before irreversible damage to the solid insulation occurs. In that context, the device described in [3] would serve well in an on-line or field-deployable application. Some manufacturers have advised retrofitting new cooling systems onto transformers that are likely to have streaming electrification problems, using lower velocity pumps and larger radiators [9]. However, this is costly and time consuming, and provides significant exposure to contamination, so should probably be limited to only the most serious cases. In the operation of the transformer, as with the design, flow rate is of foremost importance and should be maintained at the lowest possible levels that will still protect against the thermal degradation of the transformer insulation. Because the aging of the oil generally tends to increase the charging tendency[10], maintaining sufficiently low temperature is also a measure to protect against streaming electrification, which must be balanced against the flow/charge relationship. The operation of the pumps in automatic mode with a two stage thermostatic setting has been advised previously[ill. However, more levels of discretization of pump operation would also help to limit the charging, by maintaining temperatures nearer the limit. A further step would be to use variable speed drives on one or more of the pumps to provide a continuous range of oil circulation, although transformers with five or six pumps may provide a sufficiently continuous range if all pumps are operated individually. Savio observed that the bottom oil temperature of both transformers that failed at Ramapo was below 40°C at the time of failure[5]. This corresponds to the peak of leakage current as a function of temperature for some full scale transformers, so he advised the use of a thermostat to block operation of the second bank of pumps when the bottom oil temperature is below 50°C. Another option would be to operate the pumps based on the
JA. Palmer, J.K. Nelson~Journal of Electrostatics 40&41 (1997) 693-698
697
top oil temperature, and the fans based on the bottom oil temperature, so that the oil throughout the transformer would be at temperatures beyond the peak. In light of the experimental findings of streaming electrification dynamics, flow transients should be limited both in frequency and magnitude[2]. Magnitude could be limited by energizing only one pump at a time, or if a variable speed drive were used, ramping it up slowly, providing an even better avoidance of the overshoot. If any individual pump, by virtue of its location or condition, has a tendency to generate an inordinate amount of charge, as in the case cited in section 2, it should be reserved for use only as a last measure against overheating. In view of the operating issues involved in avoidance of streaming electrificaMain tion, the thermostatic control that has Al s Processor been used in the past seems insufficient. Therefore, an intelligent controller was designed taking elements of fuzzy logic and neural networks, together with recent advances in instrumentation[12]. This controller (See Fig. 5) drives the cooling system to operate in flow and temperature regimes which limit the problem of streaming Figure 5. Block diagram of intelligent controller electrification while minimizing therfor transformer cooling[12]. mal degradation of the insulation. This approach provides a more flexible,noninvasive alternative or complement to many of the other mitigation efforts described above. Rather than significantly altering the configuration of the cooling system, the controller, utilizing the base of parametric understanding of the phenomenon, changes only the operation of the system.
4
Conclusion
Streaming electrification is a phenomenon that has become increasingly well understood, thanks to extensive research, both experimental and computational. The elimination of this problem requires the implementation of all acquired understanding in practical solutions in all stages of transformer life: design, manufacture, operation, and maintenance. This paper has described measures that may be taken in all of these stages, as determined from the experience obtained from experimental and numerical studies, as well as from the literature.
