Abstract:
A device switches a direct current in a branch of a direct current voltage network node. The device contains a constant current path extending between two connection terminals, in which a mechanical switch is disposed. The device further has a switching current path bridging the constant current path and a power switching unit is disposed therein. The power switching unit has power semiconductor switches which can be switched on and off and configured to interrupt a short-circuit current in the event of a fault. The device contains a longitudinal voltage source for generating a counter-voltage in a loop formed by the constant current path and the switching current path. The device can be used economically over long periods of time to control the load flow on a network node. Accordingly, the longitudinal voltage source has a circuit for connecting and disconnecting the electrical power.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a device for switching a direct current in a branch of a DC voltage network node, having a permanent current path, in which a mechanical switch is arranged, a switched current path which bridges the permanent current path and in which a power switching unit is arranged, which power switching unit has power semiconductor switches which can be switched on and off and is set up to interrupt a short-circuit current in the event of a fault, and a longitudinal voltage source for generating a back-emf in the mesh formed by permanent current path and switched current path. 
     A device of this type is already known from WO 2011/057675 A1, for example. That document describes a DC voltage switch which has a permanent current path with a mechanical switch and a disconnection branch, which is connected in parallel with the permanent current path. A power switching unit in the form of a series circuit composed of power semiconductor switches is arranged in the disconnection branch, a freewheeling diode being connected in each case so as to be reverse-biased in parallel with said power semiconductor switches. The power semiconductor switches and freewheeling diodes of the power switching units are arranged back-to-back, that is to say the disconnectable power semiconductor switches are arranged in series, wherein, for each power semiconductor switch, a corresponding power semiconductor switch with opposite forward direction is provided. In this way, the current can be interrupted in both directions in the disconnection branch by the power switching unit. In the operating or permanent current path, in addition to the mechanical switch, an electronic auxiliary switch is also arranged in series with the mechanical switch. During normal operation, the current flows via the operating current path and thus via the electronic auxiliary switch and via the closed mechanical switch since the many power semiconductor switches of the disconnection branch represent an increased resistance to the direct current. To interrupt, for example, a short-circuit current, the electronic auxiliary switch, which can also be referred to as longitudinal voltage source, is transferred into its blocking position. As a result of this, the resistance in the operating current path increases, with the result that the direct current in the disconnection branch is commutated. The mechanical disconnection switch can therefore be opened with no current flowing. The short-circuit current conducted via the disconnection branch can be interrupted by the power switching unit. To accommodate the energy which is stored in the DC voltage network and is to be built up during switching, arresters are provided which are in each case connected in parallel with the power semiconductor switches of the disconnection branch. 
     Further DC voltage switches for switching short-circuit currents at high voltages are disclosed in DE 694 08 811 T2 and in U.S. Pat. No. 5,999,388. 
     The device mentioned at the outset has the drawback that said device cannot be used for load flow control since the losses occurring at the longitudinal voltage source then occur over long periods of time or even constantly. This is not sustainable from an efficiency or ecological point of view. 
     BRIEF SUMMARY OF THE INVENTION 
     The problem addressed by the present invention is therefore that of providing a device of the type mentioned at the outset which can also be used efficiently over long periods of time to control the load flow at a network node. 
     The invention solves this problem in that the longitudinal voltage source has means for connecting and disconnecting electrical energy. 
     Owing to the means for connecting and disconnecting electric power, the device according to the invention can also be used over long periods of time for load flow control since the means for connecting and disconnecting electric power make it possible to use in a meaningful manner for any purpose the electric power which is output, for example, by a longitudinal voltage source. On the contrary, if the longitudinal voltage source requires electric power, for example in order to reduce the load current in the branch of the longitudinal voltage source, the latter can be supplied by an independent source with the required power. Disconnected electric power can be used to supply a network situated close to the DC voltage network node, which network is connected, for example, to a further energy source or to additional consumers. Instead of a network, it is also possible to directly connect a consumer to the means for connecting and disconnecting electric power. The consumer is connected, for example, to an alternative energy source. However, at the same time, the consumer may also be an energy store, for example a hydropower store, a mechanical store, an electrical or chemical energy store. If necessary, the consumer then becomes the energy source. As has already been mentioned, the consumer can also be a classical energy consumer, for example an industrial plant, a residential complex, or the like. 
