US2882372A

US2882372A - Electrical switchgear for use in three-phase a.c. systems

Info

Publication number: US2882372A
Application number: US744405A
Authority: US
Inventors: Preissler Karl
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-06-28
Filing date: 1958-06-25
Publication date: 1959-04-14
Anticipated expiration: 1976-04-14
Also published as: BE568871A; GB858385A; DE1051357B

Description

April 14, 1959 K. PREISSLER 2,382,372

ELECTRICAL SWITCHGEAR FOR USE IN THREE-PHASE A. c. SYSTEMS Filed June 25, 1958 2 Sheets-Sheet 1 R s T L 1| 2| a Fig-7 w ul. Z 09.2

INVENTOR. h 4 IOAZ/JJLEK MJFLMM April 14, 1959 K. PREISSLER 2,882,372

ELECTRICAL SWITCHGEAR FOR USE vm THREE-PHASE A. c. SYSTEMS Filed June 25, 1958 2 Sheets-Sheet 2 INVENTOR. 26m; PM? as A /2 BY .ZW: MM

United States Patent ELECTRICAL SWITCHGEAR FOR USE IN THREE-PHASE A.C. SYSTEMS Karl Preissler, Vienna, Austria Application June 25, 1958, Serial No. 744,405

Claims priority, application Austria June 28, 1957 4 Claims. (Cl. 200-145) This invention is concerned with the problem of increasing the breaking power of a switch for three-phase A.C. either in order to reduce the dimensions of the switch or, for equal dimensions, to increase the life of the switch.

When an inductive alternating current is interrupted by a switch, the are formed in the circuit by the separation of contacts extinguishes at the natural Zero of the current and is not ignited thereafter.

The following can be shown mathematically for the interruption of a three-phase A.C.:

If the break contacts associated with the several phases do not open exactly at the same time, as is always the case owing to manufacturing inaccuracies in switchgear, the breaking power applied to each contact or, in the case of double breaks, to the pair of contacts which has opened first is equal to the total breaking power applied to both subsequent contacts or pairs of contacts. This means that the breaking power applied to the contact switching first is twice the breaking power applied to each of the following contacts. In other words: The half of the total breaking power is applied to the contact switching first Whereas only one fourth of the breaking power is applied to each of the two other contacts load. This applies to staras well as delta-connected con- 'sumers.

The switch rating of a three-phase A.C. switch is expressed by the rated current, the rated voltage and the smallest phase angle (,0 at which the switching is still properly performed. When in a three-phase A.C. switch half the total breaking power is applied to one pair of contacts whereas one fourth of this breaking power is applied to the other two pairs of contacts and all three pairs of contacts are of equal construction, two pairs of contacts will always be underloaded in the case of double breaking. For this reason each contact must be designed in practice for that extra rupturing capacity which will be required when it switches first whereas this extra capacity will not be utilized when it switches as a subsequent contact.

Because it is desired to obtain the highest possible rupturing capacity it has been proposed to increase the number of breaks. For instance, it is known that a doubling of the number of breaks in each phase will double the switch rating. The expenditure is considerable because twice the number of breaks are required.

It has also been proposed to interrupt three-phase alternating currents by breaking one phase first and breaking the two other phases subsequently and simultaneously. It has also been proposed to break the two remaining phases only when the current flow in the first phase has ceased entirely. These arrangements require special controls so that they do not lead to appreciable advantages in the construction of switchgear.

Another measure, which enables structural savings, resides in that an additional break in series with the existing one is provided only in one phase. If it is assumed 2,882,372 Patented Apr. 14, 1959 in that case that all four contacts are opened exactly at the same time, the additional contact point will cause the breaking to be effected more easily and quickly as when only one break was provided in this phase. As a result, this phase will be automatically the one that switches first and half the breaking power will be distributed to two breaks instead of one, as before. However, this construction is subject to the following objections:

The assumption that exactly half the breaking power will be applied to the phase having the two series breaks is not reliable because it depends on an exactly simultaneous opening of all breaks and on the presentation of the first current zero to the phase with double breaks. This will not always be the case, of course. Moreover, three arcs are established simultaneously for a short time and the current may be interrupted first in another phase having only one contact. For this reason the switch rating is improved only on a statistical basis, which is not exactly defined.

In a switchgear, e.g., in a cam-operated switch having two breaks in each conductor, which switchgear is intended for use in a three-phase A.C. system and which has in one phase twice as many breaks as in each of the other phases, the present invention resides in that the actuation of the breaks in the phase which has twice or approximately twice the number of breaks is positively effected before the interruption of the breaks in the other phases. For instance, in cam-operated switches, in which contact bridges are provided which are controlled by coupled cams and break each phase at two points, the advance performance of the switching at the four breaks provided in the preferred phase (resulting from the doubling of the existing double break) can be ensured by causing the controlling face of the cam or cams controlling the fourfold break to lead the control faces of the cam or cams controlling the other breaks. This enables the invention to be embodied in a very simple construction, as will be shown hereinafter. Proof that advantages are afforded even if the number of breaks in the preferred phase is only approximately doubled will be given at the end of this specification.

