US2151559A - Lightning arrester - Google Patents

Description

March 21, 1939.

K. B. M EACHRON LIGHTNING ARRESTER Filed April 24. 1936 2 Sheets-Sheet 1 m 2 W Ta n OE P tc mm KM 8 A m .5 mi

m 5 Mg H 2 Sheets- Sheet 2 Filed April 24, 1936 Inventor: Karl McEa hron, by w KM His Attorney.

Mar.

Patented 21 1939 PATENT OFFICE LIGHTNING ARRESTER Karl B. McEachron, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application April 24,

6 Claims.

The invention relates to lightning arresters such as are used for the protection of electric power transmission lines and associated apparatus from the effects of excessive voltages due to lightning,

switching operations or other causes.

A lightning arrester in the present state of the art consists in general of a gap structure in series with a current limiting element. The current limiting element should have the property of dem creasing and increasing its resistance as the discharge current increases and decreases to such a degree that the voltage across the arrester during discharge is held to safe values for connected apparatus. The gap structure performs the 5 function of a circuit breaker by interrupting the power follow current permitted to flow through the arrester by the current limiting element. The gap structure must also be such as to become conducting at a sufliciently low impulse voltage to maintain a proper margin between the gap breakdown voltage and the insulation strength of the apparatus protected by the arrester.

The general object of the invention is to provide an improved lightning arrester so constructed and arranged as to have these desirable features to a high degree.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Fig. 1

is a sectional view of a lightning arrester constructed in accordance with the invention; Figs. 2, 3 and 4 are perspective views of resistance units used in the arrester; Fig. 5 is a perspective view of a spacer or separator used in the arrester; and

Fig. 6 is a voltage-current curve diagram explanatory of the electrical characteristics of the arrester, both co-ordinate scales being loga rythmic.

Like reference characters indicate similar parts in the difierent figures of the drawings.

The lightning arrester shown in Fig. 1 includes an outer casing having a cylindrical wall portion In formed of insulating material such as porcelain. The cylinder I0 is closed at its upper end by a metal plate secured to the casing by cement so as to form a tight, protective seal. A metal cap |2 is bolted to the plate II and is provided with a line terminal l3. A metal supporting base l4 having a ground terminal I5 is bolted to a metal ring l6 which is cemented to the lower end of the insulating cylinder ID. The opening through the ring l6 into the cylinder I0 is closed by a metal plate except that small openings l8 may be provided in this plate if desired for 5 ventilation. A metal plate I9 is supported by 1936, Serial No. 76,178

screws 20 resting on the plate H, the distance of the plate I9 above the plate being adjustable by means of these screws 20.

A second cylinder 2|, formed of porcelain or other insulating material, is arranged inside the cylinder 0, the two cylinders being coaxial. This inner cylinder 2| is closed and sealed at its upper end by a metal cap 22 and at its lower end by a metal cap 23. The inner cylinder 2| is formed with an inner flange 24 forming a shoulder which supports a plurality of coaxial resistance units 25, 26, 21, 28, 29, 30 and 3|. These resistance units 25 to 3| inclusive are of the form shown in Fig. 2, although this particular number of units need not necessarily be used. Each of the'resistance units 25 to 3| inclusive is formed as a hollow ring or disk with plane upper and lower faces, as indicated in connection with the unit 25 shown in Fig. 2. Each of the plane faces of each resistance disk has a metal coating to which is soldered a metal plate 32 with a central opening. Projections from the inner edges of the two plates 32 of each resistance unit are bent toward each other, their adjacent ends being spaced to form a discharge gap 33 in the opening through the unit.

Another form of resistance unit 34 is supported in the insulating cylinder 2| between the flange 24 and the lower end cap 23, two such units being shown in Fig. 1 and one of these units being shown in detail in Fig. 3. Each resistance unit 34 is in the form of a ring open at one side, the spaced ends 35 of the open ring carrying metal projections extending into the space surrounded by the ring where they are spaced to form the electrodes of a discharge gap 36. One end of the lower resistance unit 34 is secured to a tongue 31 bent from a plate 38 clamped between the end cap 23 and the insulating cylinder 2|. One end of the upper resistance unit 34 is connected by a strap 39 to a hollow metal ring 40 resting on the flange 24 under the resistance unit 25. The other two ends of the two resistance units 34 are connected together by a strap 4|. The assembly of the resistance units 25 to 3| inclusive and the resistance units 34 with their common casing formed by the insulating cylinder 2| and its end caps 22 and 23 constitute a sealed gap unit.

