US2392679A - Lightning arrester - Google Patents

Description

Jan. 8, 1946. D. D MaCcARTHY LIGHTNING ARRESTER Filed oct. 3, 14940 M ttorneg.

M A \\\\\\\N\\\\\\\\\\\\\ Inventor. Donnell D. Mac lCar-U15,

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Patented Jan. 8, 1946 to `General -Electric C New York ompany, ,a corporation of Application October 3, 1940, Serial No. 359,535

l'Claim.

invention relates-to lightning arresters suitable fior the protection of lequipment normally subjected to an electrical potential, `and has for itsgeneral object the provision ofy an arrester that will ine/consistent in perfomance and will provide a uniform and dependable margin of protetion to-thefequipmentatall times and under the variety of conditions to which Vsuch 'protec- "tive devices are subjected.

`Someof Ythe requirements of agood lightning arrester, A'with a lbearing on the :present invention, are that `(a) the arrester should not discharge at the highest operating potentials of the equip- -inentfand under those disturbances"whichl are -well withinthefinsulation level "of-the equipment; '(b) :i

it should promptly discharge excessive potentials on the system, oiiering low 'resistance to the discharge current, when Kthese vpotentials reach a specified value with a predetermined margin of A'safety for theequipment; c) itshould promptly `interrupt the power 'follow current, in Valternating current systems preferably withinhalf a cycle `after Ytheabnormal potential is discharged; and (d) it-'should give rise to no undesirable vphe- `nomena,such 'as radio interference, corona capable-'ofleading to corrosion,etc., under thehig'hest 'operating potentials 'to which the 'equipment may'beexpected to -be subjected.

:arrester V"of the *general type here contemplated, `When reduced 'to its primary elements, consists of agap unitV designed to di'scharge'at a predetermined potential, and a resistance unit Vin 'series Ltherewith to limit the power follow current to the Amaximum value which the 'gapunit can interrupt with certainty.

Theprotection thatcan be secured from a simple gap vin series Vwith a resistance is of a very low order, because such a gap, as tha'tbetween two rods, disks or points, is Well-known tobe highly "variable or erratic in its discharge potential from time yto 1time and under various types of electrical disturbances. Moreover, 4it is not uncommon ivfor gaps of the above type to exhibit impulse ratios of the order of 2, 3, or evenhigher under vcommercial imp e 4tests required `by lthe 'American Institute of Electrical Engineers the y American'` Standards 'Associationand' other stand'- adizingorg'amz'ations. This means that by reason of theirerratic'vandvsluggish response such 'gaps permitthe exposure of the equipment to 2,

3"or' more times greater stresses under fast rising impulses than under more slowly rising fones. 'meinsulationf1eve1 which has tobe provided in thefequipmentthushasto be based on the higher 'dischargevoltage insteadf rthe-lower.

:A Aprincipal object of the present invention lis therefore to provide a new 'and'improved lightning arrester which will show'a substantially constant discharge voltage under repeated discharges and provide a dependable constant vmargin 'of safety. l

AAnother `objectof the invention is to provide an arrester which will maintain `its margin yof protection substantially unchanged over 'a `long periodof service `and independently of the condition 'of vthe electrode surfaces inf such service.

Astill further object'of the invention is toprovide an arrester'which will have `Aa lower `maximum impulse ratio than that of lightning varresters in generaluse prior to my invention.

vIn accordance with one embodiment of 'the invention, one or more 'gaps of the seriesfgap'assembly of thelightning arrester is providedwith an 'adjacent gap ionizing device `which is so `Aarranged and adjusted that 'ionizing vradiations lare produced only when the line voltage Yrises `to a predetermined value above the highest'operating voltage of the system. The ionizing device is therefore not a'sourceof radio interference'yet the impulse ratio ofthe gap is'thereby'reducedto a low value and the discharge voltage level is maintainedl consistent'for-any type of over voltage condition whichmaybe'impressed upon theline.

Further objects and advantages of my invention'will 'appear from the following description taken in :connection with the `accompanying drawin-gsin which Fig. l is a sectional view of a lightning arresterconstructed in accordance with the invention, Fig. 2'is an enlarged perspective view of one gap element of thearresterfandlh'g. 3 is an exploded view in perspective rof the Vgap ionizingdevice.

For a complete understanding of the lpresent invention, it'is necessary to consider anumber of facts which have been learned'by an extended experimental and theoretical investigation of the nature of the variability exhibited by various types of gaps 'and the probable causeof such variability.

One vof the facts 'learned isthat in general the variability of the discharge voltage or verratic per- `formance of an arrester gap `increases with an increase in the rateof rise of thelapplied voltage. This phenomenon is to be distinguished from high impulseratios and maybe considered quite properlyas a matter of the higher variability'of the higher impulses ratios.

