Vacuum circuit breaker

Abstract: A vacuum circuit breaker (1 in Fig. 1) comprising an additional coil (12) which is arranged outside a vacuum interrupter (2), and a power supply circuit (parts 13, 14 and 16) which causes current to flow through the additional coil (12). An overcurrent flowing through the main circuit of the circuit breaker (1), which leads to a load (11), is detected by an overcurrent tripping device (7) so as to separate the movable electrode (2b) of the vacuum interrupter (2) from the stationary electrode (2a) thereof and to subject a striking electric arc to a magnetic flux generated axially of the vacuum interrupter (2) by the electrodes (2a, 2b) themselves. Thereafter, a trigger gap (8) is ignited by a signal delivered from the overcurrent tripping device (7), so as to introduce a reverse current into the vacuum interrupter (2). Herein, the trigger gap (16) is ignited by a signal delivered from the overcurrent tripping device (7), in order that the zero point of currents may be formed at the point of time at which the axial magnetic flux has been canceled by the resulting current of the additional coil (12). Since the axial magnetic flux remaining at the zero point of the currents is nullified, charged particles are not hindered from diffusing radially of the vacuum interrupter (2), and hence, an enhanced breaking performance is attained.

Description: 1.FIELD OF THE INVENTION The present invention relates to a vacuum circuit breaker which employs a vacuum interrupter. The vacuum circuit breaker cuts off a current having flowed through a circuit in excess of a prescribed value, so as to protect the circuit. 2.DESCRIPTION OF THE RELATED ART A vacuum circuit breaker recovers the electrical insulation between its main electrodes at the zero point of current and cuts off the current, thereby protecting a circuit from any overcurrent. As known from EP-A- 0 411 663, Fig. 8 of the accompanying drawings illustrates the circuit arrangement of a DC (direct-current) vacuum circuit breaker (also termed "DC circuit breaker") having hitherto been conventional, while Fig. 9 illustrates the operating principles of the DC circuit breaker. Referring to Fig. 8, the DC circuit breaker 1 is constructed of a vacuum interrupter 2, a commutating capacitor 5, a commutating reactor 6, a trigger gap 8, an electromagnetic repulsion coil 3, a short-circuit ring 4, an overcurrent tripping device 7, and a zinc-oxide (ZnO) non-linear resistance element 9. In the prior-art circuit breaker 1 constructed as stated above, the commutating capacitor 5 is previously charged by a charging device in such a polarity that stored charges become negatives on the side of a DC power source 10 and positive on the side of a load 11 as shown in Fig. 8. When an overcurrent I 0 has flowed through the main circuit of the circuit breaker 1, it is detected by the overcurrent tripping device 7. Simultaneously with the detection, the overcurrent tripping device 7 generates a signal by which the electromagnetic repulsion coil 3 is excited to induce an electromagnetic repulsive force between it and the short-circuit ring 4. At a time t 0 indicated in Fig. 9, the movable electrode 2b of the vacuum interrupter 2 parts or separates from the fixed electrode 2a thereof, and an electric arc strikes across the movable electrode 2b and the fixed electrode 2a. On this occasion, the electric arc undergoes an axial magnetic flux (shown in Fig. 9) generated axially of the vacuum interrupter 2 by the fixed electrode 2a (constituting first magnetic flux generation means, and being a contact) and the movable electrode 2b (constituting the second magnetic flux generation means, and being another contact) themselves. The electric arc is therefore kept stable across both the electrodes 2a and 2b. At a time t 2 after the opening of the vacuum interrupter 2, the trigger gap 8 is ignited or sparked by a signal which is delivered from the overcurrent tripping device 7. Then, a closed circuit extending along the commutating capacitor 5 - commutating reactor 6 - trigger gap 8 - vacuum interrupter 2 is established. Thus, the charges stored in the commutating capacitor 5 beforehand are discharged, and a reverse current I C1 flows in a direction reverse to that of the current of the main circuit of the circuit breaker 1. Owing to the reverse current I C1 , a current (I 0 + I C1 ) flowing through the vacuum interrupter 2 reaches the zero point of the currents at a time t 3 . Then, the electric arc in the vacuum interrupter 2 is extinguished, and the main circuit current is commutated to a circuit path consisting