Abstract:
A device for load flow control of a direct current in a branch of a direct current voltage network node having a longitudinal voltage source which has a coupling device for connection or disconnection of electrical power. The coupling device for connection and disconnection of electrical power are connected to a coupling device for connection and disconnection of electric power of a further load flow control device which is disposed in another branch of the same direct current voltage network node. Thus the device can be used economically and flexibly for control of a load flow on or in a network node.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an apparatus for coupling in and coupling out power in a branch of a DC voltage network node comprising a longitudinal voltage source, which has means for coupling in or coupling out electric power. 
     WO2010/115453A1 proposes voltage compensation in DC transmission networks. In said document, longitudinal voltage sources are introduced into DC transmission lines in order to keep the voltage in the DC networks within a permissible range at all points. By introducing the longitudinal voltage source, unavoidably energy is supplied to or conducted away from the DC system at this point. In accordance with WO2010/115453, this energy is drawn from a three-phase system provided or is drawn from the DC line itself by means of an additional device. 
     The apparatus mentioned at the outset has the disadvantage that it severely limits the possibilities in respect of energy flow control. 
     The object of the invention therefore consists in providing an apparatus of the type mentioned at the outset which can be used economically and flexibly for controlling a load flow at a network node. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention achieves this object by virtue of the fact that the means for coupling in and coupling out electric power are coupled to means for coupling in and coupling out electric power of a further apparatus for load flow control, which further apparatus is arranged in another branch of the same DC voltage network node. By virtue of the coupling to a plurality of apparatuses for load flow control, in particular via the DC voltage network node, it is possible to achieve load flow control in a flexible manner since the means for coupling in and coupling out electric power make it possible to use, in an expedient manner, electric power output by a longitudinal voltage source, for example, for any desired purpose. For example, the longitudinal voltage source can be designed for supplying a consumer or for feeding electrical energy from a source into the DC voltage network node. Instead of the consumer or the source, a power supply system can also be provided. 
     However, the consumer may also at the same time be an energy store, for example a hydroenergy store, a mechanical store, an electrical or chemical energy store. If required, the consumer then becomes the energy source. The consumer can also be a conventional energy consumer, for example an industrial plant, a housing development or the like. 
     For complete load flow control, the number of apparatuses according to the invention at a DC voltage network node can be one fewer than the number of branches of said network node. If, therefore, n is the number of branches of the DC voltage network node, the number m of apparatuses according to the invention which is required for complete load flow control is calculated in accordance with m=n−1. 
     The apparatus according to the invention is intended for use in an HVDC network, wherein the coupling of a plurality of apparatuses via a low-voltage busbar is possible. This reduces the complexity involved in the power exchange considerably. 
     The longitudinal voltage source can be connected to a low-voltage busbar designed for alternating current. In this case, low voltage means a voltage of a few kilovolts in contrast to the operating voltages of the branch of several hundred kilovolts. 
     Within the context of one configuration, the longitudinal voltage source is connectable to a neutral point (ground potential) via a transverse current source. 
     In particular, the DC voltage node can be connected to the neutral point via the transverse current source. 
     In particular, the apparatus can have a transverse current source which is designed for connection to the neutral point. Alternatively, the apparatus can be designed for connection to the (for example one terminal or pole of the) transverse current source. 
     Preferably, the transverse current source is connectable to the DC voltage network node. 
     The transverse current source represents an option for the case where the total energy of the energy drawn by the longitudinal voltage sources and the energy fed by the longitudinal voltage sources is not equal to zero. 
     One development consists in that the longitudinal voltage source is coupled to the AC system via a transverse current source or directly (for example via a transformer). 
     A further development consists in that the longitudinal voltage source has at least one converter. For high-voltage direct-current (HVDC) transmission, so-called modular multilevel converters (MMCs) are used, whose basic module is a half-bridge comprising IGBTs and diodes, for example. This basic module is also referred to as submodule (also: converter module). It is known to connect a multiplicity of such submodules in series with one another in order to achieve HV strength. 
     An additional development consists in that the longitudinal voltage source has at least one phase module comprising at least two phase module branches connected in series, wherein a center tap between the phase module branches is connectable to an AC voltage connection, in particular to a transformer. 
     In particular, a plurality of phase modules can be connected in parallel with one another and thus realize a polyphase converter. 
