US 2365518 A
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Description (OCR text may contain errors)
Dec. 19, 1944. w BERKEY ET AL 2,365,518
ELECTRIC DISCHARGE DEVICE Filed Sept. 17, 1941 F/jad.
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Patented Dec. 19, 1944 ELECTRIC DISCHARGE DEVICE William E. Berkey, Forest Hills, and William H.
Cuttino, Wilkinsburg, Pa., assignors to Westlnghouse Electric & Manufacturing Company,
East Pittsburgh, Pa., a corporation of Pennsylanla Application September 17, 1941, Serial No. 411,142
The present invention relates in general to electric discharge devices, and more particularly to an improved spark gap device which is suitable for use as a voltage-limiting device to limit the 'across electrical'apparatus. One important example of this is in the case of series capacitors, which are connected in series in alternating current lines to improve the voltage regulation and to increase the stability limits. Since such capacitors are connected in series, they carry the line current and very high voltages may appear across the capacitor in case of a fault on the line or an abnormal overload. When this occurs, it is necessary to by-pass the capacitor immediately in order to prevent serious damage to it, or even complete destruction of the capacitor. Spark gaps are usually used for this purpose because of their substantially instantaneous operation when the voltage rises above the value for which they are set, and the spark gap device of the present invention is especially suitable for the protection of series capacitors, although it will be obvious that it is also suitable for use as a voltage-limiting device in many other applications.
The spark gap of the present invention is of the low-pressure type described and claimed in the copendin application of J. Slepian and W. E. Berkey, Serial No. 358,634, filed September 27, 1940, and assigned to the Westinghouse Electric 8: Manufacturing Company. As more fully set forth in that application, this type of spark gap has plane electrodes of relatively large area which are separated by air, or other gaseous medium, at a pressure in the range between 0.1 centimeter and iii-centimeters of mercury, and preferably less than 10 centimeters of mercury. Because of the low pressure the current density at the arc terminal is quite low, and there is little or no appreciable burning or erosion of the electrodes, so that they can be closely spaced to give a relatively low breakdown voltage, and the calibration of the gap is not changed even after repeated operation. Because of the low current density, which prevents excessive heating of the electrodes, and the large area of the electrodes, which permits the discharge to spread over their surfaces, such a spark gap has a very high discharge current capacity. The low pressure has the effect of reducing the breakdown voltage, but it has little effect on the extinction or cutoff voltage, so that the ratio of breakdown voltage to cutofl voltage is very low. For this reason, the low-pressure gap is inherently a self-extinguishing device in that the arc is extinguished when the voltage across the gap falls to a value only slightly lower than the breakdown voltage. Be-
cause of these characteristics, spark gaps of this type are very desirable for protective or voltagelimiting applications.
The principal object of the present invention is to provide a practical construction for spark gap devices of the low-pressure type which has high thermal capacity to permit the discharge of heavy currents, and which can be economically manufactured.
Another object of the invention is to provide a vacuum-tight spark gap structure having a long seal leakage path, and in which the seal can readily be made directly to an insulating containing member by means of an already developed manufacturing technique.
A further object of the invention is to provide a vacuum-tight spark gap structure having electrodes of high thermal capacity which are sealed directly to the insulating container member, and in which sufiicient flexibility is provided in the electrodes to prevent putting unduly high stresses on the seal when the electrodes are heated by a discharge.
Other objects and advantages of the invention will be apparent from the following detailed description, taken in connection with the accompanying drawing, in which:
Figure 1 is a perspective view of the spark gap device of the present invention;
Fig. 2 is a transverse sectional view of the spark l Fig. 3 is a transverse sectional view of a further embodiment of the invention; and
Fig. 4 is a wiring diagram showing a typical circuit in which the new gap device may be used.
The spark gap device shown in Figs. 1 and 2 is enclosed in a generally cylindrical container member I and has two metal electrodes 2 and 3. The container member! is preferably of porcelain, although other suitable insulating materials to which the electrodes can be sealed might be used if desired. Annular grooves 4 are provided in the end surfaces of the container memher. I to facilitate sealing the electrodes to the container as described hereinafter, and the grooves 4 are positioned far enough from the outside edge of the container to provide a long leakage path over the outside surface of the container. The electrodes 2 and 3 are preferably brass, although other metals, such as copper, might be used, and each electrode has a massive central portion 5 with a 'plane sparking surface 5 of large area, and also has a relatively thin peripheral portion 1, which is bent inward to form an annular flange 8 around the periphery of the electrode. A hole 9 may be drilled in the center of each electrode and tapped for the reception of a terminal stud of any desired type. The massive central portion of each electrode has an annular shoulder, indicated at [0, and the spark- 2 a,acs,sis
ing surfaces 6 of the electrodes are spaced apart a definite, predetermined distance by an annular porcelain spacer II, which engages the shoulders ID of both electrodes. An annular groove i2 may also be provided about the circumference of the sparking surface of each electrode to prevent the discharge from spreading beyond the central sparking area and damaging the spacer.
