US2881362A - Lightning arresters - Google Patents

Description

J. w. KALB ,8

LIGHTNING ARRESTERS April 7, 1959 Filed Jan. 20, 1954 3 Sheets-Sheet 1 Fig.1

INVENTOR. JOHN W. KALB BYW m y April 7, 1959 J. w. KALB LIGHTNING ARRESTERS 3 Sheets-Sheet 2 Filed Jan. 20, 1954 INVENTOR.

\ JOHN W. KALB April 7, 1959 J. w. KALB LIGHTNING ARRESTERS 3 Sheets-Sheet 3 Filed Jan. 20, 1954 Fig.7

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| 21' I I 29 l NON-LINEAR GRADING m TB NL EA K w N H O J LINEAR GRADING United States Patent LIGHTNING ARRESTERS John W. Kalb, Wadsworth, Ohio, assignor to The Ohio grass Company, Mansfield, Ohio, a corporation of New ersey Application January 20, 1954, Serial No. 405,159 8 Claims. (Cl. 317-40 This invention relates in general to lightning arresters and more particularly to the problems of voltage grading of lightning arresters and particularly arresters composed of a plurality of arrester units mounted in stack formation.

In conventional types of lightning arrester installations to meet high voltage requirements, the arrester stack is composed of several arrester units connected in series in stack formation, each unit having a petticoated or ribbed container or housing of insulating material. Conventional arrester units such as disclosed in Kalb Patent 2,640,096 are designed for a particular voltage rating, and when used in connection with line voltages in excess of the standard voltage rating of the unit, a number of such units are assembled end to end in stack formation and series connected to meet the voltage requirement.

This construction produces a vertical stack and when too many such units are required, the stack may become too high relative to its diameter to be self-supporting, thus requiring insulator bracing. This bracing has at least two objections, first the cost may be relatively high and second the insulator bracing may aifect adversely the desired performance of the arrester.

Another disadvantage of an arrester formed of a number of arrester units of the conventional type mounted in stack formation is the non-uniform distribution of voltages among the several units since the arrester is connected to the line and to the ground. Under some conditions too great a share of the total voltage may occur across the top or bottom unit resulting in unsatisfactory operation of the arrester stack.

The disadvantages of an excessively high stack of arrester units of conventional types are avoided by use of arrester units comprising a plurality of parallel columns of individual arrester devices mounted in an insulating housing having metal end sealing means and the individual arrester devices connected in series and to the end sealing means. An arrester unit of this description is disclosed in Rydbeck Patent 2,611,107. A unit of this type avoids the use of external insulator bracing and the inherent disadvantages stated above for the conventional type of arrester, for instance, a seven unit arrester of the conventional type would have substantially twice the height of a single unit of the Rydbeck disclosure using the same seven conventional units.

However, arresters described above, whether of the conventional type disclosed in Kalb 2,640,096, or in Kalbs copending application Serial Number 359,140, filed June 2, 1953, now Patent No. 2,825,008, or in Rydbeck Patent 2,611,107 whether used singly or in stack formation, are subject to unsatisfactory operation as a result of surface contamination and it is an object of the invention herein disclosed, among other objects, to provide means for correcting adverse operation due to surface contamination and to adverse weather and moisture conditions which may cause heavy external leakage currents over the surface of an arrester unit or a stack of ice arrester units and which currents usually are in the form of surge currents which will negatively affect the performance of the lightning arrester and may even cause external flashover. Furthermore, if lightning should strike the line to which the arrester stack is connected during these adverse conditions of surface contamination and moisture, then the division of voltage among the units may not be equal and the arrester may then be unable to extinguish the power follow current which follows after a lightning surge current.

The conventional type of arrester units, and the arrester devices of which the parallel column type of arrester unit may be composed, are provided with grading resistors connected across each gap element. The purpose of the grading resistor is to maintain relatively uniform distribution of voltage among the serially connected gaps of a complete lightning arrester. The voltage among the several gaps within a unit must be graded and the voltage between the difierent units of a stack formation must be graded.

The disadvantages of a plurality of conventional units forming a stack of unstable height to meet an extremely high operating voltage, thereby requiring bracing means, are avoided by the use of the parallel column arrester unit as stated above and the description which now follows discloses the use of additional grading means dissimilar to that referred to above and which functions to correct to a large degree the adverse effect of surface contamination to which arresters of the conventional and parallel column types are subject when exposed to atmospheric conditions.

