US3087094A - Lightning arrester - Google Patents

Description

April 1 w. H. NASH 3,087,094

LIGHTNING ARRESTER Filed July 13, 1956 2 Sheets-Sheet 1 I I 5 5 4/ 7 g 22 I IN V EN TOR. ZVZ'ZZc'am H. Msh B April 23, 1963 w. H. NASH 3,087,094

LIGHTNING ARRESTER Filed July 15, 1956 2 Sheets-Sheet 2 INVENTOR. Ground M'Zlzkzm H, JV'as/z.

United States atent lice 3,ii87,094 Patented Apr. 23, 1963 3,987,094 LIGHTNING ARRESTER William H. Nash, South Milwaukee, Wis., assignor t McGraw-Edison Company, a corporation of Delaware Filed July 13, 1956, Ser. No. 597,627 Claims. (Cl. 31770) The present invention relates to lightning arresters, and particularly pertains to an improved spark-gap assembly therefor.

Among the objects of the present invention is the provision of an improved spark-gap assembly which .is adapted to maintain relatively uniform voltage distribution across all of the insulating gaps of a series of gaps during sparkover at relatively low line frequencies, and

which assembly permits a shift to non-uniform voltage distribution under relatively high frequency surge voltage conditions to ultimately provide an arrester with an impulse ratio of one or slightly less than unity.

It is a primary object to provide insulating members for a series spark gap assembly which are cooperable with the spark gap electrodes to establish a predetermined capacitance division throughout the electrode assembly and which are of a selected dielectric constant to provide anon-uniform sparkover voltage level between gap electrodes under surge conditions.

A more specific object of this invention is to provide a spark gap assembly which includes a cooperating series connected grading resistor which may be of a helical configuration and extending substantially the axial length of said assembly to provide increased tracking distance.

In general, arrester gap requirements include the following factors:

The gap assemblies must maintain relatively high line frequency sparkover in order that they will not operate under normal voltages and surges which are below a level which might be dangerous to protected equipment.

It is to be noted that throughout the present discussion line frequency will be represented by 60 cycles per second, although it will be understood that the improved spark gap assembly will function satisfactorily under any normal line frequency supply without departing from the scope of the invention.

Gap assemblies ordinarily do not operate unless the sparkover voltage is 1 /2 to 2 times the normal system operating voltage rating of the arrester. This rating is selected in such a way that the arrester will never experience linefrequency voltage to ground in excess of the rating even under fault conditions and adverse weather conditions, such as rain or fog. In addition, the arrester should sparkover at a surge crest voltage near the 60 cycle crest sparkover, and preferably slightly below it. The spark gap assemblies should keep the impulse ratio at unity orslightly less than unity. This impulse ratio is defined as the quotient of the sparkover voltage at surge crest value divided by the sparkover voltage at line frequency, such as 60 cycles per second.

In order that the gap will perform the above functions, it should be designed so that the 60 cycle voltage will be divided uniformly among the various gaps, or in proportion to gap spacing; this voltage must be so divided, both when the arrester exterior is dry or when Wet, such as in rainy weather. This, in general, requires that the gap dimension be voltage graded. The grading is usually accomplished either by shunting the gaps with a high ohmic value resistance, or capacitively by using high reactance capacitive filaments. In general, the use of resistance elements has the advantage at 60 cycle voltage because the stray capacities introduced by moisture on the exterior of the arrester housing do not tend to upset this grading as much as capacitive type grading. The resistance spacers are constantly subjected to 60 cycle voltage, requiring them to be of such construction to also provide adequate tracking distance.

In the case of using the high ohmic resistance, either linear or non-linear resistances are used. However, there is a modern tendency to use non-linear resistances as they tend to equalize the voltages across each gap.

In addition to the above, the gap assemblies of the present invention contemplates a structure that will operate under surge conditions to provide a non-uniform voltage division, in order that the gaps will sparkover at a lower value and provide a means of piling up the voltage on certain of the gaps to accomplish this. When they do this, they will make up for the inherent impulse ratio of the gap alone, which is generally greater than one, and tend to push it back to one or less. In part, this may be accomplished by use of inherent stray capacity of the arrester to ground, but this is only partially effective,

as it is not especially controlled, nor is it high enough in capacitive value to upset the resistance grading.

