US2922914A - Preionizer - Google Patents

Description

Jan.

T. .1. CARPENTER ETAL PREIONIZER Filed May 5, 1958 v 2,912,914 iREIoNIzER Thomas J. Carpenter and Gurdon Milne, lzitts'leld, Mass.,

Yassig'nors to General Electric Company, a corporation A .of New York Application May 5, 1958, Serial No. 733,121 12 Claims. (Cl. 313-308) invention relates to a preionizer for an overvoltage protective device such as a lightning arrester.

One prior art form of lightning arrester comprises a stack of gap and valve units. Under lightning or other surge conditions the gap units are arced over to pass the surge to ground, and the valve units choke olf the power follow current. The arc across the main electrodes of the gap unit may be magnetically elongated anddn'ven nto a plurality of soft iron or magnetic material plates as in copending Snell patent application Serial No. 547,780, which was led on November 18, 1955, now abandoned; or yit may be magnetically elongated and driven into a plurality of auxiliary electrodes by a coil as in copending n Carpenter and `Olsen patent application Serial No. 496,442, which was led on March 24, 1955, now Patent No. 2,890,389. Both of these copending patent applications are assigned to the same assignee as the instant one.

'It has been well known to use a preionizer for the mainfgap of the gap unit. The purpose of the preionizer is to help break down the main gap at low and consistent impulse levels. This is done by having the preionizer preionize the main gap.

the preionizer not emit corona at normal or rated operating voltage of the arrester. This is a requirement which has been diliicult to control. `Corona at normal or rated operating voltage is undesirable because it causes radio noise and may have a corrosive effect on other lightning arrester parts as well as the preionizer itself. This requirecontrol for several reasons. gap unit for the preionizer is very small. Therefore, the preionizer must be very small as compared to other parts of the arrester. The preionizer may compise a pair of electrodesl which are spaced by an air gap or by a dielectric spacer. Accurate gap and dielectric spacer settings have been diilicult to obtain withouthigh cost, and even after they are obtained they are dihcult to maintain. -For instance, corona or arcing in the preionizer may corrode its parts which results in changed settings, or the settings can be alteredby jarring of the arrester. A further disadvantage of prior art preionizers is that they cannot be reliably calibrated and tested prior to their incorporation into the gap units.

It is an object of this invention to provide an improved preionizer which will overcome the above discussed disadvantages of prior art preionizers.

In the preferred form of our inventoin the preionizer is an integral and generally C-shaped element. The spaced ends of the C-shaped element define the preionizer gap. The element may be made by forming a ceramic composition into a closed circular, rectangular, square, oval or other equivalent shape. Thereafter, an accurate gap can be very easily and economically obtained in the formed shape by cutting the gap in one of its sides. The cut ends of the C-shaped element which define the gap are 522,914 Patented Jan. 26, 1960 integral with the main body of the C-shaped element; Therefore, the original gap setting will be self-maintained.

The invention vis not restricted to a C-shaped element. Integral members having other configurations can be lFor instance, a member could have an H-shaped slot formed in it in which the spaced integral projections in the H-shaped slot define the preionizer gap. If the slot were U-shaped then the end surface of the integral projection in the U-shaped slot and the adjacent surface which has two spaced surfaces or projections or a spaced surface and projection which together define a gap.

The features of our invention which we believe to be novel are set forth with particularity in the appended claims.

electrodes and a magnet coil to move the arc.

The magnetic plate gap unit comprises a plurality of unitary magnetic and non-magnetic material plates '1, 2.

When an overvoltage occurs across the electrodes 5 the gap 6 will arc over. The arc is illustrated by reference number 7. A magnetic eld will be created in the magnetic parts 1 as illustrated by the lines 8 when the same into the plates 1, 2.

The parts 1 have blind aligned slots 10 formed therein and the electrodes 5 are disposed in these slots. The magseries arcs. The parts 2 prevent the series arcs from being driven to the outer edge of plates 1, 2 where they could recombine into a long continuous arc.

