Abstract:
An inverter having three phase modules with an upper valve arm and a lower valve arm having each at least three two-pole subsystems connected in series, which each subsystem having a storage capacitor, is controlled in the event of failure of one or more subsystems by setting the terminal voltage of the failed subsystems permanently to zero, setting the terminal voltage of a corresponding number of fault-free subsystems in corresponding fault-free valve branches likewise to zero, and increasing the capacitor voltages of the fault-free subsystems of the failed valve branches such that their sum is equal to the sum of the capacitor voltages of the subsystems of a corresponding fault-free valve branch, while leaving the control of the fault-free phase modules unchanged. In this way, a symmetrical voltage system with maximum amplitude is obtained at the inverter outputs.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2009/055808, filed May 14, 2009, which designated the United States and has been published as International Publication No. WO 2010/015430 and which claims the priority of German Patent Application, Serial No. 10 2008 036 811.3, filed Aug. 7, 2008, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for controlling a converter with distributed energy stores. 
     DE 101 03 031 A1 discloses a converter with distributed energy stores. An equivalent circuit of a converter such as this is shown in more detail in  FIG. 1 . According to this equivalent circuit, this known converter, which is annotated  102 , has three phase modules, which are each annotated  100 . These phase modules  100  are each electrically conductively connected on the DC voltage side to a positive and a negative DC voltage busbar P 0  and N 0 . In the case of a voltage intermediate-circuit converter, a series circuit of two capacitors C 1  and C 2 , across which a DC voltage U d  is dropped, would be connected between these two DC voltage busbars P 0  and N 0 . A connection point between these two capacitors C 1  and C 2 , which are electrically connected in series, forms a virtual neutral point O. Each phase module  100  which forms a bridge arm of the polyphase converter has an upper and a lower bridge arm element, which are referred to in the following text as the respective valve arms T 1 , T 3  and T 5  as well as T 2 , T 4  and T 6 , since the bridge arm elements each represent a converter valve of the polyphase converter with distributed energy stores. Each of these valve arms T 1  to T 6  has a number of two-pole subsystems  10  which are electrically connected in series. In this equivalent circuit of the converter  102 , each valve arm T 1 , . . . , T 6  has four two-pole submodules  10 . The number of subsystems  10  per valve arm T 1 , . . . , T 6  is, however, not restricted to this illustrated number. Each junction point between two valve arms T 1  and T 2 ; T 3  and T 4  as well as T 5  and T 6  of a phase module  100  forms a respective connection L 1 , L 2  and L 3  on the AC voltage side of a phase module  100 . Since, in this illustration, the converter  102  has three phase modules  100 , a three-phase load, for example a polyphase motor, can be connected to their connections L 1 , L 2  and L 3  on the AC voltage side, which are also referred to as load connections. 
       FIG. 2  shows an equivalent circuit of one known embodiment of a two-pole subsystem  10  in more detail. The circuit arrangement shown in  FIG. 3  represents a functionally completely equivalent variant. Both embodiments of a two-pole subsystem  10  are known from DE 101 03 031 A1. These known two-pole subsystems  10  each have two semiconductor switches  1  and  3  which can be turned off, in each case two diodes  2  and  4  and in each case one unipolar energy storage capacitor  9 . The two semiconductor switches  1  and  3  which can be turned off are electrically connected in series, with this series circuit being connected electrically in parallel with the energy storage capacitor  9 . One of the two diodes  2  and  4  is electrically connected in parallel with each semiconductor switch  1  and  3  which can be turned off such that these diodes  2  and  4  are connected back-to-back in parallel with the corresponding semiconductor switch  1  or  3  which can be turned off. The unipolar energy storage capacitor  9  in the subsystem  10  consists either of a capacitor or of a capacitor bank having a plurality of such capacitors. The connection point between the emitter of the semiconductor switch  1  which can be turned off and the anode of the diode  2  forms a connecting terminal X 1  of the subsystem  10 . The connection point between the two semiconductor switches  1  and  3  which can be turned off and the two diodes  2  and  4  forms a second connecting terminal X 2  of the subsystem  10 . 
     In the embodiment of the two-pole subsystem  10  shown in  FIG. 3 , this connection point forms the first connecting terminal X 1 . The connection point between the collector of the semiconductor switch  1  which can be turned off and the cathode of the diode  2  forms the second connecting terminal X 2  of the subsystem  10 . 
