US2006053A - Gaseous discharge converter - Google Patents

Description

U. LAMM GASEOUS DISCHARGE CONVERTER June 25, 1935.

Filed July 3, 1955 Patented June 25, 1935 PATENT OFFICE GASEOUS DISCHARGE CONVERTER Uno Lamm,.Ludvka,.Sweden, assignor to Allmnna Svenska Elektriska` Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Application July 3, 1933, Serial No. 678,987 In Sweden July 4, 1932 6 Claims.

- Electric gaseous discharge converters using inert gas or metal vapor, for instance mercury vapor, as conducting medium are for medium voltages and large power values generally constructed with a main vessel of metal and with a cathode receptacle and anode bushings of insulating material. For high voltages it is generally recommended to place the anodes in the upper ends of tubular insulators arranged inside or outside the main vessel which may be of metal or of insulating material.l These tubular insulators serves to prevent the occurrence of too small distances between the anodes and the walls of the vessel. It has, however, now been found that the provision of such tubular insulators is generally not` sufficient to prevent back arcing, as the electric iield will concentrate in certain places too much if it is only determined by electrostatic action. According to the present invention, tubular anode insulators are therefore combined with tubes or rods of a high-ohmic resistance material placed inside the insulator so as to determine substantially the Vvoltage distribution therein during'the blocking interval.

In theA accompanying drawing, Fig. 1 shows a complete gaseous discharge converter (serving for instance as a rectifier) in a vertical section in which one form of the anode system is illustrated. Fig. 2 shows a modified form of the anode arrangement in a partial vertical section and Fig. 3 is a cross section on the line 3 3 in Fig. 2. Fig. 4 shows a modification of Fig. 3. Figs. 5-'7 show further modifications of the anode arrangement in vertical sections.

`'Referring to Fig. 1, I is the main Vessel of the apparatus, made for instance of iron and having a condensation dome 2, insulated cathode vessel 3 andcooling jackets 4. It has further an ignition anode 5, excitation anodes 6, one of which is shown, and main anodes 1. The main portion of current path from each anode to the cathode is surrounded by an insulating tube 8, made Afor instance of porcelain, glass, or quartz, projecting from the main receptacle and closed at its upper end by a metal cap 9 provided with gas tight sealings against the tube 8 and anode insulator bushing I0.

[Inside the tube 8 there is, according to the present invention, arranged a tube II of high resistivity conducting material, which ends at a short distance from the anode 1. The upper end of'th'e resistance tube can be kept at an appropriate voltage by connecting it either directly to theanodeor, as shown, to a suitable point of the source ofcurrent I2 of the anode (which may be the secondary winding of a transformer) so as to have at least during the blocking interval a negative potential with respect to the anode.

The lower end of the resistance tube may be provided With a similar leading-in conductor I3 for (Cl. Z-27.5)

introducing a control voltage varying according to any desired law. As the impression of such a voltage on the tube II will involve a rather large demand of power in the control circuit which in some cases is objectionable, the control voltage may instead be impressed on a separate grid I4, arranged beneath the tube II, by means of a lead I5. The lead I3 may then be connected to a point having substantially the potential of the cathode. The resistance body should at any rate have a sufficient resistance to enable it to withstand the whole voltage betweenV anode and cathode without overheating. l

As the voltage conditions closest to the anode are influenced not only by the resistance in the -tube II, but also directly by the temperature and also indirectly as the temperature generally influences the resistivity of the tube material, it is often recommendable to surround the outer insulator tube by a heat insulation or particular heating device in order to keep its temperature at an appropriate value. Such a device is diagrammatically indicated at I6.

The diameter of the resistance tube, or the mean diameter, if the tube be of irregular shape, should not be larger than a value which will permit the tube to exert a sufficient influence on the iield distribution in the conducting vapor. Its action in his respect depends on the thickness of the space charge layer (consisting of positive ions) which covers the resistance body when negative with-respect to the cathode, because outside this space charge layer the vapor which is already ionized has always practically the potential of the cathode. The thicknessrof the space charge layer rdepends in its turn on the voltage of the corresponding point of the resistance body and is even at high voltages, for the pressure and ternperature values generally occurring in apparatus of this type, only of the order of magnitude of some centimeters. In using a regularly tubular resistance body, which gives theleast losses in the current path, the tube diametershould therefore, already at a distance from the anode, be less than twice the thickness of the space charge layer which corresponds to the voltage at such distance. Such a dimensioning of the tube forms one object of the present invention. v

In Fig. 1 the space charge layer is indicated by a weak discontinuous horizontal shading and its limit opposite the surface of the resistance body by dotted lines. This limiting surface will be substantially a cone, as the thickness of the layer will be substantially proportional to the voltage and thelatter substantially proportional to the distance from the end of the tube lying nearest to the cathode (for uniform tube thickness and resistivity). yThe apex of this cone should be comparatively distant from the anode in order to give a satisfactory security against back arcing.