An object of the invention is to prevent too large a voltage appearing across any unit in an arrester stack.

Another object of the invention is to provide a means to control unequal voltage division among units in a multiunit arrester stack under heavy surface contamination and leakage current conditions.

Another object of the invention is to provide combined linear and non-linear resistors to control conduction grading of a lightning arrester.

Another object of the invention is to provide resistive means within the arrester unit which will establish a grading current greater than, and hence predominates over, that voltage grading as established by external surface conditions.

Another object of the invention is to provide a conduction grading current by a non-linear resistor operating in accordance with 1=KE", where K is a constant and where n is a positive number greater than one, and wherein the conduction grading current is not permitted to become smaller than I=KE, even where the product KE is less than unit.

Other objects and a fuller understanding of this invention may be had by referring to the following descrip tion and claims, taken in conjunction with the accompanying drawings, in which:

Figure l is a longitudinal sectional view through a lightning arrester unit according to the invention;

Figure 2 is a section on line 2-2 of Figure 1;

Figure 3 is a section on line 3-3 of Figure 2;

Figure 4 is a side elevation view of a complete arrester stack;

Figure 5 is a schematic diagram of an alternative arrangement of arrester elements within a unit;

Figure 6 is a schematic diagram of an arrester unit schematically showing the resistive and capacitive elements;

Figure 7 is a graph of current versus voltage of conduction grading current; and

Figures 8 and 9 represent the line voltage before, during, and after power follow current for non-linear linear resistance grading, respectively.

General construction Figures 1, 2, and 3 illustrate an arrester unit 15, and the Figure 4 illustrates a complete arrester stack 16 made up of three arrester units 15. The Figure 1 generally shows that the arrester unit has metal end plates, shown as a top casting 17 and a bottom casting 18, for closure and supporting purposes, and cemented in place. As best shown in Figure 2, these castings have three projecting casting lugs 19 for bolting the castings to a base or to each other.

The arrester unit 15 includes a petticoated porcelain insulator tube or housing 20 having an internal bore 21. First and second columns 22 and 23 of arrester devices are mounted within the bore 21. The column 22 includes three arrester devices 24 which are identical and includes an additional arrester device 25 which is slightly different. The column 23 includes three of the arrester devices 24 and an additional arrester device 26 which is difierent from either arrester device 24 or 25. Each of the arrester devices 24 includes a porcelain sleeve 27 and two porcelain end caps 28. Each arrester device 24 includes two non-linear resistive valve blocks 29, four linear resistors 44, two coils 30, and two gap elements 31. The valve blocks, coils, and gap elements may be similar to those shown in my copending application Serial Number 359,140, filed June 2, 1953, or as disclosed in Kalb Patent 2,640,096. Each arrester device 24 may include a compression spring 32 to maintain contact between the various parts within the arrester element.

The arrester device 25 includes two of the valve blocks 29 inside a short porcelain sleeve 33 with only a single end cap 28. A conductive pipe spacer 34 spaces yet connects the arrester device 25 with the end casting 18. Compression springs 35 are at the top of each of the columns 22 and 23 to maintain good electrical contact throughout the devices in the arrester unit. The arrester device 26 includes the shortened porcelain sleeve 33 in an end cap 28 and contains therein two of the coils and two of the gap elements 31. It will thus be seen that the complete arrester unit 15 includes fourteen each of the valve blocks 29, coils 30, and gap elements 31. At the ends of each of the arrester devices 24 within the end caps 28 there are provided contact disks 37. The

arrester devices 24, 25, and 26 are connected together in a zigzag electrical connection by connecting straps 36. The first such strap goes from the bottom contact disk 37 in the arrester device 26 to the upper contact disk 37 in the uppermost arrester element 24 in the first column 22. A similar strap 36 connects the lower end of this particular device 24 with the upper end of the adjacent device 24 in the second column 23. This zigzag electrical connection is carried throughout the arrester unit 15 with current entering through the spring and into the arrester device 26, thence through the six arrester devices 24 in series and out through the arrester device 25 and pipe spacer 34 to the bottom casting 18. The arrester device 25 taken in combination with arrester device 26 is the equivalent of one arrester device 24. This division is made in order to facilitate the aforesaid zigzag arrangement. The arrester unit shown is suitable for operating at 60 kilovolts with each of the arrester devices 24 rated at about 9 kilovolts. This unit would normally be used to protect a 69 kilovolt line. For higher line voltages the arrester units are assembled in stack formation such as stack shown in Figure 4.

