US2859373A - Lightning arrester - Google Patents

Description

1958 w. H. TEARE 2,859,373

LIGHTNING ARRESTER Filed May 21, 1954 INVENTOR. WWI/am H Teare H15 Attorney.

United States Patent LIGHTNING ARRESTER William H. Teare, Burnt Hills, N. Y., assignor to General Electric Company, a corporation of New York Application May 21, 1954, Serial No. 431,366

6 Claims. (Cl. 313-325) The present invention relates to lightning arresters and, more particularly, to lightning arresters in which the breakdown electrodes are enclosed in an evacuated space.

Lightning arresters are used to provide protection for electrical generating and transmission apparatus against high potential impulses such as those produced by a discharge of atmospheric electric charges. For proper operation a lightning arrester not only must be capable of withstanding the normal alternating voltages of the apparatus with which it is associated, but also must be designed to break down or sustain a discharge under the impulse of transient high voltage impulses at a voltage lower than that which would damage the insulation of the apparatus with which it is associated. A further requisite for a lightning arrester is the ability to interrupt the initiated breakdown path once the peak impulse load has passed so that normal operation of the associated electrical apparatus may be resumed.

Present lightning arresters often employ a semiconducting medium to form the path of impulse breakdown. Such devices may operate to interrupt the breakdown path by a chemical change within the semiconductor medium which increases the electrical resistance within the breakdown path. These devices and other present lightning arresters are, however, large and bulky. When attached to exterior parts of electrical apparatus they are easily damaged in shipment and installation. An additional disadvantage of such bulky protective devices is their physical interference with exterior wiring associated with the electrical apparatus.

It has been proposed to overcome the size disadvantages of present lightning arresters by the use of breakdown electrodes located in a vacuum. Heretofore, vacuum lightning arresters have not proved practical for, in general, the impulse-loaded breakdown voltage of vacuum gaps are much higher than their alternating current breakdown voltage. Thus a vacuum lightning arrester which would withstand normal alternating current operating voltages without breaking down would not give proper impulse-load protection.

Accordingly, one object of the invention is to provide a vacuum lightning arrester which will withstand normal operating alternating current voltages, but which will afford adequate impulse voltage protection to electrical apparatus associated therewith.

A further object of the invention is to provide a vacuum lightning arrester with an impulse load breakdown voltage as low as its alternating current breakdown voltage.

. In accordance with an important feature of the invention, there is provided a vacuum lightning arrester wherein one electrode contacts the inner surface of an evacuated hollow refractory insulating sleeve in close juxtaposition to a'second electrode. The presence of the refractory insulating sleeve in the vicinity of the electrode gap causes the impulse-loaded breakdown potential of the gap to be of the same order of magnitude as the alternatin'g current breakdown voltage for the gap. With this result achieved, practical, useful vacuum lightning arresters 2,859,373 Patented Nov. 4, 1958 'ice may be provided to protect electrical distribution and similar apparatus.

The features of the invention which I believe are novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing which illustrates, in vertical cross-section, one embodiment of my invention.

A lightning arrester 10 illustrative of one embodiment of the invention is shown in the drawing and comprises a hollow refractory insulating sleeve 11, a metallic electrode 12 disposed in spaced relation within sleeve 11, and a second electrode 13 moulded within one end 14 of sleeve 11. While insulating sleeve 11 may be any suitable dielectric material including the glasses and ceramics, the advantages of the invention are more fully obtained if the 'sleeve is formed of a suitable ceramic material such as aluminum-zirconium oxide, or zirconium silicate. Sleeve 11 is preferably of circular cross-section, but other configurations may be used. The dimensions of sleeve 11 may vary depending upon the normal alternating current voltage at which the lightning arrester 10 is to operate, and the peak arc current to be carried thereby. By way of example, however, sleeve 11 may 'have an outside diameter of l and an inside diameter of A", and a length of 2. A moulded electrode 13, located within one end 14 of sleeve 11, is in intimate contact with the inner surface of the sleeve and may conveniently be formed in the nature of a solidified pool of molten metal. The composition of electrode 13 may be any metal or metallic alloy but is preferably a metal or alloy which wets the surface of the insulating sleeve 11 and has a high room temperature vapor pressure, as for example tin or lead. While the depth of electrode 14 is not critical, it may conveniently fill approximately /3 of the inner volume of sleeve 11. A vacuum tight seal is maintained about the end of sleeve 11 containing electrode 13 by means of an adapter collar 15 and a dished plug member 18. Collar 15 has a large diameter flange 16 and a small diameter flange 17. Flange 16 encloses and is vacuumsealed to the outer surface of the end of sleeve '11 as, for example, by a silver braze. Dished member 18 fits within, and is vacuum sealed to, the smaller flange 17 of adapter collar 15, as, for example by an arc weld 25. Terminal 19, which may conveniently be copper, is connected to dished member 18 by a low resistance electrical connection, as for example by a gold braze 20.

