Abstract:
Vacuum circuit breaker, provided with a casing, in which a fixed and movable contact member are each attached to a supporting contact rod and supported therein in a mutually electrically isolated manner, and a coil coaxial to the casing and surrounding the contact members and having end connections. A first end connection of the coil is connected to one of the contact members. The contact member to which the first end connection of the coil is connected is coupled via a first coupling element to a feeder or outgoer connection of the vacuum circuit breaker. The second end connection of the coil is coupled via a second coupling element to the feeder or outgoer connection.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a vacuum circuit breaker, provided with a casing, in which a fixed and a movable contact member are each attached to a supporting contact rod and supported therein in a mutually electrically isolated manner, and a coil coaxial to the casing and surrounding the contact members and having end connections, wherein a fast end connection is electrically connected to one of the contact members. 
   DESCRIPTION OF THE RELATED ART 
   A similar device is known from European patent application EP 0.709.867 A1. The advantage in this known device is that with the assistance of a shunt, not all of the current flows through the coil but only that part necessary to generate an axial magnetic field, enabling the coil to have smaller dimensions than in the situation where the coil must be able to conduct all of the current. 
   For that purpose, with the known vacuum circuit breaker, the first end connection of the coil is electrically connected to one of the contact members, for example, the fixed contact member, whilst the second end connection of the coil is arranged as a connection strip. 
   Between the ends of the two end connections is an electrically conductive shunt by which means only pat of the total current of the vacuum circuit breaker flows through the coil that generates an axial magnetic field at the point of the contact members. The shunt has impedance and can be a resistive element and is located between the end connections of the coil. 
   By using the shunt, the coil no longer functions along the lines of the known design of vacuum circuit breakers solely in series in the main current path of the vacuum circuit breaker, but the shunt is connected in parallel so that only a part of the main current flows through the coil. This enables dissipation losses in the coil to remain limited. 
   Because the coil itself has little electrical impedance, the shunt will require only relatively low impedance to achieve the desired effect, i.e. limitation of the main current through the coil, and therefore the dimensions of the shunt can be made small. According to internationally agreed standards, vacuum circuit breakers must, however, also be momentary, in other words, able to withstand a continuous short-circuit current of 10 to 80 kA for 1 to 3 seconds. Owing to the amount of heat generated in a very short time due to the large current, the shunt must have a certain thermal capacity. In order to meet this, the shunt must, from to known vacuum circuit breaker in EP 0.709.867A1, also have large axial dimensions to meet the necessary standard for the required thermal capacity. This results in the disadvantage that the end connections of such coils must be placed far apart, which results in the coil occupying more space in the axial direction at the position of the end connections. Owing to the forces occurring with a short-circuit current, the connection conductors must therefore also be robust, resulting in yet more loss of space. 
   An additional requirement that the shunt must meet is that the change in resistance resulting from a change in temperature must be equal to the change in resistance that occurs in the coil as a result of a change in coil temperature. This is necessary to ensure that the relationship between the current through the coil and the current through the shunt always remains the same or roughly the same, not only in the event of gradual changes in the ambient temperature that will affect the coil and the shunt equally, but also in the event of abrupt and large temperature differences between coil and shunt. In practice, it seems that with a continuous short-circuit current, the temperature change in the shunt is considerably higher than the temperature change in the coil. This is caused, in particular, by the total thermal capacity of the coil being larger than that of the shunt, so the coil is more easily able to absorb the heat generated by the short-circuit current than the shunt. The difference in temperature appears, in practice, to be considerable and can be more than 100° C. The choice of material for the shunt must therefore meet a number of requirements simultaneously, thus limiting the options considerably. With the device in European patent application EP 0.709.867 A1 there is the additional restriction that results from the physical positioning between the end connections of the coil. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a vacuum circuit breaker of the type mentioned in the introduction with solutions to the abovementioned disadvantages. 