. The Figures 1 and 2 show pressboard insulator braces 40, each of which has two circular apertures 41 therein, to surround and brace the two columns 22 and 23. A vent hole 42 is provided in each of the braces 40. The Figure 2 also shows a cross section through one of the gap elements 31 and shows the spark gap electrodes 43 and two of the linear resistors 44 which are connected in parallel across each of the spark gaps 43. The linear resistors 44 are used for conduction current which may be less than a milliampere. for grading of voltage so that a generally equal distribution of voltage is provided among the serially connected gaps. The electrodes 43 are provided with arc runners or are horns 45, and the coils 30 establish a magnetic flux to force any are at the spark gap 43 out along the arc runners 45 and into the arc interrupting chamber 46 to thus extinguish such arc. The valve blocks 29 provide their usual function of furnishing a path for the conduction grading current through the linear resistors 44 during normal line voltage operation and provide a low resistance discharge path for the lightning surge to ground so that high voltage is not built up on the line which would puncture the insulation of equipment connected to the line. Also, these valve blocks 29 conduct the power follow current which is the power current determined by line voltage which follows once a lightning surge has caused an arc across the spark gaps 43 with this surge current passing through the valve blocks 29. The valve blocks 29, since they are non-linear in resistance, also provide a high resistance to limit the power follow current to a value which the gaps 31 can interrupt on the next current zero after the lightning surge.

The Figures 2 and 3 show a primary feature of the invention which is the provision of non-linear resistors 48, as applied to a parallel column unit as disclosed in Figure 1. These resistors are shown in the form of cylindrical rods of resistive material with three of these resistors 48 connected in series by electrical connector straps 49 and anchored to adjacent pressboard braces 40. The electrical connector straps 49 are flexible so that the rods 48 may move relative to each other and relative to the braces 40, without breaking or strain, which can be quite important during handling and shipping. In the arrester unit 15 shown in Figure 1 there are seven spaces for such series of three non-linear resistors; and hence, a total of twenty-one serially connected non-linear resistors 48 are provided inside the housing 20 and connected between the end castings 17 and 18. Additional vent holes 50 are provided in the braces 40 near the non-linear resistors 48. The vent holes 50 and 42 provide for circulation of air caused by convection currents or the like. The end rods of non-linear resistors 48 are electrically connected to the end castings 17 and 18 hence are in parallel with the zigzag electrically connected columns 22 and 23.

The Figure 6 schematically illustrates the various components in the arrester unit 15. The uppermost and lowermost arrester devices 24 are shown, with the intermediate devices omitted since they are merely duplicates. The uppermost and lowermost arrester devices 24 are each shown schematically as including one of the spark gaps 43, one of the linear resistors 44, one of the non linear resistive valve blocks 29 with the spark gap 43 and valve block 29 connected in series and with the linear resistor 44 connected in parallel with the spark gap 43, and the non-linear resistors 48 are shown connected across the entire unit 15 of Figure 6. The inherent capacities of the system are also shown by dotted lines with capacitance 53 representing the capacity between contact disks 37 and adjacent end caps 28, and this capacity 53 is in parallel with each of the arrester devices because of the connecting straps 36 (Figure 1). For example, referring to Figure 1, the contact disk 37 in the lower end of the arrester device 26 and the contact disk 37 immediately below, which is at the upper end of the arrester device 24 immediately below in that same column 23, will form the capacitance 53 which is in parallel with the uppermost arrester device 24 in the column 22. It will be noted that each of the arrester devices has such a capacitance connected in parallel therewith. In the Figure 6 there willalso be a capacity from the contact disks 37 to line 54, however, this is negligible compared with the capacity between adjacent contact disks 37.

The Figure 6 further shows the inherent capacitance 55 which is the capacity to ground of each of the arrester devices 24, 25, and 26. The intermediate devices, not shown in Figure 6, will also have inherent capacitances 53 and 55. The particular zigzag arrangement as shown in Figure 1 thus produces the beneficial result of the contact disks 37' which, being spaced a short distance by the two end caps 28, provide the inherent capacities 53 which establish condenser grading of voltages among the spark gaps which helps the resistance grading of the voltage as determined by the grading resistors 4.

Surface contamination The Figure 4 shows three of the arrester units 15, which may be of conventional type or as disclosed in Figure l, and assembled in an arrester stack 16, and these units have been numbered 15A, 15B, and 15C for convenience in referring to them. Also, the top and bottom castings of these arrester units have been given suflixes .A, B, and C. The arrester stack may be connected to a line 54 and to a ground bracket 56. A shielding ring 57 may also be used as is conventional.

