US2802175A - Method of and means for detecting impulse corona and preventing flashover due to impulse corona in lightning arresters - Google Patents

Description

Aug. 6, 1957 ELDRIDG T. l. METHOD OF' AND MEANS FOR DETECTING IMFULSE CORONA AND PREVENTING FLASHOVER DUE TO IMPULSFJ CORONA IN LIGHTNING ARRESTERS Filed Oct. 29, 1953 I 2 Sheets-Sheet 1 j3d" I \J6J IA'VENTO'R; EusnAs LELDRuocE H/s @from/ey Aug- 6, 1957 T. l. ELDRIDGE METHOD OF' AND MEANS FOR DETECTING IMPULSE CORONA AND PREVENTING FLASHOVER DUE TO IMPULSE CORONA 1N LxGHTNING ARRESTERS 2 Sheets-Sheet 2 Filed OCT.. 29, 1953 TAUslAs I. ELDRnocE HIS 0 TTO/l/EV 2,802,175 METHOD OF AND MEANS FOR DE-rnorlNG 1M- PULsE CORONA AND rnavnNrrNc FLASH- OVER DUE T0 IMPULSE CRGNA IN LIGHT- NING-ARRESTERS Tausias I. Eldridge, Havertown, Pa., assignor to H. K. Porter Company, a corporation of Delaware Application October 29, 1953, Serial No. 389,687

14 Claims. (Cl. 324-54) My invention is a method ofand means for detecting and overcoming a tendency of lightning arresters to flashover and fail as a result of the formation therein of an impulse corona, as distinguished from ordinary corona resulting from system voltage stress.

My invention is applicable to lightning arreste'rs having electrodes forming arc gaps in series and including a charaoteristic element which permits flow of discharge current to earth, limits the voltage at the line terminal, and suppresses follow current.

Such an arrester is ordinarily so designed that the voltage appearing across its terminals is normally balanced and distributed in direct proportion to the :several irnpedances comprised in the whole, and conducting members of one polarity are separated radially or rectilineally from conducting members of opposite polarity by an insulating tube which forms a part of the characteristic element.

In the use ofv such an arrester, a series gap electrode is connected with'a line terminal and a characteristic element electrode, of opposite polarity from the line terminal electrode, is connected with ground. The discharge path through thecharacteristic element interposed between the line electrode and the ground electrode has a much lower impedance than the flashover path around the characteristic element from any conducting member of one polarity to a conducting member of opposite polarity.

Preferably the characteristic element consists of a hard fibre protector tube containing narrow, elongated arc slots, yand encircled by a grounded grading ring. The arc slots may be formed by a series of concentric, cylindrical walls of gas forming material with -spaces between them forming multiple, parallel cylindrical arc slots each having the shape of a narrow rectangular slot bent concentrically -around and spaced from Ian axis. Electrodes are' positioned at the ends of the slots and connected respectively with a series gap electrode and with ground.

Such lightning arresters are commonly referred to as protector tubes or expulsion arresters and examples thereof are shown in the Eldridge et al. U. S. Patent No. 2,619,610, and in the Eldridge Vet al. application Serial No. 223,089, on which Letters Patent issued December 29, 1953, No. 2,664,518.

Such lightning arresters, when well designed and made, are highly eicient and dependable and the `occasional flashover before sparkover resulting in failure of such well designed and well made arresters have been generally regarded as inexplicable.

I have discovered that such failures result from the spread of impulse corona from one conducting member to another conducting member of opposite polarity within the arrester just before sparkover. Such impulse corona emanates primarily from the region adjacent to the line end of the arc slots-and particularly from the adjacent end ofthe grading ring when such ring is used. A corona of su'liicient intensity to reach a series gap electrode and cause ilashover through air impinging on a smooth surfaced States Patent O 2,802,175 Patented Aug. 6, 1957 ICC fibre protector tube results from the application across the arrester terminals of voltage of practically square wave front, positive polarity and crest voltage just below (as well as at or above) that required for sparkover in the absence 0f such corona. Only under special conditions will other .types of impulse surge cause ashover before :sparkover and l have found no instance where a ilashover before sparkover could be caused by other types of surge in an arrester immune to ilashover before sparkover on the application of an equal voltage of square wave front and positive polarity.

