Circuit arrangement having an electric device with a magnetizable magnetic core

Abstract

A circuit arrangement 1 comprises an electric device 3 having a magnetizable magnetic core 15 and at least one winding 17 configured for generating a magnetic flux in the magnetic core, and an auxilia

Description

The invention relates to a circuit arrangement comprising an electric device having a magnetizable magnetic core and at least one winding which is configured for generating a magnetic flux in the magnetic core. Furthermore, the invention relates to a method for operating such a circuit arrangement. In particular, the electric device can be a transformer. If one connects such an electric device to an electrical grid at an inopportune time (at a zero crossing of a voltage) , then the magnetic core can be driven into saturation in a half wave of the mains voltage. This brings about a high increase of the magnetomotive force of the magnetic core, which is required for generating the magnetic flux, and a high inrush current is therefore brought about. In addition to that, the remanent magnetization of the magnetic core is disadvantageous if the inrush current surge increases again. Thus, very high current surges can be caused during switching-on of the electric device. The invention is based on the object of avoiding high current surges when switching on an electric device which comprises a magnetizable magnetic core and at least one winding which is configured for generating a magnetic flux in the magnetic core. The object is achieved according to the invention by a circuit arrangement having the features of claim 1 and a method having the features of claim 8. Advantageous refinements of the invention are the subject matter of the dependent claims. A circuit arrangement according to the invention comprises an electric device having a magnetizable magnetic core and at least one winding which is configured for generating a magnetic flux in the magnetic core, and an auxiliary inductor which is electrically connected to the winding of the electric device and is configured for limiting an inrush current during switching-on of the electric device. The auxiliary inductor is dimensioned such that the inrush current of the electric device is limited to a specified maximum value even if the magnetic core of the electric device is completely saturated. The auxiliary inductor therefore enables a soft switching-on of the electric device even in the case of saturation of the magnetic core. One embodiment of the circuit arrangement according to the invention comprises a compensating device which is configured for suppressing a DC magnetic flux component in the magnetic core . Particularly in the case of an electrical transformer that is used in energy transmission grids and distribution grids, a direct current may undesirably be fed into the windings of the transformer. Even power electronics components in the electrical grid, for example the drive circuit of electric drives, inverters for flexible alternating-current transmission systems, stray currents of train systems operated using direct current or high-voltage DC transmission can bring about direct currents in the transformer. A direct current component in the windings of the transformer results in a DC magnetic flux component in the magnetic core of the transformer and the DC magnetic flux component is superposed on an AC magnetic flux. This causes an unsymmetrical magnetization of the magnetic core, which brings with it a series of disadvantages. Even a small direct current of a few amperes leads to a saturation of the magnetic core with magnetic flux. This is connected with a significant increase in the losses in the magnetic core. Furthermore, there is an increase in noise emissions during operation of the transformer, which is perceived as particularly troublesome in particular if the transformer is operated in the vicinity of a residential area. The compensating device counteracts a DC magnetic flux component in the magnetic core and thus an asymmetrical magnetization of the magnetic core. For example, the compensating device has a compensating winding for generating a magnetic flux in the magnetic core, the effect of which opposes a DC flux component. Suitable compensating devices of this kind are known for example from EP 3 783 630 Al and EP 3 786 987 Al and are therefore not described in any more detail here. If the magnetization of the magnetic core is asymmetrical after the switching-on of the electric device, the compensating device symmetrizes the magnetization of the magnetic core within a short time and therefore brings the electric device into a normal operating state. The auxiliary inductor therefore only has to be used briefly or only needs to be thermally configured for short time duty. This advantageously reduces the size and the costs of the auxiliary inductor. In a further embodiment of the circuit arrangement according to the invention, an inductance of the auxiliary inductor is variable. For example, the circuit arrangement in this case comprises a control unit which is configured for varying the inductance of the auxiliary inductor as a function of a current flowing in the at least one winding. This makes it possible in particular to reduce the inductance of the auxiliary inductor successively after the switching-on of the electric device, so that the at least one winding of the electric device creates by far the largest voltage drop. In a further embodiment of the circuit arrangement according to the invention, the auxiliary inductor is connected between a first switch and the electric device. By means of the first switch, the electric device can be switched on via the auxiliary inductor. Furthermore, the current path having the auxiliary inductor and the electric device can be interrupted by means of the first switch. A further embodiment of the circuit arrangement according to the invention comprises a second switch which is connected to the electric device and which is connected parallel to a current branch having the auxiliary inductor. It is possible to bridge the auxiliary inductor by means of the second switch. This is reasonable in particular after the magnetization of the magnetic core has been symmetrized by the compensating device and the system has stabilized, so current limitation by means of the auxiliary inductor is no longer required. In a further embodiment of the circuit arrangement according to the invention, the electric device is a transformer. The auxiliary inductor is then connected to a primary winding of the transformer . As already explained above, the invention can particularly advantageously be applied to