A general theorythatfseems to accountfor this particular vphenomenon is that the *discharging of va .gap presupposes Lthe existence "of Ja certain threshold ionization of the air in the gap capable of leading to cumulative ionization and breakdown under a continued application of a certain minimum potential. Given time enough, as afforded by slowly rising potentials, natural agencies suoh as cosmic rays, radio-active material, ultraviolet light if such should exist in sulicient intensity or could reach the gap, and various other agencies not well understood at the present time, provide this required threshold ionization sooner or later after such ionization has disappeared at any point by a recombination of ions, so that a gap will exhibit substantially the same discharge voltage under repeated operations at a very slowly rising or sustained potential. In the case of lightning impulses involving rates of rise of potential up to 1,000,00 volts per microsecond and higher, this required threshold ionization may or may not exist at the instant the potential reaches the minimum discharge value and accordingly the gap may or may not break down at that instant but may allow the potential to rise to more disruptive values. The protection afforded to the equipment is then correspondingly reduced, if not lost altogether, because the breakdown of equipment insulation, such as certain liquid and solid insulations used in hightension transformers, does not depend on the ionizing action of the above-mentioned natural agencies. In any case, whatever variability of threshold ionization that such liquid or solid or even gaseous insulation may have, such states may not be expected to be concomitant with similar states in the protective gap, and therefore lack of sufficient ionization in the gap at the instant the potential has reached the specified discharge level, constitutes an increased hazard to the insulation of the equipment. Furthermore, by actual tests, such equipment insulation is found to have denitely lower impulse ratios than the conventional air gaps under the steepest lightning impulses and thus the hazard to equipment is thereby further increased.

More specifically, therefore, it is an object of the present invention to provide means to assure the presence of a threshold ionization in the protective gap on the occurrence of potentials of discharge value regardless of the naturally provided agencies mentioned above.

For this purpose I propose to utilize the ionizing radiations of corona on a neighboring gap, and propose to provide means so arranged that the corona discharge is produced by the very potential appearing at the arrester, so that when potentials corresponding to the desired discharge level of the arrester appear, the required threshold ionization will also appear practically simultaneously therewith. While the exact explanation of the eliect of corona in this matter is not fully understood, I have verified that if a thick piece of ordinary glass is interposed between this corona source and the discharge gap, this eiiect is severely weakened; while, if a piece of quartz is interposed, the eiect is not seriously affected, which supports the theory that a large portion of the action may be due ot short wave radiation emanating from the corona.

Referring to the drawings the invention will be described in a preferred embodiment as it may be applied to a typical form of a lightning arrester now in common use and which comprises an outer cylindrical casing l of insulating material such as porcelain which is closed at its upper end Iby a cap II having a line terminal I2 and at the lower end by the base ring I3 having a grounding terminal I4 and cover plate I5. Arranged within the cylinder I0 is a current limiting unit I5 consisting of a plurality of resistance plates or disks and a series gap unit I1. The series gap unit is enclosed within a second insulating cylinder I8. which is preferably sealed at its upper and lower ends by conducting caps 2I and 22, and includes a stack of graded resistance elements or rings 23, that at the lower end of the stack being of relatively high resistance while the uppermost is of relatively low resistance. Arranged within the ring openings are pairs oi gap electrodes 24 conductively connected to the opposite sides of the rings. Arranged within the lower part of the cylinder I3 is a pair o! relatively high resistance elements 25 connected in series between the lowermost ring 23 and the lower end cap 22. The resistance elements 25, one of which is shown in greater detail in Fig. 2, are in the form of a hollow ring open at one side and are provided with metal gap electrodes 26 and 21 projecting from the ends of the ring into the central space.

The resistance material of the disks I6 and rings 23 has a high inverse voltage resistance characteristic. That is, if a voltage applied across a piece of this resistance material is increased, then the resistance of the material will decrease very greatly and the current flowing through the material will consequently increase very greatly. The resistance of each element 25 should be high as compared with that of the elements 23.

In the operation of the arrester as upon a high voltage transient, such as may be caused by lightning, being suddenly applied to the line terminal, the voltage will be distributed among the resistance units in accordance with their respective resistance values. Due to the relatively high resistance of the two units 25 as compared to that of the other units, a very large part of the voltage will appear across these two units 25. When a critical voltage is reached or shortly thereafter as determined by the time lag characteristic of the gap electrodes, the electrodes 25, 2l of the resistance element 25, located at the lower end of the gap structure, will first break down throwing the total voltage across the remainder of the resistance elements. The gap 0f the second unit 25 will immediately thereafter break down and which in turn will be followed by breakdown of the gaps 24 in progressive order throwing the entire voltage across the current limiting unit I5 so that the gaps have a predetermined sequence of operation.

The arrester thus far described is substantially the same as that disclosed and claimed in the Patent No, 2,151,559 issued March 21, 1939, to Mr. Karl B. McEachron, and assigned to the General Electric Company, the assignee of the present application. For a more complete description of the various parts and the operation thereof, reference is made to the specification of this patent.

