US1873362A - Porous lightning arrester - Google Patents

Description

Aug. 23, 1932. R. TANBERG 1,873,362

POROUS LIGHTNING ARRESTER Filed Aug. 6, 1929 I'II'IIIIIIIII'IIIII'II";

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A1ITORNEY Patented Aug. 23, 1932 UNITED STATES PATENT OFFICE RAGNAR TANBERG, OF PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA POROUS LIGHTNING ARRESTER Application filed August e, 1929. Serial No. 383,924.

My invention relates to lightning arresters, and more particularly to a laminated or striated form of arrester that operates on the porous-block principle, as described in a copending application of J. Slepian, C. E.

Krause and myself, Serial No. 379,899, filed July 22, 1929, and assigned to the lVesting' house Electric & Manufacturing Company.

In this copending application, is described a lightning arrester in which a lightning discharge, in passing therethrough to the ground, is confined within the minute pores of porous insulating material, such as porcelain, brick, or the like, but more particularly as in composite material of kaolin clay and carborundum, that has been molded into the form of a disc and fired to obtain the desired mechanical strength. It has been found that, if an electrical discharge is confined to spaces of very small size, say less than 1 mil in diameter, and preferably of the order of 10- mil,

said spaces being bounded by walls of insulating material or leaky insulating material, the voltage necessary to maintain the discharge is substantially the same as the voltage required to cause the breakdown of the confined space; and that, as soon as the discharge has passed, there is a quick extinguishing or de-ionization of the are that prevents the power current of the transmission line from following the lightning discharge to ground. Such an arrester, composed of a porous block, when applied to a transmission line, will limit very satisfactorily the over voltages of the line to safe values, by providing a discharge path to ground for any abnormal charges induced in the line by lightnin g phenomena.

One form of composite porous blocks which has been utilized with good results is a modification of the discs utilized in the well-known auto valve arresters, described in the Slepian Patent No. 1,509,493, which are composed of kaolin clay, carborundum and lampblack for giving them a resistivity of the order of 10 ohms per centimeter cube or from 1 to about 200 ohms per centimeter cube. In order to make these auto valve discs applicable to operation on the porous-block principle, they are heated to a high temperature for ohms, preferably about 30,000 ohms per disc.

Even though the increase in resistance is not to the extent that the material may be termed a perfect insulating material, it is of the order of a leaky insulator or semi-conducting.

By the provision of a slight conductivity along the surfaces of the pores, or in the walls of the pores, the breakdown potential through the pores of the material is reduced to a" lower value without lowering the cutoff voltage of the material, or the voltage at which the discharge ceases and, at the same time, the current-carrying ability of the material is increased, with consequent increase in the life during repeated passing of discharges. A very favorable performance characteristic is obtained with such a disc, surpassing, to a great and noticeable degree, the performance of arresters prior thereto. The desired conductivity when using auto valve discs may be obtained by controlling the amount of oxidation.

Though discs which may be obtained by oxidizing are preferred, it should be understood that I may utilize any porous composite disc having the desired resistivity and other electrical characteristics. For instance, it has been found that discs made of varying proportions of carborundum and clay, but omitting the lampblack constitutent, have proved very favorable. These latter discs, when made, are fired in graphite-lined furnaces and obtain their slight conductivity mainly from gaseous fumes inherent in graphite furnaces.

However, the size of the discs that can be practically manufactured can carry, without rupturing, currents less than those inherent in high-voltage transmission lines where application of this porous arrester, having the very favorable performance characteristic, is desired. Such arrester must stand currents of the order of 1000 amperes, and above, without rupturing. The natural thing to assumeis that discs made of the same 1 .composition of materials as the auto valve disc could be larger in size, particularly in area, but it is found that, when this is attempted, great difficulty is encountered in the manufacturin process, thereby providing a mechanically imperfect and weak: disc. Ber sides, it is of importance to keep the dimensionsof the arrester as small as possible.

The potentialof'thetrans'mission lines to" which the application of the arrester' is desired is also many times higher than the potential which successfully can be withstood by a single porous disc of thickness suitable for practical manufacture.

It is the objectof-my invention to provide alightning arrester, operating on the porous-block 'principleand made of composite material, which will stand currents of the order of 1000 amperes and upward and which is suitable for the-protection of highvoltage transmission lines.

