US3441800A - Electric circuit breaker comprising parallel-connected interrupters - Google Patents

Description

T. H. LEE

April 29, 1969 ELECTRIC. CIRCUIT BREAKER COMPRISING PARALLEL-CONNECTED INTERRUPTERS Sh t Filed Jan. 12, 1967 PULSE FORMING AND TIMING C/RCU/T REWQQbQ //vv/v TOR. THOMAS H. LEE,

ATTORNEY T. H. LEE ELECTRIC CIRCUIT BREAKER COMPRISING A ril 29, 1969 PARALLEL'CONNECTED INTERRUPTERS Sheet Z of 4' Filed Jan. 12, 1967 INVENTOR." THOMAS H. LEE,

TTORNEY April 29, 1969 'r. H. LEE

ELECTRICCIRCUIT BREAKER COMPRISING PARALLEL CONNECTED I NTEBRUPTERS Sheet Filed Jan 12, 1967 c: a Z Z COUNT/N6 0EV/CE INVENTOR. THOMAS H. LgE,

ATTORNEY April 29, T. H. LEE 3, r,

} ELECTRIC CIRCUIT BREAKER COMPRISING PARALLEL'CONNECTED INTERRUPTERS Filed Jan. 12. 1967 I Sheet .1 of 4 Z I as INVENTOR. THOMAS H. LEE,

A T TORNE Y United States Patent ELECTRIC CIRCUIT nIiEAKER COMPRISING PARALLEL-CONNECTED INTERRUPTERS Thomas H. Lee, Nether Providence, Pa., assignor to 1 General Electric Company, a corporation of New York Filed Jan. 12, 1967, Ser. No. 608,899 Int. Cl. H02h 3/08, 7/00 US. Cl. 31711 12 Claims ABSTRACT OF THE DISCLOSURE Background of the invention This invention relates to an alternating current electric circuit breaker and more particularly to a circuit breaker of this type which comprises a plurality of circuit interrupting devices electrically connected in parallel.

For reducing the amount of interrupting current that an interrupter is required to conduct during circuit interruption, it has been proposed that a plurality of simultaneously operable interrupters connected in parallel be provided. For obtaining a substantially equal distribution of current between these parallel connected interrupters,

it has been proposed that an inductance be connected in series with each interrupter in its individual branch of the parallel circuit. In the case of vacuum-type interrupters, a substantially equal current distribution can be obtained with relatively small inductances in view of the positive arc voltage-to-current characteristics of high-current vacuum arcs.

It is difiicult to synchronize the operation of all the parallel-connected interrupters so that their contacts part at precisely the same instant during opening. But despite this imperfect synchronization, it is usually possible to establish arcs in all the parallel interrupters if the contacts of all the interrupters part within a reasonably short span. I have found, however, that there is one particular situation where imperfect synchronization can prevent concurrently existing arcs from being initiated in all the parallel interrupters. This is the situation in which some of the contacts part just prior to current zero and others part just after current zero. Even if an arc can be establi'shed before current zero across the first set to part, this are will not usually reignite after current zero if the other contacts are still in engagement. As a result, all the current flows through the contacts that are still closed; and when these latter contacts part to establish a new are, all the current flows through this new arc.

Summary An object of the present invention is to assure that arcs will be consistently established in all the parallel-connected interrupters during a circuit breaker-opening operation despite imperfect synchronization of the instants of contact part.

In carrying out my invention in one form, I provide a circuit breaker which comprises parallel-connected inter- 3,441,800 Patented Apr. 29, 1969 rupters and opening means for parting the contacts of the interrupters substantially simultaneously during an opening operation. I also provide means for sensing the current flowing through the circuit breaker and means controlled by the sensing means for preventing the opening means from operating to part the contacts so near current zero in the alternating current through the circuit breaker that contact partings will take place on opposite sides of the current zero.

In a preferred form of the invention, the opening means is forced to part the sets of contacts during the period extending from shortly after current zero to the instant of peak current. Parting during this interval enables gaps of relatively great length to be established between the contatcs of all sets by the time current zero is reached, thereby reducing the likelihood of a reignition across any of the gaps at current zero. In the case of a vacuum-type interrupter by parting during this interval, I can establish substantially the full gap length by the time current zero is reached, assuming that a high speed-opening-operator is used for the interrupters.

