US1931475A - Vacuum circuit breaker - Google Patents

Description

Oct. 17, 1933. v BRASCH ET AL 1,931,475

VACUUM CIRCUIT BREAKER Filed Nov. 10, 1951 T0 ACUU/VPUMP 9 VALVAQL LL V 4 /l c? 1 7 Eras 0/: 4/ I 4 M Patented Oct. 17, 1933 UNITED STATES.

' 1,931,475 PATENT oF i-ci:

warms VACUUM cmoorr BREAKER Arno Brasch and Fritz Lange, Berlin, Germany Application November 10, 1931, Serial No. 574,214, and'in Germany November 11, 1930 5 Claims.

addition to the expense, also possess certain disadvantages of a practical kind, such for example as the danger of explosion, which can never be entirely overcome, efforts have been made to discover new principles of construction for the purpose of extinguishing the arcs upon the switching operation. Inter alia, vacuum switches have also been proposed, in order to obtain short path of movement and small and cheap arrangements. These switches, however, are accompanied by the disadvantage that the same will not operate in reliable fashion unless there is employed an essentially high vacuum amounting to, say, 10- mm. Hg, and since upon the switching operation there are always small amounts of vapor formed, the vacuum continues to deteriorate as the vapor is given off. Constructions of this nature are, therefore, not capable of use in practice. Beyond this, independent discharges are only to be avoided with extreme difficulty in a simple vacuum chamber in the case of potentials of more than 200 kv. It is therefore necessary to attempt on the one hand to suppress undesirable discharges in the high potential ranges, and on the other hand to employ a vacuum of such a poor kind that the small amounts of vapor produced upon the breaking action have no effect on the insulating powers of the vacuum.

It is well known that every gas is composed of a large number of single molecules, which are continuously in motion to and fro, and these molecules in turn are composed of ions. These ions are also subject to movement. It is furthermore known that the relative location of the ions within a gas varies in accordance with the particular pressure under which the gas may happen to be placed. If the gas is subjected to high pressure the ions are situated in greater proximity to each other than in the case of a low pressure. It will be obvious, therefore, that the average distance which one ion requires to traverse in order to meet against the next varies in accordance with the pressure applied to the gas. Thus, for example, if the pressure on the gas amounts to 100 mm. Hg, this mean free path will amount-f These ions and gas cordingly, in the following description, reference will be made in general to "charge carriers;

For the completion of an electric circuit there are empoyed switch contacts, which in the following description will be referred to as "electrodes, as the switching operation is performed in a chamber which is closed off against the exterior, and in which there is a certain degree of vacuum.

It has been found that an independent, i. e.,

undesirable discharge between two electrodes does not take place if the distance between the electrodes is less than the mean free path of an electron.

To be on the safe side it is desirable to make the distance between the electrodes, say, merely one-half or one-third of the path of movement of the charge carriers. The electrodes must naturally be so fitted in the vessel that a path of discharge is at no point formed which is equal to or greater than the distance traversed by the carriers,-that is to say, the electrodes in the case of a particular embodiment require to be taken right up to the wall of the discharge vessel, in order to avoid any intermediate space between the electrodes and the wall, through which space charge carriers, proceeding for example from the two rear sides of the electrodes over the free edge, might be capable of'moving to a greater extent than is represented by the mean free path of movement of the carriers with a given pressure. If the vacuum in a vessel of this nature should be subject to deterioration upon the switching operation by reason of the vapor released, the arrangement as described nevertheless remains free from discharge, provided that by proper selection of the spacial disposal of the electrodes the stated ratio between the electrode distance and the free path of movement of the charge carriers is adhered to.

Since, as already explained, the distance between the electrodes requires to be very small, a plurality of electrodes are connected in series in the case of relatively high potentials. This series connection is also accompanied by the particular advantage that in the relatively high potential ranges, for example exceeding 200,000 volts, undesirable discharges are prevented. In the case of potentials above 200 ky. surface discharges occur along the inner wall of the vacuum vessel, which form the basis for an independent discharge. A surface discharge is thus the opposite to a spark.

As to the surface discharges it is to be understood that at an insulating surface there occurs in the case of a high and more particularly tangential component of the electrical field at the vacuum limit a displacement of electricity carriers. In view of the rapid formation of the discharge it is to be assumed that electrons participate to a particular extent in this occurrence. This is referred to as a surface discharge (a kind of leakage discharge), although the same differs quite appreciably from the leakage discharges which occur in connection with hightension insulators under atmospheric pressure. The difference resides principally in the fact that due to the vacuum surrounding the insulator a dispersion of the charge carriers in the icinity of the insulator is possible to a much better degree than under atmospheric pressure. The electrodes projecting into the space initiate the discharge, and it is, therefore, necessary to sup press these surface discharges if a discharge is to be prevented in a space of poor vacuum.