Arresters are normally installed outdoors, and as such they are subject to the accumulation of external surface contamination the same as any other insulator under similar exposure. The degree of surface contamination and its influence on arrester performance varies widely, depending upon the physical proximity of the arrester to the source of contamination, prevailing air motion direction relative to the source versus the arrester, chemical nature of the contamination, degree of wetting, and exposure time between successive arrester surface cleaning operations.

Surface contamination, as long as it is not electrically conducting, will not influence the arrester performance. Except under extreme conditions, surface contamination when dry is usually high in resistance and therefore permits only very small surface leakage currents to flow. These dry surface leakage currents very likely would be lower in magnitude than the normal conduction grading current through a resistance graded arrester. When the surface leakage currents are this small, they would have very little influence on either a single unit arrester 15 or a multiple unit arrester stack 16.

When surface contaminations are wetted, the leakage currents that flow can increase dozens or hundreds of times over the dry condition value. The amount of increase depends upon the chemical nature of the deposits and the degree of wetting.

Surface leakage current investigation on insulators indicates that an insulator, which has a high surface resistance and hence only a small fraction of a milliampere leakage current when dry, can experience a rapid decrease in surface resistance when wetted. The decreased surface resistance can lead to leakage current surges reaching many milliamperes in magnitude, and durations lasting from a fraction of a cycle to several seconds. Leakage current surges may reach 50 to 100 milliamperes under extremely heavy contamination conditions. When these high order leakage current surges develop, insulation surface flashover is imminent. Such conditions can develop along seat coast exposures and near chemical plants, etc., in industrial areas.

Arrester surface fiashover would also be imminent if surface leakage currents reached 50 to 100 milliamperes. Surface flashover, however, would be a lesser problem than would be the events that might occur within the arrester as a result of these heavy leakage currents.

A problem exists in maintaining adequate grading in a multiunit arrester stack 16. In this case the surface voltage division established by surface contamination is conductively introduced into the internal grading circuit at each unit end casting, because such end castings provide an electrical path from the outside to the inside of the stack. The greater the number of units, the greater is the possibility that the external surface leakage currents will adversely influence the internal resistance grading. Therefore fewer units of higher voltage rating are desirable for any multiunit arrester rating. Fewer units of higher voltage rating have the added advantage of having fewer seals that must remain moisture and gas tight in order to assure continued freedom from internal metal electrode corrosion.

The present invention meets this problem by having each of the arrester units 15 of a greater operating voltage rating so that fewer end castings 17 and 18 are exposed to both the external and internal parts of the complete arrester stack 16.

Figure 4 illustrates a three-unit arrester stack 16. As long as the units are clean and dry the internal linear grading resistors 44 cause each unit 15 to share equally in the line-to-ground 60 cycle voltage across the arrester stack 16. Therefore each one has one-third of the total 60 cycle line-to-ground voltage across it. This means that the top unit end casting 17A has percent of the line voltage on it; the bottom casting 18A appears at 67 percent of total arrester line-to-ground voltage. Similarly the lower casting 18B will appear at 33 percent, with the bottom casting 18C of the ground unit at 0 percent.

Assume next that a heavy, uniform contamination layer accumulates on the arrester external surface. If these contamination coatings are uniformly distributed and wetted, the external surface voltage division will be identical with the internal voltage division and there will be no influence on the voltage grading within the arrester.

Next assume the condition shown in Figure 4 in which the upper two units 15A and 15B are selectively wetted and the lower unit is wetted to a lesser degree due to being sheltered by an object such as a transformer tank 58. Under this condition the surface resistance of the top two units 15A and 15B would be decreased and the lower unit 150 might stay near its original value. This has been schematically illustrated in Figure 4 by the large low value resistances 59 and 60 across the top and middle units 15A and 15B, respectively. These represent relatively low values of surface leakage resistance so that high surge surface leakage currents can occur. The lower unit 15C which is not being wetted has a high value of surface leakage resistance 61. Assuming for the moment that the non-linear resistors 48 are not included inside each of the arrester units 15, but each unit is of the conventional type, then if the conduction and wetting conditions were sufliciently severe, the external voltage division would attempt to assume a considerably different voltage division than that normally produced by the linear grading resistors 44. The external voltage division might then attempt to cause the end casting 18A to assume 90 percent of the line-to-ground voltage and the casting 183 to assume 80 percent of the line-to-ground voltage, while the internal grading resistors 44 would be attempting to cause the casting 18B to assume a 33 percent share of the 60 cycle line-to-ground voltage. If the surface leakage currents were large relative to the internal grading current, the surface grading would predominate, in which case in Figure 4 the gap structures 43 of the ground unit would probably spark over, perhaps followed with gap sparkover in the remaining two units 15B and 15A.

