US2238619A

US2238619A - Spark gap device

Info

Publication number: US2238619A
Application number: US244420A
Authority: US
Inventors: William E Berkey
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1938-12-07
Filing date: 1938-12-07
Publication date: 1941-04-15
Anticipated expiration: 1958-04-15

Description

April 15, 19410 w. E. BERKEY 2,23%,fi19

SPARK GAP DEVI CE Filed Dec. '7, 1938 VENTOR WITNESSES: r id Ma M51412] .fie i y ATTORNEY Patented Apr. 15, 1941 SPARK GAP DEVICE William E. Berkey, Forest Hills, Pa., assignor to Westinghouse Electric 8; Manufacturing Company, East Pittsburgh, Pa., a. corporation of Pennsylvania Application December 7, 1938, Serial No. 244,420

4 Claims.

My invention relates to spark gap devices, and more particularly to an improved spark gap having a low and consistent surge breakdown voltage and a low and consistent impulse ratio and which is especially suitable for use with lightning arresters or other protective discharge devices.

Lightning arresters for the protection of transmission lines or other electrical devices usually consist of a spark gap connected in series with a resistance element which has the valve-like property of changing from substantially a nonconductor, or at least a high resistance, to a conductor when a predetermined over-voltage is applied to it, so that it permits the passage to ground of relatively large currents when a high voltage, such as a lightning surge, occurs, but reduces the current to a very small value when the voltage falls to approximately the normal line voltage.

The series spark gap device performs three functions. First, under normal conditions it insulates the arrester from the line and prevents the passage of the small leakage current which would otherwise flow. Second, upon the occurrence of a voltage surge of a predetermined magnitude, the gap breaks down and connects the arrester to the line to permit the flow of current to ground. After the surge has disappeared, the series gap performs its third function by interrupting the relatively small power follow or leakage current which flows through the arrester as a result of the normal line voltage, and thus disconnects the arrest-er from-the line until the occurrence of another surge.

In order to obtain the greatest degree of protection, therefore, a lightning arrester spark gap should have as low a breakdown voltage as possible when a surge voltage is applied to it, and should have a relatively high 60-cycle or normal frequency breakdown voltage to prevent the possibility of a breakdown under normal conditions. The ratio of the breakdown voltage under a standard test surge to the normal frequency breakdown voltage is called the impulse ratio of the gap, and it is obvious that it is desirable to make this ratio as near unity as possible.

Series gap devices for lightning arresters have usually been constructed with electrodes of brass, or other conducting material having suitable arc quenching properties, separated by an annular spacer either of insulating material, usually porcelain, or of semi-conducting or high-resistance material for the purpose of controlling the voltage distribution across the individual units of a multi-gap structure. It has been found in it is necessary to insure a minimum time lag of breakdown of the gap.

I have discovered that if a spark gap is designed so that very high voltage gradients occur in localized regions in the space between the electrodes, the air in these regions is overstressed and a radiation is produced which is very effective in causing rapid breakdown of the gap. This radiation is usually, but not always, visible and appears'to be an electromagnetic radiation lying partly in the visible spectrum and partly in the ultra-violet and X-ray regions. "The portion of this radiation lying in the soft X-ray region is very effective in ionizing the air in the gap and also causes the production of large numbers of free electrons which cause rapid breakdown of the gap with a minimum time lag. In this way, low and consistent surge breakdown voltages with correspondingly low impulse ratios are obtained.

The object of the present invention, therefore,

is to provide a series spark gap device for lightning arresters or similar protective devices which will have a low and consistent surge breakdown voltage.

- More specifically, the. objectis to provide a spark gap device in which high local voltage stresses will be produced when a rapidly rising voltage is applied across the gap, in order to obtain a low breakdown voltage and a low impulse ratio.

These objects may be carried out in various ways. In a copending application Serial No.

- 244,419, filed December '7, 1938, and assigned to the Westinghouse Electric 8: Manufacturing Company, I have disclosed and claimed a gap device in which the spacer separating the electrodes is designed to cause high local voltage gradients in the gap, thus producing the radiation described above to obtain the desired low surge breakdown. The present application is directed to another form of spark gap in which the desired high voltage stresses are produced by distortion of the field between the electrodes by means of a memupper electrode.

her of insulating material of relatively high dispace. This member causes distortion of the field so that high voltage gradients occur close to the actual gap space and the radiation is emitted at the location where it is most eil'ective in causing breakdown of the gap. 4 l

Theinvention will be more fully understood from the tollowing detailed description, taken in connection with the accompanying drawing, in which Figure 1 is a sectional view of a lightning arrester spark gap with the associated apparatus shown diagrammatically, and

Fig. 2*is a plan view of the gap shown in Fig. l. The gap device shown in the drawing for purposes of illustration consists of two generally disc-shaped electrodes I and 2, preferably of brass or other suitable arc-quenching metal, separated by an annular spacer member 3. This spacer is shown as being made of semi-conducting or high resistance material, as is customary for the purpose of controlling the voltage distribution across the individual units of a multiple spark gap, but it should be understood that an insulating spacer, such as a porcelain ring, could be used if desired. Since it is usually desirable to have the spacer '3 of considerably greater thickness than the gap space, the upper electrode I is shown as being dished, so that the central part approaches more closely to the electrode 2 than the peripheral portions. The central portion of the electrode I is struck up, as indicated at 5, to form a central opening, thus forming an annular gap space 6. The upper electrode is connected to a transmission line 1, or other electrical device to be protected, and the lower electrode 2 is connected to a suitable lightning arrester valve element 8, which is connected to ground as indicated at 9. It will be understood,

oi. course, that a suitable number of gap devices may be used in series to obtain the desired voltage rating.

