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Electrical corona and specific charge on water drops from a cylindrical conductor with high d.c. voltage
Journal of Electrostatics, 8 (1980) 239--270
239
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
E L E C T R I C A L C O R O N A AND SPECIFIC C H A R G E ON WATER DROPS FROM A CYLINDRICAL C O N D U C T O R WITH HIGH D.C. VOLTAGE
MASANORI HARA*, SHINJI ISHIBE**, SATORU SUMIYOSHITANI* and MASANORI AKAZAKI**
*Department of Electrical Engineering, Faculty of Engineering, Kyushu University, Fukuoka, 812 (Japan) **Department of Energy Conversion Engineering, Graduate School of Engineering Sciences, Kyushu University, Fukuoka, 812 (Japan) (Received July 6, 1979; accepted September 11, 1979)
Summary The relationship among the waveform of corona current, the dripping manner of water drops from a conductor with high d.c. voltage, the corona discharge mechanism, the specific charge (the ratio of charge to mass of the drop) and the radius of water drops is investigated for a wide range of the drop conductivity, whilst the corona discharge mechanism on the drop is discussed. In addition to the corona and the specific charge modes described in a previous paper [1 ], Pulse-Less Corona (PLC) and the atomization of water is newly determined for highly resistive water. Moreover it is confirmed that with distilled water the maximum specific charge could reach 5 x 10 -3 C/kg and the duration of the current flowing is closely related to the mechanical parameters determinimz the deformation process of water drops.
1. Introduction
In foul weather, water drops hanging from d.c. transmission conductors cause an increase in corona activity and behave b o t h as sources of various charge carriers, audible noise and electromagnetic interference and sinks of energy. Several works on corona discharge on water drops on or near d.c. transmission conductors have been carried o u t to reduce or eliminate undesirable corona discharges and to estimate the level of RI (Radio Interference) or TVI (TV Interference) [2--11]. In connection with charge carriers from d.c. transmission conductors in foul weather, some experimental results were presented for corona discharge and the specific charge on water drops dripping from smooth conductors and ACSR conductors in a recent paper [1]. In these studies, however, the specific conductivity of the drop was fixed or unknown. On the other hand, the rain conductivity could be as low as 2 and as high as 2000/~S/cm [12--14]. Moreover, the behavior of liquid drops in a high d.c. field is strongly affected n o t only b y the size of the drop and the surface tension b u t also b y the liquid conductivity. F o r example, liquids with specific
240 conductivity lower than 10 -7 p S / c m and higher than 10 pS/cm cannot atomize from a fine glass capillary, while outside this range the atomization occurs easily [15, 16]. Therefore, it is considered that the dripping manner of water drops from d.c. transmission conductors, corona discharge mechanism and the specific charge on the drops may be markedly changed b y the rain conductivity. The purpose of this work is to investigate the effects of the water-drop conductivity on corona discharge and characteristics on the specific charge, the size and the dripping manner of drops and the relationship among the corona mechanism, the corona current waveform and the dripping manner of drops. The study described in the previous paper [1] is extended. 2. Experimental set-up The experimental set-up used is basically the same as that of the previous paper [1], Fig. 1. The rain conductivity depends strongly on the rate of rain-
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241
fall, the passage of time from the start of a rainfall and the measuring location; that is, in an inland area or in a coastal area. Figure 2 illustrates the probabili~ ties of the rain conductivity measured in USA [12] and Japan [13, 14]. The 50% cumulative frequency appears at about 40 pS/cm for both countries. The desired water with specific conductivity o in the range of 2.4--267 pS/cm at 20°0, which covers the most probable part in the distribution, was obtained by mixing distilled water and tap water. Resistors of 220 k~ and 5.1 k~ were used for the measurement of small corona current and pulses. Those correspond to 52.4 ps and 1.2 ps time constant, respectively, for the measuring circuit, because the parallel stray capacitance of the outer cylinder is 238 pF. Then the recorded peak value of individual corona pulses on an oscilloscope (Tektronix 7633 or Iwatsu 6200) may actually be smaller than the expected value. However, the ability of the circuit with the smaller resistor in identifying the individual Trichel pulses was confirmed using the inner conductor with a metal protrusion. To obtain the relationship between the current waveform and the pendant drop shape, the micro-flash lamp was normally triggered by the signal from the oscilloscope which produced the desired signal delay. The measuring methods of the specific charge and the size of water drops are the same as those described in ref. 1. 99
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Fig. 2. Rain conductivity.
3. Corona discharge and shape of water drops Detailed observations on the deformation of tap-water drops under different voltages have been described in an earlier paper [1] by the authors. Here we shall show the drop shape just before, after and during the corona discharge
3)
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b_T1
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243
period together with the corresponding oscilloscope traces of corona current to understand the relationship among the deformation process o f water drops, the corona-current waveform and corona mechanism. 3.1 Distilled water 3.1.1 Positive polarity Corona current at the onset voltage consists of a single pulse. As the voltage increases about 1% b e y o n d the onset value, the second pulse group which includes a few pulses appears several milliseconds after the first single pulse, Fig. 3(d). Figure 3(a) illustrates the drop profile at the m o m e n t of the corona start at the corona onset voltage. Figures 3(t~1) to (b-4) were taken 5.05 ms before the appearance of the first pulse of Fig. 3(d), 1.5 ~s and 500/~s after that, and 1.5 ~s after the second pulse appearance, respectively. These instants are shown in the current osciUogram by arrows. A similar representation is followed in the other photographs, in which the arrows corresponding to the first and second pulses always denote 1.5/~s after their appearance. As seen in Figs. 3(a), (b-2) and (d), the first corona pulse appears when the head of the pendant drop separates from the neck and its tip just changes from spheroid to cone. Although the distortion ratio of the main drop at the m o m e n t of its separation from the neck is about 1.9 (Taylor's distortion ratio) at the corona onset voltage [1], the surface instability and corona discharge on the main drop develop during the first period of the drop's oscillation after reaching the Taylor's distortion ratio. The conical shape is maintained for about 400/~s and gradually changes to a rounded tip as the first corona pulse decays almost to zero, Fig. 3(c-2). If the second pulse group follows the first pulse, the sharp tip develops on the tip of the neck instead of the main drop as seen in Fig. 3-
(c-3). As the voltage is raised to Mode II mentioned in ref. 1, a succession of a few streamer pulses followed by a current loop with a large rise time appears and its total duration corresponds to the interval from the instant of the occurrence of the conical tip on the pendant drop to the m o m e n t of the separation of the drop from the conductor, Figs. 4(a) and (b). An oscillogram taken with the smaller measuring resistor shows that the amplitude of the streamer pulses is at least 15 times greater than that of the following current loop, Fig. 4(c). As seen in Fig. 5, which shows the events during the first period in the corona current of Fig. 4(b), a fine filament elongates and disrupts into very fine droplets in each period of the current loops. Its elongation velocity increases with Fig. 3. Positive drops of distilled water and current oscillograms around the corona o n s e t voltage. (a) Drop profile in the case of the appearance of a single corona pulse, (E c = 4.8 kV/cm). (b-1)--(b-4) Drop profiles in the case of t h e appearance o f the pulse group following the first single pulse. Photographs w e r e t a k e n at the t i m e d e n o t e d by arrows in (d), ( E c = 4.86 kV/cm). (c-1)--(c-3) Main drops and a part of necks o f (b-2)--(b-4). (d) R = 220 k ~ , 0.909 ~A/div., 5 ms/div., (E c = 4.86 kV/cm). (e) R ffi 5.1 k ~ , 9.80 ~A/div., 1 ms/div., ( E c = 4.86 kV/cm). (f) The first pulse in (e). R = 5.1 k ~ , 9.80 ~A/div., 50/zs/div.
(a-2)
(a-4)
(a-6)
-7)
Fig. 4. Positive drops of distilled water and current oscillograms, (E c = 5.86 k V / c m ) . (b) R = 220 kG, 0 . 9 0 9 lzA/div., 10 ms/div. ( c ) R = 5.1 krL, 19.6 pA/div., 5 ms/div.
(c
(b)
)
(a-3)
(a-s) (a-7)
Fig. 5. Behavior of the drop during the period of the first current loop in Fig. 4(b). (b) R = 5.1 kft, 19.6 ~A/div., 1 ms/div.