If for some unanticipated reason, the interrupters do not properly share the interrupting current during an opening operation and one interrupter carries current exceeding its interrupting capabilities, then I immediately reclose all the interrupters and repeat the opening operation, thus offering the interrupters another opportunity to share the interrupting current. This feature can be used either with or without the trip-timing feature of the two immediately preceding paragraphs.

Brief description of drawings For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

. FIG. 1 is a schematic view of one embodiment of the Invention.

FIG. 1a is a graphic representation showing the intervals during a current wave when the contacts are permitted to part.

FIG. 2 is a schematic view of another embodiment of the invention.

FIG. 3 is a schematic view of still another embodiment of the invention.

FIG. 4 is a schematic view of still another embodiment of the invention.

Description of preferred embodiments Referring now to FIG. 1, there is shown an electric circuit breaker 10 that comprises two circuit interrupting devices 12 and 14 electrically connected in parallel in an alternating current power circuit 15. Each of the circuit interrupting devices is preferably a vacuum-type circuit interrupter. The vacuum-type circuit interrupters can be of a conventional design and are therefore shown only in schematic form. Each of the illustrated vacuum type circuit interrupters comprises a highly evacuated insulating housing 16 and a pair of relatively movable contacts 18 and 20 located within the housing. The contacts are shown in a closed and engaged position. The lower contact 20 is a movable contact which is fixed to the upper end of a vertically movable conductive operating rod 22. A flexible metallic bellows 23 provides a seal around rod 22 that permits vertical movement thereof without impairing the vacuum inside envelope 16. U.=S. Patent 3,089,936, Smith, assigned to the assignee of the present invention, can be referred to for a more detailed disclosure of such a vacuum-type circuit interrupter.

The conductive operating rods 22 of the two interrupters are connected together by a cross bar 24 which causes the rods 22 to move in unison. The cross bar 24 is biased in a downward opening direction by a suitable opening spring 25. A trip latch schematically shown at 26 holds the contacts closed against the downward bias of the opening spring. When the trip latch 26 is released, the opening spring 25 responds by rapidly driving the cross bar 24 and the two operating rods 22 in a downward-opening direction. The mechanical coupling between the two operating rods 22 assures that the contacts 18, 20 in the two interrupters will part substantially simultaneously. I have shown the operating mechanism 22-26 in a simplified schematic form to facilitate an understanding of the invention. In an actual circuit breaker, this operating mechanism is considerably more sophisticated and can include features such as a flux-shifting trip latch to provide for the desired high speed operation. See, for example, US. Patents 2,188,803, Boehne, and 2,740,859, Beatty et al., for examples of such mechanisms.

For tripping the illustrated latch 26, a normally open trip circuit 30 is provided. A normally open high-speed closing switch 32, which may, for example, be a silicon controlled rectifier, is provided for completing the trip circuit 30, when a predetermined pulse signal is delivered to the usual gate 3. of the controlled rectifier. The latch 26 releases the opening spring and causes the contacts of the two interrupters to separate at the expiration of a predetermined fixed period following delivery of the pulse signal to gate 34, e.g., two milliseconds.

In accordance with my invention, I time the application of the pulse signal to gate 34 with respect to the current flowing through power conductor in such a manner that the contacts 18, will be prevented from parting during the period immediately prior to current zero in the current through power circuit 15. This timing of the pulse signal can be accomplished by conventional means such as shown for example in US. Patent 3,265,933, Perry et al., or 3,243,656, Baude, where circuits are disclosed that will supply an output pulse at a desired point on the current wave. Since the details of the pulse-forming and timing circuit are not a part of my invention, I have shown the circuit in block form at 40. A suitable current transformer 42 coupled to power circuit 15 supplies input information to the pulse-forming and timing circuit 40 through an input circuit 43. In the aforesaid Perry et al. and Baude patents, the output pulse signal is applied to a trip-initiating device (such as my silicon controlled rectifier 32) at such an instant that the contacts of the controlled circuit breaker will part precisely at current zero. I, however, use an entirely ditferent approach, setting the circuit 40 so that the signal pulse is applied at such an instant that contact-part occurs during a period considerably ahead of the first current zero following contactpart. An important part of my invention is to prevent contact-part from occurring during the period immediately prior to current zero, and such timing of the pulse assures that this objective will be attained.

To prevent the pulse-forming circuit 40 from supplying an output pulse to the turn-on switch 32 until a predetermined overcurrent occurs, I provide an overcurrent responsive relay 50, shown for simplicity as an electromagnetic relay. This relay 50 has normally open contacts 54 and an operating coil 53 connected across the secondary winding of a current transformer 52 coupled to the power line 15. When the current through power line 15 exceeds a predetermined value, relay 50 picks up to close its normally open contacts 54. Thereafter, the next pulse developed by the control circuit 40 is supplied to gate 34 to turn on the controlled rectifier 32 to cause circuit breaker opening.

