US2279249A - Spark gap device - Google Patents

Description

INVENTOR Wa/zer 6 P0777677.

ATTORNEY April 7, 1942. w. G. ROMAN SPARK GAP DEVICE Filed Jan. 26, 1939 WES: U

Patented 7, 1942.

SPARK GAP DEVICE Walter G. Roman, Forest Hills, la., assiguor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa...

Pennsylvania a corporation of Application January as, 1939, Serial No. 252,928

5 Claims.

My invention relates in general to spark gap devices, and more particularly to a spark gap having a low and consistent breakdown voltage and a low impulse ratio, which. is particularly suitable for use in lightning arresters or other over-voltage discharge devices.

- While the spark gap of the present invention is of general application, it is especially adapted for use in connection with lightning arresters of the type in which a spark gap is connected in series with a resistance or valve element between the transmission line or other electrical device to be protected and ground. The resistance element of such an arrester preferably has the valve characteristic of being substantially a non-conductor, or at least a very high resistance, under normal voltage conditions, but of changing to a conductor when the voltage applied to it exceeds a predetermined value. Thus, when a high voltage such as a lightning surge is applied to the arrester, the valve element permits the flow of large surge currents to ground, but after the surge has passed and the voltage drops to approximately the normal line voltage, it reduces the current to a very small value which can be readily interrupted by the series gap.

The series spark gap used in such arresters performs three functions. Under normal condi tions, it insulates the arrester from the line and prevents the flow of leakage current through the arrester. When a lightning or other high voltage surge occurs, however, the gap breaks down and connects the arrester to the line, thus permitting the flow of surge current to ground.

After the surge has disappeared, the gap performs its third function of interrupting the relatively small power follow or leakage current caused by the normal line voltage. It will be evident, therefore, that in order to secure the greatest degree of protection, the gap should have a relatively high 60-cycle or normal frequency breakdown voltage in order to prevent operation under low voltage disturbances, and should have a relatively low surge breakdown voltage to insure rapid operation when a high voltage surge occurs. The ratio of the surge or impulse breakdown voltage under a standard rate of rise of the voltage to the normal frequency breakdown voltage is called the impulse ratio of the gap, and it is obviously desirable that this ratio should be as close to unity as possible.

As usually constructed, series spark gap devices for lightning arresters consist of generally disk-s aped electrodes of brass, or other suit- ,ing spacer and an electrode.

able metal, separated by an annular spacer, usually either of porcelain or of a resistance material if it is desired to control the voltage distribution across the individual units of a multiple gap structure. A suflicient number of such gaps is used in series to obtain the desired voltage rating. It has been found in practice, however, that spark gaps constructed in this manner have relatively high and inconsistent surge breakdown voltages, giving impulse ratios ranging from about 1.5 to as high as 2.0. These high breakdown voltages under surge conditions are due to a time lag after the voltage hasrisen above the static breakdown voltage before the gap actually breaks down, and this time lag may vary in different tests of the same gap, giving inconsistent results and making it difficult to predict the behavior of the gap under given conditions. It will beapparent, therefore, that in order to obtain low and consistent surge breakdowns, it is necessary to reduce this time lag to a i The object of the present invention is to pro vide a spark gap device having alow and consistent surge breakdown voltage and a low impulse ratio.

More specifically, the object of the invention is to provide a spark gap device in which the time lag of breakdown is materially reduced by causing ionization of the air in the gap, which is very effective in initiating breakdown with a minimum time lag. I have found that this result may be accomplished by causing high voltage gradients to occur in local regions of the gap adjacent the contacts between the insulat- The high voltage stresses in these regions cause electrical discharges, which may be similar to corona, and these discharges are very efiective in causing ionization of the air, and may also cause the emission of a radiation into the gap which is also effective in ionizing the air aswell as in producing free electrons in the gap. The ions and electrons thus formed are very efiective in initiatingrapid breakdown of the gap and causing it to occur with aminimum time lag.

These high voltage gradients in' local regions of the gap may be produced in various ways, and can conveniently be caused by designing the gap in such a manner that the electric field between the electrodes is distorted so that high local concentrations of voltage stress exist. This distortion of the field may be produced by reducing the area of contact between one of the electrodes and the spacer to a very small area, or

by introducing conducting elements into the gap which cause local distortion adjacent the electrode, or in any other suitable manner.

The invention will be more fully understood I from the following detailed description of certain preferred embodiments, taken in connection with the accompanying drawing in which:

Figure 1 is a vertical sectional view of a lightning arrester spark gap device, with the associated apparatus shown diagrammatically;

Fig. 2 is a perspective view of the lower electrode of the gap of Fig. 1; r

Fig. 3 is a perspective view showing a modified form of lower electrode;

Fig. 4 is a vertical sectional view of a double spark gap, illustrating another embodiment of the invention;

Fig. 5 is a plan view of the gap of Fig. 4 with the upper electrode omitted; and

Fig. 6 is a fragmentary perspective view showing the method of assembly of the gap of Fig. 4.

The spark gap device shown in Figure 1 consists of an upper electrode I and a lower electrode 2, separated by an annular spacer member 3 of porcelain, or other suitable insulating material. The electrodes may be made of brass or other suitable metal and are generally diskshaped. Since it is usually desirable to have the thickness of the spacer substantially greater than the actual gap space, the upper electrode I is shown as being dished or shaped so that its central portion approaches the lower electrode more closely than the periphery to form an annular gap space indicated at 4. The upper electrode I is connected to a transmission line conductor 5, or other electrical device to be protected, and the lower electrode 2 is connected to a lightning arrester valve element i, which is connected to ground as indicated at I. In an actual construction, of course, a sufilcient number of gaps would be used in series to obtain the desired voltage rating.