(b)
(a-1)(a-2) (a-4) I
t
246 the time and reaches a b o u t 9.3 m/s at the instant of its disruption. When the tip of the pendant drop becomes round, the current decays almost to zero in spite of the existence of the shooting droplets as seen in Fig. 5(a-7). This fact means that the current loop may be due to the pulse-less corona around the filament, b u t n o t to shooting charged droplets. When the voltage increases further, the pendant drop becomes so slender that the distinction between the main drop and the neck is not recognized. The steady increasing amplitude with no-streamer pulse follows the current with the same pattern as Fig. 5(b), Fig. 6. The development of the filament and the shooting of the fine droplets can also be observed during the periods of the pulseless current. When the field strength on the c o n d u c t o r is raised to 14.6 kV/cm (72 kV), the filament from the drop-tip is so thin that its b o t t o m appears to be smoke-like, dispersing into the conical region. The photographs with the micro-flash show that the end part of the filament vibrates laterally and disrupts into very fine droplets with approximately the same radius in the smoke region, Fig. 7(a). Corona discharge emits light along the straight filament and is in the form of pulseless corona, Fig. 7(b). As the water supply is stopped, the length of the filament becomes shorter due to the water ejection and finally becomes a conical tip with glow corona The voltage range resulting in the atomization depends on the flow rate and the conductivity of supply water. The m a x i m u m range is from about 70 kV to 100 kV for distilled water and becomes narrower for water with larger conductivity. The situation of Fig. 7(a) is very stable and is maintained continuously for hours if the flow rate of the supplied water is adjusted to a suitable value. Beyond this voltage range, the dripping manner of water drops and the corona discharge mechanism essentially return to those as shown in Fig. 6. It is concluded that, for the positive discharge on distilled water, the streamer pulses start from the conical tip and a pulseless corona grows on or around the water filament elongating from the drop-tip. This may be due to the large series resistance (the large voltage drop) along the filament and the large field gradient on its surface. Moreover, charges on the dispersed fine droplets may quench the development of the streamer. The duration of the current flow depends on the mechanical parameters relating to the drop deformation, that is, it is the same as the time interval from the formation of the conical tip to the separation of the main drop from the conductor. In the case of highly resistive water, two deformation processes of the drop were recognized as pointed out by Buraev and Vereshchagin [16]; (1) the ejection of a fine water filament from the tip of the pendant drop which deforms slowly, and (2) the necking of the base part of the pendant drop. Especially under a certain condition as shown in Fig. 7, the ejection of the filament can be maintained w i t h o u t any deformation of the pendant drop.
t
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Fig. 6. Positive drops of distilled water and current oscillograrns, ( E c ffi 9.12 kV/cm). (b) R = 220 k ~ , 2.27 ~A/div., 10 ms/div. ( c ) R = 5.1 k ~ , 39.2 ~A/div., 10 ms/div.
(a-l) (a-2) (a-3)(a-4)(a-5)(a~6)[(a-t8) (b) (a-7)
248
(a) Fig. 7. A t o m i z a t i o n o f distilled w a t e r and current oscillogram, ( E c = 14.6 kV/cm). (b) R = 5.1 kG, 9.8 uA/div., 10 ms/div.
3.1.2 Negative polarity The deformation process of water drops around the corona starting time for the negative polarity is essentially the same as for the positive one at the corona onset voltage, Fig. 8. However, the corona pulse shape is quite different from the positive one; the current pulse in the negative case consists of successive Trichel pulses with a small amplitude, in contrast to the first single pulse for the positive case. The conical shape on the main drop is maintained for a longer time than in the positive case. This may be described as follows. The remaining space charge due to the negative pulse or the discharge of the negative main drop is smaller than that in the positive case, hence the many Trichel pulses appear successively and the conical tip is maintained for a long time. Fig. 8. Negative drops of distilled w a t e r and current oscillograms around the corona onset voltage. (a) D r o p profile in the case of t h e appearance o f a pulse group, ( E c = --4.86 kV/cm). (b-1)--(b-4) D r o p profiles in the case of the appearance o f t w o pulse groups, (E c = --4.94 k V / c m ) . (c-1)--(c-3) Main drops and a part of necks o f Figs. (b-2)--(b-4). (d) R ffi 220 k ~ , 0.909 , A / d i v . , 5 ms/div., ( E c = --4.94 kV/cm). (e) R ffi 5.1 k ~ , 3.92 ~A/div., 1 ms/div. (f) The first pulse group in (e). R = 5.1 kG, 3.92 , A / d i v . , 100 ,s/div.
Fig. 10. Behavior of the drop during the period of the i'L~t current loop in Fig. 9(b).
(d)
Fig. 9. Negative drops o f distilled water and current oscillograrns, ( E c ffi --5.86 kV/cm). (c) R = 5.1 k ~ , 9.80 ~A/div., 5 ms/div. (d) R = 5.1 kG, 9.80 ~A/div., 100 ~s/div.
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When the voltage is raised, the oscillograms taken with the larger measuring resistor show a succession of loop currents, Fig. 9(b) and its duration corresponds to the interval from the instant of the occurrence of the pendant drop with conical tip to the moment of its separation from the conductor, being the same as those in the case of positive polarity. Although the relationship between drop deformation and current duration can be seen in Figs. 9(a) and (b), the current wave-form in this figure is markedly smoothed by the measuring circuit with the large time constant. Details of this relationship during the first loop was studied with a smaller measuring resistor, Figs. 9(c) and (d) and Fig. 10. One loop current shown in Fig. 9 consists of groups of succeeding Trichel pulses which have a large amplitude during the earlier period of the loop. With the time elapse, the amplitude of the first pulse in each group becomes smaller and the frequency of its appearance increases. A steady current, underlying the small Trichel pulses, and the frequency of the Trichel pulse increase with time and eventually the current becomes almost pulse-less and then declines to zero. In each group the initial pulse is the biggest and the following pulses become regularly smaller becoming about 1/5 times as big as the initial one, Fig. 9(d). The tip of the pendant drop during the period of large Trichel-pulse appearance is conical and the filament from the conical tip slightly elongates with the increase in the steady current as seen in Fig. 10(a-4). As the current decreases, though the total length of the pendant drop elongates with time, the retrogression of the filament occurs and leads to the round tip on the drop. As the voltage is further increased, the pendant drop becomes slender, as in the positive case, but the tip shape is always conical. The relationship between the current duration and the deformation process of the drop and the detailed characteristics of the current pulse are the same as those observed at a lower voltage, Fig. 11. The discharge mechanism in a group of the succeeding Trichel pulses may be following the process as described by Loeb [17] ; the large initial Trichel pulses from the conical tip develop into the gap without the space charge and create a large space charge with a low mobility which reduces the field near the drop tip so that while it moves across the gap it produces succeeding Trichel pulses with a small amplitude. As the voltage is raised to about 75 kV, a filament vibrating laterally develops from the drop tip for a short time, Fig. 12, but it is very unstable and the dripping fashion of the drops returns to those shown in Fig. 11. Vital corona appears on the base part of the filament together with corona along and at the end of the filament. These coronas are accompanied by the steady current with small pulses superimposed, Fig. 12(b). 3. 2 Tap water The duration of the current flow which corresponds to the interval from the formation of the conical tip of the pendant drop to its separation from the conductor is the same as that for distilled water. Therefore other characteristics will he described in the following section.
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Fig. 11. Negative drops o f distilled w a t e r and current oscillograms, (Ec = --9.12 k V / c m ) . (b) R = 220 k ~ , 2.27 uA/div., 10 ms/div. (c) R = 5.1 k ~ , 9.80 uA/div., 10 ms/div.
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(a) Fig. 12. Dispersion of the distilled water drops from the laterally vibrating filament and current oscillogram, (E c = --14.6 kV/cm). (b) R = 5.1 k ~ , 9.80 uA/div., 1 ms/div.
