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The Sparking of lighting arresters due to conductive contaminants
‘_ The Sparking of Lightning Arresters due to Conductive Contaminants by E.
NASSER
Department of Electrical Engineering Iowa State University, Ames, Iowa and American lJniversi[y, Beirut, Lebanon A study of lightning arrester performance
ABSTRACT:
contaminants threshold.
It has shown that under certain conditions
intermittent insulation
under t?Le effect of conductive
has revealed some aspects of the mechanism but rather persistent
breakdown.
of the various
nature
This phenomenon
types of gas discharges
It was possible
to represent
mode of operation. of a particular
Finally,
and may
of the deterioration
partial
sparking
lead to failure
was analyzed
may occur with an
of the arrester
using the nonlinear
due to
characteristics
both within the arrester and within the contaminant.
the arrester by four diflerent modela depending quantitive
surface
of the sparking
data were obtained
on the
on the percentage
particular
threshold drop
arrester due to a severe contaminant.
I. Introduction
Overvoltage protection valves and lightning arresters are devices used in a wide variety of electrical systems ranging from communication systems to power systems, and from ground to high altitude application. In spite of this multitude of applications, their basic design, the requirements they have to meet and their basic behavior is almost the same; they should be passive system elements as long as normal conditions exist. Further, when a transient overvoltage occurs, they must conduct the excessive charge to the ground and thus protect the valuable network elements from possible damage by the transient overvoltage. The flawless operation of these devices is a desirable requirement for reliability of any electric system. This paper discusses the dielectric behavior of such valves under surface contamination and provides a basis for possible further analyses. The use of multi-unit lightning arresters and valves in both communication and power systems has not been very satisfactory. Troubles and even failures of these devices are frequently experienced, usually after only a short time of service. Many of these failures take place in areas of noticeable contaminated atmosphere. This fact led to the suspicion that these troubles might be a result of surface contamination, although it was not possible to explain how the sparking threshold drops. No convincing evidence can show how contamination could result in the destruction or damage of a lightning arrester without the occurrence of a voltage transient leading some investigators to rule out this possibility (1,2).This paper will indicate how an arrester can be internally damaged only due to the presence of a conductive surface contaminant as a result of the lowering of its sparking thresholds.
469 25
E. Nasser An analysis of the effect of contamination on the arrester is given as well as an explanation of some of the complicated gas discharge phenomena involved. The sequence of events leading to a complete or partial spark breakdown is delineated at the outset, followed by the behavior of single and stacked devices. Experimental data are then given as obtained under simulated conditions.
ZZ. Spark
Breakdown
Mechanism
Most overvoltage protection devices consist of a number of air gaps connected in series with nonlinear resistors, mostly a,lternately, but sometimes with the gaps and the resistors placed together. Such a device is housed in a single insulated container, usually of porcelain or glass, for low voltage applications. At higher voltages and for economical reasons, an arrester must be placed in a number of porcelain containers. It thus consists of stacked units. The behavior of these two types under the influence of a conductive surface layer will differ basically due to the metallic connection between the surface and the air gaps of the second type. These connections can distort the voltage distribution between the units when the surface acquires a nonuniform resistivity. The threshold of spark breakdown of given electrodes is a function of the ratio of the field intensity to the pressure. At a given pressure and electrode configuration and separation, the breakdown voltage of gaps having quasiuniform fields is constant. Any field distortion will lower the threshold voltage since nonuniform fields have lower sparking voltages than uniform or quasi-uniform fields (3). Hence, the complete electric breakdown of air between two given electrodes depends only on the voltage gradient along the gap, provided all other factors such as gas pressure, external radiation and impurities remain unchanged. The sparking voltage of an arrester can only be lowered if the electric field intensity within the gap is distorted either by external fields or by surface contaminants. In order to represent the phenomena of partial and/or complete spark breakdown, the ionization processes preceding spark are briefly delineated. This digression seems justified since the whole behavior of the device depends on the gas discharge within the gaps and on the surface. A breakdown or a sequence of ionization processes is triggered by an electron avalanche produced by electron collision when a free electron is accelerated by the electric field towards the anode. When the electric field intensity is high enough, an electron avalanche may attain a certain critical concentration of positive ions from which a streamer develops in the opposite direction towards the negative electrode (3, 4). If the field intensity in the cathode region is adequate, the streamer may eventually be able to reach it. On its way in the gap, excessive branching takes place. Many of the new branches formed will eventually arrive at the cathode (3, 5).
470
Journal of The
Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants At higher fields near the cathode, the approaching streamer tip can cause excessive liberation of electrons by means of its high local field intensity. These electrons travel along the streamer channel and cause further ionization. This ionization wave is very fast and is known as the back stroke (3,6). Every one of these ionization waves, the avalanche, the streamer and the back stroke, causes an appreciable current pulse to flow; the magnitude and duration of which increase in this order. If the voltage source has a high impedance a current pulse may cause the voltage and field to drop appreciably inhibiting any further ionization which might have taken place if the voltage did not drop. At a low impedance voltage source, the back stroke, which had made the streamer channel a moderately ionized plasma, is followed by the discharge current augmenting the conductivity of the plasma channel. The voltage across this plasma channel is very low and a sparkover is known to have occurred (3, 6). When a discharge cannot develop further due to the drop of voltage between the electrodes, deionization and recombination take place in the channel and the discharge is extinguished. The current pulse declines to zero. At this instant the voltage between the electrodes returns to its original value and the whole process may be Origgered all over again by any free electron available. The frequency of these discharges depends on voltage, electrode configuration and source impedance. Such a phenomenon is called intermittent pre-breakdown streamer. Such streamers are seen in Fig. l(a). In this time exposure, only the very vigorous streamers are recognized while weak branches remain invisible. These streamers can vary in intensity, branching and range and may or may not be followed by a return stroke. They usually propagate towards the cathode at a very high velocity of the order of lo8cm/set (4) which is much higher than the drift velocity of electrons. The current pulse of these streamers has therefore a much shorter rise-time than that of electron avalanches. This short rise-time causes the radio noise of these pulses to consist of exceptionally high frequencies. Such discharges do not take place only within the spark gap of the arrester but might also develop at any point parallel to the gaps where high field intensities exist, such as at the hardware used for the assembly of gap elements. Due to the fact that these electrodes have extremely large spacings compared with the spark gaps, the field is very non-uniform. A local high field may exist at a sharp point where a streamer discharge can be launched. Such streamers are seen in Fig. l(a) as they developed on the housing of an arrester spark gap. The two photographs of Fig. 1 were taken at 1 set exposure time. Photograph (a) was taken at a total contamination resistance of 20 MS1 in series with the voltage source. Since a complete spark breakdown cannot materialize here, excessive space charge may accumulate. The negative ion space charge will distort the field to such an extent as to cause a streamer-to-glow transition to occur (6). The condition for this is a sufficiently high current or low impedance source.
