US3096496A

US3096496A - Overvoltage protective device

Info

Publication number: US3096496A
Application number: US109363A
Authority: US
Inventors: Lawrence M Burrage; Darrel D Mcstrack
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1961-05-11
Filing date: 1961-05-11
Publication date: 1963-07-02
Anticipated expiration: 1980-07-02

Description

July 2, 1963 L. M. BURRAGE ET AL 3,096,496

ovERvoLTAGE PROTECTIVE DEVICE Filed May l1, 1961 2 Sheets-Sheet 1 34 INVENToRs. Lau/@WCE M. .5a/:R465 D/WREL D. /VcSr/wck W ya/n July 2, 1963 M BURRAGE ETAL 3,096,495

OVERVOLTAGE PROTECTIVE DVICE Filed May l1, 1961 2 Sheets-Sheet 2 Current f INVENTORS. mwen/ci M. @UAW/Q65 .D4/eff; D, /VcST/Mcx Wm f @am fmewfy United States Patent O 3,096,496 OVERVOLTAGE PROTECTIVE DEVICE Lawrence M. Barrage, South Milwaukee, and Darrel D.

McStrack, New Berlin, Wis., assignors to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware Filed May 11, 1961, Ser. No. 109,363 8 Claims. (Cl. 338-21) This invention relates to improvements in valve-type excess voltage devices and more particularly to an improved valve-type resistor or valve block for lightning arresters of the type having a pluralityk of gap `and resistor elements in series with one another.

In the normal valve rtype arrester the series gap provides suiiicient insulation .at line voltages and acts somewhat as a switch, to close by sparking over, when a suflciently high voltage appears across its terminals. The series gap must also reopen the lightning arrester circuit by interrupting follow current atter the sur-ge has declined. However, in general, the series gaps .are cap-able of interrupting only limited values of follow current and means must be provided to limit the follow current to a level that the series gap can cope with. The means generally utilized is a non-linear resistance valve block and the resist-ance material from which it is made is called valve material. A valve `block of this type has a high apparent resistance at low voltages but a low apparent resistance at high voltages. Thusly, a non-linear valve element presents a low impedance to large magnitude voltages and a high impedance to power frequency voltages within the rating of the element.

The duty imposed upon the combination of series gaps and valve elements in a lightning arrester of this type can be classiiied into three categories, viz.,

(l) surges-currents of several thousand amperes or more lasting from a few microseconds to several hundred microseconds.

(2) Long tail discharge-#currents of variable magnitude lasting thousands of microseconds. y

(3) Follow current-currents at power frequency lasting approximately one half cycle in the order of hundreds of amperes.

As a result of the variety of conditions under which a yvalve type lightning arrester is called to operate, the

design of the arrester represents a compromise between the ideal and that which can be practically achieved.

In prior art valve type lightning arresters utilizing a contemporary single valve element in series with the spark gaps, the difference between the operating characteristics desired and those actually achieved has been relatively great. More exactly, the IR drops across these prior art devices have been relatively high as have been the values of follow current.

Since a single valve element of the type which is commonly utilized in prior art devices, is itself a compromise, iarresters incorporating this feature have left much Ato be desired as regards their ldurability under repeated long tailed discharges.

We have found that by providing electrically parallel valve elements in' series with the gap elements, wherein each of the parallel valve elements is designed to perform during different time portions of the arrester performance cycle, that an improvement in the over-all operating characteristics of the lightning arrester may be obtained.

It is therefore an object of this invention to provide an improved valve type lightning arrester which has lower discharge voltage (IR drop) characteristics.

Another object of this invention is to provide an imlICC proved valve type lightning arrester which experiences llower follow current.

Another object vof this invention is to provide an improved lightning arrester having a -valve element with increased long tailed discharge durability.

A further object of this invention is to provide a valve element -for a lightning arrester wherein the high irequency, high current portions of the applied wave are discharged in one physical portion of the valve element and the lower frequency, lower current magnitude components of the applied wave are discharged in another physical portion of the valve element.

A still further object of this invention is to provide an improved valve element which is capable of use i-n conjunction with conventional valve type arrester constructions with a minimum of modication thereof.

A still further object of this invention is to provide an improved valve element which, thru its use, imparts highly ldesirable electrical characteristics to the lightning arrester in which it is utilized.