The invention is diagrammatically explained in Fig. 1 of the accompanying drawings, in Figs. 2 to 5 of which some practical applications of the invention are shown. Figs. 2 and 3 show a three-phase high-voltage switch in elevation and plan, respectively. Fig. 4 shows a detail of a cam-operated switch according to the invention and Fig. 5 shows the application of the invention to electrical switchgear constructed as a contactor.

In Fig. 1 the three phases R, S, T of a three-phase A.C. system are apparent, which has a consumer Z connected thereto, which represents inductive (wL) and ohmic (W) loads on the system. In addition to the three doublebreak contact bridges 1, 2, 3 an additional double break contact bridge 4 is inserted in any desired phase, e.g., R. As contrasted to former proposals, which were satisfied with causing a more or less simultaneous operation of all bridges 1-4, the bridges 1 and 4 are intentionally and positively opened before the bridges 2 and 3. As a result, the bridges 1 and 4 will be positively and not only incidentally those that switch first and to which, therefore, half the total breaking power with respect to the consumer Z to be disconnected is applied. In this case one fourth of the breaking power is applied to each of the bridges 2 and 3.

The second half of the breaking power is applied to the two bridges 2, 3, which switch after 1, 4, and this second half is divided so that one fourth thereof is applied to each break. The time between the beginning of the opening of the bridges 1 and 4 in phase R and the beginning of the opening of the bridges 2, 3 in phases S and T must be such, as is known per se, that the bridges of phase R breaking first have completely interrupted the current in that phase before the other contact bridges 2, 3 begin their switching movement. This time will have to be about 23 periods. This mode of switching will hereinafter be referred to as preferential multibreak arrangement.

Because exactly one fourth of the total breaking power of the circuit is applied to each contact or contact pair of the switch, the contacts need not be designed for a reserve rupturing capacity which would be utilized only in one contact during the switching. With the preferential multibreak arrangement each contact is utilized to the same degree as all other switching points and the reserves which remained previously unutilized can be used entirely for increasing the switch rating of the equipment.

In a normal three-pole switch of previous design, which would be just suflicient to disconnect a load N the switch rating can be increased to 2N i.e., it can be doubled, if an additional similar break is provided in one phase, as is known per se, and if it is ensured according to the invention that this phase will be opened in advance of the others, the switch rating in the phase opening first can be increased to 2N or doubled, which represents an important advance, which is achieved with relatively small structural expenditure, as will be shown hereinafter.

The principle of construction according to the inven tion is of general application if the number of breaks in the preferred phase switching first equals the total number of breaks in the two other phases. Thus, if a switch had normally four breaks in each phase, the invention would require eight breaks in one phase and a positive opening of these contacts first in order to double the switch rating.

The principle of the preferred multi-break arrangement can be applied to high and low voltages, in three-pole circuit breakers, change-over switches, star-delta switches, reversing switches and the like and enables an increase of the switch rating also in electrical contactors (switching relays).

Figs. 2, 3 show the application of the invention to a high-voltage switch for a three-phase conductor system in elevation and plan, respectively. A base plate carries to switch

columns

11, 12, 13, which may contain known switchgear, such as gas-blast switches. The column 14 represents the additional break (e.g., also a switch of known type), which is preferentially operated and lies, e.g., in phase R. The actuating line 15 which releases or causes the switch operation includes a retarding device 16 in order to actuate the two switches in the

switch columns

11, 14 before those in the

switch columns

12, 13. This line 15 may consist of a linkage or a pressure fluid line.

Fig. 4 illustrates the application of the invention to a three-phase cam-operated switch, which may have a double-break contact bridge per pair although only the stage which effects the fourfold break (corresponding to contact bridges I, 4) is shown. The switch as a whole may be of a construction known per se. An angularsection shaft 20 carries in addition to the cams associated with the contact bridges 2 and 3 of Fig. 1 a cam 21 with two edges 25, which control the lifting movement of the plungers 22 and of the contact bridges 24 connected thereto. In known manner the cams act by means of rollers 26 on the plungers sliding in guides 27, which are lifted against the action of compression springs 28 supported in the switch casing (not shown) to open the contacts 30. The two bridges 24 are connected in series by a conductor 31. Because it is assumed that the switch is opened by a rotation of the angular section shaft 20 in the sense of the arrow, the control edges 33 of the cams associated with the bridges other than 24 must lag behind the control edges in order to ensure that the bridges 24 open before the other bridges. Thus, it is apparent that in switches of the construction shown in Fig. 4 (for the construction of such switches see also the Austrian patent specification No. 173,497) the invention can be embodied simply by providing one of the guides 27 which has previously been left free in three-phase switches with a fourth bridge 24 and the control cam for this pair of bridges is slightly staggered relative to the other cams. This refers to the construction shown in said Austrian patent specification in which the contact bridges are symmetrical with respect to a central plane which includes the switch axis. Thus, the four double-break contact bridges required are arranged in pairs on opposite sides of the central plane.