     A further configuration consists in that the longitudinal voltage source is connected in series with a mechanical switch, and a power switching unit is arranged in parallel with the longitudinal voltage source and the mechanical switch. 
     The longitudinal voltage source can be used by way of example as follows. It is firstly used as commutation voltage when, for example, short-circuit currents are intended to be interrupted. First, a high current rise and/or an excessively high current over a specific duration is detected. Then, a tripping signal is transmitted to a tripping unit of the mechanical switch in the continuous current path. At the same time, the power semiconductor switches of the power semiconductor unit in the switching path are switched on. Finally, a back-emf is generated, which generates a circulating current in the mesh formed from the continuous current path and the switching current path, which circulating current is in opposition to the short-circuit current to be switched in the continuous current path. The longitudinal voltage source actively generates a back-emf, for example. As a deviation from this, the longitudinal voltage source, as in the document mentioned at the outset, is realized as an auxiliary electronic switch. An IGBT or IGCT with a freewheeling diode which is parallel in opposition, for example, is suitable as auxiliary electronic switch. It goes without saying that it is also possible for a plurality of series-connected auxiliary electronic switches which are arranged back-to-back in series to be used. The switching-off of the auxiliary switch can be equated to the application of a back-emf, which acts in opposition to the current flow in the continuous current path. The current is commutated into the switch-off branch by means of the longitudinal voltage source, with the result that the mechanical switch is opened at zero current. The actual interruption of the short-circuit current takes place in the power switching unit. 
     Within the scope of the invention, the longitudinal voltage source can in principle have any desired design. However, particular advantages are considered to be that the longitudinal voltage source has at least one submodule, which is provided with an energy store and a power semiconductor circuit, wherein each submodule has means for coupling in and coupling out electric power. If a plurality of submodules of this type is provided, these submodules are connected in series with one another. 
     Advantageously, each submodule has a half-bridge circuit. Such half-bridge circuits are provided with a power semiconductor circuit, which consists of a series circuit comprising 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 required, a freewheeling diode in opposition is connected in parallel with the power semiconductor switches. Possible power semiconductor switches are, for example, IGBTs or IGCTs. By virtue of the half-bridge circuit, either the energy store voltage U C  which forms as a voltage drop across the energy store or else a zero voltage can be generated at the connection terminals of each submodule. In accordance with this advantageous development, the longitudinal voltage source can therefore actively generate a back-emf in only one direction. 
     Thus, half-bridge circuits are particularly suitable when the direction of the back-emf to be impressed is known. 
     In order to be able to build up a voltage in both directions, however, 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 of submodules. The submodules having a full-bridge circuit are each provided with two series circuits comprising two power semiconductor switches, wherein the potential point between the two power semiconductor switches connected in series 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. Overall, the full-bridge circuit thus has four power semiconductor switches. If required, a freewheeling diode is again connected in parallel, in opposition, with each of these power semiconductor switches. Owing to this circuit arrangement, either the energy store voltage U C  which forms as a voltage drop across the energy store, a zero voltage or else the inverse energy store voltage −U C  can be generated at the connection terminals of each submodule. Therefore, with a series circuit of such full-bridge submodules, back-emfs are built up in both directions, wherein the maximum back-emf is dependent on the number of submodules. By using pulse width modulation during the actuation of the power semiconductor switches, the back-emf can be varied quasi continuously between the maximum positive and the maximum inverse energy store voltage. 
     Expediently, an AC voltage in the low-voltage range can be generated by the means for coupling in and coupling out electric power. The AC voltage has the advantage that it can easily be coupled, for example, inductively and in a cost-effective manner to other means for coupling in and coupling out electric power generating AC voltage. 
     In accordance with an expedient development in this regard, the means for coupling in and coupling out electric power have at least one series circuit comprising two power semiconductor switches which can be switched on and off and a coil, said series circuit being connected in parallel with the energy store. The coil is connected at one of its terminals to the potential point between the power semiconductor switches of said series circuit. In accordance with a development in this regard, at least one capacitor is also used in addition to a coil. It is also possible for two series circuits in the form of a hard-switched full-bridge to 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 apparatus according to the invention which is arranged in another branch of the DC voltage network node. This inductive coupling takes place via individual transformers, for example. The individual transformers have secondary windings, which are connected to the low-voltage busbar. It is also possible to arrange all of the coils on a common transformer. 