3. 2.1 Positive polarity No great differences between the characteristics of the current pulses and the drop deformation of tap and distilled waters are recognizable at the corona onset voltage except that the time for the formation of the rounded tip on the main drop is shorter than that for distilled water, Fig. 13. As the voltage is increased, the streamer pulses appear continuously with almost the same interval during the whole current period and the tip shape during the pulses is always conical, Fig. 14. That is, there is no loop current observed. The amplitude of the first streamer pulse is about three times that of the succeeding streamer pulses, Fig. 14(d). This is due to the development of the first streamer into the clean space without the space charges. Further increase in the voltage results in a slender pendant drop but its radius is larger than that for distilled water as compared in Figs. 6 and 15. The flash photographs of the pendant drops show the conical tip during the streamer pulse and the elongation of the filament during the pulse-less current. When the field strength on the conductor reaches 14.2 kV/cm, which corresponds
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Fig. 13. Positive drops of tap water and current oscillograms around the corona onset voltage. The shape of main drop tip in Figs. (b-2) and (b-3) is approximately the same as that in Figs. 3(c-1) and (c-2), respectively. (a) Drop profile in the case of the appearance of a single corona pulse, ( E c = 4.80 kV/cm). (b-1)--(b-3) Drop profiles in the case of the appearance of the pulse group following the first single pulse, (E c = 4.86 kV/cm). (c) R = 220 k ~ , 0.909 ~A/div., 5 ms/div., (E c = 4.86 kV/cm). (d) 5.1 k ~ , 19.6 ~A/div., 1 rns/div., (E c = 4.86 kV/cm). (e) The first pulse in (c). R = 5.1 k ~ , 19.6 ~A/div., 100 ~s/div.
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(c)
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Fig. 14. Positive drops o f tap water and current oscillograms, (E c ffi 5.86 kV/cm). (b) R = 220 kG, 0.909 ~A/div., 10 ms/div. (c) R = 5.1 kG, 19.6 ~A/div., 5 ms/div. (d) R = 5.1 kG, 98.0 ~A/div., 1 ms/div.
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Fig. 15. Positive drops o f tap water and current oscillograms, (E c = 9.12 k V / c m ) . (b) R = 220 kr~, 2.27 ~A/div., 10 ms/div. (c) R ffi 5.1 kG, 39.2 ~A/div., 10 ms/div. (d) First sweep of a part d e n o t e d as (a-6) in Fig. (b). R = 5.1 kr~, 9.80 ~A/div., 2 ms/div.
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Fig. 16. Negative drops of tap water and current oscillograms around the corona onset voltage. (a) Drop profile in the case of the appearance of a pulse group, ( E c = --4.86 kV/cm). (b-1)---(b-3) Drop profiles in the case of the appearance of two pulse groups, ( E c --4.94 kV/cm). (c) R = 220 kfz, 0.909/~A/div., 5 ms/div. (d) R = 5.1 kft, 9.80 ~A/div., I ms/div. (e) The first pulse group in (d). R = 5.1 kEz, 9.80 ~A/div., 100 us/div.
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Fig. 17. Negative drops of tap water and current oseillograrns, (E c = --5.86 kV/em). (b) R = 220 k~t, 0.909 uA/div., 10 ms/div. (c) R = 5.1 kfz, 9.80 uA/div., 5 ms/div. (d) R = 5.1 kfz, 9.80 uA/div., 100 ~s/div.
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Fig. 18. Negative drops of tap water and current oscillograms (E c = - - 9 . 1 2 k V / c m ) . (b) R = 220 krz, 2.27 ~A/div., 10 ms/div. (c) R = 5.1 kr~, 19.6 ~A/div., 10 ms/div.
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to the point C denoted in ref. 1, the short filament from the tip disrupts into fine droplets but does n o t become smoke-like as in Fig. 7. The corona current at this voltage has a steady c o m p o n e n t with small pulses superimposed. Beyond the point C, the current becomes pulsed again.
3. 2. 2 Negative polarity Corona current at the onset voltage consists of a few Trichel pulses. As the voltage increases about 1% b e y o n d the onset value, the second pulse group, which includes a few pulses, appears about several milliseconds after the first pulse group, Fig. 16. The duration of the first pulse group, which corresponds with the time maintaining the conical drop-tip, is shorter than that for distilled water. The increase in the applied voltage results in a continuous train of Trichel pulses during the period from the formation of the conical drop-tip to the drop disruption, Figs. 17 and 18. T h e detailed characteristics of the pulses may be the same as those for distilled water; m a n y elements, each of which consists of the large first Trichel pulse and the succeeding small Trichel pulses, form the pulse train, Fig. 17(d). The filament does not appear from the drop tip in the voltage range of the present experiment. It may be suggested from these pulse shapes that the essential difference in the corona discharge mechanism does n o t exist between tap and distilled water in spite of the expected change of the secondary coefficient of the electron emission with the water conductivity. The amplitude of the current pulse at a given voltage for tap water is larger than that for distilled water. 4. Modes o f corona discharge and specific charge We have defined modes of corona discharge and specific charge on tap-water drops as follows: Corona Modes: No-Corona; Crackling Corona -- corona occurring only at the m o m e n t of the removal of water drops from the conductor and producing a crackling sound; Hissing Corona -- corona accompanied by a continuous hissing sound from the hanging water drop with a conical tip; Wire Corona -corona discharge from the conductor surface. Specific Charge Modes: Mode I -- the region where the specific charge gradually increases with the field strength below the corona onset voltage; Mode II -- the region at higher field strength where the specific charge has a smaller value than the m a x i m u m specific charge in Mode I; Mode III -- the region where the specific charge increases monotonically b e y o n d the field strength in Mode II; Mode IV -- the region where the specific charge decreases with an increase in the field strength beyond Mode III. As the conductivity of the drops decreases, one more mode of corona discharge and specific charge should be added for both polarities. For example, pulse-less corona and a very high peak of specific charge are developed in the boundary region between Modes III and IV under a certain flow rate of distilled water with the positive polarity. In this region, water from the
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conductor is always atomized and corona sound is very calm. This is termed Pulse~Less Corona (PLC) and Mode V for the specific charge for the positive polarity, Fig. 19(a). The region of the field strength on the conductor for Mode V becomes wider with an increase in the water resistivity and ranges from 14 kV/cm to 20 kV/cm for distilled water under a desirable water flowrate. In the experiment for the negative conductor with water having a low conductivity, the unstable filament appears and results in a sudden increase, noted by D in Fig. 19(b) in the specific charge. But the atomization of water does not appear in a stable way. The region where the filament develops, as shown in Fig. 12(a), is termed Mode V for the negative polarity. The field strength at the point D rises with an increase in the water conductivity. In general, the specific charge in Mode I and II is almost independent of the water conductivity for both polarities but the corona sound at a given voltage decreases gradually with the water conductivity because the pulse height and frequency of the corona current decrease with the conductivity as mentioned in the Section 3.2.2. The modes of specific charge for distilled and tap waters are summarized as Fig. 20. C log M
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Fig. 20. Mode of specific charge for distilled and tap water drops.
5. Specific charge and radius o f water drops Since the characteristics of the specific charge and the drop size for tap water have been reported in the earlier paper, those for distilled water are described mainly as the limit of water with a high resistivity.
5.1 Specific charge At a low applied voltage, the specific charge on tap water drops is constant in the region 10-3--10 -1 g/s of water supply and decreases with the flow rate at a higher voltage [1]. On the other hand, for highly resistive water and a
264
positive polarity there is seen to be a maximum at a certain flow rate in the voltage region corresponding to Mode V (black circles in Fig. 21). The flow rate at the maximum specific charge is determined from the balance condition between the amounts of water flowing into and out of the pendant drop; the former depends on the water-supply flow rate and the latter on the field strength on the conductor and the water conductivity. That is, to the left of 5r (C/kg)
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Fig. 21. Effect of the flow rate of supplied water on specific charge as a parameter of the applied voltage. (a) Positive conductor. (b) Negative conductor.
