Abstract:
A switching device for the interruption of a fault current includes a housing having first and second electrical connectors for connection in the path of an electrical conductor carrying a single current phase. A vacuum switching chamber is disposed in the housing. A current transformer disposed in the housing includes a primary circuit connected between the first and second terminals for conducting the single current phase and inducing a corresponding current in a secondary winding. A magnet actuator disposed in the housing is electrically coupled to the transformer secondary winding. The magnet actuator is responsive to a fault current in the transformer primary, by way of the corresponding current in the secondary, for changing from an inactive state to an actuated state. A latch disposed in the housing is connected for latching the movable switch contact in a closed position and for being unlatched in response to a change to the actuated state of the magnet actuator for moving the movable switch contact to an open position. A turn-off spring causes the latch to move the movable switch contact away from the stationary switch contact when the latch becomes unlatched.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the rights of priority with respect to application Ser. No. P 40 21 945.3 filed Jul. 10, 1990 in Germany, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a switching device for the interruption of a fault current, preferably for a transformer branch in a medium voltage switching system; and more particulary to such a switching device which is suitable for the performance of repeated interruptions and is actuated by an energy store including at least one turn-off spring and an actuating device which becomes active upon the occurrence of a fault current. 
     Such a switching device is employed with preference in closed line switching systems which are generally composed of two closed line loops and at least one switching panel for a transformer branch. A known switching device of the foregoing type is disclosed in a publication by CALOR-EMAG, entitled &#34;Schaltanlage fur Ortsnetzverteilerstationen mit Vakuum-Leistungsschalter&#34; [Switching System For Local Network Distributor Stations Equipped With A Vacuum Power Switch], by Klaus Bottger and Bruno Schemann, Special Printing from Elektrotechnische Zeitschrift {Electrical Engineering Magazine], Volume 106, No. 10, (1985). The closed line terminals are each equipped with a power circuit breaker and the transformer tap with a vacuum power switch. The latter, in conjunction with an excess current relay fed by a current transformer, provides short-circuit protection for this branch. 
     The cited switching system, which is insulated by SF 6  gas, is considered to be fully insulated since the short-circuit protection is no longer provided by EHV (extra high voltage) fuses disposed outside the encapsulation. Moreover, the switching system is considered to be maintenance free since, after the interruption of a short-circuit current, a possibly remote, controllable renewed turn-on of the power switching device permits resumption of operation without having to exchange an operating medium, such as, for example, the fuses. The absence of EHV fuses additionally precludes further sources of error right from the start, such as the existence of a critical current range in which a fuse fails to respond, or responds only after a very long period of melting, and in which therefore the danger of overheating and explosion exists. Moreover, it is noted, among others, that safety characteristics fluctuate considerably and that the power circuit breaker is overloaded in the borderline region of the fuse. 
     On the other hand, in the cited switching system, the vacuum power switch must have the full insulating capability as specified, for example, by applicable VDE [Vorschriftenwerk Deutscher Elektrotechniker=Rules for German Electrical Engineers] rules and must be able, by the layout of its contacts and the energy store, to turn off short-circuit currents without welding. 
     In the connection region, that is, outside of the encapsulation of the switching system, sufficient space must be provided for the necessary cable transformers. The excess current relays must additionally be checked a certain time intervals in routine monitoring which is possible only if the branch can be separated. With the prior art switching device, a three-phase interruption of the branch is made even if the fault is only in a single phase, although in many cases it would be of advantage to maintain two-phase emergency operation. This applies primarily for rigidly grounded networks as they are used in many countries overseas. If one employs the prior art switching device in a switching system that is insulated by air, all its elements are fully exposed to the climate of the environment. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a switching system that can be employed as a single-phase, autonomous operating means for the repeated interruption of short-circuit currents. 
     It is a further object of the present invention to provide a switching system in which all elements of the switching device are fully protected against environmental influences, that is, also against a corrosive climate. 