I have further discovered that the formation and extent of such impulse corona is detectable by positioning sheet material having a surface coated with a photographically sensitive emulsion in juxtaposition to the exterior of the characteristic element and between conducting members of Opposite polarities, e. g. a characteristic element electrode and a series gap electrode, and discharging across the arrester terminals a voltage of very steep wave front, positive polarity and a crest just below normal sparkover voltage. The photographic development of the emulsion reveals a feathery design generally resembling fronds having their bases adjacent to the top of the grading ring, or at the level of the upper electrode of the characteristic element.

It would appear that positive and negative charges are produced in the hard fibre tube, between conducting members of opposite polarities, and that the electrostatic field due -to the voltage stress causes these charges to move more or less rapidly in the hard fibre, lthereby causing changes in current-s in the dielectric and a tendency to upset the voltage distribution or balance and the' eman'ation of corona with its base in the vicini-ty of the electrode on the line side of the characteristic element.

The grading ring, when used, is of opposite polarity from the electrode on the' ground side o'f the series gap and facilitates the establishment of an electrostatic field and the frond images produced when a grading ring is used are generally larger and denser than the images produced when no grading ring is employed.

I have further discovered that the formation of impulse corona between the grading ring and the electrode of opposite polarity may be minimized and its deleterious effects eliminated by preventing free passage of the impulse corona along the surface of the 'hard fibre tube. This may be effected by knurling the surface of the hard libre tube between the ring and electrode members or by encircling the libre tube and the upper portion of the grading ring with a strong, elastic insulator immune to the effects of ozone, such as a polyvinyl chloride or soft rubber tube or a tightly Wound polyvinyl chloride or soft rubber tape, which constantly exerts inward radial pressure.

The principles of my invention and the bes-t methods contemplated by me of applying such principles will furither appear in the following description and the accompanying drawings in illustration thereof.

In the drawings, Fig. l is a lightning arrester equipped for the detection of impulse corona in accordance with my invention; Figs. 2 and 3 show enlarged views of the unrolled and developed photographic films encircling the upper portion of the characteristic element in Fig. 1 after such films have been subjected to impulse corona generated by applying across the arrester terminals a surge voltage just lbelow normal sparkover voltage of the arrester and of steep wave front and positive polarity across the arrester terminals; Fig. 4 shows the arrester of Fig. l

equipped with photographic films for detecting impulse corona and with a sleeve preventing the generation of impulse corona of suflicient intensity to reach from the grading ring to a series gap electrode; Figs. 5 and 6 show the unrolled and developed photographic films of Fig. 4 after applying across the arrester terminals of Fig. 4 the same voltages that produced the imagery of Figs. 2 and 3 when applied across the terminals of Fig. 1; Fig. 7 1s afragmentary View of a portion of the arrester shown 1n Fig. l (with films omitted) after wrapping the exposed protector tube and part of the grading ring with tightly drawn layers of polyvinyl chloride or soft rubber tape; and Fig. 8 is a fragmentary View of a portion of the arrester shown in Fig. l (with films omitted) after knurling the exposed external periphery of the potector tube.

The arrester shown in the drawings comprises a wet process porcelain, cylindrical, hollow ,body 1 having its top closed by a porcelain plug 2 which supports a line terminal 3 from which depends a series gap electrode 4.

A flanged, threaded conducting cup 6 is cemented in the countersunk bottom of the body v1 and terminates in a ground conductor '7 containing a discharge vent S for arc slots hereinafter described. The flanges of the cup 6 and the cement securing the same contain vent passages 9 for Ventilating the interior of the body 1.

A hard fibre tube 10 is threaded into the cup 6 and tightly encircled `by a grounded grading ring 11 consisting of a steel reinforcing sleeve conductively seated in a `groove in the top of the cup 6.

A circular, -grounded arc electrode 12 is conductively seated in the cup 6 and supports a hard fibre tube 13 in close, radially spaced relationship to `the tube 10. The electrode 12 also supports a hard fibre rod 14 in close, radially spaced relationship to the tube 13. The upper ends of the tube 13 and rod 14 support, and are radially positioned by, an intermediate electrode 1S whose periphery makes a close fit with the interior of the libre tube 10 at approximately the level of the top of the grading ring 11. The space between the tubes 1t) and 13 and the space between the tube 13` and rod 14 form elongated arc slots 16 and 16 of narrow cross section and of unequal lengths between the electrodes 15 and 12. The electrode 12 contains apertures 17 through which such slots 16, 16 discharge to the vent 8.