A plurality of resistance plates or disks 42 are arranged in a stack between the upper cap 22 of the gap unit and the top plate I I, a spring 43 between the plate and the upper resistance unit 42 being provided to hold the resistance units 42 and the gap unit in place. The pressure of the spring 43 on the resistance units 42 may be regulated by means of the adjusting screws 20. The

resistance disks 42 are spaced from each other by thin metal plates or spacers 44 which are considerably smaller in diameter than the resistance disks. Under normal conditions there is thus no conductive connection between adjacent disks except between those areas of the disks covered by the spacers 44. The thickness of the disks 44 and therefore the spacing between the outer portions of the resistance disks 42 is suificient to prevent any electrical discharge across these spacers under normal voltage conditions, the spacing oi the disks being preferably of the order of 20 mils. Any current flowing through the disks 42 under normal voltage conditions will therefore be confined substantially to those portions of the disks covered by the spacers 44. The entire upper surface of the upper disk 42 should be covered with a metal coating to provide good conductive contact between this resistance disk and the spring 43. The entire lower surface of the lower resistance disk 42 should also be covered with a metal coating to provide good conductive contact with the metal cap 22. Those portions of the suriaces of the resistance disks 42 which are in contact with the spacers 44 should also be covered with a metal coating 45, as indicated in Fig. 4, to provide good conductive contact between the resistance disks and their metal spacers.

The resistance material in the resistance units 25 to 3| inclusive and 42 must have a high inverse voltage-resistance characteristic. That is, if a voltage applied across a piece of this resistance material is increased, then the resistance 01" the material will decrease very greatly and the current flowing through the material will consequently increase very greatly. It is also important that there be very little, it any, time lag between the change in the voltage and the corresponding changes in resistance and current. A suitable resistant material for this purpose is disclosed in my U. S. Patent No. 1,822,742, issued September 8, 1931, and assigned to the General Electric Company. As disclosed in that patent, the resistance material described therein may have dlfierent predetermined characteristics. Thus, although the resistance units 25 to 3| inclusive are shown as being oi. similar size and shape, they may have diflerent resistances under the same voltage conditions. As indicated in Fig. 1, the resistances of these resistance units decreases from the unit 25 to the unit 3|, the unit 25 having the highest resistance and the unit 3| having the lowest resistance. It is not necessary, however, that the resistances be all diflerent but the units may be arranged in groups, the resistances of the units in each group being of the same value but the resistances of the units in difierence groups having diflerent values. As shown in Fig. l, the units 25 and 23 have the highest resistance, the units 21 and 23 have resistances somewhat lower than that of the units 25 and 26,

and the units 23, 33 and 3| have still lower resistances. The resistance of each unit 34 should be high and should vary little if any with applied voltage. That is, the inverse voltage-resistance characteristic of the units 34 should be much lower than that of the units 25 to 3| and 42. The high resistance of each unit 34 may be partly due to the characteristic of the material used in the unit but it is also largely due to the considerably smaller cross section and longer current path as compared to the units 25 to 3|. The combined resistance of the two units 34 under high impulse voltage conditions may even be comparable to the entire resistance of all the other resistance units 25 to 3| and 42.

The operation of the arrester and the relation of its parts may be clearly understood by referring to Fig. 6 in which the curves show the characteristics of the resistance units and the discharge gaps and how these characteristics determine the flow of current through the arrester when a high voltage transient, such as may be caused by lightning, is applied to the line terminal l3 of the arrester. The terminal |3 of the arrester may be connected directly to a high voltage transmission line conductor such as one phase of a three-phase line. There is then a direct path from this line terminal l3 through the cap l2, the plate I I, the spring 43, the resistance disks 42 and their spacers 44, the cap 22, the resistance units 25 to 3| inclusive, the resistance units 34, the plate 38, the cap 23, the supporting plate I! and its adjusting screws 20, the plate H, the ring l6 and the base H to the ground terminal IS. The resistance material used in the resistance units and the number of units are so selected that under normal line voltage conditions the resistance of the units will be so high that the losses due to the current through the arrester will be negligible. The very slight current which does of course necessarily flow is really beneficial because it tends to keep the arrester and particularly the gap unit warm and dry. Assuming that the arrester is connected to one phase of a 9400 volt three-phase transmission line, then the leakage current to ground under normal voltage conditions will be only about 0.001 ampere, assuming that the resistance characteristics of the resistance units 25 to 3| inclusive, 34 and 42 are represented by the corresponding curves C25 to C3| inclusive, C34 and C42 of Fig. 6. If one of the other phases of the three-phase circuit becomes accidentally grounded," then the voltage of the phase to which the arrester is connected will rise to about 16,200 volts. This increase in voltage across the resistance units will cause the resistances of the units 25 to 3| and 42 to decrease and the current to increase as shown by the curves in Fig, 6, and the current will then become about 0.01 ampere which is still not nearly great enough to injure the arrester.