265 the maximum specific charge in Fig. 21(a) the water-supply flow rate is less than the rate of the outflow and then the length of the filament from the drop tip as shown in Fig. 7 becomes shorter than that at the maximum specific charge. To the right of the maximum specific charge the surface instability on the base part of the slender pendant drop occurs more easily on the drop tip, therefore the filament does not grow. This results in larger droplets and a reduction of the specific charge. With an increase in the water conductivity, the flow rate corresponding to the peak specific charge shifts gradually to the low value, Fig. 22(a). For negative polarity, the specific charge in the high field region changes in a complicated way with the flow rate, Fig. 22(b), because the development of the unstable filament described in the Section 3.1.2 depends on the applied voltage, the water conductivity and the flow rate. ~dl~ttl led water)
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266
The plots in Fig. 23 show the specific charge taken under a given flow rate of water. Although the effect of the water conductivity is small below about 9 kV/cm, it becomes significant in the higher field strength on the conductor. The vertical bars shown in Fig. 23(a) indicate the maximum specific charge obtained by changing the applied voltage and the water-supply flow rate. Even if the filament as Fig. 7(a) becomes unstable for one second during the measuring time of about 30 minutes, this leads to a remarkable reduction of the specific charge. Hence the measured values in Mode V disperse as in Fig. 19(a) and the vertical bars as in Fig. 23(a). The solid and dotted lines in Fig. 23(a) indicate respectively whether the stable atomization appears or not. It can be seen clearly that the specific charge of the order of 5 × 10 -4 C/kg or more results from the stable atomization. The ejection of unstable smoke-like mist and fine droplets in the region of 50--267 ~S/cm for the positive polarity is markedly influenced by humidity in the ambient air. In room air with the relative humidity under 30% they rarely grow and the streamer corona develops easily from the water drop, but both of them always appear in humid air which is produced by boiling water under the outer mesh cylinder. This may be due to the suppression effect of the steam on the (a)
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268
development of the streamer corona. It has been suggested that the ejection of the filament in highly conductive liquids is prevented by the streamer corona [18]. From the present experimental results, it is argued that the streamer corona prevents the growth of the filament b u t the pulse-less corona (glow corona) does n o t affect the atomization of liquids and that the ejection manner of water drops is related to meteorological conditions since the corona mechanism depends on the properties of the ambient gas. For a negative polarity, since the production of the unstable filament leads to a large specific charge and its starting field strength increases with the water conductivity as described in Section 4, the specific charge decreases suddenly with an increase in the water conductivity at a given field strength and the value of the water conductivity for the sudden change shifts to the larger value with the field strength, Fig. 23(b).
5. 2 Radius o f water drop The radius of a mother drop and its satellites was measured by the same methods described in ref. 1, and a cumulative distribution of the radius was approximated by the logarithmic-normal distribution (Fig. 24). A cumulative value of 50% for distilled water and tap water is illustrated b y a circle or a cross and a d o t t e d line, and a range of a cumulative value from 5% to 95% by a vertical line, respectively. The general tendency with respect to the field strength on the c o n d u c t o r is nearly the same b u t in the range from the high field region in Mode III to Mode IV the radius decreases with an increase in the water resistivity. For a positive polarity, the maximum difference between them reaches a factor of 10 or more, Fig. 24(a). This reduction of the radius with water resistivity closely relates to the increase in the specific charge as expected from the condition for the surface instability on water drops. 6. Conclusions (1) For high resistivity water, there is a field range to cause the atomization of water and the pulse-less and calm corona. This region is termed 'Pulse-Less Corona' (PLC) for corona m o d e and Mode V for the specific charge. (2) At the corona onset voltage, corona discharge starts from the drop-tip which separates from the neck. While, at the higher voltage, corona discharge grows on the conical tip of the pendant drop. (3) Corona mechanism on the positive water drops depends on the water conductivity. The pulse-less corona occurs around the filament elongating from the pendant drop with a low conductivity over a wide range of the field strength on the conductor. On the other hand, with highly conductive water the streamer corona starts always from the conical drop-tip. (4) In the case of negative polarity, the effect of the conductivity on corona discharge appears as a change of the pulse amplitude. The amplitude of Trichel pulses increases with the water conductivity under a given voltage. (5) Duration of the corona current is closely related to the mechanical
269 parameters determining the deformation of water drops and coincides with t h e t i m e i n t e r v a l f r o m t h e f o r m a t i o n o f t h e conical tip t o t h e s e p a r a t i o n o f t h e d r o p f r o m t h e c o n d u c t o r ; t h a t is, t h e basic p r o c e s s e s o n i o n i z a t i o n , dei o n i z a t i o n a n d m o t i o n o f c h a r g e d p a r t i c l e s in air a n d c o r o n a m e c h a n i s m w o u l d
not be so important. (6) There is a marked polarity effect on the ejection manner of high resistivity water under high applied voltage. The stable atomization and the elongation of the fine filament occur easily for the positive drops with smaller conductivity than about 20/~S/cm under a desirable flow rate of the supplied water. The atomization and the formation of very fine droplets are affected by the meteorological conditions too because the mechanism of corona discharge depends on the properties of the ambient gas. (7) The m a x i m u m specific charge reaches 5 X 10 -3 C/kg and the conditions for its occurrence depend on the field strength on the conductor, the watersupply flow rate and the water conductivity. (8) In M o d e V or the higher field region, the drop size decreases with the water conductivity under a given voltage. For distilledwater it becomes one tenth or less than that of tap water and the m i n i m u m radius of the droplets reaches 10 ~ m in M o d e V. Acknowledgements T h e a u t h o r s wish t o t h a n k Mr. S a k a i f o r t e c h n i c a l help. This w o r k was s u p p o r t e d in p a r t b y a Grant~in-Aid f o r Scientific R e s e a r c h f r o m t h e M i n i s t r y o f E d u c a t i o n , Science a n d C u l t u r e , J a p a n .
References 1 M. Hara, S. Ishibe and M. Akazaki, Corona discharge and electrical charge on water drops dripping from d.c. transmission conductors -- An experimental study in laboratory, J. Electrostatics, 6 (1979) 235. 2 Y. Sato, Y. Tsunoda and K. Arai, Corona pulses from a water drop on a cylindrical conductor surface, JIEE Jpn., 81 (1961) 1606. 3 L. Boulet and B.J. Jakubczyk, AC corona in foul weather: I - - Above freezing point, IEEE Trans. Power Appar. Syst., PAS-83 (1964) 508. 4 Y. Tsunoda and K. Arai, Corona discharge from water drops on a cylindrical conductor surface, JIEE Jpn., 84 (1964) 1430. 5 M. Akazaki, Corona discharge from water drops on smooth conductors under high direct voltage, IEEE Trana Power Appar. Syst., PAS-84 (1965) 1. 6 M. Akazaki and S. Lin, Corona discharge from water drops dripping onto inner conductor of co-axial cylinder, JIEE Jpn., 88 (1968) 909. 7 H.H. Neweli, T.W. Liao and F.W. Waburton, Corona and RI caused by particles on or near EHV conductors: II -- Foul weather, IEEE Trans. Power Appar. Syst., PAS-87 (1968) 911. 8 M. Akazaki and M. Hara, Corona discharge from water drops passing near a conductor, IEEE PES Summer Meeting, Portland, 71CP633 (1971).
270 9 C.L. Phan and A. Mansiaux, Corona and charge transfer on water drops in proximity of a conductor, IEEE P E S S u m m e r Meeting, San Francisco, A75-565-2 (1975). 10 F. Inna, G.L. Wilson and J.D. Bosack, Spectral characteristicsof acoustic noise from metallic protrusions and water drops in high electricfield,IEEE PES Winter Meeting, N e w York, C73-164-1 (1973). 11 J.E. Houburg and J.R. Melcher, Current driven, corona-terminated water jets as sources of charged droplets and audible noise, IEEE PES Winter Meeting, N e w York, C73-165-8 (1973). 12 IEEE Committee Report, Rainfall Resistivities,IEEE Trans. Power Appar. Syst., PAS-83 (1964) 520. 13 JIEE Committee, Standard of the Japanese ElectricalCommittee-I (in Japanese), No. 112 (1975) 44. 14 JIEE Committee, Standard of the Japanese ElectricalCommittee-II (in Japanese), No. 48 (1976) 5. 15 V.G. Drozin, The electricaldispersion of liquids as aerosols, J. Colloid Sci., 10 (1955) 158. 16 T.K. Burayev and I.P. Vereshchagin, Physical processes during electrostatic atomization of liquids, Fluid Mech. Soy. Res., 1 (1972) 56. 17 L.B. Loeb, personal communication, May, 1971. 18 H. Schene, Untersuchungen fiber die electrostatische Zerstaubbarkeit yon Lacken (2), Ind. Lack. Betrieb., 34 (1966) 471.
239
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
E L E C T R I C A L C O R O N A AND SPECIFIC C H A R G E ON WATER DROPS FROM A CYLINDRICAL C O N D U C T O R WITH HIGH D.C. VOLTAGE
MASANORI HARA*, SHINJI ISHIBE**, SATORU SUMIYOSHITANI* and MASANORI AKAZAKI**
*Department of Electrical Engineering, Faculty of Engineering, Kyushu University, Fukuoka, 812 (Japan) **Department of Energy Conversion Engineering, Graduate School of Engineering Sciences, Kyushu University, Fukuoka, 812 (Japan) (Received July 6, 1979; accepted September 11, 1979)
Summary The relationship among the waveform of corona current, the dripping manner of water drops from a conductor with high d.c. voltage, the corona discharge mechanism, the specific charge (the ratio of charge to mass of the drop) and the radius of water drops is investigated for a wide range of the drop conductivity, whilst the corona discharge mechanism on the drop is discussed. In addition to the corona and the specific charge modes described in a previous paper [1 ], Pulse-Less Corona (PLC) and the atomization of water is newly determined for highly resistive water. Moreover it is confirmed that with distilled water the maximum specific charge could reach 5 x 10 -3 C/kg and the duration of the current flowing is closely related to the mechanical parameters determinimz the deformation process of water drops.