     However, particular advantages are afforded when the means for connecting and disconnecting electric power are coupled to means for connecting and disconnecting electric power of at least one further device for switching a direct current, which further device for switching a direct current is arranged in another branch of the same DC voltage network node. According to this advantageous configuration of the invention, for example the power drawn in one branch in the longitudinal voltage source can be connected to an identically constructed device which is arranged in another branch. In this connection, it is by no means necessary that all branches of the DC voltage network node are equipped with a device according to the invention; rather, it is sufficient if at least two branches have a DC voltage network node of a device according to the invention and the two devices according to the invention are coupled to one another via the means for connecting and disconnecting electric power. In this case, too, it is again conceivable to provide an independent energy source or an energy store which intermediately stores and, if appropriate, returns to the overall system again the excess energy or power occurring despite the coupling of the two devices. A consumer may also be supplied with the excess power. For complete load flow control, the number of devices according to the invention at one DC voltage network node is one lower than the number of branches of said network node. Therefore, if n is the number of branches of the DC voltage network node, the number m of devices according to the invention necessary for complete load flow control is calculated according to m=n−1. 
     According to the invention, a device is provided which can be coupled, via its means for connecting and disconnecting electrical energy, to other devices which likewise have means for connecting and disconnecting electrical energy. The device according to the invention is provided for use in a high-voltage direct current network, wherein it is possible to couple a plurality of devices via a low-voltage busbar. This significantly reduces the expenditure for power exchange. The longitudinal voltage source of the device according to the invention is used in two cases within the scope of the invention. Thus, said longitudinal voltage source is firstly used as commutation voltage if, for example, short-circuit currents are to be interrupted. First of all, a high current increase and/or a current which is too high over a particular interval is detected. Then, a trigger signal is sent to a trigger unit of the mechanical switch in the permanent current path. At the same time, the power semiconductor switches of the power semiconductor unit in the switched path are switched on. Finally, a back-emf is generated which generates a circulating current in the mesh formed by permanent current path and switched current path, which back-emf is opposed to the short-circuit current to be switched in the permanent circuit path. The longitudinal voltage source generates, for example actively, a back-emf. In a departure therefrom, the longitudinal voltage source is realized as electronic auxiliary switch, as in the document mentioned at the outset. An IGBT or IGCT with freewheeling diode connected so as to be reverse-biased in parallel is suitable as electronic auxiliary switch. Of course, a plurality of series-connected electronic auxiliary switches which are arranged back-to-back can also be used. The disconnection of the auxiliary switch can be compared with applying a back-emf which opposes the flow of current in the permanent current path. By means of the longitudinal voltage source, the current in the disconnection branch is commutated, with the result that the mechanical switch is opened with no current flowing. The actual interruption of the short-circuit current takes place in the power switching unit. However, this is known, for example, from WO 2011/057675 A1, and so a more precise description can be dispensed with at this point. 
     According to the invention, the longitudinal voltage source is connected to a low-voltage busbar which is configured for alternating current. In this connection, low-voltage means a voltage of a few kilovolts in contrast to the operating voltages of the branch of a few hundred kilovolts. 
     According to a preferred configuration of the invention, the longitudinal voltage source is arranged in the permanent current path. In this case, the energy required to generate a back-emf, for example to charge capacitors or to operate the power electronics, can be taken directly from the branch in which the device according to the invention is connected in series. 
     In principle, the longitudinal voltage source can have any design within the scope of the invention. However, particular advantages are seen in that the longitudinal voltage source has at least one submodule which is equipped with an energy store and a power semiconductor circuit, wherein each submodule has means for connecting and disconnecting electric power. If a plurality of submodules of this type is provided, said submodules are connected in series with one another. Submodules such as this are also used in so-called modular multilevel converters for energy transmission and energy distribution. 
     Advantageously, each submodule has a half-bridge circuit. Half-bridge circuits such as this are equipped with a power semiconductor circuit which consists of a series circuit composed of two power semiconductor switches, wherein the series circuit is connected in parallel with an energy store, for example a unipolar storage capacitor. The potential point between the two power semiconductor switches of the series circuit is connected to a first connection terminal, wherein a pole of the energy store is connected to a second connection terminal of the submodule. 