Inpracticing my invention, I stack a rality of composite, high-resistance discs, one

on top of another. I so treat the discs prior:

to stacking, with a solution of va conducting material, as to render the endsurfaces, 1n WlllCll. the conductingmaterial has penetrated to something of the order of one-half or three-quarters of a millimeter, relatively highly conducting as compared to the bulk ofthe block. I thereby provide the effect.

of a graded resistance from the surface of the disc to the interior. The discs are than:

cemented together in.a stack by a conducting, cementitious material. The current, in passing through this stacked or laminated arrester, upon reaching a conducting region, is well distributed over the entire cross-section, thereby eliminatingconcentration of current which might .cause puncture of a porous disc. To further improve the results,

' I cement a disc of relatively low-resistance.

such as an auto valve disc to each end of the stack, as will be subsequently described. Such an arrester carries heavy surge currents over 1000 amperes without failure and, in doing so, gives the desired characteristic. At the same time it can be made suitable for any line voltage by cementing the proper. number of discs together. I v

For a better understanding of my invention, reference may be had to the following potential characteristic of a given porous arrester disc is influenced by varying the resistance.

In Fig. 1, myinvention is illustrated in a laminated arrester element of a size capable of protecting a transmission line. with 23,00v volts; (RJML S.) to (ground. QIn making my laminated arrester, I utilize a plurality of high-resistance porous discs, 1a, 1b and 1c, the resistivityof each of said discs being from megohm to 10 megohms per centimeter cube, and the number of said high-resistance discs depending upon the po-. tential rating of the system to be protected as will hereinafter appear. My laminated arrester has end discs 2a and 2b of relatively low resistance, onlyone being shown on each end, but a plurality might be used ofdifi'erent resistance to provide a graded resistance, the resistivity of these discs being of the order of a few ohmsper disc, or of the order of, 10 ohms per centimeter cube. 7

The several high and low-resistance discs are held together by conducting cement layers 3' for solidly holding the discs in stacked relation and, at the same time, providing conductivity over .the surfaces of the discs in order'to effect an equal distribution of current and, in this way to make it possible for the discs to pass heavy surge currents without being punctured. or destroyed. a

Metallic. contact plates 4a and 4b are disposed on each end of; the stack to serve to equalize the distribution of the current, as well as to provide a good contacting surface. In addition to the metalplates, it is preferable that the low- resistance end discs 2a and 2?) shall have a copper-plated contact surface for engagement with the metallic plates 4a and 4b. v

In order to prevent flashover offthe discs along the edges, I coat the periphery of the integralstack with some insulating material 5. For example, shellac provides a good insulation and does not crack.

I preferably treat the high-resistance discs 1a, 1b and 1c, in order tov provide them with the thin, highly conductin surface layer pre viously mentioned, and, or this purpose, I prefer to utilize a thin water-glass solution having mixed with it a large quantity of powdered conducting particles, such as lampblack. I allow the discs to soak in this solution for about an hour, which is suflicient time for the solution to penetrate into the discs to a distance of one half of a millimeter or thereabouts, filling the pores, near the surfaces of the discs, with conducting lampblack, and giving each of the discs a conducting surface, whereby good distribution of current is obtained. At the same time, the rating of the disc is not lowered because the penetration of v the conducting solution is so ver 'sinall, as

compared with the total disc-thic inch or about 3 millimeters.

ess of 'For cementing the discs together, I also utilize a mixture of li uid water-glass and lampblack, but one WhlCl'l. is thicker or less diluted. This cement layer 3 provides additional conductivity between adjacent discs, and hence it also aids in securing the desired distribution of current over the entire crosssection of each of the discs or lamination of my laminated arrester. After the discs are stacked, any superfluous water-glass solution, that may be left on the edges of the discs, may be washed off with water. Since the impregnation is so slight, it is found that ordinary washing with water is sufiicient, but, in addition, if desired, the edges may be ground on a stone. The assembled discs are then allowed to dry for about six hours at 70 C. and thereafter baked for about six hours at 150 C.

Although I have only mentioned water 29 glass and carbon, I find that solutions or suspensions of almost any other conducting liquid may be utilized with a fair degree of success.

