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Photographic investigations of electric discharges in Sandy media
Journal of Electrostatics, 30 (1993) 47-56 Elsevier
Photographic Sandy Media
Investigations of
47
Electric Discharges in
V i c t o r M. Cabrera M.
Centro de Graduados e Investigaci6n del I. Tecnol6gico de la Laguna, Boulevard Revoluci6n y Calz. Cuauhtdmoc, Torre6n, Coah., Mdxico CP 27000 Abstract A complementary work to the accompanying paper of discharges properties in sand is presented here by means of photographing techniques. Photographic records of electric discharges in sand of three types are displayed. A few pictures were recorded under DC voltages. Most of the investigation was for lightning impulses. Streamer paths were recorded for very dry sand (large resistivities) and wet sand (low resistivities). Under DC voltages photographic cameras were mainly used. When applying lightning impulses the recording was made by placing X-ray sensitive films along the paths of the discharges. The observed features of the streamers are briefly described. A rare and new type of discharge has been observed. Positive streamers initiated at a certain radial distance (low field region) from the conductor and propagated towards the central conductor (high field region). This mechanism took place only for larger sand particles and application of negative lightning impulses under high resistivities of sand. Clear differences between the streamers were found, specially for high resistivities. The discharging shapes were mainly dependent on the particle size and the impulse polarity.
1.- INTRODUCTION The present investigation was carried out with the purpose of obtaining more information about electric discharges in sandy media. Earlier studies [1,2] of discharges in sand have motivated further investigations to understand the processes involved on the breakdown phenomena. The study identifies some features which could give inputs for future theories of discharge development in sand. The identification of some properties of the discharges by photographic methods may support or reject some of the theoretical suggestions, available at present. A more detailed description of results and larger number of pictures is presented in [3]. 2.- P H O T O G R A P H I C TECHNIQUE The sand samples used in this experiment were also defined as in ref. [1,2], according to the size of the sand particles. S a n d A was a very coarse sand, S a n d B coarse and S a n d C fine sand. 0304-3886/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
48 Frays film /Sand / ~:~!:~i~I~:'~':!;!:-'.T~:::':,:-:::~: ):,j
glass tube sand particles
.-4" Fig. 1 Arrangement for the first and second tests.
~
::~-~=::!~..:-;~.-:?..'!~',::.~i ,-.:,:...-..~;::~.-:.-.~.
~}!~j~...~:.!.i.:i~ill!~(.,~ I :.":--,.-..~v~2:.:-.-:~.::.:...~7.~::~;:,::~..,..:.:i N
V '.'3¢:..:'~,~-:::'."."~:.::'.":.,...-..~.':::.':~::.'-'~..~.~:"
central ~onduct~r ~ u t e r cylinderl' d=Imm D=/00mm
Fig.2 Arrangement for recording of streamers tracks in sand.
Three tests were carried out to obtain the photographic information of discharges in sand described in this report. In the f i r s t test, the gravel particles (Sand A) were placed in transparent boron silicate glass tubes, with extension coefficient 3.3, see Fig. 1. The tubes had an internal diameter of 13 mm and a length of a few cm. Application of DC voltages followed. Recordings were made at visible light with an ordinary camera. Polaroid instantaneous films were used to calibrate the exposition settings. Some pictures were taken with ultraviolet pass filters. The s e c o n d test was in the same small glass tube, with the same particles but applying standard lightning impulses(1.2/50ps) and wetting the particles. The t h i r d test was carried in a cylindrical coaxial cell filled with sand. The three mentioned types of sand were used and applied standard lightning impulses only. The same cell as used earlier for experiments on breakdown voltages in sand was employed, see ref. [1,2] for a more detailed information. Sensitive X ray films were placed along the central conductor, immersed in the sand. The first streamer traces were recorded under very dry sand (equivalent bulk resistivity of M~-m), while the last experiments under wetted sand(a few hundred gt.m). The reduction of sand resistivity was made by spraying water solution and rotating in a concrete mixing machine, see ref. [2]. Some of the features described in this work, are difficult to reproduce in paper prints. They were inferred from the recorded films under the experiments, which captured many details. The diffÉculty of printing is because the photographic paper available today has lower photographic resolution than the films (negatives). Extremely high contrasts and different light conditions cannot be reproduced by simple methods. With special techniques those conditions might be illustrated, but in this report this has not been achieved. The pictures presented in section 3.3 (coaxial cell) are negatively printed. I.e. they are reproduced exactly as in the negative films. This was done to improve the reproduction of details. Accordingly the black traces represent the light produced by discharges.
