Abstract:
A method for inhibiting a converter having at least two phase modules is disclosed. Each phase module has an upper and a lower valve branch, with each upper and lower valve branch having a plurality of two-pole submodules which are electrically connected in series and each have a unipolar energy storage capacitor, with a series connection of two turn-off semiconductor switches each being connected in parallel with an antiparallel connected diode. With the method, the submodules in an upper and a lower valve branch in each phase module in the converter are controlled to a switching state III, staggered in time. This considerably reduces the voltage load for the converter and a connected polyphase motor, or a connected power supply system.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2010 030 078.0, filed Jun. 15, 2010, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a relates to a method for inhibiting a converter with distributed energy stores having at least two phase modules, which each have an upper and a lower valve branch, which each have a multiplicity of two-pole submodules, which are electrically connected in series and each have a unipolar energy storage capacitor, with which a series circuit of two semiconductor switches which can be turned off, each with a diode connected back-to-back in parallel, is connected electrically in parallel. 
     The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
     A conventional converter with distributed energy stores is illustrated schematically in  FIG. 1 . According to  FIG. 1 , this converter  2  has three phase modules  4   1 ,  4   2  and  4   3 , which each have an upper and a lower respective valve branch P 1  and N 1 , P 2  and N 2  as well as P 3  and N 3 . These two valve branches P 1 , N 1 ; P 2 , N 2  and P 3 , N 3  in each phase module  4   1 ,  4   2  and  4   3 , respectively, are connected to form one bridge arm. A junction point between an upper and a lower valve branch P 1  and N 1 , P 2  and N 2  as well as P 3  and N 3  is passed out as a respective connection L 1 , L 2  or L 3 , respectively, on the AC voltage side of the respective phase module  4   1 ,  4   2  or  4   3 . A polyphase motor  6  or a power supply system is connected to these connections L 1 , L 2  or L 3  on the AC side. The phase modules  4   1 ,  4   2  and  4   3  are electrically connected in parallel with one another and are connected to form a DC voltage feed device, which is not illustrated in any more detail but is connected to the DC voltage connections P 0  and N 0  of the converter  2  with distributed energy stores C SM . A generated DC voltage U dc  is present between these DC voltage connections P 0  and N 0 . 
     This illustration of the converter  2  with distributed energy stores C SM  likewise shows that each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  has a multiplicity of two-pole submodules SM 1 , SM 2 , . . . , SMn, which are electrically connected in series. Each two-pole submodule SM 1 , SM 2 , . . . , SMn, based on the illustration of the submodule SM 1 , has a unipolar energy storage capacitor C SM , two semiconductor switches S 1  and S 2  which can be turned off, and two diodes D 1  and D 2 . The two semiconductor switches S 1  and S 2  which can be turned off are electrically connected in series, and this series circuit is electrically connected in parallel with the unipolar energy storage capacitor C SM . A respective diode D 1  or D 2  is connected back-to-back in parallel with the semiconductor switches S 1  and S 2  which can be turned off. These diodes D 1  and D 2  therefore each form a freewheeling diode. A junction point between the two semiconductor switches S 1  and S 2  which can be turned off is passed out as the module connection X 2 . The negative connection of the unipolar energy storage capacitor C SM  forms a second module connection X 1 . When the unipolar energy storage capacitor C SM  has been charged, a capacitor voltage U SM  is dropped across it. 
     These capacitor voltages U SM1 , U SM2 , U SMn  of the two-pole submodules SM 1 , SM 2 , . . . , SMn in each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  are respectively added to form a valve voltage U ZP1 , U ZN1 , U ZP2 , U ZN2 , U ZP3  and U ZN3 . The addition of in each case two valve voltages U ZP1 , U ZN1  as well as U ZP2  and U ZN2  as well as U ZP3  and U ZN3  of a respective phase module  4   1 ,  4   2  or  4   3  results in the DC voltage U dc  which is present between the DC voltage connections P 0  and N 0 . 