     In both embodiments of the two-pole subsystem  10  shown in  FIGS. 2 and 3 , Insulated Gate Bipolar Transistors (IGBT) are used as the semiconductor switches  1  and  3  which can be turned off. It is likewise possible to use MOS Field-Effect Transistors, also referred to as MOSFETs. It is also possible to use Gate Turn Off Thyristors, also referred as GTO thyristors, or Integrated Gate Commutated Thyristors (IGCT). 
     According to DE 101 03 031 A1, the two-pole subsystems  10  in each phase module  100  of the converter  102  as shown in  FIG. 1  are switched to a switching state I, II and III. In the switching state I, the semiconductor switch  1  which can be turned off is switched on, and the semiconductor switch  3  which can be turned off is switched off. The terminal voltage U X21  which is present at the connecting terminals X 1  and X 2  in the two-pole subsystem  10  is therefore equal to zero. In the switching state II, the semiconductor switch  1  which can be turned off is switched off, and the semiconductor switch  3  which can be turned off is switched on. In this switching state II, the terminal voltage U X21  which is present is equal to the capacitor voltage U C  across the energy storage capacitor  9 . In the switching state III, both semiconductor switches  1  and  3  which can be turned off are switched off, and the capacitor voltage U C  across the energy storage capacitor  9  is constant. 
     In order to allow this converter  102  to operate in redundant form with distributed energy stores  9  as shown in  FIG. 1 , it is necessary to ensure that a faulty subsystem  10  is permanently short-circuited at its terminals X 1  and X 2 . This means that the terminal voltage U X21  of the faulty subsystem  10  is zero irrespective of the current direction through the terminals X 1  and X 2 . 
     The failure of one of the semiconductor switches  1  and  3  which can be turned off and are present in the subsystem  10 , or of an associated control circuit, results in this subsystem  10  not operating correctly. Further possible causes of malfunctions are, inter alia, faults in the associated control circuit for the semiconductor switches, their power supply, communication and measured-value detection. That is to say, the two-pole subsystem  10  can no longer be controlled as desired in one of the possible switching states I, II or III. The short-circuiting of the subsystem  10  at its connections X 1  and X 2  means that no more power is supplied to this subsystem  10 . This reliably precludes consequential damage such as overheating and fire resulting from continued operation of the converter  102 . 
     A conductive connection like a short circuit such as this between the connecting terminals X 1  and X 2  of a faulty two-pole subsystem  10  has to reliably carry at least the operating current of one valve arm T 1 , . . . , T 6  in the phase module  100  in which the faulty two-pole subsystem  10  is connected, without overheating. DE 10 2005 040 543 A1 discloses how a faulty subsystem  10  can be reliably short-circuited in order that this known converter  102  with distributed energy stores  9  can continue to be operated in a redundant form. 
     The following explanation is based on the assumption that the energy storage capacitors  9  in all of the two-pole subsystems  10  each have the same voltage U C . Methods for initially producing this state and for maintaining it during operation are likewise known from DE 101 03 031 A1.  FIG. 4  shows a graph of a profile of the potential difference U PL  between the terminal P of a phase module  100  and a load connection L, plotted against the time t.  FIG. 5  shows a graph of a profile of the potential difference U LN  between the load connection L and the potential at the terminal N, plotted against the time t. According to these potential profiles U PL  and U LN , in each case one subsystem  10  of the eight two-pole subsystems  10  of the valve arms T 1  and T 2  is connected or disconnected at each of the times t 1 , . . . , t 8 . Switching on in this case corresponds to a change from the switching state I to the switching state II. Turning off corresponds to a change from the switching state II to the switching state I. These two graphs each show one period T P  of a fundamental oscillation of the potential profile U L0  ( FIG. 6 ) between the load connection L and the virtual neutral point O of a phase module  100  of the converter  102  with distributed energy stores  9 , for the potential profiles U PL  and U LN . 