If a transient voltage should cause the arrester series gaps in Figure 4 to spark over while the severely nonuniform external voltage division existed, or if the arrester units 15 should spark over internally one at a time, as discussed in the previous paragraph, too great a share of the recovery voltage transient would appear across the ground unit 15C after power follow current zero. If the ground unit series gaps failed to interrupt power follow current at current zero, the remaining two units probably could not also and the arrester stack would be expected to fail.

assrsea 17 Non-linear resistor A principal novel feature of the invention is the in clusion of the highly non-linear impedances or resistors 48 connected between the end castings of each arrester unit 15. The purpose of the non-linear resistors is to control unequal voltage division between units in a multiunit arrester stack under heavy surface contamination and leakage current conditions.

The highly non-linear resistors 48 connected in parallel with the block and gap structures in each unit 15 prevent power frequency voltages materially in excess of unit rating from appearing across any unit in the stack. There are now three continuously connected electrical paths in parallel between the end castings of each unit. These are: (l) the series gap linear grading impedances or resistors 44 and valve blocks 2?, (2) the newly added and separate non-linear resistors 43 extending from line-to-ground caps, and (3) the surface leakage resistances 59, 60, and 61.

When the voltage across the ground unit 15C attempts to increase in response to non-uniform external surface voltage grading, the voltage is automatically increased across the extra internal non-linear resistance 43, and due to its highly non-linear properties its resistance rapidly decreases with increase in voltage. The external surface leakage current is then partly diverted through it to ground with only a minor increase in voltage across the unit. If only non-linear resistors are used to provide resistor grading current, whether among gaps within a unit or among units within a stack, this current is so small at the low voltages associated with current interruption recovery transients that it is not able to establish equal voltage distribution at this critical operating condition.

The combined use of linear type grading resistors within the series gap structure plus a completely separate non-linear grading resistor connected in parallel within the arrester unit offers the distinct advantage of automatic voltage equalization between or among a plurality of stacked units under heavy surface contamination conditions without the inherent disadvantage of detracting from the interrupting ability of the series gap structure in the short interval of time following power follow current zero. This disadvantage would exist if non-linear type series gap grading resistors were used in place of linear types.

The parallel column and zigzag connection arrangement of low voltage arrester devices in the present design makes possible a completely self-supporting arrester up to ultra high voltage ratings, such as 276 kilovolts.

It thereby eliminates the bracing costs and the inherent electrical disadvantages of any bracing arrangement that introduces bracing insulator leakage current voltage divisions into the arrester stack.

The Figures 8 and 9 show voltage conditions before,

during, and after power follow current. The Figure 8 shows the conditions for non-linear grading, such as if non-linear resistors were used in place of the linear resistors 44, and without the resistors 48. The Figure 9 shows voltage conditions for linear grading resistors 44 as used in the present invention, and without the resistors 48. In Figure 8 the solid curve 65 indicates the voltage on the top unit 15A in the arrester stack 16. The dash line curve 66 indicates the voltage on the bottom unit 150. The dotted curve 67 indicates the ideal condition. The reason that these voltages on the various units 15A, 15B, and 150 are not in phase is because the grading resistors of the upper unit are supplying the capacity current to ground of all the units underneath it, while the resistors of the bottom unit 15C have no such current to supply. At a point 68 the lightning surge is assumed to strike, momentarily with respect to the sixty cycle current. The power follow current flows during the time 69 of the curve from point 68 until point 70, which is the next current zero. All the voltages are in phase at the portion 69, since the capacity current is then negligible relative to the power follow current. At this point 70 the valve blocks 29 must have sufficient resistance to limit the power follow current to a valve which the gaps can interrupt. Immediately following the point 70, after the interruption of the current, small magnitude voltage transients, as shown at 71, are experienced on the voltages appearing across each of the units. Since the gap structures 43 have previously been arcing over, these gap structures are filled with an ionized medium and these transient voltages 71, if sufiiciently severe, can cause the arc to restrike in which case arrester failure may be expected.