     The above and other objects are accomplished according to the invention by the provision of a switching device for the interruption of a fault current, comprising: a housing including first and second electrical connectors for connection in the path of an electrical conductor carrying a single current phase, the housing being associated solely with the single current phase; a vacuum switching chamber disposed in the housing; a stationary switch contact and a movable switch contact disposed in the vacuum switching chamber and connected, respectively, to the first and second electrical connectors; a current transformer disposed in the housing and including a secondary circuit and a primary circuit connected between the first and second terminals for conducting the single current phase and inducing a corresponding current in the secondary circuit; magnet actuator means disposed in the housing and electrically coupled to the secondary circuit of the transformer, the magnet actuator means having an inactive state and an actuated state and being responsive to a fault current in the primary circuit of the current transformer, by way of the corresponding current in the secondary circuit, for changing from the inactive state to the actuated state; latch means disposed in the housing, connected to the movable switch contact for latching the movable switch contact in a closed position with respect to the stationary switch contact and for being unlatched in response to a change to the actuated state of the magnet actuator means for moving the movable switch contact to an open position relative to the stationary switch contact; a turn-off spring for storing a spring force and operatively associated with the latch means for causing the latch means, under influence of the spring force, to move the movable switch contact away from the stationary switch contact when the latch means becomes unlatched; and reset means operatively connected to the latch means for causing the latch means to latch the movable contact in the closed position and for restoring the spring force of the turn-off spring subsequent to the latch means being unlatched in response to a fault current. 
     The switching device according to the invention contains all operationally significant elements in a single-phase housing and is therefore suitable for a phase-wise interruption of fault currents. Additionally, the housing may also be made gas tight so that the protective device according to the invention also results in a noticeable improvement in the service life of air-insulated switching systems, primarily in humid or corrosion endangered environments. 
     The switching device according to the invention is autonomous because it includes a power store and an actuating system as well as a resetting device which can be operated easily from the operator&#39;s side even in encapsulated, gas insulated switching systems. Due to the built-in vacuum switching chamber, the switching device has an almost unlimited service life. 
     The resetting device is preferably in the form of a screw spindle so that the vacuum switching chamber can be turned on again after each interruption due to a fault and the turn-off spring can be latched again. According to one aspect of the invention, a shaft is attached to the movable switch contact and the shaft, or an extension thereof, of the movable switch contact transfers the switching movement toward the outside of the housing and there enables the indication of the switch position or, by way of connecting means, the actuation of an associated power circuit breaker. 
     According to another aspect of the invention, the secondary winding of the current transformer and a toroidal coil of the magnet actuator means are connected in series without using an outside energy source. If a fault current is present in the primary of the current transformer, the induced secondary current is utilized to actuate the magnet actuator means and to thus release the latch means. The turn-off spring then turns the vacuum switching chamber off. Upon removal of the fault, the screw spindle is employed to turn the vacuum switching chamber on again. Because the switching device according to the invention includes a built-in vacuum switching chamber, it is able to interrupt an almost unlimited number of faults without access to the interior of the switching device. It can therefore be installed within the encapsulation of gas insulated switching systems and is thus at the same high insulation level as the operating means present in the gas atmosphere. On the other hand, the housing of the switching device according to the invention can be made gas tight so that its internal functions are not influenced by environmental influences such as corrosion, humidity, etc. when the device is employed in air insulated systems. 
     For a better understanding of the invention, reference is made to the following drawing figures: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal, sectional view of a switching device according to the invention, showing the ON position on the left of the center line and the OFF position on the right of the center line. 
     FIG. 2 is a schematic which shows the incorporation of a switching device of the invention in a metal encapsulated, gas insulated switching system (single-phase illustration). 
     FIG. 3 is a circuit diagram showing three single-phase switching devices of the invention and a three-phase power circuit breaker linked together by way of an electrical control according to another aspect of the invention. 
     FIG. 4 is a circuit diagram for the secondary circuit including a capacitance. 
     FIG. 5 is a partial longitudinal sectional view which shows the switching device of the invention with a wound capacitor. 