of the commutating capacitor 5 - commutating reactor 6 - trigger gap 8. Consequently, energy having been stored in the inductance of the load (11) side changes into energy for charging the commutating capacitor 5, so that the terminal voltage of the commutating capacitor 5 rises. When the terminal voltage has reached the operating voltage of the ZnO non-linear resistance element 9, this non-linear resistor 9 conducts to discharge the stored charges of the commutating capacitor 5. Then, the breaking operation of the circuit breaker 1 is completed. With the prior-art technique, as illustrated in Fig. 9, the attenuation rate of the axial magnetic flux  0 generated between the electrodes 2a and 2b by these electrodes themselves on the basis of the overcurrent I 0 flowing through the main circuit is low with respect to the period of the reverse current I C1 which begins to be introduced at the time t 2 . As indicated at symbol  0 ', therefore, a flux  r remains even at that zero point of the sum current (I 0 + I C1 ) of the main circuit which is developed at the time t 3 by the reverse current I C1 . On account of the residual flux  r , charged particles existing between the electrodes 2a and 2b are hindered from diffusing radially of the vacuum interrupter 2 at the current zero point at the time t 3 , and a recovery rate for the insulation between these electrodes lowers. As a result, the electrodes 2a and 2b fail to withstand a transient recovery voltage, and they strike an electric arc again. Thus, the breaking performance of the circuit breaker 1 is suppressed disadvantageously. From US-3,372,258 is known a vacuum circuit interrupter having a first and second electrode for switching an electric circuit by moving one of said electrodes, thus defining a primary arc gap. These electrodes are surrounded by a coil electrode comprising a pair of series-connected coils wound in opposite directions so that the magnetic field produced by the two coils oppose each other. The first electrode is mainly influenced by the magnetic field of the first coil and the second electrode is mainly influenced by the opposite magnetic field of coil. Both coils are coil electrodes connected to the respective electrodes. Their magnetic field has a high flux density when the current to the contact electrodes is high. The vacuum circuit interrupter of this document further comprises an auxiliary electrode connected to the first electrode surrounding the second electrode and defining a second arc gap. Opening of the interrupter initiates an arc across the primary gap which is transferred to the second arc gap by the force of the magnetic field generated by the coils and because of the increasing distance between the electrodes. It is noted, that the magnetic field is transversal to the arc between the two electrodes. The transferred arc between the movable electrode and the surrounding electrode is subjected to an intense magnetic field substantially parallel to the axis of the arc, thereby reducing the arc voltage. The arc continues to burn in the gap until a natural current zero is reached, at which time the gap recovers its dielectric strength and prevents re-ignition of the arc. SUMMARY OF THE INVENTION An object of the present invention is to eliminate the disadvantage of the prior art stated above, and to provide a vacuum circuit breaker which exhibits a high breaking performance between electrodes (or contacts). In order to accomplish the object, the present invention proposes a vacuum circuit breaker comprising a pair of contacts for switching an electric circuit, a coil electrode for generating a first magnetic flux in response to a current through said contacts, a further coil for generating a second magnetic flux so as to cancel said first magnetic flux in absence of a current through said contacts, said coil having its own power supply circuit for supplying current to flow through said coil. The magnetic flux generated between the contacts is canceled in advance of the zero point of the current between these contacts, whereby charged particles existing between these contacts are not hindered from diffusing at the current zero point. Consequently, the characteristic of dielectric recovery after the interruption of the current can be enhanced to improve the breaking performance of the vacuum circuit breaker. