Abstract:
The invention relates to a method for controlling a power converter comprising at least two phase modules, each of which is provided with an upper and a lower valve leg that is equipped with at least two serially connected bipolar subsystems, respectively. According to the invention, the switching actions in the two valve legs (T 1 , T 2 ; T 3 , T 4 ; T 5 , T 6 ) of each phase module ( 100 ) of the multiphase power converter having distributed energy stores are performed at a freely selected interval (ΔTZ) rather than synchronously. The inventive control method for a multiphase power converter having distributed energy stores thus makes it possible to dynamically regulate valve leg currents (i 11 , i 12 , i 21 , i 31 , i 32 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for controlling a converter having at least two phase modules, which each have an upper and a lower valve branch, which each have at least two series-connected two-pole subsystems, with a constant, freely variable number of subsystems of each phase module being operated such that their terminal voltages are in each case equal to a capacitor voltage across the energy storage capacitor in the associated subsystem, with the remaining subsystems of this phase module being operated such that their terminal voltages are equal to zero. 
     A polyphase converter is known from DE 101 03 031 A1.  FIG. 1  illustrates a circuit arrangement of a converter such as this, in more detail. According to this circuit arrangement, this known converter circuit has three phase modules, which are each annotated  100 . These phase modules  100  are each electrically conductively connected on the DC voltage side by a respective connection P or N to a positive and a negative DC voltage busbar P 0  and N 0 . There is a DC voltage U d  between these two DC voltage busbars P 0  and N 0 . Each phase module  100  has an upper and a lower valve branch T 1 , T 3  and T 5 , as well as T 4  and T 6 , respectively. Each of these valve branches T 1  to T 6  has a number of two-pole subsystems  11  which are electrically connected in series. Four of these subsystems  11  are shown for each valve branch T 1 , . . . , T 6  in this equivalent circuit. Two-pole subsystems  12  ( FIG. 3 ) can also be electrically connected in series instead of the two-pole subsystems  11  ( FIG. 2 ). Each junction point between two valve branches T 1  and T 2 , T 3  and T 4  or T 5  and T 6  of a phase module  100  forms a respective connection L 1 , L 2  or L 3  of this phase module  100  on the AC voltage side. Since, in this description, the converter has three phase modules  100 , a three-phase load, for example a three-phase motor, can also be connected to their connections L 1 , L 2  and L 3 , which are also referred to as load connections, on the AC voltage side. 
       FIG. 2  shows one embodiment of a two-pole known subsystem  11  in more detail. The circuit arrangement shown in  FIG. 3  represents a functionally completely equivalent variant, which is likewise known from DE 101 03 031 A1. These known two-pole subsystems  11  and  12  each have two semiconductor switches  1 ,  3  and  5 ,  7  which can be switched off, two diodes  2 ,  4  and  6 ,  8 , and a unipolar energy storage capacitor  9  and  10 . The two semiconductor switches  1  and  3 , as well as  5  and  7 , respectively, which can be switched off are electrically connected in series, with these series circuits being connected electrically in parallel with a respective energy storage capacitor  9  or  10 . One of the two diodes  2 ,  4  and  6 ,  8  is electrically connected in parallel with each semiconductor switch  1  and  3 , or  5  and  7 , respectively, which can be switched off such that these diodes  2 ,  4  and  6 ,  8  are electrically connected back to back in parallel with the corresponding semiconductor switches  1 ,  3 ,  5  or  7  which can be switched off. The unipolar energy storage capacitor  9  or  10  in the respective subsystem  11  or  12  comprises either a capacitor or a capacitor bank composed of a plurality of such capacitors with a resultant capacity C 0 . The connecting point of the emitter of the respective semiconductor switch  1  or  5  which can be switched off and the anode of the respective diode  2  or  6  forms a connecting terminal X 1  of the respective subsystem  11  or  12 . The connecting point of the two semiconductor switches  1  and  3  which can be switched off and of the two diodes  2  and  4  form a second connecting terminal X 2  of the subsystem  11 . The connecting point of the collector of the semiconductor switch  5  which can be switched off and the cathode of the diode  6  forms a second connecting terminal X 2  of the subsystem  12 . 
