US2002042A - Excess-voltage protective structure - Google Patents

Description

' y 1935. R, R. PITTMAN EXCESS VOLTAGE PROTECTIVE STRUCTURE Filed Jan. 11, 1954 3 Sheets-Sheet 2 INVENTOR VMG W y 1935- R. R. PITTMAN 2,002,042

EXCESS VOLTAGE PROTECTIVE STRUCTURE Filed Jan. 11, 1934 5 Sheets-Sheet 3 INVENTOR @w PM Patented May 21, 1935 PATENT OFFICE excess-venues raorsc'nva srauoruas Ralph a. Pittman, Pine mun, Ark.

Application January 11, 1934, Serial No. 708,170

13Clalms.

This invention relates generally to excess-voltage protective devices for use with high voltage alternating current electrical systems, and has for its principal object the provision of means for positively preventing an excessive superimposed voltage rise on a conductor or connected apparatus, such as might be caused by lightning.

It is a particular object to provide a novel arrangement for the purpose of effecting the cascade flashover of a series arrangement of spark gaps formed by a plurality of normally insulated conducting electrodes supported in spaced relation.

A further object is the provision of a novel means for reducing the time lag of flashover of such a series arrangement.

A further object is the provision in an are extinguishing device embodying one of the spark gaps of such a series arrangement, of means for 20 reducing the rate of recovery voltage rise, thereby facilitating the extinguishment of the arc.

A further object is to provide means for reducing the voltage stress normally imposed on the insulating members of the arc extinguishing device to prevent thermal damage to such members from creepage current.

A yet further object is the provision in an arc extinguishing device of the expulsion type, of a novel discharge path a providing a low initial breakdown voltage, andv an arcpath which may subsequently be taken by an arc following sparkover which varies both in length and cross-sectional area, the latter quantities being so related under all conditions as to effect are extinguishment.

With the above and other objects in view which will appear from the description, my invention resides in a novel electrostatic field control arrangement, in combination with a novel type of arc extinguishing device in which is embodied certain members which assist in arc extinguishment, and at the same time effect the desired electrostatic field control, thus making possible mum of parts, and greatly simplifying the structure.

In the drawings:

Figure 1 illustrates an assembly of one embodiment of the structure, shown in perspective.

Figure 2 is a diagram of a curve showing a volttime characteristic of my structure, in comparison with similar characteristics of certain well-known types ofprotective gaps.

Figure 3 is an elevation of the arc ext nguishing 65 device, shown in section.

the attainment of the desired results with a mini- Figure 4 is another view of the arc extinguishing device, as seen from the plane indicated by the line 4-4 in Figure 3.

Figure 5 is an additional view of the are extinguishing device, as seen from the plane indi- 5 cated by the line ll in Figure 3.

, Figure 6 is a fragmentary plan view of the mounting base of the arc extinguishing device, illustrating the venting means.

Figure 7 is a perspective of the cylindroid 10 forming the upper electrode of the arc extinguishing device.

Figure 8 is an elevational view of another embodiment iii the invention, shown partly in section.

Figure 9 is an elevational view of yet another embodiment of the invention, also shown partly in section.

Referring now in detail to the embodiment of the invention illustrated in Figure l, a portion of ausual type of steel substation supporting framework, comprising the steel members 35, 36, and 31 is designated in its entirety by the numeral II. In accordance with usual practice, this framework is maintained at substantially ground potential, and is here shown connected to the ground 50. An insulating member, or bus support, which may be an assembly of several insulating units, is designated by i l, and extends in a downwardly direction from the steel member 31, being attached thereto at its upper end in any suitable manner, as by the cap screws 38. An upper conducting member i1, here shown as a metal ring is rigidly supported in coaxial relation with the insulating member ll, by means of a pair of outwardly and downwardly extending metal arms 28, having their respective ends attached to the ring I! by the bolts IO, and to the lower end of the insulating member II by the bolts ll. One of the bolts ll also provides a means for connecting a line conductor II to the lower end of the insulating member II.

A conducting electrode 2|, here shown as a metal rod, is also supported in any suitable manner at the lower end of the insulating memher H, and extends downwardly therefrom coaxially with the ring I1 and the insulating member ll.