1. Introduction
In foul weather, water drops hanging from d.c. transmission conductors cause an increase in corona activity and behave b o t h as sources of various charge carriers, audible noise and electromagnetic interference and sinks of energy. Several works on corona discharge on water drops on or near d.c. transmission conductors have been carried o u t to reduce or eliminate undesirable corona discharges and to estimate the level of RI (Radio Interference) or TVI (TV Interference) [2--11]. In connection with charge carriers from d.c. transmission conductors in foul weather, some experimental results were presented for corona discharge and the specific charge on water drops dripping from smooth conductors and ACSR conductors in a recent paper [1]. In these studies, however, the specific conductivity of the drop was fixed or unknown. On the other hand, the rain conductivity could be as low as 2 and as high as 2000/~S/cm [12--14]. Moreover, the behavior of liquid drops in a high d.c. field is strongly affected n o t only b y the size of the drop and the surface tension b u t also b y the liquid conductivity. F o r example, liquids with specific
240 conductivity lower than 10 -7 p S / c m and higher than 10 pS/cm cannot atomize from a fine glass capillary, while outside this range the atomization occurs easily [15, 16]. Therefore, it is considered that the dripping manner of water drops from d.c. transmission conductors, corona discharge mechanism and the specific charge on the drops may be markedly changed b y the rain conductivity. The purpose of this work is to investigate the effects of the water-drop conductivity on corona discharge and characteristics on the specific charge, the size and the dripping manner of drops and the relationship among the corona mechanism, the corona current waveform and the dripping manner of drops. The study described in the previous paper [1] is extended. 2. Experimental set-up The experimental set-up used is basically the same as that of the previous paper [1], Fig. 1. The rain conductivity depends strongly on the rate of rain-
~ W e tso maan friu dlppyl
Oue tcr
y
l
i
~
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toeelcrtomee tr Fig. 1. Experimental set-up. R: resiztor for current measurement, 5.1 k12 or 29.0 ir~. Inner conductor: smooth cylinder of 3.2 cm ¢ (2rc). Outer mesh cylinder: 70 cm ¢ diameter and 150 cm long.
241
fall, the passage of time from the start of a rainfall and the measuring location; that is, in an inland area or in a coastal area. Figure 2 illustrates the probabili~ ties of the rain conductivity measured in USA [12] and Japan [13, 14]. The 50% cumulative frequency appears at about 40 pS/cm for both countries. The desired water with specific conductivity o in the range of 2.4--267 pS/cm at 20°0, which covers the most probable part in the distribution, was obtained by mixing distilled water and tap water. Resistors of 220 k~ and 5.1 k~ were used for the measurement of small corona current and pulses. Those correspond to 52.4 ps and 1.2 ps time constant, respectively, for the measuring circuit, because the parallel stray capacitance of the outer cylinder is 238 pF. Then the recorded peak value of individual corona pulses on an oscilloscope (Tektronix 7633 or Iwatsu 6200) may actually be smaller than the expected value. However, the ability of the circuit with the smaller resistor in identifying the individual Trichel pulses was confirmed using the inner conductor with a metal protrusion. To obtain the relationship between the current waveform and the pendant drop shape, the micro-flash lamp was normally triggered by the signal from the oscilloscope which produced the desired signal delay. The measuring methods of the specific charge and the size of water drops are the same as those described in ref. 1. 99
(%)
_ D i s t i l l e d water • L
Tap water
~7o ~-60 ~30
5
I
I
I
I
J
l
i
I
I
2
5
10
2
5
102
2
5
103
Rain conductivity
(lJS/cm)
Fig. 2. Rain conductivity.
3. Corona discharge and shape of water drops Detailed observations on the deformation of tap-water drops under different voltages have been described in an earlier paper [1] by the authors. Here we shall show the drop shape just before, after and during the corona discharge
3)
~-4)
(b.
b_T1
t',C,
243
period together with the corresponding oscilloscope traces of corona current to understand the relationship among the deformation process o f water drops, the corona-current waveform and corona mechanism. 3.1 Distilled water 3.1.1 Positive polarity Corona current at the onset voltage consists of a single pulse. As the voltage increases about 1% b e y o n d the onset value, the second pulse group which includes a few pulses appears several milliseconds after the first single pulse, Fig. 3(d). Figure 3(a) illustrates the drop profile at the m o m e n t of the corona start at the corona onset voltage. Figures 3(t~1) to (b-4) were taken 5.05 ms before the appearance of the first pulse of Fig. 3(d), 1.5 ~s and 500/~s after that, and 1.5 ~s after the second pulse appearance, respectively. These instants are shown in the current osciUogram by arrows. A similar representation is followed in the other photographs, in which the arrows corresponding to the first and second pulses always denote 1.5/~s after their appearance. As seen in Figs. 3(a), (b-2) and (d), the first corona pulse appears when the head of the pendant drop separates from the neck and its tip just changes from spheroid to cone. Although the distortion ratio of the main drop at the m o m e n t of its separation from the neck is about 1.9 (Taylor's distortion ratio) at the corona onset voltage [1], the surface instability and corona discharge on the main drop develop during the first period of the drop's oscillation after reaching the Taylor's distortion ratio. The conical shape is maintained for about 400/~s and gradually changes to a rounded tip as the first corona pulse decays almost to zero, Fig. 3(c-2). If the second pulse group follows the first pulse, the sharp tip develops on the tip of the neck instead of the main drop as seen in Fig. 3-
(c-3). As the voltage is raised to Mode II mentioned in ref. 1, a succession of a few streamer pulses followed by a current loop with a large rise time appears and its total duration corresponds to the interval from the instant of the occurrence of the conical tip on the pendant drop to the m o m e n t of the separation of the drop from the conductor, Figs. 4(a) and (b). An oscillogram taken with the smaller measuring resistor shows that the amplitude of the streamer pulses is at least 15 times greater than that of the following current loop, Fig. 4(c). As seen in Fig. 5, which shows the events during the first period in the corona current of Fig. 4(b), a fine filament elongates and disrupts into very fine droplets in each period of the current loops. Its elongation velocity increases with Fig. 3. Positive drops of distilled water and current oscillograms around the corona o n s e t voltage. (a) Drop profile in the case of the appearance of a single corona pulse, (E c = 4.8 kV/cm). (b-1)--(b-4) Drop profiles in the case of t h e appearance o f the pulse group following the first single pulse. Photographs w e r e t a k e n at the t i m e d e n o t e d by arrows in (d), ( E c = 4.86 kV/cm). (c-1)--(c-3) Main drops and a part of necks o f (b-2)--(b-4). (d) R = 220 k ~ , 0.909 ~A/div., 5 ms/div., (E c = 4.86 kV/cm). (e) R ffi 5.1 k ~ , 9.80 ~A/div., 1 ms/div., ( E c = 4.86 kV/cm). (f) The first pulse in (e). R = 5.1 k ~ , 9.80 ~A/div., 50/zs/div.
(a-2)
(a-4)
(a-6)
-7)
Fig. 4. Positive drops of distilled water and current oscillograms, (E c = 5.86 k V / c m ) . (b) R = 220 kG, 0 . 9 0 9 lzA/div., 10 ms/div. ( c ) R = 5.1 krL, 19.6 pA/div., 5 ms/div.
(c
(b)
)
(a-3)
(a-s) (a-7)
Fig. 5. Behavior of the drop during the period of the first current loop in Fig. 4(b). (b) R = 5.1 kft, 19.6 ~A/div., 1 ms/div.
