US2985788A - Spark gap device - Google Patents

Description

May 23, 1961 A. M. oPsAHL ETAL 2,985,788

SPARK GAP DEVICE Filed Jan. 19, 1956 Fig.|.

\\\\\v\ \\\m\\\\\\\\\\\\\m\\\\\\\ United States Patent @dice SPARK GAP DEVICE Alert M. Opsahl, Forest Hills, and Tom L. Dyer, Jr., North IIuntlngdon Township, Westmoreland County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 19, 1956, Ser. No. 560,146

1 Claim. (Cl. 313-325) The present invention relates to spark gap devices and, more particularly, to a spark gap for lightning arresters of the valve type.

Valve type lightning arresters consist essentially of a spark gap, or a series of spark gaps, connected in series with a resistance element having valve or nonlinear characteristics; that is, a resistor which has very high resistance under normal voltage conditions, but which sharply reduces its resistance under a predetermined over voltage, to permit a surge to be discharged to ground with low discharge voltage, and which is capable of thereafter increasing its resistance to reduce the power follow current to a small value. The series gap device normally insulates the arrester from the line to which it is connected, but sparks over under a predetermined over voltage to connect the arrester between line and ground to allow the surge to be discharged to ground. After discharge of the surge, the valve element of the lightning arrester reduces the power follow current, which tends to flow due to the normal line voltage, to a small value which is interrupted by the gap to again insulate the arrester from the line.

Recent improvements in lightning arresters of this type have resulted in improved protective characteristics of the valve element and, in particular, it reduces discharge voltage. The effect of this lower discharge voltage is to increase the power follow current which flows through the arrester after a discharge, and which must be interrupted by the series gap. With moderate power currents, the arc in the gap tends to move over the face of the electrodes, which facilitates interruption of the arc and prevents localized overheating of the electrodes. With the higher power currents resulting from the improved protective characteristics mentioned above, however, the arc tends to remain in one place on the electrode. This concentration of current at one particular spot on the electrode for an appreciable time is highly undesirable because it raises the temperature of the electrode, which is usually made of brass, above its melting point and thus causes deformation or serious burning of the electrode at that particular point. Such deformation or burning is likely to cause a suiicient change in the shape of the electrode surface to reduce the effective spacing between the electrodes, thus lowering the spark-over voltage of the gap and also reducing its ability to withstand the recovery voltage after a surge is discharged, so that interruption of the arc is made more diicult. This intense heating of a particular spot on the electrode further increases the diiculty of interrupting the arc by making it more likely to restrike after a current zero. Movement of the arc over the surface of the electrode is, therefore, very desirable, or even necessary, to insure that the series gap will reliably interrupt the power current and that its characteristics will not be changed by a discharge.

The principal object of the present invention, therefore, is to provide a spark gap device in which the arc ,isV positively moved over the sparking area of the elec-` 2,985,788 Patented May 23, 1961 trodes to facilitate interruption of the arc and to prevent localized overheating of the electrodes.

Another object of the invention is to provide a spark gap device which includes means for providing a magnetice field of substantially constant strength in the gap in a direction to cause continuous movement of the arc until it is extinguished.

A further object of the invention is to provide a spark gap device in which a permanent magnet is associated with at least one of the electrodes, the magnet being magnetized in a manner to produce a magnetic field which extends throughout the gap space between the elec-y trodes in a direction generally perpendicular to the path of a discharge, so as to cause continuous movement of the arc over the surface of the electrode.

Other objects and advantages of the invention will beapparent from the following detailed description, taken in connection with the accompanying drawing, in which:`

lFigure 1 is a view of a lightning arrester, partly in ele` vation and partly in vertical section; and

Fig. 2 is a sectional view on an enlarged scale of a spark gap assembly embodying the invention.

