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Experimental study on the effect of applying a crossed electric field on the breakdown behavior of low vacuum
Journal of Electrostatics 70 (2012) 174e177
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Short communication
Experimental study on the effect of applying a crossed electric field on the breakdown behavior of low vacuum K. Abu-Elabbasa, *, A. Nossierb, A. El-Zeinc a
Academy for Special Studies, Technology Department, Worker University, El-Mansoura, Egypt Electrical Power and Machine Department, Faculty of Engineering, Ain-Shams University, Cairo, Egypt c Electrical Power and Machine Department, Faculty of Engineering, Zagazig University, Zagazig, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2011 Received in revised form 12 September 2011 Accepted 18 September 2011 Available online 29 September 2011
The paper presents experimental results on the effect of applying a transverse electric field on the breakdown behavior of low vacuum. The results show that the breakdown voltage BDV increases as the transverse voltage TV increases. But on the other hand the leakage current decreases as TV increases. The increase in BDV of the main gap MG is large for AC TV rather than under DC TV. The effect of TV on the breakdown characteristics shows dependence on the separation of MG and the transverse gap as well as the type of applied TV, and the air pressure of vacuum gap. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Low vacuum Transverse electric field
1. Introduction
2. Experimental technique and test cell
The influence of applying a crossed magnetic field on the breakdown characteristic of different material has been previously done [1e15]. Also, the use of transverse electric fields for studying the BDV of dielectric liquids and solids has been done by the authors [16e19]. However, for the author’s best knowledge, the influence of a crossed electric field in low vacuum has not yet been widely introduced. A knowledge of the effect of applying a crossed electric field on the breakdown behavior of low vacuum is very important for high voltage HV technology because of practical applications such as vacuum interrupters for contactors and circuitbreakers. This can be carried out if we put transverse gap configuration in the circuit-breaker design, by connecting the phase voltage on one electrode and connecting the other with earth. In the present study, an attempt is made to investigate the effect of applying transverse electric field, to the main tested gap, on breakdown characteristics of low vacuum. The pressure of low vacuum used in this study was extend from 1 105 to 0.03 105 Pa. The great improvements in the experimental technique employed in the present work have led to new contributions being added to our knowledge of breakdown characteristics in the presence of a crossed electrical field.
The test container consists of rectangular parallelepiped container and having dimensions of: length, width and height of: 150, 150 and 200 mm respectively. All container sides were made of fiber material except the front side which was made of transparent material (Perspex), to enable the MG and transverse gap TG to be observed from outside the test container, during the test. The container has been prepared to study the effect of applying transverse electric fields to the main tested gap, on breakdown in low vacuum as shown in Fig. 1. The applied lest voltage of the main gap had been obtained from 60 kV AC source between its two terminals, but for the leakage current measurement 150 kV DC negative polarity has been used. While the additional transverse voltage was obtained either from 40 kV DC source (40 kV þve or 40 kV ve) with respect to earth simultaneously or from 33 kV AC source between one terminal and earth. All tests are carried out inside test container (vacuum chamber) evaluated by vacuum pump of maximum value of 0.03 105 Pa. The cell has two transverse inlets. The vertical inlets are used for the main gap needle e needle electrode fixation with the possibility of adjusting the tested gap distance to the required value. The horizontal inlets are used for the transverse gap needleeneedle electrode fixation with the possibility of adjusting the transverse gap distance to the required value also. The main and transverse gaps used were needleeneedle brass electrodes. The needle was a circular cone of height 36 mm and apex of 48 .
* Corresponding author. E-mail address: [email protected] (K. Abu-Elabbas). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.09.002
K. Abu-Elabbas et al. / Journal of Electrostatics 70 (2012) 174e177
obtained for different TG distances of 5, 10, 15, 20 mm, as shown in Fig. 3. It is clear that BDV increases as TV increases. The BDV-TV characteristics assume a linear relation with increasing slop as the transverse gap distance decreases.