698 5
JA. Palmer, JK. Nelson~Journal of Electrostatics 40&41 (1997) 693-698 Acknowledgements
This work was undertaken under the auspices of a contract from the Electric Power Research Institute and the authors are indebted to Mr. S.R.Lindgren who managed the program. Thanks are also due to the Edison Electric Institute who supported one of the authors (JAP) through their Power Engineering Education Foundation. References [1] J. A. Palmer and J. K. Nelson, "Simulation of short-term streaming electrification dynamics," IEEE Transactions on Dielectrics and Electrical Insulation, submitted for review, 1996. [2] J. A. Palmer, Dynamics of Streaming Electrification in Large Power Transformers. PhD thesis, Rensselaer Polytechnic Institute, Troy, New York, 1996. [3] J. A. Palmer and J. K. Nelson, "A method for the repetitive measurement of the electrostatic charging tendency of liquid dielectrics," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 3, pp. 70-74, Feb. 1996. [4] J. Cross, "The failure of 2 core-form generator transformers in NSW.," in Workshop Proceedings: Static Electrification in Power Transformers, pp. 1-4-1 - 1-4-10, EPRI EL6918, July 1990. [5] L. J. Savio, "Ramapo substation Westinghouse 500/345 kV 1000 MVA autotransformer failure," in Proceedings: Static Electrification in Power Transformers, pp. 1-1-1 - 1-1 14, EPRI TR-105019, May 1995. [6] J. H. Ugo and H. R. Moore, "Static electrification in a 700 MVA 23.7/345 kV generator step up transformer," in Proceedings: Static Electrification in Power Transformers, pp. 1-4-1 1-4-11, EPRI TR-105019, May 1995. [7] L. Peyraque, C. Boisdon, A. Beroual, and F. Buret, "Static electrification and partial discharges induced by oil flow in power transformers," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 2, pp. 40-45, February 1995. [8] J. A. Palmer and J. K. Nelson, "Streaming electrification dynamics in oil/cellulose systems," in Conference on Electrical Insulation and Dielectric Phenomena, (Arlington, Texas), pp. 895-900, October 23-26, 1994. [9] G. Arndt, D. Smyers, and D. Withers, "History and circumstances for Palo Verde GSU transformers involved in static electrification incidents," in Proceedings: Static Electrification in Power Transformers, pp. 1-2-1 - 1-2-20, EPRI, May 1993. [10] T. V. Oommen, "Static electrification properties of transformer oil," IEEE Transactions on Electrical Insulation, vol. 23, pp. 123-128, February 1988. [11] H. R. Moore, "Suggested guidelines for pump operation to reduce risk of static electrification," in Proceedings: Static Electrification in Power Transformers, pp. A-1 A-3, EPRI TRo105019, May 1995. [12] J. A. Palmer and J. K. Nelson, "Intelligent control of large power transformer cooling pumps," IEE Proceedings: Generation, Transmission, and Distribution, September 1996.
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 693-698
P r a c t i c a l I m p l i c a t i o n s of T r a n s f o r m e r F l o w E l e c t r i f i c a t i o n S t u d i e s J. A. Palmer ~ and J. K. Nelson b Colorado School of Mines, Division of Engineering, Golden, CO 80401 b Rensselaer Polytechnic Institute, Electric Power Engineering Dept., Troy, NY 12180
1
Introduction
In the past, the issue of streaming electrification has been very politically charged, with utilities, manufacturers and suppliers of materials all blaming each other for failures. In fact, all parties are partially responsible, and each may contribute to the eradication of this problem as a cause of failure. The need for further action is underscored by the continued occurrence of failures, as recently as last year. As has been shown in recent papers [1] [2], there are issues relating to all aspects of transformers, both in design, manufacturing, maintenance and operation which influence streaming electrification. Many of these factors will be addressed in this paper in the context of practical improvements to reduce the incidence of failure. The application of some experimental work to the full-scale transformer is justified because while the details may not scale uniformly, many of the features observed in the laboratory have been likewise observed in the full-scale transformer. For example, a short term transient in streaming electrification was observed in a small scale model (See Fig. 1)[3], as well as in actual transformer leakage current measurements [4]. Likewise, behavior consistent with a longer-term transient seen in the same model were observed in testing of another transformer(See Fig. 2).