Assume now that'a high voltage transient, such as may be caused by lightning, is suddenly applied to the line terminal |3 of the arrester, the voltage across the arrester of course increases with extreme rapidity along the upper curve C34 of Fig. 6. This voltage will be distributed among the resistance units in accordance with the other curves of Fig. 6.

The resistance units 42 have been assumed to be all alike and the single curve C42 shows the characteristics of this entire group of units 42. Each of the resistance units 34 and 25 to 3| inclusive is shunted by the discharge gaps 36 and 33. Due to the relatively high resistance of the two units 34 as compared to that of the other resistance units, a very large part of the voltage will appear across these two units 34. As the voltage across the entire arrester increases, the two units 34 will assume an increasingly larger proportion of total voltage. This is because the resistance or the units 34 is not only higher than that of the other units but because these units 34 have a considerably lower inverse voltage-resistance characteristic than that of the other units. These two units 34 may be so proportioned that even something like half the total voltage may appear across them under high impulse voltage conditions. The two discharge gaps 36 are proportioned to break down when the 'voltage across the arrester has reached a predetermined value such as some value around 40,000 volts, as indicated in Fig. 6. Of course it is unlikely that both discharge gaps 36 will break down simultaneously but just as soon as one of them breaks down the arc across it will short circuit its resistance unit and consequently cause the entire voltage to be impressed across the remaining resistance units. Assuming that the discharge gap 36 of the lower resistance unit 34 is the first one to break down, then the breakdown of this gap is indicated by the vertical line C36 of Fig. 6 corresponding to a current of somewhat less than one ampere and a decrease in the total voltage across the arrester from something like 40,000 volts to something like 30,000 volts. As the voltage now increases along the second curve C34 the current will also increase until it reaches something like 40,000 volts again when the second discharge gap 36 will break down, reducing the voltage again to something like 30,000 volts and causing the total voltage across the arrester to be applied to the remaining resistance units 42 and 25 to 3| inclusive. This total voltage again is not uniformly distributed across the resistance units because of their non-uniform resistances. Thus a higher voltage will appear across the units 25 and 26 than across the remaining units, and the discharge gaps 33 of these units 25 and 26 will be the first to break down and short circuit their units. This action is continued, the discharge gaps 33 breaking down in succession as indicated by the vertical lines C33 of Fig. 6 until the only efiective resistance remaining is the combined resistance of the resistance units 42. If the transient has not been already dissipated through the arrester before the last discharge gap 33 break down, then by this time it will be seen from Fig. 6 that the current has reached a value of nearly 1000 amperes. If this current increases, its density through the portions of the resistance units 42 between the spacers 44 may become so great that it will spread out into the spaces beyond the spacers 44, the current arcing across these spaces and the cross section of its path increasing correspondingly. Thus the current is permitted to increase with extreme rapidity until the voltage and current again decrease toward their normal values and the power follow current will be interrupted at the first current zero.