The present invention provides an arrester which accentuates the non-uniform stray capacity by intentionally introducing non-uniform capacity. This is accomplished by adding capacitive elements of relatively high dielectric constants and having a 60 cycle capacitive reactance that is high enough not to effect the 60 cycle voltage grading of the resistors. However, under frequencies associated with surges, the capacitive reactance of the elements will become relatively low so that they will predominate over the resistors which are used for grading purposes. Therefore, almost all of the voltage will appear across the gaps at the lower dielectric constant spacer end.

The present invention will be more clearly explained with attention being given to the following specific description aided by the drawing, in which:

FIG. 1 is a vertical view, in partial section through a lightning arrester, which includes a spark-gap assembly of the present invention;

PEG. 2 isa cross section of the arrester taken on lines 22 of FIG. 1, and showing only components of the spark-gap assembly contained therein;

FIG. 3 is a vertical section taken through the sparkgap assembly along lines 3-3 as indicated on P16. 2;

FIG. 4 is an elevational view, in partial section, of a spacer member of this invention.

FIG. 5 is a representative electrical circuit diagram.

indicating the arrangement of resistors and capacitive elements providing the uniform voltage distribution across the series gaps at line frequency and the shift to nonuniform voltage distribution at surge frequencies provided by the intentional introduction of non-uniform capacitive elements plus stray capacities to ground.

Although the spark-gap assembly 19 may be accommodated to a lightning arrester of any type, or to other suitable equipment, it is shown herein as enclosed in a porcelain housing 1 including a series of axially spaced skirts or petticoats 2, and including metal end caps 3 and 4. The porcelain housing 1 is sealed at the end by means of compressing and retaining a heavy resilient gasket 5 between the housing and a conducting cap 6, which is spun in place over the annular flange portion 7 of the housing while the assembly is under pressure in an hydraulic press. The end caps are held in place adjacent the housing by means of cement 8 poured between the end cap and the housing.

The electric line 9 to be protected by the lightning arrester is connected to the conducting end cap 3 by means of conventional connectors (not shown) received in an aperture in one of the integral laterally projecting cars 10. Connection with ground G may be madev in like manner with the projecting ear 11 of the conducting end cap Contained within the housing 1 is a conventional valve material of suitable quantity and generally comprising valve blocks or solid mass of a negative resistance material, such as silicon carbide.

It is to be noted, that if it is desired, the arrester may contain spark-gap assemblies 19 at one end or at both ends thereof, the latter having the valve material 15 interposed therebetween.

The improved spark-gap assembly is clearly shown in the relatively enlarged views of FIGS. 2 and 3, wherein the assembly 19 comprises an are or spark-gap pile which comprises a plurality of spaced, plate-like electrodes 20, and each electrode is substantially identical to each of the others. Each electrode is formed from sheet material, and each is formed that when the electrodes are properly assembled, there will be found one or more are gaps 21 (see FIG. 3) between each twoadjacent electrodes.

As illustrated in FIG. 2, the electrodes may be provided with a radially extending mounting portion 22 having an aperture for receiving a machine screw 23. For further support, each of the various electrodes are arranged in alternate relationship on a rod-like member or core 24 preferably positioned in a centra opening 25 in each electrode. The member 24 is of insulating material, such as steatite which, for the present purpose, has a relatively low dielectric constant of about 4 to 7 above unity.

Each electrode consists of a disk, preferably circular, including a plurality of embossments or impressions, which are formed, as viewed in FIGS. 2 and 3, with three depressions 26 and three projections 27 on each face of the disk. It will be noted from FIG. 2 that the depression on one face is preferably diametrically opposed to the projection on that face. The number of impressions per disk may vary, but six have been found to be a convenient number, especially for the arrangement of the grading resistors, as will hereinafter be described. When the electrodes are assembled in superimposed relation, they are so related that the projections on the face of one disk will be adjacent or opposite those of the adjacent disk to form the spaced spark gaps 21 therebetween. The depressions on the said face of the one disk will be opposite those on the adjacent disk to form recesses for retaining the insulating spacer members 30 therebetween.