It is difficult to precisely synchronize contact-part of the two interrupters; but despite imperfect synchronization, it is usually possible to establish concurrently existing arcs in both the interrupters if the contacts part within a reasonably short time. I have found, however, that there is one particular situation where imperfect synchronization can prevent concurrent arcs from being initiated in both interrupters. This is a situation in which one set of contacts parts just before current zero, and the other parts just after current zero. Parting of the first set of contacts will usually establish an arc therebetween, but this are will be extinguished at current zero. After current zero, the set of contacts still in engagement will carry all the current until they part, and this tends to prevent an are from reigniting across the already parted set of contacts. When the second set of contacts does part, an arc is drawn thereacross; but the gap between the first set of contacts remains intact, thus forcing the second set to carry all the current.

By preventing either of the parallel-connected interrupters from opening during the period immediately prior to current zero, as described hereinabove, I am able to prevent the staggering of contact-partings on opposite sides of current zero and am therefore able to avoid the above-described situation where the total arcing current is carried by only one of the two interrupters. The time span immediately preceding current zero during which I prevent contact-parting is sufficiently long to prevent contact-part at the two sets of contacts on opposite sides of current zero. Accordingly, the contacts of the two sets are consistently forced to part on the same side of a current zero.

To illustrate more specifically the period during which contact part is prevented, reference may be had to FIG. 1a. If the first set of contacts part between instants A and G, then there is a significant probability that imperfect synchronization will result in the second set parting after current zero is reached at G. Thus, I force the first set to part ahead of instant A, thereby assuring that the second set will part prior to the current zero point G.

In a preferred form of my invention, I force the two sets of contacts to part during the approximately cycle interval extending between an instant B shortly after the current zero H and an instant C of pea current. Parting between instant B and C enables gaps of relatively great length to be established between the contacts of both sets by the time current zero is reached at G. This greater length results in greater dielectric strength across the gaps at current zero, and there is thus a substantially reduced likelihood that either of the two gaps will reignite following current zero. When vacuum type interrupters, such as illustrated, are used with an operating mechanism of moderately high speed, substantially the full gap length can be established by the time current zero is reached at G. In a preferred form of the invention, the contacts of the vacuum interrupters are parted sufliciently early between instants B and C that a gap of inch or longer is attained in both interrupters by the time current zero is reached at G. By thus reducing the likelihood of a reignition across either gap immediately following the current zero point G, I can further reduce the possibility that only one of the two parallel connected interrupters will carry the total arcing current.

Although the present circuit breaker control system requires some precision in controlling the instant of contact part, the requirements for precision are not nearly as stringent as they are in those circuit breaker systems that attempt to force the contacts to part precisely a current zero. In comparison to such systems, the present system can tolerate contact-part over a very wide range. It is only during the period just prior to current zer'o that it is important with the present system to prevent contact-part.

Vacuum-type circuit interrupters lend themselves especially Well to parallel operation since vacuum arcs have a positive arc voltage-to-current relationship which forces a substantially equal distribution of current between the parallel vacuum arcs. But the present invention is not limited to circuit breakers that comprise vacuum-type circuit interrupters. It can also be used, for example, in circuit breakers that comprise fluid-blast type circuit interterupters. Although this positive arc voltage-to-current relationship is not present in such interrupters, inductors can be connected in series with each interrupter in its parallel branch to force substantially equal current distribution between the parallel branches. Such inductors are usually desirable even when vacuum-type interrupters are used, but they can be of a considerably smaller size than the inductors needed for nonvacuum type interrupters. Such inductors are shown at 60 in the drawing. Although I have shown the invention applied in FIG. 1 to a single-phase circuit breaker, it also has application to polyphase cricuit breakers. FIG. 2 illustrates one way of applying the invention to a polyphase circuit breaker, shown for simplicity as a two-phase circuit breaker. Here there is provided an assembly for each phase substantially identical to that illustrated in FIG. 1. The added phase is designated a and the same numerals are used for corresponding parts of the assembly 10 in phase 15a as for the assembly in phase 15. The timing circuit 40 for each phase allows the circuit breaker assembly for that particular phase to part its contacts only during a period considerably ahead of current zero in that phase. The overcurrent responsive relays 50 in the respective phases are shown mechanically coupled together so that the contacts 54 associated with all the phases close whenever any one of the overcurrent relays 50 picks up.