Within sleeve 11, metallic electrode 12 is centrally located in spaced relation to the inner surface of sleeve 11. Metallic electrode 12 is held in position by means of an adapter collar 21 having a large diameter flange 22 which encloses and is fastened to end 11a of sleeve 11 by a suitable vacuum tight seal, as for example, a silver braze 24,

and a smaller diameter end 23 which encloses metallic electrode 12. Adapter collar 21 may be any suitable alloy as, for example chrome-iron or nickel-iron, which may be approximately /s of the length of sleeve 11 and so that: the gaps between electrode 12 and moulded electrode 13 and between electrode 12 and sleeve 11 are between; 0.050" and 0.100 and preferably between 0.060" and.

0.080. Electrode 12 may preferably be of iron, but. may also be made from any metal or alloy which will sustain a high current arcwithout melting. While the external configuration of end 12a of electrode 12 is not critical, the configuration shown in the drawing has been satisfactory. As shown in the drawing, electrode 12 has an elongated stem piece 26 at its exterior end. Stern piece 26 may be any diameter consistent with mechanical strength, depending upon the metal of which electrode 12 is formed. A circular passage 28 is cut through the entire length of electrode 12 along the longitudinal axis thereof. A vacuum lineconnection 29 which may, for example, be fernico, is fastened within the exterior end of passage 28 of electrode 12, and fastened thereto with a vacuum tight seal 31 which may, for example, be'a gold braze. A thin cylindrical wall 32 which comprises an extension of the outer lateral face of electrode 12 is formed by cutting an annular recess 33 from the body of electrode 12. The wall 32 of electrode 12 andthe adapter collar 21 are Welded together, to form a vacuum tight seal 34. The herein-described arrangement allows weld 34 to be made after the seal has been otherwise assembled without melting braze 31 or heating sleeve 11 unnecessarily. When lightning arrester is assembled vacuum connection 29 is connected to a vacuum pump (not shown) and the interior of the arrester is evacuated to a pressure of 0.001 micron or less. Vacuum connection 29 is then sealed oif and the unit is ready for use.

As stated herein, vacuum gaps have heretofore been unsatisfactory for use as lightning arresters. One function of a lightning arrester is to become conducting when an impulse voltage having a steep wave front is impressed across its terminals. Ordinarily the impulse breakdown voltage of a vacuum gap is many times greater than the root-mean-square alternating current breakdown voltage. As an example of this difference, one conventional vacuum gap tested possessed a root-mean-square alternating current breakdown voltage of 16,000 volts, but did not break down under steep wave front impulse voltages until the peak impulse voltage reached 132,000 volts. As opposed to the characteristics of the conventional vacuum gap as noted herein, a lightning arrester constructed according to this invention, as described hereinbefore, breaks down under steep wave front impulse voltages of the same order of magnitude as the rootmeansquare alternating current breakdown voltage characteristic of the device. As an example of this result, one lightning arrester constructed according to the invention and having a root-mean-square alternating current breakdown voltage of from 7 to 10 kilovolts was tested on a surge generator and displayed an impulse breakdown voltage of 10 kilovolts with the moulded electrode 13 negative. With the moulded electrode positive, impulse breakdown occurred at 27 kilovolts.