Vol. 294, No. 6. December
1972
471
E. Nasser Figure l(b) shows a typical glow discha,rge at the same arrester gap housing. The contamination resistance was reduced to 1.1 MO, thus allowing enough space charge to accumulate and to cause a streamer-glow transition. The voltage applied to the arrester was the same in both cases and was less than the dry sparkover voltage. All these phenomena, particularly the intermittent streamers and the transition to a glow, are usually not detected by the operator. They develop only when the arrester is contaminated as the contamination layer has a resistance which varies continuously and abruptly over many orders of magnitudes depending on the amount of moisture content. Thus the voltage source impedance vary from low to high impedance with layer resistance which is low when moistened by atmospheric effects and high when it dries out due to the heating of the surface or leakage current. III.
Insulated
Arresters
With respect to surface contamination, arrester devices may be classified in two major groups; the insulated and the noninsulated. Here the insulation is from the surface that is susceptible to conductive deposits. The latter type, which is usually stacked, is connected electrically to the surface only at the junction points. d!lultiple Gap Type In the insulated single-unit arrester, there is no metallic connection between the surface which is susceptible to contamination of infinite multitude and of a resistivity varying in many orders of magnitude and with all possible distributions. The surface contaminant may distort the field between the electrodes by shifting the equipotential surfaces, i.e. by changing the stray capacitances. This is schematically illustrated in Fig. 2 using the equipotential surfaces concept. The distortion of the equipotential surfaces and HIGH
VOLTAGE
LINE
T 7
Electrodes<
:I
Conductive
GROUND
contaminant on the configuration of equipotential surface of a standard spark gap. Note region denoted by H where a high voltage gradient was produced by field distortion and may exhibit corona discharges. FIG.
472
2. Schematic illustration of the effect of a conductive
Journal of The Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants field lines by the nonuniformly conductive layer may produce high field intensities at the areas of the electrodes which are adjacent to the surface. The local high fields may produce a locally confined discharge, such as corona, glow, an intermittent pre-breakdown streamer or even a complete spark breakdown. From the system viewpoint, a predischarge is not accompanied by a drop of voltage across the electrodes as a spark breakdown. Thus, the knowledge of the gas discharge type affects the analysis of system behavior to a great extent. When a predischarge (streamer or glow) develops to a spark, the voltage between the two electrodes will drop to a small value, thus enhancing the voltages across all other series gaps. This does not necessarily cause them to spark unless their fields are also distorted. Since the discharge current associated with the breakdown of the first gap is very low, a highly ionized plasma channel cannot materialize. After the electrode charge has been discharged, the current becomes zero and the channel is gradually deionized by natural diffusion and recombination. Then the whole process starts all over again as explained previously. If, however, in general a number of m gaps had locally distorted fields, their successive breakdown would cause the voltage across the remaining n-m gaps to increase by a factor of n/(n - m) which may eventually be high enough to make all the remaining n -m gaps break down too. This runaway condition results in a complete spark breakdown at the normal service voltage. The grave consequences of reducing the sparking voltage below the service voltage on the system are similar to those of a short circuit. The partial, or incomplete intermittent breakdown, on the other hand, is accompanied by current pulses that cause an undesirable high-level radio interference. The existence of ionization for a longer period may lead to a complete deterioration and tracking of the internal insultation.
Effect of Shieldin,g In arresters having so-called grading resistors (RB1, R,, and R,, in Fig. 3) that are physically placed closely around or along the gaps, such a phenomenon becomes less pronounced because the resistors keep the field within the gaps almost independent of surface layer potential distribution. Or, in other words, it reduces the capacitances between surface and electrodes. This shielding effect of the “grading” resistors has been recognized and is utilized in some arresters. This effect was demonstrated experimentally by removing them and measuring the breakdown voltage of the contaminated single unit and comparing it with the value when the resistor is inserted. For a particular European-made arrester, it was found that when the shielding grading resistor was removed, the sparkover voltage of the contaminated arrester dropped to about 70 per cent of its original value without a contaminant. Replacing the grading resistor, the sparkover voltage was only 3 per cent below the original value. This test was performed on single unit arresters
Vol.
294, No. 26
6, December
1972
473
E. Nasser using artificial contamination and Kieselguhr (SO,) dust. I .
42 12
l
P
c
2
ISI
z0
l
I
t
Ra2
E
2 E cc
Rw
v23
G2’
: T:
. 413
.zw .c_ E b-4
1
8 ‘=
1
2
Is2
E 5 a
13
.
chloride
I
.i
GI w 2 0
of sodium
i
RCII
%I
of a mixture
I
.
1
consisting
Rg3
5
v30
Is3
1
g 7 P
!
.
(:I
Normal
condition
v, = v, = v, . . . = v, Igl = Igz = Is3 . . . I,,
I,, = I,, = I,, 1.. I,, = 0 Severe contamination and partial
breakdown
v, = I,, Ra3
0
0
(b)
(c)
(d)
No breakdown v,#v,#v,...#v,
I,IfI.z#Ia Iilk& I,, I*,zII,,rI,, Complete
. ..#I., ’ . ZII,, breakdown
I,, = I,, = I,, = I,, = 0 I,, = I,, = I,, = I,, I,, = VIZ’ Ra,
I,, = 0 I,1 = I82 = Ia
FIG. 3. Arrester model under the basic (4) modes of operation under the effect of surface contamination. These models are for low frequency SC. or for d.c. arresters.
IV.
Noninsulated
Arresters
These are stacked devices having one or more connections This is usually the result of stacking insulated single units.
to the surface.