(b)
(a-1)(a-2) (a-4) I
t
246 the time and reaches a b o u t 9.3 m/s at the instant of its disruption. When the tip of the pendant drop becomes round, the current decays almost to zero in spite of the existence of the shooting droplets as seen in Fig. 5(a-7). This fact means that the current loop may be due to the pulse-less corona around the filament, b u t n o t to shooting charged droplets. When the voltage increases further, the pendant drop becomes so slender that the distinction between the main drop and the neck is not recognized. The steady increasing amplitude with no-streamer pulse follows the current with the same pattern as Fig. 5(b), Fig. 6. The development of the filament and the shooting of the fine droplets can also be observed during the periods of the pulseless current. When the field strength on the c o n d u c t o r is raised to 14.6 kV/cm (72 kV), the filament from the drop-tip is so thin that its b o t t o m appears to be smoke-like, dispersing into the conical region. The photographs with the micro-flash show that the end part of the filament vibrates laterally and disrupts into very fine droplets with approximately the same radius in the smoke region, Fig. 7(a). Corona discharge emits light along the straight filament and is in the form of pulseless corona, Fig. 7(b). As the water supply is stopped, the length of the filament becomes shorter due to the water ejection and finally becomes a conical tip with glow corona The voltage range resulting in the atomization depends on the flow rate and the conductivity of supply water. The m a x i m u m range is from about 70 kV to 100 kV for distilled water and becomes narrower for water with larger conductivity. The situation of Fig. 7(a) is very stable and is maintained continuously for hours if the flow rate of the supplied water is adjusted to a suitable value. Beyond this voltage range, the dripping manner of water drops and the corona discharge mechanism essentially return to those as shown in Fig. 6. It is concluded that, for the positive discharge on distilled water, the streamer pulses start from the conical tip and a pulseless corona grows on or around the water filament elongating from the drop-tip. This may be due to the large series resistance (the large voltage drop) along the filament and the large field gradient on its surface. Moreover, charges on the dispersed fine droplets may quench the development of the streamer. The duration of the current flow depends on the mechanical parameters relating to the drop deformation, that is, it is the same as the time interval from the formation of the conical tip to the separation of the main drop from the conductor. In the case of highly resistive water, two deformation processes of the drop were recognized as pointed out by Buraev and Vereshchagin [16]; (1) the ejection of a fine water filament from the tip of the pendant drop which deforms slowly, and (2) the necking of the base part of the pendant drop. Especially under a certain condition as shown in Fig. 7, the ejection of the filament can be maintained w i t h o u t any deformation of the pendant drop.
t
t
t
t
t
(c)
Fig. 6. Positive drops of distilled water and current oscillograrns, ( E c ffi 9.12 kV/cm). (b) R = 220 k ~ , 2.27 ~A/div., 10 ms/div. ( c ) R = 5.1 k ~ , 39.2 ~A/div., 10 ms/div.
(a-l) (a-2) (a-3)(a-4)(a-5)(a~6)[(a-t8) (b) (a-7)
248
(a) Fig. 7. A t o m i z a t i o n o f distilled w a t e r and current oscillogram, ( E c = 14.6 kV/cm). (b) R = 5.1 kG, 9.8 uA/div., 10 ms/div.
3.1.2 Negative polarity The deformation process of water drops around the corona starting time for the negative polarity is essentially the same as for the positive one at the corona onset voltage, Fig. 8. However, the corona pulse shape is quite different from the positive one; the current pulse in the negative case consists of successive Trichel pulses with a small amplitude, in contrast to the first single pulse for the positive case. The conical shape on the main drop is maintained for a longer time than in the positive case. This may be described as follows. The remaining space charge due to the negative pulse or the discharge of the negative main drop is smaller than that in the positive case, hence the many Trichel pulses appear successively and the conical tip is maintained for a long time. Fig. 8. Negative drops of distilled w a t e r and current oscillograms around the corona onset voltage. (a) D r o p profile in the case of t h e appearance o f a pulse group, ( E c = --4.86 kV/cm). (b-1)--(b-4) D r o p profiles in the case of the appearance o f t w o pulse groups, (E c = --4.94 k V / c m ) . (c-1)--(c-3) Main drops and a part of necks o f Figs. (b-2)--(b-4). (d) R ffi 220 k ~ , 0.909 , A / d i v . , 5 ms/div., ( E c = --4.94 kV/cm). (e) R ffi 5.1 k ~ , 3.92 ~A/div., 1 ms/div. (f) The first pulse group in (e). R = 5.1 kG, 3.92 , A / d i v . , 100 ,s/div.
Fig. 10. Behavior of the drop during the period of the i'L~t current loop in Fig. 9(b).
(d)
Fig. 9. Negative drops o f distilled water and current oscillograrns, ( E c ffi --5.86 kV/cm). (c) R = 5.1 k ~ , 9.80 ~A/div., 5 ms/div. (d) R = 5.1 kG, 9.80 ~A/div., 100 ~s/div.
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251
When the voltage is raised, the oscillograms taken with the larger measuring resistor show a succession of loop currents, Fig. 9(b) and its duration corresponds to the interval from the instant of the occurrence of the pendant drop with conical tip to the moment of its separation from the conductor, being the same as those in the case of positive polarity. Although the relationship between drop deformation and current duration can be seen in Figs. 9(a) and (b), the current wave-form in this figure is markedly smoothed by the measuring circuit with the large time constant. Details of this relationship during the first loop was studied with a smaller measuring resistor, Figs. 9(c) and (d) and Fig. 10. One loop current shown in Fig. 9 consists of groups of succeeding Trichel pulses which have a large amplitude during the earlier period of the loop. With the time elapse, the amplitude of the first pulse in each group becomes smaller and the frequency of its appearance increases. A steady current, underlying the small Trichel pulses, and the frequency of the Trichel pulse increase with time and eventually the current becomes almost pulse-less and then declines to zero. In each group the initial pulse is the biggest and the following pulses become regularly smaller becoming about 1/5 times as big as the initial one, Fig. 9(d). The tip of the pendant drop during the period of large Trichel-pulse appearance is conical and the filament from the conical tip slightly elongates with the increase in the steady current as seen in Fig. 10(a-4). As the current decreases, though the total length of the pendant drop elongates with time, the retrogression of the filament occurs and leads to the round tip on the drop. As the voltage is further increased, the pendant drop becomes slender, as in the positive case, but the tip shape is always conical. The relationship between the current duration and the deformation process of the drop and the detailed characteristics of the current pulse are the same as those observed at a lower voltage, Fig. 11. The discharge mechanism in a group of the succeeding Trichel pulses may be following the process as described by Loeb [17] ; the large initial Trichel pulses from the conical tip develop into the gap without the space charge and create a large space charge with a low mobility which reduces the field near the drop tip so that while it moves across the gap it produces succeeding Trichel pulses with a small amplitude. As the voltage is raised to about 75 kV, a filament vibrating laterally develops from the drop tip for a short time, Fig. 12, but it is very unstable and the dripping fashion of the drops returns to those shown in Fig. 11. Vital corona appears on the base part of the filament together with corona along and at the end of the filament. These coronas are accompanied by the steady current with small pulses superimposed, Fig. 12(b). 3. 2 Tap water The duration of the current flow which corresponds to the interval from the formation of the conical tip of the pendant drop to its separation from the conductor is the same as that for distilled water. Therefore other characteristics will he described in the following section.
252
!
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.4)
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Fig. 11. Negative drops o f distilled w a t e r and current oscillograms, (Ec = --9.12 k V / c m ) . (b) R = 220 k ~ , 2.27 uA/div., 10 ms/div. (c) R = 5.1 k ~ , 9.80 uA/div., 10 ms/div.
{b)
•
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254
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o
(a) Fig. 12. Dispersion of the distilled water drops from the laterally vibrating filament and current oscillogram, (E c = --14.6 kV/cm). (b) R = 5.1 k ~ , 9.80 uA/div., 1 ms/div.
3. 2.1 Positive polarity No great differences between the characteristics of the current pulses and the drop deformation of tap and distilled waters are recognizable at the corona onset voltage except that the time for the formation of the rounded tip on the main drop is shorter than that for distilled water, Fig. 13. As the voltage is increased, the streamer pulses appear continuously with almost the same interval during the whole current period and the tip shape during the pulses is always conical, Fig. 14. That is, there is no loop current observed. The amplitude of the first streamer pulse is about three times that of the succeeding streamer pulses, Fig. 14(d). This is due to the development of the first streamer into the clean space without the space charges. Further increase in the voltage results in a slender pendant drop but its radius is larger than that for distilled water as compared in Figs. 6 and 15. The flash photographs of the pendant drops show the conical tip during the streamer pulse and the elongation of the filament during the pulse-less current. When the field strength on the conductor reaches 14.2 kV/cm, which corresponds
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Fig. 13. Positive drops of tap water and current oscillograms around the corona onset voltage. The shape of main drop tip in Figs. (b-2) and (b-3) is approximately the same as that in Figs. 3(c-1) and (c-2), respectively. (a) Drop profile in the case of the appearance of a single corona pulse, ( E c = 4.80 kV/cm). (b-1)--(b-3) Drop profiles in the case of the appearance of the pulse group following the first single pulse, (E c = 4.86 kV/cm). (c) R = 220 k ~ , 0.909 ~A/div., 5 ms/div., (E c = 4.86 kV/cm). (d) 5.1 k ~ , 19.6 ~A/div., 1 rns/div., (E c = 4.86 kV/cm). (e) The first pulse in (c). R = 5.1 k ~ , 19.6 ~A/div., 100 ~s/div.