3.- R E S U L T S OF PHOTOGRAPHIC OBSERVATIONS 3.1.- Application of DC voltages to a small cell of s a n d A, Fig.3 Applied voltages were in the region of 40kV for both polarities. The particles were dry and current measurements were in the range of 500 ~A to lmA.
49 The discharges appeared always in the gaps between the particles and later they extended over their surfaces. Streamers were generated all the time in the direction from the positive electrode towards the negative one. If the applied DC was positive, the streamers propagated from the HV electrode to the grounded one. But, if the applied DC was negative then the streamers propagated from the grounded electrode to the HV electrode. The above observation is probably due to generation of positive streamers in both cases of applied high voltages. This suggests the better possibility of positive streamers to initiate and propagate through the sand particles whatever the applied polarity of the source.
IV
+)
0 I
0
I
10 i
I
10 I
mm
I
20 I
30 '-,
20 I
i
30 I
Fig.3 Enlargement of discharges under positive and negative DC voltages, =40kV. 3.2.- S t a n d a r d l i g h t n i n g impulses o n a small cell o f s a n d A, Fig. 4
The sand particles were moistened and placed in the same glass tube used for the DC tests. Voltages in the range of 80 kV of both polarities were applied. Current peaks around 100 A were measured across the channel. A very brilliant channel discharge was recorded. Under application of positive impulses the discharge channel always had a single stem touching the HV electrode and frequently branched on the grounded electrode. When applying negative impulses the discharge channel was branched at the HV electrode, but a unique stem on the grounded side was observed. These results can be interpreted
50 as follows. The main discharge channel is propagating from the positive electrode to the negative one. Some times a branched channel was observed in regions of the gap filled with the small stones.
HV
(÷) 0
I
10
I
I
I mm
20 I
30
i
I
HV
(-) 0
10 I
I
I
20 I mE
I
31 i
I
Fig.4 Enlargement of the discharge channels under application of a positive and negative standard lightning impulse. =80 k V, 1001k 3.3.- Application of standard lightning impulses 1.2/50 ~s t o a c o a x i a l cell of sand a) In a i r a n d high resistivities of sand, Fig. 5, 6, 7 a n d 8 Measurements of dry sand samples gave equivalent resistivities in the range of several M~.m. The capacitance of the empty cell was 7pF. The following were the measured resistivities and capacitances. SandA; p= 10 M~.m, 107pF Sand B; p= 9 M~.m, 88 pF Sand C; p= 11 M~.m, 80 pF Air; p= 13 T~.m, 7 pF In air, the streamer tracks were recorded very clearly. Positive streemers were very long, of high branching (up to 4th order) and with high density. Many of t h e m went over the length of the film. The separation between the roots of streamers was from 2, to 15 ram. Negative s t r e a m e r s were few and much shorter. The separation between them was also large. Larger areas of glow-like discharges were also observed in the negative impulses. They were probably due to avalanches which did not reach the streamer stage or due to the light of streamers nearby from the other side of the central conductor. In both positive and negative cases the branching was tree-like.