Parallel or Grading Resistors One of the main features of these arresters is the grading resistors parallel to arrester assembly that are in this case designed to obtain a uniform distribution of voltage among the single units. Such resistors are shown as Rgl, R,, and R,, in Fig. 3 and may be linear or nonlinear. The use of nonlinear resistors parallel to gaps carrying gas discharges of nonlinear character makes it extremely difficult to analyze the behavior even qualitatively because the nonlinear characteristics of both resistors and discharges are not known. Linear resistors usually have high resistances in order to keep the heat dissipated as low as possible. Usual values for the grading current lie between
474
Journal
of The Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants O-5 and 5 mA which is much lower than surface currents of severe contamination (7). However, such a grading current is comparable to surface currents under standard wet tests and much less than surface currents under natural rain or drizzle (8). To increase the effectiveness of the grading current and at the same time keep the heat dissipated low, nonlinear resistors are being used. They have a high resistance at normal service voltage and a small fraction thereof at twice this value. The relation between voltage and current for such resistors can be expressed as
v = bia, where b and a are constants depending on the physical dimension and on the history of the sample [see Ref. (9)]. The value of the grading current near spark threshold should be in the order of the surface current. Usually, however, they become thermally instable when carrying excessively high currents. They either change their characteristics or fail completely. As wa’s shown in the previous section, the use of shielded spark gaps tends to suppress the field distortion due to contamination. At high voltages the use of a single insulator is an expensive method of combatting contamination and probably not always feasible because of the difficulty in manufacturing and handling very large porcelain housings. Even if it were feasible, the versatility and economy of a stackable unit need not be emphasized. The contaminant layer on the surface does not consist usually of a closed path but includes periodical &continuities covering all or part of the circumference. The noncontaminated areas are those washed away by rain and also those protected by the sheds against deposits. They have very high resistances and are represented in the circuit by open gaps. As expected, a random number of such gaps will exist along the surface. The resistances shown on the surface are in the order of some megohms or even kiloohms. If the open gaps would be bridged by sparks the surface current reaches some tenths of 1 amp. Arrester Behavior Figure 3 illustrates the behavior of the arrester under service voltage V and under varying contamination conditions. This figure depicts a simple equivalent circuit showing the arrester components and neglecting all stray capacitances whose influence was already analyzed by considering the electric field configuration. In this figure R,, R, and R, denote the grading, surface and arrester resistors, respectively. The second subscript denotes the position of the unit considered starting from the top of the stack. When the surface contaminant is dry and nonconductive, its resistance is extremely high and can therefore be represented by an open circuit as shown in Fig. 3(a). If the contamination becomes highly conductive, it is bound to be nonuniform resulting in a number of series resistances of different orders of magnitude. Some of these will dry and attain very high resistances that carry a greater proportion of the voltage applied and will thus soon
Vol. 294, No. 6. December
1972
475
E. Nasser breakdown to gas discharges of low resistance. Such gaps are shown bridged by discharges in Fig. 3(b). The voltage across each unit will be determined by the instantaneous value of the surface resistance. Thus for any unit k, where Isk is the surface current. This current can be considered the same along the surface of all units if .lg< I, which is usually the case. The behavior of the arrester under these conditions can be characterized by the equations v,#v,...
#v,,
(I)
ISI+ $1 = I,, + $2 = . . . = Isn+ Ign
(2)
Ial - Igl = Ia - I@ = . . . = I,, - Ign = 0.
(3)
and
The latter condition will prevail as long as the gaps G,, G,, . .., G, are nonconductive which is the case as long as the voltages across them are less than their breakdown threshold. Or, l&v, ,...) V,
(4)
Equations (l)-(4) describe arrester behavior with a contaminated surface but with the voltages across each unit less than the breakdown voltage of the spark gaps G. Figure 3(c) illustrates what happens when this condition is no longer fulfilled and when unit No. 3, for example, becomes internally conductive by the ignition of a spark in its gap G, as soon as V, = V,. After ignition, V, = Ia3,Ra3, which is very low compared with V,. Therefore the collapse of the voltage V, due to spark ignition will be accompanied by a voltage surge across the remaining units. This voltage transient reaches a steady state within a few microseconds. The rise in steady-state voltage will be V, (before ignition)
- V, (after ignition) E V,
over the remaining units. V, will be distributed among them according to the ratio of the shunt resistances. Figure 4 is a diagram of the envelopes of voltages Vl,,,V,, and V&,of the 3-unit arrester, of the total current I,+ I,, and of I,, as measured by inserting meters on the grounded end of the device. This oscillogram is of low time resolution and does not therefore show any transients due to the intermittent sparking of unit No. 3. Starting with no surface current when the conductivity of the contaminant is negligible, I, is zero and the applied voltage V is uniformly distributed among the three units. When the amount and/or conductivity of contamination start to increase, I, increases and the voltage across each unit is determined by the random value R, of each unit assumes. It should be noted at this point that the resistance of the surface contaminant is affected by various factors acting in opposite directions. While the deposit of soluble contaminants and of water moisture increases the
476
Journal
of The Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants conductivity, the current flowing in the layer will dissipate sufficient heat to evaporate moisture and thus reduce the conductivity.* If the layer on unit No. 3 happens to have less contaminant and/or to dry out first, Rs3 will increase faster than Rs2 or R,, and so will the voltage V,, as Fig. 4 shows during the first 2.9 set where the voltages and currents are ta--Contominotion
period _c1
TIME
t
t ARRESTER CURRENT ba + 193
:
f%3
1
c
t
FIQ. 4. Voltages and current envelopes during a typical cantamination period. Time scale of a laboratory test is in seconds whereas under service conditions the time scale is in min.
as described in Fig. 3(b). At t = 2.9 set, V,, attains the value of V, and gap G, breaks down reducing V,, to a small value. Thus the equations characterizing the arrester behavior at this point are v;, Z 0,
(5)
43 = 0,
(6)
Za = &+I,,
and
= &l-J71
K,vZ<&.
(7) (3)
This condition will persist as long as after the instantaneous zero value of Iaa3every half-cycle, the spark is re-ignited making G, conductive for another half-cycle. Re-ignition will therefore take place as long as the voltage is capable of reproducing the spark. As soon as this ceases to be the case, the surface current 1& returns to flow by bridging any gaps with surface discharges. Thus the condition returns to that of Fig. 3(b) and to Eqs. (I) * It is not possible at this point to describe all the processes taking place in and at the surface layer since these alone would require one or more papers. The reader is referred to Ref. (6).