Vacuum gauge
Vent tap
Vacuum tap
High voltage
Main Gap Electrodes High voltage
175
35
Suction branch
Transverse Gap Electrodes
30 25 20
High voltage
Test Container (Vacuum Chamber)
10
M.G = 10 mm
5
Vacuum pump
0
T.G, mm
a.c
15
Delivery branch
5 10 15 20
a.c
P = 0.4*105 Pa
a.c 0
5
10
15
20
Fig. 1. Schematic diagram of the experimental setup. Fig. 3. Effect of transverse voltage variation on the breakdown voltage in low vacuum under different transverse gap spacing.
3. Experimental procedures and test results The test procedures were carried out under different cases, as follows: 3.1. Breakdown voltage test in low vacuum This test has been achieved under different procedures: 3.1.1. First test This test was carried out for a fixed TG distance of 5 mm and the MG distances were varied. The degree of vacuum used in this test was 0.4 105 Pa. The BDV for different transverse applied voltages TV’s of 5, 10, 15 and 20 kV were obtained for different M.G distances of 5, 10, 15 and 20 mm as shown in Fig. 2. From this figure it is clear that the increase of the TV has the effect of increasing the BDV of the MG. The BDV-TV characteristics assume a linear relation, the slop of this relation increase as the MG distance decreases. 3.1.2. Second test This test was carried out under a fixed MG distance of 10 mm whilst the TG distances were varied. The degree of vacuum used in this test was 0.4 105 Pa. The BDV for different transverse applied voltages of 5, 10, 15 and 20 kV were obtained. The results were
35
3.1.3. Third test This test is carried out by connecting the MG needleeneedle electrodes to 60 kV AC source supply and connecting The TG needleeneedle electrodes to 40 kV DC source supply or 33 kV AC source supply. With the MG at 10 mm and TG at 5 mm, the test results are taken and given in Fig. 4. From this figure, it is clear that the BDV of the main tested gap increases as the TV of the TG increases, the increase in the BDV of the MG is larger for AC TV rather than under DC TV. 3.1.4. Fourth test With the MG at 10 mm and TG at 5 mm the BDV was measured for different gap pressure of 0.8, 0.6, 0.4 and 0.2 105 Pa under specified TV. The above procedure was repeated for different TV’s of 0, 5, 10, 15 and 20 kV. The obtained results are presented in Fig. 5. From this figure it is clear that the BDV increases as the TV increases. Also, the increases in the BDV value is more pronounced at low gap pressures. The effect of TV increases as the gap pressure decreases. The BDV become insensitive to any increase of TV for gap pressures above 0.8 105 Pa nearly. The gap pressure range in this figure ranges from 1 105 to 0. 2 105 Pa under constant main gap distance of 10 mm, if we use pressure gap scale pd which ranges from pd ¼ 2 105 to 10 105 Pa$mm, according to curve 2 of Paschen curves for various gases [20, page 108] for air which is to the right of Paschen minimum. Therefore our results are correct.
30 35
25
M.G, mm
20
25 20 15 10 5
a.c
15
a.c
10
T.G = 5 mm P = 0.4*105 Pa
5 0
0
5
a.c
10
15
a.c
30
a.c
25 20
a.c Type of Transverse Voltage
15 10
M.G = 10 mm T.G = 5 mm 5 P = 0.4*105 Pa
20
Fig. 2. Effect of transverse voltage variation on the breakdown voltage in low vacuum under different main gap spacing.
0
0
5
AC voltage to earth + VE D.C Voltage to earth -VE D.C Voltage to earth + VE D.C Voltage to - VE D.C Voltage 10
15
20
Fig. 4. The relation between breakdown voltage of the main gap and transverse voltage of different modes in low vacuum.