     The configuration of each two-pole submodule SM in the converter  2  with distributed energy stores C SM  allows each submodule SM each to be controlled to three switching states, specifically the switching states I, II and III. In the switching state I, the semiconductor switch S 1  which can be turned off is in the ON-state, and the semiconductor switch S 2  which can be turned off is in the OFF-state. The capacitor voltage U SM  is therefore present as the terminal voltage U X2X1  at the module connections X 2  and X 1  of the submodule SM, independently of the direction of a branch current i Z  flowing. In the switching state II, the semiconductor switch S 1  which can be turned off is in the OFF-state, and the semiconductor switch S 2  which can be turned off is in the ON-state, thus resulting in a terminal voltage U X2X1  with the amplitude zero being present at the module connections X 2  and X 1  of the submodule SM, likewise independently of the direction of a branch current i Z  flowing. In the switching state III both semiconductor switches S 1  and S 2  which can be turned off are in the OFF-state. The amplitude of the terminal voltage U X2X1  of each submodule SM when in the switching state III is dependent on the direction of a branch current i Z  flowing. If the branch current is greater than zero, then the amplitude of the terminal voltage U X2X1  of the submodule SM corresponds to the amplitude of the capacitor voltage U SM  in this submodule SM. In contrast, if the branch current is less than zero, the amplitude of the terminal voltage is equal to zero. If no branch current i Z  is flowing and if the voltage split between the semiconductor switches S 1  and S 2  which can be turned off in the submodule SM is symmetrical, the amplitude of the terminal voltage U X2X1  corresponds to half the amplitude of the capacitor voltage U SM  in the submodule SM. 
     In the conventional converter  2  with distributed energy stores C SM , only the switching states I and II of the submodules SM are used during normal operation of this converter  2 . The switching state III is used only in the event of defects, for example a short at its DC voltage connections P 0  and N 0 , for a deliberate open circuit (interruption of converter operation) and for negligibly short switching delay times for the semiconductor switches S 1  and S 2  which can be turned off in a submodule SM, when a switching state change occurs. 
     It is known that a so-called pulse inhibitor can be activated, in order to turn the converter off in critical operating states, for example overcurrent, overvoltage, failure of a drive, failure of a control system, failure of communications between the converter valve and the modulator, . . . , such that this converter is in a safe state after operation of the pulse inhibitor. This pulse inhibitor can be implemented by inhibiting all the converter valves in the self-commutated converter (inverter). This is preferably done by interruption of the supply voltage, which is derived from an external voltage, for the optocouplers in the associated drive circuits. 
     When a pulse inhibitor in a converter  2  with distributed energy stores C SM  is triggered, then all the drive signals for the semiconductor switches S 1  and S 2  which can be turned off in all the submodules SM 1 , SM 2 , . . . , SMn in all the valve branches P 1 , N 1 , P 2 , N 2 , P 3  and N 3  in the phase modules  4   1 ,  4   2  and  4   3  in the converter  2  with distributed energy stores C SM  must be inhibited at the same time, as shown in  FIG. 1 . 
       FIG. 2   a  shows for sake of clarity in more detail only the phase module  4   1  of the converter  2  with distributed energy stores C SM  depicted in  FIG. 1 . The submodules SM 1 , . . . , SM 4  in the upper and lower valve branches P 1  and N 1  in this phase module  4   1  illustrate a switching state distribution during normal operation of this converter  2 . Of the four submodules SM 1 , . . . , SM 4  in the upper valve branch P 1 , the submodules SM 2  to SM 4  are in the switching state I, and the submodule SM 1  is in the switching state II. Of the submodules SM 1 , . . . , SM 4  in the lower valve branch N 1 , the submodules SM 1  to SM 3  are in the switching state II, and the submodule SM 4  is in the switching state I. The amplitude of the DC voltage U dc  which is present at the DC voltage connections P 0  and N 0  of the converter  2  is therefore U dc =4·U SM . The voltage u ZP  in the upper valve branch P 1  with respect to a virtual neutral point is given by u ZP =3·U SM , while, in contrast, the voltage u ZN  in the lower valve branch N 1  is given by u ZN =1·U SM . 
     Once a pulse inhibitor is triggered, all the submodules SM 1  to SM 4  in the upper and lower valve branches P 1  and N 1  are switched to the switching state III. The phase module  4   1  with the submodules SM 1  to SM 4  in the switching state III is illustrated in  FIG. 2   b . The pulse inhibitor can be set on the one hand by a fault occurring (for example an overcurrent) by an open-loop and closed-loop control device, which is not illustrated in any more detail, in the converter  2 , whereas on the other hand autonomously by the submodules SM 1  to SM 4  (disturbance with or breakdown of communication, overvoltage). Since the time at which a pulse inhibitor is set cannot be predicted, the voltages u ZP  and u ZN  and/or their rates of change du ZP /dt and du ZN /dt over the valve branches P 1  and N 1  in a phase module  4   1  are governed solely by the direction of the corresponding branch current i ZP1  and i ZN1  when the pulse inhibitor is set. 
     On the assumption that the sum of the two branch voltages u ZP  and u ZN  in a respective phase  4   1 ,  4   2  or  4   3  corresponds on average to the DC voltage U dc  during normal operation, this results in the voltages and voltage changes as shown in the following table after a pulse inhibitor has been set. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Direction of the 
                   
