Abstract:
The invention relates to a converter circuit comprising at least one phase module ( 100 ) having an upper and a lower converter valve (T 1 , . . . , T 6 ), wherein each converter valve (TI, . . . , T 6 ) has at least one two-pole subsystem. According to the invention, each two-pole subsystem ( 14 ) has four semiconductor switches ( 21, 23, 25, 27 ), which can be switched off and are connected electrically in series, four diodes ( 22, 24, 26, 28 ), which are each connected electrically back-to-back in parallel with one semiconductor switch ( 21, 23, 25, 27 ) which can be switched off, two unipolar storage capacitors ( 29, 30 ), which are connected electrically in series and in parallel with the series circuit comprising the semiconductor switches ( 21, 23, 25, 27 ), and an electronic system ( 32 ), whose reference potential connection (M) is electrically conductively connected to a common potential (P 0 ). This results in a subsystem ( 14 ), at whose connection terminals (X 2 , XI) it is possible to generate a terminal voltage (UX 21 ) having four potential stages, which requires only one electronic system ( 32 ), whose energy supply takes place symmetrically, and which does not require any increased complexity in terms of potential isolation.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a converter circuit. 
   A converter circuit of this type is disclosed in DE 101 03 031 A1, and an equivalent circuit of such a converter circuit is shown in greater detail in  FIG. 1 . As shown in this equivalent circuit, this known converter circuit comprises three phase modules, each of which are denoted by  100 . These phase modules  100  are each electrically connected on the DC side to a positive and a negative DC busbar P 0  and N 0 . A DC voltage, which is not described in further detail, lies across these two DC busbars P 0  and N 0 . Each phase module  100  comprises an upper and a lower converter valve T 1  or T 3  or T 5  and T 2  or T 4  or T 6  respectively. Each of these converter valves T 1  to T 6  comprises a number of two-terminal subsystems  11  electrically connected in series. Four of these subsystems  11  are shown in this equivalent circuit. Two-terminal subsystems  12  ( FIG. 3 ) can also be electrically connected in series in place of the two-terminal subsystems  11  ( FIG. 2 ). Each node between two converter valves T 1  and T 2  or T 3  and T 4  or T 5  and T 6  of a phase module  100  forms an AC-side terminal L 1  or L 2  or L 3  respectively of this phase module  100 . Since in this diagram the converter circuit has three phase modules  100 , a three-phase load, for example an AC motor can be connected to its AC-side terminals L 1 , L 2  and L 3 , also known as load terminals. 
     FIG. 2  shows in greater detail an equivalent circuit of a known embodiment of a two-terminal subsystem  11 . The circuit arrangement of  FIG. 3  shows a version that is completely identical in function, which is also disclosed in DE 101 03 031 A1. These known two-terminal subsystems  11  and  12  each comprise two turn-off capable semiconductor switches  1 ,  3  and  5 ,  7 , two diodes  2 ,  4  and  6 ,  8  and one unipolar capacitor  9  and  10 . The two turn-off capable semiconductor switches  1  and  3  or  5  and  7  respectively are electrically connected in series, with these series connections being electrically connected in parallel with one storage capacitor  9  or  10  respectively. Each turn-off capable semiconductor switch  1  and  3  or  5  and  7  respectively is electrically connected in parallel with one of the two diodes  2 ,  4  and  6 ,  8  in such a way that this diode is connected in antiparallel with the corresponding turn-off capable semiconductor switch  1 ,  3 ,  5  or  7 . The unipolar storage capacitor  9  of the subsystem  11  or  12  respectively comprises either one capacitor or a capacitor bank containing a plurality of such capacitors having a resultant capacitance C 0 . The junction between the emitter of the respective turn-off capable semiconductor switch  1  or  5  and the anode of the respective diode  2  or  6  forms a connecting terminal X 1  of the subsystem  11  or  12 . The junction between the two turn-off capable semiconductor switches  1  and  3  and the two diodes  2  and  4  form a second connecting terminal X 2  of the subsystem  11 . The junction between the collector terminal of the turn-off capable semiconductor switch  5  and the cathode of the diode  6  forms a second connecting terminal X 2  of the subsystem  12 . 