     In both illustrations of the embodiments of the two subsystems  11  and  12 , as illustrated in  FIGS. 2 and 3 , insulated gate bipolar transistors (IGBTs) are used as semiconductor switches  1 ,  3  and  5 ,  7  which can be switched off. Furthermore, MOS field-effect transistors, also referred to as MOSFETs, can be used. Gate turn-off thyristors (GTO thyristors) or integrated gate commutated thyristors (IGCTs) can likewise be used as semiconductor switches  1 ,  3  and  5 ,  7  which can be turned off. 
     According to DE 101 03 031 A1, the respective subsystems  11  and  12  of each phase module  100  of the polyphase converter shown in  FIG. 1  can be controlled in a switching state I, II or III, respectively. In the switching state I, the respective semiconductor switch  1  or  5  which can be turned off is switched on, and the respective semiconductor switch  3  or  7  which can be turned off in the subsystem  11  or  12  is switched off. This results in a terminal voltage U X21 , at the connecting terminals X 1  and X 2 , in the respective subsystem  11  or  12  being equal to zero. In the switching state II, the respective semiconductor switch  1  or  5  which can be turned off is switched off, and the respective semiconductor switch  3  or  7  which can be turned off in the subsystem  11  or  12  is switched on. In this switching state II, the terminal voltage U X21  that occurs is equal to the capacitor voltage U C  across the respective energy storage capacitor  9  or  10 . In the switching state III, both the respective semiconductor switches  1 ,  3  and  5 ,  7  which can be turned off are switched off, and the capacitor voltage U C  across the respective energy storage capacitor  9  or  10  is constant. 
       FIG. 4  shows a circuit arrangement of a further embodiment of a subsystem  14 , in more detail. This two-pole subsystem  14  was registered in a prior national patent application with the official file reference 2005P12105 DE, and has four semiconductor switches  21 ,  23 ,  25  and  27  which can be turned off, four diodes  22 ,  24 ,  26  and  28 , two unipolar capacitors  29  and  30  and electronics  32 , also referred to in the following text as the electronic assembly  32 . The four semiconductor switches  21 ,  23 ,  25  and  27  which can be turned off are connected electrically in series. Each of these semiconductor switches  21 ,  23 ,  25  and  27  has a diode  22 ,  24 ,  26  and  28  electrically connected back-to-back in parallel with it. One respective unipolar capacitor  29  or  30  is electrically connected in parallel with two respective semiconductor switches  21 ,  23  and  25 ,  27  which can be turned off. The respective unipolar capacitor  29  or  30  in this subsystem  14  comprises either a capacitor or a capacitor bank composed of a plurality of such capacitors with a resultant capacitance C 0 . The connecting point of the two semiconductor switches  21  and  23  which can be turned off and of the two diodes  22  and  24  forms a second connecting terminal X 2  of the subsystem  14 . The connecting point of the two semiconductor switches  25  and  27  which can be turned off and of the two diodes  26  and  28  forms a first connecting terminal X 1  of this subsystem  14 . The connecting point of the emitter of the semiconductor switch  23  which can be turned off, of the collector of the semiconductor switch  25  which can be turned off, of the anode of the diode  24 , of the cathode of the diode  26 , of the negative connection of the unipolar capacitor  29  and of the positive connection of the unipolar capacitor  30  forms a common potential             which is electrically conductively connected to a reference-ground potential connection M of the electronics assembly  32 . This electronics assembly  32  is linked for signalling purposes by means of two optical waveguides  34  and  36  to a higher-level converter control system, which is not illustrated in any more detail. The common potential           is used as a reference ground potential for the electronics assembly  32 .
     This subsystem  14  can be controlled in four switching states I, II, III and IV. In the switching state I, the semiconductor switches  21  and  25  which can be turned off are switched on, and the semiconductor switches  23  and  27  which can be turned off are switched off. In consequence, the terminal voltage U X21  at the connecting terminals X 2  and X 1  in the subsystem  14  is equal to the capacitor voltage U C  across the capacitor  29 . In the switching state II, the semiconductor switches  21  and  27  which can be turned off are switched on while, in contrast, the semiconductor switches  23  and  25  which can be turned off are switched off. The terminal voltage U X21  of the subsystem  14  now corresponds to the sum of the capacitor voltages U C  across the unipolar capacitors  29  and  30 . In the switching state III, the semiconductor switches  23  and  25  which can be turned