51 Discharges on large particles (Sand A) showed, a large number (more than in air) of thin streamers, Fig. 6. Streamer diameters rarely exceed 200 ~m. They were uniform and of high density, but of low intensity. The tree-like shape was not clearly maintained. They were more "brush"-like, having a slightly appearance of parallel lines. Streamers under positive and negative impulses were similar in shape, but the direction of propagation and their length were different. Under positive impulses they started from the central conductor and propagated outwards in the direction of low electric field regions. The streamers under negative impulses, started far from the central conductor and propagated towards the central conductor in the direction of high electric field. This was a very unusual discharge, where streamers initiated in a region of lower electric field, see Fig.6 and 7. Near to the central conductor the appearance of spots of discharges was clear. These spots were the discharges developed around the sand particles. A hot region of streamers is detected near to the central conductor. Under positive impulses this region reaches about 4mm from the conductor, while under negative impulses up to 6mm.
0
I
I
10 I
I
2O I
mJTl
Fig.5 Partial enlargement of positive(above) a n d negative(below) s t r e a m e r s in air. Standard lightning impulses 1.2/50ps, +50.15 and -80.57kV peak.
The discharges on s m a l l e r p a r t i c l e s (Sand B), had characteristics quite similar to the discharges on large particles: "brush"-like streamers, opposite direction of propagation for positive and negative impulses, thin tracks of most of the discharges, spots of discharges near to the conductor, etc. However, there were two main differences:
52 - first, the length of the discharges were shorter and of lower density for the same magnitude of potentials. - second, the appearing of additional s t r o n g s t r e a m e r s , specially clear under positive impulses. The strong streamers were of higher light intensity, more branched than the original thin streamers but maintaining small diameter of 100 pm. A clear distinction between weak streamers and strong streamers was identified. Because of this new strong streamers the whole set of discharges looks less uniform. The branching in positive strong discharges was observed tree-like.
Oi
I
1~
t 20] rain
!
30i
Fig. 6 Partial enlargement of discharges under p o s i t i v e (above) a n d n e g a t i v e (below) standard lightning impulses in dry Sand,4. +78.25 and - 66.5k V peak. On f i n e p a r t i c l e s (Sand C), the strong streamers were clearly established, while the weak streamers almost disappeared and were confined to surrounding regions of the central conductor. The trend of decreasing intensity and density of
53
10 I
I mn'l
20 I
Fig. 7 Partial enlargement of discharges under positive(above) a n d negative(below) lightning impulses in dry Sand B. + 67.14 and -64.4k V peak.
0
I
I
10 I
I TTIITI
20
I
Fig. 8 Partial amplification of discharges under positive(above) and negative(below) lightning impulses in dry Sand C. +60 and -63.16kVpeak.
54 thin (weak) streamers for smaller particles was confirmed. The smaller the particle size, the shorter the weak streamers and the larger the amount of strong streamers. The strong discharges had almost 1 mm diameter, thus it is more proper to refer them as leaders or spark channels. A clear distinction of positive and negative streamers was showed, growing both from the central conductor towards the regions of lower electric fields. I.e. they did not show an opposite direction of propagation as in the case of Sand A and Sand B. The branching increased very much in both positive and negative streamers. While the positive streamers branching was tree-like of high order (up to 5th), the negative streamers were much branched but with a feather-like shape and low order (2nd), see Fig.8. The negative channels had small diameter, 100 ~m, and a similar size in the main channel and the branches. However the main channels were more brilliant (hot).
