Abstract:
Power switches are provided with measures for preventing a recommendation of the arc from the secondary current path to the main current path and/or measures for returning the current limiter during the recommutation of the light art. A corresponding arrangement for carrying out the method is provided with means in the power switch and/or current limiter for preventing a switch failure of the current limiter disposed in the secondary current path of the power switch. The current limiter can be especially a PTC current limiter.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE02/01229 which has an International filing date of Apr. 4, 2002, which designated the United States of America and which claims priority on German Patent Application number DE 101 18 746.7 filed Apr. 17, 2001, the entire contents of which are hereby incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The invention generally relates to a method for operation of a switching device having a switchable current limiter. In this case, the expression switching device may include, in particular, circuit breakers or contactors, and possibly semiconductor switches or the like, as well. In addition, the invention also generally relates to an arrangement for carrying out the method. 
   BACKGROUND OF THE INVENTION 
   Switching devices protect electrical power supply systems and loads in the event of a short circuit by rapidly building up a sufficiently high switching voltage. As a result of this, the short-circuit current is limited, and is interrupted after a short time. In order to increase the current limiting effect, the switching voltage that is used can be increased by connecting the switching device in series with a separate current limiter. According to the prior art, the limiter is for this purpose connected in the main circuit, so that the load current flows through it all the time, both during normal operation and in the event of a short circuit. 
   There are various technical solutions for limiters. In addition to conventional mechanical switches which produce switching arcs, PTC limiters are used for current limiting, in which the build up of voltage when switching occurs is produced by increasing the electrical resistance of the limiter material and/or by way of a gas discharge with a high burning voltage. 
   In comparison to mechanical switches, PTC limiters have the advantage that the switching voltage is built up very quickly. The disadvantage is the greater cold electrical resistance. As a result of this, the rated current must be limited during rated operation in order to prevent unacceptable heating of the PTC material, for example as a result of motor starting currents, and unintentional response of the limiter. In one known commercial product (ABB PROLIM), a circuit breaker, acting as a switching device, and a PTC limiter are specifically electrically connected in series in the main circuit. 
   Another possible solution for the rated current problem is described in EP 0 657 062 B1, in which a limiter for a circuit breaker is connected in an auxiliary current path, through which current flows only briefly, when switching occurs. The auxiliary current path is formed from the arc guide rails and the quenching chamber, and is connected by the commutation of the arc from the switching contacts onto the guide rails. 
   In comparison to conventional circuits in which a circuit breaker and a limiter are connected in series, there is, however, a risk of switching failure, with the switching arc possibly being commutated back to the main current path, when the limiter is connected in the auxiliary current path. When commutation back such as this occurs, the switching arc or the arc attachment point is moved back from the auxiliary current path to the main current path, for example to the switching contacts. As a result of this, although no current passes through the limiter, it does, however, generally retain its resistance at that time. If another attempt is made for the arc to commutate into the switching chamber, the limiter switching voltage must then be overcome in addition which, in some circumstances, makes the commutation process so difficult that it can fail. 
   DE 42 43 314 A discloses a current-limiting circuit breaker with an arc quenching device and an auxiliary current path with at least one PTC thermistor and an overvoltage suppressor associated with it. In both devices, the switching from the current-limiting mode to the non-current-limiting mode takes place in a corresponding manner to the overcurrent decay. Furthermore, U.S. Pat. No. 5,777,286 discloses an electrical switching device with separate contacts, which can be disconnected mechanically, and with an arc switching contact associated with this, in which case a PTC thermistor or the like can be connected in the auxiliary circuit. Furthermore, EP 0 350 825 A2 discloses an electrical switching device with an arc quenching device and a current limiting device in the auxiliary circuit. 
   SUMMARY OF THE INVENTION 
   An object of an embodiment of the invention is to specify a method for operation of a switching device having a current limiter in the auxiliary current path, in which there is no possibility of a switching failure caused by commutation back to the main current path. A further aim is to provide associated arrangements. 
   Advantageous embodiments include embodiments of the current limiter, which is, in particular, in the form of a PTC limiter, on the one hand, and/or of the switching device, which is in the form of a circuit breaker, on the other hand. 