This security may be said to be substantially proportional to the thickness of the space charge layer which separates the anode from a vapor quantity of cathode potential. If the apex of the limiting cone should fall at the same or smaller distance from the anode than the thickness of the space charge layer which the anode alone would create, the safety would not be appreciably greater than without the tube. The distance of the apex from the anode should therefore be substantially greater than the layer thickness which the anode would create, which in practice implies that the radius of the tube should be essentially smaller than said layer thickness. The condition can also be expressed so that the radius of the tube should at the most be equal to the thickness of the space charge layer at the voltage for which an unprotected anode has a tendency of back-arcing. The length of the tube is then determined by the real voltage between anode and cathode and should be about so much larger than the length of the cone shown in Fig. 1 as the real voltage is higher lthan thel highest permissible one for an unprotected anode. As an approximative practical measure of maximum diameter of the tube 50 millimeters or 2 inches may serve. This maximum diameter, however, is valid only for the main portion of the tube, as small Wider portions do not affect the field distributions as a Whole.

A consequence of the necessary maximum diameter of the tube or sleeve will be that the current per anode tube will be limited, and in practice for apparatus using low pressure mercury vapor as conducting medium this limitation will be at about amperes per anode or anode tube. For large apparatus it will therefore be necessary to divide the current between different parallel anodes 01 anode tubes.

In Fig. 2, the anode 22 is inside the main Vessel 2|. It is surrounded by an insulating tube 28 shown as integral with the insulator bushing for the anode bolt. Inside the tube 28 there is a resistance body 23 shown in a side view in Fig. 2 and in a cross section in Fig. 3. At the upper and lower ends, this body is by means of leads 24, 25 connected to voltage sources. The leads 24, 25 may terminate in metal discs 25, l21| serving to distribute the current over the cross-section of the resistance body 23 which may consist for instance of silicon carbide.

Fig. 4 shows a modied cross-section of the resistance body which is in this form divided into ve parallel bars 29 instead of forming one bar, star-shaped in cross-section, in Fig. 2. The practical result will be essentially the same.

In Fig. 5, the anode 52 is inside an insulating tube 58 projecting from the main vessel 5|. The longitudinal resistance body consists in this case of a lining 53 of resistance material on the inside of the tube 58. As this lining may be rather thin, it can consist of a material of comparatively low resistivity, as for instance graphite.

In Fig. 6, the lower part of the outer tube consists of a metal tube B3 projecting from the main vessel 6|, and only the upper part surrounding the anode 62 consists of an insulating tube 64. The tubular resistance body 65 extends through both the outer tubes.

In Fig. '7, the outer tube 14 is again inside the vessel 1| and consists entirely of insulating material. The anode l2 is concave, and the tubular resistance body l5 forms a direct extension thereor. The lower end of the resistance tube is in this case supposed to assume the right potential only by Contact with the ionized gas, and as the current leaks gradually from the tube to the gas, the thickness of the tube diminishes towards the lower end in order to give a substantially uniform voltage drop. The insulator tube has on its inside, just below the anode, an annular ange 'I6 for preventing access of the arc to the outside of the anode.

I claim as my invention:

l. In gaseous discharge converters using gas of low pressure as a conducting medium, a gas receptacle, a cathode therein, tubes of insulating material projecting from said receptacle, anodes in the outer ends of said tubes, and conductors extending longitudinally inside said tubes having a sufficient resistance for supporting between their ends practically the maximum potential difference between anode and cathode without being overheated.

2. In gaseous discharge converters using gas of low pressure as a conducting medium, a gas receptacle, a cathode therein, tubes of insulating material projecting from said receptacle anodes in the outer ends of said tubes, and longitudinal tubular conductors inside said insulating tubes, said conductors having a suiiicient resistance for supporting between their ends practically the maximum potential difference between anode and cathode without being overheated.

3. In gaseous discharge converters using gas of low pressure as a conducting medium, a gas receptacle, a cathode therein, tubes of insulating material communicating with the interior of said receptacle, anodes inside said tubes, and conductors extending longitudinally inside said tubes having a suihcient resistance for supporting between their ends practically the maximum potential dii'erence between anode and cathode without being overheated.

4. In gaseous discharge converters using gas of low pressure as a conducting medium, a gas receptacle, a cathode and an anode therein, and a resistance conductor extending from the anode towards the cathode, said conductors forming sufficiently narrow passages from the anode to allow the space charge layer protecting the anode to be essentially deeper than that created by the anode itself.

5. In gaseous discharge converters using gas of low pressure as a conducting medium, a gas receptacle, a cathode and an anode therein, and a tubular resistance conductor surrounding the current path from anode to cathode and having a diameter less than the thickness of space charge layer corresponding to the normal voltage of the valve.

6. In gaseous discharge converters using gas of low pressure as a conducting medium, a gas receptacle, a cathode and an anode therein, a resistance body extending longitudinally from said anode towards said cathode and having a resistance sufficient to withstand substantially the operating voltage of the valve without overheating, and a conductor for impressing on the end of said conductor adjacent to the anode a voltage near that of the anode.

UNO LAMM.

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