   In both diagrams of the embodiments of the two subsystems  11  and  12 , insulated gate bipolar transistors (IGBT) are used as the turn-off capable semiconductor switches  1  and  3  as shown in  FIGS. 2 and 3 . MOS field effect transistors, also known as MOSFETs, can also be used. In addition, gate turn-off thyristors, also known as GTO thyristors, or integrated gate commutated thyristors (IGCT) can be used as the turn-off capable semiconductor switches  1  and  3 . 
   According to DE 101 03 031 A1, the subsystems  11  or  12  of each phase module  100  of the converter circuit shown in  FIG. 1  can be driven in a control state I and II. In control state I, the respective turn-off capable semiconductor switch  1  or  5  is switched on, and the respective turn-off capable semiconductor switch  3  or  7  of subsystem  11  or  12  is switched off. As a result, a terminal voltage U X21  of the subsystem  11  or  12  that exists across the connecting terminals X 1  and X 2  is equal to zero. In control state II, the respective turn-off capable semiconductor switch  1  or  5  is switched off and the respective turn-off capable semiconductor switch  3  or  7  of the subsystem  11  or  12  is switched on. In this control state II, the terminal voltage U X21  that exists equals the capacitor voltage U C  across the respective storage capacitor  9  or  10 . 
   As shown in the equivalent circuit of the converter circuit of  FIG. 1 , this converter circuit comprises eight two-terminal subsystems  11  or  12  per phase module  100 , with four per respective converter valve T 1 , T 2  or T 3 , T 4  or T 5 , T 6 , these subsystems being electrically connected in series by their connecting terminals X 1  and X 2 . The number of two-terminal subsystems  11  or  12  electrically connected in series depends both on a DC voltage lying between the two DC busbars P 0  and N 0  and on the turn-off capable semiconductor switches  1 ,  3 ,  5  and  7  that are used. Another factor here is to what extent a sinusoidal AC voltage at the AC-side terminal L 1 , L 2  or L 3  is supposed to follow a sinusoidal curve. 
   For the configuration and operation of a converter circuit designed as shown in  FIG. 1 , it is necessary to use a suitable circuit to drive the respective turn-off capable semiconductor switches  1 ,  3  or  5 ,  7  of each two-terminal system  11  or  12 , and to acquire various measurements of the two-terminal subsystem  11  or  12 , for example the capacitor voltage U C  lying across the respective storage capacitor  9  or  10 , and to transfer them to a higher-level converter controller. Hence, each two-terminal subsystem  11  or  12  comprises an electronic circuit, which is not shown explicitly in the diagrams of the subsystems  11  and  12  shown in  FIGS. 2 and 3  for reasons of clarity. This electronic circuit, also referred to below as an electronic module, performs the following functions:
         bidirectional communication with the higher-level converter controller   acquisition of various measurements and status/fault signals   driving the turn-off capable semiconductor switches  1 ,  3  or  5 ,  7     processing all incoming and outgoing signals.       

   In addition, it is advantageous but not essential to tap the power for operating the electronic module of a two-terminal subsystem  11  or  12  directly from its respective storage capacitor  9  or  10 . If two optical fibers are used for the data transmission between the electronic module of each two-terminal subsystem  11  or  12  and the higher-level converter controller, then this operation is electrically isolated. The reference potential of the electronic module of each two-terminal subsystem  11  or  12  is generally connected to a negative terminal of its respective unipolar storage capacitor  9  or  10 . 
   When a plurality of two-terminal subsystems  11  or  12  are connected in series for a phase module of a converter circuit, one embodiment is generally used for the subsystems  11  or  12 , i.e. the phase modules  100  of the converter circuit shown in  FIG. 1  comprise either subsystems in the embodiment of  FIG. 2 , or subsystems in the embodiment shown in  FIG. 3 . For a three-phase converter circuit according to  FIG. 1 , forty-eight optical fibers must be run between a higher-level converter controller and the twenty-four two-terminal subsystems  11  or  12 . If the number of two-terminal subsystems  11  or  12  used is increased by one subsystem per converter system T 1 , . . . , T 6 , the number of optical fibers increases by 12. 