2
T r a n s f o r m e r D e s i g n a n d M a n u f a c t u r i n g Issues
Of all the aspects of the experimental results obtained by previous researchers, only flow rate and AC electric fields, seem to universally enhance leakage and streaming currents in varying degrees. Clearly, in a transformer, issues of field strengths have been seriously considered by designers and efforts have been made to minimize irregularities in field distributions. It would certainly do no harm to consider electric field intensity in regions of high flow, but there are not likely to be significant gains in this aspect. On the other hand, altering the flow rate may provide significant gains against streaming electrification. Leakage currents have been shown to depend on flow rate by as much as a 0304-3886/97/$17.00 © ElsevierScience B.V. All rights reserved.
Pll S0304-3886(97)00117-4
694
J.A. Palmer, J..K. Nelson~Journal of Electrostatics 40&41 (1997) 693-698
:-i _ l Figure 1. Leakage current resulting from initiation of flow through an experimental model [2].
g
0=
~ -~' i
~
~ _30 .~o~--
~
- -
~
-40 I
2
3
4
6
8
7
8
9
10
11
12
13
14
15
16
17
18
19
20
"nm e~ou=)
Figure 2. Long term change in quasi-steady-state leakage current in an experimental model [2]. power of four in the steady state, and the short term transient was also strongly enhanced by the initiation of higher flows. Reduction of flow rate has already been used as a defense against streaming electrification [5]. Nevertheless, it is likely that further reductions may be possible in key areas such as in the lower plenum and near the entry to the flow ducts. In particular, the high velocity jet from the pump outlet may have strong effects on the streaming electrification. In one case, the flow f~om one pump into the end of the transformer caused static discharges, but those discharges were eliminated when a second pump drove a perpendicular stream [6]. Although the average flow rate was doubled, the interference between the pump jets appears to have significantly reduced the maximum local flow velocity. A full scale shell-form transformer that was tested with flow rates up to four times rated without any static discharges was unique in its unusually large lower plenum[7], which probably provided significant damping for the pump discharge jet, reducing the velocity of oil impacting the insulation structure. Whether by altering the pump outlet, enlarging the lower plenum, or some other method, damping should be provided in the lower plenum (without surfaces against which to generate more charge) to eliminate the presence of high speed jets or eddies caused by those jets. Another flow related issue that should be considered in design is the flow in channels. The abrupt opening into a channel as the basic geometry of the computational model presented in [1] is representative of the entry into the transformer cooling ducts, as well as the flow through the blocking structure. The problems of flow electrification in the ducts may be reduced by geometric changes of the spacers. The sharp leading edge of a spacer may produce an eddy (See Fig. 3), which significantly enhances the charge generation, as evidenced by the abundant tracking on and around the blocking structure of many of the failures.
J..A. Palmer, J..K. Nelson~Journal o f Electrostatics 40&41 (1997) 693-698
695
This enhancement may be reduced by eliminating sharp corners in the flow channels. A typical blocking structure for Y a shell4orm flow duct is shown 2.0 in Fig. 4 with the spacer corners 1-5 rounded. This would not totally ...................................... "~,, ",,j~i eliminate the flow eddies, but it 1 , 0 - - - would eliminate the highest flow and field gradients at the corners ==================== . . . . . . : : : : : ; : : 0.5 of the spacers, reducing the leakage currents and likelihood of par0.0 tial discharges. The same principle -2.4 0.0 2.4 -4.8 4.S applies at the entry to the ducts, X edges of the washers, etc. Figure 3. Streamlines for flow impacting the leading edge of a channel with Reynolds Number 565. DimenThe importance of several transsions are normalized by channel half height[2] former manufacturing issues is also brought out in the recent studies. The most critical issue with respect to manufacturing and streaming electrification is the selection of materials. Oil should be selected with minimal electrostatic charging tendency (ECT). If the charging tendency is not sufficiently low, filtering through an ion exchange filter or Fullers earth may be used to remove the excessive ions. The use of an additive such as benzotriazole (BTA) has been controversial, but merits further investigation.