To avoid these surface discharges there may be employed the means set forth in the United States patent application Ser. No. 525,244, filed March 25, 1931, via, the inner wall of the vessel may be sub-divided in such fashion that the same, for example, is composed of conductive or semiconductive materials alternating with layers of insulating material. Particularly good results have been obtained by making the slideways between the single sections as long as possible. This may be accomplished by the use of two or ,more insulating rings of different inner diameter between the single chambers or sections. The assembly is preferably such that alternate discs of metal and rings of insulating material, such as mica, board or the like, are placed one against the other, and the bounding faces covered with a material offering the greatest possible pro- .tection against rupture.

the movement of the switch contacts employed,

in the breaker according toFig. 1.

Fig. 3 shows diagrammatically in conjunction with Fig. 4 the principle of the circuit breaker,

Fig. 4 being the diagrammatical view of a vacuum vessel, having depicted therein the movement of the charge carriers of a gas.

By way of explanation as regards the assembly of the circuit breaker, reference is first made to Figs. 3 and 4. In these two figures 1 is a vessel which is enclosed on all sides, and in which there prevails a certain degree of vacuum. In the vessel there are situated the two electrodes 2. These electrodes may be so moved in relation to each other that finally the same are caused to touch, thus completing a circuit connected with the electrode leads. Suitable means are provided to ensure that the maximum distance between the electrodes conforms with the distance a, i. e., the distance between the electrodes amounts to a when the switch is inoperative. In this the part of the wall between the electrodes designated a would be subjected, owing to.the tangential disposal of the electrical field, to sufficient potentials to cause surface discharges to take place. These would then initiate a discharge between the electrodes. In Fig. 4 there are depicted the gas molecules or ions which, as stated above, are in continuous motion. The arrows indicate the exact movements performed. The pressure ,of the gas within the vessel 1 is so chosen that, for example, the distance between the ions conforms with the distances 1) in Fig.

4. One ion or charge carrier moving in the vessel 1 is, in other words, required to traverse the distance h before it strikes against the next one. The distance b may accordingly be referred to as the average free path of movement of the charge carriers. When comparing Figs. 3 and 4, it will be seen that the distance a between the electrodes is smaller than the average free path of movement I) of the charge carriers within the vessel. The conditions above referred to are accordingly fulfilled in a circuit breaker of this description, and the space between the two electrodes 2 is, for all practical purposes, free from discharge.

The construction of the circuit breaker is illustrated in Figs. 1 and 2. With regard to Fig. 1, there are provided metallic discs 4 having burings 5 near the edge, which borings in the particular form of embodiment shown are moved into alignment, i. e., are situated one above the other to form a common passage. The metallic discs 4 serve, together with the insulating rings 6 and '7, to constitute the switch. The outer edges of the discs 4, the rings '1 and the discs 6 represent the wall of the vessel. At the centre each disc 4 is furnished with an orifice 8 covered by means of a tightly fitting diaphragm 9, which in turn carries on its outer face the actual switch contact 10. Counter-contacts 11 are furnished on the lower faces of the discs 4. The distance a in Fig. 3 is, in the case of the circuit breaker according to Fig. 1, produced between the contacts l0 and 11 by reason of the superimposed arrangement of the discs 4. By means of the rings 6 and 7 there are formed pockets or chambers 12, and into these there project the inner edges of the discs 6, so that a certain divisioning of the wall results on the lines set forth in the United States patent application Ser. No. 525,244. The borings 5 enable the pockets or chambers 12 to be pumped out in common, so

that the same vacuum prevails in each. This vacuum is naturally so chosen that the corresponding free paths of the charge carriers are greater than the spacing between the contacts 10 and 11.

To prevent any discharges, caused for example by reason of sliding effect, from extending along the wall through the borings 5, the latter, as shown in Fig. 1, may be filled out with small balls or the like, such as shot 13. The energy of any charge carriers striking against this metallic filling is immediately compensated and destroyed, in such fashion that a discharge is unable to take place. Regarding the operation of the balls, it will be recognized in Fig. 1 that the space 12 is bounded on both sides by the metallic surfaces of the members 4. In disregarding the disturbances which are caused by the projection of the contacts, the lines of force of the electrical field are accordingly disposed for the greater part vertical to the surface from the one member to the other. The spacing between the two members is made to be such that in the case of the particular gas pressure prevailing in the space 12 the average free distance of the charge carriers is greater than the distance between the surfaces measured in the direction of the lines of force.