   In order to reduce this outlay, the number of electronic modules per converter valve T 1 , . . . T 6  must be reduced. This reduction can be achieved if, for example, two two-terminal subsystems  11  or  12  are combined into one subsystem module, it then being possible to combine two electronic modules into one. When using one electronic module for at least two two-terminal subsystems  11  or  12  combined into one subsystem module, the question of the power supply for this electronic module arises. If the required power is only tapped from one unipolar storage capacitor of at least two two-terminal subsystems  11  or  12  combined into a subsystem module, then the power is supplied asymmetrically. Once again, this results in greater outlay for providing electrical isolation for driving the respective turn-off capable semiconductor switches  1 ,  3  or  5 ,  7  of the combined subsystems  11  or  12  and for acquiring the capacitor voltages U C , and results in a disadvantageous, asymmetric voltage division. 
   By combining at least two two-terminal subsystems  11  or  12  in such a way in one subsystem module, the number of optical fibers used is halved, but this is paid for by more costly electrical isolation and involves an asymmetric power supply. This means that two subsystems of simple design are replaced in each case by one subsystem module of more complex design. 
   SUMMARY OF THE INVENTION 
   Hence the object of the invention is to define a two-terminal subsystem for a converter circuit that avoids the cited disadvantages and reduces the outlay for such a converter circuit. 
   According to one aspect of the invention, this object is achieved by a converter circuit having at least one phase module comprising an upper and a lower converter valve, with each converter valve comprising at least one two-terminal subsystem, wherein each two-terminal subsystem comprises four turn-off capable semiconductor switches, four diodes, two unipolar storage capacitors and an electronic circuit, wherein a diode is electrically connected in antiparallel with each turn-off capable semiconductor switch, wherein these four turn-off capable semiconductor switches are electrically connected in series, wherein the two unipolar storage capacitors are electrically connected in series, with this series connection being electrically connected in parallel with the series connection of the turn-off capable semiconductor switches, wherein each junction between two turn-off capable semiconductor switches forms a respective connecting terminal of the two-terminal subsystem, and wherein a junction between the two storage capacitors that are electrically connected in series is electrically connected to a reference-potential terminal of the electronic circuit. 
   According to another aspect of the invention, this object is achieved by a converter circuit According at least one chase module comprising an upper and a lower converter valve, with each converter valve comprising at least one two-terminal subsystem, wherein each two-terminal subsystem comprises four turn-off capable semiconductor switches, four diodes, two unipolar storage capacitors and an electronic circuit, wherein a diode is electrically connected in antiparallel with each turn-off capable semiconductor switch, wherein pairs of turn-off capable semiconductor switches are electrically connected in series, wherein each series connection is electrically connected in parallel with a unipolar storage capacitor, wherein a junction between two turn-off capable semiconductor switches of a first series connection forms a connecting terminal of the two-terminal subsystem, with an emitter of a second turn-off capable semiconductor switch of the two turn-off capable semiconductor switches of a second series connection forming a second connecting terminal of the two-terminal subsystem, wherein a junction between two turn-off capable semiconductor switches of the second series connection is electrically connected to an emitter of a second turn-off capable semiconductor switch of the two turn-off capable semiconductor switches of the first series connection, and wherein this junction is electrically connected to a reference-potential terminal of the electronic circuit. 
   According to yet another aspect of the invention, this object is achieved by a converter circuit having at least one phase module comprising an upper and a lower converter valve, with each converter valve comprising at least one two-terminal subsystem, wherein each two-terminal subsystem comprises four turn-off capable semiconductor switches, four diodes, two unipolar storage capacitors and an electronic circuit, wherein a diode is electrically connected in parallel with each turn-off capable semiconductor switch, wherein pairs of turn-off capable semiconductor switches are electrically connected in series, wherein each series connection is electrically connected in parallel with a unipolar storage capacitor, wherein a junction between two turn-off capable semiconductor switches of a second series connection forms a connecting terminal of the two-terminal subsystem, with a collector of a first turn-off capable semiconductor switch of the two turn-off capable semiconductor switches of a first series connection forming a second connecting terminal of the subsystem, and wherein a junction between two turn-off capable semiconductor switches of the first series connection is electrically connected to a collector of a first turn-off capable semiconductor switch of the two turn-off capable semiconductor switches of the second series connection, and wherein this junction is electrically connected to a reference-potential terminal (M) of the electronic circuit. 