176
K. Abu-Elabbas et al. / Journal of Electrostatics 70 (2012) 174e177
40
30
T.V, kV
20
20 15 10 5 0
10
0
1
a.c a.c M.G = 10 mm T.G = 5 mm
a.c 0.8
0.6
0.4
0.2
5
10 Pa Fig. 5. Variation of the breakdown voltage with gap pressure in low vacuum for various transverse voltages.
3.2. Leakage current test This test was carried out in low vacuum (under main voltage near breakdown). For leakage current measurement 150 kV DC negative polarity source for the main tested gap was used. The leakage current can be read directly on the micro-ammeter. The test was carried out under constant MG distance of 10 mm, and constant TG distance of 5 mm. All tests were carried out under a fixed main applied voltage of 20 kV DC ve. The obtained results are presented in Fig. 6. The leakage current was measured for different gap pressure under specified TV’s. The above procedure was repeated for different TV’s of 0, 5, 10, 15 and 20 kV, as shown in Fig. 6. From this figure it is clear that as the TV increase the leakage current decreases. The reduction in leakage current value is more pronounced as the air pressure decreases. The transverse electric field make the actual gap distance increase because the inclination of the resultant electric field with respect to main axis, which leads to the minimization of the leakage current. 4. Results and discussions From the results obtained throughout this work, the transverse electrical field increases the BDV of the MG, which is accordance with the findings of previous workers [16e19]. Also, the transverse electrical field reduces the leakage current. The effect of TV on the BDV characteristics shows a marked dependence on the separation of the main and transverse gaps as well as the type of applied voltage, and the air pressure of vacuum gap. 60 50 40
The improvement in the BDV and leakage current in the presence of a transverse electrical field may be due to elimination of charges by the sweeping action of the transverse field (out of gap) which may lead to retardation in the complete bridging of the gap which is actual breakdown. This will increase the BDV and reduce the leakage current, which agrees with the results given in [16e19]. It can be said that transverse electric field is similar in its effect to the influence of transverse magnetic field [1e4]. Also the improvement in breakdown characteristics in the presence of transverse field can be discussed on the basis given after [1e4], that in a transverse field the electric field E along which the electron travel is reduced from E to E cosq where q is the angle of avalanches inclination to the electric field, while on the other hand the path length in the electric field direction is increased by 1/cosq. Another way of looking at the effect of transverse electric field on the breakdown characteristics in compressed gas is to consider the electron mean free path l. This is inversely proportional to the gas number density N so that, the larger N, the smaller is the l and hence the smaller is the bending influence due to the transverse electrical field, which is in agreement with [1]. The present results show that the BDV under þve TV is higher than that under AC or ve which is in accordance with that reported in literature [20]. From our new results the effect of the transverse field decreases as the MG distance increases, this is because parts of the MG shall be outside the influence zone of the transverse field. Also, the effect of transverse field increases with the decrease of the TG, this results is expected since the transverse field acts as if it was a barrier to the moving charged particles and tends to holdback the volume charges [3], to overcome this retarding action of the barrier a higher voltage is required for breakdown. Also, as TG is small, the transverse field is in good position to holdback the volume charge traversing the gap, and then a higher voltage is needed to cause breakdown. Also, from our new results the leakage current decreases in the MG as the TV increase. The decrease in leakage current can be taken as a pre-breakdown event which when it increases give indication that the BDV is imminent. The reduction of the leakage current is due to elimination of charges out of tested gap by the sweeping action of the transverse field, or in other words forcing the charged particles for away from the MG. Thus in a crossed electric field the electron swarm is bowed and then, is pushed away from the main tested gap due to the change in the direction of the drifting electrons which takes place when electric field is applied, which are in good agreement with application of co-field and cross-field on the mineral oil flow [21]. Finally, an attempt has been made to investigate the effect of applying transverse electric field, to the main tested gap, on breakdown characteristics of low vacuum. The great improvements in the experimental technique employed in the present work have led to new contributions being added to our knowledge of breakdown characteristics in the presence of a crossed electrical field. 5. Conclusions
30
T.V, kV 0 5 10 15 20
-ve
20 M.V = 20 kV d.c M.G = 10 mm T.G = 5 mm
10
a.c
0 1
0.8
0.6
0.4 5
0.2
Gap Pressure, 10 Pa Fig. 6. Variation of the leakage current with gap pressure in low vacuum for various transverse voltages.
The effect of transverse electric field on low vacuum breakdown, as outlined in this study, show a marked dependence on the separation of the main and transverse gaps as well as the type of transverse voltage and air pressure of vacuum gap. The transverse electrical field increases the BDV of the main tested gap. Also, the transverse electrical field reduces the leakage current. The effect of TV increases as the main and transverse gaps decreases. Also, the effect of TV increases as the gap pressure decreases.