It is also to be noted that grading resistance spacers have also been provided in series relationship with the spaced apart gaps along the circumference of the electrodes. A preferred form of the resistors provide sectoral members arranged relative to one another to provide a helical path circumjacent to the electrode disks throughout the entire length of the spark-gap assembly 19. This is accomplished by arranging the resistors 35 in juxtaposed relationship with each of the ends being in overlap arrangement and joined by the machine screw 23.

It will be apparent that it is preferred to arrange alternately spaced electrodes 20 with their respective mounting ears 22 spaced substantially 120 relative to one another to provide a convenient assembly for combined mounting with the resistor segments 35 and the respective mounting screws 23.

Although, the resistors have been shown and described as being in helical configuration circumjacent to the various spark gaps, it will be apparent that other conventional ring type resistors (not shown) or cylindrical resistor members substituted for one or more of the spacer members 30 (not shown) may be substituted there for without departing from the scope of this invention. Such resistors may be voltage dependent or of a simple ohmic variety.

The entire assembly 19 is positioned to be seated on a contact plate 36 and is retained in position means of the machine screw 37 threadingly engaging the core 24. As viewed in PEG. 1, it will be noted that the assembly 19 is positioned in the housing 1 with the contact plate 36 being positioned in electrical en agement with the valve material 15. From FIG. 1, it will be seen that the upper end of the spark gap assembly is in electrical and mechanical contact with the helical spring 40, the opposite end of which spring engages an embossment all impressed in the conducting cap 6. The conducting cap 6 is also in mechanical and electrical engagement with the end cap 3. During the process of spinning on the cap 6, as the entire unit is held in compression, the assembly 1.9 is maintained in relative operating position by the component cooperating parts.

Attention is now directed to the insulating spacer members 30, an example of which is particularly shown in FIG. 4. The present invention contemplates the use of insulating spacer members of relatively high dielectric constants and differing from one another in those constants, with insulating members having the relatively lower dielectric constant preferably being located at the end adjacent the valve material 1.5. As the spacers are positioned relatively remote from the said end, the dielectric constant of each is preferably of relatively higher value. For instance, the spacers may be made of titanate ceramic compositions having dielectric constants in the order of 5,000 for the spacer 30,, (see FIG. 3), which decreases in value to a constant of approximately at the spacer 3%.

However, as a practical matter, from a manufacturing cost standpoint, it is preferable to provide the spacers with the first few sets adjacent the valve material being of the high value of, for instance, approximately 100 and the remaining remotely relative spacers being at a value of, for instance, 5,000. Obviously, if it is expeditious, the spacers may be continually decreasing in dielectric constant, or may include variou intermediate values as cost permits.

It will be apparent from the above description that the present spark gap assembly will operate to provide relatively uniform sparkover between gaps at frequencies approximating line frequencies, and will provide desirable non-uniformity in sparkover of the gaps during surge conditions.

As an example, assuming two sections of spacers 30; for instance, spacers 30,, being of a material having a dielectric constant of 5,000 and spacers 30 of a dielectric constant of approximately 500, the capacitive reactance of 60 cycle current might approximate 10 and 100 times, respectively, of the normal ohmic value of the resistors 35. However, at surge, which might have as a primary component a frequency of approximately 60,000 cycles per second, the capacitive reactance of the respective spacers would be divided by 1,000 (60,000/60) or 0.01 to 0.1 times, respectively, of the ohmic value of the resistors 35. Thus, the capacitive elements will determine the grading, in addition to the minor effect provided by stray capacitance-to-ground.

It will also be apparent that the desired end result may be obtained with uniform capacitive grading coupled with non-uniform gap spacing with resistor grading being in relation to the gap spacing. The resistor voltage division would be selected to be in proportion to the gap spacing provided by varying length capacitive spacers.

Accordingly, the present arrester will accentuate the non-uniform stray electrostatic capacities, by intentionally introducing additional non-uniform capacity. At 60 cycles, or line frequency, the capacitive reactance of the added spacers 30 will be high enough that they do not affect the 60 cycle voltage grading provided by the resistors 35. But, under the frequencies associated with surges, the capacitive reactance of the elements Will become low enough that they will predominate over the resistors 35. Accordingly, almost all of the voltage will appear across the gaps at the end of the spark gap assembly incorporating the lower dielectric spacers, in this 5. case 30 After the sparkover has occurred, the voltage will tend to distribute itself relatively uniformly throughout the various gaps 21 of the assembly.