The preionizer 11 is located adjacent tothe open ends of the slots 10. It is connected electrically in parallel with the electrodes .5. This electrical connection and the support for the preionizer may comprise a pairof metallic and resilient conducting strips 12 which are connected as by welding to the insides of plates 4. The free ends of strips 12 may have slots 13 formed therein which are intended to have the end tabs 14 of the preionizer 11 snapped therein. The opposite ends of the preionizer 1'1 adjacent to tabsV 14 may be silver plated or the like to obtain good electrical contact between preionizer 11 and strips 12.

The preionizer 11 is generally C-shaped and its spaced ends 15 define a preionizer gap 16 for preionizing the main gap 6. The preionizer 11 is positioned to have its slotted side or gap 16 face toward the main gap 6. If the preionirzer were turned around about 90 degrees about its lengthwise axis the line of sight through the gaps 16 and 6 would be free and clear of any obstructions and by the term facing each other or one gap facing the other or similar expressions is meant an arrangement in which the line of sight through the two gaps is free and clear of any obstructions.

When the gap unit is subjected to an overvoltage such as a lighting impulse the gap 16 will break down and ionize the gas space in the gap 6 which will then arc over. Of course, it will be appreciated that this sequence of events occurs practically instantaneously.

In one commercial form of the invention the gap unit y was rated at 3 kv.; the preionizer was about 1" long, 1/2 wide and 1/s" thick; and the gap 16 wasA about 3%000 long and the gap 6 was about 8%000" long. However, the invention is not limited to` these dimensions. Rather, they are given to illustrate one practical working embodiment of the invention.

The preionizer was made from fired barium titanate although other ceramic compositions have been used. The preionizerwithstood l0 kv. R.M.S. at 60 cycles without flashing over and there was no corona at the ends 15 below the 3 kv. operating rating of the gap unit.

The uncut side ofthe preionizer is the electrical equivalent of capacitance between strips 12 and their respective electrodes 5. The cut side of the preion-izer also provides capacity. Therefore, the gap 16 is capacitively in shunt with the iirst mentioned capacitance and the main gap electrodes 5. While the preionizer gap 16 could be resistance coupled, capacity coupling is de,- sirable for when gap 16 is arced over, the gap current increases as frequency increases and more effective preionization of main gap 6 results. The gap spacing 16 can be accurately controlled dimensionally which results inaccurate control of the breakdown voltage of gap 16. After breakdown of the gap 16 the current as a function of voltage and frequency can be controlled by thedimen- 4sions and by the composition of the material of the preionizer.

A further improvement which we have found to be useful is to coat the ends 15 with graphite. When the ends 15 are coated with graphite the consistency of the spark over level is better than when uncoated. Although graphite is more eifective the invention is not restricted to this particular material since other coatings such as lead and metallic oxides ca-n also be used to, 0btain low and consistent sparkover voltages.

In many prior art preionizers the photons for illuminating the main gap are produced by a corona discharge by overstressing the dielectric about a metal electrode that contacts a solid electrical insulating materialfplate,

spacer, pad' or the like. In'these devices the corona starting level as a function of the applied voltage is difiicult to control and therefore the level of current in the device must' be Very 10W. The lack of control over the corona Y starting level may causeV radio noise below the Vrated voltage .of the arrester and promote corrosion of the arrester elements. Also, the eifectiveness of the ioniza# tion is reduced `due to the severe limitation ony the current through thejdevice. 11 l t `disposed adjacent to and facing In the invention the voltage level at which the preionizing discharge occurs can be accurately controlled by the gap spacing. In actual practice this voltage is between about to 125 percent of the rated voltage of the gap unit. Consequently, there is no corona below the rating of the gap unit. Therefore, radio noise and corrosion is eliminated. Also, because the voltage level at which the preionzing device becomes effective is above the rated voltage of the gap unit the impedance of thecapacity coupling of the cut sides of device can be decreased by changing the dielectric constant or 'shapeof the ceramic body. By decreasing the y'impedance more effective ionization is obtained due to the higher current discharge through the gap of the device.