From the above description it will be apparent that both the ring I! and the electrode 20 60 are at the same potential as the conductor l3. For reasons which will later appear, the diameter of the ring II should be large in comparison to the diameter of the electrode 20.

A second conducting member, as the metal ring l8, of substantially the same size as the ring I1, is rigidly supported in coaxial spaced relation therewith by means of the metal arm 22, the latter having its respective ends attached to the ring l8 by the bolt 23, and to the steel framework member 36 by the bolts 24. Since the supporting framework is maintained at substantially ground potential, it will'be apparent that the voltage of the conductor with respect to ground potentialis definitely limited I to the value determined by the spacing of the rings l1 and It.

An arc extinguishing device, for convenience designated in its entirety in Figure i by-the numeral l2, and shown more in detail in Figures 3, 4, 5, 6 and 7, is supported coaxially with the electrode 20 by means of the flange 32, which may be attached by means of cooperating threads to the lower end of the device l2, and the bolts 34 cooperating with the flange 33 and thexcon-q ducting member,

- The arc extinguishing'device I 2, as shown in Fig. 3. and later-described more in detail, in-

is preferably determinedby 'the' insulation cludes the upper electrode 29 and the lower electrade 32, which are normally insulated from each other, and means for extinguishing an arc-which may occur therebetween. At the upper end of the arcextinguishingdevice l2, 'andelectrically ing metal arms '26, at the respective ends of which, and in coaxial spaced. relation with the rings l1 and I8, and the'arc extinguishing device I2, is supported, by means of the bolts 21,

V a third conducting member or metal ring 19.

The latter is substantially the same size as the rings l1 'and 18. An electrode 2|, here shown as a metal rod, is carried in any suitable manner at the upper end of the arc extinguishing device I2, and extends coaxially inanupward direction toward but in spaced relation with the electrode 20, to form a spark gap therebetween.

From the above description, it will be apparent conductorto ground; the first between the line conducting member or ring I1 and the ground conducting member or ring. 18, and the second from the electrode 20 to the electrode 2|, and

thence through the arc extinguishing device I 2 to the grounded member 35.

Assuming now that a highvoltage with respect to ground potential is impressed upon the conductor l3, it will be apparent that the distribution of the resultant electrostatic field between the rings l1 and 18 will retardthe formation of streamersfrornthe electrodes and 2|, thus reducing the time lag of flashover therebetween.

It will also be seen that all of the flux passing.

from the electrode 20V to the electrode 2| is further divided between the rings l8 and I9, and

from line to ground, one of which is embodied in the arc extinguishing device l2. As the voltage division between the two above-mentioned series gaps is dependent upon the relative flux division, or the relative shunt capacitance of the gaps, it will beseen that the voltage required to sparkover the series arrangement may bemade substantially equal to that required to sparkover the longer of the two gaps. The spacing between the rings 11 and I8 must therefore be somewhat greater than-the electrode spacing of the longer of the two series gaps, but may be made substantially less than the sum of the spacings of the electrodes forming the series arrangement.

It will be seen'that the ring l'I cooperates with the ring II to provide a capacitance in shunt" strength of the apparatus to be protected, thus providing a definite coordination gap having a desirable volt-time characteristic. The spacing of the electrodes forming the series arrangement to ground may then be adjusted to prevent 'sparkover of the coordination gap, since the series arrangement embodies the arc extinguishing device for preventing a circuit outage which would occur should the coordination gap sparkover.

better illustrated by Figure 2, in which the line a may represent the v'olt-time breakdown characteristic or an electrical apparatus to be protected. The line b represents the characteristic of the above-mentioned coordination gap formed by vthe'spaced rings IT and II, the slope of the line being determined by the physical dimensions' of'the rings employed, and it is thereby adjusted to lie below and substantially parallel to, the characteristicof the apparatus 'tobe protected. Theline c of the protective structure, due to the .novel arrangement of .the conducting members, may be adjusted to flashover at substantially lessvoltage than either the apparatus or the coordination gap for those time intervals characteristic of the rate of voltage application due to natural lightning; that is, the characteristic of the protective structure in which is em-, bodied the arc extinguishing device lies.below and substantially parallel to that of the apparatus and coordination gap to be protected.