     The invention likewise relates to a mains voltage node (DC voltage network node) having branches, wherein an apparatus in accordance with the present invention is arranged in at least two branches. 
     Expediently, the means for coupling in and coupling out electric power of the at least two apparatuses in accordance with the invention are coupled to one another via a low-voltage busbar. The low-voltage busbar is designed 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 matter of the description below relating to exemplary embodiments of the invention with reference to the figures in the drawing, wherein the same reference symbols refer to functionally identical component parts, and wherein 
         FIG. 1  shows a DC voltage network node comprising a series of branches, in which in each case one exemplary embodiment of the apparatus according to the invention is illustrated schematically, 
         FIG. 2  shows an exemplary embodiment of the apparatus according to the invention in conjunction with an interruption circuit, 
         FIG. 3  shows an exemplary embodiment of a longitudinal voltage source of the apparatus, 
         FIG. 4  shows a submodule of a longitudinal voltage source including the means for coupling in and coupling out electric power, 
         FIG. 5  shows, on the basis of  FIG. 1 , a bipolar DC voltage transmission network comprising apparatuses according to the invention and two optional transverse current sources, 
         FIGS. 6-11  each show an exemplary embodiment of the means for coupling in and coupling out electric power, schematically, 
         FIGS. 12-14  show exemplary embodiments of transformers for coupling the AC voltages which can be generated by the means for coupling in and coupling out electric power, 
         FIG. 15  shows, on the basis of  FIG. 1 , an exemplary realization of the apparatus according to the invention comprising an optional transverse current source using a converter, 
         FIG. 16  shows an exemplary realization of an arrangement which can be used as longitudinal voltage source or as transverse current source, 
         FIG. 17  shows a submodule as a half-bridge circuit, 
         FIG. 18  shows a submodule as a full-bridge circuit, 
         FIG. 19  shows an exemplary realization comprising two longitudinal voltage sources and a transverse current source, which are coupled to one another via the busbar. 
     
    
    
     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 one exemplary embodiment of the apparatus  5  according to the invention is arranged. Each apparatus  5  has a longitudinal voltage source comprising means for coupling in and coupling out electric power, wherein the means for coupling in and coupling out electric power are connected to one another via a low-voltage busbar  6 . In this way, power exchange between the apparatuses  5   a ,  5   b  and  5   c  is made possible, for example. Optionally, a transverse current source  40  is provided, which can be connected to the DC voltage network node  1  and the low-voltage busbar  6 . Furthermore, a neutral point  41  (ground potential) can be provided, which is connected to the apparatuses  5   a ,  5   b  and  5   c  and the transverse current source  40 . 
       FIG. 5  shows, on the basis of the illustration shown in  FIG. 1 , the case of a bipolar DC voltage transmission network 
     comprising a positive star point  1   a  and a negative star point  1   b  and a busbar  6   a  for apparatuses  5   a  to  5   c  and a busbar  6   b  for apparatuses  5   d  to  5   e.    
     Optionally, two transverse current sources  40   a  and  40   b  can be provided, which are coupled via the neutral point  41 . The transverse current source  40   a  is connected to the positive star point  1   a  and to the busbar  6   a , and the transverse current source  40   b  is connected to the negative star point  1   b  and the busbar  6   b.    
       FIG. 15  shows an exemplary embodiment on the basis of  FIG. 1 , wherein the apparatuses  5   a  to  5   c  and the transverse current source  40  are each in the form of a converter. The AC connections of the apparatuses  5   a  to  5   c  are coupled to the AC connection of the transverse current source  40  via a transformer. 