       FIG. 6  shows a profile of the difference between the potential profiles U LN  and U PL  as shown in  FIGS. 4 and 5 , in the form of a graph plotted against the time t. This resultant potential profile U LO  occurs between a connection L 1 , L 2  or L 3  on the AC voltage side of a phase module  100  in the converter  102  with distributed energy stores  9  as shown in  FIG. 1  and a virtual neutral point O, which is formed by the connection point between the two capacitors C 1  and C 2  in a voltage intermediate circuit having two capacitors C 1  and C 2 . Corresponding components of harmonics or DC voltage components in each of the output voltages U LXO  of the phase modules  100  in the polyphase converter  102  with distributed energy stores  9  as shown in  FIG. 1  are cancelled out in the case of a balanced polyphase voltage system in the difference voltages between in each case two phase-shifted output voltages U L10 , U L20  or U L30 . These two potential profiles U PL  and U LN  likewise show that the sum of the potentials at any time is 4·U C . This means that the value of the DC voltage U d  between the DC voltage busbars P 0  and N 0  always corresponds to a constant number of subsystems  10  in the switching state II multiplied by the value of the capacitor voltage U C  across the capacitor  9 . In the situation illustrated by way of example, this number corresponds to the number of two-pole subsystems  10  in the valve arms T 1 , . . . , T 6  in the converter  102  as shown in  FIG. 1 . 
       FIG. 7  shows the output voltages U L10 , U L20  and U L30  of the converter  102  with distributed energy stores  9  and the associated line voltages U L12 , L L23  and U L31  together. In this sound situation, the output voltages U L10 , L L20  and U L30  and their line voltages U L12 , U L23  and U L31  form a balanced polyphase system. This means that the phase shift between the output voltages U L10 , L L20  and U L30  and their line voltages U L12 , U L23  and U L31  in the three phase modules  100  in the converter  102  with distributed energy stores  9  with respect to one another is 120° electrical. 
     DE 10 2005 045 091 A1 discloses a method for controlling a converter with distributed energy stores as shown in  FIG. 1 , by means of which the balance conditions are maintained in the event of a malfunction of at least one subsystem in a phase module of this converter. According to this known method, one valve arm of one of the three phases in which one or more of the two-pole subsystems is or are faulty is first of all determined. Each faulty subsystem is controlled such that the terminal voltage is in each case zero. A number of subsystems corresponding to the number of determined two-pole subsystems in a further valve arm of the faulty phase module are controlled such that the terminal voltage is in each case equal to a capacitor voltage. This control of the subsystems in the faulty phase module is likewise carried out in subsystems in the valve arms of the sound phase modules. 
       FIG. 8  shows a graph of a profile of the potential difference U PL1  between the terminal P in a phase module  100  and a load connection L 1 , plotted against the time t, with one faulty two-pole subsystem  10  in the lower valve arm T 2 .  FIG. 9  shows a graph of a profile of the potential difference U L1N  between the terminal L 1  and the potential of the terminal N, plotted against the time t. As can be seen from the profile of the potential difference U PL1  in  FIG. 8 , a subsystem  10  in the upper valve arm T 1  of the phase module  100  is controlled such that its terminal voltage U X21  is always equal to the capacitor voltage U C  across the energy storage capacitor  9 . Of the four subsystems  10  illustrated by way of example in the upper valve arm T 1 , there are now only three remaining subsystems  10  which can be connected and disconnected. As can be seen from the time profile of the potential difference U L1N  of the lower valve arm T 2  in the phase module  100 , one of the four subsystems  10  illustrated by way of example is controlled such that its terminal voltages U X21  are always equal to zero. As shown in  FIG. 1 , of these lower valve arms T 2 , T 4  and T 6  in the three phase modules  100 , the valve arm T 2  has a faulty two-pole subsystem  10 , identified by shading. The value of the amplitude of the voltage U L1N  of the valve arm T 2  can therefore now be only at most 3·U C . This known method results in the number of subsystems  10  used in the event of a fault being equal to the number of subsystems  10  used when no faults are present. The profile of the amplitude of the sum of the potential differences U PL1  and U L1N  is shown by means of a dashed line in the graph in  FIG. 9 . In comparison to the situation when there are no faults, the voltages U L10 , U L20  and U L30  each have a lower maximum amplitude when a fault is present. In the illustrated example, these voltages U L10 , U L20  and U L30  when no fault is present have a maximum voltage amplitude of ½·U d  each, while in contrast the maximum amplitude when a fault is present is only ⅜·U d . This means that this known method results in a balanced three-phase voltage system, with a lower maximum amplitude, when a fault is present. 