   An embodiment of the invention provides in particular the functional reliability of a combination specifically comprising a circuit breaker and a limiter in the auxiliary circuit. However, embodiments of the invention are also applicable to other switching devices and current limiters. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details and advantages of the invention will become evident from the following description of the figures relating to exemplary embodiments, and with reference to the drawings, in conjunction with the patent claims. In the figures: 
       FIG. 1  shows the combination of a circuit breaker with a current limiter in the auxiliary current path of the circuit breaker, 
       FIG. 2  shows a current limiter in the form of a PTC limiter, 
       FIG. 3  shows the profile of the electrodes and of the resistance body in the PTC limiter as shown in  FIG. 2 , 
       FIG. 4  shows an illustration, in the form of a graph, of the resistance as a function of time for an arrangement as shown in  FIGS. 2 and 3 , 
       FIG. 5  shows a symmetrical circuit breaker with a switching link and limiter added to it, 
       FIG. 6  shows a detail from  FIG. 5 , with switching chambers in the circuit breaker, which is in the form of a double-interrupting circuit breaker, and 
       FIG. 7  shows a plan view of the outer guide rails of the switching chambers, with apertures. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Parts which are identical or have the same effect have the same reference symbols in the figures. Two measures according to an embodiment of the invention are essentially described, by which, individually or in conjunction, the problem of switching failure of PTC limiters in the auxiliary current path of circuit breakers can be avoided. 
     FIG. 1  shows a schematic illustration of the arrangement of a limiter in the auxiliary current path of a circuit breaker, as is described by way of example in EP 0 657 062 B1. 
   In  FIG. 1 , a current-limiting circuit breaker  20  contains at least two contacts  22  and  23 , at least one of which is designed to move and can be opened and closed via a switching mechanism  24  which can be tripped by a thermal and/or magnetic release  25  or  26 , respectively. Each contact  22  and  23  has a respectively associated arc guide rail  27  and  27 ′, which surround an initial chamber area  28  and open into a quenching chamber  21  with a large number of splitter plates  29  for quenching an arc, which is not illustrated in  FIG. 1 . The initial chamber area  28  and the quenching chamber  21  may together form a switching chamber. Once the arc foot points have been struck, the arc moves on the guide rails  27  and  27 ′ into the quenching chamber  21  with splitter plates  29 , where an arc voltage is built up that is sufficiently high for current limiting and arc quenching. 
   In the case of very high short-circuit currents, for example I K =50 to 100 KA, the increase in the arc voltage is no longer sufficient to limit the current flowing through the switch to non-critical levels. It is then possible for the switching device to be damaged or destroyed. In order to avoid these undesirable consequences, a limiter  1  is connected upstream of the circuit breaker  20  in the auxiliary current path in  FIG. 1 . 
   The limiter  1  is a current-limiting element which is not connected in the main current path in a corresponding manner to that in EP 0 657 062 B1, but forms an auxiliary current path in the switching device  20 , being passed to an arc guide rail  27 . The auxiliary current path for a commutation current I com  is defined in  FIG. 1  as a current path in parallel with the main current path for a current i that is to be switched. Current flows through it when the arc attaches itself to this guide rail as a result of striking of a foot point. 
   The limiter  1  as shown in  FIG. 1  is advantageously in the form of a PTC limiter. A PTC limiter  1  such as this is illustrated schematically in  FIG. 2  and comprises two planar electrodes  10 , between which a resistance body  5  composed of some suitable material (e.g., a polymer material) is clamped in, with a force K acting on it. The resistance body  5  has surfaces  2  and  3  and the electrodes  10  have surfaces  11 . A PTC limiter such as this for current limiting operates as explained in detail in EP 0 657 062 B1. 
   As can be seen from  FIG. 3 , the flat electrode  10  has profiling  15  rather than having a smooth surface  11 , and this profiling  15  has a square-wave structure with a web width b and web height h. The web width b may be between 0.1 and 1 mm, and the web height h may likewise be between 0.1 and 1 mm. In particular, the web width b and the web height h are of the same order of magnitude, preferably between 0.3 and 0.6 mm. The resistance body  5  has complementary profiling  7  on both surfaces  2  and  3 . The resistance body  5  and the flat electrodes  10  are connected to one another, such that they cannot be detached, via the profiling  7  and  15 . 