       FIG. 2  shows an exemplary embodiment of the apparatus according to the invention in conjunction with an interruption circuit. In accordance with this example, the apparatus  5  comprises, by way of example, a continuous current path  7  and a switch-off current path  8 , in which a power switching unit  9  is arranged. The power switching unit  9  is designed to interrupt high short-circuit currents which are driven by high voltages of up to 500 kV. Such power switching units are known to a person skilled in the art, and therefore details are not given of the configuration of said power switching unit at this juncture. They have, for example, a series circuit of power semiconductor switches, IGBTs, IGCTs, GTOs or the like, with in each case one freewheeling diode being connected in parallel, in opposition, therewith. In this case, the power semiconductor switches and therefore the freewheeling diodes assigned thereto are not all oriented in the same direction of current flow. Instead, arranging the power semiconductor switches back-to-back in series with one another makes it possible to switch currents in both directions. In order to dissipate the energy released during switching, surge arrestors are used in the parallel circuit with the power semiconductor switches. A quick-response mechanical switch  10  which is closed during normal operation is arranged in the continuous current path  7 . In addition, a longitudinal voltage source  11  is illustrated schematically as well as the low-voltage busbar  6  for coupling the apparatuses  5   a ,  5   b  and  5   c . As has already been explained, the longitudinal voltage source actively generates a back-emf in the mesh consisting of the continuous current path  7  and the switch-off path  8 . The back-emf ensures a circulating current in the mesh which is in opposition to the normal direct current in the continuous current path. The resultant current in the continuous current path can therefore be limited or completely suppressed. 
       FIG. 3  shows an exemplary embodiment of the longitudinal voltage source  11  contained in the apparatus, which longitudinal voltage source in this case consists of a series circuit as submodules  12 , by way of example. 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 continuous current path  7 , where they conduct 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 . In this case, for example, inductances, transformers or the like can be used, with further details in this regard being given later in the text. 
       FIG. 4  shows an exemplary embodiment of a submodule  12 , wherein the submodule illustrated in  FIG. 4  has a full-bridge circuit, by way of example. In other words, the submodule  12  has a first series circuit comprising two power semiconductor switching units  18 . Each power semiconductor switching unit  18  consists of an IGBT  19  as power semiconductor switch and a freewheeling diode  20 , which is connected in parallel therewith, in opposition. In addition, a second series circuit  21  is shown, which is likewise formed from two power semiconductor switching 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 in the form of a storage capacitor. The potential point between the power semiconductor switching units  18  of the first series circuit  17  is connected to the first connection terminal  13 , and the potential point between the power semiconductor switching units  18  of the second series circuit  21  is connected to the second connection terminal  14  of the submodule  12 . In addition, means for coupling in and coupling out electric power  23  are provided, which means have a series circuit  24  comprising two power semiconductor switching units  18 , wherein the potential point between the power semiconductor switching units  18  of the series circuit  24  is connected to the first AC voltage connection terminal  15 . This second AC voltage connection terminal  16  is connected to the pole of the energy store  22  via a capacitor  25 . In addition, the means for coupling in and coupling out electric power  23  have an inductance  26 , which is coupled to an inductance  27  of the low-voltage busbar  6 . The inductance or coil  26  does not need to be designed for high voltages in the region of 500 kV, owing to a lack of grounding. Owing to its connection to one of the poles of the energy store, i.e. to the DC link, with a capacitor interposed, the AC voltages which can be generated thereby are within the low-voltage range. 
       FIG. 6  once again shows a submodule  12  with a full-bridge, the details of which have already been given in connection with  FIG. 4 .  FIG. 6  shows the submodule  12  without means for coupling in and coupling out electric power, however. Instead, connection points P and N for the connection of the means  23  for coupling in and coupling out electric power on the DC voltage side are illustrated. 
       FIGS. 7, 8, 9, 10 and 11  show different configurations of the means for coupling in and coupling out electric power  23 . 
       FIG. 7  shows a first exemplary embodiment of the means for coupling in and coupling out electric power  23 , which has already been explained in connection with  FIG. 4 . 
       FIG. 8  shows a further exemplary embodiment of the means for coupling in and coupling out electric power  23 , which are configured as a half-bridge, corresponding to the example shown in  FIG. 7 , and have a series circuit  24  comprising two power semiconductor switching units  18 , wherein a series circuit comprising two capacitors C DC  are connected in parallel with the series circuit  24 . 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 switching units  18  of the series circuit  24 . As in the configuration of the means for coupling in and coupling out electric power illustrated in  FIG. 7 , the exemplary embodiment shown in  FIG. 8  is a hard-switching half-bridge. In comparison with  FIG. 7 , however, the DC link is realized by the capacitors C DC  as voltage dividers. 