     FIG. 6 is a similar view as FIG. 5 showing the switching device of the invention with a plate capacitor. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a particularly advantageous embodiment of a switching device according to the invention which has a cylindrically shaped housing 50 having an interior which is divided into two parts or chambers 51 and 52 by a partition 8. In the first chamber 51, shown in FIG. 1 above partition 8, there is a vacuum switching chamber 1 having a stationary contact 1a attached to a stationary shaft 1a&#39;, and a movable contact 1b attached to a movable shaft 1b&#39;. Contacts 1a and 1b are disposed within a metallic dome element 1c which is sealed to shaft 1a&#39;. A cylindrical part 1d has a radially outwardly extending flange 1e connected at its periphery to the rim of dome element 1c. A ceramic tube 54, comprised for example of porcelain, is sealingly attached at one end to flange 1e and at its opposite end to a bottom closing cover 55 which is sealed to movable shaft 1b&#39; by a bellows 56 to provide an air-tight vacuum switching chamber as is known in the art. 
     Stationary shaft 1a&#39; is connected by way of a conductor coil 23 with a first electrical connector 2. Conductor coil 23 generates a magnetic field in a gap G in the opening between separated switch contacts 1a and 1b which is almost parallel to the switch axis A and thus improves in a known manner the switching capability of vacuum switching chamber 1. 
     A second chamber 52 disposed below partition 8, accommodates a current transformer 4, a magnet actuator 5, a turn-off spring 9 and a latch 10, following in the listed order below partition 8. Second chamber 52 is terminated by a second electrical connector 3. 
     First chamber 51 is encased essentially by an insulating tube 6, while second chamber 52 is encased by a metal tube 7. In the open state (contacts 1a and 1b separated) of vacuum switching chamber 1, a voltage differential is present across switch contacts 1a, 1b, at the ends of ceramic tube 54 of the vacuum switching chamber 1 and at the ends of insulating tube 6. Since the environment in the interior of the housing is free of contaminants for vacuum switching chamber 1, the length of ceramic tube 54 may be noticeably shorter than the length of insulating tube 6 which may also be subjected to unfavorable environmental influences. 
     Current transformer 4 has a secondary circuit composed of a toroidal core 4a and a secondary coil 4b supported thereon, and a primary circuit comprised of the shaft of the movable switch contact 1b or, more precisely, its extension 25. The latter is in communication with second connector 3 by way of a sliding contact 24 and is thus traversed by primary current during operation. Magnet actuator 5 includes a toroidal coil 5a and a ferromagnetic body 5b surrounding it on three sides and having a lower leg 5d configured so that it creates a minimum air gap 5e relative to a magnet armature 5c that is movable in the vertical direction between the free end of lower leg 5d and the inner surface of metal tube 7. Magnet armature 5c is under the influence of a reset spring 14 which, if toroidal coil 5a is not excited, urges magnet armature 5c into its rest position where it forms a vertical air gap 26 relative to ferromagnetic body 5b (left side of FIG. 1). Current transformer 4 and magnet actuator 5 form an annular gap relative to extension 25 in which, in the present example, turn-off spring 9 is disposed. Turn-off spring 9 has an upper end supported at partition 8 and a lower end which lies against a pressure member 10b. In the ON (closed) state of vacuum switching chamber 1 (left side of FIG. 1), pressure member 10b has a sloped face 10d which is axially supported by way of clamping bodies 10a seated on a support plate 10c, which in turn is seated on an interior radial surface 3b of electrical connector 3. In the ON state, pressure member 10b additionally is radially supported by way of clamping bodies 10a which lie against magnet armature 5c. Pressure member 10b also supports a lower end of a contact spring 12, by way of a tube 11 surrounding shaft extension 25. In the disclosed embodiment, contact spring 12 is a plate spring with an upper end which acts by way of a step 21 on shaft 1b&#39; of movable switch contact 1b. 
     A pin 15 inserted into extension 25 ensures that the two switch contacts 1a and 1b, when in the OFF state (right side of FIG. 1), do not exceed a stroke h. Pin 15 may either cooperate with the lower face of pressure member 10b or, as shown in FIG. 1, with a groove 30 worked into the pressure member. 
     At lower connector 3, there is a screw spindle 18 with the aid of which vacuum switching chamber 1 can be turned on again after each interruption of current and turn-off spring 9 can be tightened (compressed) until it is latched by way of pressure member 10b and clamping bodies 10a as shown on the left side of FIG. 1. However, the switching device according to the invention is ready to switch only after screw spindle 18 has been turned back into its starting position, namely after a required distance has been established between the lower face of pressure member 10b and the upper edge of screw spindle 18. 