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing the first embodiment of the present invention; Fig. 2 is a diagram for explaining the operating principles of the first embodiment of the present invention; Fig. 3 is a circuit diagram showing the second embodiment of the present invention; Fig. 4 is a circuit diagram showing the third embodiment of the present invention; Fig. 5 is a circuit diagram showing the fourth embodiment of the present invention; Fig. 6 is a circuit diagram showing the fifth embodiment of the present invention; Fig. 7 is a circuit diagram showing the sixth embodiment of the present invention; Fig. 8 is a circuit diagram of a DC (direct-current) circuit breaker in the prior art; and Fig. 9 is a diagram for explaining the operating principles of the prior-art DC circuit breaker. PREFERRED EMBODIMENTS OF THE INVENTION According to the vacuum circuit breaker of the present invention, a coil (an external coil or second magnetic flux generation means) is disposed outside a vacuum interrupter in such a manner as to surround the main electrodes (or contacts) of the vacuum interrupter, and current is conducted to the coil in time with the introduction of a reverse current (reverse in direction to a main current which flows through a main circuit including the main electrodes) into the vacuum interrupter. In this regard, the structure of the coil and the value of the current to be conducted to the coil are properly set so as to cancel or nullify the residual magnetic flux between the main electrodes at the zero point of the main current. As a result, charged particles existing between the main electrodes are not hindered from diffusing radially of the vacuum interrupter at the current zero point. Therefore, the rate of dielectric recovery between the main electrodes is not suppressed, and the breaking performance of the vacuum circuit breaker can be enhanced. Now, the preferred embodiments of the present invention will be described in conjunction with the accompanying drawings. Fig. 1 is a circuit diagram showing one embodiment of the present invention, while Fig. 2 is a diagram for explaining the operating principles of the embodiment shown in Fig. 1. Referring to Fig. 1, a DC (direct-current) circuit breaker 1 is constructed having a vacuum interrupter 2, a commutating capacitor 5, a commutating reactor 6, a trigger gap 8, an electromagnetic repulsion coil 3, a short-circuit ring 4, an overcurrent tripping device 7, and a zinc-oxide (ZnO) non-linear resistance element 9. It comprises an external coil 12 which is arranged outside the vacuum interrupter 2 in order to cancel a residual magnetic flux, and a capacitor 13, a reactor 14 and a trigger gap 16 which constitute a power supply circuit for conducting current to the external coil 12. Numeral 10 designates a DC power source, and numeral 11 a load. The circuit breaker 1 thus constructed operates as explained below, reference being made also to Fig. 2. The commutating capacitor 5 is previously charged by an unshown charging device so as to store charges in an illustrated polarity. When an overcurrent I 0 flows through the main circuit of the circuit breaker 1 (including main electrodes 2a and 2b), it is detected by the overcurrent tripping device 7. Simultaneously with the detection, the overcurrent tripping device 7 generates a signal by which the electromagnetic repulsion coil 3 is excited to induce an electromagnetic repulsive force between it and the short-circuit ring 4. At a time t 0 , the movable electrode 2b of the vacuum interrupter 2 parts or separates from the fixed electrode 2a thereof, and an electric arc strikes across the movable electrode 2b and the fixed electrode 2a. On this occasion, the electric arc undergoes an axial magnetic flux generated axially of the vacuum interrupter 2 by coil electrodes arranged at the back of the fixed electrode 2a and the movable electrode 2b. The electric arc is therefore kept stable across both the electrodes 2a and 2b. At a time t 1 after the opening of the vacuum interrupter 2, the trigger gap 16 is ignited or sparked (i. e., is electrically closed by arcing) by a signal which is delivered from the overcurrent tripping device 7. Then, a closed circuit extending through the capacitor 13 - reactor 14 - trigger gap 16 - external coil 12 constituting the power supply circuit of the external coil 12 is established. Thus, charges stored in the power supply capacitor 13 are discharged, and a current I C2 flows through the external coil 12. Owing to this current I C2 , an axial magnetic flux  C2 opposite in polarity to the axial magnetic flux  0 generated by the main electrodes 2a, 2b themselves is applied between these electrodes. Herein, the trigger gap 8 is ignited or sparked (i. e., is electrically closed by arcing) by a signal which is delivered from the overcurrent tripping device 7 at a