=
The pressboard is also a domain in which further investigation is needed. The longer term transient has been hypothesized to be due to the formation of deposits on the surface of the cellulose, by the electrolysis action of streaming electrification, leading to an over all reduction in the surface charge density, and hence a reduction in streaming electrification activity. Based on this, further research should be undertaken into processes which will accelerate the formation of such insoluble deposits without entering streaming electrification regimes that would pose a danger to the cellulose. It is likely that greater care in the selection of the oil used for the initial impregnation of the paper and pressboard may also reduce charging.
3
Figure 4. Modified version of the shell-form blocking structure for reduced charging.
I m p a c t of Maintenance and Operational Procedures
Transformer design and manufacturing issues are Important, but are limited to new transformers. Maintenance and operational issues, on the other hand, apply to both new and
696
J..A. Palmer, JK. Nelson~Journal of Electrostatics 40&41 (1997) 693-698
existing transformers. The fact that many of the flow electrification related failures have occurred in a short time following installation or maintenance indicates that these areas merit some attention in prevention efforts. In view of the extreme sensitivity of streaming electrification to contamination, any time the sealed transformer is entered, every precaution should be taken to prevent contamination, including airborne contamination. After the transformer has been resealed, or if the transformer has been off line for an extended outage, the pumps should each be run individually, and then in pairs to ensure that any ionic contaminant that might become entrained in the flow is detected in a leakage current or ECT measurement and eliminated prior to bringing the transformer on line. The pumps should not all be energized simultaneously under any circumstances, as the evidence seen in [8] demonstrated that the short term transient yields considerably higher peaks when the initiated flow is higher. Furthermore, in light of the information on the long term transient, it is advisable that the oil be circulated at a low ftow rate for several days prior to increasing the flow to higher levels. An additional maintenance issue would be the implementation of an ECT monitoring program. While not a prevention technique per se, such a program would alert the utility to increases in electrostatic charging tendency before irreversible damage to the solid insulation occurs. In that context, the device described in [3] would serve well in an on-line or field-deployable application. Some manufacturers have advised retrofitting new cooling systems onto transformers that are likely to have streaming electrification problems, using lower velocity pumps and larger radiators [9]. However, this is costly and time consuming, and provides significant exposure to contamination, so should probably be limited to only the most serious cases. In the operation of the transformer, as with the design, flow rate is of foremost importance and should be maintained at the lowest possible levels that will still protect against the thermal degradation of the transformer insulation. Because the aging of the oil generally tends to increase the charging tendency[10], maintaining sufficiently low temperature is also a measure to protect against streaming electrification, which must be balanced against the flow/charge relationship. The operation of the pumps in automatic mode with a two stage thermostatic setting has been advised previously[ill. However, more levels of discretization of pump operation would also help to limit the charging, by maintaining temperatures nearer the limit. A further step would be to use variable speed drives on one or more of the pumps to provide a continuous range of oil circulation, although transformers with five or six pumps may provide a sufficiently continuous range if all pumps are operated individually. Savio observed that the bottom oil temperature of both transformers that failed at Ramapo was below 40°C at the time of failure[5]. This corresponds to the peak of leakage current as a function of temperature for some full scale transformers, so he advised the use of a thermostat to block operation of the second bank of pumps when the bottom oil temperature is below 50°C. Another option would be to operate the pumps based on the
JA. Palmer, J.K. Nelson~Journal of Electrostatics 40&41 (1997) 693-698
697
top oil temperature, and the fans based on the bottom oil temperature, so that the oil throughout the transformer would be at temperatures beyond the peak. In light of the experimental findings of streaming electrification dynamics, flow transients should be limited both in frequency and magnitude[2]. Magnitude could be limited by energizing only one pump at a time, or if a variable speed drive were used, ramping it up slowly, providing an even better avoidance of the overshoot. If any individual pump, by virtue of its location or condition, has a tendency to generate an inordinate amount of charge, as in the case cited in section 2, it should be reserved for use only as a last measure against overheating. In view of the operating issues involved in avoidance of streaming electrificaMain tion, the thermostatic control that has Al s Processor been used in the past seems insufficient. Therefore, an intelligent controller was designed taking elements of fuzzy logic and neural networks, together with recent advances in instrumentation[12]. This controller (See Fig. 5) drives the cooling system to operate in flow and temperature regimes which limit the problem of streaming Figure 5. Block diagram of intelligent controller electrification while minimizing therfor transformer cooling[12]. mal degradation of the insulation. This approach provides a more flexible,noninvasive alternative or complement to many of the other mitigation efforts described above. Rather than significantly altering the configuration of the cooling system, the controller, utilizing the base of parametric understanding of the phenomenon, changes only the operation of the system.