     In order to facilitate installation of the switching device according to the invention, lower connector 3 is preferably screwed into metal tube 7. 
     In the simplest embodiment of the switching device according to the present invention, the provision of a coiled conductor 23 to generate an axial magnetic field may be omitted. 
     In accordance with the invention, no additional outside energy source is required for the secondary circuit of transformer 4. Rather, secondary winding 4b of current transformer 4 is connected directly in series with toroidal coil 5a (connection not shown in FIG. 1). 
     If a fault current occurs in the primary circuit of transformer 4, the following occurs successively within the switching device: 
     (a) Induction of a corresponding secondary current in secondary winding 4b, simultaneously inducing a magnetic field in ferromagnetic body 5b of toroidal coil 5a, pulling magnet armature 5c into air gap 26 against the force of reset spring 14 until magnet armature 5c comes in contact with the lower edge of ferromagnetic body 5b. 
     (b) Clamping bodies 10a, which are now no longer supported radially by magnet armature 5c, are moved toward the outside by sloped face 10d of pressure member 10b and release the turn-off stroke of the pressure member. 
     (c) Under action of turn-off spring 9, pressure member 10b moves downward until it encounters a stop (not shown) and carries along by way of groove 30 and pin 15 the movable switch contact 1b until switching stroke h has been completed between switch contacts 1a and 1b. 
     Thus the turn-off process is completed. 
     The left side of FIG. 1 shows the turned-on state of the switching device and the right side the turned-off state. Since extension 25 of shaft 1b&#39; of movable switch contact 1b, penetrates the lower delimiting face of connector 3, its end face 25a may be provided either with an indicator for the completed turn-off or with a mechanism with the aid of which an associated circuit breaker is moved into a break (tripped) position. As described below in connection with FIGS. 2 and 3, this may be effected by a mechanical rod assembly for actuating a turn-off energy store of the circuit breaker, or by way of an auxiliary contact which gives an electrical signal to turn-off the energy store for tripping the breaker. 
     The reclosing of the switching device according to the invention will now be described. 
     (d) Screw spindle 18 is actuated to cause pressure member 10b to move upward, simultaneously compressing turn-off spring 9 and bringing switch contacts 1a and 1b in contact. 
     (e) After renewed galvanic contact of switch contacts 1a and 1b, the further rotation of screw spindle 18 causes latch 10 to be brought into its blocking position (left side of FIG. 1). This is done by means of reset spring 14 which acts on magnet armature 5c, thus moving clamping bodies 10a over sloped face 29 of magnet armature 5c into their starting position below sloped face 10d of pressure member 10b. During this process, a small excess stroke occurs between groove 30 and pin 15 which is required by contact force spring 12 in order to reliably generate the necessary contact force. 
     (f) Screw spindle 18 is turned back into its starting position so as to re-establish the free path required by pressure member 10b. 
     As a further feature of the present invention, screw spindle 18 may be configured so that, in addition to performing its function as a turn-on mechanism for vacuum switching chamber 1, it also serves as a stop for pressure member 10b during each turn-off process. 
     FIG. 1 further shows that connectors 2 and 3 have connecting faces 2a and 3a, respectively, which can be employed as adapters for the clamp contacts of EHV fuses. However, other connecting faces can be employed just as well. 
     With the switching device according to the invention it is possible, for example, to produce a closed loop switching system according to FIG. 2 wherein there is shown an encapsulation 31 with two cables 62, 64 entering a lower wall of the encapsulation 31 through suitable gas-tight penetrations 66, 68, respectively, and a third cable 65, for example for a transformer branch, entering an upper wall through a similar penetration 69. Cables 62 and 64 may be connected to a bus bar 37 or ground by respective cable connections 69, 70 as shown in the lower portion of the encapsulation. In the upper portion of the encapsulation, bus bar 37 is coupled to the transformer branch by way of another cable connection 71 and a switching device 33 of the type shown in FIG. 1. The fault current protection is here integrated in the encapsulation without requiring the use of a complete operational power switch. Since the switching device according to the present invention is an autonomous unit which requires no outside energy and in which it is not necessary to exchange any components after the successful interruption of a fault current, the device can be fully incorporated in an encapsulated switching system. In this case, the same high insulation level then applies for it as for all other operating means included in the switching system. 