transformers, since high inrush currents and asymmetrical magnetization of magnetic cores can occur in transformers, which are caused by direct current components of currents flowing in the windings of the transformers . The method according to the invention is used for operating a circuit arrangement which comprises an electric device having a magnetizable magnetic core and at least one winding which is configured for generating a magnetic flux in the magnetic core, an auxiliary inductor which is electrically connected to the winding of the electric device, and a compensating device which is configured for suppressing a DC magnetic flux component in the magnetic core. In the method, during switching-on of the electric device, an inrush current of the electric device is conducted through the auxiliary inductor and after the switching-on of the electric device, a DC magnetic flux component in the magnetic core is suppressed using the compensating device. If the magnetic core is saturated in the case of an asymmetrical magnetization, for example after the switching-on of the electric device, the inductance of the electric device is low. The magnetization of the magnetic core is symmetrized by means of the compensating device, as a result of which the inductance of the electric device adopts its large setpoint again and the large part of the voltage in the circuit arrangement drops across the at least one winding of the electric device. In the method according to the invention also, the electric device is for example a transformer. One embodiment of the method according to the invention relates to a circuit arrangement, the auxiliary inductor of which has a variable inductance. In this embodiment of the method according to the invention, the inductance of the auxiliary inductor is varied as a function of a current flowing in the at least one winding. A further embodiment of the method according to the invention relates to a circuit arrangement having a second switch which is connected to the electric device and parallel to a current branch having the auxiliary inductor. In this embodiment of the method according to the invention, the second switch is closed if the DC magnetic flux component in the magnetic core falls below a specified threshold value. The features of the method according to the invention correspond to features of a circuit arrangement according to the invention. Therefore, the advantages of the method according to the invention also correspond to advantages of a corresponding circuit arrangement which have already been mentioned above. The above-described properties, features and advantages of this invention and the manner in which they are achieved become more clearly and distinctly comprehensible in connection with the following description of exemplary embodiments which are explained in more detail in connection with the drawings. In the figures : FIG 1 shows a circuit diagram of an exemplary embodiment of a circuit arrangement according to the invention, FIG 2 shows a flowchart of an exemplary embodiment of the method according to the invention. Figure 1 (FIG 1) shows a circuit diagram of an exemplary embodiment of a circuit arrangement 1 according to the invention. The circuit arrangement 1 comprises an electric device 3, an auxiliary inductor 5, a compensating device 7, a first switch 9, a second switch 11, and a control device 13. The electric device 3 is a transformer for example and has a magnetizable magnetic core 15 and a winding 17 which, if the electric device 3 is a transformer, is a primary winding of the transformer . The auxiliary inductor 5 is connected between the first switch 9 and the electric device 3. The auxiliary inductor 5 has a variable inductance. For example, the inductance of the auxiliary inductor 5 is variable between the value zero and a maximum inductance. The maximum inductance is dimensioned such that, in the case of the maximum inductance, the auxiliary inductor 5 limits an inrush current of the electric device 3 to a specified maximum value even if the magnetic core 15 of the electric device 3 is completely saturated. The compensating device 7 is configured for suppressing a DC magnetic flux component in the magnetic core 15. For this purpose, the compensating device 7 has a compensating winding 19 for generating a magnetic flux in the magnetic core 3, the effect of which opposes a DC flux component. The compensating device 7 is designed in detail like a device described in EP 3 783 630 Al or EP 3 786 987 Al and is therefore not described in any more detail here. By means of the first switch 9, the electric device 3 can be switched on via the auxiliary inductor 5. Furthermore, the current path having the auxiliary inductor 5 and the electric device 3 can be interrupted by means of the first switch 9. The second switch 11 is connected to the electric device 3 and parallel to a current branch having the auxiliary inductor 5 and the first switch 9. It is possible to bridge the auxiliary inductor 5 by means of the second switch 11. The control unit 13 is configured for varying the inductance of the auxiliary inductor 5 as a function of a current flowing in the winding 15. The current flowing in the winding 15 is detected for example by a current measuring unit of the compensating device 7 and supplied to the control unit 13. Figure 2 (FIG 2) shows a flowchart 20 of an exemplary embodiment of the method according to the invention having method steps 21, 22, 23 for operating a circuit arrangement 1 that is described with reference to figure 1. In a first method step 21, during switching-on of the electric device 3, the first switch 9 is closed and the second switch 11 is opened, so that an inrush current of the electric device 3 is conducted through the auxiliary inductor 5 and limited by the auxiliary inductor 5. In a second method step 22, after the switching-on of the electric device 3, a DC magnetic flux component in the magnetic core 15 is counteracted using the compensating device 7. At the same time, the inductance of the auxiliary inductor 5 is reduced with falling current flowing in the winding 17 of the electric device 3. In a third method step 23, after the magnetization of the magnetic core 15 has been symmetrized by the compensating device 7 and the system has stabilized, the second switch 11 is closed and thus the auxiliary inductor 5 is bridged. At the same time, the first switch 9 can be opened. Although the invention has been illustrated and described in more detail by preferred exemplary embodiments, the invention is not restricted by the disclosed examples and other variations may be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.