The lines d and-e of Figure2 represent respectively the characteristics of ring-ring and rod-rod gaps which have anelectrode separation equal to the ,sum .of the electrode spacings of series gaps embodied in the invention. It is apparent that neither of these gaps ofiers desir- The volt-time characteristic of the invention is able protection to the connected apparatus, not. I

only because the voltage required to cause-sparks over is much higher than that of the protective such gaps are not inherently self-extinguishing. The are extinguishing device v illustrated in Figures 3, 4, 5, 6, and '7 wil now'be described indetail. a tubular housing a, of suitable insulating material, is supported with its principal axis normal to the metal mountlng'member 2i, h

and is provided at its lower end'with the metal flange 33. The bolts 34- extend through the flange 33 and the mounting member "to provide 5 arrangement herein disclosed, but also because II. .by means of cooperating threads on the r the top at the midpoint of the cap II and coaxial with the housing Ill, together withfcooperating threads on the lower end of the spark gap elec- ,trode 2| provides a convenient means for rigidly mounting the latter on the upper end of the device. An additional pair of tapped holes 50 in' the cap it, located laterally of the electrode II, are provided for receiving the cap screws 25, shown in Fig. 1. Eccentrically disposed within the housing 30, and slidably supported with its base resting on the member 35, is the hollow tubular insulating element 3 I, having its upper end completely closed by an integral portion thereof, and open at its lower end. The inner surface of the element II is provided with a conducting coating 32, which is preferably of arelatively high resistance as compared to an equivalent section of copper. The coating may be in the form of a conducting graphite or metallic paint, or anyconvenient ma- .terial having the desired resistance characteristic. In addition to covering the entire inner surface of the element ii, the conducting coating extends over the lower end of the wall of the element 3|, to provide a lower electrode for the device. The coating 32, being in contact with the supporting member 36, is thereby maintained at substantially ground potential.

A hollow metal cylindroid 29, better shown in Figure 7, closed at the top and open at the bottom, is closely fitted over the top of the element II, in such a manner that it extends downwardly from the top for a portion of the length of the element SI and closelyengages the top and outer.

surface thereof. The outer surface of the cylindroid 2! is aligned with the outer surface of the element I I, so that a continuous smooth external surface is provided on the assembly comprising these parts. The top of the cylindroid is held in engagement with the upper end of the element 8! by means ofthe resilient conducting member II, the latter being positioned between the cap ll and'the cylindroid 29. The resilient conducting member 44 also serves toeleetricall'y connect these parts, to the end that the cylindroid 28 is maintained at substantially the same potential as the cap II and theelectrode 2i The cylindroid 29, in cooperation with the conducting coating 32, provide respectively the upper and lower electrodes of the arc extinguishing device, and are so spaced longitudinally of thehousing 30 that the weakest electrical path is internally of the housing, and along the inner wall thereof. I

, The wall of the" housing 30 is provided with a pair ofopenings, into each of which is fitted the hollow metal cylinders 4 I, a flange being provided adjacent one end of each of the cylinders for en? gaging the inner surface of the housing It. The

respective members. The coil springs 42, each having one end thereof engaging the caps 48, and the other ends engaging the slidable members ll, are housed within the cylinders II, and the slidable members 40 engage the outer surface of the element 3| to resiliently urge it in a lateral direction so that it engages in abutting relationthe inner surface of the housing 30 along a vertical line passing through the lowermost point of the cylindroid 28. Q

From the above description it will be seen that the shortest path through the device, and there-- fore the one of lowest dielectric strength, will be along a line extending vertically from the lowermost-point of the cylindroid or upper electrode 20 to the conducting coating or lower electrode 32, and in a space between the inner surface of the housing and the outer surface of the element Ii. It will amo be seen that the pressure resulting from an are following this path will be the highest per'unlt length of path, since this is the path of maximum arc restriction. The pres sure incident to an are over this path urges the member ii in a lateral direction, this motion being opposed by the springs 62. Following such a movement, the arc may follow any path from the upper electrode 29 to the lower electrode II, but, due to the novel arrangement of the electrodes. as the restriction of the are becomes les as it moves around the periphery of the element 3 I, the length of the arc becomes correspondingly greater. The device is in this manner arranged to extinguish the arc regardless of the path taken between the electrodes. Obviously the advantage of the arrangement lies in the fact that the device is adapted to sparkover at a very much lower initial voltage than would be possible in a device having an arc path-of constant arc length and constant cross-sectional area of arc path, this result being accomplished without sacrifice of arcextinguishing qualities. In addition, the impact resulting from the operation of the device, due to the resilient mounting of the element II, is much less than that attending the operation of a rigid structure of the same arc-extinguishing effectiveness.