     More particulary, the closed loop switching system according to FIG. 2 is provided, for example, in its lower portion with two three-position switches 32 with which cables 62 and 64, respectively, can be connected with bus bar 37 or with which each individual cable can be separated or grounded. Toward the top, bus bar 37 is connected with a further three-position switch 32&#39; whose switch contacts are connected to switching device 33 according to the invention for the interruption of fault currents. Switching device 33 is provided with two clamping contacts 35 and associated insulators on encapsulation 31. Switching device 33 is oriented so that second connector 3 is oriented toward one of the walls of the encapsulation. Screw spindle 18 (not shown in FIG. 2) is extended in the form of an insulated shaft 19, which is brought to the exterior of the switching system in a gas tight manner. With the aid of insulated shaft 19, switching device 33 can be turned on again and turn-off spring 9 can be set in compression and latched either manually or by way of a motor drive 34 once the fault current has been interrupted. 
     The switching device according to the invention is a single-phase switching device. End face 25a (see FIG. 1) of switching device 33 according to the invention may be connected with one end of a rod assembly 36 which is connected at another end to three-position switch 32&#39; in order to separate the branch circuit after a fault current has been interrupted. In networks having a rigid neutral point ground connection or, if it should be necessary for other reasons, three-position switch 32&#39; may be equipped with single-phase actuation. 
     FIG. 3 shows how, by means of an electrical control, three single-phase switching devices 33 and a three-phase power circuit breaker 32a are linked together and a fault current interruption in only one phase need not lead to the separation of the power circuit breaker 32a. That is, according to the embodiment of FIG. 3, only a two-phase or three-phase fault current interruption leads to the final breaking of the circuit. Each movable switch contact 1b of the three switching devices 33 is connected with a pair of two-pole auxiliary switches 39 that are disposed outside of encapsulation 31. The three pairs of auxiliary switches 39 are interconnected so that one auxiliary contact of each interrupted phase is connected in series with an auxiliary contact of another interrupted phase and is in communication with an operating current actuator of a control device 38. Control device 38 may be actuated intentionally by way of a push button 41, and also automatically in the case of a fault by way of auxiliary switches 39. That is, in the case of automatic actuation, the connecting lines between auxiliary switches 39 of the individual phases are connected so that the operating current actuator of control device 38 has voltage applied to it only if two current phases are interrupted by way of two vacuum switching chambers. If there is a single-phase interruption, so that only one auxiliary switch contact is closed, no control voltage is present at control device 38. 
     The inventive switching device according to FIG. 1 may be employed to great advantage also in air insulated switching systems. In such cases, the housing is configured and sealed so that corrosive influences from the environment are unable to penetrate into the interior of the switching device. The length of insulating tube 6 is also adapted to the harder environmental influences. 
     In some cases it is desirable not to have an immediate response in the case of a fault. For this purpose, the conventional switching systems employ an adjustable excess current relay with which the response period of the actuating devices can be varied. However, such relays have the disadvantage that they must be checked out at certain time intervals. Yet, this is permissible only if the branch has been separated. 
     Extended actuation times can be realized with a further feature of the switching device according to the invention with the addition of a capacitance 28, preferably parallel with toroidal coil 5a of magnet actuator 5 as shown in FIG. 4. The capacitance may be configured, as shown in FIG. 5, as a cylindrical capacitor 28a. Cylindrical capacitor 28a is advantageously disposed in a gap between secondary winding 4b of current transformer 4 and metal tube 7 of the housing. FIG. 4 shows the interconnections of capacitor 28a with secondary winding 4b and toroidal coil 5a. 
     According to another feature of the invention, a plate capacitor 28b can also be employed as the capacitance for influencing the turn-off time of the switching device. This plate capacitor 28b may advantageously be disposed between current transformer 4 and magnet actuator 5. Here again the interconnections are made according to the circuit diagram of FIG. 4. In all cases, openings 42 are provided in ferromagnetic body 5b through which the electrical connection to the capacitors and to the secondary coil 4b of current transformer 4 are drawn. 
     Obviously, numerous and additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically claimed.