     If necessary, a freewheeling diode is connected so as to be reverse-biased in parallel with the power semiconductor switches. By way of example, IGBTs and IGCTs are used as power semiconductor switches. Owing to the half-bridge circuit, either the energy store voltage U c  which drops across the energy store or a zero voltage can be generated across the connection terminals of any submodule. According to this advantageous further development, the longitudinal voltage source can therefore actively generate a back-emf in only one direction. Thus, half-bridge circuits are particularly suitable if the direction of the back-emf to be impressed is known. 
     However, in order to be able to build up a voltage in both directions, submodules which have a full-bridge circuit are advantageous. These submodules are also connected in series, with the result that the longitudinal voltage source consists of a series circuit composed of submodules. The submodules having a full-bridge circuit are in each case equipped with two series circuits composed of two power semiconductor switches, wherein the potential point between the two series-connected power semiconductor switches of the first series circuit is connected to the first connection terminal and the potential point between the two power semiconductor switches of the second series circuit is connected to the second connection terminal. Both series circuits are connected in parallel with an energy store. In total, the full-bridge circuit therefore has four power semiconductor switches. If necessary, a freewheeling diode is connected so as to be reverse-biased in parallel with each of these power semiconductor switches. Owing to this circuit arrangement, either the energy store voltage U c  which drops across the energy store, a zero voltage or the inverse energy store voltage −U c  can be generated across the connection terminals of any submodule. Thus, back-emfs can be built up in both directions using a series circuit composed of such full-bridge submodules, wherein the maximum back-emf depends on the number of submodules. By using pulse-width modulation in the case of controlling the power semiconductor switches, the back-emf can be practically continuously varied between the maximum positive and the maximum inverse energy store voltage. 
     Expediently, an AC voltage in the low-voltage range can be generated using the means for connecting and disconnecting electric power. The AC voltage has the advantage that it can be simply, for example inductively, and inexpensively coupled to the other AC voltage-generating means for connecting and disconnecting electric power. 
     According to another related expedient further development, the means for connecting and disconnecting electric power have at least one series circuit composed of two power semiconductor switches which can be switched on and off and a coil, which series circuit is connected in parallel with the energy store. One of the terminals of the coil is connected to the potential point between the power semiconductor switches of said series circuit. According to a related further development, in addition to a coil, at least one capacitor is also used. In addition, two series circuits in the form of a hard-switched full-bridge can be used, wherein the potential points between the two power semiconductor switches of the two series circuits are connected to different terminals of the coil. 
     Expediently, the coil is inductively coupled to a coil of a longitudinal voltage source of a further device according to the invention, which is arranged in another branch of the DC voltage network node. Said inductive coupling is done via single transformers, for example. The single transformers have secondary windings which are connected to the low-voltage busbar. It is also possible to arrange all coils on a common transformer. 
     The invention likewise relates to a network voltage node with branches, wherein a device according to the present invention is arranged in at least two branches. Expediently, the means for connecting and disconnecting electric power of the at least two devices according to the invention are coupled together via a low-voltage busbar. The low-voltage busbar is set up for AC voltages, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       Further expedient configurations and advantages of the invention are the subject of the following description of exemplary embodiments of the invention with reference to the figures of the drawing, wherein identical reference signs denote identically acting components, and wherein 
         FIG. 1  shows a DC voltage network node with a series of branches, in which in each case an exemplary embodiment of the device according to the invention is schematically illustrated, 
         FIG. 2  schematically illustrates an exemplary embodiment of the device according to the invention, 
         FIG. 3  illustrates an exemplary embodiment of a longitudinal voltage source of the device according to  FIG. 2 , 
         FIG. 4  illustrates a submodule of a longitudinal voltage source including the means for connecting and disconnecting electric power, 
         FIG. 5  shows another exemplary embodiment of a DC voltage network node according to the invention with three branches, in which in each case an exemplary embodiment of the device according to the invention is arranged, 
         FIGS. 6-11  schematically illustrate in each case an exemplary embodiment of the means for connecting and disconnecting electric power, and 
         FIGS. 12-14  illustrate exemplary embodiments of transformers for coupling the AC voltages which are generated by the means for connecting and disconnecting electric power. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary embodiment of a DC voltage network node  1  according to the invention which has a series of branches  2 ,  3 ,  4 , in which in each case an exemplary embodiment of the device  5  according to the invention is arranged. Each device  5  has means—not shown in the figure—for connecting and disconnecting electric power, wherein the means for connecting and disconnecting electric power are connected to one another via a low-voltage busbar  6 . In this way, a power exchange between the devices  5   a ,  5   b  and  5   c  is made possible. 