Other objects and advantages of our invention will be lapparent 4from the following description of the preferred embodiments of the invention taken in connection with the accompanying drawings in which:

FIG. 1 is a partially cutaway View in elevation of a light-ning arrester incorporating one embodiment of the valve element of the invention;

FIG. 2 is a schematic view of the invention;

FIG. 3 is a partially cutaway view in elevation of one embodiment of the invention;

FIG. 4 is `a partially cutaway plan view of the embodiment of FIG. 3;

FIG. 5 is ia partially ycutaway View in elevation of another embodiment of the invention;

FIG. 6 is a partially cultaway plan View of the embodiment of FIG. 5;

IFIG. 7 is `a partially cutaway view in elevation of still another embodiment of the invention; and

FIG. 8 is a graph illustrating the slopes of the voltagecurrent characteristics of a typical valve block of the invention.

Referring now lto FIGURE 2, which is a schematic 'diagram of the invention, 10v indicates spaced apart electrodes deiining thereinbetween a spark gap 11. In series electrical relation with the electrodes 10` and gap 11 and between pairs of electrodes defining the gap 11 are two

valve elements

12 and 13 which are in parallel electrical relation with one another. It can be seen that the basic structure involves a gap, a pair of unlike valve elements, a gap etc., which combination of elements is placed between line (L) and ground (G).

The valve elements 12 may be composed of a bonding agent of any commonly utilized type and a line siliconcarbide grain and the valve element |13 may be composed of a similar bonding agent and a primarily coarse silicon carbide grain. While bonded silicon carbide is preferred as the valve material unbonded silicon carbide and/or other granular semi-conductors such as boron carbide and the like may be utilized provided the grain size as between the elements 12 and E13, varies.

The purpose of having coarse granular silicon carbide (or other valve materials) in one element and line granular silicon carbide in the other element is so -that a division of electrical duty, as between the valve elements may be obtained.

The element composed of coarse granular silicon carbide is used to discharge the low frequency low current components of the applied wave while the element composed of line granular silicon carbide is used to discharge the higher frequency, higher current por-tions of the applied wave. Basically the division of electrical duty between the

elements

12 and 13 is determined and controlled by the electrical and thermal characteristics of the granular material alone or as modified by the presence of a bonding agent.

The operation of the invention will now be explained. The coarse grain element 13 can be represented by a series of particles (C) in number and the ine(er) grain element 12 can be represented by a series of particles (1F) in number where F is greater than C. The discharge voltage (IR drop) across the (C) and (F) particles under a /20 its. current wave shape is essentially the same.

Upon application of a 10/ 2O ns. current wave (1x50 its. voltage wave) the coarse grain element (C) will in herently start conducting sooner than the line grain element (F). Within a few microseconds both elements will be conducting to at least some degree as a result of the rapid voltage rise. Since the fine particles (F) have a smaller heat sink available than the (C) particles the temperature of these particles (F) will rise much more quickly than the temperature of the (C) particles. Consequently the resistance of the (F) particles decreases more rapidly than the corresponding resistance decrease in the (C) particles. Since the more rapid change to lower resistance occurs in the (F) particles the fine grain element will carry a larger portion of the high frequuency current wave. At some time after these initial conditions, as a result of a portion of the current being carried by the (C) particles, the temperature of these particles will increase to the point where the total resistance of the (C) particles becomes 4less than the total resistance of the (F) particles. Therefore the langer portion of the current will pass thru the (C) particles. On the basis of the foregoing it can be seen that there is a continual change in the current division between the (C) and (F) particles throughout the passage of the current wave with the line element discharging the high frequency, high current portions of the wave and the coarse element discharging the lower frequency lower current portions of the applied wave.

Upon the application of lower frequency, longer duration surges (long tail discharge) to the combination (line and coarse elements) the (C) particles have sufficient time to change their resistance to a much lower value therefore permitting them (C) to carry a larger portion of the current than do the (F) particles.

During a very low frequency, very long duration current wave such as that characterizing follow current, the coarse element carries a very great portion of the current.