Vol. 294, No. 6, December
1972
477
E. Nasser and (4). Again, when V, = 5, G, becomes conductive and the conditions will be that of Fig. 3(c) and Eqs. (5)-(8). In Fig. 4 this intermittent ignition is clearly illustrated by the envelope curves. The actual oscillogram of Fig. 5 delineates this further. It shows how conditions can change even for one half-cycle (see letter a) and how intermittent the sparking of G, can actually be. This condition of intermittent partial breakdown of the arrester was not expected to occur prior to the execution of these tests. It is highly undesirable since it is associated with the production of high intensity radio noise. It also may heat up the arrester elements and lead to the deterioration of the insulation and the complete failure of the arrester. Again, the voltage I?&across the second unit may also reach V, and ignite a spark in G, at a time when G, is already conducting. This will cause the current to increase abruptly during one-half only or for a longer period. This condition is marked on the oscillogram by the letter b. Finally, the last gap G, may also break down and the whole arrester fails without the occurrence of a voltage surge. The current flowing may not be interrupted and a short-circuit current will destroy the arrester components in a fraction of a second. This is illustrated in Fig. 3(d). V. Test Results
To determine the voltage at which a partial or a complete breakdown takes place, laboratory tests using an artificial contaminant were carried out by applying a uniform dry surface layer of silicon oxide containing an amount of sodium chloride depending on the degree of conductivity, or the severity of pollution, expected. The method used was described earlier by the author (7). After the dry surface layer was applied, the arresters were cooled at 0°C (32’F) in a refrigerator. They were taken to the humid test chamber to be moistened before the test voltage was applied. The test chamber was kept at 23°C and 100 per cent r.h. The surface layer attained its maximum conductivity after about 18 min. Then the test voltage was applied and voltage, surface and arrester current were recorded on an oscillograph and observed on a fast dual-beam oscilloscope. The voltage of the lowest unit was measured by means of an electrostatic voltmeter to prove the occurrence of partial sparkover as explained above. The test voltage was kept constant throughout each single test. To determine the lowest possible voltage at which partial or complete breakdown may occur, tests were carried out several times lowering the test voltage successively until no sparkover was found to occur. Figure 5 shows a typical oscillogram of the two currents IS3 and Ia + Igg3 and the applied voltage V as recorded during the tests. The results are summarized in Fig. 6 in which the breakdown voltage is plotted against the number of arrester units under test. The single unit has almost no decrease in breakdown voltage due to shielding of the spark gap
478
Journal of The Franklin Institute
Tlie Sparking of Lightning Arresters Due to Conductive Contaminants by the grading resistor surrounding it. Removing the grading resistor resulted in a drop in the breakdown voltage of 30 per cent or more. The minimum voltage at which breakdown either of the whole device or of one unit is plotted in Fig. 6 for the dry and clean condition, as reference, and for the contaminated case. In the absence of a contaminant, partial breakdown cannot occur and only complete breakdown is observed. The higher the number of units, the great,er is the decrease in breakdown voltage.
160-
Complete Ereokdown
Portiol Breokdown
Contominoted
I I
I
I
I
I
I
2
3
4
5
6
NUMBER OF ARRESTER UNITS
FIG. 6. Arrester breakdown voltages as & function of n at two extreme surface conditions. Figure 7 illustrates the relative breakdown voltage as referred to the clean and dry value for different unit number. The values of minimum breakdown as given by the curves of Figs. 6 and 7 are for severe contamination of high conductivity as in the vicinity of chemical plants or near the sea coast. The surface conductivity in these tests was about 30 pmho per unit square area. Conclusions
In this paper, some of the complex current and gas discharge processes involved in the contaminated arrester have been presented. The partial spark breakdown of lightning arresters was found to occur at very low voltages. The phenomena described may serve to elucidate some of the failures and unexpected performance of arrester devices. The observed partial breakdown, beside generating excessive radio noise, may eventually lead to the deterioration of the insulation by forming surface tracks that
Vol. 298, Jo. 6. December 1972
479
E. Nasser conducts the current to ground and finally heats up the elements to destruction. Moreover, complete breakdown of the device was found to occur at voltages much lower than those of the clean arrester. This happens by a chain-like reaction triggered by the partial breakdown of one of its units. These processes, it is hoped, might contribute to the clarification of some anomalous arrester behavior.
I
I
I
2
3
NUMBER FIG.
7. The breakdown
voltages
I
I
4
5
OF ARRESTER
UNITS
of Fig. 6 as referred to ideal conditions of infinite surface resistivity.
Acknowledgements
The experimental work described in this paper was carried out during the author’s association with the Siemens Company, West Berlin, to whom the author is grateful for the permission to publish this material. The author wishes to express his thanks to the Insulator laboratory staff for their assistance ; he also wishes to thank the Iowa Engineering Research Institute, Iowa State University, for the continued support of the studies.
References
(1) W. H. Eason and E. C. Saksaug, “Contamination
testing of lightning arresters”, IEEE Winter Power Mtg., Paper No. 31 CP 65-72, 1965. (2) E. Beck, “Lightning arresters for high voltage systems”, Utility Eng. Conf., 1961. (3) E. Nasser, “Fundamentals of Gaseous Ionization”, Chapter 11, New York, WileyInterscience, 197 1. im ungleichformigen Feld bei positiver Spitze”, (4) E. Nasser, “Entladungsaufbau Archiv f. Elektrotechnik, Vol. 44, pp. 157-167 and 168-176, 1959.
480
Journal of The Franklin Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants (5) L.‘B. Loeb and E. Nasser, “Impulse streamer branching from Lichtenberg figure studies”, J. appl. Phys., Vol. 34, pp. 3340-3348, Nov. 1963. (6) L. B. Leob, “Electrical Coronas, Their Basic Physical Mechanisms”, Univ. of Cdif. Press, 1965. (7) E. Nasser, “The problem of flashovers on contaminated insulators”, Electkrotech. Z., Pt. A, Vol. 83, pp. 356-365, May, 1962. (8) L. D. Whitehead, “Rainfall rate and resistivity at East Pittsburgh”, AIEE Trans., Part III, Vol. 80, pp. 952-954, 1963. (9) E. Nasser, “Lightning arrestor, and method for using same”, U.S. Patent No. 537,892, March 1969.
Vol.