Although I have illustrated only two interrupters connected in parallel in a given phase, it is to be understood that a greater number can, if desired, be connected in parallel.

Although I have shown the two parallel pairs of contacts in separate envelopes, it is to be understood that the invention is also applicable to circuit breakers in which the parallel pairs of contacts are in a single envelope.

If for some unanticipated reason, only one of the interrupters should carry the interrupting current, then there is a significant possibility that the circuit breaker will be unable to clear a very high short circuit current. To minimize the possibility of a failure under such circumstances, I provide means for immediately reclosing the circuit breaker should there be an improper sharing of the current which results in a single interrupter carrying an excessive amount of current which is too high for it to interrupt. Such means is illustrated in FIG. 3, where the reclosing device is shown as a high-speed fluid motor 70. This motor 70, which may be of a conventional form, comprises a cylinder 71 and a vertically movable piston 72 shown in FIG. 3 in its lowermost or reset position. A suitable 3-way control valve 73 controls the operation of piston 72. This control valve normally vents the space heneath piston 72, but, upon operation, closes its vent and admits pressurized fluid to the space beneath piston 72 to drive the piston upward. Assuming that the circuit breaker is then open, this drives its operating mechanism 24 upwardly to reengage the contacts and reclose the circuit breaker.

For sensing whether either of the interrupters has carried an excessive share of a high fault current during an attempted interruption, I provide a current-balance relay 75 which is sensitive to the difference between the currents carried by the two interrupters. If this difference is greater than a predetermined value, the current-balance relay 75 (operating through components 93, 94) completes a closure-initiating circuit 80 after a short time that is, however, made long enough for the circuit breaker to reach its fully open position and have an adequate opportunity to interrupt, e.g., one cycle. If at the expiration of this short period, one of the interrupters is still carrying an excessive share of this high current, then relay 75 completes closure-initiating circuit 80, operating valve 73, and

causing reclosing device 70 to immediately reclose the circuit breaker.

To assure that the circuit breaker is fully open and has had an opportunity to interrupt before a reclosing signal can be imparted to the circuit breaker, a suitable b switch 82 is provided in the closure-initiating circuit 80. When the circuit breaker reaches its fully open position, the switch 82 closes; and the circuit 80 can thereafter be completed by the current-balance relay 75 should one of the interrupters be carrying an excessive share of a high current.

A reclosing operation results in the contacts of both interrupters reengaging, and immediately thereafter, a second opening operation is initiated by the operation of trip latch 26. This second opening operation provides another opportunity for the interrupters to share the total interrupting current. If this current is properly shared, then the interruption will be successfully completed, and no further reclosings will occur. The current-balance relay 7-5 will again sense whether the current is properly shared, and if it is, will not again pick up to reinitiate another closing operation.

If the interrupting current is not properly shared during the second opening operation, then the reclosing device will again reclose the circuit breaker. Thereafter, further opening operations are prevented by a suitably controlled lock-in switch 83 that permanently opens the trip-controlling circuit 34 until deliberately reset. The lock-in switch 83 is opened by a suitable counting device (shown in block form at 85) that responds to closely successive operations of the circuit breaker to open switch 83 after a predetermined number of such closely succes sive operations. If fewer operations occur in close succession, the counting device 85 will reset to its normal condition and will not open trip-controlling circuit 34. Patent 3,114,079, Sofianek et al., assigned to the assignee of the present invention, may be referred to for an example of a suitable counting or sequencing device usable for this purpose.

The current-balance relay can be of a conventional type and has been shown in schematic form as comprising a core 89 linked to each of the parallel branches through a pair of windings 90. These windings 90 are wound to produce flux in opposite directions in the core 89. When the currents through windings 90 are equal, no net flux is conducted around the core 89. But when the currents are unequal, flux proportional to the difference in current is developed in core 89. A second winding 92 is coupled to core 89 and develops a voltage across its output terminals proportional to the flux developed. This voltage is used for energizing the operating coil 93 of a relay, which responds to voltage above a predetermined value to close its normally open contacts 94 after a suitable short time delay.

A modification of the circuit breaker of FIG. 3 is shown in FIG. 4. Here, reclosing is initiated by overcurrent relays 96 and '97 which respond to the magnitude of the current through either interrupter, rather than the difference in currents. If the interrupting current through either interrupter exceeds a predetermined value and the interrupter is open, the closure-initiating circuit will be completed through [2 switch 82 and the contacts 94 of the appropriate overcurrent relay to initiate a circuit breaker reclosing operation. The relays 96 and 97 are coupled to the respective parallel branches of the power circuit by means of current transformers 98 and 99.