                   
                   
                   
               
               
                   
                 branch currents 
               
               
                   
                 i ZP /i ZN   
                 pos/pos 
                 pos/neg 
                 neg/pos 
                 neg/neg 
               
               
                   
                   
               
             
             
               
                   
                 u ZP   
                 U dc   
                 U dc   
                 0 
                 0 
               
               
                   
                 u ZN   
                 U dc   
                 0 
                 U dc   
                 0 
               
               
                   
                 u ZP  + u ZN   
                 2 U dc    
                 U dc   
                 U dc   
                 0 
               
               
                   
                 Δ(u ZP  + u ZN ) * 
                 +U dc   
                 0 
                 0 
                 −U dc   
               
               
                   
                   
               
               
                   
                 * Assumption: mean sum of the branch voltages before the pulse inhibitor (u ZP  + u ZN ) = U dc   
               
             
          
         
       
     
     It is also assumed that the capacitor voltages U SM  in each submodule SM on average have a value of U SM =U dc /n sub , where n sub  represents the number of series-connected submodules SM 1 , . . . , SMn in each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  in the converter  2  with distributed energy stores C SM . 
     As can be seen from this table, two worst-case scenarios occur with respect to the voltage change in the branches in one phase when a pulse inhibitor is set. The maximum voltage change in the sum of the branch voltages u ZP  and u ZN  in a phase module  4   1 ,  4   2  and  4   3  is ±U dc  and occurs when both branch currents i ZP  and i ZN  in a phase module  4  have the same mathematical sign. This state is maintained until one of the branch currents has been commutated down to zero. 
     Switching on the switching state III results in commutation processes from the semiconductor switch S 1  which can be turned off to a diode D 2  in said submodules SM 2 , SM 3 , SM 4  in the upper valve branch P 1  and the submodule SM 4  in the lower valve branch N 1  in the phase module  4   1 , if, before setting of the pulse inhibitor, the sum of the branch voltages u ZP  and u ZN  in a phase module  4   1  is on average equal to the DC voltage U dc  between the DC voltage connections P 0  and N 0 , and the branch currents i ZP  and i ZN  have a negative mathematical sign. When the switching state III is switched on, no commutation processes occur in the submodule SM 1  in the upper valve branch P 1  and the submodules SM 1 , SM 2 , SM 3  in the lower valve branch N 1  in the phase module  4   1  if, before the setting of the pulse inhibitor, the sum of the branch voltages u ZP  and u ZN  in a phase module  4   1  is on average equal to the DC voltage U dc  and the branch currents i ZP  and i ZN  have a negative mathematical sign since the diode D 2  carried the corresponding branch current before switching on the switching state III. 
     In contrast, commutation processes take place from the semiconductor switch S 2  which can be turned off to the diode D 1  in said submodules as a result of switching on the switching state III in the submodule SM 1  in the upper valve branch P 1  and the submodules SM 1 , SM 2 , SM 3  in the lower valve branch N 1  in the phase module  4   1  if, before setting of the pulse inhibitor, the sum of the branch voltages u ZP  and u ZN  of a phase module  4   1  is on average equal to the DC voltage U dc  between the DC voltage connections P o  and N o , and the branch currents i ZP  and i ZN  have a positive mathematical sign. No commutation processes take place when the switching state III is switched on in the submodules SM 2 , SM 3 , SM 4  in the upper valve branch P 1  and the submodule SM 4  in the lower valve branch N 1  in the phase module  4   1  if, before setting of a pulse inhibitor, the sum of the branch voltages u ZP  and U ZN  of a phase module  4   1  is on average equal to the DC voltage U dc  and the branch currents i ZP  and i ZN  have a negative mathematical sign, since the diode D 1  carried the corresponding branch current before switching on the switching state III. 
     A voltage rate of change which, for example, may be 4 kV/μs can be assumed for a semiconductor switch S 1  or S 2  which can be turned off in each submodule SM, for every voltage change which occurs during a commutation process. This then results in a voltage rate of change across the two valve branches P 1  and N 1  in the phase module  4   1  with a value of 16 kV/μs, because there are four submodules SM in the phase module  4   1  in the switching state I before the setting of a pulse inhibitor. The greater the number of submodules SM which are used in each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  in the converter  3  with distributed energy stores C SM , the higher is the value of the voltage change per phase module  4   1 ,  4   2  and  4   3 . 
     In order to obtain a respective output voltage u L10 , U L20 , U L30  with as sinusoidal a waveform as possible at the respective output L 1 , L 2  or L 3  of a respective phase module  4   1 ,  4   2  or  4   3  in the converter  2  with distributed energy stores C SM , twelve or more submodules SM, for example, are used for each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3 . When there are twelve submodules SM for each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3 , the voltage rate of change is already 48 kV/μs. 
     If the DC voltage U dc  which is present at the DC voltage connections P 0  and N 0  of the converter  2  with distributed energy stores C SM  is assumed to be constant, then said voltage rate of change acts not only on a branch inductor L Z  but also on a parasitic inductor L dc  in the DC voltage circuit. This voltage load on the branch inductor L Z  leads to a large physical size, because of the use of reinforced insulation. 
     With respect to the output voltages u L10 , u L20  and u L30  of the converter  2  with distributed energy stores C SM , different worst-case conditions occur in comparison to the converter-internal voltages u ZP  and u ZN . A worst-case situation will be explained below with reference to  FIGS. 3   a  and  3   b , with respect to the voltage changes in the phase voltages u L10  in the phase module  4   1  in the converter  2  as shown in  FIG. 1 . 
     Because of the switching state distribution of the submodules SM 1  to SM 4  in the valve branches P 1  and N 1  in the phase module  4   1  in the converter  2  with distributed energy stores C SM  as shown in  FIG. 1 , the submodules SM 1  to SM 4  in the upper valve branch P 1  are all in switching state II. In contrast, the submodules SM 1  to SM 4  in the lower valve branch N 1  are all in the switching state I. The phase voltage u L10 , which is equal to half the difference between the valve voltages u ZN  and u ZP , is U dc /2. If a pulse inhibitor is now set, then this results in a voltage change in the phase voltage U L10  which is dependent on the instantaneous current direction of the branch currents i ZP  and i ZN . The following table provides an overview of the voltages and voltage changes over the valve branches P and N in a phase module  4  after a pulse inhibitor has been set. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Direction of the 
                   