K. Abu-Elabbas et al. / Journal of Electrostatics 70 (2012) 174e177
References [1] A.E.D. Heylen, Electrical ionization and breakdown of gases in a crossed magnetic field, IEE Proceeding 127 (1980) 221e244. [2] P.R. Emtage, C.W. Kimbline, J.G. Gorman, F.A. Holmes, J.V. Heberlein, R.E. Voshall, P.G. Slade, Interaction between vacuum arcs and transverse magnetic fields with application to current limitation, IEEE Trans. Plasma Sci. 8 (1980) 314e319. [3] C.E. Speck, J.J. Lee, The physics of retrograde motion of the electric Arc", Japan, J. Appl. Phys 20 (1981) 1027e1037. [4] P. Lamp, G. Buschhoron, Measurement of electron transport in liquid argon in crossed electric and magnetic fields, IEEE Transaction on Electrical Insulation 1 (1994) 407e411. [5] G. Petraconi, H.S. Maciel, R.S. Pessoa, G. Murakami, M. Massi, C. Otani, W.M.I. Uruchi, B.N. Sismanoglu, Longitudinal magnetic field effect on the electrical breakdown in low pressure gases, Braz. J. Phys. 34 (4-B) (December, 2004). [6] S. Dujko, R.D. White, K.F. Ness, Z.L. Petrovic, R.E. Robson, Non-Conservative electron transport in CF4 in electric and magnetic fields crossed at arbitrary angles, J. Phys. D: Appl. Phys. 39 (2006) 4788e4798. [7] B. X. Du, N. Chen, Y. Gao, Xiangjin Zhang, “Carbonization Migration of Polymer Insulating Material Under Magnetic Field” 8th International Conference on properties and applications of dielectric materials, PP. 423e426, 2006. [8] P.G. Slade, Wang-Pei Li, Stephen Mayo, R.K. Smith, Erik D. Taylor, Vacuum interrupter, high reliability component of distribution switches, circuit breakers and contactors, J. Zhejiang Univ. Sci. A 8 (3) (2007) 335e342. [9] Z. Buntat, I.R. Smith, N.A.M. Razali, Effect of crossed magnetic field on ozone production in air, Elektrika 11 (2009) 16e19. [10] D.F. Alferov, D.V. Evsin, V.P. Ivanov, Raising the interrupting capacity of vacuum gap with transverse magnetic field by means of shunting resistor, High Temperature 47 (4) (2009) 489e493.
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[11] A.P. Ershov, E.B. Kolesnikov, A. Logunov, V.A. Chernikov, Parameters of electrode discharges in supersonic air flows, High Temperature 47 (12) (2009) 165e174. [12] A.P. Dostanko, D.A. Golosov, Neutralization of an ion beam from the end-Hall ion source by a plasma electron source based on a discharge in crossed E H fields, Tech. Phys. 54 (10) (2009) 1461e1466. [13] E.V. Ilyakov, I.S. Kulagin, V.E. Nechaev, Experimental study of a saturated multifactor in crossed fields, Radio Physics Quantum Electronics 52 (12) (2009) 885e891. [14] S. Pek’areka, DC corona discharge ozone production enhanced by magnetic field, Eur. Phys. J. D 56 (2010) 91e98. [15] Du Boxue, Zhu Xiaohui, Gao Yu, Surface breakdown of printed circuit board under magnetic field with reduced pressure, Trans. Tianjin Univ. 16 (1) (2010) 6e10. [16] A. Nosseir, A. El-Zein, and K. Abu-Elabbas “Experimental Study of Transverse Electric Field Effect on the Breakdown in Dielectric liquids”, 5th International Conference, MEPCON, Vol. 2, PP. 684e687, 1997. [17] A. Nosseir, A. El-Zein, and K. Abu-Elabbas “Breakdown Characteristics of Solids Insulation Under the Presence of Non-uniform Transverse Electric Fields” 10th ISH Montreal, Quebec, Canada, 1997. [18] A. El-Zein, A. Nosseir, and K. Abu-Elabbas "Experimental Breakdown Analysis of Transformer Oil in a Transverse Electric Field Using AC and DC High Voltage Sources”, 6th International Conference, MEPCON, Vol. 1, PP. 395e400, 1998. [19] K. Abu-Elabbas, “Effect of Transverse Fields on Electrical Characteristics of Insulating Materials”, Ph.D. thesis, Zagazig University, Egypt, 2000. [20] Dieter kind, Karner Hermann, High Voltage Insulation Technology. Vieweg, Weishaden, 1985. [21] Abdallah, M. “Dielectric Properties of Insulating Liquids Under High Electrical Stress and Non-uniform Fields” Ph.D. thesis, Faculty of Engineering, Alexandria University, Egypt, 1995.