       FIG. 9  shows a further exemplary embodiment of the means for coupling in and coupling out electric power  23 , which, in addition to a first series circuit comprising two power semiconductor switching units, has a second series circuit  29  comprising two power semiconductor switching units  18 . The potential point between the power semiconductor switching units  18  of the first series circuit  24  is connected to a first terminal of the coil  26  via the first AC voltage connection terminal  15 , while the potential point between the power semiconductor switching units  18  of the second series circuit  29  is connected to the other terminal of the coil  26  via the second AC voltage connection terminal  16 . The circuit shown in  FIG. 9  can be referred to as a hard-switched full-bridge. 
     The exemplary embodiment shown in  FIG. 10  largely corresponds to the exemplary embodiment shown in  FIG. 9 , but a capacitor C r  is arranged in the first AC voltage connection terminal  15 , i.e. on the AC voltage side. This is accordingly a full-bridge with resonant switching. 
       FIG. 11  largely corresponds to the exemplary embodiment shown in  FIG. 7 , but the capacitor is arranged on the AC voltage side of the coil  26  as resonant capacitor. The means  23  for coupling in and coupling out electric power shown in  FIG. 11  can therefore be referred to as half-bridge with resonant switching. 
     In respect of the exemplary embodiments 7 to 11, it can be stated by way of summary that the means for coupling in and coupling out electric power in the form of a half-bridge or full-bridge can have both hard switching and resonant switching. The resonant switching has the advantage of the higher achievable clock frequency and therefore a smaller size of the transformer(s) for coupling the inductances of different apparatuses according to the invention. 
     Possible realizations of such transformers  30  are shown in  FIGS. 12, 13 and 14 . In the exemplary embodiment shown in  FIG. 12 , a multiplicity of individual transformers  30  is provided. The primary winding of each individual transformer  30  is formed by a coil  26  of the means for coupling in and coupling out electric power  23 . It is coupled to a secondary winding  32  via a core  31  of the transformer  30 . All of the secondary windings are connected to one another via the busbar  6 . 
       FIG. 14  shows an exemplary embodiment comprising three transformers  30 . The secondary windings of the transformers  30  are connected to one another again via a busbar  6 . However, the secondary windings are each coupled to a plurality of inductances or coils  26  of the means for coupling in and coupling out electric power  23 . In contrast to the exemplary embodiments 12 and 14, the transformer  30  shown in  FIG. 13  does not have any secondary windings or a busbar 
       6 . The inductances of the means for coupling in and coupling out electric power are instead only connected to one another via the core  31  of the transformer. The exchanged powers in this case add up to zero. 
       FIG. 16  shows an exemplary realization of a longitudinal voltage source in the form of an arrangement  50 , as can be used, for example, as component  5   a  to  5   c . The arrangement  50  is connected to the DC voltage node  1 , on one side, and to one of the branches  2 ,  3  or  4  on the other side. To this extent, the longitudinal voltage source can be represented as a two-port network, which is connected between the two poles  1  and  2  (or  3  or  4 ) of a DC voltage. 
     The arrangement  50  is a modular converter comprising three phase modules  51 ,  52  and  53 , of which each has two phase module branches  54  and  55 ,  56  and  57 , and  58  and  59 . An AC voltage connection  60 ,  61 ,  62  is provided between the respective phase module branches. 
     Each phase module branch  54  to  59  has a multiplicity of series-connected submodules  63 . 
     The submodules  63  can be embodied as a half-bridge circuit, as shown in  FIG. 17 , with an energy store  64 , which is arranged in parallel with a series circuit comprising two power semiconductor switches  65 ,  66 , with in each case one freewheeling diode  67 ,  68  being provided in parallel therewith and in opposition thereto. 
     As shown in  FIG. 18 , the submodule  63  can be embodied alternatively as a full-bridge circuit. In this case, four power semiconductor switches  70  to  73  are provided, of which in each case two are connected in series, and two such series circuits are arranged in parallel with an energy store  69 . A freewheeling diode  74  to  77  is arranged in parallel, in opposition, with each of the power semiconductor switches. In the case of the half-bridge circuit, only one polarity of the voltage is possible on the DC side, whereas the full-bridge circuit can generate both voltage polarities. 
     The AC voltage connections  60 ,  61  and  62  can be connected to a three-phase transformer. By way of example, the circuit shown in  FIG. 16  is embodied with three phase modules. Correspondingly, it is possible to provide a single phase module or any desired number of phase modules. At least two phase modules then having two AC voltage connections are provided for the connection of a transformer. 