                   
                   
                   
               
               
                 Pulse 
                 branch currents 
               
               
                 inhibitor 
                 i ZP /i ZN   
                 pos/pos 
                 pos/neg 
                 neg/pos 
                 neg/neg 
               
               
                   
               
             
             
               
                 Before 
                 u ZP   
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 u ZN   
                 U dc   
                 U dc   
                 U dc    
                 U dc   
               
               
                   
                 u L10   
                     U dc /2 
                     U dc /2 
                 U dc /2 
                  U dc /2 
               
               
                 After 
                 u ZP   
                 U dc   
                 U dc   
                 0 
                 0 
               
               
                   
                 u ZN   
                 U dc   
                 0 
                 U dc    
                 0 
               
               
                   
                 u L10   
                 0 
                  −U dc /2 
                 U dc /2 
                 0 
               
               
                   
                 Δu L10   
                  −U dc /2 
                 −U dc   
                 0 
                 −U dc /2 
               
               
                   
                 Δ(u ZP  + u ZN ) * 
                 U dc   
                 0 
                 0 
                 −U dc    
               
               
                   
               
               
                 * Assumption: Capacitor voltage of a submodule U SM, x  = U dc /n sub   
               
             
          
         
       
     
     The worst-case situation with respect to the voltage change in the phase voltage U L10 , u L20  or U L30  after setting a pulse inhibitor occurs when the following conditions occur before setting of the pulse inhibitor:
         all the submodules in a valve branch, for example in the valve branch N 1 , in a phase module are in the switching state I   all the submodules in a corresponding valve branch, for example the valve branch P 1 , in a phase module are in switching state II   the branch current, for example the branch current i ZN , in the valve branch with the submodules which are in switching state I has a negative mathematical sign, and   the branch current, for example the branch current i ZP , in the valve branch with the submodules which are in switching state II has a positive mathematical sign.       