The housing I0 and the element 3| are preferably constructed of some material which will evolve gas in the presence of an arc, as hard fiber, since the device depends for its arc-extinguishing qualities upon the pressure resulting from an are within the device. The vents 38 in the mounting member 35, as shown in Figure 6, are provided to relieve the stress due to such pressure, and the circumferential reinforcing bands 39, located between the ends of the housing Iii, are desirable for providing additional wall strength to enable the device to withstand the heavy pressures attending the interruption of large currents.

. The hard fiber, mentioned specifically above as a suitable material for use in constructing the housing 30 and the element II, is, in common with numerous other insulating materials of organic base, quite'susceptible to damage from creepage currents, particularly if subiected to high voltages for long periods of time. To prevent such damage, the structure embodies means for normally impressing substantially all the voltage-between the electrodes 2! and II, and in addition provides means for preventing the concentration within the housing of any flux entering the arc extinguishing device.

To further increase the arc-extinguishing qualof the element 3 I"interposed therebetween, forms a capacitance in shunt relation to the voltage tending to maintain the are within the device.

In an alternating current are, the current passes through zero twice each cycle, and the recovery voltage tending to maintain the arc increases as a transient voltage at a rate determined by the constants of the circuit of which the arc is a part. Obviously any capacity in shunt relation to the arc must be charged before the transient recovery voltage tending to sustain the arc'can increase.

A small capacitance in shuntcircuit relation with V respective series gaps from electrode 20 to the grounded member 35 through the arc extinguishthe arc has therefore considerable effect in reducing the rate of recovery voltage, and any suchreduction assists in arc-extinguishment.

desirable, however, that such a capacitance be so arranged and of such design that in discharging it will not set up voltage oscillations of such ma nitude as to defeat its, purpose. For this reason, the coating 32 is preferably of a sufficiently high resistance to provide the required damping effect to suppress such oscillations. I

It will be seen from the above description that the important considerationsin applying the de-,

vice for operation on a circuit having a given normal voltage are (1) the weakest electrical path must-be internally of the housing 30, and in the space between the housing and the element 3|,

and (2) the length of any path between the upper.

electrode 29 and the lower electrode 32 must be suflicient that, under the conditions of operation, the normal circuit voltage will be incapable of sustaining an are over any path within the housing; 1

Additional embodiments of the invention here-, in described are illustrated inFiguresB and 9, in each of which has been incorporated the above described advantages over prior art structures.

Referring in detail to Figure 8, it will be seen that the structure here illustrated is similar in many respects to that shown in Figure l, the essential difference being the substitution of a dielectric material having a greater dielectric permittivity than air for accomplishing a further dstribution of the flux passing between the electrodes 20 and 2|. p

The ground ring [8 in the embodiment shown by Figure 8 is supported by a plurality of pairs of insulating units. Each unit includes the porcelain body 45 having rigidly secured in insulated relation the conducting metal cap 55 and the conducting base member 46 at the respective ends thereof. The bolts 48, extending through the base member 46 and the framework member 35, rigidly secure the lower unit of each pair on either side-of the arc extinquishing device l2. The clip angles 50, togetherwith the cap screws 5| cooperating with the caps 55 of the upper units, and the bolts 52 cooperating with the ring I8, are provided for rigidly attaching the latter at the top of each pair of units. The ring l8 in this embodiment is maintained at ground potential by the conducting member 22, the respective ends of which are attached to the framework l by the bolts 24, and to the clip angle 50 by the bolt The cap screws 41 extend through the base members 46 of each upper unit, and into the caps 55 of each lower unit, rigidly connecting the two units of each pair together. One end of each of the conducting members 49 is fixed to the base member 46 of each upper unit, and the cap 55 of each lower unit by means of the cap screws 41,

and the other end is attached to the cap l5 of the It is' are extinguishing device 12 by the cap screws-25.

