US2959704A - Overvoltage protective device - Google Patents

Description

Nov. 8, 1960 J. H. sNELl., JR., ETAL 2,959,704

OVERVOLTAGE PROTECTIVE DEVICE Filed Oct. 9. 1958 l @Juif m2, du,

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United States Patent O OVERVOLTAGE PROTECTIVE DEVICE James H. Snell, Jr., Pittsfield, Francis J. Charewicz,

Lanesboro, and John R. Lucek, Pittsfield, Mass., assignors to General Electric Company, a corporation of New York Filed Oct. 9, 1958, Ser. No. 766,250

2 Claims. (Cl. 313--309) This invention relates to an improved overvoltage protective device and also to an improved electrode construction.

In overvoltage protective devices such as lightning arresters established standards require that the power frequency breakdown -voltage of the lightning arrester gap be not less than 1.5 times the kv. rating of the lightning arrester. In order for the lightning arrester to satisfactorily perform its intended purpose the impulse ratio should be as close to unity as possible and an impulse ratio of about 1.1 is considered satisfactory. By the term impulse ratio is meant the ratio of the breakdown voltage of the lightning arrester gap under impulse conditions to the power frequency breakdown voltage. In the United States power frequency is usually 60 cycles. There are many satisfactory ways of obtaining the desired impulse Iratio in conventional lightning" ar'resters. However, this is not true with respect to lightning arresters in which the arc gap operates in a vacuum.

It is one object of this invention to provide improved means in a vacuum overvoltage protective device for bringing the breakdown voltage at impulse conditions close to the power frequency breakdown voltage. That is to say, to provide an improved means in a vacuum arrester which will provide an impulse ratio of'one or thereabout. Generally speaking, the prior art has been unsuccessful in reducing the impulse ratio to much less than two in vacuum arresters and also of maintaining such reduced impulse ratio after repeated use of the device.

In one form of our invention the surfaces of the electrodes which define the arc gap of an overvoltage protective device have particles embedded or otherwise attached thereto. These particles have sharp edges which protrude from the surfaces of the electrodes, and the surfaces of the electrodes are relatively large with respect to the size of the particles.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which we regard as our invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawing in which is shown a side sectional view of one form of our invention.

The device illustrated in the drawing is a vacuum overvoltage protective device. However, it is believed that our invention will provide beneficial results even in an overvoltage protective device which does not operate in vacuum. The vacuum device comprises .a pair of electrodes 1 which are housed in a vacuum tight insulating housing 2 which provides a vacuum chamber 3 for an arc gap 4. An arc vapor shield 8 to protect the insulatting walls from metal vapor deposit may be used. As will be obvious to those skilled in the art the electrodes 1 are adapted to be connected to different electrical potentials. That is to say, if the overvoltage protective device is to be used as a lightning arrester one of the electrodes would be connected to a line and the other of the electrodes would be connected to ground. If the overe ICC with the capacitor it was intended to protect.

and the shield may be supported adjacent its central portion from a central portion of the housing. The shield may be made from insulating or conducting' material.

The arc gap 4v is defined by surfaces 5 of the electrodes 1 which are positioned vadjacent to each other'in closely spaced relationship. The surfaces 5 are provided by special arcing contacts or inserts 6 which are inserted into or otherwise connected to the ends of the electrodes which are positioned adjacent to each other within the vacuum chamber 3. However, the surfaces 5 can be provided directly by the electrodes 1 themselves in which event the special inserts 6 could be omitted. The surfaces 5 have minute particles 7 embedded therein. The particles 7 have sharp edges which protrude from or through the surfaces 5.

The electrodes 1 and inserts 6 are constructed from electrically conductive material. Particles 7 can be conductive, semi-conductive or non-conductive. However particles 7 are preferably.semi-conductive material. Some beneficial results may be obtained from conductive or nonconductive materials. Therefore, in the claims by the term conductive material is meant material which is semi-conductive as well as conductive. In the preferred form of the invention the particles 7 are made from materials which are crystalline. This is because particles made from' crystalline materials typically provide many sharp edges so that if many particles are embedded at randoml in the surfaces 5 there will always be many sharp edges of the particles protruding from or throughV the surfaces 5. This means that if arcing across the. gap 4 blunts some of the particle edges there will always be a great number of sharp particle edges remaining.

However, the invention is not necessarily restricted to crystalline particles inasmuch as some amorphous sub-y stances will also provide particles having sharp edges or configurations which will simulate or approximate crystal# line particles.