294,
No.
6, December
1972
481
NASSER
Department of Electrical Engineering Iowa State University, Ames, Iowa and American lJniversi[y, Beirut, Lebanon A study of lightning arrester performance
ABSTRACT:
contaminants threshold.
It has shown that under certain conditions
intermittent insulation
under t?Le effect of conductive
has revealed some aspects of the mechanism but rather persistent
breakdown.
of the various
nature
This phenomenon
types of gas discharges
It was possible
to represent
mode of operation. of a particular
Finally,
and may
of the deterioration
partial
sparking
lead to failure
was analyzed
may occur with an
of the arrester
using the nonlinear
due to
characteristics
both within the arrester and within the contaminant.
the arrester by four diflerent modela depending quantitive
surface
of the sparking
data were obtained
on the
on the percentage
particular
threshold drop
arrester due to a severe contaminant.
I. Introduction
Overvoltage protection valves and lightning arresters are devices used in a wide variety of electrical systems ranging from communication systems to power systems, and from ground to high altitude application. In spite of this multitude of applications, their basic design, the requirements they have to meet and their basic behavior is almost the same; they should be passive system elements as long as normal conditions exist. Further, when a transient overvoltage occurs, they must conduct the excessive charge to the ground and thus protect the valuable network elements from possible damage by the transient overvoltage. The flawless operation of these devices is a desirable requirement for reliability of any electric system. This paper discusses the dielectric behavior of such valves under surface contamination and provides a basis for possible further analyses. The use of multi-unit lightning arresters and valves in both communication and power systems has not been very satisfactory. Troubles and even failures of these devices are frequently experienced, usually after only a short time of service. Many of these failures take place in areas of noticeable contaminated atmosphere. This fact led to the suspicion that these troubles might be a result of surface contamination, although it was not possible to explain how the sparking threshold drops. No convincing evidence can show how contamination could result in the destruction or damage of a lightning arrester without the occurrence of a voltage transient leading some investigators to rule out this possibility (1,2).This paper will indicate how an arrester can be internally damaged only due to the presence of a conductive surface contaminant as a result of the lowering of its sparking thresholds.
469 25
E. Nasser An analysis of the effect of contamination on the arrester is given as well as an explanation of some of the complicated gas discharge phenomena involved. The sequence of events leading to a complete or partial spark breakdown is delineated at the outset, followed by the behavior of single and stacked devices. Experimental data are then given as obtained under simulated conditions.
ZZ. Spark
Breakdown
Mechanism
Most overvoltage protection devices consist of a number of air gaps connected in series with nonlinear resistors, mostly a,lternately, but sometimes with the gaps and the resistors placed together. Such a device is housed in a single insulated container, usually of porcelain or glass, for low voltage applications. At higher voltages and for economical reasons, an arrester must be placed in a number of porcelain containers. It thus consists of stacked units. The behavior of these two types under the influence of a conductive surface layer will differ basically due to the metallic connection between the surface and the air gaps of the second type. These connections can distort the voltage distribution between the units when the surface acquires a nonuniform resistivity. The threshold of spark breakdown of given electrodes is a function of the ratio of the field intensity to the pressure. At a given pressure and electrode configuration and separation, the breakdown voltage of gaps having quasiuniform fields is constant. Any field distortion will lower the threshold voltage since nonuniform fields have lower sparking voltages than uniform or quasi-uniform fields (3). Hence, the complete electric breakdown of air between two given electrodes depends only on the voltage gradient along the gap, provided all other factors such as gas pressure, external radiation and impurities remain unchanged. The sparking voltage of an arrester can only be lowered if the electric field intensity within the gap is distorted either by external fields or by surface contaminants. In order to represent the phenomena of partial and/or complete spark breakdown, the ionization processes preceding spark are briefly delineated. This digression seems justified since the whole behavior of the device depends on the gas discharge within the gaps and on the surface. A breakdown or a sequence of ionization processes is triggered by an electron avalanche produced by electron collision when a free electron is accelerated by the electric field towards the anode. When the electric field intensity is high enough, an electron avalanche may attain a certain critical concentration of positive ions from which a streamer develops in the opposite direction towards the negative electrode (3, 4). If the field intensity in the cathode region is adequate, the streamer may eventually be able to reach it. On its way in the gap, excessive branching takes place. Many of the new branches formed will eventually arrive at the cathode (3, 5).
470
Journal of The
Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants At higher fields near the cathode, the approaching streamer tip can cause excessive liberation of electrons by means of its high local field intensity. These electrons travel along the streamer channel and cause further ionization. This ionization wave is very fast and is known as the back stroke (3,6). Every one of these ionization waves, the avalanche, the streamer and the back stroke, causes an appreciable current pulse to flow; the magnitude and duration of which increase in this order. If the voltage source has a high impedance a current pulse may cause the voltage and field to drop appreciably inhibiting any further ionization which might have taken place if the voltage did not drop. At a low impedance voltage source, the back stroke, which had made the streamer channel a moderately ionized plasma, is followed by the discharge current augmenting the conductivity of the plasma channel. The voltage across this plasma channel is very low and a sparkover is known to have occurred (3, 6). When a discharge cannot develop further due to the drop of voltage between the electrodes, deionization and recombination take place in the channel and the discharge is extinguished. The current pulse declines to zero. At this instant the voltage between the electrodes returns to its original value and the whole process may be Origgered all over again by any free electron available. The frequency of these discharges depends on voltage, electrode configuration and source impedance. Such a phenomenon is called intermittent pre-breakdown streamer. Such streamers are seen in Fig. l(a). In this time exposure, only the very vigorous streamers are recognized while weak branches remain invisible. These streamers can vary in intensity, branching and range and may or may not be followed by a return stroke. They usually propagate towards the cathode at a very high velocity of the order of lo8cm/set (4) which is much higher than the drift velocity of electrons. The current pulse of these streamers has therefore a much shorter rise-time than that of electron avalanches. This short rise-time causes the radio noise of these pulses to consist of exceptionally high frequencies. Such discharges do not take place only within the spark gap of the arrester but might also develop at any point parallel to the gaps where high field intensities exist, such as at the hardware used for the assembly of gap elements. Due to the fact that these electrodes have extremely large spacings compared with the spark gaps, the field is very non-uniform. A local high field may exist at a sharp point where a streamer discharge can be launched. Such streamers are seen in Fig. l(a) as they developed on the housing of an arrester spark gap. The two photographs of Fig. 1 were taken at 1 set exposure time. Photograph (a) was taken at a total contamination resistance of 20 MS1 in series with the voltage source. Since a complete spark breakdown cannot materialize here, excessive space charge may accumulate. The negative ion space charge will distort the field to such an extent as to cause a streamer-to-glow transition to occur (6). The condition for this is a sufficiently high current or low impedance source.