Claims

1. A circuit arrangement, comprisingan electric device having a magnetizable magnetic core and at least one winding which is configured for generating a magnetic flux in the magnetic core, andan auxiliary inductor which is electrically connected to the winding of the electric device and is configured for limiting an inrush current during switching-on of the electric device.

2. The circuit arrangement as claimed in claim 1, having a compensating device which is configured for suppressing a DC magnetic flux component in the magnetic core.

3. The circuit arrangement as claimed in claim 1 or 2, wherein an inductance of the auxiliary inductor is variable.

4. The circuit arrangement as claimed in claim 3, having a control unit which is configured for varying the inductance of the auxiliary inductor as a function of a current flowing in the at least one winding.

5. The circuit arrangement as claimed in any one of the preceding claims, wherein the auxiliary inductor is connected between a first switch and the electric device.

6. The circuit arrangement as claimed in any one of the preceding claims, having a second switch which is connected to the electric device and which is connected parallel to a current branch having the auxiliary inductor.7 . The circuit arrangement as claimed in any one of the preceding claims, wherein the electric device is a transformer.

8. A method for operating a circuit arrangement which comprises an electric device having a magnetizable magnetic core and at least one winding which is configured for generating a magnetic flux in the magnetic core, an auxiliary inductor which is electrically connected to the winding of the electric device, and a compensating device which is configured for suppressing a DC magnetic flux component in the magnetic core, whereinduring switching-on of the electric device, an inrush current of the electric device is conducted through the auxiliary inductor andafter the switching-on of the electric device, a DC magnetic flux component in the magnetic core is suppressed using the compensating device.

9. The method as claimed in claim 8, wherein the inductance of the auxiliary inductor is variable and is varied as a function of a current flowing in the at least one winding.

10. The method as claimed in claim 8 or 9, wherein a second switch, which is connected to the electric device, is connected parallel to a current branch having the auxiliary inductor and is closed if the DC magnetic flux component in the magnetic core falls below a specified threshold value.T +44(0)30 0300 2000A

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