(a)
(c)
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Fig. 14. Positive drops o f tap water and current oscillograms, (E c ffi 5.86 kV/cm). (b) R = 220 kG, 0.909 ~A/div., 10 ms/div. (c) R = 5.1 kG, 19.6 ~A/div., 5 ms/div. (d) R = 5.1 kG, 98.0 ~A/div., 1 ms/div.
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Fig. 15. Positive drops o f tap water and current oscillograms, (E c = 9.12 k V / c m ) . (b) R = 220 kr~, 2.27 ~A/div., 10 ms/div. (c) R ffi 5.1 kG, 39.2 ~A/div., 10 ms/div. (d) First sweep of a part d e n o t e d as (a-6) in Fig. (b). R = 5.1 kr~, 9.80 ~A/div., 2 ms/div.
(a-l)
i
e
t~
(b-l)
(b-2)
(b-3) _
Fig. 16. Negative drops of tap water and current oscillograms around the corona onset voltage. (a) Drop profile in the case of the appearance of a pulse group, ( E c = --4.86 kV/cm). (b-1)---(b-3) Drop profiles in the case of the appearance of two pulse groups, ( E c --4.94 kV/cm). (c) R = 220 kfz, 0.909/~A/div., 5 ms/div. (d) R = 5.1 kft, 9.80 ~A/div., I ms/div. (e) The first pulse group in (d). R = 5.1 kEz, 9.80 ~A/div., 100 us/div.
(a)
(c)
=
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Fig. 17. Negative drops of tap water and current oseillograrns, (E c = --5.86 kV/em). (b) R = 220 k~t, 0.909 uA/div., 10 ms/div. (c) R = 5.1 kfz, 9.80 uA/div., 5 ms/div. (d) R = 5.1 kfz, 9.80 uA/div., 100 ~s/div.
(d)
ed~
Fig. 18. Negative drops of tap water and current oscillograms (E c = - - 9 . 1 2 k V / c m ) . (b) R = 220 krz, 2.27 ~A/div., 10 ms/div. (c) R = 5.1 kr~, 19.6 ~A/div., 10 ms/div.
t~ O
261
to the point C denoted in ref. 1, the short filament from the tip disrupts into fine droplets but does n o t become smoke-like as in Fig. 7. The corona current at this voltage has a steady c o m p o n e n t with small pulses superimposed. Beyond the point C, the current becomes pulsed again.
3. 2. 2 Negative polarity Corona current at the onset voltage consists of a few Trichel pulses. As the voltage increases about 1% b e y o n d the onset value, the second pulse group, which includes a few pulses, appears about several milliseconds after the first pulse group, Fig. 16. The duration of the first pulse group, which corresponds with the time maintaining the conical drop-tip, is shorter than that for distilled water. The increase in the applied voltage results in a continuous train of Trichel pulses during the period from the formation of the conical drop-tip to the drop disruption, Figs. 17 and 18. T h e detailed characteristics of the pulses may be the same as those for distilled water; m a n y elements, each of which consists of the large first Trichel pulse and the succeeding small Trichel pulses, form the pulse train, Fig. 17(d). The filament does not appear from the drop tip in the voltage range of the present experiment. It may be suggested from these pulse shapes that the essential difference in the corona discharge mechanism does n o t exist between tap and distilled water in spite of the expected change of the secondary coefficient of the electron emission with the water conductivity. The amplitude of the current pulse at a given voltage for tap water is larger than that for distilled water. 4. Modes o f corona discharge and specific charge We have defined modes of corona discharge and specific charge on tap-water drops as follows: Corona Modes: No-Corona; Crackling Corona -- corona occurring only at the m o m e n t of the removal of water drops from the conductor and producing a crackling sound; Hissing Corona -- corona accompanied by a continuous hissing sound from the hanging water drop with a conical tip; Wire Corona -corona discharge from the conductor surface. Specific Charge Modes: Mode I -- the region where the specific charge gradually increases with the field strength below the corona onset voltage; Mode II -- the region at higher field strength where the specific charge has a smaller value than the m a x i m u m specific charge in Mode I; Mode III -- the region where the specific charge increases monotonically b e y o n d the field strength in Mode II; Mode IV -- the region where the specific charge decreases with an increase in the field strength beyond Mode III. As the conductivity of the drops decreases, one more mode of corona discharge and specific charge should be added for both polarities. For example, pulse-less corona and a very high peak of specific charge are developed in the boundary region between Modes III and IV under a certain flow rate of distilled water with the positive polarity. In this region, water from the
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Fig. 19. Specific charge o f w a t e r d r o p d r i p p i n g f r o m a s m o o t h i n n e r c o n d u c t o r . (a) Pomtlve c o n d u c t o r . ( b ) Negative c o n d u c t o r .
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263
conductor is always atomized and corona sound is very calm. This is termed Pulse~Less Corona (PLC) and Mode V for the specific charge for the positive polarity, Fig. 19(a). The region of the field strength on the conductor for Mode V becomes wider with an increase in the water resistivity and ranges from 14 kV/cm to 20 kV/cm for distilled water under a desirable water flowrate. In the experiment for the negative conductor with water having a low conductivity, the unstable filament appears and results in a sudden increase, noted by D in Fig. 19(b) in the specific charge. But the atomization of water does not appear in a stable way. The region where the filament develops, as shown in Fig. 12(a), is termed Mode V for the negative polarity. The field strength at the point D rises with an increase in the water conductivity. In general, the specific charge in Mode I and II is almost independent of the water conductivity for both polarities but the corona sound at a given voltage decreases gradually with the water conductivity because the pulse height and frequency of the corona current decrease with the conductivity as mentioned in the Section 3.2.2. The modes of specific charge for distilled and tap waters are summarized as Fig. 20. C log M
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Fig. 20. Mode of specific charge for distilled and tap water drops.
5. Specific charge and radius o f water drops Since the characteristics of the specific charge and the drop size for tap water have been reported in the earlier paper, those for distilled water are described mainly as the limit of water with a high resistivity.
5.1 Specific charge At a low applied voltage, the specific charge on tap water drops is constant in the region 10-3--10 -1 g/s of water supply and decreases with the flow rate at a higher voltage [1]. On the other hand, for highly resistive water and a
264
positive polarity there is seen to be a maximum at a certain flow rate in the voltage region corresponding to Mode V (black circles in Fig. 21). The flow rate at the maximum specific charge is determined from the balance condition between the amounts of water flowing into and out of the pendant drop; the former depends on the water-supply flow rate and the latter on the field strength on the conductor and the water conductivity. That is, to the left of 5r (C/kg)
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Fig. 21. Effect of the flow rate of supplied water on specific charge as a parameter of the applied voltage. (a) Positive conductor. (b) Negative conductor.
265 the maximum specific charge in Fig. 21(a) the water-supply flow rate is less than the rate of the outflow and then the length of the filament from the drop tip as shown in Fig. 7 becomes shorter than that at the maximum specific charge. To the right of the maximum specific charge the surface instability on the base part of the slender pendant drop occurs more easily on the drop tip, therefore the filament does not grow. This results in larger droplets and a reduction of the specific charge. With an increase in the water conductivity, the flow rate corresponding to the peak specific charge shifts gradually to the low value, Fig. 22(a). For negative polarity, the specific charge in the high field region changes in a complicated way with the flow rate, Fig. 22(b), because the development of the unstable filament described in the Section 3.1.2 depends on the applied voltage, the water conductivity and the flow rate. ~dl~ttl led water)
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F i g . 2 2 . Effect o f f l o w rate o f supplied water on specific charge as a parameter o f the water conductivity. (a) Positive conductor. (b) Negative conductor.