off are switched on, and the semiconductor switches  21  and  27  which can be turned off are switched off. In this switching state, the terminal voltage U XZ1  of the subsystem  14  is equal to 0. In the switching state IV, the semiconductor switches  23  and  27  which can be turned off are switched on while, in contrast, the semiconductor switches  21  and  25  which can be turned off are switched off. In consequence, the terminal voltage U X21  of the subsystem  14  changes from the potential level “zero” to the potential level “capacitor voltage U C ” which is the voltage across the unipolar capacitor  30 . In the switching states I and IV, the respective energy store  29  or  30  receives or emits energy depending on the terminal current direction. In the switching state III, the capacitors  29  and  30  receive or emit energy depending on the terminal current direction. In a switching state III (“zero”), the energy in the capacitors  29  and  30  remains constant. This subsystem  14  according to the invention therefore corresponds, in terms of its functionality, to the known subsystem  11  being connected in series with the known subsystem  12 . 
     The maximum number of respective energy stores  9  and  10  which can in fact be connected in series between a positive terminal P and the connection Lx, where x=1, 2, 3, on the AC voltage side of each phase module  100  of the polyphase converter as shown in  FIG. 1  is referred to as the series operating cycle n. The maximum number of respective energy stores  9  and  10  which are actually connected in series between a positive terminal p and the connection Lx, where x=1, 2, 3, on the AC voltage side is reached when all the subsystems  11 ,  12  and/or all the subsystems  14  of this valve branch T 1 , T 3  or T 5  have been switched to the switching state II (U 11 =n·U C  and U 21 =n·U C  and U 31 =n·U C , respectively). It is advantageous, but not absolutely essential, to provide the same series operating cycle n between the connection Lx on the AC voltage side and a negative terminal N of each phase module  100 . The subsystems  11  and  12  shown in  FIGS. 2 and 3  have a respective energy storage capacitor  9  or  10 , while the subsystem  14  shown in  FIG. 4  contains two energy storage capacitors  29  and  30 . This therefore results in a series operating cycle of n=4 for the polyphase converter shown in  FIG. 1 , when four subsystems  11  and  12  are electrically connected in series in each case between the positive terminal P and the connection Lx, on the AC voltage side of each phase module  100 . However, if four subsystems  14  as shown in  FIG. 4  are connected in series between the positive terminal P and the connection Lx on the AC voltage side of each phase module  100 , then this results in a series operating cycle of n=8, since eight energy stores  29  and  30  can then be electrically connected in series. In applications in the field of power distribution, a polyphase converter such as this with distributed energy stores for each phase module  100  has at least 20 energy storage capacitors  9 ,  10  or  29 ,  30  connected electrically in series. Converters such as these are used for high-voltage direct-current transmission systems (HVDC system) or for flexible AC transmission systems, so-called FACTS. 
     The following explanatory notes are based on the assumption that all the energy stores in the subsystems  11 ,  12  or  14  of each valve branch T 1 , T 2 : T 3 , T 4  or T 5 , T 6 , respectively, of each phase module  100  of the polyphase converter and shown in  FIG. 1  each have the same capacitor voltage U C . Methods for initial production of this state and for maintaining it during operation of a converter such as this are known from DE 101 03 031 A1. 
       FIG. 5  shows an electrical equivalent circuit of the polyphase converter shown in  FIG. 1 . In this electrical equivalent circuit, the individual equivalent circuit components of each subsystem of a valve branch T 1  . . . , T 6  are combined to form an electrical equivalent circuit of one valve branch T 1 , . . . , T 6 . 
     In general, it is advantageous to design the polyphase converter such that, averaged over time, a suitable number of the systems  11 ,  12  and/or  14  are always being operated, such that the sum of their terminal voltages is given by: ΣU X21 =n·U C  (switching state II). This corresponds to precisely half of the energy stored in the series-connected subsystems  11 ,  12  and/or  14 , and leads to a mean intermediate-circuit voltage of U d =n·U C . This corresponds to a drive level b on the DC voltage side of 0.5, with the drive level b representing the ratio of the actual intermediate-circuit voltage U d  to the maximum possible intermediate-circuit voltage U dmax . This drive level is calculated using the following equation: 
     