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Short communication
Experimental study on the effect of applying a crossed electric field on the breakdown behavior of low vacuum K. Abu-Elabbasa, *, A. Nossierb, A. El-Zeinc a
Academy for Special Studies, Technology Department, Worker University, El-Mansoura, Egypt Electrical Power and Machine Department, Faculty of Engineering, Ain-Shams University, Cairo, Egypt c Electrical Power and Machine Department, Faculty of Engineering, Zagazig University, Zagazig, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2011 Received in revised form 12 September 2011 Accepted 18 September 2011 Available online 29 September 2011
The paper presents experimental results on the effect of applying a transverse electric field on the breakdown behavior of low vacuum. The results show that the breakdown voltage BDV increases as the transverse voltage TV increases. But on the other hand the leakage current decreases as TV increases. The increase in BDV of the main gap MG is large for AC TV rather than under DC TV. The effect of TV on the breakdown characteristics shows dependence on the separation of MG and the transverse gap as well as the type of applied TV, and the air pressure of vacuum gap. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Low vacuum Transverse electric field
1. Introduction
2. Experimental technique and test cell
The influence of applying a crossed magnetic field on the breakdown characteristic of different material has been previously done [1e15]. Also, the use of transverse electric fields for studying the BDV of dielectric liquids and solids has been done by the authors [16e19]. However, for the author’s best knowledge, the influence of a crossed electric field in low vacuum has not yet been widely introduced. A knowledge of the effect of applying a crossed electric field on the breakdown behavior of low vacuum is very important for high voltage HV technology because of practical applications such as vacuum interrupters for contactors and circuitbreakers. This can be carried out if we put transverse gap configuration in the circuit-breaker design, by connecting the phase voltage on one electrode and connecting the other with earth. In the present study, an attempt is made to investigate the effect of applying transverse electric field, to the main tested gap, on breakdown characteristics of low vacuum. The pressure of low vacuum used in this study was extend from 1 105 to 0.03 105 Pa. The great improvements in the experimental technique employed in the present work have led to new contributions being added to our knowledge of breakdown characteristics in the presence of a crossed electrical field.
The test container consists of rectangular parallelepiped container and having dimensions of: length, width and height of: 150, 150 and 200 mm respectively. All container sides were made of fiber material except the front side which was made of transparent material (Perspex), to enable the MG and transverse gap TG to be observed from outside the test container, during the test. The container has been prepared to study the effect of applying transverse electric fields to the main tested gap, on breakdown in low vacuum as shown in Fig. 1. The applied lest voltage of the main gap had been obtained from 60 kV AC source between its two terminals, but for the leakage current measurement 150 kV DC negative polarity has been used. While the additional transverse voltage was obtained either from 40 kV DC source (40 kV þve or 40 kV ve) with respect to earth simultaneously or from 33 kV AC source between one terminal and earth. All tests are carried out inside test container (vacuum chamber) evaluated by vacuum pump of maximum value of 0.03 105 Pa. The cell has two transverse inlets. The vertical inlets are used for the main gap needle e needle electrode fixation with the possibility of adjusting the tested gap distance to the required value. The horizontal inlets are used for the transverse gap needleeneedle electrode fixation with the possibility of adjusting the transverse gap distance to the required value also. The main and transverse gaps used were needleeneedle brass electrodes. The needle was a circular cone of height 36 mm and apex of 48 .
* Corresponding author. E-mail address: [email protected] (K. Abu-Elabbas). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.09.002
K. Abu-Elabbas et al. / Journal of Electrostatics 70 (2012) 174e177
obtained for different TG distances of 5, 10, 15, 20 mm, as shown in Fig. 3. It is clear that BDV increases as TV increases. The BDV-TV characteristics assume a linear relation with increasing slop as the transverse gap distance decreases.