Since the magnitude of the follow current is to an extent determined by the initiating surge current magnitude or by the surge current immediately preceding the follow current a high(er) magnitude surge current will cause a high(er) magnitude follow current. Therefore because of the thermal-time relationship between the coarse and fine elements the majority of the surge is initially carried through the fine element. The coarse grain element is changing its resistance to a lower value near the end of the surge thereby causing follow current to fiow therethrough at a still lower magnitude than would be the case if the surge were being discharged exclusively through the coarse element. Some follow current also passes thru the fine element, but since some of the surge passed thru the coarse element, there is yless than normal follow current passing through the fine element. The total follow current passing through the combination is therefore less than would be the `follow current passing through a contemporary single element valve block.

If relative particle size, material or bonding agent, or block processing method is somewhat altered the combination may be tailored to meet almost any combination of expected operating regions.

`It should be noted that irrespective of the surge impressed on conventional or contemporary elements (which utilize relatively homogeneous grain sizes or an admixture Vof heterogeneous grain sizes) that current is always passed CII through substantially the same path. Hence an overload condition of the finer particles `during the `follow current portion of the operating cycle occurs. This condition results in premature failure of the finer particles and consequently subsequent failure of the entire valve element and lightning arrester. The fact that this does not happen in our combination element provides the important advantage of increased durability.

One example of the relative effect of varying the grain sizes in the two parallel

connected valve elements

12 and 13 will now be explained. The element 12 may for instance be composed of 180i ygrit silicon carbide bonded with a suitable agent and having a discharge voltage thereacross under -a ilOXZ frs. current wave equal to the discharge voltage of element 13 under the same wave shape and current magnitude while element 13 may be composed of grit silicon carbide plus bonding agent. When elements of .the type -described 'are subjected to l0 20 lis. current surges, approximately 66% of the surge current is carried by the nner grain element (12) up to 27 its. at which time the current becomes equally divided as between the

elements

12 and 13. After this point as time progresses an increasing portion of the current is carried by the coarse grain element (13). On long tailed current discharges of the nature of 501 its. to crest by 2000 as. to half crest, the coarse grain element carries the greater portion of the current for the entire duration of the discharge. Under follow current conditions the coarse grain element carries the major portion of the current.

Since the follow current magnitude can be a function of `the surge current magnitude the `fact that most of the surge is normally carried by the line element (i12) means that lower follow currents will be encountered in the parallel combination of coarse and fine elements than would be the case in a single compromise element.

Therefore! by utilizing fine and coarse valve elements in parallel with one lanother and in series with gap structure lower discharge voltages and lower values of follow current are achieved, together with improved long tailed discharge durability as compared to single contemporary valve elements. For desired application where specific duty requirements are necessary the relative crosssec-tional areas of the elements may also be modified to achieve a particular result.

Referring now to fFIGURE 8 a measure of the division of duty between theV parallel valve elements may be based on the ratio of the voltage-current characteristic of the granular material. As can be seen from FIG. 8 the slope of the Sr grit curve is ngo=lnAV1/1nAI1 and the slope of the i grit curve is 1z130=i1nAV2/1nAI2. The greater the ratio of nwo/1180 the larger will be portion of the duty carried by the coarse grain element and conversely, the lower the ratio of 11180/1180 the greater will be the portion of the duty carried by the tine grain element. The intersection of Ithe curves marks the point at which the duty as between the elements is equal.

Referring now to FIGURE 1 there is shown a typical valve type lightning arrester which is indicated generally `as 15 and which comprises -a skirted cylindrical housing 16 of suitable insulating material, such as porcelain or glass, closed at its ends by cup-shaped sealing caps 17 and hermetically sealed by resilient annular gaskets (not shown) which are compressed by cups l17 against annular seats (not shown) provided on the outer periphery of each end of the housing 16. End castings 19 are provided on each end of housing 16 and serve as seats for clamp .type terminals which connect with the electrical power system to be protected L and ground G, respectively, but these are not shown.

At the base of the housing 16 is a hollow cylindrical spacerl member 23 which may be of aluminum or other suitable conductive material. Atop the member 23 and within the Ibore of the housing 16 are positioned a plurality of gap and valve elements 30 which include a cylindrical valve element 24 having resilient insulating material 25 disposed on the curvato periphery thereof of the type described in Patent No. 2,891,194 issued .Tune 16, 1959 to the co-inventor-s of the present application. Other nonresilient insulating materials may also be used without modifying the inventive concept. The valve element 24 may be any one of the embodiments shown in FIGS. 3-7 which will be subsequently discussed.