       
         
           
             
               
                 
                   b 
                   = 
                   
                     
                       
                         U 
                         d 
                       
                       
                         U 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           max 
                         
                       
                     
                     = 
                     
                       
                         U 
                         d 
                       
                       
                         2 
                         · 
                         n 
                         · 
                         
                           U 
                           c 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Equivalent capacitance value of each valve branch T 1 , . . . , T 6 , averaged over time, is therefore C/m, where m=n/2. In order to prevent large uncontrolled equalizing currents flowing through the DC voltage busbars P 0  and N 0  between the individual phase modules  100  of the polyphase converter with distributed energy stores, the same nominal value is generally predetermined in each case between the terminals P and N of each phase module  100  for the respective voltages U 11 , U 12 , and U 21 , U 22 , and U 31 , U 32  and this means that:
 
 U   11   +U   12   =U   21   +U   22   =U   31   +U   32   =U   d .  (2)
 
     If the respective semiconductor switches  1 ,  3 ;  5 ,  7  and  21 ,  23 ,  25 ,  27  of all the phase modules  100  of the polyphase converter with distributed energy stores are operated in a balanced form, then, for balancing reasons, the arithmetic mean values of the valve branch currents i 11 , i 12 , i 21 , i 22 , i 31  and i 32  become:
 
 ī   11   =ī   12   ī   21   ī   22   =ī   31   =ī   32 =⅓· I   d .  (3)
 
     Because of the effective impedances of the phase modules  100  of the polyphase converter when the phases are being operated and loaded in a balanced form, these values are passive. The time profiles of the valve branch currents i 11 (t), i 12 (t), i 21 (t), i 22 (t), i 31 (t) and i 32 (t) therefore correspond to the following equations:
 
 i   11 ( t )˜⅓ ·I   d +½ ·i   L1 ( t ),
 
 i   12 ( t )˜⅓ ·I   d +½ ·i   L1 ( t ),
 
 i   21 ( t )˜⅓· I   d +½ ·i   L2 ( t ),
 
 i   22 ( t )˜⅓ ·I   d +½ ·i   L2 ( t ),
 
 i   31 ( t )˜⅓ ·I   d +½ ·i   L3 ( t ),
 
 i   32 ( t )˜⅓ ·I   d −½ ·i   L3 ( t ),  (4)
 
     According to these equations, the valve branch currents i 11 (t), i 12 (t), i 21 (t), i 22 (t), i 31 (t) and i 32 (t) each have corresponding fundamental profiles comprising a DC component ⅓·            and an AC component which corresponds to half the output current i Lx (t). This combination results from the balanced operation and the identical impedances, resulting from this, in all the valve branches T 1 , . . . , T 6  ( FIG. 5 ).
     In order to ensure the passive setting of these valve branch currents i 11 (t), i 12 (t), i 21 (t), i 22 (t), i 31 (t) and i 32 (t) the following rules should be observed with regard to the operation of the semiconductor switches  1 ,  3 ;  5 ,  7  and  21 ,  23 ,  25 ,  27  which can be turned off in a respective subsystem  11 ,  12  or  14 :
         Within one phase module  100 , care should always be taken to ensure that a constant number of energy stores in the subsystems  11 ,  12  and/or  14  are connected in series at any given time.       

     This means that, when a switching state change occurs from the switching state I to the switching state II in any given subsystem  11  or  12 , or a change from the switching state I to II; IV to II; III to IV or III to I in any given subsystem  14 , or from the switching state II to the switching state I in any given subsystem  11  or  12 , or a change occurs from the switching state II to I; II to IV; IV to III or I to III in any given subsystem  14  in an upper or lower respective valve branch T 1 , T 3 , T 5  or T 2 , T 4 , T 6  of a phase module  100 , a corresponding switching state change must also take place from the switching state II to the switching state I of any given subsystem  11  or  12  or a change from the switching state II to I; II to IV; IV to III or I to III of any given subsystem  14  or from the switching state I to the switching state II of any given subsystem  11  or  12  or a change from the switching state I to II; IV to II; III to IV or III to I of any given subsystem  14  in a lower or upper respective valve branch T 2 , T 4 , T 6  or T 1 , T 3 , T 5 . With a drive level b of 0.5 on the DC voltage side, this means that the subsystems  11 ,  12  and/or  14  of a phase module  11  must always be switched such that n and only n energy stores in the subsystems  11 ,  12  and/or  14  are actually connected in series (U d =n·U C ). 
     If this condition is not satisfied, then this leads to undesirable and uncontrolled equalizing currents between the phase modules  100  of the polyphase converter with distributed energy stores as shown in  FIG. 1 . These equalizing currents are excited by a voltage/time integral ΔU ph , which can be calculated using the following equation:
 