   According to the invention this object is achieved by the following means. The contact member to which the first end connection of the coil is connected is coupled via a first coupling element to a feeder or outgoer connection of the vacuum circuit breaker, and the second end connection of the coil is coupled via a second coupling element to the feeder or outgoer connection. The resistance of the first and second coupling element are set in order to achieve a desired current through the coil. 
   The advantage of the invention is that the shunt no longer needs to be physically present between the end connections of the coil, so more parameters for adjusting the current through the coil can be used, enabling more freedom in the dimensioning of the coil and therefore increased flexibility. This allows for better adjustment of the current-generated magnetic field to the desired optimum strength. From the literature (including the article “Interaction between a vacuum arc and an axial magnetic field” by H. C. W. Gundlach and included in the Proceedings 8 th  Int. Symposium Discharges and Electrical Insulation in Vacuum”, held in Albuquerque, U.S.A., Sep. 5–7, 1978 p. A2-1–11/see FIG. 2 and the article “Vacuum arc under an axial magnetic field and its interrupting ability” by S. Yanabu et al. published in Proc. IEE Vol. 126, No. 4, April 1979/see FIG. 4, 5 and 6) it is known that, depending upon several parameters, for favourable operation on the interrupting ability of a vacuum circuit breaker, there is an optimum vale of magnetic field. Higher and lower values reduce this favourable operation. In general, the optimum value lies between 3 and 10 mT per kA. 
   In order to guarantee that the set resistances result in a current in the coil which produces the desired optimum magnetic field even with the temperature changes and differences to be expected during operation, the materials of the first and second coupling elements and the coil must be such that they are resistive so that the set relationship between the resistances of the first and second coupling elements and coil remain the same or almost the same even with the large temperature changes and differences to be expected. 
   Preferably, the first and second coupling elements and the coil materials are chosen such that the set relationship between the resistances of the first and second coupling elements and the coil exhibits the same or almost the same change in resistance for the temperature changes which occur both during working-current conditions and with fault-current conditions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further embodiments of the invention are described in the dependent claims. 
     The invention will be explained further by means of drawings in which: 
       FIG. 1  shows a cross-section of a preferred vacuum circuit breaker to be used according to the invention in partial cross-section, 
       FIG. 2  is a bottom view of the coil shown in  FIG. 1 , 
       FIG. 3  shows a cross-section through the coil of  FIG. 2 . 
   

   Table 1 gives the data measured during a practical test of a switch. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The cross-section of the embodiment displayed in  FIG. 1  is an example of a certain type of vacuum switch; however, the invention can also be applied to every other type of switch, in which an axial field is applied to improve the arcing behaviour of the switch. 
   The connection set-up shown in  FIG. 1  includes a vacuum tube  1 , comprising an encasement  2  which is closed off by two end walls  3  and  4  situated opposite each other. 
   The fixed contact member  5  is fastened to and forms an electrically conductive connection with contact rod  6 . This contact rod  6  is fixedly supported in the end wall  4  of vacuum tube  1 . The movable contact member  7  is fastened to and forms an electrically conductive connection with the contact rod  8  which is supported such that it can move in the vacuum tube  1 . 
   The connection set-up shown includes, moreover, a coil  10 , of which one end connection  11  is electrically connected to the contact rod  6  of the fixed contact member  5 . 
   The vacuum circuit breaker furthermore forms an electrically conductive connection with a feeder or outgoer connection  12  with which the vacuum circuit bier can be incorporated in an electrical circuit. The other of these connections is not shown and is connected to the movable contact rod  8 . 
   The contact rod  6  of the fixed contact member  5  forms, via a first coupling element, which has the form of a rod  14  in  FIG. 1 , an electrically conductive connection with the feeder or outgoer connection  12 . 
   The other end connection  13  of coil  10  is, in principle, coupled via a second coupling element to the feeder or outgoer connection  12 . This coupling element can be a strip, for example, or can have another form. 