It will now be seen that any flux passing from the electrode to the electrode 2| is divided, in accordance with the respective permittivities of the various multiple paths, through the dielectric of the four insulating units in parallel, and in addition, as in each of the other embodiments herein described, through the dielectric. of. the element 3| between the cylindroid' 29 and the conducting coating 32. Asa result, the flux longitudinally of the arc path of the arc extinguishing device I2 is much less than that passing between the air-spaced electrodes and 2!, thus providing the unbalance of voltage desired across the above described, provides the means for preventing that fiuxpassing between the electrodes 20 and 2! from concentrating between the upper and lower electrodes of the arcextinguishing device I2. In this structure the ground ring 18 is supported by means of a pairof outwardlyand,

upwardly extending conducting arms 54, the lower ends of which are attached to the flange 33 by the bolts 34, and the upper ends to the ring.

i8 by means of the bolts 53. In this arrangement, it will be seen that the ground ring I8 is so positioned in the electrostatic field of the structure as to intercept substantially all- .of the flux which would otherwise pass from the upper ring I 1 to the cap 15 of. the arc extinguishing device, thereby efiectively preventing any flux entering the device i2 except that passing between the electrodes 20 and 2|. V

From the several embodiments of my invention herein described, other equivalent arrangements will be readily apparent, and it is to be understood that the invention herein disclosed is to be limited only as is necessitated by the prior art and the appended claims. I

What I claim as new, and desire to secure by Letters Patent, is:

1. An excess-voltage protective arrangement adapted to be interposed between a conductor and ground, comprising a pair of conducting members rigidly supported in spaced-apart relation, one of said members being electrically connected to said conductor and the other of saidv bers associated with said electrodes for causing cascade flashover of said spark gaps.

2.. An excess-voltage protective arrangement adapted to be interposed between a conductor and ground, comprising a pair of conducting members'rigidly supported inspaced-apart relation at a definite distance one from the other, one of said members being electrically connected tosaid conductor and the other of said members being electrically connected to ground, said pair of members forming a spark gap adapted to'limit to a predetermined value the potential difference between said conductor and ground, and

9,009,049 means for preventing fiashover between said members. said means including a plurality of normally insulated conducting electrodes rigid- Ly supportedin spaced relation to provide a plurality oispark gapsin series circuit relation be- ;tween said conductor and ground and capacity .meansincluding said conducting members asso- .-ciated with said-electrodes for causing cascade fiashover of said spark gaps, the-saiddefinite distance being less than the sum oi'the electrode spacings oi said plurality of spark gaps. .3. Inan. excess-voltage protective structure summit to a voltage from a conductor to ground. insulating means supporting a plurality of noroi said 'gapssubstantially all of said voltage on said first spark gap.

4. 1n excess-voltage protective structure, means supporting a plurality of normally insulated conducting electrodes providing a plurality oi spark gaps in series relation from conductor to ground, a hollow tubular insulator having gas evolving wall material therein housing an upper and a lower conducting electrode associated with said insulator to provide one of said spark gaps, the relationship between said upper and lower electrodes being such as to cause sparkover therebetween to occur within said insulator, means for venting said insulator, means for connecting said lower electrode to ground, and capacity means including said upper and lower electrodes disposed within said hollow insulator, a grounded conducting member, and means for supporting said conducting member at such a position in the electrostatic field of said structure as to restrict the entrance of fiux to said capacity means except through said upper electrode.

5. In an excess voltage structure having a plurality oi spark gaps in series circuit relation from conductor to ground, a hollow tubular insulator having gas evolving wall material therein, an upper and a lower conducting electrode associated with said insulator to form one of said spark gaps, structural means for causing sparkcver between said electrodes to accur within said insulator to provide an enclosed spark gap, means for venting said insulator, means for maintaining-said lower electrode at substantially ground potential and capacity means electrically connected in shunt relation with said enclosed spark gap, said capacity means including a grounded metal shield supported at such a position in the electrostatic field as to prevent the concentration 01' fiux between the electrodes of said enclosed gap.