time t 2 , in order that a sum current (I 0 + I C1 ) flowing through the vacuum interrupter 2 may form the zero point of currents at a time t 3 at which the sum axial magnetic flux between the main electrodes 2a, 2b becomes sufficiently low. Then, a closed circuit extending along the commutating capacitor 5 - commutating reactor 6 - trigger gap 8 - vacuum interrupter 2 is established. Thus, the charges stored in the commutating capacitor 5 beforehand are discharged, and a reverse current I C1 flows in a direction reverse to that of the current of the main circuit of the circuit breaker 1. Owing to the reverse current I C1 , the current (I 0 + I C1 ) flowing through the vacuum interrupter 2 reaches the zero point at the time t 3 . Then, the electric arc in the vacuum interrupter 2 is extinguished. At this time, the axial magnetic flux between the main electrodes 2a, 2b as denoted by ( 0 +  C2 ) is suppressed to a sufficiently low level, and charged particles existing between these electrodes are not hindered from diffusing radially of the vacuum interrupter 2. Therefore, the circuit breaker 1 demonstrates a favorable dielectric recovery characteristic. After the main circuit current has been cut off, it is commutated to a circuit path consisting of the commutating capacitor 5 - commutating reactor 6 - trigger gap 8. Consequently, energy having been stored in the inductance of the load (11) side changes into energy for charging the commutating capacitor 5, so that the terminal voltage of the commutating capacitor 5 rises. When the terminal voltage has reached the operating voltage of the ZnO non-linear resistance element 9, this resistor 9 conducts to discharge the stored charges of the commutating capacitor 5. Then, the breaking operation of the circuit breaker 1 is completed. As stated above, the axial magnetic flux between the main electrodes is canceled before the introduction of the reverse current, whereby the dielectric recovery characteristic after the interruption of the current can be enhanced to improve the breaking performance of the vacuum circuit breaker. In Figs. 3 thru 7 to be referred to below, constituents identical or corresponding to those in Fig. 1 are respectively denoted by the same symbols as in Fig. 1, and they shall not be repeatedly explained. Fig. 3 is a circuit diagram showing the second embodiment of the present invention. This embodiment consists in that the reactor (14 in Fig. 1) in the power supply circuit of the external coil 12 is dispensed with by appropriately setting the inductance of the external coil 12. Since the number of parts is reduced, the circuit breaker 1 of this embodiment can have its cost curtailed and its reliability heightened. Even with this embodiment, a function and an effect similar to those of the embodiment shown in Fig. 1 can be attained. Fig. 4 is a circuit diagram showing the third embodiment of the present invention. This embodiment consists in that the power supply circuit of the external coil 12 is constituted by the capacitor 13, a resistor 15 and the trigger gap 16. In this embodiment, the semi-steady part of the current I C2 to be conducted to the external coil 12 can be set longer than in the embodiment shown in Fig. 1 or Fig. 3. Therefore, the circuit breaker 1 of this embodiment has the feature that the resultant magnetic flux ( 0 +  C2 ) in the axial direction of the vacuum interrupter 2 can be nullified in semi-steady fashion for a longer time period. Fig. 5 is a circuit diagram showing the fourth embodiment of the present invention. In this embodiment, the power supply circuit of the external coil 12 is constituted by the trigger gap 16 and a π (pi) network in which capacitors 13a ∼ 13d and reactors 14a ∼ 14d are connected. Thus, this embodiment has the feature that the semi-steady part of the current I C2 to flow through the external coil 12 can be made still longer than in the embodiment of Fig. 4, so the time t 2 at which the reverse current I C1 is introduced into the vacuum interrupter 2 as illustrated in Fig. 2 can be set more freely. Fig. 6 is a circuit diagram showing the fifth embodiment of the present invention. This embodiment is an example in which the current I C2 to be conducted to the external coil 12 is fed through feed terminals 61 from a DC power source 17 disposed outside the circuit breaker 1. The circuit breaker 1 in this embodiment has the feature of a curtailed cost because the capacitor (13 or the like) for feeding the current I C2 to the external coil 12 need not be included within the circuit breaker 1. In this embodiment, the supply voltage of the external