As stated above, the discs themselves are of composite material and may be made from the relatively low-resistance auto valve discs described in the above-mentioned Slepian patent. These auto valve discs have an initial resistance of the order of two ohms, for a 30 cross section of 3 square inches and a length of inch. They are heated sufiiciently to burn out or oxidize most of the lampblack, leaving but a small amount in the discs and preferably just enough to ensure satisfactory 5 action of a porous arrester, the resistance of each disc being increased to something of the order of 30,000 ohms. These treated discs have fine pores, having an average diameter of about .03 mil, and have a resistivity of 49 something of the order vof 2 megohms per centimeter cube.

The incorporation of a small trace of conducting material in a porous-block arrester has been found to have two very important effects, which may very probably be explainable as follows. The addition of conducting material has the effect of lowering the breakdown potential without appreciably changing the cutoff potential, thereby im- 50 proving the characteristic of the arrester considerably. The addition of the conduct- .ing material has also the effect of flattening -'out the voltage-current characteristic of the arrester, by making the break-down voltages of the pores, more nearly equal, thereby also causing the discharge to be distributed over a larger number of pores which will increase the life and current-carrying ability of the arrester.

The manner in which these two results are effected may very probably be explained somewhat as follows. For a given length of pore, in an insulating material, the breakdown voltage is effected but very slightly by the size or diameter of the pore, but the voltage gradient of the discharge in the pore, once break-down has occurred, increases very rapidly as the diameter of the pore is decrease or, conversely, the arcing voltage or maintaining voltage or cut-off voltage is very much lower for large pores than for small pores. In any natural or inherently porous material, there are some slight irregularities in the lengths of the various porous paths from one surface to another, so that the break-down voltages of all of the pores, in an insulating material, are not the same.

The effect of the slight traces of conducting material, which incorporated into the porous-block arrester material, is to produce extremely attenuated chains or needles of conducting particles disposed in the walls of the pores, so that the potential gradient or electrostatic field, in the pores between the opposite terminals of the arrester, is rendered non-uniform. These needles are believed to produce needle discharges which initiate the ionization of the gaseous material in the pores, thereby lowering the breakdown voltage and making the break-down voltages of all of the porous paths more nearly equal. In other words, the effect of the needle discharge is more noticeable in the porous paths of high break-down voltage than in those having the lower break-down voltages. The cross-sections of these needles or chains of minute conducting particles, probably making loose contacts with each other, are so small that no material current can be carried by the needles, so that, when the discharge is once initiated, by breaking down the gaseous space by reason of the ionization produced by the needle discharges, the discharge current is distributed throughout the entire volume of the pores without being affected by the presence of the needles, because the current carried by the needles is wholly negligible, as compared with the current carried by the pores.

This explanation, which seems to be the proper explanation of the phenomena involved, also explains the fact that a larger quantity of conducting material is required for porous arresters having large pores than for the more finely porous materials, because the voltage gradient necessary to maintain an arc in a large pore is much smaller than in a small pore, so that more conducting material must be utilized,'in the case of the large pore, in order to bring down the break down voltage sufliciently close to the maintaining or cut-off voltage. Preferably, only enough conducting material is added, for any type of \porous-block arrester, to bring down the break-down voltage to the vicinity of the cut-off voltage. Too much conducting material will needlessly increase the leakage loss, which is measured in milli-amperes, or fractions of milli-amperes and, as the quantity of conducting material is increased more and more until it becomes excessive,.it will begin to short-circuit the ,arcmg paths through the pores because the voltage dro 1n the conductlng paths will become sma ler than the minimum voltage drop necessary to maintain the arcs in the small ores.-

In general,therefore, the desirable or permissible resistivity of the porous-block arrester material depends upon the, fineness of the pores. Nevertheless, the resistiyity of the porous-arrestermaterial is always so hlfh that it, is of the order of the resistivity o a leaky insulator, being measurable in units of the order of 10* or 10 ohms per centimeter cube, as against something like 100 ohms for the auto valvearrester disc, 10' ohm, for car bon, and 10* ohm for copper.- A widerange of diameters of-the .pores seems to beavail: able, ran ing probably, anywhere from a di-. ametero the order of l mil,.or possibly more, todiameters of the .orderof 10? mils, or pos sibly less. Thesevalues are notabsolute and sharply fixed, as desirable results may be obtained with aten-foldor even a. hundred-fold departure from any of the values stated.