While the mechanism of the lowered potential impulse breakdown of the lightning arresters disclosed herein is not understood to a mathematical certainty, it is believed that the greatly improved impulse breakdown characteristics of the invention are a result of the field configuration within sleeve 11 in the vicinity of the junction of moulded electrode 13 and sleeve 11. It is believed that when breakdown occurs between electrodes 12 and 13 the initial spark is a result of field emission. This type emission involves the emission of electrons from a cold cathode under the stress of an electric field only. The dielectric constant of the vacuum within lightning arrester 10 is essentially unity. The dielectric constant of sleeve 11"is substantially greater than unity, and may approach a value 'of 10. For this reason there is a sharp field dis- {continuity at the interior surface 35 of sleeve 11 in the vicinity of electrodes 12 and 13. This discontinuity causes a concentration of the field strength in the vacuum gap immediately adjacent surface 35 between electrodes 12 and 13. It is believed that initial field emission of the vacuum gap occurs in this highly slIessed portion of the gap rather than in the central portion 36 of the gap where the electric field between electrodes 12 and 13 is more uniform. This breakdown mechanism is offered only as a possible scientific explanation of the improved characteristics of the invention andis not to be considered to restrict the scope of the invention or to impair the validity of the claims if a different explanation should ultimately prove more accurate or comprehensive.

It will be appreciated that, although I have disclosed one specific embodiment of the invention, many modifications may be made, and I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A lightning arrester comprising an evacuated hollow sleeve member of refractory insulating material, a first electrode disposed within and spaced from said sleeve member, and a second metallic electrode within said sleeve member contacting the inner surface thereof in close juxtaposition to said first electrode, wherein said first and second electrodes occupy substantially all of the interior of said hollow sleeve member.

2. The device of claim 1 wherein the second electrode comprises a solidified pool of molten metal which wets thesurfa'ce of the hollow sleeve member.

3. A lightning arrester comprising an evacuated hollow ceramic sleeve, and a spark gap therein comprising a first electrode disposed in spaced relation within one end of said ceramic sleeve and a second metallic electrode moulded within the remaining end of said ceramic sleeve and contacting the inner surface thereof in close juxtaposition to the periphery of the gap end of said first electrode, wherein said first and second electrodes occupy substantially all of the interior of said hollow sleeve member.

4. A lightning arrester comprising an evacuated hollow sleeve member of refractory insulating material, a first electrode disposed within and spaced from said sleeve member, and a second metallic electrode within said sleeve member contacting the inner surface thereof in close juxtaposition to said first electrode and providing a gap between said electrodes, wherein the spacing of said first electrode from said sleeve in the region of said gap is substantially the same as the spacing between said first electrode and said second electrode, and wherein the width of said first electrode in the region of said gap is considerably greater than the gap distance between said electrodes.

5. The device of claim 4 wherein said second electrode comprises a solidified pool of molten metal.

6. A lightning arrester comprising an evacuated hollow sleeve member of refractory insulating material, a first electrode disposed within and spaced from said sleeve member, and a second metallic electrode within said sleeve member contacting the inner surface thereof and spaced from said first electrode a distance considerably less than the width of said first electrode, wherein said electrodes occupy substantially all of the interior of said hollow sleeve member.

References Cited in the file of this patent UNITED STATES PATENTS 882,218 Thomas Mar. 17, 1908 1,011,538 Weintraub Dec. 12, 1911 1,017,649 Chapman Feb. 20, 1912 1,156,227 Hewitt Oct. 12, 1915 1,182,291 Meikle May 9, 1916 1,394,971 Creighton et a1 Oct. 25, 1921 1,483,540 Allcutt Feb. 12, 1924 1,701,840 Donle Feb. 12, 1929 1,874,407 Young Aug. 30, 1932 1,900,577 Moore Mar. 7, 1933 2,141,654 Kott Dec. 27, 1938 2,365,518 Berkey et al Dec. 19, 1944' 2,395,991 Chilcot et al Mar. 5, 1946

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