The reclosing arrangements of FIG. 3 or 4 can be used either with the trip-timing arrangement of FIG. 1 or with a conventional tripping arrangement that trips at any point on the current wave.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An alternating current electric circuit breaker comprising:

(a) a pair of circuit interrupters connected in parallel circuit relationship and adapted to be connected in one phase of an alternating current circuit,

(b) each circuit interrupter comprising a pair of contacts relatively movable between an engaged closed position and a disengaged open position,

(c) opening means for parting the contacts of said pair of interrupters substantially simultaneously,

((1) means for sensing the current flowing through said one phase,

(e and means controlled 'by said sensing means for consistently preventing the opening means for operating to part said pairs of contacts during the period immediately prior to current zero in the alternating current through said one phase.

2. An alternating current electric circuit breaker comprising:

(a) a pair of circuit interrupters connected in parallel circuit relationship and adapted to be connected in one phase of an alternating current circuit,

(b) each circuit interrupter comprising a pair of contacts relatively movable between an engaged closed position and a disengaged open position,

(c) opening for parting the contacts of said pair of interrupters substantially simultaneously,

(d) means for sensing the current flowing through said one phase,

(e) and means controlled by said sensing means for consistently initiating operation of said opening means at a time which will result in both pairs of said contacts parting on the same side of any current zero in the alternating current through said phase.

3.'The circuit breaker of claim 2 in which said circuit interrupters are vacuum type circuit interrupters.

4. The circuit breaker of claim 2 in which said circuit interrupters are fluid blast type circuit interrupters.

5. The circuit breaker of claim 2 in which operation of said opening means is consistently initiated at a time which results in both pairs of contacts parting during the approximately A cycle period extending from shortly after a current zero to the instant of peak current.

6. The circuit breaker of claim 2 in which:

(a) said circuit interrupters are vacuum-type interrupters,

(b) operation of said opening means is consistently initiated at a time which results in both pairs of contacts parting sufficiently ahead of current zero to produce gaps of inch or longer by the time the first current zero is reached following contact-part.

7. The circuit breaker of claim 6 in which substantially the full gap length is established at both contact pairs by the time the first current zero is reached.

8. An alternating current electric circuit breaker comprising:

(a) a pair of circuit interrupters connected in parallel circuit relationship and adapted to be connected in one phase of an alternating current circuit,

(h) each circuit interrupter comprising a pair of contacts relatively movable between an engaged closed position and a disengaged open position,

(c) opening means operable in response to an overcurrent in said one phase for parting the contacts of said pair of interrupters substantially simultaneously,

(d) closing control means for sensing whether said interrupters have properly shared said overcurrent during an attempted high current interruption and for developing a predetermined closing signal in response to improper sharing of the high current,

(e) reclosing means operable in response to said clos ing signal for immediately returning the contacts of said pair of interrupters to their engaged position upon receiving said closing signal, thereby permitting another opening operation in the event said overcurrent condition still prevails.

9. The circuit breaker of claim 8 in which said closing control means senses the difference in the currents passing through said two interrupters during an attempted interruption and develops said closing signal in response to this difference exceeding a predetermined value.

10. The circuit interrupters of claim '8 in which said closing control means senses the magnitude of the current passing through either interrupter and develops said closing signal in response to the magnitude of the current through either interrupter exceeding a predetermined value.

11. The circuit breaker of claim 8 in combination with lock-in means for preventing further opening operations in the event of a predetermined number of closely successive circuit breaker operations.

12. The circuit breaker of claim 2 in which:

(a) said opening means is operable in response to an overcurrent in said one phase,

(b) closing control means is provided for sensing whether said interrupters have properly shared said overcurrent during an attempted high current interruption and for developing a predetermined closing signal in response to improper sharing of the high current,

(0) and reclosing means responsive to said closing signal is provided for immediately returning the contacts of said pair of interrupters to their engaged position upon receiving said closing signal, thereby permitting another opening operation in the event said overcurrent condition still prevails.

References Cited UNITED STATES PATENTS 2/ 1966 Coburn 317-11 8/1966 Perry et a1. 317-11 12/1966 Induni 3l7-11 6/ 1968 Greenwood 317--1l US. Cl. X.R.

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