       FIG. 2  shows an exemplary embodiment of the device  5  according to the invention in more detail. The device  5  has a permanent current path  7  and a disconnection current path  8 , in which a power switching unit  9  is arranged. The power switching unit  9  is set up to interrupt high short-circuit currents which are driven by high voltages of up to 500 kV. Power switching units such as this are known to the person skilled in the art, and therefore the configuration thereof need not be described in detail at this point. By way of example, they have a series circuit composed of power semiconductor switches, IGBTs, IGCTs, GTOs, or the like, a freewheeling diode being in each case connected so as to be reverse-biased in parallel therewith. In this case, the power semiconductor switches and thus also the freewheeling diodes associated therewith are not all oriented in the same current direction; rather, a back-to-back arrangement of the power semiconductor switches enables currents to be switched in both directions. To build up the energy which is released during switching, arresters connected in parallel with respect to the power semiconductor switches are used. 
     In the permanent current path  7 , a rapid mechanical switch  10  is arranged, which switch is closed during normal operation. Furthermore, a longitudinal voltage source  11  is schematically illustrated and the low-voltage busbar  6  for coupling the devices  5   a ,  5   b,    5   c . As has already been explained, the longitudinal voltage source actively generates a back-emf in the mesh consisting of permanent current path  7  and disconnection path  8 . The back-emf ensures a circulating current in the mesh, which circulating current is opposed to the operating direct current in the permanent current path. The resultant current in the permanent current path can thus be limited or completely suppressed. 
       FIG. 3  shows an exemplary embodiment of the longitudinal voltage source  11  which consists here of a series circuit composed of submodules  12 . The submodules  12  each have a first connection terminal  13  and a second connection terminal  14 . In this case, the connection terminals  13  and  14  are arranged in the permanent current path  7 , where they cause a direct current during normal operation. Each submodule also has a first AC voltage connection terminal  15  and a second AC voltage connection terminal  16 . The two AC voltage connection terminals  15  and  16  are each connected to the low-voltage busbar  6 . By way of example, inductances, transformers or the like can be used here; this will be discussed in more detail below. 
       FIG. 4  shows an exemplary embodiment of a submodule  12  in more detail, wherein the submodule illustrated in  FIG. 4  has a full-bridge circuit. In other words, the submodule  12  has a first series circuit composed of two power semiconductor switch units  18 . Each power semiconductor switch unit  18  consists of an IGBT  19  as power semiconductor switch and a freewheeling diode  20  connected so as to be reverse-biased in parallel therewith. Furthermore, a second series circuit  19  can be identified, which is likewise formed by two power semiconductor switch units  18 . The first series circuit  17  and the second series circuit  21  are each connected in parallel with an energy store  22 , which is designed as storage capacitor. The potential point between the power semiconductor switch units  18  of the first series circuit  17  is connected to the first connection terminal  13  and the potential point between the power semiconductor switch units  18  of the second series circuit  21  is connected to the second connection terminal  14  of the submodule  12 . Furthermore, means for connecting and disconnecting electric power  23  are provided, which have a series circuit  24  composed of two power semiconductor switch units  18 , wherein the potential point between the power semiconductor switch units  18  of the series circuit  24  is connected to the first AC voltage connection terminal  15 . The second AC voltage connection terminal  16  is connected via a capacitor  25  to the pole of the energy store  22 . Furthermore, the means for connecting and disconnecting electric power  23  have an inductance  26 , which is coupled to an inductance  27  of the low-voltage busbar  6 . Owing to a lack of grounding, the inductance or coil  26  must not be designed for high voltages in the region of 500 kV. 
     Owing to its connection to one of the poles of the energy store, that is to say to the intermediate circuit, by interconnection of a capacitor, the AC voltages which can be generated using said inductance fall in the low-voltage range. 
       FIG. 5  shows a further exemplary embodiment of the DC voltage network node  1  according to the invention with three branches  2 ,  3 ,  4 , in which in each case an exemplary embodiment of the device  5   a ,  5   b  or  5   c  according to the invention is arranged. As has already been mentioned in connection with the description of  FIGS. 1 to 4 , each device  5   a ,  5   b  or  5   c  has in each case a power switching unit  9 , which is arranged in the switched current path  8 . In the permanent current path  7 , a mechanical isolator  10  and a longitudinal voltage source  11 , which consists of a series circuit composed of submodules  12 , are again arranged, wherein each submodule is equipped with means for connecting and disconnecting electric power. Said means for connecting and disconnecting electric power are coupled to one another via a low-voltage busbar. 