     In these conditions, the phase voltage u L10  jumps from U dc /2 to −U dc /2, or from −U dc /2 to U/dc/2. In these conditions, the change in the phase voltage u L10  is therefore ±U dc . If it is now assumed that a voltage rate of change of, for example, 4 kV/μs occurs across each submodule SM in the phase module  4   1  in each submodule SM 1  to SM 4  in each valve branch P 1  and N 1  in a phase module  4   1 , because of the commutation from a semiconductor switch S 1  or S 2  which can be turned off to a respective diode D 2  or D 1  then this results in a value of 16 kV/μs for the voltage rate of change of the phase voltage u L10 /dt, u L20 /dt and u L30 /dt when there are four submodules SM in each valve branch P and N, and in a value of 48 kV/μs when there are twelve submodules SM in each valve branch P and N in a phase module  4   1 . 
     This means in the worst-case situation, in which two phase voltages change suddenly through ±U dc  in opposite senses, for a line voltage u L1L2 , that a voltage change of Δu L1L2 =±2U dc  and a voltage rate of change of 32 kV/μs occurs across an output impedance (stator winding of a connected polyphase motor  6 ) when four submodules SM are used in each valve branch P and N in a phase module  4   1 , or of 96 kV/μs when twelve submodules SM are used in each valve branch P and N in a phase module  4   1 . In order to prevent a DC voltage feeder on the power supply system side and a polyphase motor  6  connected on the load side from being excessively damaged when worst-case situations occur, these components must be designed for a very much greater voltage rate of change, thus resulting in additional costs to a not inconsiderable level. 
     It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved method for inhibiting a converter with distributed energy stores, in which the voltage load in worst-case situations is significantly reduced. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method for inhibiting a converter having at least two phase modules, which each phase module having an upper and a lower valve branch, with each upper and lower valve branch having a plurality of two-pole submodules, which are electrically connected in series and each have a unipolar energy storage capacitor, with a series connection of two turn-off semiconductor switches each being connected in parallel with an antiparallel connected diode, includes the steps of a) triggering a pulse inhibitor in response to a fault occurring during operation of the converter, b) controlling a switching state of one two-pole submodule in each valve branch to a switching state III after a pulse inhibitor has been set, c) controlling an additional submodule in each valve branch to a switching state III after a predetermined time interval has elapsed, and d) repeating step c) until all two-pole submodules in each valve branch are controlled to the switching state III. 
     According to an advantageous feature of the present invention, controlling the switching state of an additional submodule includes controlling either an outer additional submodule or an inner additional submodule in an upper and a lower valve branch in each phase module. 
     According to another advantageous feature of the present invention, the predetermined time interval is equal to a turn-off delay time of a turn-off semiconductor switch in a two-pole submodule. 
     According to another advantageous feature of the present invention, the two turn-off semiconductor switches of a two-pole submodule are switched of when the two-pole submodule is in the switching state III. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
         FIG. 1  shows an equivalent circuit of a conventional converter with distributed energy stores, 
         FIGS. 2   a ,  2   b  show switching state distributions in the submodules in a phase module in the converter as shown in  FIG. 1 , before and after setting a pulse inhibitor, 
         FIGS. 3   a ,  3   b  show switching state distributions in the submodules in a phase module in the converter as shown in  FIG. 1  before and after setting a pulse inhibitor, and 
         FIGS. 4   a  to  4   e  each show switching state distributions in the submodules in a phase module in the converter as shown in  FIG. 1 , which result by means of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
     The method according to the invention for inhibiting a converter  2  with distributed energy stores C SM  as shown in  FIG. 1  will now be explained in more detail with reference to  FIGS. 4   a  to  4   e . According to the method according to the invention, after a pulse inhibitor has been set, the submodules SM 1  to SM 4  in an upper and lower valve branch P 1  and N 1  in each phase module  4   1 ,  4   2  and  4   3  in the converter  2  are no longer controlled at the same time to the switching state III, but staggered in time. This time staggering of the processing of a pulse inhibitor which has been set is illustrated by four switching state distributions of the submodules SM 1  to SM 4  in the valve branches P 1  and N 1  in the phase module  4   1 , with the arrows between two respective switching state distributions in the phase module  4   1  as shown in  FIGS. 4   b  to  4   e  each symbolizing a predetermined time interval Δt for the time-staggered or stepped implementation of the processing of a pulse inhibitor which has been set. 