Vol. 294, No. 6. December
1972
471
E. Nasser Figure l(b) shows a typical glow discha,rge at the same arrester gap housing. The contamination resistance was reduced to 1.1 MO, thus allowing enough space charge to accumulate and to cause a streamer-glow transition. The voltage applied to the arrester was the same in both cases and was less than the dry sparkover voltage. All these phenomena, particularly the intermittent streamers and the transition to a glow, are usually not detected by the operator. They develop only when the arrester is contaminated as the contamination layer has a resistance which varies continuously and abruptly over many orders of magnitudes depending on the amount of moisture content. Thus the voltage source impedance vary from low to high impedance with layer resistance which is low when moistened by atmospheric effects and high when it dries out due to the heating of the surface or leakage current. III.
Insulated
Arresters
With respect to surface contamination, arrester devices may be classified in two major groups; the insulated and the noninsulated. Here the insulation is from the surface that is susceptible to conductive deposits. The latter type, which is usually stacked, is connected electrically to the surface only at the junction points. d!lultiple Gap Type In the insulated single-unit arrester, there is no metallic connection between the surface which is susceptible to contamination of infinite multitude and of a resistivity varying in many orders of magnitude and with all possible distributions. The surface contaminant may distort the field between the electrodes by shifting the equipotential surfaces, i.e. by changing the stray capacitances. This is schematically illustrated in Fig. 2 using the equipotential surfaces concept. The distortion of the equipotential surfaces and HIGH
VOLTAGE
LINE
T 7
Electrodes<
:I
Conductive
GROUND
contaminant on the configuration of equipotential surface of a standard spark gap. Note region denoted by H where a high voltage gradient was produced by field distortion and may exhibit corona discharges. FIG.
472
2. Schematic illustration of the effect of a conductive
Journal of The Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants field lines by the nonuniformly conductive layer may produce high field intensities at the areas of the electrodes which are adjacent to the surface. The local high fields may produce a locally confined discharge, such as corona, glow, an intermittent pre-breakdown streamer or even a complete spark breakdown. From the system viewpoint, a predischarge is not accompanied by a drop of voltage across the electrodes as a spark breakdown. Thus, the knowledge of the gas discharge type affects the analysis of system behavior to a great extent. When a predischarge (streamer or glow) develops to a spark, the voltage between the two electrodes will drop to a small value, thus enhancing the voltages across all other series gaps. This does not necessarily cause them to spark unless their fields are also distorted. Since the discharge current associated with the breakdown of the first gap is very low, a highly ionized plasma channel cannot materialize. After the electrode charge has been discharged, the current becomes zero and the channel is gradually deionized by natural diffusion and recombination. Then the whole process starts all over again as explained previously. If, however, in general a number of m gaps had locally distorted fields, their successive breakdown would cause the voltage across the remaining n-m gaps to increase by a factor of n/(n - m) which may eventually be high enough to make all the remaining n -m gaps break down too. This runaway condition results in a complete spark breakdown at the normal service voltage. The grave consequences of reducing the sparking voltage below the service voltage on the system are similar to those of a short circuit. The partial, or incomplete intermittent breakdown, on the other hand, is accompanied by current pulses that cause an undesirable high-level radio interference. The existence of ionization for a longer period may lead to a complete deterioration and tracking of the internal insultation.
Effect of Shieldin,g In arresters having so-called grading resistors (RB1, R,, and R,, in Fig. 3) that are physically placed closely around or along the gaps, such a phenomenon becomes less pronounced because the resistors keep the field within the gaps almost independent of surface layer potential distribution. Or, in other words, it reduces the capacitances between surface and electrodes. This shielding effect of the “grading” resistors has been recognized and is utilized in some arresters. This effect was demonstrated experimentally by removing them and measuring the breakdown voltage of the contaminated single unit and comparing it with the value when the resistor is inserted. For a particular European-made arrester, it was found that when the shielding grading resistor was removed, the sparkover voltage of the contaminated arrester dropped to about 70 per cent of its original value without a contaminant. Replacing the grading resistor, the sparkover voltage was only 3 per cent below the original value. This test was performed on single unit arresters
Vol.
294, No. 26
6, December
1972
473
E. Nasser using artificial contamination and Kieselguhr (SO,) dust. I .
42 12
l
P
c
2
ISI
z0
l
I
t
Ra2
E
2 E cc
Rw
v23
G2’
: T:
. 413
.zw .c_ E b-4
1
8 ‘=
1
2
Is2
E 5 a
13
.
chloride
I
.i
GI w 2 0
of sodium
i
RCII
%I
of a mixture
I
.
1
consisting
Rg3
5
v30
Is3
1
g 7 P
!
.
(:I
Normal
condition
v, = v, = v, . . . = v, Igl = Igz = Is3 . . . I,,
I,, = I,, = I,, 1.. I,, = 0 Severe contamination and partial
breakdown
v, = I,, Ra3
0
0
(b)
(c)
(d)
No breakdown v,#v,#v,...#v,
I,IfI.z#Ia Iilk& I,, I*,zII,,rI,, Complete
. ..#I., ’ . ZII,, breakdown
I,, = I,, = I,, = I,, = 0 I,, = I,, = I,, = I,, I,, = VIZ’ Ra,
I,, = 0 I,1 = I82 = Ia
FIG. 3. Arrester model under the basic (4) modes of operation under the effect of surface contamination. These models are for low frequency SC. or for d.c. arresters.
IV.
Noninsulated
Arresters
These are stacked devices having one or more connections This is usually the result of stacking insulated single units.
to the surface.