As previously indicated, the present invention is particularly suitable for use in lightning arrester spark gaps and is shown in the drawing embodies in such a device, although it will be understood that its usefulness is not necessarily limited to this specific application. The invention is shown in the drawing, for the purpose of illustration, embodied in a series gap device for a valve type lightning arrester shown in Fig. l. The complete arrester is contained in a porcelain housing 1 which is closed at the ends by metal end fittings 2, of any suitable type, cemented or otherwise secured to the housing 1, which provides for electrical connection to the arrester and for mechanical mounting of the arrester. The arrester consists of a spark gap assembly 3 disposed in series relation with a valve element which consists of a plurality of valve blocks 4 disposed in a series column in the housing l. The valve blocks 4 may be of any suitable type and are preferably made of granular silicon carbide molded with a suitable binder, such as sodium silicate, and baked to provide blocks having the desired nonlinear or valve characteristics. The valve blocks 4 are disposed in a column in the housing 1, any suitable number of blocks being used depending on the desired voltage rating of the arrester.

The spark gap assembly 3 is disposed in the housing 1 in series relation with the column of valve blocks 4 and is shown as being inserted in the center of the column, although it could equally well be placed at the top or at the bottom of the column of valve blocks. The gap assembly 3 is contained in a tube S of porcelain, or other insulating material, which is closed at the ends of metal end caps 6 which are sealed to the tube 5. The gap assembly consists of a plurality of sparg gap devices 7 disposed in a series column in the tube 5, any suitable number of gaps being provided depending on the voltage rating of the device.

The construction of the individual spark gap devices 7 is shown in detail in Fig. 2, which shows two double gap assemblies in series relation. Each of the gap assemblies 7 consists of a pair of formed electrodes 8 which cooperate with flat electrode plates 9 on each side. The electrodes 8 and 9 are preferably made of brass but may be made of any suitable conducting material. Each of the formed electrodes S has an `annular ridge 10 formed in its surface, and the electrodes 8 are placed together with their ridges 10 extending oppositely towards the electrodes 9 to form annular gap spaces or spark gaps 11 extending continuously raround the electrodes 8. The electrodes 8 and 9 are separated by annular spacers 12 which determine the spacing of the gaps 11. The spacers 12 might be made of a suitable insulating material, but are preferably made of a high resistance material to control the voltage distribution `across the series of spark gaps. Each gap is preferably also provided with a preioniz'ing button 13 which is made of an insulating material of high dielectric constant, such as rutile ceramic. The buttons 13 are preferably attached to the formed electrodes 8 by a rivet 14 which also serves to join together the two adjacent electrodes 3. The buttons 13 extend close to the liat electrodes 9, and under a rapidly rising surge voltage, an ionizing radiation occurs which preionizes the gaps 11 to obtain low and consistent sparkover voltage. If desired, an insulating barrier 15 may also be provided adjacent each gap to protect the spacer 12 from the arc.

As previously explained, when a discharge occurs in a spark gap of this type, with moderate current, the arc in the gap 11 tends to move over the annular sparking surfaces -dened by the ridge portions 10 of the electrodes and thus localized overheating of the electrodes is avoided and interruption of the arc is facilitated. With the iniproved protective characteristics of modern lightning arresters, however, the power currents which must be interrupted by the gap may be relatively high, and with these higher currents the arc tends to remain in one spot on the electrodes, with the disadvantages previously discussed of local overheating and deformation of the electrodes.

The present invention provides means for insuring that the `arc will move over the surface of the electrodes regardless of the magnitude of the current. For this purpose, a permanent magnet is provided in each gap assembly to produce `a substantially constant magnetic eld extending across the gap space in a direction to cause continuous movement of the arc until it is extinguished. In the preferred embodiment of the invention shown in the drawing, generally `annular permanent magnet 16 is provided for each double gap assembly, the magnet being placed in the space formed by the ridges 10` of the two electrodes 3, so that it requires no extra space and is positively held in the desired position. The annular magnet is magnetized in a radial direction, las indicated in Fig. 2, in such a manner that the magnetic pole of one polarity extends around the outer periphery of the magnet, while the pole of opposite polarity extends around the inner periphery. The magnetic iield produced by such a magnet extends continuously around the annular spark gap 11 in a manner such that the magnetic field is generally perpendicular to the path of a discharge at all points in the gap space, the configuration of the field at any point being substantially as indicated by the lines of force shown on Fig. 2. Thus, when a discharge occurs in the gap 11, the arc extends across a strong magnetic field so that it is caused to move in a direction perpendicular to its path, and since the magnetic iield extends continuously around the annular ridge 10, the arc tends to follow the ridge and moves around the spark gap continuously until it is extinguished. Thus, the arc is positively moved regardless of the magnitude of the current and localized overheating of the electrodes is prevented.