       FIG. 10  shows a profile of the difference in the voltage differences U PL1  and U L1N  as shown in  FIGS. 8 and 9 , plotted against the time t. As can be seen from this time profile of the potential U L10  between the load connection L 1  and a virtual neutral point O, this potential no longer oscillates symmetrically about a null position. This null position is shifted through ⅛· d . This means that this potential profile has a DC voltage component. 
       FIG. 11  shows a vector diagram of a three-phase voltage system in the converter  102  as shown in  FIG. 1  when one subsystem  10  is faulty. As can be seen from this voltage system, the amplitudes of the output voltages U L20  and U L30  have not changed in comparison to the voltage system shown in  FIG. 7 . Since one subsystem  10  (shaded) has failed in the valve arm T 2  the amplitude of the output voltage U L10  of this faulty phase module  100  has been reduced in magnitude by the magnitude of a capacitor voltage U C . The magnitudes of the line voltages U L12 , U L23  and U L31  are therefore no longer the same. The amplitudes of the two line voltages U L12  and U L31  are the same, but are less than the line voltage U L23 . The failure of at least one subsystem  10  in a valve arm T 1 , . . . , T 6  results in an unbalanced voltage system from a balanced voltage system of line voltages U L12 , U L23 , U L31 . The unbalance which occurs depends on the number of subsystems  10  which have failed and on the number of valve arms T 1 , . . . , T 6  which are affected. 
     SUMMARY OF THE INVENTION 
     The invention is now based on the object of specifying a method for controlling a three-phase converter with distributed energy stores, by means of which a balanced three-phase voltage system can be generated when at least one energy store fails. 
     In accordance with the method according to the invention, the number of failed subsystems and therefore the faulty valve arms in the phase modules of the converter with distributed energy stores are determined first of all. The faulty subsystems and the subsystems of sound valve arms in faulty phase modules are then controlled such that their terminal voltages are equal to zero. In consequence, all the faulty subsystems and subsystems in the sound valve arms of faulty phase modules are short-circuited, corresponding to the number of faulty subsystems. The output voltage of a faulty phase module therefore has a reduced amplitude which is symmetrical with respect to a null position. This means that this output voltage has no DC voltage component. An unbalanced voltage system is formed from a balanced three-phase voltage system, which is present at the output terminals of the converter with distributed energy stores, as a result of the failure of at least one subsystem in one valve arm of one phase module. 
     The fundamental idea on which this invention is based is that the output voltage from a faulty phase module which has been reduced because of the failure of at least one subsystem must be raised to its previous amplitude value again. In doing so, the unbalanced voltage system will become balanced again, and the amplitudes of this balanced voltage system would at the same time be a maximum. 
     This raises the question as to how an amplitude which is intended to correspond to the amplitude of an output voltage of a phase module during operation without any faults can be generated with a reduced number of subsystems. The solution is to charge the energy stores in the remaining subsystems in the valve arms of a faulty phase module to a higher extent such that the sum of the increased capacitor voltages in the subsystems in a valve arm of a faulty phase module is equal to the sum of the capacitor voltages in the subsystems in a valve arm of a sound phase module. The greater the number of subsystems that there are in each valve arm, the less is the increase in each capacitor voltage in the remaining subsystems in a valve arm of a faulty phase module if one subsystem fails. 
     In one advantageous method, the remaining subsystems in a faulty phase module are electrically conductively connected for a predetermined time interval successively to an energy source which provides at least the increased capacitor voltage. During this time interval, a current flows out of the energy source into the connected subsystem, thus recharging its energy store. When a capacitor voltage which is present in the energy store reaches a predetermined value, then this subsystem is disconnected from the energy source which is linked to a next subsystem in a faulty phase module. Since a voltage source such as this is already provided in a converter such as this for precharging of the energy stores in the subsystems in the converter, there is no need to add any hardware to an existing converter. 
     In a further advantageous method, an additional valve arm current is set which flows successively for a predetermined time interval through the remaining subsystems in a faulty phase module. This also increases the capacitor voltages of the remaining subsystems in a faulty phase module. 
     The method for generation of additional valve arm currents by means of additional voltage/time integrals is known from DE 10 2005 045 090 B4. 