The Figure 9 shows similar conditions but not the circuit and structure of the present invention, since the effect of the resistors 48 is not included. Here the curves and points have been denoted by the same reference numerals with the sufiix A. It will here be noted that the crest voltages of the curves 65A and 66A are not as well controlled as with the non-linear resistor grading of Figure 8. However, the transient voltages at 71A are more nearly equal in magnitude just after current interruption and are better controlled by the linear resistors 44. This diagram of actual voltages obtained for linear and non-linear resistor grading shows the superiority of using the linear resistors 44, under conditions of low instantaneous voltage.

The present invention uses the linear grading resistors 44 which have the characteristics shown in Figure 9.. Also, by using the non-linear grading resistors 48, the desirable,

'but not the undesirable, characteristics shown in Figure 8 are incorporated in the complete arrester.

The Figure 7 illustrates a further novel feature of the invention. Here the curve 74 illustrates the curve of current versus voltage for the linear resistors 44. These linear resistors 44 have considerably higher resistance than the valve blocks 29 for normal voltage conditions and thus primarily determine the resistor grading current. The curve 74 may be expressed by the formula I==KE wherein K is a constant. The curve 75 illustrates the curve of the non-linear resistors 48 and may be expressed by the formula I=KE, where n is a positive number greater than one. In typical non-linear resistors in use today in arresters the value of 11 may be in the order of 5. This curve 75 has thus been plotted as I KE The .curve 76 is the summation of the two curves 74 and 75.

The combined curve 76 therefore illustrates the resistance curve which the complete arrester stack 16 follows. The point 77 may illustrate the R.M.S. value of the total line voltage on the complete arrester stack. The point 78 may therefore illustrate the crest 60 cycle voltage. The point 79 therefore illustrates the value of current introduced by the linear resistors 44 and point 80 illustrates the current produced by the non-linear resistors 48. The point 81 illustrates the combined current which is primarily caused by the non-linear resistors 48. At a particular voltage value the two curves 74 and 75 cross at a point 82, and for all voltage values below this point the grading current caused by the linear resistors 44 is predominant. Referring again to the Figures 8 and 9, it will be noted that if non-linear grading resistors were used in place of the linear grading resistors 44, as shown in Figure 8, then the grading current through such an arrester would attempt to follow a curve similar to the curve 75. This would mean that the grading current for the low voltages near current zero would establish a negligible conduction grading current and thus jeopardize the otherwise uniform voltage division among the series gap elements that is so highly important at current zero and the short period thereafter. The amount of conduction grading current caused by non-linear resistors in the place of the resistors 44 is so small that these non-linear resistors are essentially insulators. However, in the present invention it will be seen that the conduction grading current follows the curve 76 rather than the curve 75 and therefore the conduction grading current can never 'be less than the curve 74 which is a current determined by I=KE.

The voltage transients 71 or 71A occurring immediately after power follow current zero may be somewhere in the range between a point 83 and a point 84. The point 83 may be near zero voltage. At these points it will be seen that on the curve 76 sutficient conduction current grading is provided to maintain uniform voltage division of voltage among the series gap elements, whereas if the curve 75 were to be followed, practically negligible conduction grading currents would flow and thus it could not be certain that uniform voltage division would be achieved with consequent possibility that the series gaps would restrike and therefore the arrester would fail.

. The combined resistors 44 and 48 provide means at the point 78 to establish generally equal voltage division among each spark gap which resistive or impedance means is primarily that from the non-linear resistors 48; and the same impedance or resistors 44 and 48 provide means immediately following power follow current zero to maintain generally uniform voltage drops across each spark gap by maintaining a conduction grading current at least equal to I=KE, even where the product KE is less than unity This may be at the point 84, for example, and it will there be seen that the resistive means which provides this conduction grading current is primarily that of the linear resistors 44.

Another way of expressing the same thing is that the combined grading resistors 44 and 48 provide the grading current under normal line voltage conditions. The nonlinear resistors 48 primarily provide a means for controlling the grading current primarily during crest line voltage conditions generally according to I=KE", and the linear resistors 44 primarily maintain the grading current at or near current zero at least equal to I=KE. It will be seen that the grading resistors 44 and 48 have dissimilar characteristics, with the linear resistors being directly associated with the spark gaps 31 to control the distribution of voltage relative to the spark gaps, and the non-linear resistors 48 control the distribution of voltage with respect to the external surface of the housing 20.