   According to still another aspect of the invention, this object is achieved by a converter circuit having at least one phase module comprising an upper and a lower converter valve, with each converter valve comprising at least one two-terminal subsystem, wherein each two-terminal subsystem comprises four turn-off capable semiconductor switches, four diodes, two unipolar capacitors and an electronic circuit, wherein a diode is electrically connected in antiparallel with each turn-off capable semiconductor switch, wherein pairs of turn-off capable semiconductor switches are electrically connected in series, wherein each series connection is electrically connected in parallel with a unipolar storage capacitor, wherein the junctions between each pair of turn-off capable semiconductor switches are connected together, wherein a collector of a first turn-off capable semiconductor switch of a first series connection and an emitter of a second turn-off capable semiconductor switch of a second series connection form a respective connecting terminal of the two-terminal subsystem, and wherein a reference-potential terminal of the electronic circuit is electrically connected to an emitter of a second turn-off capable semiconductor swich of the first series connection. 
   The fact that, according to the invention, four turn-off capable semiconductor switches are connected in a circuit of associated diodes connected in antiparallel and two unipolar capacitors, means that a common electronic module can be used to drive these turn-off capable semiconductor switches and to acquire the capacitor voltages, without needing to accept an increased outlay for the electrical isolation. In addition, the power can also be tapped symmetrically. From the outside, such a subsystem according to the invention has two connecting terminals and two terminals for two optical fibers. Hence this subsystem according to the invention is equivalent to a known system in terms of the connections. This subsystem can be driven so that a terminal voltage is generated across the two connecting terminals that now has four potential levels instead of just two potential levels. Hence only half so many subsystems compared with a known embodiment are required for a converter circuit for a defined high voltage, with the number of optical fibers required also being halved. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is explained in greater detail with reference to the drawing, which illustrates schematically a plurality of embodiments of a two-terminal system according to the invention. 
       FIG. 1  shows an equivalent circuit of a known converter circuit comprising distributed energy stores, 
       FIGS. 2 and 3  each show in greater detail an equivalent circuit of a first and second embodiment of a known two-terminal subsystem, 
       FIG. 4  shows an equivalent circuit of a first embodiment of a two-terminal subsystem according to the invention, 
       FIG. 5  shows an equivalent circuit of a second embodiment of a two-terminal subsystem according to the invention, 
       FIG. 6  shows an equivalent circuit of a third embodiment of a two-terminal subsystem according to the invention, and 
       FIG. 7  shows an equivalent circuit of a fourth embodiment of a two-terminal subsystem according to the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 4  shows in greater detail the equivalent circuit of a first embodiment of a subsystem  14  according to the invention. This two-terminal subsystem  14  according to the invention comprises four turn-off capable semiconductor switches  21 ,  23 ,  25  and  27 , four diodes  22 ,  24 ,  26  and  28 , two unipolar capacitors  29  and  30  and one electronic circuit  32 , also referred to below as an electronic module  32 . The four turn-off capable semiconductor switches  21 ,  23 ,  25  and  27  are electrically connected in series. A diode  22 ,  24 ,  26  and  28  is electrically connected in antiparallel with each of these semiconductor switches  21 ,  23 ,  25  and  27 . One unipolar capacitor  29  or  30  respectively is electrically connected in parallel with each pair of turn-off capable semiconductor switches  21 ,  23  or  25 ,  27  respectively. The unipolar capacitor  29  or  30  of this subsystem  14  comprises either one capacitor or a capacitor bank containing a plurality of such capacitors having a resultant capacitance C 0 . The junction between the two turn-off capable semiconductor switches  21  and  23  and the two diodes  22  and  24  form a second connecting terminal X 2  of the subsystem  14 . The junction between the two turn-off capable semiconductor switches  25  and  27  and the two diodes  26  and  28  form a first connecting terminal X 1  of this subsystem  14 . The junction between the emitter of the turn-off capable semiconductor switch  23 , the collector of the turn-off capable semiconductor switch  25 , the anode of the diode  24 , the cathode of the diode  26 , the negative terminal of the unipolar capacitor  29  and the positive terminal of the unipolar capacitor  30  forms a common potential P 0 , which is electrically connected to a reference-potential terminal M of the electronic module  32 . This electronic module  32  is linked to a higher-level converter controller (not shown in greater detail) by two optical fibers  34  and  36  in a manner allowing signal transmission. 