A further advantage of the device is that since it is 'essentially an integral single piece element it has dimensions or a setting which will not be affected by handling, assembly, shipment or storage. It also can be tested before assembly into a lightning arrester gap unit. These' advantages are not true of Yprior art preionizers which are assembled from a plurality of parts since variations in these parts or their relative spacing during or after assembly may affect the performancel ofthe preionizer.

While there has been shown and described a particular embodiment 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, there-` fore, 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 desireto secure by Letters Patent ofthe United States is: v

l. In an overvoltage protective device which has a pair of spaced electrodes, a preionizer for the gap of said electrodes, said preionizer being'connected electrically in shunt with said electrodes, said preionizer comprising an integrally formed element which has two ysurfaces which together define a `gap which is disposed adjacent to and facing the gap of said electrodes.

2. In an overvoltage protective device which has a pair of spaced electrodes, a preionizer for the gap of said electrodes, said preionizer being connected electrically in shunt with said electrodes, said preionizer comprising an integrally formed element which has two projections which together deine a gap which is ldisposed adjacent to and facing the gap of said electrodes.A

3.` In an overvoltage protective device which has a pairV of spaced electrodes, a preionizer for the gap of said electrodes, said preionizer being connected electrically in shunt with said electrodes, said preionizerrcomprising an integrally formed element which has a projection and surface which together define a gap which is the gap of said electrodes. 4. In an overvoltage protective device which has a pair of spaced electrodes, a preionizer for the gap of said electrodes, said preionizer being connected electrically in shunt with said electrodes, said preionizer'comprising an integral generally C-shaped element which has its spaced ends disposed adjacent to and facing said gap. 5. In an'overvoltage protective device which has a pair of spaced electrodes, a device for controllingthe impulse breakdown of the gap of said electrodes to a low and consistent level without producing corona below the normal rated voltage ofKs-aid gap,.said device being connected electrically in shunt with said lelectrodes and comprising an integral generally C-shaped'impedance element whose spaced ends are facing said gap. i y

6. In an overvoltage protective device whichrhjas a pair of spaced electrodes,-a preionizer for the gap-of said electrodes, said preionizer being connected electrically in shunt with said electrodes, said preionizer consisting essentially lof Van integral generally Cshaped impedance element whose spaced ends are disposed adjacent to and facing-.said gap, -said .spaced ends being spacedfrom each disposed adjacentto and other by a gap whose length ,-is small with respect to the length of said element and the length of the gap of said electrodes.

7. In an overvoltage protective device which has a pair of spaced electrodes, a device for controlling the impulse breakdown of the gap of said electrodes to a low and consistent level without producing corona below the normal rated voltage of said gap, said device being connected electrically in shunt with said electrodes and consisting essentially of an integral generally C-shaped ceramic element whose spaced ends are disposed adjacent to `and facing said gap, said spaced ends being spaced from each other by an air gap whose length is short relative to the length of said element and the length of the gap of said electrodes, and said spaced ends being coated with graphite.

8. In a lightning arrester which has a pair of electrodes which are spaced from each other by an air gap, a preionizer for said air gap consisting essentially of an integral generally C-shaped high dielectric ceramic element whose ends are spaced from each other by an air gap which is disposed adjacent to and faces said first mentioned air gap, said element being connected electrically in parallel with said rst mentioned air gap, said preionizer air gap having a length which is short relative to the length of said element and said iirst mentioned air 6 gap, and said preionizer gap ends being coated with graphite.

9. A preionizer which consists essentially of an integrally formed impedance element which has two surfaces which are positioned opposite to each other in spaced relationship to define a gap therebetween.

l0, A preionizer which consists essentially of an integrally formed impedance element which has two projections which are positioned opposite to each other in spaced relationship to define a gap therebetween.

ll. A preionizer which consists essentially of an i11- tegral generally C-shaped impedance element Whose ends are spaced from each other by a distance which is short with respect to the length of said element.

12. A preionizer which consists essentially of an integral generally C-shaped high dielectric constant ceramic element whose ends are spaced from each other by a distance which is short with respect to the length of said element and whose ends are coated with graphite.

References Cited in the tile of this patent UNITED STATES PATENTS

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