' The linear resistors 44 may typically be composed principally. of carbon and a binder, and the non-linear resistors may be composed principally of silicon carbide and a binder, so that each of these resistors may be shaped in the form of cylindrical rods.

' The Figure shows an alternative arrangement of valve blocks 29, coils 30, and gap elements 31 over that shown in. Figure 1.' The Figure 5 shows schematically that the gap elements 31 and coils 30 may all be stacked in a first column.22A and all the valve blocks 29 may be stacked in the second column 23A. The Figure 5 also schematically shows the non-linear resistors 48 connected between the top and bottom castings 17 and 18.

The arrester units 15 of Figures 1 or 5 are preferably filled with an inert gas, such as nitrogen.

The-complete arrester 16 thus provides one or more units 15 with means for better maintaining uniform voltage grading among units at all times and under all conditions. The units 15 may have any form of arrester devices inside, and the devices 24, 25, and 26 have been described as an example. Such arrester units have one or more valve blocks in series with one or more spark gaps. In parallel with thespark gaps are the linear impedances or resistors, which primarily establish the conduction grading current at or near current zero. Also in parallel with the spark gaps are the non-linear impedances or resistors 48 which primarily establish the conduction grading current at or near crest line voltage, and despite any external leakage current. Also, the non-linear resistor grading could be accomplished by a continuous resistor, such as a semiconductor glaze on the internal or external surface of the housing 20, rather than using the discrete resistors 48.

It will be apparent that the non-linear resistor herein disclosed can be used in combination with conventional arresterunits such as disclosed in Kalb Patent 2,640,096, applying the non-linear resistor thereto in the same manner as disclosed in Figures 2 and 3 herein, that is, connected between the end closing members, or by mounting the non-linear resistor in a housing of its own and connecting the end plates thereof to the end plates of the conventional unit. In either case the resistor would function in the same manner as resistor 48 herein disclosed to grade the voltage among units.

Further, it will be evident in the construction disclosed in Figures 2 and 3 herein or in the unit construction disclosed in Kalb copending application, that one of the linear resistors 44 may be replaced by a non-linear resistor in which case the two dissimilar resistors will be in parallel and both in direct parallel with the associated gap. This construction would achieve the major benefit of the invention, that of giving good voltage distribution at rated voltage plus adequate grading at low transient recovery voltages, and would permit the non-linear resistors to supply the capacity to ground currents of the devices 24, Figure 6, through the capacities 55. The construction illustrated in Figures 1 to 4 results in lower arrester impulse sparkover voltages, however, because the substantial non-linear grading resistor current immediately preceding gap sparkover does not flow through the valve blocks, thus avoiding generating an IR discharge voltage to be added to the gap sparkover voltage.

The term arrester component shall be construed to include any of the parts of the complete arrester, such as the units 15, devices 24, 25, and 26, valve blocks 29, coils 30, and gap elements 31.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A lightning arrester stack comprising, a plurality of lightning arrester components serially connected end to end, said stack including electrically conductive plates between adjacent components and providing an electrical path from the exterior to the interior of said components, each of said components including at least one spark gap connected between said plates, resistor means in each of said components to establish a small conduction grading current to establish a substantially equal voltage distribution among said spark gaps, means connected in series with said gaps having a changeable impedance under surge voltage conditions, said components being subject to external surface contamination to. establish an external surface leakage current having a. tendency to upset uniform voltage grading between components, and means separate from said resistor means and connected between said plates in parallel with said serially connected spark gaps to maintain substantially uniform voltage grading between components despite nonuniform external surface leakage current.

2. A lightning arrester stack comprising, a plurality of lightning arrester units serially connected end to end, said stack including electrically conductive plates between adjacent units and providing an electrical path from the exterior to the interior of said units, each of said units including a plurality of serially connected spark gaps connected between said plates, means connected in series with said gaps having a changeable impedance under surge voltage conditions, and linear and non-linear resistor means, one of said resistor means being connected between said electrically conductive plates and in parallel with said serially connected spark gaps, and the other of said resistor means being connected in parallel with the series combination of said gaps and changeable impedance means.

3. A lightning arrester stack comprising, a plurality of lightning arrester units serially connected end to end, said stack including electrically conductive plates between adjacent units and providing an electrical path from the exterior to the interior of said units, each of said units including linear resistors and a plurality of serially connected spark gaps connected between said plates, said units being subject to external surface contamination to establish an external surface leakage current having a tendency to upset uniform voltage grading between units, means connected in series with said gaps having a change able impedance under surge voltage conditions, and nonlinear resistor means connected between said electrically conductive plates and in parallel with said serially connected spark gaps and changeable impedance means to limit the increase in voltage across any unit as effected by non-uniform external surface leakage current.