It is important that the discharge across the discharge gaps 33 and 36 occur with as little time delay as possible after the voltages across these gaps have risen to the predetermined value at which the discharges should take place. As shown in Fig. 1, the electrodes of the discharge gaps 33 and 36 are arranged side by side so that a discharge across any gap will illuminate the adjacent gap or gaps. This illumination has the efiect of ionizing the adjacent gap or gaps and reducing the time lag between the application of oltage and the discharge. Thus, after the first gap 36 breaks down, the remaining gaps will break down in succession with substantially no time lag. In order that the discharge gaps 33 may break down in succession, the discharge gap 33 of the resistance unit 25 is a little shorter than that of the resistance unit 26. If, however, the lengths of the gaps are successively increased, they will soon become so long that they will not tend to extinguish the arcs across them after the arrester has discharged and dissipated the transient. As the size of a gap increases with a corresponding increase in the voltage applied to it,- the efliciency of the gap from the point of view of its ability to interrupt a discharge or flow of current across it decreases. In order therefore that the discharge gap 33 of the resistance unit 21 may not be too long this gap may be made as short as that of the gap 25 but the resistance of the resistance unit 21 is smaller than that of the resistance unit 25. The voltage at any time across the discharge gap of the resistance unit 25 will thus be greater than that across the gap of the resistance unit 21 and the former gap will thus break down first. In like manner the resistances of the resistance units 21 and 28 may be of the same value but the length of the discharge gap of the resistance unit 28 may be slightly greater than that of the discharge gap of the resistance unit 21. As shown in Fig. 2, the resistances of the three resistance units 29, 30 and 3| are of equal values but the lengths of the discharge gaps are difierent, that of the gap of the resistance unit 29 being shortest and that of the gap of the resistance unit 3| being longest.

The metal electrodes forming the discharge gaps 33 and 36 are protected from moisture by their sealed casing formed by the cylinder 2| and its end caps 22 and 23. These electrodes are also shielded by the surrounding resistance units 25 to 3| and 34 so as to prevent the formation of corona on the electrodes during normal voltage conditions. Corrosion and deterioration of electrodes is not particularly serious in the presence of either moisture or corona but the electrodes are corroded rapidly if both moisture and corona are present at the same time. A long and effectively useful life of the electrodes is doubly assured by both shielding and sealing the electrodes because the protection against both corona and moisture must fall before corrosion of the electrodes can take place. Another important advantage of the sealed gap unit is that the slight current continually flowing through the resistance elements 25 to 3| and 34 produces a small amount of heat which tends to keep the electrodes dry even if the sealing of the casing of the gap unit fails to be perfect. The structure described provides a current limiting element formed by the resistance units 42 and the gap unit formed by the gaps 33 and 36 shunted across their resistance units 25 to 3| and 34. This combination has the advantage that the total resistance of the arrester may be high enough to limit the current flowing continually through the resistance elements under normal voltage conditions to a negligible value from the standpoint of losses. At the same time the total resistance decreases with extreme rapidity under transient voltage conditions so that apparatus with which the arrester is associated is effectively protected.

The invention has been explained by describing and illustrating a particular form thereof but it will be apparent that changes may be made without departing from the spirit of the invention and the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A lightning arrester including a discharge gap, a resistance unit shunting the discharge gap, a sealed casing enclosing the discharge gap and resistance unit to form a discharge gap unit, a current limiting element outside of said sealed casing, and a casing enclosing the current limiting element and the sealed discharge gap unit.

2. A-lightning arrester including a plurality of discharge gaps connected in series, and a resistance unit shunting each discharge gap, said resistance units being arranged in groups, the units of each group having resistances of the same value and units of different groups having resistances of different values, the discharge gaps associated with each group of resistance units having different breakdown characteristics, and said discharge gaps being proportioned to cause breakdown of the gaps successively through the series.

3. A lightning arrester including a plurality of superposed resistance plates, and good conductive connections between portions of adjacent surfaces of the plates, the remaining portions of said adjacent surfaces being spaced at least about twenty mils.

4. A lightning arrester including a plurality of superposed resistance plates, metal spacers hetween portionsoi. adjacent surfaces of the plates, the remaining portions 01' said adjacent surfaces being spaced at least about twenty mils by said spacers.

5. A lightning arrester including a hollow ring of resistance material open at one side,'and metal electrodes projecting from the ends 01' the open ring to form spaced electrodes within the ring.

6. A lightning arrester including a pair of coaxial hollow rings of resistance material, a metal plate with a central opening secured to each face of each ring, projections from the inner edges of the two plates associated with each ring, the ends of the projections associated with each ring forming spaced electrodes within the ring, and each pair of electrodes being 50 disposed that an are between them will illuminate the gap between the other pair of electrodes.

KARL B. MCEAC'IiRON.

DISCLAIMER 2,l51,5 59.--Karl B. .McEachron, Pittsfield, Mass. LIGH'INING Annns'rea. Patent dated March 21, 1939. General Electric Company.

Disclaimer filed April 22, 1942, by the assignee,

Hereby enters this disclaimer of claim 1 of said patent.

[Oficial Gazette May 19, 1942.]

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