In applying the present invention, I provide means for ionizing the series gap at the lower end of the gap unit. This means or ionizing device comprises a pair of electrodes 3| and 32 fixed upon the ends of resilient arms 33 and 34 and which arms are secured upon a piece 35 of insulating material, such as porcelain, by means of rivets 36 and 3l extended through cooperating openings in the piece 35 and bent portions of the arms 33 and 34. The electrodes are separated by a dielectric insert or plate 38 which is provided with an opening 4I at one' end for cooperatively fitting over the supporting piece 35. The electrode arms 33 and 34 are provided with integral extensions 43 and 44, respectively, for mounting the device upon the opposite ends of the lower resistance ring and whereby the electrodes 3| and 32 are connected in parallel with the gap electrodes 26 and 21.

The invention requires that this ionizing device shall produce corona at potentials below the breakdown level of the adjacent gap and also that the device shall not break down in the sense of becoming a discharge path in preference to the gap under any normal or abnormal condition of operation of the arrester. This double requirement is met by choosing such a material for the insert 38 as will have a high dielectric strength and a high dielectric constant the former to avoid breakdown, the latter to enhance corona. I find that mica is particularly suitable for such an insert by virtue of its high dielectric strength and dielectric constant. For a given kind of material for the insert and a given length of gap, the dimensions, such as the thickness and radial width of insert and the sharpness of the edge of the electrodes in contact with it are so adjusted, preferably experimentally, as to assure generation of corona at the electrode edges and on the surface of the insert at potentials below the breakdown voltage level of the gap but definitely above the highest operating potential to which the gap may be subjected continuously under service conditions. The dimensions, such as the thickness of the insert, may be adjusted or Varied by means of small circular rods or disks 39, cemented to the end of strip 38 concentric with the aligned electrodes 3| and 32.

'I'he ionizing device is preferably supported in a laterally spaced relation with respect; to the associated air gap so that the device and particularly the dielectric thereof will not be injured by the heat of the arc which tends to rise upwardly therefrom. The device is also so mounted that the dielectric sheet is substantially coplanar with the gap so that the ionizing radiations will be directed toward the gap space. It -will further be noted that since only the outer edge of the dielectric insert faces the arc gap, minute particles of metal which may be fused from the surface of the gap electrodes 26, 21 by the arc heat will not be splattered upon the flat surface of the dielectric to lower the creepage resistance between the electrodes 3l, 32 of the ionizing device. With normal line voltage conditions obtaining, the voltage across the gap ionizing device is materially below the corona starting point thereof, hence, it is not a source of radio interference during such periods.

The gap ionizing device as described not only reduces the impulse ratio of the main gap but also establishes a relatively constant breakdown voltage therefor. Thus since the maximum impulse voltage to which the after connected apparatus may be subjected is definitely determined, the matter of providing insulation for such ap paratus is materially simplified.

In the application of the invention to a lightning arrester of the general type shown only a single gap ionizing device is required since the shunting resistances are so graded that the various gaps will break down with a definite sequence. The discharge characteristics of the arrester are therefore determined by the first gap of the series since once it breaks down, the remaining gaps are illuminated and the spaces thereof amply ionized by the arc to hold the impulse ratios of the remaining gaps to minimum values. While in the illustrated embodiment the starting gap happens to be the lowermost one of the series, this is merely a matter of design.. It is to be understood that similar ionizing devices may be applied, if desired, to each of the various gaps or to any selected group or number. It will be obvious, however, that an ionizing device should be applied at least to those gaps which by reason of their particular location in the resistance and capacitance network of the arrester are most likely to be subjected to a larger share of the applied impulse voltages than the remainder and precede the others in reaching the predetermined critical breakdown potential.

I have explained my invention in the foregoing with the help of most probable theories, well-established principles and certain preferred structural details, and I have shown how simply the invention can be embodied in a structure utilizing materials and elements of a type generally available in the industry, but I wish to add that my invention is not limited to any theory or structural detail, and that various modifications thereof will be apparent to those skilled in the art.

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

A lightning arrester comprising a pluralityof series gap elements, a gap ionizing device connected in parallel with one of said series gap elements having the largest potential drop during the operation of said arrester at normal operating potential, said ionizing device comprising a pair of electrodes and an insert having a high dielectric strength and a high dielectric constant arranged therebetween in engagement with adjacent surfaces of said electrodes, said insert extending beyond the edges of said electrodes so that the breakdown voltage of said device around the edges of said insert is materially greater than the breakdown voltage for said associated gap element, said device being laterally displaced with respect to said gap of the associated element, the edges of said device electrodes facing the gap of the associated element and adjacent said dielectric insert being relatively sharp, the thickness of said insert being such that a corona discharge takes place at said sharp edges above the highest operating voltage for said gap and below the breakdown voltage of said series gap element.

c DONNELL D. MACCARTHY.

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