     The longitudinal voltage sources  5   a  to  5   c  can be constructed from a series circuit of submodules  12 , which can be embodied as four-port networks, as is illustrated by way of example in  FIG. 3 . 
     It is noted by way of addition that the transverse current source  40 ,  40   a  or  40   b  can also be embodied in accordance with the arrangement in  FIG. 3 . Alternatively, the transverse current source  40 ,  40   a  or  40   b  can be embodied corresponding to the arrangement shown in  FIG. 16 . 
     Advantageously, the transverse current source can be realized by means of half-bridge circuits, and the longitudinal voltage source can be realized advantageously by means of full-bridge circuits. 
       FIG. 19  shows a schematic circuit arrangement comprising a longitudinal voltage source  80  (cf. longitudinal voltage source  5   a  in  FIG. 1 ), a longitudinal voltage source  81  (cf. longitudinal voltage source  5   b  in  FIG. 1 ) and a transverse current source  82  (cf. transverse current source  40  in  FIG. 1 ). 
     The longitudinal voltage sources  80 ,  81  and the transverse current source  82  have a similar design corresponding to the statements in relation to arrangement  50 : 
     Thus, the longitudinal voltage source  80  comprises a plurality of phase modules  83  to  86 , wherein the phase module  83  is connected in series with the phase module  84 . Furthermore, the phase modules  85  and  86  are connected in series. The two series circuits comprising the phase modules are connected in parallel with one another, wherein a center tap between the phase modules which are in each case connected in series is connected to the low-voltage busbar  6  via a transformer  87 . The longitudinal voltage source  80  is arranged between the branch  2  and the DC voltage node  1 . 
     These statements apply correspondingly to the longitudinal voltage source  81 , which is arranged between the branch  3  and the DC voltage node  1 . This longitudinal voltage source  81  comprises phase modules  88  to  91 , which are arranged corresponding to the longitudinal voltage source  81 . In addition, a transformer  92  is provided, which is connected to the low-voltage busbar  6 . 
     The transverse current source  82  also comprises phase modules  93  to  96 , which are arranged correspondingly. A transformer  97  of the transverse current source  82  is connected to the AC voltage connections and to the low-voltage busbar  6 . The transverse current source  82  is connected on one side to the DC voltage node  1  and on the other side to the neutral point  41 . 
     The longitudinal voltage sources  80 ,  81  and the transverse current source  82  can exchange energy via the low-voltage busbar  6 . 
     As has already been mentioned in respect of  FIG. 16 , the longitudinal voltage sources  80 ,  81  and/or the transverse current source  82  can also have a plurality of phase module branches and correspondingly also provide a plurality of AC voltage connections, of which in each case two are coupled via a transformer. 
     LIST OF REFERENCE SYMBOLS 
       1  DC voltage network node 
       2  Branch 
       3  Branch 
       4  Branch 
       5  Apparatus 
       5   a - 5   f  Apparatus 
       6  Low-voltage busbar 
       7  Continuous current path 
       8  Switch-off current path 
       9  Power switching unit 
       10  Switch 
       11  Longitudinal voltage source 
       12  Submodule 
       13  Connection terminal 
       14  Connection terminal 
       15  AC voltage connection terminal 
       16  AC voltage connection terminal 
       17  Series circuit 
       18  Power semiconductor switching unit 
       19  Power semiconductor switching unit (for example IGBT) 
       20  Freewheeling diode 
       21  Series circuit 
       22  Energy store 
       23  Means for coupling in and coupling out electric power 
       24  Series circuit 
       25  Capacitor 
       26  Coil 
       27  Inductance 
       29  Series circuit 
       30  Transformer 
       31  Core of transformer 
       32  Secondary winding 
       40  Transverse current source 
       40   a ,  40   b  Transverse current source 
       41  Neutral point 
       50  Arrangement 
       51 - 53  Phase module 
       54 - 59  Phase module branch 
       60 - 62  AC voltage connection 
       64  Energy store 
       65 ,  66  Power semiconductor switch 
       67 ,  68  Freewheeling diode 
       69  Energy store 
       70 - 73  Power semiconductor switch 
       74 - 77  Freewheeling diode 
       80 ,  81  Longitudinal voltage source 
       82  Transverse current source 
       83 - 86  Phase module 
       87  Transformer 
       88 - 91  Phase module 
       92  Transformer 
       93 - 96  Phase module 
       97  Transformer