Parallel or Grading Resistors One of the main features of these arresters is the grading resistors parallel to arrester assembly that are in this case designed to obtain a uniform distribution of voltage among the single units. Such resistors are shown as Rgl, R,, and R,, in Fig. 3 and may be linear or nonlinear. The use of nonlinear resistors parallel to gaps carrying gas discharges of nonlinear character makes it extremely difficult to analyze the behavior even qualitatively because the nonlinear characteristics of both resistors and discharges are not known. Linear resistors usually have high resistances in order to keep the heat dissipated as low as possible. Usual values for the grading current lie between
474
Journal
of The Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants O-5 and 5 mA which is much lower than surface currents of severe contamination (7). However, such a grading current is comparable to surface currents under standard wet tests and much less than surface currents under natural rain or drizzle (8). To increase the effectiveness of the grading current and at the same time keep the heat dissipated low, nonlinear resistors are being used. They have a high resistance at normal service voltage and a small fraction thereof at twice this value. The relation between voltage and current for such resistors can be expressed as
v = bia, where b and a are constants depending on the physical dimension and on the history of the sample [see Ref. (9)]. The value of the grading current near spark threshold should be in the order of the surface current. Usually, however, they become thermally instable when carrying excessively high currents. They either change their characteristics or fail completely. As wa’s shown in the previous section, the use of shielded spark gaps tends to suppress the field distortion due to contamination. At high voltages the use of a single insulator is an expensive method of combatting contamination and probably not always feasible because of the difficulty in manufacturing and handling very large porcelain housings. Even if it were feasible, the versatility and economy of a stackable unit need not be emphasized. The contaminant layer on the surface does not consist usually of a closed path but includes periodical &continuities covering all or part of the circumference. The noncontaminated areas are those washed away by rain and also those protected by the sheds against deposits. They have very high resistances and are represented in the circuit by open gaps. As expected, a random number of such gaps will exist along the surface. The resistances shown on the surface are in the order of some megohms or even kiloohms. If the open gaps would be bridged by sparks the surface current reaches some tenths of 1 amp. Arrester Behavior Figure 3 illustrates the behavior of the arrester under service voltage V and under varying contamination conditions. This figure depicts a simple equivalent circuit showing the arrester components and neglecting all stray capacitances whose influence was already analyzed by considering the electric field configuration. In this figure R,, R, and R, denote the grading, surface and arrester resistors, respectively. The second subscript denotes the position of the unit considered starting from the top of the stack. When the surface contaminant is dry and nonconductive, its resistance is extremely high and can therefore be represented by an open circuit as shown in Fig. 3(a). If the contamination becomes highly conductive, it is bound to be nonuniform resulting in a number of series resistances of different orders of magnitude. Some of these will dry and attain very high resistances that carry a greater proportion of the voltage applied and will thus soon
Vol. 294, No. 6. December
1972
475
E. Nasser breakdown to gas discharges of low resistance. Such gaps are shown bridged by discharges in Fig. 3(b). The voltage across each unit will be determined by the instantaneous value of the surface resistance. Thus for any unit k, where Isk is the surface current. This current can be considered the same along the surface of all units if .lg< I, which is usually the case. The behavior of the arrester under these conditions can be characterized by the equations v,#v,...
#v,,
(I)
ISI+ $1 = I,, + $2 = . . . = Isn+ Ign
(2)
Ial - Igl = Ia - I@ = . . . = I,, - Ign = 0.
(3)
and
The latter condition will prevail as long as the gaps G,, G,, . .., G, are nonconductive which is the case as long as the voltages across them are less than their breakdown threshold. Or, l&v, ,...) V,
(4)
Equations (l)-(4) describe arrester behavior with a contaminated surface but with the voltages across each unit less than the breakdown voltage of the spark gaps G. Figure 3(c) illustrates what happens when this condition is no longer fulfilled and when unit No. 3, for example, becomes internally conductive by the ignition of a spark in its gap G, as soon as V, = V,. After ignition, V, = Ia3,Ra3, which is very low compared with V,. Therefore the collapse of the voltage V, due to spark ignition will be accompanied by a voltage surge across the remaining units. This voltage transient reaches a steady state within a few microseconds. The rise in steady-state voltage will be V, (before ignition)
- V, (after ignition) E V,
over the remaining units. V, will be distributed among them according to the ratio of the shunt resistances. Figure 4 is a diagram of the envelopes of voltages Vl,,,V,, and V&,of the 3-unit arrester, of the total current I,+ I,, and of I,, as measured by inserting meters on the grounded end of the device. This oscillogram is of low time resolution and does not therefore show any transients due to the intermittent sparking of unit No. 3. Starting with no surface current when the conductivity of the contaminant is negligible, I, is zero and the applied voltage V is uniformly distributed among the three units. When the amount and/or conductivity of contamination start to increase, I, increases and the voltage across each unit is determined by the random value R, of each unit assumes. It should be noted at this point that the resistance of the surface contaminant is affected by various factors acting in opposite directions. While the deposit of soluble contaminants and of water moisture increases the
476
Journal
of The Franklin
Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants conductivity, the current flowing in the layer will dissipate sufficient heat to evaporate moisture and thus reduce the conductivity.* If the layer on unit No. 3 happens to have less contaminant and/or to dry out first, Rs3 will increase faster than Rs2 or R,, and so will the voltage V,, as Fig. 4 shows during the first 2.9 set where the voltages and currents are ta--Contominotion
period _c1
TIME
t
t ARRESTER CURRENT ba + 193
:
f%3
1
c
t
FIQ. 4. Voltages and current envelopes during a typical cantamination period. Time scale of a laboratory test is in seconds whereas under service conditions the time scale is in min.
as described in Fig. 3(b). At t = 2.9 set, V,, attains the value of V, and gap G, breaks down reducing V,, to a small value. Thus the equations characterizing the arrester behavior at this point are v;, Z 0,
(5)
43 = 0,
(6)
Za = &+I,,
and
= &l-J71
K,vZ<&.
(7) (3)
This condition will persist as long as after the instantaneous zero value of Iaa3every half-cycle, the spark is re-ignited making G, conductive for another half-cycle. Re-ignition will therefore take place as long as the voltage is capable of reproducing the spark. As soon as this ceases to be the case, the surface current 1& returns to flow by bridging any gaps with surface discharges. Thus the condition returns to that of Fig. 3(b) and to Eqs. (I) * It is not possible at this point to describe all the processes taking place in and at the surface layer since these alone would require one or more papers. The reader is referred to Ref. (6).