   In an embodiment different to that shown in  FIG. 3 , the profiling  7  and  15  may also be at an inclination angle with respect to the surface of the flat electrode. The configuration of the profiling influences the way in which the PTC limiter  1  operates. 
     FIG. 4  shows a switching oscillogram for the PTC limiter  1  with profiling  15  on the electrodes  10  and with a complementary surface profile  7  on the PTC resistance body  5 , as is described in detail in EP 0 717 876 B1. The time profile of the limiter resistance R when the PTC limiter  1  is interrupting a short circuit can be seen from the curve  17 . At the commencement of the short-circuit current, the limiter resistance starts from its initial value R0≈4 mΩ, and increases slightly. After about 300 μs, it reaches a first plateau level P at about 8 mΩ. While the short-circuit current rises further and reaches the value of 5 kA, 500 μs after the start of the short circuit, the resistance curve changes to a steep rise at this time, and remains for about 300 μs at resistance values which are considerably greater than 100 mΩ. About 900 μs after the start of the short circuit, the limiter resistance once again falls back to a low resistance value of about 15 mΩ, and then decreases to its initial value. 
   The combination of the characteristics of the arrangement comprising a circuit breaker and current limiter as shown in  FIG. 1  and the embodiment of the PTC limiter  1  as a current limiter as shown in  FIGS. 2 to 4  makes it possible for the limiter  1  to be reset in a time period of a few tenths of a millisecond from the high-resistance state, that is to say the switched state, to the low-resistance state by reducing the current, when an arc is commutated back from the auxiliary current path to the main current path. The additional voltage required for renewed commutation of the switching arc from the main current path to the auxiliary current path is quantitatively produced by the product of the instantaneous current and the resetting resistance of the limiter. 
   In the example shown in  FIG. 4 , the resetting resistance is about two to four times the cold resistance. In order not to exceed the additional commutation voltage of, for example, 50 V with a resetting resistance of about 10 mΩ, the current passing through the short circuit must therefore not exceed 5 kA. Thus, the resetting resistance of the PTC limiter  1  must therefore be designed to match the magnitude of the current being passed through it and the maximum commutation voltage. 
     FIG. 5  shows a circuit breaker  20  with a PTC limiter  1  in the auxiliary current path, as is shown in a comparable manner in  FIG. 1 . The major difference is the symmetrical configuration of the switching part  30  of the circuit breaker  20 , with a switching link  32  and a double switching chamber, with respective guide rails  36 ,  36 ′ and splitter plates  29 ,  29 ′, and with the limiter being connected in this symmetrical switching chamber arrangement. The designations of the functional parts correspond essentially to those in  FIG. 1 . For example, each switching chamber may include an initial chamber area  28  (or  28 ′) and an arc splitter chamber  21  (or  21 ′). 
   As in  FIG. 1 , the limiter  1  in  FIG. 5  is also loaded by the arrangement of the current limiter  1  in the auxiliary current path of the circuit breaker  20  only during switching operations. The switching chamber current path is used as the auxiliary current path and, once the switching link  32  has been opened, is connected by the arc commutating from the switching link  32  to the adjacent guide rails  36  and  36 ′. 
   In  FIG. 5 , the current limiter has its own enclosure  50 , which is fitted to the enclosure  30  of the circuit breaker  20  and includes an extension  52  for mechanical operation of the switching mechanism  24 . 
   When the limiter  1  is not connected, then the circuit breaker  20  contains, instead of this, a guide rail link  39  for connection of the two guide rails  36 ,  36 ′.  FIG. 5  shows the switching link  32  in the closed position by way of a solid line, and in the open position by way of a dashed line. The current path passes from one of the connections  47 ,  47 ′ into the drive part  40  of the circuit breaker  20 , which in turn, as shown in  FIG. 1 , contains the switching mechanism  24 , the overcurrent release  25  and the short-circuit release  26 . In consequence, when a short circuit occurs, the short-circuit release  26  can open the switching link  32  of the circuit breaker  20  without any delay. 