In assembling the gap, the electrodes 2 and I are sealed directly to the porcelain container member i with a gas-tight seal. For this purpose the annular grooves 4 in the end surfaces of the container member are coated with a metallic glare as described in the Smede and Shand Patent No. 1,852,093, issued April 5, 1932. The annular flanges 8 of the electrodes are then soldered to this glaze, as indicated at l2, forming a strong gas-tight seal. A small metal tube i4 is soldered in the electrode 2 to permit partial evacuation of the gap, and after the electrodes have been sealed to the container I, the air pressure within the container is reduced to a pressure less than centimeters of mercury, after which the tube 14 is sealed off. To facilitate this operation, the tube H may be coated with solder on the inside, so that it can be sealed off merely by pinching it together and heating sufflciently to melt the solder. The exact pressure to which the gap is evacuated is determined by the desired breakdown voltage, which is dependent on the gas pressure and on the spacing of the electrodes. It is also to be understood that the gap may be substantially completely evacuated, and then filled with an inert gas, such as nitrogen or argon, or a mixture of such gases, at the desired pressure.
Since the new gap operates at low pressure, it can be calibrated for a relatively low breakdown voltage, such as a few hundred volts, and it will retain its calibration unchanged even after repeated operation, because of the absence of burning of the electrodes. The large sparking area of the plane electrodes, which permits the discharge to spread over a large surface, together with the low current density and the high thermal capacity of the massive electrodes, which prevent excessive heating of the electrodes, gives the gap 2. very high current carrying capacity, so that it can be used safely in applications where very large currents are to be discharged. The thin peripheral portions 1 of the electrodes provide sufilcient flexibility to avoid placing any undesirably large stresses on the seal when the electrodes expand as a result of heating by a discharge. At the same time, the thin, flexible peripheral portions 1 are held under sufiicient strain,
and have sufficient stiffness, to hold the shoulders ill of the electrodes firmly against the spacer H, as previously described. The electrodes are sealed directly to the porcelain container to provide a very simple and mechanically reliable gastight structure which can be economically manufactured because of its simplicity and because it employs a sealing technique which is already developed and in commercial use. The design of the container member i with the vacuum-tight seal made in grooves in its end surfaces is such that a long leakage path is provided across the outside of the container, as clearly seen in Fig. 2, thus obviating any danger of flashover between the electrodes on the outside of the container. It will be apparent, therefore, that a spark gap device has been provided which is of simple and inexpensive construction, and which has the advantages of low and unchanging breakdown voltage, high discharge current capacity, and low ratio of breakdown voltage to cutofl voltage.
For applications in which a higher breakdown voltage is desired than can readily be obtained with the single gap of Figs. 1 and 2, the multiple gap structure of Fig. 3 may be utilized. In this embodiment of the invention, the gap has a cylindrical container member ll of porcelain, which is generally similar to the containermember I described above, and which has annular grooves I. in its end surfaces disposed so as to provide a long leakage path over the outside of the container. The gap device also has top and bottom electrodes I1 and I0, which are similar to the electrodes 2 and I previously described, having massive central portions ill, with large plane sparking surfaces In, and thin peripheral portions 2 l. The outer edge of each electrode is bent down to form an annular flange 22 which is soldered to a metallic glaze in the grooves It of the container l5, as indicated at 23. Tapped holes 24 may be provided in the electrodes l1 and II for the reception of terminal studs, and a metal tube 25 is soldered in the electrodes ll to permit partial evacuation of the device after the electrodes have been scaled to the container.
Within the container IS a plurality of intermediate electrodes 28 are placed between the top and bottom electrodes H and I8. Each of the intermediate electrodes 28 has a plane sparking surface on both sides of the same area as the sparking surfaces 20 of the electrodes l1 and I2, and a shallow peripheral groove 21 to prevent the discharge from spreading beyond the sparking surface. Each of the electrodes 26 also has a small hole 28 drilled in it at one side to provide communication between the enclosed spaces between the electrodes, so as to insure that the gas pressure will be the same in all parts of the gap device. The electrodes 28 are separated by annular spacers 29 of porcelain, or other suitable insulating material. Each of the spacers 29 has an annular internal rib 20 which engages the adjacent electrodes and spaces them the desired distance apart, while the thickness of the spacers 29 is such that the adjacent spacers are in contact so as to form a solid stack which prevents any possibility of flashover between the outer edges of the electrodes 28. The top and bottom spacers 29 bear against annular porcelain spacers 3| which engage shoulders 32 on the electrodes i1 and i8 and which space the sparking surfaces 20 of the electrodes l1 and II the desired distance from the top and bottom intermediate electrodes 26.
It will be seen that with this construction the spacers 29 and 3i form a rigid stack between the electrodes I! and is in which the intermediate electrodes 26 are firmly held. After the assembly of electrodes and spacers has been placed in position in the container IS, the top and bottom electrodes I1 and I! are sealed to the container, as described above, with a gastight seal, and the device is then evacuated to the desired pressure through the tube 25, which is then sealed off. It will be apparent that this device has the same advantages as the single gap of Fig. 2 and operates in the same manner, but it is suitable for applications where a relatively high breakdown voltage is desired, because it provides a number of individual gaps in series.