   When the vacuum circuit breaker with coil  10  has to be incorporated in an electrical circuit, this electrical circuit is connected on one side to connection  12  and on the other side to the connection on the upper contact rod  8 , not shown. The main current path is from connection  12  via the first coupling element (for example rod  14 ), the fixed contact member  5 , the movable contact member  7  and the movable contact rod  8  to the connection, not shown, on the upper contact rod  8  of the vacuum circuit breaker. The vacuum circuit breaker is opened because the upper movable contact rod  8  moves upward, separating contacts  5  and  7 . Between the two contact members  5  and  7 , an arc is then created and part of the main current to be interrupted subsequently flows from connection  12  over the first coupling element, the fixed contact member  5 , the arc created, the movable contact member  7  and the movable contact rod  8  to the other connection of the vacuum circuit breakers From connection  12 , another part of the main current runs over a second current path via the second coupling element, end connection  13  of the coil, coil  10 , end connection  11  of the coil, contact rod  6  and subsequently joins the main current path mentioned earlier. The current flowing through the coil generates an axial magnetic field at the contact members  5  and  7 . As known from the above articles, the axial magnetic field has an optimum value and it is the intention for the current flowing through the coil to be such that the axial magnetic field approaches this optimum value as closely as possible. The resistances of the first and second coupling elements are therefore chosen to ensure that the current flowing through the coil is such that the desired axial field of optimum strength is obtained. Compared with the known switch, the second coupling element provides an additional possibility of sending the right amount of current through the coil and therefore creating an optimum magnetic field. 
   In another embodiment (not shown), the end section  15  of the second end connection  13  runs transversely to the first coupling element, for example rod  14 , but ends before this rod  14 , so that the said end section  15  does not make contact with rod  14 . In the embodiment which is not show, the second coupling element can be incorporated between the said transverse end section  15  of the second end connection  13  of coil  10  and connection  12 , so that these three components, i.e. end section  15 , second coupling element (for example in the form of a strip, rod or such like) and connection  12 , can be pressed into conductive contact with one another by any suitable means. 
   In the preferred embodiment to be used shown in  FIG. 1 , the transverse end section  15  of the second end connection  13  of the coil  10  extends beyond rod  14 . The transverse end section  15  and rod  14  must not come into contact with each other, so therefore end section  15  of second end connection  13  of coil  10  has a hole  16  through which rod  14  passes and is thereby insulated. The second coupling element has the form of a bush  17  which is arranged coaxially with and insulated from rod  14  and which can be a moulded piece. In the preferred embodiment to be used from  FIG. 1 , rod  14  is a tie bar which is electrically connected on one end to the contact rod  6  and on the other end is connected to the connection  12  such that the contact rod  6 , the end section  18  of the end connection  11  of the coil  10 , an insulating layer  19  which can be an insulating washer, the end section  15  of the second end connection  13  of the coil  10 , the second coupling element of bush  17  and the connection  12  are pushed together and onto each other with an electrical contact pressure of sufficient strength. Here, rod  14  fulfils a combined electrical and mechanical function. In addition, this design embodiment has the advantage hat the first coupling element  14  is concentrically arranged in relation to the second coupling element, allowing use to be made of the so-called ‘skin effect’ whereby large currents, in particular, will flow along the outer edge of a conductor. Thus, this can also be used to influence the current distribution through the coil. 
   The equivalent circuit between the fixed contact member  5  and connection.  12  consists of a parallel circuit formed by the impedances of tie bar  14  and the impedance of coil  10  and the second coupling element or bush  17  connected in series. The invention makes it possible to choose from a large number of parameters in order to set the current through the coil at an optimum value to create an optimum axial magnetic field. These parameters are the material of the tie bar  14 , the material of coaxial coupling element  17 , coil  10 , the length and cross-sectional dimensions of tie bar  14 , coaxial coupling element  17  and coil  10 . 