In order to obtain the desired high voltage gradients in the gap, a column or button ill of insulating material having a high dielectric constant is inserted in the central opening of the This button extends across the gap 8 and preferably makes light contact with the electrode 2, or, if desired, its lower extremity may be spaced a slight distance from the electrode. The button I is preferably soarranged that the flashover distance along the button is at least twice the spacing of the gap 6. If desired, a conducting coating may be applied to those portions of the button which make contact with the electrodes l and 2.

The button I 0 may be made of any suitable insulating material having a sufliciently high dielectric constant, but one material which has been found to be especially suitable is rutile ceramic. Rutile is a natural mineral consisting chiefly of titanium dioxide, usually with small amounts of impurities. This material is preferably flrst purified, and then ground and mixed with about 10% of water. The mixture thus formed is molded to the desired shape under heavy pressure and fired to produce a ceramic material resembling porcelain in appearance.

This material has good insulating characteris-' tics and also has the rather unusual property of having an extremely high dielectric constant,

ranging from 20 to as high as 110, depending on the treatment given it.

electric constant which is substantially higher than that of air.

When a surge voltage is applied to this gap device, the presence of the button l0 causes a high concentration of voltage stress adjacent the junctions between the button and the electrodes. The high voltage gradients caused by this concentrated stress result in local overstressing of the air in these regions and the emission of a radiation as described above. This radiation causes ionization of the air and the production of electrons, which are very effective in initiating breakdown of the gap. It will be seen that, with the arrangement described, the radiation is produced closely adjacent the gap space 6 where it is most eifective in reducing the time lag of breakdown of the gap, and for this reason, good results can be obtained even with materials of dielectric constant as low as 6, such as porcelain. Although the radiation may not be as intense as would be produced by materials of higher dielectric constant, it is concentrated at the regions where it is very effective in causing breakdown of the gap. For this reason, any suitable insulating material having a reasonably high dielectric constant may be used for the button I0, although the effect will be greater with materials of higher dielectric constants.

It will be seen therefore, that a spark gap device has been provided which is of relatively simple and inexpensive construction and which has a low and consistent impulse ratio. It will also be apparent that various modifications of this arrangement are possible without departing from the spirit of the invention. Thus, the insulating button I0 may be of any desired shape, and may be associated with either the upper or lower electrode in any desired relation. Also, in the case of a multiple gap structure, the button I could be made sumciently long to extend through two or more of the individual gaps.

It is to be understood, therefore, that although a specific embodiment of the invention has been illustrated and described, it is not limited to the exact arrangement shown, but that in its broadest aspect 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 a pair 01 spaced electrodes arranged to provide a gap space between them of smaller area than that 01 the electrodes, and a dielectric member of high dielectric constant secured in one of said electrodes outside the gap space and spaced a small distance therefrom, said dielectric member extending towards the other electrode and terminating closely adjacent thereto, and the length of said dielectric member between the electrodes being greater than that of the gap space and great enough to prevent any discharge over the dielecspaced electrodes arranged to provide a gap space between them of smaller area than that of the electrodes, and a dielectric member of high dielectric constant secured in one of said electrodes outside the gap space and spaced a. small distance therefrom, said dielectric member extendin'g towards the other electrode and terminating closely adjacent thereto, and the length of said dielectric member between the electrodes being at least twice that of the gap space to prevent any discharge over the dielectric member from one electrode to the other, whereby the initial breakdown of the gap always occurs in said gap space.

3. A spark gap device comprising a pair of spaced electrodes arranged to provide a gap space between them of smaller area than that of the electrodes, and a dielectric member of high' being greater than that of the gap space and great enough to prevent any discharge over the dielectric member from one electrode to the other, whereby the initial breakdown of the gap always occurs in said gap space.

4. A spark gap device comprising a pair of spaced electrodes arranged to provide a gap space between them of smaller area than that of the electrodes, and a dielectric member of high dielectric constant secured in one of said electrodes outside the gap space and spaced a small distance therefrom, said dielectric member extending towards the other electrode and being lightly in contact therewith, and the length of said dielectric member between the electrodes being at least twice that of the gap space to prevent any discharge over the dielectric member from one electrode to the other, whereby the initial breakdown of the gap always occurs in said gap 20 Space.

WILLIAM E. BERKEY.