Vacuum gauge
Vent tap
Vacuum tap
High voltage
Main Gap Electrodes High voltage
175
35
Suction branch
Transverse Gap Electrodes
30 25 20
High voltage
Test Container (Vacuum Chamber)
10
M.G = 10 mm
5
Vacuum pump
0
T.G, mm
a.c
15
Delivery branch
5 10 15 20
a.c
P = 0.4*105 Pa
a.c 0
5
10
15
20
Fig. 1. Schematic diagram of the experimental setup. Fig. 3. Effect of transverse voltage variation on the breakdown voltage in low vacuum under different transverse gap spacing.
3. Experimental procedures and test results The test procedures were carried out under different cases, as follows: 3.1. Breakdown voltage test in low vacuum This test has been achieved under different procedures: 3.1.1. First test This test was carried out for a fixed TG distance of 5 mm and the MG distances were varied. The degree of vacuum used in this test was 0.4 105 Pa. The BDV for different transverse applied voltages TV’s of 5, 10, 15 and 20 kV were obtained for different M.G distances of 5, 10, 15 and 20 mm as shown in Fig. 2. From this figure it is clear that the increase of the TV has the effect of increasing the BDV of the MG. The BDV-TV characteristics assume a linear relation, the slop of this relation increase as the MG distance decreases. 3.1.2. Second test This test was carried out under a fixed MG distance of 10 mm whilst the TG distances were varied. The degree of vacuum used in this test was 0.4 105 Pa. The BDV for different transverse applied voltages of 5, 10, 15 and 20 kV were obtained. The results were
35
3.1.3. Third test This test is carried out by connecting the MG needleeneedle electrodes to 60 kV AC source supply and connecting The TG needleeneedle electrodes to 40 kV DC source supply or 33 kV AC source supply. With the MG at 10 mm and TG at 5 mm, the test results are taken and given in Fig. 4. From this figure, it is clear that the BDV of the main tested gap increases as the TV of the TG increases, the increase in the BDV of the MG is larger for AC TV rather than under DC TV. 3.1.4. Fourth test With the MG at 10 mm and TG at 5 mm the BDV was measured for different gap pressure of 0.8, 0.6, 0.4 and 0.2 105 Pa under specified TV. The above procedure was repeated for different TV’s of 0, 5, 10, 15 and 20 kV. The obtained results are presented in Fig. 5. From this figure it is clear that the BDV increases as the TV increases. Also, the increases in the BDV value is more pronounced at low gap pressures. The effect of TV increases as the gap pressure decreases. The BDV become insensitive to any increase of TV for gap pressures above 0.8 105 Pa nearly. The gap pressure range in this figure ranges from 1 105 to 0. 2 105 Pa under constant main gap distance of 10 mm, if we use pressure gap scale pd which ranges from pd ¼ 2 105 to 10 105 Pa$mm, according to curve 2 of Paschen curves for various gases [20, page 108] for air which is to the right of Paschen minimum. Therefore our results are correct.
30 35
25
M.G, mm
20
25 20 15 10 5
a.c
15
a.c
10
T.G = 5 mm P = 0.4*105 Pa
5 0
0
5
a.c
10
15
a.c
30
a.c
25 20
a.c Type of Transverse Voltage
15 10
M.G = 10 mm T.G = 5 mm 5 P = 0.4*105 Pa
20
Fig. 2. Effect of transverse voltage variation on the breakdown voltage in low vacuum under different main gap spacing.
0
0
5
AC voltage to earth + VE D.C Voltage to earth -VE D.C Voltage to earth + VE D.C Voltage to - VE D.C Voltage 10
15
20
Fig. 4. The relation between breakdown voltage of the main gap and transverse voltage of different modes in low vacuum.