4. A lightning arrester stack to protect electrical equipment to which connected, comprising, a plurality of lightning arrester units serially connected end to end, each of said units including an electrically conductive end plate at least at one end thereof to interconnect adjacent units and providing an electrical path from the exterior to the interior of said units, first resistor means connected between said end plates and having a relatively high resistance for normal line voltages to establish a small conduction grading current, spark gaps connected in said units and connected to said first resistor means to establish a substantially equal voltage distribution among said spark gaps, said units being subject to external surface contamination to establish an external surface leakage current having a tendency to upset uniform voltage grading among units at said end plates, said first resistor means including as part thereof non-linear resistor means in series with said spark gaps and to establish a relatively low resistance therein during lightning surges to protect electrical equipment to which the arrester stack is connected, said non-linear resistor means having the property to increase the resistance thereof after the lightning surge and during power follow current to enable said gaps to interrupt the are across said spark gaps at the next current zero following the lightning surge, and second non-linear resistor means within each of said units connected between said electrically conductive end plates and in parallel with said first resistor means to limit the increase in voltage across any unit as effected by non-uniform external surface leakage current.

5. A lightning arrester stack, comprising a plurality of lightning arrester units serially connected end to end, each of said units including a hollow insulating tube, the external surface of said tube being subject to surface contamination from the ambient atmosphere to thus establish an external surface leakage resistance, electrically conductive plates at the junction of adjacent units and interconnecting the inside and outside of each tube, a plurality of first non-linear resistor valve blocks, a plurality of spark gaps, conductor means for connecting in series said valve blocks and said spark gaps inside said units between the two plates at the ends of each unit, a linear resistor connected in parallel with each of said spark gaps to establish substantially equal voltage grading among all spark gaps, and second non-linear resistor 12 means connected between said plates to establish substantially equal voltage grading among all units.

6. An arrester comprising, a hollow insulator housing, end plates closing. the ends of said housing, a plurality of valve blocks, metallic arc plates having a plurality of dished indentations on both sides thereof, linear resistor spacers, non-linear resistor spacers, said are plates being stacked with said spacers therebetween to establish arc gaps by adjacent convex faces of said indentations, there being at least one linear and one non-linear resistor spacer between each of said arc plates, and means for connecting said stack of are plates and spacers in series with said valve blocks and between said end plates.

7. A lightning arrester stack comprising a plurality of lightning arrester units serially connected end to end, said stack including electroconducting plates between adjacent units for mechanically supporting and electrically connecting the units and for providing an electrical path from the interior to the exterior of each of the units, each of the said lightning arrester units comprising a ceramic housing, a plurality of serially connected spark gaps and a plurality of valve blocks connected in series with the spark gaps between the plates and arranged in a multiplicity of columns within the ceramic housing and a plurality of electrical conductors arranged between the columns to form a series connection for the spark gaps and valve blocks, said units being subject to surface contamination on the exterior of the ceramic housings to establish external surface leakage currents between the plates and thereby upset uniform voltage division between the units, and separate non-linear resistor means in each unit comprising a plurality of series connected non-linear resistors disposed within the housing between the columns and the wall of the housing electrically connected between the electroconducting plates of that unit to limit the increase in voltage across any unit due to non-uniform external surface leakage currents in the remaining units.

8. A plurality of arrester units serially connected in an arrester stack, each unit comprising a hollow housing, a plate at each end of said housing, with said plate extending from the interior to the exterior of said housing in the said stack, a plurality of columns inside said housing between said end plates, each column composed of a plurality of arrester devices, means for connecting all of said devices in series and to said end plates, said devices including a plurality of spark gaps connected in series, a linear resistor connected in parallel with each spark gap to establish a conduction grading current for said gap, and non-linear resistor means connected to said plates to provide an additional conduction grading current for said arrester unit.

References Cited in the file of this patent V UNITED STATES PATENTS 2,135,085 Ludwig Nov. 1, 1938 2,151,559 McEachron Mar. 21, 1939 2,253,361 Berkey Aug. 19, 1941 2,600,149 Yonkers June 10, 1952 2,611,107 Rydbeck Sept. 16, 1952 FOREIGN PATENTS 560,306 Great Britain Mar. 24, 1944 669,000 Great Britain Mar. 26, 1952

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