Switching Device

Abstract

The switching device has three spaced electrodes with a gas-filled annular space therebetween. When an axial magnetic field above a certain value is applied to the gas-filled space, and after initiation, cascading ionization occurs for conduction. The electrodes can be electrically serially connected for higher holdoff voltage during nonconduction, or can be electrically connected in parallel for higher current capacity in the same envelope.

Description

BACKGROUND

This invention is directed to a switching device of the crossed field type, employing Penning discharge, wherein three spaced electrodes define two spaces in which conduction can occur.

Switching devices of this general type are known in the art. Penning U.S. Pat. No. 2,182,736 describes such a switching device, while Boucher et al., U.S. Pat. No. 3,215,893 and Boucher U.S. Pat. No. 3,215,939 describe improvements thereon. All three of these devices are primarily directed to rectifier type switching and the Boucher and Boucher et al., patents are directed to an improvement wherein the shape of the magnetic field improves rectifying action by providing a lower breakdown voltage in one direction than the other between the two electrodes which define the gas-filled space. These structures suffer from the problem that there is only one gas-filled space. Therefore, maximum voltage is limited by the single interelectrode space and maximum current is limited by the area of one of the electrodes.

With continually increasing electric power demands, there is increased need to exploit sources of power farther away from the users of large amounts of electric power, with the consequent need for transporting the electric power over greater distances. In the United States, a number of our larger electric power-consuming areas are at some distance from primary power sources, such as sites for generation of hydroelectric power, coal deposits and oil deposits. Accordingly, it becomes necessary to transport electricity over greater distances. It is known that, to transport high powers over long distances, DC can be economically superior to AC. This has already led to a number of high power DC transmission lines, such as the Pacific Intertie presently under construction between the Columbia River and Los Angeles. One limitation to the wide use of DC is the lack of practical high power DC switching devices. The present device permits higher standoff voltage in a single envelope or, when so connected, permits higher current flow in a single envelope. The present invention thus provides improved means by which high power can be transmitted over long distances.

SUMMARY

In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a switching device having first, second and third electrodes which define first and second interelectrode spaces. The interelectrode spaces are gas-filled to such a pressure that the length of electron path is below a critical value when an electric field is applied without a magnetic field, and is above a critical value when a magnetic field above a critical value is applied at right angles to the electric field. The three electrodes are connectable in series to provide a switching device having a total standoff voltage equal to the sum of the standoff voltages of the first and second interelectrode spaces when the magnetic field is below a critical value, and are alternatively connectable in parallel for increased current capacity during conduction.

Accordingly, it is an object of this invention to provide a switching device of the crossed field type suitable for a large nonconducting standoff voltage in a single envelope. It is another object to provide a switching device which has first and second interelectrode spaces between three electrically separate electrodes so that the device can be connected in series for higher standoff voltages or in parallel for higher current conduction in a single envelope. It is a further object to provide a switching device wherein a single magnetic field controls the field strength in two annular spaces to thus be able to control the Penning discharge characteristics in separate annular spaces, with a single magnetic field source. Other objects and advantages of this invention will become apparent from a study of the following parts of the specification, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A is a schematic drawing of a portion of a power system of the nature in which the switching device of this invention is employed.

FIG. 1B is a schematic drawing of another portion of such a power system.

FIG. 2 is an external view of the switching device, in accordance with this invention.

FIG. 3 is an enlarged longitudinal section of the switching device of this invention.

FIG. 4 is a transverse section, principally schematic, showing series connection of the electrodes.

FIG. 5 is similar to FIG. 4, showing parallel connection of the electrodes.

FIG. 6 is a Paschen curve showing the conductive and nonconductive conditions as a function of voltage versus pd.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The switching device is generally indicated at 10 in FIG. 2. Referring to FIGS. 1A and 1B, which illustrate the manner in which the switching device 10 is employed in a circuit, two different applications of the switching device are indicated at 10A and 10B. In FIG. 1A, power source 12 drives generator 14. Power source 12 can be of any conventional type, including hydroelectric, internal combustion engine, or steam, including nuclear heated steam. Generator 14 generates alternating current electricity of suitable voltage and frequency for that portion of the system. It supplies alternating current transformer 16 which changes the voltage to one suitable for rectification and direct transmission. When direct current is employed for economic, long distance power transmission, this usually requires an increase in voltage at the transformer output, as compared to its input. Transformer 16 supplies rectifier 18, which preferably includes a plurality of rectifiers arranged in bridge form, depending on the plurality of phases at the output of transformer 16.