b) Low resistivities of sand, Fig.9 and 10 The following were the resistivities of the sand under wet conditions. Sand A; p= 250 ~.m, Sand B; p= 160 ~.m, SandC; p=410 ~.m, The discharges were much stronger than those taking place at high resistivities of sand. The reason is because larger currents flew through the channels. The number of individual streamers (or leaders) were less than those in dry sands, however they were more intense (hotter and more ionized channels). In sand A, a clear difference in the discharges' shape was identified for positive and negative polarities. Positive discharges were more branched than the negatives. Small isolated discharges were induced, presumably on the sand particles. These discharges had a star shape but they were also largely deviated outwards, in opposite direction to the central conductor, or towards nearby strong branch channels. Positive channel diameters vary from about 50 pm (at the tip of branches) up to 1 mm (main stem). Most of positive discharges had between 600 and 800 ~m. The largest diameter of negative channels was 600 pm. The results of discharges in sand B showed very blurred figures. Around the central conductor no clear tracks were recorded. By some reason the film records were distorted (might be water heating and an early development). A special importance of positive streamers or leaders is maintained through all the observed discharges. When discharges are induced on the sand particles, as for example on Sand A and Sand B, the positive discharges are dominating. If the applied voltages are positive the induced discharges are outwards, but if the applied voltages are negative the induced discharges are inwards. In Sand C not only positive but also negative streamers have been observed to develop. The general appearance of these discharges was similar for both polarities. In most cases positive and negative streamers have been created and then joined to build the breakdown channels. 4.- CONCLUSIONS Photographic records of electric discharges in sand were presented. Three types of sand with different particle sizes were tested. Pictures of discharges under DC voltages for large sand particles were taken. Most of the work carried out was
55
~,: ~
~ i,!.! t
0
10
20
30
Fig. 9 Partial enlargement of discharges under p o s i t i v e ( a b o v e ) and negative(below) lightning impulses in wet S a n d A. +39.28 and - 40. 78k V peak.
i¸¸¸ ,
~ .~ ,:~
0
I
I
10 I
I mill
20 I
Fig. 10 Partial enlargement of discharges under p o s i t i v e ( a b o v e ) and negative(below) lightning impulses in wet S a n d C. +27.27 and - 32.5k V peak.
55 for lightning impulses in a coaxial cylindrical cell. Streamer paths were recorded for very dry sand (large resistivities) and wet sand (low resistivities). Under DC voltages the pictures were taken mainly by photographic cameras. When applying lightning impulses the recording was made by placing X-ray sensitive films on the paths of the discharges. A short description of the observed features of the streamers was carried out. An unusual type of discharge has been observed, when positive streamers initiated at certain radial distance (low field region) from the conductor and later propagated towards the central conductor (high field region). This mechanism took place only for bigger sand particles, with application of negative lightning impulses and for high resistivities of sand. Clear differences of the streamers shapes were found, specially for high resistivities of sand. They were mainly dependent on the particle size and the impulse polarity. 5.- ACKNOWLEDGEMENTS The experimental work was carried out at the Institute of High Voltage Research (IHVR), Uppsala University, Sweden. I acknowledge my gratitude to Prof. Stig Lundquist for his invaluable advising and support. His very motivating questions about the initiation and appearance of discharges in sand particles suggested the investigations of this work. The facilities made available by Prof. Viktor Scuka, head of the IHVR for this investigation are very much appreciated. I also appreciate his interest and general support in this project. The economical support granted by Vattenfall Utveckling AB and ABB HV Switchgear from Sweden is greatly appreciated. Special thanks to Dr. Kjell Isaksson and Mr. Per ,~ke Hellman. I thank very much to Doc. Vernon Cooray for his kind help, comments and review of the manuscript. The advising and help in photographing techniques are acknowledge to Erik L6tberg, from the staff of the Institute of High Voltage Research. I thank to"Sistema Nacional de Investigadores" for the given support. 6.- REFERENCES [1] Victor M. Cabrera M., E x p e r i m e n t a l Results of Discharges in Sand under Lightning Impulse Voltages and a Physical Interpretation. Paper 3.3, 20th International Conference on Lightning Protection (ICLP-90), Sept 24-28th, Interlaken, Switzerland 1990. [2] Victor M. Cabrera M., Stig Lundquist and Vernon Cooray, O n the Physical Properties o f Discharges in Sand Under Lightning Impulses. Accepted to the 7th International Conference on Electrostatics, 11-14 May, 1993. Lahnstein nr. Koblenz, Germany. [3] Victor M. Cabrera M., A Photographic Investigation o f Discharges in Sand Particles. UURIE: 241-92, Institute of High Voltage Research, Uppsala University, 1992. [4] Victor M. Cabrera M. and Vernon Cooray, On the m e c h a n i s m o f space charge generation and neutralization in a coaxial cylindrical configuration, paper accepted for publication to the Journal of Electrostatics, April 1992.