     The method according to the invention is therefore subdivided into three sections, specifically the determination of failed subsystems and their short circuits, wherein subsystems are also short-circuited corresponding to the number of failed subsystems in a valve arm of a valve arm, which corresponds to the faulty valve arm, in a faulty phase module. The output voltage of the faulty phase module is therefore reduced corresponding to the number of failed subsystems, but has no DC voltage component. In the second section, the capacitor voltages of the remaining subsystems in a faulty phase module are increased such that the sum of these capacitor voltages is equal to the sum of the capacitor voltages of a sound phase module. A third section of the method according to the invention deals with the control of the subsystems in the sound phase modules. This control does not differ from the control of the subsystems in the converter with distributed energy stores during operation when no faults are present. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In order to explain the invention further, reference is made to the drawing, which schematically illustrates one embodiment of a method according to the invention for controlling a three-phase converter with distributed energy stores. 
         FIG. 1  shows an equivalent circuit of a known converter with distributed energy stores, 
         FIG. 2  shows an equivalent circuit of a first embodiment of a known two-pole subsystem in the converter shown in  FIG. 1 , 
         FIG. 3  shows an equivalent circuit of a second embodiment of a known two-pole subsystem in the converter as shown in  FIG. 1 , 
         FIGS. 4 to 6  show potential profiles of a phase module of a converter as shown in  FIG. 1  when no faults are present, in each case in the form of a graph plotted against the time t, 
         FIG. 7  shows a vector diagram of a balanced three-phase voltage system in the converter as shown in  FIG. 1  when no faults are present, 
         FIGS. 8 to 10  show potential profiles of a phase module in a converter as shown in  FIG. 1  when a fault is present, in each case in the form of a graph plotted against the time t, 
         FIG. 11  shows a vector diagram of an unbalanced three-phase voltage system in the converter as shown in  FIG. 1 , when a fault is present, 
         FIG. 12  shows a block diagram of a control system according to the invention for a converter as shown in  FIG. 1 , and 
         FIGS. 13 to 15  show voltage profiles of a faulty phase module in the converter as shown in  FIG. 1 , in each case in the form of a graph plotted against the time t, generated by means of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 12  shows a block diagram of a control system for a converter  102  with distributed energy stores  9  as shown in  FIG. 1 . In this block diagram,  104  denotes a device for production of control signals S v ,  106  denotes a device for determination of faulty subsystems  10 ,  108  denotes a memory apparatus and  110  denotes a low-voltage energy source which can be connected. On the output side, the device  104  is electrically conductively connected to control connections of the semiconductor switches  1  and  3  in the two-pole subsystems  10  in the valve arms T 1  to T 6  in the converter  102 . The output voltages U L10 , U L20  and U L30  which are present at the connections L 1 , L 2  and L 3  on the AC voltage side, also referred to as the output terminals of the converter  102 , are supplied to the device  106  for determination of faulty two-pole subsystems  10 . On the output side, this device  106  is linked on the one hand to an input of the device  104  for production of control signals S v , and on the other hand to an input of the memory unit  108 . On the output side, this memory apparatus  108  is connected to an energy source  110  which can be connected. The device  104  for production of control signals S v  is supplied with the determined output voltages U L10 , U L20  and U L30 , and with a nominal voltage U* L . 
     There is a further option for determination of faulty subsystems  10  in the valve arms T 1 , T 2 ; T 3 , T 4  and T 5 , T 6  in each phase module  100  of the converter  102  with distributed energy stores  9 . This is done using a device  112  which is linked on the input side to each two-pole subsystem  10  in the converter  102 . Each subsystem  10  sends a feedback signal S R  to this device  112  which indicates whether the associated subsystem  10  has or has not changed its switching state correctly. These μ=6 m feedback signals S R  are used to generate a fault signal S F , which is supplied to the device  104 . Since this represents a further option for determination of faulty two-pole subsystems  10 , this is illustrated in the control system shown in  FIG. 12  by means of a dashed line. 
     As already mentioned, an output voltage U L10 , U L20  or U L30  of the converter  102  with distributed energy stores  9  as shown in  FIG. 1  decreases as soon as one two-pole subsystem  10  fails in a valve arm T 1 , . . . , T 6  in one of the three phase modules  100  in the converter  102 . The value of the amplitude reduction in this case corresponds to the value of a capacitor voltage U C  across the energy store  9 . 
     It is now assumed that one two-pole subsystem  10  in the valve arm T 2  of the phase module  100  in the converter  102  with distributed energy stores  9  as shown in  FIG. 1  has been safely short-circuited because of some fault. This faulty subsystem  10  is indicated by means of shading in the equivalent circuit of the converter  102  shown in  FIG. 1 . 