The rectifier, in turn, supplies transmission lines 20, through switch 10A. The presence of switch 10A, which can also serve as circuit breaker in an appropriate circuit combination, permits the use of uncontrolled rectifiers for the rectifier 18. This can lead to substantial savings over the use of controlled rectifiers, such as are required by the present state of the art in the absence of a DC switch, such as 10A. Transmission lines 20 are supported on a plurality of towers 22, which support the lines in insulated fashion away from the terrain, from the area of generation to the area where the electric power is to be employed. In some cases, transmission lines 20 may be buried and, in some cases, they will be underwater transmission lines. Furthermore, while two transmission lines are preferable so that the voltage to ground can be divided between them, some systems may employ a ground return, but such is not preferred for high power systems.

Referring to FIG. 1B, switch 10B is connected between transmission lines 20 and load 24. While a simple switch and simple load are indicated, there are preferably two switches at 10B, in order to switch the power coming from each of transmission lines 20. Furthermore, load 24 may be a direct current load operating at transmission line voltage, or it may be an inverter-transformer-load system. Switch 10B, with its load 24, illustrates the use of switch 10B for a tap on the transmission line. In the appropriate circuit combination, switch 10B can also serve as a circuit breaker for the tap.

In FIG. 1B, the termination of transmission lines 20 can be in an inverter in the nature of switch 10C. The switch of this invention can be operated in an inverter mode to produce alternating current of appropriate frequency. Accordingly, sufficient switches are incorporated in the inverter 10C to supply an alternating current output. The output is connected to transformer 26, which has its output connected to the ultimate load 28.

Referring to FIGS. 2 and 3, the switching device 10 is shown as having a bottom flange 3 which stands on supporting foot 32 to serve as a physical support for the switching device and as an electrical connection to one of the electrodes. Ceramic envelope 34 is in the form of a cylindrical tube and serves as the main housing member. Flange 30 is secured to envelope 34 by means of flange ring 36, which is snapped into an exterior annular groove in envelope 34 and is retained therein by ring 38. Gasket 40 between flange 30 and envelope 34 serves as a vacuum seal therebetween.

Tubular electrode 42 serves as the outer electrode. It is mounted upon bottom flange 30 and is electrically connected thereto. Electrode 42 is upstanding and is generally concentric with ceramic envelope 34, both having a substantially common cylindrical axis. Outer electrode 42 has an inner surface 44 which acts in the electric discharge, as is explained hereinafter.

Disc 46 supports intermediate electrode 48, which has an outer surface 50 and an inner surface 52, which act in the discharge. Kovar rings 53 are soldered to the ceramic envelope and to disc 46 to provide structural and vacuum integrity. Intermediate electrode 48 is shown as having a closed bottom. This closed bottom and the cans in the bottom and on flange 30 reduces spaces to maintain the electron path lengths short to prevent breakdown. The two rings on the inside of disc 46 perform the same function. Holes are provided for pumpdown. The intermediate electrode 48 is in the form of a cylindrical tube, preferably having its axis coincidental with the central axis of the switching device 10, upon which the axis of ceramic envelope 34 lies. This defines a uniform radial space D between the outer surface 50 of electrode 48 and inner surface 44 of electrode 42. Corona shield 54 is electrically connected to disc 46, for external electric connection to electrode 48 and to help reduce corona discharge.

At the top end of ceramic envelope 34 is top flange 56. Flange 56 carries guide ring 58, which has a lip which engages exteriorly of ceramic envelope 34, to centralize top flange 56. The top flange and guide ring are clamped onto the top of the ceramic envelope by means of flange ring 60. Gasket 62 is engaged therebetween to assure vacuum integrity to the interior of the envelope. Corona shield 64 is mounted on flange ring 60 and is at the potential of top flange 56. Cable 66 passes through an opening in the corona shield and is electrically connected to top flange 56 by means of angle connector 68.