Photographic Sandy Media
Investigations of
47
Electric Discharges in
V i c t o r M. Cabrera M.
Centro de Graduados e Investigaci6n del I. Tecnol6gico de la Laguna, Boulevard Revoluci6n y Calz. Cuauhtdmoc, Torre6n, Coah., Mdxico CP 27000 Abstract A complementary work to the accompanying paper of discharges properties in sand is presented here by means of photographing techniques. Photographic records of electric discharges in sand of three types are displayed. A few pictures were recorded under DC voltages. Most of the investigation was for lightning impulses. Streamer paths were recorded for very dry sand (large resistivities) and wet sand (low resistivities). Under DC voltages photographic cameras were mainly used. When applying lightning impulses the recording was made by placing X-ray sensitive films along the paths of the discharges. The observed features of the streamers are briefly described. A rare and new type of discharge has been observed. Positive streamers initiated at a certain radial distance (low field region) from the conductor and propagated towards the central conductor (high field region). This mechanism took place only for larger sand particles and application of negative lightning impulses under high resistivities of sand. Clear differences between the streamers were found, specially for high resistivities. The discharging shapes were mainly dependent on the particle size and the impulse polarity.
1.- INTRODUCTION The present investigation was carried out with the purpose of obtaining more information about electric discharges in sandy media. Earlier studies [1,2] of discharges in sand have motivated further investigations to understand the processes involved on the breakdown phenomena. The study identifies some features which could give inputs for future theories of discharge development in sand. The identification of some properties of the discharges by photographic methods may support or reject some of the theoretical suggestions, available at present. A more detailed description of results and larger number of pictures is presented in [3]. 2.- P H O T O G R A P H I C TECHNIQUE The sand samples used in this experiment were also defined as in ref. [1,2], according to the size of the sand particles. S a n d A was a very coarse sand, S a n d B coarse and S a n d C fine sand. 0304-3886/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
48 Frays film /Sand / ~:~!:~i~I~:'~':!;!:-'.T~:::':,:-:::~: ):,j
glass tube sand particles
.-4" Fig. 1 Arrangement for the first and second tests.
~
::~-~=::!~..:-;~.-:?..'!~',::.~i ,-.:,:...-..~;::~.-:.-.~.
~}!~j~...~:.!.i.:i~ill!~(.,~ I :.":--,.-..~v~2:.:-.-:~.::.:...~7.~::~;:,::~..,..:.:i N
V '.'3¢:..:'~,~-:::'."."~:.::'.":.,...-..~.':::.':~::.'-'~..~.~:"
central ~onduct~r ~ u t e r cylinderl' d=Imm D=/00mm
Fig.2 Arrangement for recording of streamers tracks in sand.
Three tests were carried out to obtain the photographic information of discharges in sand described in this report. In the f i r s t test, the gravel particles (Sand A) were placed in transparent boron silicate glass tubes, with extension coefficient 3.3, see Fig. 1. The tubes had an internal diameter of 13 mm and a length of a few cm. Application of DC voltages followed. Recordings were made at visible light with an ordinary camera. Polaroid instantaneous films were used to calibrate the exposition settings. Some pictures were taken with ultraviolet pass filters. The s e c o n d test was in the same small glass tube, with the same particles but applying standard lightning impulses(1.2/50ps) and wetting the particles. The t h i r d test was carried in a cylindrical coaxial cell filled with sand. The three mentioned types of sand were used and applied standard lightning impulses only. The same cell as used earlier for experiments on breakdown voltages in sand was employed, see ref. [1,2] for a more detailed information. Sensitive X ray films were placed along the central conductor, immersed in the sand. The first streamer traces were recorded under very dry sand (equivalent bulk resistivity of M~-m), while the last experiments under wetted sand(a few hundred gt.m). The reduction of sand resistivity was made by spraying water solution and rotating in a concrete mixing machine, see ref. [2]. Some of the features described in this work, are difficult to reproduce in paper prints. They were inferred from the recorded films under the experiments, which captured many details. The diffÉculty of printing is because the photographic paper available today has lower photographic resolution than the films (negatives). Extremely high contrasts and different light conditions cannot be reproduced by simple methods. With special techniques those conditions might be illustrated, but in this report this has not been achieved. The pictures presented in section 3.3 (coaxial cell) are negatively printed. I.e. they are reproduced exactly as in the negative films. This was done to improve the reproduction of details. Accordingly the black traces represent the light produced by discharges.