A conducting metal tube 18 is sleeved on a boss on 'the top of the electrode 15 and on a boss on the bottom of a conducting metallic plug 19 which is threaded into the top of the fibre Itube 10. An electrode 19 is adjustably seated in a socket in the top of the plug 19 and forms with the electrode 4 a series gap 2t) for normally isolating the protector tube from line voltage.

The application of sparkover voltage of usual shape and polarity across the arrester terminals sets up an electrostatic field which causes sufficient ionization of air in the discharge path to cause sparkover across the isolating gap 20 and through the arc slots 16 and 16 and permit discharge of the surge to ground. The ionization of the air in the discharge path also permits follow current to flow to ground from the system and maintain arcs across the gaps. The action of the arcs on the hard fibre walls of the arc slots 16, 16' generates deionizing gas and builds up high pressures in these slots. The deionizing particles and pressure effect suppression of the arcs in the slots 16, 16 and consequent quenching of the arc across the isolating gap 20.

By my invention the liability to failure of a particular design of arrester under extreme conditions may be detected by encircling the upper portion of the grading ring 11 and of the protector tube il@ with films 21 and 2,2 coated with a sensitive photographic emulsion. Such emulsion may be of the type ordinarily used for photographing in black and white contrast or of the type used to obtain color contrast. There is then impressed across the arrester lterminals an impulse or surge voltage which is preferably just `below normal sparkover voltage and of steep, e. g. substantially square, wave front and positive polarity. The Voltage stresses involving a voltage wave of positive polarity generate more intense impulse `corona than those of a voltage wave of negative polarity, and the steeper the wave front the shorter is the time available for the actions which must take place in initiating sparkover of the arrester. An arrester, which will withstand ashover under the foregoing conditions, will not ashover upon application of surges of negative polarity and of more gradual wave front even though of higher voltages, and will sparkover and perform its intended functions upon application of sparkover voltage of any shape or polarity.

The greater the steepness of the voltage wave, the shorter is the time to sparkover and the higher is the voltage needed to initiate sparkover. For example, a 500 kv. voltage wave having a rising front averaging 100 kv. per microsecond ywhen reaching an arrester that normally sparkover'at 75 kv. will reach 75 kv. in about threefourths of a microsecond and if for any reason sparkover does not occur immediately, the continued rise creates a stress resulting in an impulse corona which develops in a fractional part of a millionth of a second and is ditiicult to identify. As the ltime period approaches zero, the more important become the coincident effects and/ or secondary actions caused by the stress, and particularly the generation of impulse corona in the region of the protector tube encircling the electrode at the upper end of the arc slots. Such impulse corona is not to 1be confused with corona of the steady state type such as might be produced by system voltage adjacent to the isolating gap.

Since the crest Value of voltage required to initiate sparkover of the arrester gaps increases with the steepness of front of the Voltage wave, viz. the time required yfor the wave to rise from zero to crest, a very intense corona normally results from a voltage of square wave front and positive polarity.

The space relationship of the intermediate electrode 15 with the grounded grading ring 11 and the separation of these parts by the outer fibre tube 10 helps to develop an intense electric field and facilitates fast sparkover from the outer intermediate electrode 15 to the terminal electrode 12 through the are slots 16, 16', since the dielectric strength of the tube 19 prevents a discharge from passing therethrough.

Since the intermediate electrode 15, plug 19 and series gap electrode 19 are conductively connected together, the same difference of potential which establishes stress between the intermediate electrode 15 and the grading ring 11 will also exist between the plug 19 and grading ring 11 along the smooth, outer surface of the tube 10 of Fig. 1, hence the distance between these parts is always made sufficiently long to provide a factor of safety sufciently high to prevent discharge, under normal sparkover conditions, from the plug 19 to the grading ring 11 instead of from the intermediate electrode 15 through the slots 16, 16 to the terminal electrode 12'.