The above described operation of the various components will be further clarified when studied in connection with the diagram of FIG. 5. The circuit through the gap assembly, when used in combination with a valve type arrester element, is shown in the diagram with the symbol C representing the capacitive values provided by the various spacer members 30- and their respective electrodes, plus any-stray capacities to ground. It will be understood that the intentionally added capacitance provided by the preselected spacer members, will predominate in any case. The symbol- R relates to the respective resistance values provided by the resistors 35, whereas the symbol G represent each of the repective gap dimensions defined by the electrodes 20. R relates to the non-linear resistance of the valve material. Although the resistance of the valve material is indicated for purposes of explanation, it has very little bearing on the voltage division throughout the gap assembly.

For purposes of explanation, in the particular case, it is assumed that the capacitance is graded in gradual steps throughout the assembly length.

Assuming that R =R =R =R :R and that each of these values denoted by R is very much greater than R and that C C C C C and further assuming that C is never equal to C 1:

It will then be apparent that at line frequency, such as 60 cycles per second, any one of the resistance values R will be much less than the quotient of It will also be apparent that at the relatively high surge frequencies denoted by i that here R will be very much greater than the quotient of Inasmuch as R is very much smaller than the same quotient at line frequency, it will be apparent that the voltage division across the spark gap assembly will be accomplished by the various resistors, whereas at the higher surge frequencies, the capacitive reactance will be smaller and thus the voltage division will be determined by capacitance.

It will also be noted that the preferred helical arrangement of the resistors 35 will greatly increase the tracking distance providing an extremely long resistor creepage resistance to prevent flashover. Thus, the 60 cycle voltage distribution is primarily controlled by the grading resistor and will be uniform. The overall effect of the construction will consequently provide a desirable low impulse ratio.

I claim as my invention:

1. In a lightning arrester, a spark gap assembly comprising a plurality of spaced electrode plates defining opposed spark gap surfaces, a plurality of uniform in size insulating electrode spacing members being received and retained between each of said electrode plates and providing a means of establishing said predetermined gap spacing between said spark gap surfaces, said support members each being of an insulating material having a predetermined dielectric constant and being positioned relative to one another in said stacked relationship to provide a gradual variation in dielectric values in those members positioned remotely relative to one end of said assembly, and a plurality of grading resistors comprising a series of generally arcuate segments, means electrically connecting each of said segments to respective ones of said electrode plates, said segments joining one another in end to end relationship with the ends of adjacent segments in juxtaposed endwise overlapping relationship to provide a staggered helical configuration, and means for 6 supporting said staggered helical configuration circumjacent to said spark gap surface throughout the axial length of said assembly.

2. In a lightning arrester, a spark gap assembly comprising a plurality of spaced electrode plates defining opposed uniform spark gap surfaces, each of said electrode plates further defining an opening therein, an insulating rod traversing the said opening in each of said electrodes, a plurality of uniform in size insulating electrode spacing members being received and retained between each of said electrode plates and providinga means of establishing said predetermined gap spacing between said spark gap surfaces, said support members each being of an insulating material having a predetermined dielectric constant and being positioned relative to one another in said stacked relationship to provide a gradual variation in dielectric values in those members positioned remotely relative to one end of said assembly, and a plurality of voltage dependent grading resistors comprising a series of generally arcuate segments, the ends of each ofsaid segments being supported by and electrically connected to alternate ones of said electrode plates and said segments joining one another in end to end relationship with the ends of adjacent segments in juxtaposed endwise overlapping relationship to provide a staggered helical configuration circumjacent to said spark gap surfaces throughout the axial length of said assembly.

3. In a lightning arrester, a spark gap assembly comprising a plurality of spaced electrode plates defining opposed spark gap surfaces, a plurality of uniform in size insulating electrode spacing members being received and retained between each of said electrode plates and providing a means of establishing said predetermined gap spacing between said spark gap surfaces, and a plurality of grading resistors comprising a series of generally arcuate segments, the ends of each of said segments being supported by and electrically connected to alternate ones of said electrode plates and said segments joining one another in end to end relationship with the ends of adjacent segments in juxtaposed endwise overlapping relationship to provide a staggered helical configuration circumjacent to said spark gap surfaces throughout the axial length of said assembly.