4
Conclusion
Streaming electrification is a phenomenon that has become increasingly well understood, thanks to extensive research, both experimental and computational. The elimination of this problem requires the implementation of all acquired understanding in practical solutions in all stages of transformer life: design, manufacture, operation, and maintenance. This paper has described measures that may be taken in all of these stages, as determined from the experience obtained from experimental and numerical studies, as well as from the literature.
698 5
JA. Palmer, JK. Nelson~Journal of Electrostatics 40&41 (1997) 693-698 Acknowledgements
This work was undertaken under the auspices of a contract from the Electric Power Research Institute and the authors are indebted to Mr. S.R.Lindgren who managed the program. Thanks are also due to the Edison Electric Institute who supported one of the authors (JAP) through their Power Engineering Education Foundation. References [1] J. A. Palmer and J. K. Nelson, "Simulation of short-term streaming electrification dynamics," IEEE Transactions on Dielectrics and Electrical Insulation, submitted for review, 1996. [2] J. A. Palmer, Dynamics of Streaming Electrification in Large Power Transformers. PhD thesis, Rensselaer Polytechnic Institute, Troy, New York, 1996. [3] J. A. Palmer and J. K. Nelson, "A method for the repetitive measurement of the electrostatic charging tendency of liquid dielectrics," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 3, pp. 70-74, Feb. 1996. [4] J. Cross, "The failure of 2 core-form generator transformers in NSW.," in Workshop Proceedings: Static Electrification in Power Transformers, pp. 1-4-1 - 1-4-10, EPRI EL6918, July 1990. [5] L. J. Savio, "Ramapo substation Westinghouse 500/345 kV 1000 MVA autotransformer failure," in Proceedings: Static Electrification in Power Transformers, pp. 1-1-1 - 1-1 14, EPRI TR-105019, May 1995. [6] J. H. Ugo and H. R. Moore, "Static electrification in a 700 MVA 23.7/345 kV generator step up transformer," in Proceedings: Static Electrification in Power Transformers, pp. 1-4-1 1-4-11, EPRI TR-105019, May 1995. [7] L. Peyraque, C. Boisdon, A. Beroual, and F. Buret, "Static electrification and partial discharges induced by oil flow in power transformers," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 2, pp. 40-45, February 1995. [8] J. A. Palmer and J. K. Nelson, "Streaming electrification dynamics in oil/cellulose systems," in Conference on Electrical Insulation and Dielectric Phenomena, (Arlington, Texas), pp. 895-900, October 23-26, 1994. [9] G. Arndt, D. Smyers, and D. Withers, "History and circumstances for Palo Verde GSU transformers involved in static electrification incidents," in Proceedings: Static Electrification in Power Transformers, pp. 1-2-1 - 1-2-20, EPRI, May 1993. [10] T. V. Oommen, "Static electrification properties of transformer oil," IEEE Transactions on Electrical Insulation, vol. 23, pp. 123-128, February 1988. [11] H. R. Moore, "Suggested guidelines for pump operation to reduce risk of static electrification," in Proceedings: Static Electrification in Power Transformers, pp. A-1 A-3, EPRI TRo105019, May 1995. [12] J. A. Palmer and J. K. Nelson, "Intelligent control of large power transformer cooling pumps," IEE Proceedings: Generation, Transmission, and Distribution, September 1996.