The contactor shown in Fig. 5 has a movable contact bridge 40 with

resilient contacts

41, 42, 43 and 44. The countercontacts are shown at 41, 42', 43 and 44'. The contacts 43' and 44 are connected in series by a series connector 45 (corresponding to 31 in Fig. 4). The two contact sets contact each other when the circuit is closed. When the contact bridge 40 is upwardly displaced along the guide columns 46 by the magnetic system (not shown) in order to break the circuit, the

contacts

43 and 44 will open first because they are intentionally set back by an appropriate distance a relative to the

contacts

41 and 42. The dimensioning of the distance a or of the angle of lead of cam-operated switches is-not critical.

In the examples shown the bridge which was opened first is also closed first when the switches are closed. As can be shown mathematically this is of no consequence, particularly also because in electrical switchgear electrical problems occur mainly in interrupting rather than in establishing a flow of current.

It has been stated hereinbefore that the desired distribution of the breaking power to the two groups of contacts opening with a time interval if one phase contains twice as many contacts as any of the remaining phases. This requirement need not be exactly complied with. For instance, in the case of 15 breaks in one phase and 8 breaks in each of the other phases, i.e., if one phase contains only approximately twice the number of breaks of each of the other phases, a substantial improvement in the contact utilization will also be obtained. This is essential if the construction of the switch in view of the switching program for which it is designed permits of accommodating of, e.g., 15 breaks in one phase whereas the accommodation of a 16th break would require the provision of an additional contact unit only for this one contact. The breaking power applied to a preferentially opened contact is 5' 1 wherein n is the number of contacts points in the preferentially opened phase. In View of the above the breaking power applied to the breaks in the remaining phases will be wherein n is the number of contact points in each of these phases. Ideal conditions are obtained for 2n =n if in differs from 2,, the utilization of the contacts will deteriorate in proportion to this difference. The effect of this deterioration, however, will be relatively smaller if I1 and 11 are relatively large numbers. Applied to the above example, the optimum contact utilization of 162l6=1 would be deteriorated to 16:15=1.065, which means an overloading of the contacts in the preferentially opening phase by about 6.5% compared to those in any other phase. As contrasted with earlier constructions the advantage is still retained that such differences in the utilization of the contacts will always occur in one and. the same phase, which in the present case is preferentially opened.

Because the present invention is applicable to any elec- .trical switchgear which controls a three-conductor A.C.

.5 system, the invention can be embodied in numerous forms. As a rule, the basic construction of the switchgear will not be changed and an additional switch unit of an existing or contemplated type is added to the device and provision is made for this additional switch unit to break reliably before the other switch units in the device.

I claim:

1. Electrical switchgear for a three-phase A.C. system, which comprises three sets of contacts, each of which sets is associated with one phase and forms a number of breaks in said phase, one of said sets forming a number of breaks which is at least approximately twice the number of breaks formed by each of the others of said sets, and means for operating said contacts in such a manner that the breaks of said one set are opened positively in advance of the breaks of said other sets.

2. Electrical switchgear as set forth in claim 1, in which said one set comprises at least four breaks and each of said other sets comprises at least two breaks and in which said means for operating said contacts comprise a plurality of contact bridges each of which carries one of said contacts of two breaks of one of said sets, said contact bridges being operable to open said breaks associated therewith, and a plurality of cams, at least one of which is associated with the contact bridges associated with each of said sets, each of said cams having a working portion adapted to operate each of said contact bridges with which said cam is associated upon a predetermined movement of said cam, said cams being coupled for joint movement and said working portions of said cams associated with said contact bridges of said one set leading the working portion of the others of said cams in the direction of said predetermined movement to cause said breaks of said one set to be opened in advance of said breaks of said other sets.

3. Electrical switchgear as set forth in claim 2, in which said contact bridges are arranged in pairs on opposite sides of and symmetrically with respect to the center plane of said switchgear.

4. Electrical switchgear as set forth in claim 1, in which said means for operating said contacts comprise a contact bridge carrying one of said contacts of each of said breaks and electrically operable means for moving said contact bridge to open said breaks, said contacts of said one set being resilient and arranged so that the breaks formed thereby open after a smaller movement of said contact bridge that the breaks formed by the contacts of said other sets.

Italy Sept. 17, 1953 Germany Sept. 5, 1957