In order to cause high concentrations of voltage stress adjacent the points of contact between the lower electrode and the spacer, the area of contact between these elements is made very small. As shown in Figs. 1 and 2, this is accomplished by providing a plurality of projections on the upper surface of the electrode to space it a small distance from the spacer and which have a very small area of contact with the spacer. These projections may be conveniently provided by securing short lengths of wire {to the upper surface of the electrode 'by soldering, or other suitable means, as shown in Fig. 2. Projections may also be provided on the electrode by forming it .with an annular ridge 9, as shown in Fig. 3, or by forming itwith projecting portions of any suitable configuration which will space it a small distance from the spacer 3 and have a very small area of contact therewith.

The effect of this construction is to'cause a high concentration of voltage stress adjacent the small areas of contact so that high voltage gradients exist along the spacer and in the air near these points. These high local voltage gradients, as explained above, cause discharges to occur which result in ionization of the air in the gap and cause breakdown to occur with a evident that these projections might be provided on the upper electrode if desired, although the efl'ect would probably not be as great, since the ionization of the air would occur at points farther removed from the actual gap space.

Another embodiment of the invention is shown in Figs. 4 and 5. A double gap device is shown in these figures, although it will be understood that the arrangement shown may be used equally well with a single gap. The gap device consists of an upper electrode IO, anintermediate electrode or gap plate II, separated from the electrode III by an annular spacer I2, and a lower electrode I3, separated from the electrode I I by an annular spacer I4. The spacers are made of porcelain or other insulating material, while the electrodes are preferably made of brass, and the upper and lower electrodes are dished in the usual manner to permit the outer peripheries to be spaced apart a greater distance than the actual gap space. In this embodiment of the invention, the high local voltage gradients are produced by introducing a conducting element into the gap space for the purpose of distorting the electric field between the electrodes. For this purpose, spring members I5 are placed in the gap on each side, in contact with the intermediate electrode and with the inner surfaces of the spacing members. The spring members It may be made of brass,.phosphor bronze or other suitable metal, and, as more clearly shown in Fig. 6, are generally H- shaped. A slot I8 is formed in the electrode II on each side for the reception of the springs. In assembling the gap, the springs are inserted in the slots of the electrode I I as shown in Fig. 6, and the spacer members I2 and II are then placed in position, causing the springs to assume the position shown inFig. 5, with their outer ends tightly in contact with the inner surfaces of the spacers.

When a voltage is applied across the gap, the presence of these springs will cause the electric field to be badly distorted in the regions around the points of contact between the springs and the spacer which are adjacent to the intermediate electrode II. This distortion causes high voltage gradients to exist in these regions, which results in ionization of the air in the gap in the manner described above, and this ionization is very eflective in causing breakdown of the gap to occur with a minimum time lag.

It will be seen that in both embodiments of the invention means are provided for causing a high concentration of voltage stress in local regions of the gap adjacent the points of contact between an electrode'and the spacer memminimum time lag. Tests have shown that the improvement in performance is, to a large extent, a function of the height of the projections, and impulse ratios as low as 1.00 to 1.05 have been obtained with gaps of this type. It will be ber. In both cases. these high stresses cause electrical discharges in the nature of corona which cause ionization of the gas and which may also, cause the emission of a radiation which causes further ionization and the production of free electrons in the gap. The effect of this ionization is to greatly facilitate breakdown of the gap and to cause it to occur with a minimum time lag, thus giving low and consistent breakdown voltages. It will be apparent that the high voltage gradients necessary for causing this effect can be produced in other ways than by the specific arrangements illustrated, and any other suitable means for causing distortion of the field in the gap to produce high local voltage gradients may be-used without departing from the spirit of the invention.

It will be seen, therefore, that a spark gap devention have been illustrated and described, it 1 is not limited to the particular arrangements shown, but that in its broadest aspects it includes all equivalent embodiments and modifications which come within the scope of the appended claims.

I claim as my invention:

1. A spark gap device comprising spaced electrodes and an insulating spacer member separat ing the electrodes, one of said electrodes having means thereon tor spacing it from the spacer member, said means having a very small area of contact with the spacer member, whereby high voltage gradients occur in the regions adjacent the points of contact.

2. A spark gap device comprising spaced electrodes and an insulating spacer member separating the electrodes, one or said electrodes having projecting portions which space it from the spacer member and have a very small area of contact therewith, whereby high voltage gradientsoccur in the regions adjacent the points of contact.

3. A spark gapdevice comprising spaced electrodes and an insulating spacer member separating the electrodes, one of said electrodes having projections of small area thereon, said projections being in contact with the spacing member and maintaining the electrode spaced a small distance therefrom.

4. A spark gap device comprising a pair of spaced electrodes and an insulating spacer member separating the electrodes, one of said electrodes having a generally plane surface and having a plurality of projecting elements thereon, said projecting elements spacing the electrode a small distance away from the spacer member and being of such configuration that they have very small area of contact with the spacer mem-' ber.

5. A spark gap device comprising'a pair-of generally disc-shaped electrodes and an annular insulating spacer member separating the

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