At the point of the borings, which require to be provided for the purpose of evacuation, this disposal of the field suffers disturbance. The equipotential "faces move further apart, and there is the danger that the charge carriers reach the speed, which is necessary for ionization by collision, before they strike against the metallic confinement of the space. It is usual in the art to ensure an even disposal of the equipotential faces in the case of vacuum apparatus also at those points at which openings are necessarily provided by covering these openings by nets or grids. These nets, however, are relatively diflicult to produce, and under certain circumstances possess a very considerable electrostatic reciprocal. For this reason in the invention the nets are replaced by metal balls. These metal balls are simple to produce and to arrange within the pumping aperture, and fulfill their object of compensating the potential faces to a considerably better extent than a net. In this connection it is unimportant that the metal balls move into contact with the insulating ring 6, as a decrease in the spacing between the equipotential faces (such as also exists for example between the electrodes) is always permissible. On the other hand increase in the spacing between the equipotential faces must be avoided. In lieu of the shot it is, of course, also possible to employ other metallic bodies. For example,

. wire fabric may be inserted in one or more layers, so that, for example, the fabric passes from one edge to the other. When using wire fabric, it is desirable to dispose the single pieces in such fashion that the meshes are situated in staggered relation to eachother, thus increasing the possibility of the charge carriers striking against a piece of metal.

Fig. 2 shows diagrammatically the means for actuating the circuit breaker. All spaces 8 of the discs 4 communicate by means of pipes 14 and a common feed pipe 15 with a source of pressure. This, for example, may be oil under pressure. As already mentioned above, the cover portions 9 for the spaces 8 may be constructed in the form of diaphragms. For this purpose the cover portions 9 may be constituted by corrugated metal rings. Immediately oil under pressure flows through the pipes 15, 14 into the spaces 8 there is a simultaneous movement on the part of all of the contacts 10 in the direction of the arrow P, so that the contacts 10 and 11 move together and the circuit is completed. The mechanical movement. of the switch contacts may'naturally also be produced in other fashion.

The connection of the single pockets orchambers .12 may also be performed by the provision of specially shaped passages in the discs 4, which passages, for example, may be inclined, helical or curved in order to increase the path of sliding discharge.

The arrangement described with reference to Fig. 1 may, for example, also be fitted in a special vessel evacuated accordingly. In this case there are provided in lieu of the borings 5 in the metallic discs 4 and insulating rings 7 radial borings of such nature that the vacuum is capable of even distributionover the pockets or chambers 12 between the inner face of the outer vessel and the inner boundary of the rings 4, '1 and 6.

Under certain circumstances it is also possible to specially seal the system according to Fig. 1 situated in an outer vessel, or to pour oil into the annular space, in order to obtain additional secure insulation against the exterior.

The actuating mechanism may be constructed 1 on various lines. The metallic discs may consist of an elastic material, such as steel. At the centre, for example, there may be provided small projections h, and the entire system, since only small differences in length are concerned, may be pressed together. To actuate all sections simultaneously there may be furnished a throughgoing insulating rod, which is firmly connected with all of the metallic discs. In particular constructions it may be of importance to bore holes in the metal discs at certain distances apart so as to form an inclined passage. In this passage there may be moved the actuating lever, and the field distribution is then such that the charge carriers are nevertheless unable to pass freely through.

Experiments have shown that staggered disposal of the borings alone is, in the majority of cases, not sufficient to perform effective divisioning of the total space. such as to preclude possibility of a discharge. Possibly owing to unequal distribution of the field, or'also by reason of capacitative action caused by charging of the insulated surface, reflected or secondary and possibly also direct charge carriers have been found to pass in roundabout fashion through the single systems, resulting in independent discharge. It is, therefore, always preferable, as stated above, to fill out the borings with metal balls, wire fabric or other suitable means.

It will be understood that no restriction is made to the specific forms of embodiment shown, and that various modifications are quite possible within the meaning of the above description and the annexed claims without departing from the spirit of the invention.

What we claim as new and desire to secure by Letters Patent is:

l. A switch, comprising an evacuated vessel, partitions therein defininga plurality of chambers, relatively movable contacts within each chamber, the opposing partitions being spaced a distance less than the mean free path of an electron at the pressure existing within the chambers and being of such extent and so positioned that discharges in vacuum between chambers are prevented, means adapted to connect in series the contacts in the several chambers, and means to actuate the contacts to make and break the circuit through the switch.

2. A vacuum circuit breaker as claimed in claim 1, in which the partitions are spaced by insulating members, the discharge path over the surfaces of the insulating members exceeding the distance between the adjacent partitions.

3. A vacuum circuit breaker as claimed in claim 1, in which portions of the partitions are flexible.

4. A vacuum circuit breaker as claimed in claim 1, in which passages are provided in the partitions for evacuating said chambers and means in said passages permitting the removal of air and preventing the penetration of charge carriers, said last mentioned means including granular conducting material.

5. A switch comprising an evacuated vessel, as in claim 1, in which portions of the partitions are flexible anda contact is carried by each flexiblev portion, and in which the means for actuating the contacts comprises means for admitting fluid under pressure to one side of said flexible portions.

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