Δ U   ph   =k·U   C   ·ΔT   (5)
 
     In this case, ΔT is a difference time interval which can occur when a switching state change occurs. This difference time interval ΔT is very much less than 1 μs. The factor k is a constant indicating the difference between the number of energy stores actually connected in series in the subsystems  11 ,  12  and/or  14  and the series operating cycle n. If the drive level b on the DC voltage side is 0.5, then: −n≦k≦n. The equalizing currents which are excited by this voltage/time integral ΔU ph  can be calculated using the electrical equivalent circuit shown in  FIG. 5 . In order to prevent high voltage/time integrals ΔU ph  resulting in the excitation of high equalizing currents, the drive for the polyphase converter with distributed energy stores should be designed such that only one or only a small number of subsystems  11 ,  12  and/or  14  of one valve branch T 1 , . . . , T 6  can have their switching states changed at any one time. 
     This measure limits the constant k to low values. 
     Basic profiles of the valve branch voltages U x1  and U x2 , where x=1, 2, of an upper respective valve branch T 1 , T 3  or T 5  and a lower respective valve branch T 2 , T 4  or T 6  of a phase module  100  of a polyphase converter with distributed energy stores are each illustrated, by way of example, in a graph plotted against time t in  FIGS. 6 and 7 . The graph in  FIG. 8  shows the profile of the sum of the two valve branch voltages U x1  and U x2  plotted against time t. In accordance with the control method described above, the sum of the two valve branch voltages U x1  and U x2  is always constant and corresponds to the intermediate-circuit voltage U d . The switching operations illustrated in  FIGS. 6 and 7  are required in order to allow the illustrated profile of the valve branch voltages U x1  and U x2  to be set. These valve branch voltages U x1  and U x2  of a phase module  100  are controlled by a higher-level control system. 
     According to the known control method, when the number of energy stores which are actually connected in series in the upper respective valve branch T 1 , T 3  or T 5  is changed, a corresponding number of subsystems  11 ,  12  and/or  14  in the lower respective valve branch T 2 , T 4  or T 6  have their switching state changed such that, in each phase module  100 , a constant number n of energy stores are still connected in series in the subsystems  11 ,  12  and/or  14  for a drive level b of 0.5 on the DC voltage side. This results in a constant DC voltage of U d =n·U C . 
     If this known method is used in all the parallel-connected phase modules  100  of the polyphase converter with distributed energy stores, this generally leads to there being no significant equalization processes in the form of equalizing currents between these phase modules  100 . However, this is also dependent on the impedance relationships illustrated in  FIG. 5 . 
     SUMMARY OF THE INVENTION 
     The invention is now based on the idea of being able to influence the valve branch currents i 11 , i 12 , i 21 , i 22 , i 31  and i 32  differently from their passively set profile. 
     In principle, additional valve branch currents i Zxy (t) can be set and controlled as required in each valve branch T 1 , T 2 ; T 3 , T 4  or T 5 , T 6 , respectively, in a time profile for a valve branch current i 11 (t), i 12 (t), i 21 (t), i 22 (t), i 31 (t) and i 32 (t). These additional valve branch currents i Zxy (t) result in the time profiles of the valve branch currents, according to equation system ( 4 ), becoming:
 
 i   11 ( t )=⅓ ·I   d +½ ·i   L1 ( t )+ i   Z11 ( t ),
 
 i   12 ( t )=⅓ ·I   d +½ i   L1 ( t )+ i   Z12 ( t ),
 
 i   21 ( t )=⅓ ·I   d +½ ·i   L2 ( t )+ i   Z21 ( t ),
 
 i   22 ( t )=⅓ ·I   d +½ ·i   L2 ( t )+ i   Z22 ( t ),
 
 i   31 ( t )=⅓ ·I   d +½ ·i   L3 ( t )+ i   Z31 ( t ),
 
 i   32 ( t )=⅓ ·I   d +½ ·i   L3 ( t )+ i   Z32 ( t )  (6)
 