It is our theory that for an arc gap to have low and consistent breakdown there should be a high field or electron emission. In our invention the primary source of electrons is the many sharp points or projections in the iniserts 6 which are provided by the particles 7. Ar'cing preferably should not melt the particles but should either knock them out of the inserts 6 to expose new ones or cleave them to expose new sharp edges. In order to avoid melting the particles they should 1have a melting temperature of 2000 C. or above. requirement of the material of the particles is that it have a low vapor pressure in its original and decomposed state. One material which will meet all these requirements is a high temperature refractory semi-conductive crystalline material such as silicon carbide. It is also our theory that another source of electrons may be the interface that exists between materials having very different dielectric constants when volt-age is applied. For this reason, non-conducting particles may be used in combination with conductive material electrodes which includes semi-conductive material electrodes.

In one form of the device illustrated in the drawing the electrodes 1 were constructed from steel and the inserts 6 comprised a mixture of 10 parts silicon carbide par ticles, l part copper, 7 parts nickel, and 3 parts titanium. The three metals of copper, nickel and titanium were in granular form and the four ingredients of the inserts were pressed and then air dried at 300 C. to remove all absorbed moisture. Then the air dried inserts were fired at 1100a C. and 5x10*5 mm. of mercury for 30 minutes Another preferred Y.

to sinter the metal ingredients together to give some strength to the inserts. Obviously in such a procedure for providing sharp particles in the arcing surfaces of the electrodes the sintering temperature of the metals will have to be less than the melting point of the silicon carbide particles so that their sharp edges are not blunted or otherwise lost. The vacuum pressure in the chamber 3 was of the order of 10-4 millimeters of mercury. Silicon carbide is a crystalline material and the particles have many cleavage edges so that many sharp particle edges protrude through the surfaces S even though the particles are interspersed in the inserts 6 at random. An advantage of using the inserts 6 which have particles 7 interspersed throughout the whole thereof is that even if the exposed or protruding particles become blunted evaporation of the surfaces due to electrical arcing will result in the exposure of new particle edges. In one form of the device the particles had sharp edges even when viewed under a microscope having a magnification power of 100. The particles 7 are relatively very small with respect to the arcing surfaces 5. However, the optimum particle size in specic cases would not be controlled by the actual size of the electrode surfaces but would be determined by trial based on experimental data.

Electrodes which are ground down to a single point or wire emitters are well known in the art. However, such electrodes are not satisfactory inasmuch as a single discharge of very low current will blunt the pointed electrode or wire emitter by melting of the sharp point. Electrodes whose arcing surfaces are scored or serrated mechanically or etched to provide projections thereon will not be satisfactory inasmuch as present day scoring, serrating or etching techniques will not provide as sharp edges as are provided by our particles 7. 'Ihe feature which these electrodes have in common is that the points or projections thereon are integral portions of the electrodes themselves. That is to say, the points or serrations are not constructed from a material which is different from the material of the electrode surface proper. In our invention the particles are not an integral part of the electrodes themselves but have a different composition from the electrodes and are embedded or otherwise attached thereto. However, when mechanical scoring, etching or other methods are developed to provide as sharp projections or points as is provided by our particles then the invention could be practiced by having many sharp projections and points on relatively large arcing surfaces with the points or projections and arcing surfaces being integral with each other. That is to say, constructed from the same material.

The electrodes 1 need not necessarily be constructed from steel but could be made from other electrically conductive materials such as graphite, copper, molybdenum, and other substances. Another way of attaching the particles of our invention to the arcing surfaces of an electrode is to roll or otherwise press the particles into the electrode surfaces. For instance, silicon or boron carbide particles could be embedded into the surfaces of a copper electrode by rolling them in the relatively soft surfaces of the copper. Another way of attaching the particles to the electrode is to bond them thereto with an eutectic composition. For instance, 400- grit silicon carbide particles can be readily bonded to a molybdenum electrode with a nickel-titanium eutectic.

In the preferred form of the invention the surfaces 5 are flat and parallel to each other inasmuch as this will give uniform gap spacing between the arcing surfaces. However, the invention is not necessarily restricted to any particular kind of electrode arrangement or arcing surface configuration. For instance, the arcing ends of the electrodes could be `rounded or semi-spherical, and the electrodes would not necessarily have to be positioned end to end in a line, but could be disposed at an angle with respect to each other.

While there have been shown and described particular embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and, therefore, it is intended by the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

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

1. A vacuum gap comprising an evacuated sealed envelope containing two metal electrodes respectively supporting composite arcing members of substantial thickness, said composite arcing members having spaced facing surfaces defining an arc gap, said composite arcing members comprising a titanium-nickel alloy containing uniformly dispersed relatively tine particles of silicon carbide fused therein.

2. A vacuum gap comprising a sealed envelope containing two metal electrodes and evacuated to a pressure of the order of 10-4 millimeters of mercury, said electrodes each having an arcing portion of substantial thickness, said arcing portion each containing about ten parts of uniformly distributed silicon carbide particles of the order of 400 grit size in an alloy of about one part copper, seven parts nickel and three parts titanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,290,526 Berkey et al -a July 21, 1942 2,661,439 Stoelting Dec. l, 1953

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