   Table 1 gives the data recorded in a practical test of a switch. This relates to a switch which, according to internationally set standards, must be able to resist a continuous short-circuit current of 16 kA for 1 second. In the choice of material for coil  10 , tie bar  14  and coupling element  17 , account has also been taken of the influence of changes in temperature on the resistance and the effect thereof on the interrelationship of the currents through coil  10 , tie bar  14  and coupling element  17 . From the available materials used in practice, a copper alloy has been chosen for coil  10  and coupling element  17  and a brass alloy for tie bar  14 . It is, of course, also possible to use completely different materials as long as these meet the requirement that changes in resistances resulting from swings in ambient temperature and changes in temperature as a result of load or fault currents do not or hardly influence the relationship of the currents through coil  10 , tie bar  14  and coupling element  17 . 
   For the test, three operational situations were used, namely a minimum operating temperature of −40° C., nominal operating temperature of 20° C. and a maximum operating temperature of 105° C. Subsequently, in all three of these situations, a fault-current situation was simulated in which a current of 16 kA was conducted through the switch for 1 second. 
   From the minimum operating temperature, the fault current appeared to cause an increase in temperature of 118.2° C. in tie bar  14  and an increase of 26.3° C. in coil  10 . This temperature difference caused a deviation m the current relationship of 4.5% so that the initial field strength of the axial magnetic field of 6.5 mT per kA was found to have risen to 6.8 mT per kA. 
   Based on the nominal operating temperature, the rise in temperature was found to be 146° C. and 29.2° C., respectively, so that the initial optimum field strength of the axial magnetic field of 5.9 was found to have increased to 6.3 mT per kA. 
   Finally, the temperature increase measured from the maximum operating temperature was 184° C. and 33° C., respectively, with an increase of the axial magnetic field from 5.3 to 5.8 mT per kA. 
   From these measurements, it can be deduced that the optimum axial magnetic field set for the nominal operating temperature to 5.9 mT per kA only deviated by 0.6 mT per kA or by approx. 10% from the optimum value dog the variation from minimum to maximum operating temperature. In the fault-current situations, the deviation was found to vary from 0.1 to 0.9 mT per kA, i.e. a maximum deviation of approx. 15%. The conclusion drawn from this is that the deviations in the actual magnetic field generated in relation to the optimum magnetic field have remained within acceptable limits in all situations. 
   Because a phase shift between the current through the coil and the current through the switch also influences the axial magnetic field, this was also looked at during the measurements. It was found that there is hardly any phase shift so that there is no negative influence on the optimum axial magnetic field. 
   Because only an insulating layer  19  has to be applied between the end connections  11 ,  13  of coil  10 , the distance between those end connections is only the thickness of the insulating layer  19 . Depending on the material of this insulating layer, this need only be a few millimetres. Another advantage is at it is now possible to use a spring washer in this location as well, which can absorb any expansion differences. Since the measurement has shown that short-term temperature differences which can amount to 200° C. can occur under fault-current conditions, expansion differences can also be considerable. With the known switch, a spring washer cannot readily be used in this location because a current runs between the ends of the coil which causes the aforementioned high temperature increase, thereby affecting the resilient properties of the spring washer. 
   It should be noted that in the invention, the shunt is not physically located between the end connections  11 ,  13  of coil  10  but outside them. This has the advantage that the dimensions of coil  10  are not influenced thereby that the choice of dimensions of the shunt can be selected for optimum resistance, temperature coefficient and heat absorption ability. Although the first coupling element  14  in the embodiment shown has been fitted completely outside vacuum tube  1 , the invention is not limited thereto. For example, if the design of the vacuum tube allows it, it is also possible to fit the coupling element partially or completely in the vacuum tube, thus allowing the axial dimensions to be reduced. 
     FIG. 2  shows a bottom view of coil  10  and  FIG. 3  depicts a cross-section of this coil. 
   As indicated, the coil consists of one turn  20 . However, the coil can also have more turns or consist of a number of partial turns which form one or more turns. The coil is provided with end connections  11  and  13  having turn(s)  20  running perpendicular to end sections ( 18  and  15  respectively), which open out into rings  21  and  22 .