176
K. Abu-Elabbas et al. / Journal of Electrostatics 70 (2012) 174e177
40
30
T.V, kV
20
20 15 10 5 0
10
0
1
a.c a.c M.G = 10 mm T.G = 5 mm
a.c 0.8
0.6
0.4
0.2
5
10 Pa Fig. 5. Variation of the breakdown voltage with gap pressure in low vacuum for various transverse voltages.
3.2. Leakage current test This test was carried out in low vacuum (under main voltage near breakdown). For leakage current measurement 150 kV DC negative polarity source for the main tested gap was used. The leakage current can be read directly on the micro-ammeter. The test was carried out under constant MG distance of 10 mm, and constant TG distance of 5 mm. All tests were carried out under a fixed main applied voltage of 20 kV DC ve. The obtained results are presented in Fig. 6. The leakage current was measured for different gap pressure under specified TV’s. The above procedure was repeated for different TV’s of 0, 5, 10, 15 and 20 kV, as shown in Fig. 6. From this figure it is clear that as the TV increase the leakage current decreases. The reduction in leakage current value is more pronounced as the air pressure decreases. The transverse electric field make the actual gap distance increase because the inclination of the resultant electric field with respect to main axis, which leads to the minimization of the leakage current. 4. Results and discussions From the results obtained throughout this work, the transverse electrical field increases the BDV of the MG, which is accordance with the findings of previous workers [16e19]. Also, the transverse electrical field reduces the leakage current. The effect of TV on the BDV characteristics shows a marked dependence on the separation of the main and transverse gaps as well as the type of applied voltage, and the air pressure of vacuum gap. 60 50 40
The improvement in the BDV and leakage current in the presence of a transverse electrical field may be due to elimination of charges by the sweeping action of the transverse field (out of gap) which may lead to retardation in the complete bridging of the gap which is actual breakdown. This will increase the BDV and reduce the leakage current, which agrees with the results given in [16e19]. It can be said that transverse electric field is similar in its effect to the influence of transverse magnetic field [1e4]. Also the improvement in breakdown characteristics in the presence of transverse field can be discussed on the basis given after [1e4], that in a transverse field the electric field E along which the electron travel is reduced from E to E cosq where q is the angle of avalanches inclination to the electric field, while on the other hand the path length in the electric field direction is increased by 1/cosq. Another way of looking at the effect of transverse electric field on the breakdown characteristics in compressed gas is to consider the electron mean free path l. This is inversely proportional to the gas number density N so that, the larger N, the smaller is the l and hence the smaller is the bending influence due to the transverse electrical field, which is in agreement with [1]. The present results show that the BDV under þve TV is higher than that under AC or ve which is in accordance with that reported in literature [20]. From our new results the effect of the transverse field decreases as the MG distance increases, this is because parts of the MG shall be outside the influence zone of the transverse field. Also, the effect of transverse field increases with the decrease of the TG, this results is expected since the transverse field acts as if it was a barrier to the moving charged particles and tends to holdback the volume charges [3], to overcome this retarding action of the barrier a higher voltage is required for breakdown. Also, as TG is small, the transverse field is in good position to holdback the volume charge traversing the gap, and then a higher voltage is needed to cause breakdown. Also, from our new results the leakage current decreases in the MG as the TV increase. The decrease in leakage current can be taken as a pre-breakdown event which when it increases give indication that the BDV is imminent. The reduction of the leakage current is due to elimination of charges out of tested gap by the sweeping action of the transverse field, or in other words forcing the charged particles for away from the MG. Thus in a crossed electric field the electron swarm is bowed and then, is pushed away from the main tested gap due to the change in the direction of the drifting electrons which takes place when electric field is applied, which are in good agreement with application of co-field and cross-field on the mineral oil flow [21]. Finally, an attempt has been made to investigate the effect of applying transverse electric field, to the main tested gap, on breakdown characteristics of low vacuum. The great improvements in the experimental technique employed in the present work have led to new contributions being added to our knowledge of breakdown characteristics in the presence of a crossed electrical field. 5. Conclusions
30
T.V, kV 0 5 10 15 20
-ve
20 M.V = 20 kV d.c M.G = 10 mm T.G = 5 mm
10
a.c
0 1
0.8
0.6
0.4 5
0.2
Gap Pressure, 10 Pa Fig. 6. Variation of the leakage current with gap pressure in low vacuum for various transverse voltages.
The effect of transverse electric field on low vacuum breakdown, as outlined in this study, show a marked dependence on the separation of the main and transverse gaps as well as the type of transverse voltage and air pressure of vacuum gap. The transverse electrical field increases the BDV of the main tested gap. Also, the transverse electrical field reduces the leakage current. The effect of TV increases as the main and transverse gaps decreases. Also, the effect of TV increases as the gap pressure decreases.
K. Abu-Elabbas et al. / Journal of Electrostatics 70 (2012) 174e177
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