     In order to ensure that the output currents i Lx (t) do not change, the additional valve branch currents i Zxy (t) are set such that the additional valve branch currents i Zxy (t) of each phase module  100  are the same. This means that:
 
 i   Z11 ( t )= i   Z12 ( t ),
 
 i   Z12 ( t )= i   Z22 ( t ),  (7)
 
 i   Z31 ( t )=i Z32 ( t ),
 
     The invention is now based on the object of developing the known control method for a polyphase converter with distributed energy stores such that predetermined additional valve branch currents occur. 
     According to one aspect of the invention, this object is achieved by a method for controlling a polyphase converter having at least two phase modules, which have an upper and a lower valve branch, which each have at least two series-connected two-pole subsystems, with switching operations in the upper valve branch and corresponding switching operations in the lower valve branch of each phase module being carried out with a freely variable time interval between them. 
     According to another aspect of the invention this object is achieved by a method for controlling a polyphase converter having at least two phase modules, which each have an upper and a lower valve branch, which each have at least two series-connected two-pole subsystems, with at least two further switching operations, which are offset with respect to one another for a predetermined time interval, being carried out between time-synchronized switching operations in the upper and lower valve branch of each phase module ( 100 ), in an upper and/or a lower valve branch of each phase module. 
     Since additional voltage/time integrals are used in the valve branch voltages of a phase module as a manipulated variable to influence the valve branch currents, the valve branch currents can be influenced deliberately. 
     Voltage/time integrals such as these are produced, according to the invention, by the switching operations in the two valve branches of each phase module of the polyphase converter with distributed energy stores no longer being carried out synchronized in time, but with a freely variable time interval. 
     Voltage/time integrals such as these are also produced according to the invention by providing a further switching operation between the switching operations which are synchronized in time. 
     These further switching operations can be carried out in an upper and/or a lower valve branch of each phase module of the polyphase converter with distributed energy stores. This results in a balanced drive at the times of the additional switching operations in the upper and/or lower valve branches of each phase module of the polyphase converter with distributed energy stores. 
     In one advantageous method, the switching operations of an upper valve branch of a phase module are carried out delayed and/or advanced with respect to switching operations of a lower valve branch of this phase module. This allows a predetermined additional voltage/time integral to be set dynamically over one period of the valve branch voltages of a phase module. 
     In a further advantageous method, the two methods are combined with one another in order to generate additional voltage/time integrals. This means that a required predetermined voltage/time integral can be generated at any desired time. 
     A valve branch current can in each case be calculated as a function of the additional voltage/time integrals in conjunction with the electrical equivalent circuit of the valve branches of the polyphase converter with distributed energy stores. If the valve branch currents of the individual phase modules of the polyphase converter with distributed energy stores are measured, then an additional voltage/time integral can be determined at any time, ensuring that the existing valve branch currents are changed such that equalizing currents can no longer flow between the phase modules of the polyphase converter with distributed energy stores. 
     The use of the control method according to the invention results in dynamic control of the valve branch currents of a polyphase converter with distributed energy stores. Inter alia, this use results in a number of advantages:
         damping of current oscillations, for example caused by:
           transient load change processes   faults, for example unbalances in a power supply system or a machine, ground faults, lightning strikes, switching overvoltages, . . .   inadequate damping of capacitive networks by the inductances and resistances provided in the design.   