Fig. 4 shows a typical circuit in which the new sap may be used. As previously explained, the low pressure type of spark gap is very well suited for voltage-limiting protective applications because of the fact that it can be accurately calibrated for relatively low breakdown voltages, and because of its high discharge current capacity as well as its low ratio of breakdown voltage to cutoff voltage. These characteristics make it very well suited for the protection of series capacitors against overvoltages. In Fig. 4, an alternatingcurrent line is indicated at 35 and a capacitor 36 is connected in series with the line 35. A lowpressure spark gap device 31, such as is shown in either Fig. 2 or Fig. 3, is connected across the capacitor 36 to protect it from overvoltages. In some cases where the maximum current that may pass through the gap 31 is not too high, the gap may be connected directly across the terminals of the capacitor 36, since the gap will cease discharging and restore the capacitor to service after the occurrence of an abnormal voltage as soon as the line current ha fallen to its normal value, because of the inherent selfextinguishing characteristics of the low-pressure gap. In most cases, however, it is desirable to short-circuit the gap and relieve it from carrying the heavy short-circuit current for more than a few half-cycles. For this purpose, a contactor 38 is provided with its operating coil 39 connected in series with the gap 31 and its contacts 40 connected to short-circuit both the gap and the capacitor. Thus, if the overvoltage persists for more than a few half-cycles, the contactor 38 will have time to close its contacts 40 and shortcircuit the gap, extinguishing the arc. Preferably, the contacts 40 are connected to a tap ll in the coil 39, so that after the contacts close, a part of the coil remains energized to function as a holding coil which maintains the contacts closed until the line current drops to its normal value, when the contactor drops out to restore the capacitor 36 to service. 'Thus, by the use of the new gap device, a simple and inexpensive 'means is provided for the protection of series capacitors, instead of the complicated and expensive arrangements previously used, involving auxiliary spark gaps, transformers and special contactors, which have greatly restricted the use of series capacitors because of the cost the necessary protection equipment. It is to be understood that, although the new gap device is especially suitable for the protection of series capacitors, as just described, its usefulness is not restricted to this application, but it is also suitable for many other applications as a voltagelimiting or protective device.
It should now be apparent that a low-pressure spark gap device has been provided which is of simple and inexpensive construction, and which provides a practical structure embodying all the advantages of the low-pressure type of spark gap, as described above. It is to be understood that, although certain specific embodiments of the invention have been shown and described. it is not restricted to the precise details of construction shown, since various changes and modifications may be made without departing from the spirit of the invention, and the invention, therefore, includes all equivalent embodiments and modifications which come within the scope of the appended claims.
We claim as our invention:
1. A spark gap device comprising a generally cylindrical container member of insulating mate rial, said container member having an annular groove in each of its end surfaces, a pair of metal electrodes disposed at opposite ends of the container member, each of said electrodes having a massive central portion which extends into the container and a relatively thin peripheral portion, the central portion of each electrode having a plane sparking surface of large area within the container, and the peripheral portions of the electrodes having annular flanges which extend into the grooves in the end surfaces of the container, means for fastening the annular flanges of the electrodes in the grooves of the container to form a gas-tight seal, and insulating spacing means within the container for spacing the sparking surfaces of the electrodes a predetermined, uniform distance apart.
2. A spark gap device comprising a generally cylindrical container member of insulating material, said container member having an annular groove in each of its end surfaces, a .pair of metal electrodes'disposed at opposite ends of the container member, each of said electrodes having a massive central portion which extends into the container and a relatively thin peripheral portion, the central portion of each electrode having a plane sparking surface of large area within the container, and the peripheral portions of the electrodes having annular flanges which extend into the grooves in the end surfaces of the container, means for fastening the annular flanges of the electrodes in the grooves of the container to form a gas-tight seal, and insulating spacing means within the container for spacing the sparking surfaces of the electrodes 9. predetermined, uniform distance apart, the container being filled with a gaseous medium at a pressure less than 15 centimeters of mercury.
3. A spark gap device comprising a generally cylindrical container member or insulating material, said container member having an annular roove in each of its end surfaces, a pair of metal electrodes disposed at opposite ends of the container member, each of said electrodes having a massive central portion extending into the container member and a. relatively thin peripheral portion, the central portion of each electrode having a plane sparking surface of large area within the container, and the peripheral portion of the electrodes having annular flanges extending into the grooves in the end surfaces of the container, and means for fastening the annular flanges in said grooves to seal the electrodes to the container with a gas-tight seal.
4. A low-pressure, partially evacuated spark gap device comprising an annular housing member of an insulating material which is susceptible of having a closely adhering metallic coating oppliedto its surface, said housing member having anannulargrooveineachofitsendsurfacee, the surface of each groove having a closely edhering metallic coating thereon, a pair of terminal electrode members of metal, each electrode member having an inwardly bent peripheral flange extending into the annular groove at its 'endofthehousingmembenandamassotsolder filling each groove for making an air-tight soldered seal between theelectrode and the hous-