266
The plots in Fig. 23 show the specific charge taken under a given flow rate of water. Although the effect of the water conductivity is small below about 9 kV/cm, it becomes significant in the higher field strength on the conductor. The vertical bars shown in Fig. 23(a) indicate the maximum specific charge obtained by changing the applied voltage and the water-supply flow rate. Even if the filament as Fig. 7(a) becomes unstable for one second during the measuring time of about 30 minutes, this leads to a remarkable reduction of the specific charge. Hence the measured values in Mode V disperse as in Fig. 19(a) and the vertical bars as in Fig. 23(a). The solid and dotted lines in Fig. 23(a) indicate respectively whether the stable atomization appears or not. It can be seen clearly that the specific charge of the order of 5 × 10 -4 C/kg or more results from the stable atomization. The ejection of unstable smoke-like mist and fine droplets in the region of 50--267 ~S/cm for the positive polarity is markedly influenced by humidity in the ambient air. In room air with the relative humidity under 30% they rarely grow and the streamer corona develops easily from the water drop, but both of them always appear in humid air which is produced by boiling water under the outer mesh cylinder. This may be due to the suppression effect of the steam on the (a)
5 (C/kg)
F = 0.004 g/sec (normally) Fc= 1.6 cm
I i
10-3
5 E
Maximum value obtained by changing of applied voltage and flow rate of supplied water
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Fig. 23. Effect of the water conductivity on specific charge. (a) Positive conductor. (b) Negative conductor.
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.
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Fig. 24. Drop size for distilled and tap waters. (a) Positive conductor. (b) Negative conductor.
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268
development of the streamer corona. It has been suggested that the ejection of the filament in highly conductive liquids is prevented by the streamer corona [18]. From the present experimental results, it is argued that the streamer corona prevents the growth of the filament b u t the pulse-less corona (glow corona) does n o t affect the atomization of liquids and that the ejection manner of water drops is related to meteorological conditions since the corona mechanism depends on the properties of the ambient gas. For a negative polarity, since the production of the unstable filament leads to a large specific charge and its starting field strength increases with the water conductivity as described in Section 4, the specific charge decreases suddenly with an increase in the water conductivity at a given field strength and the value of the water conductivity for the sudden change shifts to the larger value with the field strength, Fig. 23(b).
5. 2 Radius o f water drop The radius of a mother drop and its satellites was measured by the same methods described in ref. 1, and a cumulative distribution of the radius was approximated by the logarithmic-normal distribution (Fig. 24). A cumulative value of 50% for distilled water and tap water is illustrated b y a circle or a cross and a d o t t e d line, and a range of a cumulative value from 5% to 95% by a vertical line, respectively. The general tendency with respect to the field strength on the c o n d u c t o r is nearly the same b u t in the range from the high field region in Mode III to Mode IV the radius decreases with an increase in the water resistivity. For a positive polarity, the maximum difference between them reaches a factor of 10 or more, Fig. 24(a). This reduction of the radius with water resistivity closely relates to the increase in the specific charge as expected from the condition for the surface instability on water drops. 6. Conclusions (1) For high resistivity water, there is a field range to cause the atomization of water and the pulse-less and calm corona. This region is termed 'Pulse-Less Corona' (PLC) for corona m o d e and Mode V for the specific charge. (2) At the corona onset voltage, corona discharge starts from the drop-tip which separates from the neck. While, at the higher voltage, corona discharge grows on the conical tip of the pendant drop. (3) Corona mechanism on the positive water drops depends on the water conductivity. The pulse-less corona occurs around the filament elongating from the pendant drop with a low conductivity over a wide range of the field strength on the conductor. On the other hand, with highly conductive water the streamer corona starts always from the conical drop-tip. (4) In the case of negative polarity, the effect of the conductivity on corona discharge appears as a change of the pulse amplitude. The amplitude of Trichel pulses increases with the water conductivity under a given voltage. (5) Duration of the corona current is closely related to the mechanical
269 parameters determining the deformation of water drops and coincides with t h e t i m e i n t e r v a l f r o m t h e f o r m a t i o n o f t h e conical tip t o t h e s e p a r a t i o n o f t h e d r o p f r o m t h e c o n d u c t o r ; t h a t is, t h e basic p r o c e s s e s o n i o n i z a t i o n , dei o n i z a t i o n a n d m o t i o n o f c h a r g e d p a r t i c l e s in air a n d c o r o n a m e c h a n i s m w o u l d
not be so important. (6) There is a marked polarity effect on the ejection manner of high resistivity water under high applied voltage. The stable atomization and the elongation of the fine filament occur easily for the positive drops with smaller conductivity than about 20/~S/cm under a desirable flow rate of the supplied water. The atomization and the formation of very fine droplets are affected by the meteorological conditions too because the mechanism of corona discharge depends on the properties of the ambient gas. (7) The m a x i m u m specific charge reaches 5 X 10 -3 C/kg and the conditions for its occurrence depend on the field strength on the conductor, the watersupply flow rate and the water conductivity. (8) In M o d e V or the higher field region, the drop size decreases with the water conductivity under a given voltage. For distilledwater it becomes one tenth or less than that of tap water and the m i n i m u m radius of the droplets reaches 10 ~ m in M o d e V. Acknowledgements T h e a u t h o r s wish t o t h a n k Mr. S a k a i f o r t e c h n i c a l help. This w o r k was s u p p o r t e d in p a r t b y a Grant~in-Aid f o r Scientific R e s e a r c h f r o m t h e M i n i s t r y o f E d u c a t i o n , Science a n d C u l t u r e , J a p a n .
References 1 M. Hara, S. Ishibe and M. Akazaki, Corona discharge and electrical charge on water drops dripping from d.c. transmission conductors -- An experimental study in laboratory, J. Electrostatics, 6 (1979) 235. 2 Y. Sato, Y. Tsunoda and K. Arai, Corona pulses from a water drop on a cylindrical conductor surface, JIEE Jpn., 81 (1961) 1606. 3 L. Boulet and B.J. Jakubczyk, AC corona in foul weather: I - - Above freezing point, IEEE Trans. Power Appar. Syst., PAS-83 (1964) 508. 4 Y. Tsunoda and K. Arai, Corona discharge from water drops on a cylindrical conductor surface, JIEE Jpn., 84 (1964) 1430. 5 M. Akazaki, Corona discharge from water drops on smooth conductors under high direct voltage, IEEE Trana Power Appar. Syst., PAS-84 (1965) 1. 6 M. Akazaki and S. Lin, Corona discharge from water drops dripping onto inner conductor of co-axial cylinder, JIEE Jpn., 88 (1968) 909. 7 H.H. Neweli, T.W. Liao and F.W. Waburton, Corona and RI caused by particles on or near EHV conductors: II -- Foul weather, IEEE Trans. Power Appar. Syst., PAS-87 (1968) 911. 8 M. Akazaki and M. Hara, Corona discharge from water drops passing near a conductor, IEEE PES Summer Meeting, Portland, 71CP633 (1971).
270 9 C.L. Phan and A. Mansiaux, Corona and charge transfer on water drops in proximity of a conductor, IEEE P E S S u m m e r Meeting, San Francisco, A75-565-2 (1975). 10 F. Inna, G.L. Wilson and J.D. Bosack, Spectral characteristicsof acoustic noise from metallic protrusions and water drops in high electricfield,IEEE PES Winter Meeting, N e w York, C73-164-1 (1973). 11 J.E. Houburg and J.R. Melcher, Current driven, corona-terminated water jets as sources of charged droplets and audible noise, IEEE PES Winter Meeting, N e w York, C73-165-8 (1973). 12 IEEE Committee Report, Rainfall Resistivities,IEEE Trans. Power Appar. Syst., PAS-83 (1964) 520. 13 JIEE Committee, Standard of the Japanese ElectricalCommittee-I (in Japanese), No. 112 (1975) 44. 14 JIEE Committee, Standard of the Japanese ElectricalCommittee-II (in Japanese), No. 48 (1976) 5. 15 V.G. Drozin, The electricaldispersion of liquids as aerosols, J. Colloid Sci., 10 (1955) 158. 16 T.K. Burayev and I.P. Vereshchagin, Physical processes during electrostatic atomization of liquids, Fluid Mech. Soy. Res., 1 (1972) 56. 17 L.B. Loeb, personal communication, May, 1971. 18 H. Schene, Untersuchungen fiber die electrostatische Zerstaubbarkeit yon Lacken (2), Ind. Lack. Betrieb., 34 (1966) 471.