   The common potential P 0  is used as the reference potential for the electronic module  32 . The fundamental principle in selecting reference potentials is to select those potentials that do not produce unnecessarily high voltage stresses for driver chips of the turn-off capable semiconductor switches  21 ,  23 ,  25  and  27  or of the module packages for these turn-off capable semiconductor switches  21 ,  23 ,  25  and  27 . 
   This subsystem  14  according to the invention can be driven into four control states I, II, III and IV. In control state I, the turn-off capable semiconductor switches  21  and  25  are switched on, and the turn-off capable semiconductor switches  23  and  27  are switched off. As a result, a terminal voltage U X21  across the connecting terminals X 2  and X 1  of the subsystem  14  equals the capacitor voltage U C  across the capacitor  29 . In control state II, the turn-off capable semiconductor switches  21  and  27  are switched on, whereas the turn-off capable semiconductor switches  23  and  25  are switched off. The terminal voltage U X21  of the subsystem  14  now equals the sum of the capacitor voltages U C  across the unipolar capacitors  29  and  30 . In control state III, the turn-off capable semiconductor switches  23  and  25  are switched on and the turn-off capable semiconductor switches  21  and  27  are switched off. In this control state, the terminal voltage U X21  of the subsystem  14  equals zero. In control state IV, the turn-off capable semiconductor switches  23  and  27  are switched on, whereas the turn-off capable semiconductor switches  21  and  25  are switched off. As a result, the terminal voltage U X21  of the subsystem  14  changes from potential level “zero” to potential level “capacitor voltage U C ”, which lies across the unipolar capacitor  30 . In control state I or IV, the energy store  29  or  30  receives or releases energy depending on a current direction across the terminals. In control state II, the capacitors  29  and  30  receive or release energy depending on a current direction across the terminals. In a control state III (“zero”), the energy in the capacitors  29  and  30  remains constant. Hence, in terms of functionality, this subsystem  14  according to the invention is equivalent to connecting in series the known subsystem  11  and the known subsystem  12 . In contrast, however, this subsystem  14  according to the invention does not have the disadvantages of such a series connection. 
   When the two known subsystems  11  and  12  are connected in series, each subsystem  11  and  12  also has its own reference potential and its own electronic module. If only one of these two electronic modules is to be used, and the power for this electronic module is only to be tapped from one corresponding capacitor, the power is tapped asymmetrically and creates asymmetries in the voltage division. In addition, a greater outlay must be made for electrical isolation for driving the turn-off capable semiconductor switches and acquiring the capacitor voltages. These disadvantages no longer arise with the two-terminal subsystem  14  embodied according to the invention. 
     FIG. 5  shows an equivalent circuit of a second embodiment of a two-terminal subsystem  16  according to the invention. This subsystem  16  differs from the subsystem  14  shown in  FIG. 4  in that just pairs of turn-off capable semiconductor switches  21 ,  23  and  25 ,  27  are electrically connected in series. As in subsystem  14 , a diode  22 ,  24 ,  26  and  28  is electrically connected in antiparallel with each turn-off capable semiconductor switch  21 ,  23 ,  25  and  27 . A respective unipolar capacitor  29  or  30  is electrically connected in parallel with each series connection. The junction between the emitter of the turn-off capable semiconductor switch  23 , the anode of the diode  24  and the negative terminal of the unipolar capacitor  29  is electrically connected to a junction between the two turn-off capable semiconductor switches  25 ,  27  that are electrically connected in series. This junction forms a common potential P 0 , to which the reference-potential terminal M of the electronic module  32  is electrically connected. In addition, the junction between the emitter of the turn-off capable semiconductor switch  27 , the anode of the diode  28  and the negative terminal of the unipolar capacitor  30  forms the connecting terminal X 1  of the subsystem  16 . Hence, in terms of functionality, this embodiment of the two-terminal subsystem  16  is equivalent to connecting in series the two known subsystems  11 . Instead of using the negative terminal of the capacitor  29  as the reference potential for the electronic module  32 , other terminals can also be used as the reference potential. The fundamental principle in selecting reference potentials is to select those potentials that do not produce unnecessarily high voltage stress for the driver chips of the turn-off capable semiconductor switches  21 ,  23 ,  25  and  27  or of their module packages. 