     In accordance with the method according to the invention, the number of faulty subsystems  10  is first of all determined. Since it is assumed that only one subsystem  10  is faulty, a number “1” is determined. Furthermore, that valve arm T 1 , . . . , T 6  in which the faulty system  10  is arranged is determined. In the assumed case, this is the valve arm T 2 . The faulty subsystem  10  in the valve arm T 2  and a subsystem  10  in the sound valve arm T 1 , which corresponds to the faulty valve arm T 2 , in the faulty phase module  100  are controlled such that their terminal voltages U X21  are each zero. If a plurality of subsystems  10  in one valve arm T 1  . . . , T 6  or in a plurality of valve arms T 1 , . . . , T 6  are faulty, then control action is taken corresponding to the number of faulty subsystems  10  in the valve arms T 1 , . . . , T 6  of faulty phase modules  100  which correspond to the faulty valve arms T 1 , . . . , T 6 , such that the terminal voltages U X21  of these subsystems  10  are also each zero. This means that 2n subsystems  10 , where n=the number of faulty subsystems  10 , are short-circuited. 
     With the number of faulty subsystems  10  and knowledge of the faulty valve arms, it is possible to determine the voltage dip in each case at an output L 1 , I 2  or L 3  of the phase modules in the converter  102  with distributed energy stores  9 . The number of subsystems  10  used per valve arm T 1 , . . . , T 6  and the capacitor voltage U C  which is in each case present across the energy storage capacitor  9  in each subsystem  10  is known, as a result of which the amplitude of each voltage U L10 , U L20  and U L30  which is present at the output terminals L 1 , L 2  and L 3  is known. This amplitude is equal to mU C /2, where m is the number of subsystems  10  used per valve arm T 1 , . . . , T 6 . The greater the number m of subsystems  10  used, the less is the amplitude dip in the event of failure of one subsystem  10  in a valve arm T 1 , . . . , T 6 . 
     The aim of the invention is now to compensate for this voltage dip in a faulty phase module  100  by increasing the capacitor voltages U C  of the subsystems  10  in the faulty phase module  100 . For this purpose, the energy stores  9  in these subsystems  10  are charged further such that the sum of the increased capacitor voltages U* C  is equal to the sum of the capacitor voltages U C  in a valve arm T 3 , T 4  or T 5 , T 6  in a sound phase module  100 . This increased capacitor voltage U* C  is given by:
 
 U*   C   =U   C   +ΔU  where Δ U=U   C   /m−n.  
 
     In the assumed case, this means that:
 
Σ U*   C =4/3 U   C  
 
     This means that the capacitor voltages U C  in the energy stores  9  in the three subsystems  10  which are still present in the two valve arms T 1  and T 2  of the faulty phase module  100  must be increased by one third of their value. If eight subsystems  10  are used instead of four subsystems  10  per valve arm T 1 , . . . , T 6 , then the capacitor voltages U* C  in the subsystems  10  of each valve arm T 1  and T 2  in a faulty phase module  100  must need each be increased only by 1/7 of their value according to the cited equation for an increase to the capacitor voltage U *   C . 
     With the increase in the capacitor voltage U C  to the value U* C  across the energy store  9  in each subsystem  10  in a faulty phase module  100 , the voltage load on the two semiconductor switches  1  and  3  and the two diodes  2  and  4  likewise in each case rises. In order to ensure that these semiconductors  1  to  4  in each subsystem  10  in a faulty phase module  100  withstand this voltage load, the number m of subsystems  10  used in each valve arm T 1 , . . . , T 6  in the converter  102  with distributed energy stores  9  should be as high as possible, for example eight, in particular, twelve. The greater the number m of subsystems  10  used in each valve T 1  . . . , T 6  of the converter  102  with distributed energy stores  9  is, the less is the increase in the capacitor voltage U C  in the remaining subsystems  10  in a faulty phase module  100 , and the greater the number of faulty subsystems  10  in a valve arm T 1 , . . . , T 6  in a phase module  100  which can be compensated for by increasing the capacitor voltages U C  in the remaining subsystems  10  in the two valve arms T 1 , T 2 ; T 3 , T 4  or T 5 , T 6  in a faulty phase module  100 . 