3.- R E S U L T S OF PHOTOGRAPHIC OBSERVATIONS 3.1.- Application of DC voltages to a small cell of s a n d A, Fig.3 Applied voltages were in the region of 40kV for both polarities. The particles were dry and current measurements were in the range of 500 ~A to lmA.
49 The discharges appeared always in the gaps between the particles and later they extended over their surfaces. Streamers were generated all the time in the direction from the positive electrode towards the negative one. If the applied DC was positive, the streamers propagated from the HV electrode to the grounded one. But, if the applied DC was negative then the streamers propagated from the grounded electrode to the HV electrode. The above observation is probably due to generation of positive streamers in both cases of applied high voltages. This suggests the better possibility of positive streamers to initiate and propagate through the sand particles whatever the applied polarity of the source.
IV
+)
0 I
0
I
10 i
I
10 I
mm
I
20 I
30 '-,
20 I
i
30 I
Fig.3 Enlargement of discharges under positive and negative DC voltages, =40kV. 3.2.- S t a n d a r d l i g h t n i n g impulses o n a small cell o f s a n d A, Fig. 4
The sand particles were moistened and placed in the same glass tube used for the DC tests. Voltages in the range of 80 kV of both polarities were applied. Current peaks around 100 A were measured across the channel. A very brilliant channel discharge was recorded. Under application of positive impulses the discharge channel always had a single stem touching the HV electrode and frequently branched on the grounded electrode. When applying negative impulses the discharge channel was branched at the HV electrode, but a unique stem on the grounded side was observed. These results can be interpreted
50 as follows. The main discharge channel is propagating from the positive electrode to the negative one. Some times a branched channel was observed in regions of the gap filled with the small stones.
HV
(÷) 0
I
10
I
I
I mm
20 I
30
i
I
HV
(-) 0
10 I
I
I
20 I mE
I
31 i
I
Fig.4 Enlargement of the discharge channels under application of a positive and negative standard lightning impulse. =80 k V, 1001k 3.3.- Application of standard lightning impulses 1.2/50 ~s t o a c o a x i a l cell of sand a) In a i r a n d high resistivities of sand, Fig. 5, 6, 7 a n d 8 Measurements of dry sand samples gave equivalent resistivities in the range of several M~.m. The capacitance of the empty cell was 7pF. The following were the measured resistivities and capacitances. SandA; p= 10 M~.m, 107pF Sand B; p= 9 M~.m, 88 pF Sand C; p= 11 M~.m, 80 pF Air; p= 13 T~.m, 7 pF In air, the streamer tracks were recorded very clearly. Positive streemers were very long, of high branching (up to 4th order) and with high density. Many of t h e m went over the length of the film. The separation between the roots of streamers was from 2, to 15 ram. Negative s t r e a m e r s were few and much shorter. The separation between them was also large. Larger areas of glow-like discharges were also observed in the negative impulses. They were probably due to avalanches which did not reach the streamer stage or due to the light of streamers nearby from the other side of the central conductor. In both positive and negative cases the branching was tree-like.