       FIG. 6  shows once again a submodule  12  with full-bridge, which has already been mentioned in connection with  FIG. 4 . In  FIG. 6 , however, the submodule  12  is shown without means for connecting and disconnecting electric power. Instead, DC voltage-side connection points P and N for connecting the means  23  for connecting and disconnecting electric power are illustrated. 
     In  FIGS. 7, 8, 9, 10 and 11 , different configurations of the means  23  for connecting and disconnecting electric power are shown.  FIG. 7  shows a first exemplary embodiment of the means  23  for connecting and disconnecting electric power, which has already been mentioned in connection with  FIG. 4 .  FIG. 8  shows a further exemplary embodiment of the means  23  for connecting and disconnecting electric power, which is configured as a half-bridge according to the example according to  FIG. 7  and has a series circuit  24  composed of two power semiconductor switch units  18 , wherein the series circuit  24  is connected in parallel with a series circuit composed of two capacitors C DC . The potential point between the two capacitors is connected to the first terminal of the coil  26 , wherein the other terminal of the coil  26  is connected to the potential point between the power semiconductor switch units  18  of the series circuit  24 . Like the configuration of the means for connecting and disconnecting electric power illustrated in  FIG. 7 , the exemplary embodiment according to  FIG. 8  is a hard switching half-bridge. In comparison to  FIG. 7 , however, the intermediate circuit is realized by the capacitors C DC  as voltage divider. 
       FIG. 9  shows another exemplary embodiment of the means  23  for connecting and disconnecting electric power, which, in addition to a first series circuit composed of two power semiconductor switch units, has a second series circuit  29  composed of two power semiconductor switch units  18 . The potential point between the power semiconductor switch units  18  of the first series circuit  24  is connected via the first AC voltage connection terminal  15  to a first terminal of the coil  26 , while the potential point between the power semiconductor switch units  18  of the second series circuit  29  is connected via the second AC voltage connection terminal  16  to the other terminal of the coil  26 . The circuit according to  FIG. 9  can also be referred to as hard switched full-bridge. 
     The exemplary embodiment according to  FIG. 10  corresponds to the greatest extent to the exemplary embodiment according to  FIG. 9 , wherein a capacitor C r  is arranged in the first AC voltage connection terminal  15 , that is to say on the AC voltage-side, however. Accordingly, this is a resonant switched full-bridge. 
       FIG. 11  corresponds to the greatest extent to the exemplary embodiment according to  FIG. 7 , wherein the capacitor is arranged on the AC voltage-side of the coil  26  as resonant capacitor, however. The means  23  for connecting and disconnecting electric power according to  FIG. 11  can thus be referred to as resonant switched half-bridge. 
     With respect to the exemplary embodiments 7 to 11, it can be summarized that the means for connecting and disconnecting electric power as half-bridge or full-bridge can be embodied both as hard switching and as resonant switching. The resonant circuit has the advantage of higher clocking frequencies being achievable and thus a smaller size of the transformer or transformers for coupling the inductances of different devices according to the invention. 
     Possible realizations of such transformers  30  are shown in  FIGS. 12, 13 and 14 . In the case of the exemplary embodiment according to  FIG. 12 , a multiplicity of single transformers  30  are provided. The primary winding of each single transformer  30  is formed by a coil  26  of the means  23  for connecting and disconnecting electric power. It is coupled to a secondary winding  32  via a core  31  at the transformer  30 . All secondary windings are connected to one another via the busbar  6 . 
       FIG. 14  shows an exemplary embodiment with three transformers  30 . The secondary windings of the transformers  30  are again connected to one another via a busbar  6 . However, the secondary windings are in each case coupled to a plurality of inductances or coils  26  of the means  23  for connecting and disconnecting electric power. In contrast to the exemplary embodiments 12 and 14, the transformer  30  according to  FIG. 13  does not have any secondary windings or a busbar  6 ; rather, the inductances of the means for connecting and disconnecting electric power are coupled to one another solely via the core  31  of the transformer. In this case, the exchanged powers add up to zero.