When, however, a surge voltage of positive polarity and practically square wave front is applied across the line and ground terminals of an arrester containing photographically sensitive lms 21 and 22 (with due precautions to prevent any elect by such media on the electric field and current development) and the voltage wave reaches crest value in practically zero time, an impulse corona is generated which extends upward from the tube zone surrounding the electrode 1S and activates the photographic emulsion to form thereon images which, when photographically developed in the usual manner, generally have the form of branches or fronds, e. g. as shown in Figs. 2 and 3. Fig. 2 shows a typical design produced on the upper film of Fig. 1 and Fig. 3 shows a typical design produced on the lower film of Fig. 1 when an arrester is subject to voltage slightly below normal sparkover voltage and of positive polarity and substantially square wave front, with the grading ring 11 in place. If the grading ring is omitted, the images produced on the upper and lower films more nearly resemble scattered clumps of grass or ferns. Where colored film is used, the shorter fronds usually appear in blue color whereas the longer fronds usually appear in yellow color. If the volt- 'age sfraised lto-or about normal sparkover voltage, hashover occurs before sparkover and the film isseared. If the steepness of the wave front is dimlnished the imagesdecrease in density and amplitude and generally disappear from the upper 'film even though the voltage be increased. j

I believe ,that the `generation of impulse corona produces positiveand negative charges in the hardibre tube and that such charges tend to move more or less rapidly and 'cause changes in currents in the tube 10, resulting in a tendency to upset the voltage distribution or balance between the parts of the arresterV and particularly between theplug 19 and the grading ring 11`.

vThe generation of impulse corona of limited intensity might not be too serious, but in conjunction therewith the grading ring 11 tends to facilitate the establishment of a flashover dueto the mobility of ions discharged from its sharp edge as a secondary transmitter due to rapid movement of charges within the adjacent tube 10 under the inuence of the almost instantaneous establishment of the electric eld.

Circuit constants of distributionlines tend to determine the steepness of a wave front that travels along the circuit and square fronts* are less likely to be present than fronts y controlof impulse corona and its effects under such con-` ditions will eliminate ashover under conditionsinvolving the discharge of the more prevalent surges of negative polarity and gradual slope of the wave front.

I have found that the emanation of impulse corona from the grading ring 11 and from the tube periphery adjacent thereto may be minimized, and its ilashover effects eliminated, by covering the upper portion of the grading ring 11 and the smooth surface of the tube 10 between the grading ring 11 and plug 19 with a medium of high dielectric strength which constantly exerts inward radial pressure on the outer periphery of the tube 10 and the upper portion of the grading ring 11, as shown in Figs. 4 and 7.

As shown in Fig. 4, the upper portion of the grading ring 11 and the peripheral surface of the tube 10 are encased in constricting tubing 25 of polyvinyl chloride, rubber, or the like. The tubing is normally of slightly smaller diameter than the outside diameter of the tube 10, and is expanded and slipped over the tube 10 and ring 11 and then allowed to shrink. Such tubing 25 should be of suicient thickness and length to prevent the formation of any images extending all the way from the grading ring 11 to the plug 19 on the photographically sensitive films 21a, 22a when a surge voltage of positive polarity and substantially square wave front is impressed across the terminals 3 and 7 of the arrester. The absence of images across the height of the upper film 21a, when the arrester has voltage of square wave front and positive polarity impressed across its terminals as heretofore described, is a determinant that the thickness, length and quality of the tubing is suicient to prevent ashover before sparkover under any conditions.

In lieu of the tubing 25, the upper portion of the grading ring 11 and the peripheral surface of the tube 10 may be tightly wrapped in layers of polyvinyl chloride or soft rubber tape 26 so as to form a tight sleeve of sufiicient thickness, length and constrictiveness to prevent the production of images on a photographically sensitive film positioned as shown at 21 and 21a in Figs. l and 4, but omitted from Fig. 7.

I have further found that knurling or forming closely spaced transversely extending narrow grooves 27 in the peripheral surface of the upper portion of the tube 10 will give substantially the same protective effect against the upward travel of impulse corona as the application lof the tight sleeves shown in Figs. 4 and 7.

In all cases it is desirable tocontrol the circulation of air and ions between the tube 10 and the inner wall of the body 1 by inserting layers of glass libre batting 28, but this in itself is insucient 'to prevent the formation and upward' travel of impulse corona and ashover under conditions of surge voltage of positive polarity and very steep wave front.

Having described my invention, I claim:

I. A method of detecting impulsecorona in a lightning arrester having a protective tube including-series gap electrodes and arc slot electrodes, said series gap electrodes being spaced by a hard libre tube from one of said arc slot electrodes, which comprises encircling said tube with material having a surface coated with a photographically sensitive emulsion, and impressing across said arrester an impulse voltage of positive polarity and substantially square wave front.

2. A methodl of detecting impulse corona in a lightning arrester comprising a protective tube having series gap electrodes and arc slot electrodes which comprises positioning material having a surface coated with a photographically sensitive emulsion between one of said series gap electrodes and one of said arc slot electrodes, and impressing across the arrester a surge voltage of substantially square wave front.