   The limiter  1  is connected to a connection point between the two guide rails  36  and  36 ′, which have an associated switching link  32  and are used as arc guide rails. Current does not flow through the limiter  1  until the arc attachment has commutated from the link contact to the adjacent guide rail in both switching chambers. The necessity for simultaneous arc commutation results in the additional voltage requirement being distributed between the two switching paths, as a result of the voltage drop across the limiter  1 . This splitting effect also makes it easier for repeated commutation from the main current path to the auxiliary current path once the arc has commutated back onto the switching link. 
   As a further effect, the double interruption indicates that the switching arc cannot move back from the auxiliary current path to the main current path unless this backward movement takes place in both switching chambers. 
   As a particular measure to prevent arcs from commutating backwards in this way, the configuration of the guide rails  36 ,  36 ′ creates an area which is largely screened from the respective arc splitter chamber  21  or  21 ′ and the associated initial chamber area  28  or  28 ′, and this area  34 ,  34 ′ holds the link contacts  23 ,  23 ′ when the switching device  20  is in the open position. 
   In  FIG. 6 , an aperture  38  or  38 ′ has been incorporated in the guide rails  36 ,  36 ′, respectively, which are associated with the switching link  32  being in the open position, and this can clearly be seen in detail in the plan view in  FIG. 7 . The aperture  38  or  38 ′ in the guide rails screens the switching link  32  in the open position from the splitter plates  29 ,  29 ′ and from the initial chamber area  28 ,  28 ′, thus preventing arcs from restriking on the switching link  32 . This ensures that the limiter function does not cease if arcs are restruck in the initial chamber area  28 ,  28 ′. 
   As mentioned,  FIG. 6  shows the switching link in the open position, in which the distance between the link contacts  23 ,  23 ′ and the stationary contacts  22 ,  22 ′ is considerably greater than the distance between the guide rails  36 ,  36 ′ and the stationary contacts. This result in the arc burning voltage creates a voltage difference which assists the arc commutation and makes it harder for the arc to commutate backwards. 
   The screening geometry of the guide rails  36 ,  36 ′ prevents arc plasma from being able to flow out of the arc splitter chambers  21 ,  21 ′ or from the initial chamber areas  28 ,  28 ′ directly to the switching link  32 , and causing flashovers from the switching link  32  to the guide rails  36 ,  36 ′ or to the stationary contacts  23 ,  23 ′. 
   As can clearly be seen from  FIG. 7 , the switching link  32  passes through the aperture  38 ,  38 ′ in the guide rails  36 ,  36 ′ during the opening movement. The chosen geometry results, in a known manner, in a magnetic field being formed, by means of which the arc or arcs is or are driven onto the cutout edge, and is or are split. The link mount  45  for movement of the switching link  32  is at the same time used for electrical isolation between the two switching chambers of the double-interrupting circuit breaker  20 . 
   Particularly in the case of the example described with reference to  FIGS. 5 to 7 , the circuit breaker is in the form of an individual switching device with the capability for connection of a limiter. The limiter connecting point is for this purpose connected by means of a guide rail link. A mechanical extension  52  for switching the circuit breaker on and off is provided, if required, for the combination of a circuit breaker and limiter. 
   Instead of fitting the limiter to the circuit breaker, a high-current version of the circuit breaker can be provided with a limiter that is integrated in the breaker enclosure. 
   Examples have been used to show that a circuit breaker is particularly suitable for the combination according to the invention of the switching device with a suitable current limiter. However, a contactor or a semiconductor switch can also be used in a corresponding manner as the switching device. However, arc switching elements are required, in particular for switching without any arcs. 
   For the practical implementation of the invention, the switching device and the current limiter can also advantageously include system engineering means. For example, the current commutation can be improved by isolating media, such as moving slides, a cover on the main current path/contact point. Use with single-interrupting and/or double-interrupting contact arrangements has been described. In this case, the switching contacts can be provided with a linear opening movement, or else with a rotary opening movement. Additionally or alternatively, current limiters which have been described in detail with reference to the figures may also be used in the form of a limiter with an additional switching chamber/contact point, or else a solid-state limiter. Special quick-action releases, for example a piezo-element for switching to the auxiliary current path at low power levels, can be used for early identification of short circuits. Finally, electronic tripping is also possible. 
   The described arrangements also allow communication with monitoring of switching states and/or of the life of the contacts or an indication of the remaining life, as well as an indication of the limiter life by addition of the short circuits. 
   Exemplary embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.