   In a third embodiment of the two-terminal subsystem  18  according to the invention, in the same way as in the subsystem  16  shown in  FIG. 5 , pairs of turn-off capable semiconductor switches  21 ,  23  and  25 ,  27  are electrically connected in series. Unlike the subsystem  16 , the junction between the two turn-off capable semiconductor switches  21  and  23  that are electrically connected in series is now electrically connected to a junction between the collector of the turn-off capable semiconductor switch  25 , the cathode of the diode  26  and the positive terminal of the capacitor  30 . The junction between the two turn-off capable semiconductor switches  25  and  27  that are electrically connected in series now forms a first connecting terminal X 1 , whereas the junction between the collector of the turn-off capable semiconductor switch  21 , the cathode of the diode  22  and the positive terminal of the capacitor  29  forms a second connecting terminal X 2  of this subsystem  18 . As in the subsystem  16 , the negative terminal of the unipolar capacitor  29  is again provided as the reference potential for the electronic module  32  of this subsystem  18 . In terms of functionality, this subsystem  18  is equivalent to connecting in series the two known subsystems  12 . 
   In the two-terminal subsystem  20  corresponding to the equivalent circuit shown in  FIG. 7 , once again, pairs of the four turn-off capable semiconductor switches  21 ,  23  and  25 ,  27  are electrically connected in series, with a diode  22 ,  24 ,  26  and  28  being electrically connected in antiparallel with each turn-off capable semiconductor switch  21 , . . . ,  27 . A respective capacitor  29  or  30  is electrically connected in parallel with a series connection of the turn-off capable semiconductor switches  21 ,  23  or  25 ,  27 . The junction between the two turn-off capable semiconductor switches  21  and  23  that are electrically connected in series is electrically connected to the junction between the two turn-off capable semiconductor switches  25  and  27  that are electrically connected in series. The junction between the collector of the turn-off capable semiconductor switch  21 , the cathode of the diode  22  and the positive terminal of the capacitor  29  forms a second connecting terminal X 2  in this subsystem  20 . The junction between the emitter of the turn-off capable semiconductor switch  27 , the anode of the diode  28  and the negative terminal of the capacitor  30  forms a first connecting terminal X 1  of the subsystem  20 . In terms of functionality, this subsystem  20  is equivalent to connecting in series a known subsystem  12  with a known subsystem  11 . 
   By means of this embodiment according to the invention of the subsystems  14 ,  16 ,  28  and  20  for a converter circuit for high voltages, in particular in the field of drive technology and power engineering, the number of optical fibers between a converter circuit, comprising a multiplicity of series-connected subsystems, and a higher-level converter controller, is halved compared with a known converter circuit for high voltages. Each subsystem can be driven in such a way that a terminal voltage U X21  appears across its connecting terminals X 2 , X 1  that can assume four different potential levels. Such a terminal voltage U X21  can only be achieved with conventional subsystems  11  and  12  if two series-connected subsystems  11 ,  12  or  12 ,  12  or  11 ,  11  or  12 ,  11  are used. Compared with merely connecting in series two known subsystems  11  and  12 , with these being housed in one module, the subsystem  14  or  16  or  18  or  20  according to the invention requires just one electronic module  32 , and, in addition, its power can be supplied symmetrically from the capacitors  29  and  30 . As a result, no further outlay is required for electrical isolation for the drive and for acquiring a capacitor voltage U C .