Along the upper and lower face of all but the top and bottom valve blocks is positioned 4an electrode member 26 having an embossed portion 27 which defines with the next succeeding electrode embossed portion a spark gap 28. To provide a stacked configuration of spark gap assemblies and valve elements -annular spacer blocks or rings 29 which may be of resistive material and which may also function as grading resistors are positioned between the electrode on the top of one valve element 24 and the electrode -on the bottom of the next successive valve element.

As is customary a compression spring 31 is biased between the sealing cap #17 and a relatively fiat conductive plate 32 atop the uppermost valve element so as to maintain the stacked configuration of the gaps `and valve elements 30. While one rnode of providing a stacked electrode configuration is discussed the valve elements of this invention may be modified in such a manner that they may be accommodated in a wide variety of stacked coniigurations.

Referring now to FIGURES 3 and 4 one embodiment of valve block is shown which -comprises a cylindrical valve element 35 having the central cylindrical portion 36 thereof made up of corase grain silicon carbide or other material and the remaining area of the block 35 surrounding portion 36 made up of a fmeer) granular silicon carbide 37. As can be seen the electrodes 26 on the ends of `the element 3S are in contact with both the fine and coarse portions of the block. Normally the element 3S is provided with an insulating peripheral portion 25 and a conducting (metallic) coating (generally sprayed on) on the two circular faces of the element which are in contact with electrodes 26.

While 180 -grit :and 80 grit size grain elements are discussed in the example we do not intend to be limited by this since :a variety lof grain size combinations may be utilized to obtain the advantages of this invention, provided there is a difference in the grain sizes and therefore a difference in their (elements) thermal properties or voltage-current characteristic slope.

The block of -FGURES and 6 is similar -to that of FIGURES 3 and 4 except that the boundary between the coarse grit 34 and the fine grit 33 is a pair of chordal segments. The particular areas or shape of the areas of the fine and coarse portions of the block is of course subject to change 'and therefore this embodiment merely suggests one other method of `distinct apportionment of fine and coarse grains through the block.

In FIGURE 7 still another embodiment of the invention is shown which comprises a cylindrical block 41 having a raised annular peripheral portion y4Z of coarse valve material and a recessed cylindrical portion 43 of fine granular material. As can be seen the electrodes 44 are so configured that they -contact both the fine and coarse granular portions of the block. Insulating material 39 surrounds portions of the block las before to minimize the possibility of flashover between electrodes around the blocks. Resilient insulating material may be used to serve the dual purpose of fiashover prevention and to protect the valve blocks against damage during shipping and handling.

Asis evident from the foregoing objects and description our invention as herein described has improved discharge voltage and follow current characteristics as well as improved long tailed discharge durability, 'and results in a compact easily assembled lightning arrester having improved performance characteristics.

While only a few embodiments of the' invention have been illustrated and described, many modifications and variations thereof will become obvious to those skilled in the art, and as a result it is intended in the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the' invention.

We claim as our invention:

l. A valve element for `a lightning rarrester of the type having at least a pair of spaced apart electrodes which conjointly define a spark gap therebetwene, said element being in series electrical relation with :at least one of said pair of electrodes and comprising first and second discrete portions which are in parallel electrical relation with one another, said first portion composed in part of large grain non-linear resistance valve material, said second p-ortion composed in part of relatively smaller grain non-linear resistance valve material, said valve element having a pair of opposed contact -faces and each of said first and second portions extending across said element between said contact faces and defining ra portion of each of said contact faces.

2. A valve element for a lightning arrester of the type having at least a pair of spaced apart conducting members which conjointly define a spark gap therebetween, said element being in series electrical relation with at least one of said pair of conducting members yand comprising first `and second portions which are in parallel electrical relation with one another, said first portion composed of a bonding agent and large grained non-linear resistance material, said second portion composed of a bonding agent and relatively smaller grained non-linear resistance material.

3. Valve element means for a lightning arrester of the type having at least one pair of spaced apart conducting members which conjointly define a spark gap therebetween, said valve element means being in series electrical relation with at least one of said pair of conducting inembers and comprising, a first element composed in part of large grained non-linear resistance material and a second element composed in part of relatively smaller grained non-linear resistance material, said first and second elements being in parallel electrical relation with one another, whereby each of said elements predominates during different time portions of the arrester performance cycle.