coil 12 is low, and the current I C2 to flow therethrough does not have a zero point naturally, so that the current is controlled by a switch 18. This embodiment, per se, consists in locating the power supply of the external coil 12 outside the vacuum circuit breaker 1. Alternatively, the external coil 12 may well be fed with the current I C2 from the power supply of the electromagnetic repulsion coil 3 or the power supply for the commutating circuit (at the numerals 5, 6 and 8) while the phase of the power supply is being controlled. With this measure, the cost of the circuit breaker 1 can be further curtailed. Fig. 7 is a circuit diagram showing the sixth embodiment of the present invention. Numeral 19 designates an AC (alternating-current) power source. This embodiment is an example in which an AC circuit breaker adopts residual-magnetic-field cancellation means configured of the external coil 12, and the capacitor 5, reactor 6 and gap switch 8 constituting the power supply of the coil 12. In the case of alternating current, the rate of change thereof at a current zero point is proportional to the magnitude thereof. In cutting off a large current, therefore, the problem of a residual magnetic flux is posed by the same phenomenon as in the DC circuit breaker. Accordingly, when the axial magnetic flux between the electrodes 2a and 2b is canceled in advance of the current zero point, a favorable dielectric recovery characteristic can be attained to enhance the breaking performance of the circuit breaker 1. As thus far described, according to the present invention, the axial magnetic flux between the main electrodes is canceled in advance of the zero point of the current between these electrodes, whereby the charged particles existing between these electrodes are not hindered from diffusing radially of the vacuum interrupter at the current zero point. This brings forth the effect that the dielectric recovery characteristic after the interruption of the current can be enhanced to improve the breaking performance of the vacuum circuit breaker. As set forth above, the present invention can provide a vacuum circuit breaker which exhibits a high breaking performance between electrodes (or contacts).

Claims: A vacuum circuit breaker, comprising: a pair of contacts (2a, 2b) for switching an electric circuit, a coil electrode for generating a first magnetic flux in response to a current through said contacts (2a, 2b), characterised by a further coil (12) for generating a second magnetic flux so as to cancel said first magnetic flux in absence of a current through said contacts (2a, 2b), said coil (12) having its own power supply circuit (13, ..., 17) for supplying current to flow through said coil (12). The circuit breaker of claim 1, having a vacuum interrupter (2) including the pair of contacts (2a, 2b) and coil electrodes, wherein the pair of contacts (2a, 2b) are a pair of main electrodes which are arranged in a vacuum vessel, a coil electrode is arranged at the back of each of the pair of contacts (2a, 2b), said coil electrodes generating a first axial magnetic flux component between themselves, and the coil (12) is adapted to generate a second axial magnetic flux component opposite in sense to said first axial magnetic flux component. The circuit breaker of claim 1 or 2, wherein said power supply circuit (13,...,17) includes a capacitor (13) and a trigger gap (16). The circuit breaker of claim 3, wherein said power supply circuit (13,...,17) further includes a coil (14). The circuit breaker of claim 3, wherein said power supply circuit (13,...,17) further includes a resistor (15). The circuit breaker of claim 1 or 2, wherein said power supply circuit (13,...,17) includes a trigger gap (16) and a circuit with capacitors (13a...,13d) and coils (14a...14d) connected in a π (Pi) shaped network. The circuit breaker of claim 1 or 2, wherein said power supply circuit (13,...,17) includes feed terminals (61) for conducting current from an outside supply (17). The circuit breaker of any of the preceding claims, further comprising: a reverse current generation circuit (5, 6, 8) for generating a current cancelling the current flowing through the pair of contacts (2a, 2b). The circuit breaker of claim 8, wherein the reverse current generation circuit (5, 6, 8) includes a capacitor (5), a coil (6) and a trigger gap (8).

Inventors: ENDO SHUNKICHI, HASHIMOTO AKIRA, KUROSAWA YUKIO, SATO TAKASHI, SUZUKI KOJI, ENDO, SHUNKICHI, HASHIMOTO, AKIRA, KUROSAWA, YUKIO, SATO, TAKASHI, SUZUKI, KOJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: H01H33
IPC codes: H01H33