Gas source 70 is mechanically supported from top flange 56 by connectors 72 and 74. These connectors are electrical feedthroughs which serve to both mechanically support the gas source and to permit the supply of electricity thereto. Connectors 72 and 74 are connected by flexible cable led through a central opening in the top of the corona shield, so that selected potential can be supplied from an exterior source to produce gas from gas source 70. The gas source can comprise a material such as titanium hydride ribbon or sponge, so that the temperature thereof can be controlled by the amount of current passing through the gas source. At elevated temperature, titanium hydride gives off hydrogen. The hydrogen thus produced passes from the interior of support base 76 into the general interior of ceramic envelope 34 to supply gas needs. As is conventional in Penning discharge devices, net electron flow from the cathode to the anode results in collisions with gas atoms in the interelectrode space to cause ionization. A certain number of these ionizing collisions cause the ions to be driven into the surface of the cathode. Gas pumping by ion implantation and by adsorption onto freshly-sputtered material occurs, with the result that the amount of ionized and neutral gas decreases after the switching device has been conducting for a period of time. The gas ultimately decreases to a point where conduction cannot be maintained, if no gas source is provided. This causes unwanted or premature off-switching of the device, when the only gas available is that in the interelectrode space. Gas source 70 is energized to produce gas to overcome this problem.

Support 76 is mechanically secured to top flange 56 and is in electrical contact therewith. Support 76 serves as the support for inner electrode 78. Inner electrode 78 has an outer surface 80 and is preferably in the form of a cylindrical tube. The hollow interior provides an increased net gas space within the ceramic envelope 34 to aid in avoiding gas depletion by implantation and adsorption. The interior gas space provides additional volume from which gas may move to supply the demands of the interelectrode spaces. The cylindrical, tubular character of inner electrode 78 also defines a uniform interelectrode space D between the outer surface 80 and the inner surface 52 of electrode 48. Thus, there is an interelectrode space interiorly of and one exteriorly of electrode 48. The intermediate electrode is shaped and positioned so that the inner and outer electrodes cannot "see" each other. Thus, ions and electrons cannot directly pass between the inner and outer electrodes. They are electrically separate in the sense that the same plasma or other electrically conductive medium cannot contact both of them.

Surrounding the envelope 34 outside of these interelectrode spaces is solenoid 82. Corona shield 84 is mounted thereover. Solenoid 82 is illustrated as being an electromagnet, because it can conveniently provide the desired field strength. The magnet is positioned in such a manner as to provide magnetic lines of force in the interelectrode spaces which are substantially parallel to the axis of the electrodes of the switching device 10 over at least a part of the electrode length. The magnetic field strength of magnet 82 is such as to provide a field between 50 and 100 gauss in the interelectrode space. Seventy gauss is found to be a preferred value for the dimensions illustrated below, used in the experiments to date.

The interelectrode space is filled with a gas to an appropriate pressure. Referring to FIG. 6 the Paschen curve is shown therein. This curve illustrates that, at a certain critical product of the interelectrode pressure times the average electron path length d, the voltage to breakdown is fairly low. It also illustrates at point A that, for a lower product, voltage to cause breakdown is considerably higher. This is because, at lower pressure, the ionization mean-free path exceeds the average electron path length d, and the ionization rate decreases, which makes it more difficult to sustain the discharge and makes it possible to withstand higher voltage between electrodes before breakdown occurs.

When the magnetic field is off, electron flow is only under the influence of the electric field from the cathode to the anode so that the average electron path length d is substantially equal to the interelectrode space D and is less than the mean-free path length. Thus, there is no sustained ionization, electron flow is low, and the switching device can withstand a high standoff voltage, for it is conditioned approximately below point A on the Paschen curve. However, when the magnetic field is applied to the interelectrode space by magnet 82, the axial magnetic field causes the electron path to follow an inward spiral more circumferential than radial in the interelectrode space to increase electron path length d. In this longer path caused by the magnetic field effect, there are sufficient collisions to maintain ionization, because the path length d is longer than the mean-free path length. Thus, so long as a sufficient magnetic field is applied, once electrons start flowing, the flow is maintained until the magnetic field is cut off. When cut off, the electrons again flow radially so that ionization soon stops.

However, the ionized conduction cannot start again without ionizing ignition. Thus, the presence of the magnetic field above the critical value and the presence of an electric field above the conducting voltage drop of the device does not cause conduction in the absence of ionizing ignition to initiate ionization. Any convenient ionizing device can be employed.