6. In an excess-voltage protective structure subject to a voltage from conductor to ground, insulating means supporting normally insulated conducting electrodes in spaced relation to provide a first spark gap and a second spark gap in series relation from conductor to ground, a hollow insulator comprising insulating material within said insulator subject to thermal damage from creepase current, structural means for causing sparkover 01' said second gap to occur within said insulator, and fiux control means including a conducting shield at ground potential associated with said structure for reducing both the time whereby damage to said insulating lag of sparkover of said structure and the pormal voltage gradient across said second gap, material due to said creepage current is prevented.

,7. An arc extinguishing device comprisinga first hollow insulator, upper and lower conductm ing electrodes at the respectiveends thereof, said elcctrodesbeing so arranged withrespect to said hollow insulator as to cause sparkover between said electrodes to occur withinsaid first hollow insulator, a second hollow insulator supported within said first hollow insulator, means ior increasing the capacity of the device including an. integral portion oi said lower electrode disposed within said second hollow insulator to tr'ode and-said integral portion of said lower electrodewith the, wall of said second hollow insulator. interposed therebetween, said integral portion of said, lower electrode having a relaprovide a condensercomprising said upper electively high resistance adapted to suppress voltage oscillations resulting irom a discharge between said electrodes.

8. An arc extinguishing device comprising a tubular insulator closed]; at the upper end and open at the lower end, a cylindrical element 0! insulating material within said insulator, means supporting said element oi insulating material in lateral abutting engagement with said insulator, a first conducting electrode carried by said element and extending downwardly from the closed end of said insulator and a second conducting electrode positioned adjacent the open end of said insulator, said electrodes being normally insulated from each other and so related that the shortest path therebetweenis along the line of abutment of said element of insulating material with the inner surface 01' said tubular insulator.

9. An arc extinguishing device comprising a means holding said element oi insulating material in lateral abutting relation with said tube, and conducting electrodes spaced apart to provide a preferential spark path therebetween internally 01' said tube and normally insulated from each other, said electrodes being so arranged that the weakest electrical path therebetween is along the line of abutment of said element of insulating material with said hollow tube, at least one oi said electrodes being positioned in said annular space.

10. An arc extinguishing device comprising a tubular insulator closed at the upper end and open at the lower end, an element of insulating material eccentrically supported in lateral abutting engagement with the inner surface of said insulator, a first conducting electrode carried 7 by said element and extending downwardly from the closed end of said insulator and a second conducting electrode positioned in spaced relationship with said first electrode, the arrangement oi said electrodes being such that the weakest electrical path therebetween is along the line oi! abutment between said insulator and said insulating element, and resilient means opposing the movement of said element in a lateral direction away from said weakest electrical path.

11. An arc extinguishing device comprising a tubular insulator closed at the upper end and open at the lower end, an element of insulatabutting engagement with the *inner' suria'ce of said insulator, a'first conducting electrode carried by said elementand' extending "downwardlly 'from thefclosedend of said-insulatorand'a "second conducting electrode positionedins'pabe'd relationship with said first' electrode; the ar- 'said' electrodes being such that rangement of r r eakest' electrical path therebetween is along the the line of abutment between said insulator and said insulating'element, and resilient means arranged to fcontinuouslybias said element of insulating material toward weakest electrical 4 path.

' 12; An are extinguishingdevice comprising a tubulaifinsulatorclosed at the upper end and 7 open at the lower end, a cylindrical elementof v insulating support n material within said insulator; means said-element or insulating material H v. i 9 3 4 ing' material eccentrically supported in lateral end-of said that the shortest path ther'ebetween is along the line of' abutment'of said element of] insulatin material with the inner surface of said tubular insulator, and means including an integral portion ofon'of 'said electrodes 'disposed within said cylindrical element of insulating material for increasing the electrical capacity'ot the"de-- vice. 1 r v r "13. An arcextinguishing' device comprising a first hollow insulator, a second hollow'insulator contained by said first hollow insulator, normal- -ly insulated conducting electrodes associated --'with said second hollow insulator, saidelectrodes beingsorelated that the weakest electrical path therebetween 'isin aspajce between the inner surface or said first hollow insulator and the outer surface or said second hollow insulator;-' and v means including integral portions of saidial'ectrodes-disposed respectively on the inner and outer surfaces of said second insulator tor increasing. the' electrical capacity of the -device. a RALPH R. PrrrMAN.

r insulatonsaid'electrodes being nor- 'mally insulated from each'otlierand so-related

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