51 Discharges on large particles (Sand A) showed, a large number (more than in air) of thin streamers, Fig. 6. Streamer diameters rarely exceed 200 ~m. They were uniform and of high density, but of low intensity. The tree-like shape was not clearly maintained. They were more "brush"-like, having a slightly appearance of parallel lines. Streamers under positive and negative impulses were similar in shape, but the direction of propagation and their length were different. Under positive impulses they started from the central conductor and propagated outwards in the direction of low electric field regions. The streamers under negative impulses, started far from the central conductor and propagated towards the central conductor in the direction of high electric field. This was a very unusual discharge, where streamers initiated in a region of lower electric field, see Fig.6 and 7. Near to the central conductor the appearance of spots of discharges was clear. These spots were the discharges developed around the sand particles. A hot region of streamers is detected near to the central conductor. Under positive impulses this region reaches about 4mm from the conductor, while under negative impulses up to 6mm.
0
I
I
10 I
I
2O I
mJTl
Fig.5 Partial enlargement of positive(above) a n d negative(below) s t r e a m e r s in air. Standard lightning impulses 1.2/50ps, +50.15 and -80.57kV peak.
The discharges on s m a l l e r p a r t i c l e s (Sand B), had characteristics quite similar to the discharges on large particles: "brush"-like streamers, opposite direction of propagation for positive and negative impulses, thin tracks of most of the discharges, spots of discharges near to the conductor, etc. However, there were two main differences:
52 - first, the length of the discharges were shorter and of lower density for the same magnitude of potentials. - second, the appearing of additional s t r o n g s t r e a m e r s , specially clear under positive impulses. The strong streamers were of higher light intensity, more branched than the original thin streamers but maintaining small diameter of 100 pm. A clear distinction between weak streamers and strong streamers was identified. Because of this new strong streamers the whole set of discharges looks less uniform. The branching in positive strong discharges was observed tree-like.
Oi
I
1~
t 20] rain
!
30i
Fig. 6 Partial enlargement of discharges under p o s i t i v e (above) a n d n e g a t i v e (below) standard lightning impulses in dry Sand,4. +78.25 and - 66.5k V peak. On f i n e p a r t i c l e s (Sand C), the strong streamers were clearly established, while the weak streamers almost disappeared and were confined to surrounding regions of the central conductor. The trend of decreasing intensity and density of
53
10 I
I mn'l
20 I
Fig. 7 Partial enlargement of discharges under positive(above) a n d negative(below) lightning impulses in dry Sand B. + 67.14 and -64.4k V peak.
0
I
I
10 I
I TTIITI
20
I
Fig. 8 Partial amplification of discharges under positive(above) and negative(below) lightning impulses in dry Sand C. +60 and -63.16kVpeak.
54 thin (weak) streamers for smaller particles was confirmed. The smaller the particle size, the shorter the weak streamers and the larger the amount of strong streamers. The strong discharges had almost 1 mm diameter, thus it is more proper to refer them as leaders or spark channels. A clear distinction of positive and negative streamers was showed, growing both from the central conductor towards the regions of lower electric fields. I.e. they did not show an opposite direction of propagation as in the case of Sand A and Sand B. The branching increased very much in both positive and negative streamers. While the positive streamers branching was tree-like of high order (up to 5th), the negative streamers were much branched but with a feather-like shape and low order (2nd), see Fig.8. The negative channels had small diameter, 100 ~m, and a similar size in the main channel and the branches. However the main channels were more brilliant (hot).