The permanent magnets 16 may be made of any suitable permanent magnet material. It is preferred, however, to use a ceramic material, such as one of the ferfrites, having low permeability and, in particular, barium ability of the preferred material, however, allows a more .uniform distribution of flux about the magnet, so that the conguration of the field is substantially `as shown in Fig.

2, with a large part of the magnetic ux passing through the gap 11 where it is desired. Thus, the low permeability of the preferred material is very advantageous. Another advantage of this material is its great resistance to demagnetization by transient external magnetic fields or other external iniiuences such as heat or shock. Most permanent magnet mater-ials are quite easily dem-agnetized by such infiuences yand must be very carefully handled and protected to prevent demagnetization. The magnetic stability of the preferred material makes it very desirable for the purposes of the invention since it is not subject to any substantial loss of magnetic strength under any usual conditions of handling or use. A further advantage of this material is its excellent electrical insulating property. If conventional permanent magnets of steel or other metallic magnetic material are used for the permanent magnet of this invention, it would be preferable to insulate the magnet from the electrodes. The preferred ceramic material, therefore, is desirable since it eliminates the need for insulation between the electrodes and the permanent magnet. While barium ferrite is the preferred material because of its peculiarly suitable properties, it

will be understood that any other suitable permanent Y magnet material might be used for the magnets 16, a material of low permeability and good magnetic stability being the most suitable.

It Will now be apparent that a spark gap device has been provided which has very desirable characteristics, since it has means for positively moving the are over the electrode surface. It has been proposed to accomplish this result by means of coils through which the discharge current passes and which produce a magnetic field for the purpose of moving the arc. Such an arrangement has many disadvantages, however, and does not positively insure movement of the arc because the strength of the magnetic field varies with the magnitude of the current, so that a constant, strong eld is not provided. The necessary coil is fairly bulky and requires a relatively large amount of space which is not readily available, and also requires an auxiliary gap to protect the coil itself from surges of steep wave front. Thus, the use of a magnetic coil' to produce a field for moving the arc is not a satisfactory solution of the problem because of the numerous disadvantages involved. The present invention avoids these disadvantages in a simple and very effective manner by using a permanent magnet associated with at least one electrode of the gap in a manner which requires no additional space. The use of a permanent magnet makes possible the provision of a strong, constant magnetic field which is independent of the magnitude of the discharge current, and no auxiliary gaps or other protective means are required. Thus, a very effective and desirable means is provided for positively producing movement of the arc in a gap device.

A particular embodiment of the invention has been shown and described for the purpose of illustration, but it will be apparent that various modifications and other embodiments are possible within the scope vof the invention. Thus, the electrodes may he of any desired configuration and the permanent magnet itself may be of any desired shape to conform to the electrode contiguration and may be magnetized in any desired direction to produce a magnetic ield which extends throughout the gap space in a direction generally perpendicular to the path of a discharge. It is to be understood, therefore, that the invention is not limited to the specific embodiment shown for the purpose of illustration, but includes all equivalent embodiments and modifications.

We claim as our invention:

A double gap structure comprising an intermediate electrode structure, annular spacers surrounding said intermediate electrode structure, said intermediate electrode structure comprising a pair of 'formed electrodes each having an annular ridge thereon, said intermediate electrodes placed together with their ridges extending in opposite directions, a at plate electrode facing each of said annular ridges and spaced therefrom by said annular spacers to form an annular spark gap, a generally annular permanent magnet disposed Within said ridges, said permanent magnet consisting of electrical insulating magnetic ferrite of low permeability, and being magnetized in a radial direction to produce a magnetic eld extending across the spark gap generally perpendicular to the path of discharge, whereby a large part of the magnetic eld passes through the gap.

References Cited in the le of this patent UNITED STATES PATENTS Dyer Aug, 21, 1951 Birkbeck Mar. 27, 1956 FOREIGN PATENTS France Dec. 23, 1953

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