InFig'. 3, isshown the manner in which the cut-off voltage changes, with varlations in the resistance of discs having a diameter of 2 inches, a length or thickness of inch, and pore diameters of about .03 mil. For resistances up to. about. 35,000 ohms per d sc, the

curve takes the form of an approximately straight line, having a s10 e of approximate- 1y 40 volts per thousando ms. At about the 35 thousand-ohm point, the curve begins to. straighten vout, so that there is no material advantage in utilizing materials of much higher resistance. With this resistance of 35 thousand ohms per disc, or about 2 megohms per centimeter cube, good. current-carrying abilityis ensuredjr A single disc has a cut-oif voltage of,1600 volts, as may be seen from the curve in Fig.3. A single disc, without means for ensuring good distribution of the discharge, willdischarge a momentary current of the order of amperes, but probably could not pass more current without becoming impaired or unctured. a

By stacking tl ie high-resistance discs just described, and providing for good distribution of current at the surfaces of the discs, I obtain a characteristic, suchas shown in Fig. 2. Discs so stacked will carrycurrent of the order jof 1000 amperes. A stack utilizing three high-resistance discs, as shown in Fig. 1, including the'relatively low-resistance end discs, gives a maximum voltage of about 12,000 volts, anda cut-01f voltage of about 3800volts crest value. Such an element is capable of protecting a transmission line having a root-mean-square voltage of 2300 volts from line to ground. To provide an arrester for higher-voltage systems, a larger number of high-resistance iscs may beutilized or several elements may be stacked.

Y As is usual in, arresters ofthis type, myarrester .is preferably provided with a series ga device 10, as indicated in Fig-1. f 7 d he term cut-oftv yoltage orr-cut-ofi' potential, as, used in my description denotes the lowest voltage that willmalntain the arc discharge through the pores of the semi-conductmg material.

AI-(318.1111 as my invention a 1. A striated porous-block arrester having a plurality of porous discharge discs stacked and cemented together with conducting cement, said discsbeing made. of semi-cone ducting porous material operatingson the principle of discharges through the pores, said pores being so ,fineithat the jvol tagegradient necessaryyto maintain a discharge is measured in thousands of volts per inch, and the amountof conducting matter in said porous material being barely sufficient .to bring the breakdown voltage. substantially down to the cutoff voltage in each of the pores carrylng current. 2, A laminated porous-block arrester having a central portion of semisconducting porous material operating on the principle of discharges through the pores,-said pores being so line that the voltage-gradient necessary to maintain a discharge is measured in thousands-of volts perinch, and the amoutof conducting matterin said porous materialber, ing barely suflicient to bring the. breakdown voltage substantially down to the cutoff volt-. age in each of the pores carrying current, said arrester having top and bottom terminal portions, each ofsaid terminal portions having .a plurality of layers of conduct-ing material of, progressively increasing conduc tlvity, starting with an inner layer disposed next to the said central portion and having more conductivity than said central portion, and including a metallic end terminal plate.

3. .A laminated porous-block arrester having a central portion of semi-conducting porous material operated on the principle of dischar es through the pores, said pores being so e that thevoltage-gradient necessary to maintain a discharge is measured in thousands of volts per inch, and the amount of conductingmatter in said porous material being barely suflicient to bring the breakdown voltage substantially down to the cutoif voltage ineach of thepores carrying current,

said arrester having top and bottom terminal portions, each of said terminal portions having a plurality of layers of conducting material or progressively increasing conduc tivity, starting with an inner layer disposed next to the said central portion and'having more conductivity than saidcentral portion,

and includinga metallic end terminal plate, the said layers and central portion being all cemented together into a single solid bod y.

4. In the manufacture of porous-disc lightning arrester's the method which comprises soaking semi-conducting porous discs in a conducting solution, rovidmg a, layer of rela- V tively high conductmg cement on the end surfaces of the discs, stacking said discs together, drying and baking the assembled structure.

In testimony whereof, I have hereunto subscribed my name this 30th day of July, 1929.

RAGNAR TANBERG.

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