     The amount ΔU by which the capacitor voltages U C  of the subsystems  10  in the valve arms T 1  and T 2  in the faulty phase module  100  must in each case be increased is called up from the memory apparatus  108 , as a function of the determined number of faulty subsystems  10  which are found. A signal S L  is produced at the output of this memory apparatus  108 , by means of which the energy source  110  which can be connected is connected to the terminals X 1  and X 2  of the subsystems  10  in the faulty phase module  100  such that their capacitor voltages U C  are increased by a predetermined amount ΔU. In order to achieve this, each sound subsystem  10  which is still present in the two valve arms T 1  and T 2  in the faulty phase module  100  can be controlled for the charging purposes, as follows: of the (2m−2n) subsystems which are still present (m=the number of subsystems  10  per valve arm; n=the number of faulty subsystems per valve arm) in the converter  102 , (2m−2n−1) subsystems  10  are switched to the switching state I, and the respectively remaining sound subsystem  10  is switched to the switching state II or III. A next sound subsystem  10  in the faulty phase module  100  is cyclically successively switched to the switching state II, and the previous one is switched back to the switching state I. The energy source  110  which provides the increased capacitor voltage U* C  is also required for precharging of the energy stores  9  in the subsystems  10  in the converter  102 . This means that this energy source  110  is already a component of this converter  102  with distributed energy stores  9 . Such precharging is described in DE 101 03 031 A1, which has already been cited in the introduction. 
     The capacitor voltages U C  of the energy stores  9  in the subsystems  10  in the two valve arms T 1  and T 2  in the faulty phase module  100  can also be increased with the aid of an additional valve arm current. In order to generate an additional valve arm current, additional voltage/time integrals must be produced in the valve arm voltages of a phase module, according to DE 102005045090 B4. Voltage/time integrals such as these can be applied by no longer carrying out the switching operations of the two valve arms in a phase module synchronously in time, but with a freely variable time interval. This means that the switching operations in an upper valve arm, for example T 1 , in a phase module  100  are carried out with a lag and/or lead with respect to the switching operations in a lower valve arm T 2  in this phase module  100 . This modification of the switching operations in the two valve arms T 1 ,  12  in the faulty phase module  100  dynamically adjusts a predetermined additional voltage/time integral. In order to avoid repeating the majority of this patent specification, reference is made to this patent specification for a more detailed explanation of the generation of additional voltage/time integrals. 
       FIGS. 13 and 14  each show potential profiles U PL1  and U L1N  of the valve arms T 1  and T 2  in the faulty phase module  100 , in each case in the form of a graph plotted against the time t. The profile in the graph shown in  FIG. 14  corresponds precisely to the qualitative potential profile in the graph shown in  FIG. 9 . In accordance with the method according to the invention, a subsystem  10  in the sound valve arm T 1 , which corresponds to the faulty valve arm T 2 , in the faulty phase module  100  is likewise controlled such that its terminal voltage U X21  is zero. The potential profile U PL1  in the graph in  FIG. 13  therefore corresponds to the qualitative potential profile U L1N  in the graph in  FIG. 14 , which are in opposite senses to one another. A potential profile U L10  which is produced at the output L 1  of the faulty phase module  100  in the converter  102  with distributed energy stores  9  is illustrated in  FIG. 15  in the form of a graph plotted against the time t. Without the second part of the method according to the invention, specifically the increase in the capacitor voltages U C  by ΔU in the subsystems  10  which are still present, the peak value L 1  output voltage U L10  would be 3/2 U* C  in comparison to 2U C  for the output voltage U L20  or U L30  of a sound phase module  100 . This amplitude difference is compensated for by increasing the capacitor voltages U C  by ΔU in the subsystems  10  which are still present in the two valve arms T 1 ,  12  in the faulty phase module  100 . This increase in the capacitor voltages U C  by ΔU results in the unbalanced vector diagram shown in  FIG. 11  being changed back to the balanced vector diagram as shown in  FIG. 7 . 
     The method according to the invention even allows redundant further operation of the converter  102  with distributed energy stores  9  in the case of a so-called double fault. A double fault is a fault in which two systems  10  in valve arms which do not correspond in two phase modules  100 , for example valve arms T 1  and T 4 , have failed. In order to keep the voltage load on the semiconductors  1  to  4  in each subsystem  10  in the converter  102  with distributed energy stores  9  within limits, the number m of subsystems  10  should be chosen to be as great as possible, in which case m=12 per valve arm T 1 , . . . . , T 6  should be sufficient.