Vol. 294, No. 6, December
1972
477
E. Nasser and (4). Again, when V, = 5, G, becomes conductive and the conditions will be that of Fig. 3(c) and Eqs. (5)-(8). In Fig. 4 this intermittent ignition is clearly illustrated by the envelope curves. The actual oscillogram of Fig. 5 delineates this further. It shows how conditions can change even for one half-cycle (see letter a) and how intermittent the sparking of G, can actually be. This condition of intermittent partial breakdown of the arrester was not expected to occur prior to the execution of these tests. It is highly undesirable since it is associated with the production of high intensity radio noise. It also may heat up the arrester elements and lead to the deterioration of the insulation and the complete failure of the arrester. Again, the voltage I?&across the second unit may also reach V, and ignite a spark in G, at a time when G, is already conducting. This will cause the current to increase abruptly during one-half only or for a longer period. This condition is marked on the oscillogram by the letter b. Finally, the last gap G, may also break down and the whole arrester fails without the occurrence of a voltage surge. The current flowing may not be interrupted and a short-circuit current will destroy the arrester components in a fraction of a second. This is illustrated in Fig. 3(d). V. Test Results
To determine the voltage at which a partial or a complete breakdown takes place, laboratory tests using an artificial contaminant were carried out by applying a uniform dry surface layer of silicon oxide containing an amount of sodium chloride depending on the degree of conductivity, or the severity of pollution, expected. The method used was described earlier by the author (7). After the dry surface layer was applied, the arresters were cooled at 0°C (32’F) in a refrigerator. They were taken to the humid test chamber to be moistened before the test voltage was applied. The test chamber was kept at 23°C and 100 per cent r.h. The surface layer attained its maximum conductivity after about 18 min. Then the test voltage was applied and voltage, surface and arrester current were recorded on an oscillograph and observed on a fast dual-beam oscilloscope. The voltage of the lowest unit was measured by means of an electrostatic voltmeter to prove the occurrence of partial sparkover as explained above. The test voltage was kept constant throughout each single test. To determine the lowest possible voltage at which partial or complete breakdown may occur, tests were carried out several times lowering the test voltage successively until no sparkover was found to occur. Figure 5 shows a typical oscillogram of the two currents IS3 and Ia + Igg3 and the applied voltage V as recorded during the tests. The results are summarized in Fig. 6 in which the breakdown voltage is plotted against the number of arrester units under test. The single unit has almost no decrease in breakdown voltage due to shielding of the spark gap
478
Journal of The Franklin Institute
Tlie Sparking of Lightning Arresters Due to Conductive Contaminants by the grading resistor surrounding it. Removing the grading resistor resulted in a drop in the breakdown voltage of 30 per cent or more. The minimum voltage at which breakdown either of the whole device or of one unit is plotted in Fig. 6 for the dry and clean condition, as reference, and for the contaminated case. In the absence of a contaminant, partial breakdown cannot occur and only complete breakdown is observed. The higher the number of units, the great,er is the decrease in breakdown voltage.
160-
Complete Ereokdown
Portiol Breokdown
Contominoted
I I
I
I
I
I
I
2
3
4
5
6
NUMBER OF ARRESTER UNITS
FIG. 6. Arrester breakdown voltages as & function of n at two extreme surface conditions. Figure 7 illustrates the relative breakdown voltage as referred to the clean and dry value for different unit number. The values of minimum breakdown as given by the curves of Figs. 6 and 7 are for severe contamination of high conductivity as in the vicinity of chemical plants or near the sea coast. The surface conductivity in these tests was about 30 pmho per unit square area. Conclusions
In this paper, some of the complex current and gas discharge processes involved in the contaminated arrester have been presented. The partial spark breakdown of lightning arresters was found to occur at very low voltages. The phenomena described may serve to elucidate some of the failures and unexpected performance of arrester devices. The observed partial breakdown, beside generating excessive radio noise, may eventually lead to the deterioration of the insulation by forming surface tracks that
Vol. 298, Jo. 6. December 1972
479
E. Nasser conducts the current to ground and finally heats up the elements to destruction. Moreover, complete breakdown of the device was found to occur at voltages much lower than those of the clean arrester. This happens by a chain-like reaction triggered by the partial breakdown of one of its units. These processes, it is hoped, might contribute to the clarification of some anomalous arrester behavior.
I
I
I
2
3
NUMBER FIG.
7. The breakdown
voltages
I
I
4
5
OF ARRESTER
UNITS
of Fig. 6 as referred to ideal conditions of infinite surface resistivity.
Acknowledgements
The experimental work described in this paper was carried out during the author’s association with the Siemens Company, West Berlin, to whom the author is grateful for the permission to publish this material. The author wishes to express his thanks to the Insulator laboratory staff for their assistance ; he also wishes to thank the Iowa Engineering Research Institute, Iowa State University, for the continued support of the studies.
References
(1) W. H. Eason and E. C. Saksaug, “Contamination
testing of lightning arresters”, IEEE Winter Power Mtg., Paper No. 31 CP 65-72, 1965. (2) E. Beck, “Lightning arresters for high voltage systems”, Utility Eng. Conf., 1961. (3) E. Nasser, “Fundamentals of Gaseous Ionization”, Chapter 11, New York, WileyInterscience, 197 1. im ungleichformigen Feld bei positiver Spitze”, (4) E. Nasser, “Entladungsaufbau Archiv f. Elektrotechnik, Vol. 44, pp. 157-167 and 168-176, 1959.
480
Journal of The Franklin Institute
The Sparking of Lightning Arresters Due to Conductive Contaminants (5) L.‘B. Loeb and E. Nasser, “Impulse streamer branching from Lichtenberg figure studies”, J. appl. Phys., Vol. 34, pp. 3340-3348, Nov. 1963. (6) L. B. Leob, “Electrical Coronas, Their Basic Physical Mechanisms”, Univ. of Cdif. Press, 1965. (7) E. Nasser, “The problem of flashovers on contaminated insulators”, Electkrotech. Z., Pt. A, Vol. 83, pp. 356-365, May, 1962. (8) L. D. Whitehead, “Rainfall rate and resistivity at East Pittsburgh”, AIEE Trans., Part III, Vol. 80, pp. 952-954, 1963. (9) E. Nasser, “Lightning arrestor, and method for using same”, U.S. Patent No. 537,892, March 1969.
Vol.
294,
No.
6, December
1972
481