           Faults coped with better.   Poor operating points coped with such as:
           operating points at low output frequencies.   
           Capabilities to optimize the design of the subsystems and of the polyphase converter in terms of capacitor complexity and the need for power semiconductors.   A uniform load ensured on all semiconductor switches which can be turned off.   Balancing of highly unbalanced voltage on the individual converter elements after fault disconnection.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The rest of the explanation of the invention refers to the drawing, which schematically illustrates a plurality of embodiments of one method according to the invention for controlling a polyphase converter with distributed energy stores, and in which: 
         FIG. 1  shows a circuit arrangement of a known converter with distributed energy stores, 
         FIGS. 2 to 4  each show a circuit arrangement of one embodiment of a known subsystem, 
         FIG. 5  shows an electrical equivalent circuit of the valve branches of the converter shown in  FIG. 1 , 
         FIGS. 6 and 7  each use a graph plotted against time t to show a valve branch voltage of an upper and lower valve branch of a phase module of the known converter shown in  FIG. 1 , 
         FIG. 8  uses a graph plotted against time t to show the sum voltage of the two valve branch voltages shown in  FIGS. 6 and 7 , 
         FIGS. 9 and 10  each use a graph plotted against time t to show the valve branch voltages of a phase module of the converter shown in  FIG. 1 , when using a first embodiment of the control method according to the invention, 
         FIG. 11  uses a graph plotted against time t to show the sum voltage of the two valve branch voltages shown in  FIGS. 9 and 10 , 
         FIGS. 12 and 13  each use a graph plotted against time t to show valve branch voltages of a phase module of the converter shown in  FIG. 1  when using a second embodiment of the control method according to the invention, 
         FIG. 14  uses a graph plotted against time t to show the associated sum voltage, 
         FIGS. 15 and 16  each use a graph plotted against time t to show a valve branch voltage of a phase module of the converter shown in  FIG. 1 , with these being the valve branch voltages which occur when using a combination of the two embodiments of the control method according to the invention, and 
         FIG. 17  shows a graph plotted against time t of the associated sum voltage. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The graph in  FIG. 9  shows the profile of a valve branch voltage U x1  of an upper valve branch T 1 , T 3  and T 5  of a phase module  100  of the converter shown in  FIG. 1 , plotted against time t. The time profile of a valve branch voltage U x2  of a lower valve branch T 2 , T 4  or T 6  of this phase module  100  is illustrated in more detail in the graph in  FIG. 10 . The sum voltage of these two valve branch voltages U x1  and U x2  of a phase module  100  of the converter shown in  FIG. 1  is illustrated, plotted against time t, in the graph in  FIG. 11 . If this sum voltage is compared with the sum voltage in  FIG. 8 , it is evident that the sum voltage shown in  FIG. 11  has additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4 . These additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  occur because the switching operations in the upper and lower respective valve branches T 1  and T 2 ; T 3  and T 4  as well as T 5  and T 6  of a phase module  100  are no longer carried out synchronized in time. Any given subsystem  11 ,  12  changes from the switching state I to the switching state II at the time t 1 , or any given subsystem  14  in the lower respective valve branch T 2 , T 4  or T 6  of the phase module  100  changes from the switching state I to II or IV to II, or III to IV, or III to I at the time t 1  in comparison to the change of the switching state II to the switching state I of any given subsystem  11 ,  12  or the change from the switching state II to I, II to IV, IV to III, or I to III of any given subsystem  14  in the upper respective valve branch T 1 , T 3  or T 5  of this phase module  100 , delayed by a time interval ΔT 1 . The additional voltage/time integral ΔU ph1  resulting from this is calculated using the following equation:
 