4. A valve element for 'a lightning arrester of the type having at least a pair of spaced apart conducting members which conjointly define la spark gap therebetween, said element being in series electrical relation with at least one of said pair of conducting members and cornprising first and second portions which `are in parallel electrical relation with one another, said first portion composed of a bonding agent and large grained nonlinear resistance material, said second portion composed of a relatively smaller grained non-linear resistance material.

5. A valve element for a lightning arrester of the type having at least a pair of spaced apart conducting members which conjointly define a spark gap therebetween, said element being in series electrical relation with at least one of said pair of conducting members and comprising first and second portions which are in parallel electrical relation with one another, said first portion composed of large grained non-linear resistance material, said second portion composed of `a bonding agent and relatively smaller grained non-linear resistance material.

6. A valve disc for a lightning arrester having a plurality of spaced apart electrodes defining spark gap means, said element having an axial portion and an outer portion surrounding said axial portion, one of said portions having large grain non-linear resistance material and the other portion having relatively smaller grain non-linear resistance material, said portions defining opposed contact faces on said element generally transverse to the axis of said disc, each of said contact faces being adapted to elec- 7 trically engage one of said electro-des so that each of said portions is in electrical contact with said electrode, whereby said `axial and outer portions are in parallel circuit relation with one another when said electrodes are disposed against said opposed contact faces of said disc.

7. In a lightning arrester having a plurality of spaced apart electrodes delinirig spark gap means, a threedimen sional valve element in series with said spark gap means and having a pair of opposed contact faces and comprising first and second discrete portions, said iirst portion being ycomposed substantially of large grain, non-linear resistance material `and said second portion being com posed substantially of relatively smaller grain non-linear resistance material, each of said portions extending across said valve element between said opposed contact faces `and defining -a portion of each of said faces, whereby said first and second portions are in parallel circuit relation, said second portion carrying the greater percentage of the higher frequency, higher current components of a surge impressed upon said lightning arrester and said iirst portion carrying the greater percentage of the relatively low frequency, lower current components of said surge.

8. In an overvoltage protective device having spark 8 gap means, a cylindrical valve disc in series with said spark gap means Iand having opposed contact faces and a peripheral surface therebetween, electricalinsulation surrounding said peripheral -surface and in intimate contact therewith, -said valve disc having iirst and second discrete portions each of which extends between said opposed contact lfaces land defines a portion of each of said faces and said rst and second portions being positioned and arranged so that both said portions are adapted to electrically engage conductive contact means disposed against one of said faces, whereby said first and second por-tions are in parallel circuit relation with one another, one of `said portions having large grain non-linear resistance material and the other portion having relatively smaller" References Cited in the tile of this patent UNITED STATES PATENTS Ludwig et al Mar. 17, 1942 Ridgway Sept. 7, 1943

Claims

1. A VALVE ELEMENT FOR A LIGHTNING ARRESTER OF THE TYPE HAVING AT LEAST A PAIR OF SPACED APART ELECTRODES WHICH CONJOINTLY DEFINE A SPARK GAP THEREBETWEEN, SAID ELEMENT BEING IN SERIES ELECTRICAL RELATION WITH AT LEAST ONE OF SAID PAIR OF ELECTRODES AND COMPRISING FIRST AND SECOND DISCRETE PORTIONS WHICH ARE IN PARALLEL ELECTRICAL RELATION WITH ONE ANOTHER, SAID FIRST PORTION COMPOSED IN PART OF LARGE GRAIN NON-LINEAR RESISTANCE VALVE MATERIAL, SAID SECOND PORTION COMPOSED IN PART OF RELATIVELY SMALLER GRAIN NON-LINEAR RESISTANCE VALVE MATERIAL, SAID VALVE ELEMENT HAVING A PAIR OF OPPOSED CONTACT FACES AND EACH OF SAID FIRST AND SECOND PORTIONS EXTENDING ACROSS SAID ELEMENT BETWEEN SAID CONTACT FACES AND DEFINING A PORTION OF EACH OF SAID CONTACT FACES.