Referring to FIGS. 4 and 5, they respectively illustrate connections of the electrodes for high voltage standoff and for high current conduction. FIG. 4 illustrates the connection of electrode 78 to the positive line 86 and the connection of electrode 42 to the negative line 88. Electrode 48 is electrically connected therebetween in the center of a voltage-dividing resistance network 90.

When voltage is applied to the device 10 when it is connected in the configuration of FIG. 4, sufficient magnetic field is applied to cause a long enough electron path in conjunction with gas pressure to cause cascading ionization, and ionization is caused in the interelectrode space, crossed field discharge occurs. In such a case, the inner surfaces of electrodes 42 and 48 act as cathode surfaces and the outer surfaces of electrodes 48 and 78 act as anode surfaces. By this means, the interelectrode radial distance can be maintained at a proper value for conduction, but the effective interelectrode spacing between the interior of electrode 42 and the exterior of electrode 78 is longer for higher nonconducting standoff voltages. In effect, two serial vacuum gaps are provided in the same envelope.

Referring to FIG. 5, the manner in which the electrodes are connected provides greater current capacity than is otherwise found in a single gap device in an envelope of that size. Positive line 92 is connected to electrodes 42 and 78 and negative line 94 is connected to electrode 48. When so connected, and under crossed field discharge conditions, the interior surface of electrode 42 and exterior surface of electrode 78 acts as anode surfaces, while both the inner and outer surfaces of electrode 48 act as cathode surfaces. In this way, parallel operation is obtained. By this means, the switching device of this invention can be connected either for increased voltage standoff during nonconduction, or increased current during conduction, in a convenient envelope.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty.

Claims

What is claimed is:

1. A switching device, said switching device comprising:

envelope means for maintaining a reduced pressure within said envelope means;

an outer, an intermediate and an inner electrode positioned within said envelope means, each of said electrodes being unheated, said outer and said intermediate electrodes being tubular and said inner electrode having an exterior surface, said inner electrode being positioned within said intermediate electrode, and said intermediate electrode being positioned within said outer electrode for electrically separating said inner and outer electrodes and for defining substantially uniform interelectrode distances between said inner electrode and said intermediate electrode, and between said intermediate electrode and said outer electrode;

gas at a reduced pressure in the interelectrode space;

separate electrical connection means connected to each of said electrodes;

magnetic field means positioned to induce a magnetic field in the interelectrode spaces;

said switching device passing current in the cold cathode crossed field discharge mode through the interelectrode spaces between said electrodes, when a magnetic field is applied to the interelectrode spaces to cause the average electron path length to exceed the ionization mean-free path of the interelectrode gas.

2. The switching device of claim 1 wherein said intermediate electrode and said outer electrode are cylindrical tubes and said inner electrode has a cylindrical outer surface, the axes of said tubes and said surface being substantially coincident.

3. The switching device of claim 2 wherein the radial interelectrode space between said inner and said intermediate electrodes is substantially equal to the interelectrode space between said intermediate and said outer electrodes.

4. A switching device, said switching device comprising:

envelope means for maintaining a reduced pressure within said envelope means, said envelope means being comprised of substantially tubular cylindrical sidewalls and having closed ends, said sidewalls being made of dielectric material, and said ends being made of electrically conductive metallic material;

an outer, an intermediate, and an inner electrode positioned within said envelope means, said intermediate and said outer electrodes being cylindrical tubes and said inner electrode having a cylindrical outer surface, the axes of said tubes and said surface being substantially coincident to define substantially uniform interelectrode distances between said inner electrode and said intermediate electrode, and between said intermediate electrode and said outer electrode, said inner electrode being mounted on one of said ends and said outer electrode being mounted on the other of said ends;

gas at a reduced pressure in the interelectrode space;

separate electrical connection means connected to each of said electrodes;

magnetic means positioned to induce a magnetic field in the interelectrode spaces;

said switching device passing current in the crossed field discharge mode through the interelectrode spaces between said electrodes when a magnetic field is applied to the interelectrode spaces to cause the average electron path length to exceed the ionization mean-free path of the interelectrode gas.

5. The switching device of claim 4 wherein said intermediate electrode is supported by said envelope sidewalls intermediate the ends of said envelope.

6. The switching device of claim 4 wherein the radial interelectrode space between said inner and said intermediate electrodes is substantially equal to the interelectrode space between said intermediate and said outer electrodes.