       FIG. 4   a  illustrates the phase module  4   1  in the converter  2  as shown in  FIG. 1  with a random switching state distribution of the submodules SM 1  to SM 4  in its upper and lower valve branches P 1  and N 1 . Of the submodules SM 1  to SM 4  in the valve branches P 1  and N 1  in the phase module  4   1 , the submodules SM 1  and SM 2  are each in the switching state II while, in contrast, the submodules SM 3  and SM 4  are each in the switching state I. If a pulse inhibitor is now set, then, in a first step at the time t 1  ( FIG. 4   b ), one submodule SM 1  in the upper and lower valve branches P 1  and N 1  is in each case controlled to the switching state III. After a predetermined time interval Δt has elapsed, that is to say at the time t 2  ( FIG. 4   c ), a further submodule SM 2  in the upper and lower valve branches P 1  and N 1  in the phase module  4   1  is in each case controlled to the switching state III. After a predetermined time interval Δt has elapsed, specifically at the time t 3  ( FIG. 4   d ), a further submodule SM 3  in the upper and lower valve branches P 1  and N 1  in this phase module  4   1  is in each case controlled to the switching state III. After a further time interval Δt has elapsed, at the time t 4  ( FIG. 4   e ), a further submodule SM 4  in the upper and lower valve branches P 1  and N 1  in this phase module  4   1  is controlled to the switching state III. At the time t 4 , all the submodules SM 1  to SM 4  in each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  in the converter  2  with distributed energy stores C SM  are therefore in the switching state III, with a pulse inhibitor which has been set being implemented in a staggered form, according to the invention. 
     There is a predetermined time interval Δt in each case as the time stagger between the individual method steps ( FIGS. 4   b  to  4   e ), and this preferably corresponds to a so-called “delay time” of a semiconductor switch S 1  or S 2  which can be turned off in a submodule SM. This delay time of a semiconductor switch S 1  or S 2  which can be turned off in a submodule SM is the minimum time interval Δt which can be implemented. After a minimum time interval Δt has in each case elapsed, a switching state change of a submodule SM is complete. This ensures that the voltage load in each time step of the stepped processing of a pulse inhibitor which has been set corresponds at most only to the voltage rate of change of two submodules SM. 
     For example, if the voltage rate of change in each submodule SM is 4 kV/μs, the maximum du/dt load during each switching state change is only 8 kV/μs in comparison to 16 kV/μs when a pulse inhibitor is processed in the conventional manner. This means that the method according to the invention at least halves the voltage load for converter-internal voltages and phase output voltages. 
     When a pulse inhibitor which has been set is processed staggered in time according to the invention, there is no need to control in each case one submodule SM in an upper and a lower valve branch to the switching state III at the same time, while, instead it is also possible to control only one submodule SM in each phase module  4   1 ,  4   2  and  4   3  to the switching state III. It is irrelevant which of the submodules SM in a phase module  4   1 ,  4   2  and  4   3  is started with. The sequence on the basis of which the submodules SM in an upper and a lower valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  and a phase module  4   1 ,  4   2  and  4   3  are controlled to the switching state III is also irrelevant for the reduction in the du/dt load. 
     It is important to provide a time offset between the switching state changes of in each case one submodule SM in an upper and a lower valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  and a phase module  4   1 ,  4   2  and  4   3 . 
     If only one submodule SM in a phase module  4   1 ,  4   2  and  4   3  in the converter  2  with distributed energy stores C SM  as shown in  FIG. 1  is in each case controlled to the switching state III when a pulse inhibitor which has been set is processed staggered in time, twice as many time steps are required instead of the four time steps shown in  FIG. 4 , that is to say eight time steps are required, before a pulse inhibitor which has been set has been implemented. Considerably more time is accordingly required to implement a pulse inhibitor which has been set. If there are twelve or more submodules in each valve branch P 1 , N 1 , P 2 , N 2 , P 3  and N 3  in the converter  2  as shown in  FIG. 1 , it is necessary to check, depending on the application of the converter, whether the protection functions which are initialized with the pulse inhibitor can be satisfied. 
     The pulse inhibitor is set in order to turn off the converter  2  with distributed energy stores C SM  in critical operating states, for example overcurrent, overvoltage or failure of the drive, such that the converter  2  is in a safe state after the pulse inhibitor has been applied. The time available for turning off the converter  2  is not unlimited, because of these fault situations. 
     The method according to the invention is therefore used, wherein two submodules, specifically one submodule in the upper valve branch P 1 , P 2 , P 3  and one submodule SM in the lower valve branch N 1 , N 2 , N 3 , are controlled to the switching state III at the same time in each time stagger. 
     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.