b) Low resistivities of sand, Fig.9 and 10 The following were the resistivities of the sand under wet conditions. Sand A; p= 250 ~.m, Sand B; p= 160 ~.m, SandC; p=410 ~.m, The discharges were much stronger than those taking place at high resistivities of sand. The reason is because larger currents flew through the channels. The number of individual streamers (or leaders) were less than those in dry sands, however they were more intense (hotter and more ionized channels). In sand A, a clear difference in the discharges' shape was identified for positive and negative polarities. Positive discharges were more branched than the negatives. Small isolated discharges were induced, presumably on the sand particles. These discharges had a star shape but they were also largely deviated outwards, in opposite direction to the central conductor, or towards nearby strong branch channels. Positive channel diameters vary from about 50 pm (at the tip of branches) up to 1 mm (main stem). Most of positive discharges had between 600 and 800 ~m. The largest diameter of negative channels was 600 pm. The results of discharges in sand B showed very blurred figures. Around the central conductor no clear tracks were recorded. By some reason the film records were distorted (might be water heating and an early development). A special importance of positive streamers or leaders is maintained through all the observed discharges. When discharges are induced on the sand particles, as for example on Sand A and Sand B, the positive discharges are dominating. If the applied voltages are positive the induced discharges are outwards, but if the applied voltages are negative the induced discharges are inwards. In Sand C not only positive but also negative streamers have been observed to develop. The general appearance of these discharges was similar for both polarities. In most cases positive and negative streamers have been created and then joined to build the breakdown channels. 4.- CONCLUSIONS Photographic records of electric discharges in sand were presented. Three types of sand with different particle sizes were tested. Pictures of discharges under DC voltages for large sand particles were taken. Most of the work carried out was
55
~,: ~
~ i,!.! t
0
10
20
30
Fig. 9 Partial enlargement of discharges under p o s i t i v e ( a b o v e ) and negative(below) lightning impulses in wet S a n d A. +39.28 and - 40. 78k V peak.
i¸¸¸ ,
~ .~ ,:~
0
I
I
10 I
I mill
20 I
Fig. 10 Partial enlargement of discharges under p o s i t i v e ( a b o v e ) and negative(below) lightning impulses in wet S a n d C. +27.27 and - 32.5k V peak.
55 for lightning impulses in a coaxial cylindrical cell. Streamer paths were recorded for very dry sand (large resistivities) and wet sand (low resistivities). Under DC voltages the pictures were taken mainly by photographic cameras. When applying lightning impulses the recording was made by placing X-ray sensitive films on the paths of the discharges. A short description of the observed features of the streamers was carried out. An unusual type of discharge has been observed, when positive streamers initiated at certain radial distance (low field region) from the conductor and later propagated towards the central conductor (high field region). This mechanism took place only for bigger sand particles, with application of negative lightning impulses and for high resistivities of sand. Clear differences of the streamers shapes were found, specially for high resistivities of sand. They were mainly dependent on the particle size and the impulse polarity. 5.- ACKNOWLEDGEMENTS The experimental work was carried out at the Institute of High Voltage Research (IHVR), Uppsala University, Sweden. I acknowledge my gratitude to Prof. Stig Lundquist for his invaluable advising and support. His very motivating questions about the initiation and appearance of discharges in sand particles suggested the investigations of this work. The facilities made available by Prof. Viktor Scuka, head of the IHVR for this investigation are very much appreciated. I also appreciate his interest and general support in this project. The economical support granted by Vattenfall Utveckling AB and ABB HV Switchgear from Sweden is greatly appreciated. Special thanks to Dr. Kjell Isaksson and Mr. Per ,~ke Hellman. I thank very much to Doc. Vernon Cooray for his kind help, comments and review of the manuscript. The advising and help in photographing techniques are acknowledge to Erik L6tberg, from the staff of the Institute of High Voltage Research. I thank to"Sistema Nacional de Investigadores" for the given support. 6.- REFERENCES [1] Victor M. Cabrera M., E x p e r i m e n t a l Results of Discharges in Sand under Lightning Impulse Voltages and a Physical Interpretation. Paper 3.3, 20th International Conference on Lightning Protection (ICLP-90), Sept 24-28th, Interlaken, Switzerland 1990. [2] Victor M. Cabrera M., Stig Lundquist and Vernon Cooray, O n the Physical Properties o f Discharges in Sand Under Lightning Impulses. Accepted to the 7th International Conference on Electrostatics, 11-14 May, 1993. Lahnstein nr. Koblenz, Germany. [3] Victor M. Cabrera M., A Photographic Investigation o f Discharges in Sand Particles. UURIE: 241-92, Institute of High Voltage Research, Uppsala University, 1992. [4] Victor M. Cabrera M. and Vernon Cooray, On the m e c h a n i s m o f space charge generation and neutralization in a coaxial cylindrical configuration, paper accepted for publication to the Journal of Electrostatics, April 1992.