3. A method of detecting impulse corona in a lightning arrester having series gap electrodes and a characteristic element containing spaced electrodes which comprises positioning-material having a surface coated with a photographically sensitive emulsion adjacent to said characteristic element between one of the electrodes thereof and one of said series gap electrodes, and impressing across said arrester an impulse voltage of steep wave front.

4. A lightning arrester comprising a protector tube including series gap electrodes and arc slot electrodes, and material having a surface coated with a photographically sensitive emulsion bent around said tube between one of said series gap electrodes and one of said arc slot electrodes.

5. A lightning arrester comprising a protector tube having series gap electrodes and arc slot electrodes, and means for preventing the activation of a photographically sensitive emulsion on sheet material positioned adjacent to said tube between one of said series gap electrodes and one of said arc slot electrodes when an impulse voltage of positive polarity and substantially square wave front is impressed across the arrester.

6. A lightning arrester comprising a protector tube having series gap electrodes and arc slot electrodes and a constricting sleeve of non-conducting material encircling said tube between one of said series gap electrodes and one of said arc slot electrodes and of sufficient thickness to prevent activation of a photographically sensitive emulsion on sheet material adjacent thereto when a surge voltage of positive polarity and substantially square wave front is impressed across said arrester.

7. A lightning arrester comprising a protector tube including series gap electrodes and arc slot electrodes, a grounded grading ring surrounding said protector tube and a constricting sleeve of non-conducting exible elastic material immune to the effects of ozone encircling at least a part of said grading ring and encircling said tube from said grading ring to one of said series gap electrodes, said constricting sleeve exerting inward pressure throughout the length thereof on said tube and grading ring.

8. A lightning arrester comprising a protector tube including series gap electrodes and arc slot electrodes, the outer surface of said tube being knurled between one of said series gap electrodes and one of said arc slot electrodes.

9. A method of testing a lightning arrester having electrodes forming arc gaps and having a characteristic element which comprises applying across the arrester terminals a surge voltage slightly below normal arcover voltage' and having a substantially square wave front and i including a series gap electrode and an arc slot electrode between which impulse corona tends to, form on the dischargeY between them of positive surges having steep wave fronts, and an expanded tube of elastic polyvinyl chloride tightly encircling said tube substantially the entire distance between said electrodes and exerting inward pressure throughout the length of said expanded tube.

l1. A lightning arrester comprising a protective tube including a series gap electrode and an arc slot electrode between which impulse corona tends to form on the discharge between them of positive surges having steep wave fronts, and an expanded tube of rubber tightly encircling said tube substantially the entire distance between said electrodes and exerting inward pressure throughout the length of said expanded tube.

12. A lightning arrester comprising a protective tube including a series gap electrode and an arc slot electrode between which impulse corona tends to form on the discharge between them of positive surges having steep wave fronts, and a band of polyvinyl chloride tightly wound in stretched condition around said tube substantially the entire distance between said electrodes, said band exerting inward pressure throughout the winding thereof.

13. A lightning arrester comprising a protective tube including a series gap electrode and an arc slot electrode between which impulse corona tends to form on the '8 dischargejbetween'them of positive surges having-steep wave fronts, and a band of soft rubber tape tightly wound in' stretched condition around said tube substantially the entire distance between said electrodes, said 'tape exerting inward pressure throughout the winding thereof.

14. A lightning arrester comprising a protective tube including series gap relectrodes and arc slot electrodes, and a Anon-conducting sleeve of flexible elastic material immune to the effects of ozone and selected from the group consisting of polyvinyl chloride and soft rubber encircling said tube throughout substantially the entire distance between the gap forming portion of one of said series vgap electrodes and one of said arc slot'electrodes; said sleeve exerting inwardpressure throughout the length thereof.

ReferencesCited in the le of this patent UNITED STATES PATENTS 1,957,306 vogel May 1, 1934 2,170,813 Doble a- Aug. 29, 1939 2,338,479 Ackermann Jan. 4, 1944 2,372,035 Wade Mar. 20, 1945 2,603,685 Bychinsky July l5, 1952 2,677,072 De Val Apr. 27, 1954 OTHER REFERENCES Enclosed Spark Gaps, pages 429-432, American Institute of Electrical Engineers Transactions, volume 59, August 1940.

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