4. In a lightning arrester, a spark gap assembly, comprising a plurality of stacked electrode plates, each of said plates being formed with a mounting arm portion and a plurality of concavo-convex portions arranged to face alternately in opposed directions to provide a series of opposed convex faces and a series of opposed concave faces in staggered relationship along the axial length of the electrode stack, said mounting arm portions disposed to extend laterally from said stack with mounting arm portion of adjacent ones of said electrode plates being relatively spaced around the periphery of said stack, a plurality of uniform in size insulating electrode spacing members being received and retained between each of said opposed concave faces and further providing a means of establishing a predetermined uniform gap distance between said opposed convex faces, and a plurality of grading resistors comprising a series of generally arcuate segments, said segments joining one another in end to end relationship with the ends of adjacent segments in juxtaposed endwise overlapping relationship and supported on and electrically connected to said mounting arms to provide a staggered helical configuration circumjacent to said concave-convex portions throughout the axial length of said assembly.

5. In a lightning arrester, a spark gap assembly comprising a plurality of stacked electrode plates, each of said plates being formed with a mounting arm portion and a plurality of concavo-convex portions arranged to face alternately in opposed directions to provide a series of opposed convex faces and a series of opposed concave faces in staggered relationship along the axial length of the electrode stack, said mounting arm portions disposed to extend laterally from said stack with mounting arm portions of adjacent ones of said electrode plates being relatively spaced around the periphery of said stack, each of said electrode plates further defining an opening therein, an insulating rod traversing the said opening in each of said electrodes, a plurality of uniform in size insulating electrode spacing members being received and retained between each of said opposed concave faces and further providing a means of establishing a predetermined gap distance between said opposed convex faces, said support members each being of an insulating material having a predetermined dielectric constant and being positioned relative to one another in said stacked relationship to provide a gradual variation in dielectric values in those members positioned remotely relative to one end of said stacked electrode plates, and a plurality of voltage dependent grading resistors comprising a series of generally arcuate segments, said segments joining one another in end to end relationship with the ends of adjacent segments in juxtaposed endwise overlapping relationship with one of said mounting arm portions disposed therebetween, said mounting arm portions electrically connecting said segments to said electrode plates and supporting said segments to provide a staggered helical configurartio-n circumjacent to said concavoconvex portions throughout the axial length of said assembly.

References Qited in the file of this patent UNITED STATES PATENTS 1,902,510 McEachron Mar. 21, 1933 2,151,559 McEachron Mar. 21, 1939 2,324,108 Pyk July 13, 1943 2,611,108 Rydbeck Sept. 16, 1952 2,615,145 Rydbeck Oct. 21, 1952 2,640,096 Kalb May 26, 1953 2,659,842 Teszner Nov. 17, 1953 2,670,398 Sheadel Feb. 23, 1954 FOREIGN PATENTS 698,352 Great Britain Oct. 14, 1953 730,709 Great Britain May 25, 1955 847,379 France June 26, 1939 215,001 Switzerland Aug. 16, 1941

Claims (1)

1. IN A LIGHTNING ARRESTER, A SPARK GAP ASSEMBLY COMPRISING A PLURALITY OF SPACED ELECTRODE PLATES DEFINING OPPOSED SPARK GAP SURFACES, A PLURALITY OF UNIFORM IN SIZE INSULATING ELECTRODE SPACING MEMBERS BEING RECEIVED AND RETAINED BETWEEN EACH OF SAID ELECTRODE PLATES AND PROVIDING A MEANS OF ESTABLISHING SAID PREDETERMINED GAP SPACING BETWEEN SAID SPARK GAP SURFACES, SAID SUPPORT MEMBERS EACH BEING OF AN INSULATING MATERIAL HAVING A PREDETERMINED DIELECTRIC CONSTANT AND BEING POSITIONED RELATIVE TO ONE ANOTHER IN SAID STACKED RELATIONSHIP TO PROVIDE A GRADUAL VARIATION IN DIELECTRIC VALUES IN THOSE MEMBERS POSITIONED REMOTELY RELATIVE TO ONE END OF SAID

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