Δ U   ph   =k·U   C   ·ΔT   Z   (8)
 
     In this case, the factor k indicates the difference between the energy stores (in the switching state II in subsystems  11 ,  12  and in the switching state I or II or IV in the subsystem  14 ) which are actually connected in series and through which current passes during the time interval ΔT Z , and the series operating cycle n. In this example, the series operating cycle is n=4. This results in a factor of k=−1 for the time interval ΔT 1 . At the time t 4 , any given subsystem  11 ,  12  changes from the switching state I to the switching state II, or any given subsystem  14  in the upper valve branch T 1 , T 3  or T 5  changes from the switching state I to II, IV to II, III to IV, or III to I, with an advance corresponding to the time interval ΔT 2  with respect to any given subsystem  11 ,  12  changing from the switching state II to the switching state I or any given subsystem  14  in the lower valve branch T 2 , T 4 , or T 6  changing from the switching state II to I, II to IV, IV to III or I to III. The factor is therefore k=+1 during the time interval ΔT 2 . The magnitude of the additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  can be determined using the freely variable time interval ΔT Z . The mathematical sign of the additional voltage/time integral ΔU ph  and therefore the mathematical sign of an additional valve branch current i Zxy (t) are determined by means of the factor k. The additional valve branch current i Zxy (t) can be varied by generating a plurality of additional voltage/time integrals ΔU ph  distributed over the period of the fundamental frequency of the valve branch voltage U x1  or U x2 , respectively, of a respective upper or lower valve branch T 1 , T 3 , T 5  or T 2 , T 4 , T 6 . The valve branch currents i xy (t) can be dynamically controlled by means of this method according to the invention for controlling a polyphase converter with distributed energy stores as shown in  FIG. 1 . 
     The graph in  FIG. 12  shows the profile of a valve branch voltage U x1  of an upper valve branch T 1 , T 3  or T 5  of a phase module  100  of a converter shown in  FIG. 1 . The profile of a valve branch voltage U x2  of a corresponding respective valve branch T 2 , T 4  or T 6  of this phase module  100  is plotted against time t in the graph in  FIG. 13 . The associated sum voltage of these two valve branch voltages U x1  and U x2  is illustrated plotted against time t in the graph in  FIG. 14 . These two valve branch voltages U x1  and U x2  differ from the two valve branch voltages U x1  and U x2  shown in  FIGS. 6 and 7  by additional switching operations being carried out in addition to the switching operations that are synchronized in time. Two switching operations have been inserted in the profile of the valve branch voltage U x1  in the time period t 2 -t 1 , resulting in connection of a further respective subsystem  11  or  12  or a further energy store of a subsystem  14  of the respective upper valve branch T 1 , T 3  or T 5  of a phase module  100  for a time interval ΔT 1 . Further switching operations such as these are carried out in the time period t 5 -t 4  for a time interval ΔT 2 . Two switching operations have been inserted in the profile of the valve branch voltage U x2  in the time period t 8 -t 7 . These switching operations result in two respective subsystems  11  and  12  or two respective energy stores in the subsystems  14  being turned off for a time interval ΔT 3  in the lower valve branch T 2 , T 4  or T 6 , respectively, of a phase module  100 . In the time period t 11 -t 10 , respective further switching operations are carried out in the upper and lower valve branch T 1 , T 3 , T 5  and T 2 , T 4 , T 6 . As a result of these switching operations, a respective subsystem  11  or  12  or an energy store in a subsystem  14  of a phase module  100  is turned off for this time interval ΔT 4  in the upper respective valve branch T 1 , T 3  or T 5  and a respective subsystem  11  or  12  or an energy store in a subsystem  14  is likewise turned off for the same time interval ΔT 4  in the lower respective valve branch T 2 , T 4  or T 6 . These further switching operations in the upper and/or lower valve branches T 1 , T 3 , T 5  and/or T 2 , T 4 , T 6  result in additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  being generated, which each generate additional valve branch currents i Zxy (t) in the respective valve branches T 1 , T 2 ; T 3 , T 4  or T 5 , T 6  of each phase module  100  of the converter shown in  FIG. 1 . These additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  can be obtained from the sum voltage of the two valve branch voltages U x1  and U x2 . The magnitude of these additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  depends on which additional valve branch currents i Zxy (t) are required in the respective valve branches T 1 , T 2 ; T 3 , T 4  or T 5 , T 6  of each phase module  100 . These additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  are calculated using the equation (7). The additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  obtained from this can also be distributed over time over one period of the fundamental frequency of the valve branch voltage U x1  or U x2 , respectively, in the method for additionally introduced switching operations. 
     A combination of the methods for producing additional voltage/time integrals ΔU ph1 , . . . , ΔU ph4  by delayed and/or advanced switching operations with additional switching operations leads to the profiles of the valve branch voltages U x1  and U x2  of an upper and lower respective valve branch T 1 , T 2 ; T 3 , T 4  or T 5 , T 6  of a phase module  100  of the converter shown in  FIG. 1 . These valve branch voltages U x1  and U x2  are respectively shown in a graph plotted against time t in  FIGS. 15 and 16 . An associated sum voltage of these valve branch voltages U x1  and U x2  plotted against time is illustrated in the graph in  FIG. 17 .