Patent Publication Number: US-2016226480-A1

Title: Modular Multipoint Power Converter for High Voltages

Description:
The invention relates to a two-pole sub module for constructing a converter. Said sub module here comprises a first subunit that comprises a first energy store, a first series circuit connected in parallel with the first energy store, said series circuit having two power semiconductor switching units, each of which comprises a power semiconductor that can be switched on and off and having the same forward conduction directions, and each of which is capable of conducting in a direction opposite to said forward conduction direction, and a first connecting terminal which is connected to the potential node between the power semiconductor switching units of the first series circuit. The sub module furthermore comprises a second subunit that comprises a second energy store, a second series circuit connected in parallel with the second energy store, said series circuit having two power semiconductor switching units, each of which comprises a power semiconductor that can be switched on and off and having the same forward conduction directions, and each of which is capable of conducting in a direction opposite to said forward conduction direction, and a second connecting terminal which is connected to the potential node between the power semiconductor switching units of the second series circuit. The first subunit and the second subunit are, moreover, connected to one another via connecting means. Said connecting means comprise an emitter connecting branch that connects an emitter of a first power semiconductor switching unit of the first series circuit to an emitter of a first power semiconductor switching unit of the second series circuit, a collector connecting branch that connects a collector of the second power semiconductor switching unit of the first series circuit to a collector of the second power semiconductor switching unit of the second series circuit, and a switching branch in which a switching unit is arranged and which connects the emitter connecting branch to the collector connecting branch. 
     The invention relates furthermore to a converter with a series circuit of such two-pole sub modules, wherein the series circuit of the sub modules is arranged between an AC voltage terminal and a DC voltage terminal of the converter. 
     The use of power electronic systems in the field of very high voltages and powers has become increasingly important. The power electronic systems are primarily used for controlling the flow of energy between various energy supply networks (network couplings, high voltage direct current transmission (HVDC)). In particular for spatially extended, branched high voltage direct current networks to which several converters are connected (“multi-terminal”), the secure and fast handling of possible faults can be of crucial significance. 
     In the past, primarily power converters with thyristors and impressed direct current have been employed for the very high powers that are required. These, however, do not satisfy the requirements, rising in the future, for highly dynamic reactive power compensation, network voltage stabilization, favorable usability of DC voltage cables and the ability to realize branched HVDC networks. Power converters with impressed DC voltage are therefore primarily developed as the preferred type of circuitry. This type of power converter is also known as a voltage source converter (VSC). A disadvantage of some of the usual voltage source converters is in particular that, in the event of a short-circuit on the DC voltage side of the converter, extremely high discharge currents flow from the capacitor bank on the DC voltage side, which can cause destruction as a result of the action of extremely high mechanical forces and/or the effect of arcs. 
     This disadvantage of known voltage source converters is a topic of document DE 10 103 031 A1. The converter described there comprises power semiconductor valves connected to one another in a bridge circuit. Each of these power semiconductor valves has an AC voltage terminal and a DC voltage terminal, and consists of a series circuit of two-pole sub modules, each of which comprises a unipolar storage capacitor and a power semiconductor circuit connected in parallel with the storage capacitor. The power semiconductor circuit consists of a series circuit of power semiconductor switches oriented in the same sense, such as IGBTs or GTOs, each of which has a freewheeling diode of the opposite polarity connected in parallel with it. One of two connecting terminals of one of these sub modules is connected to the storage capacitor, and the other connecting terminal is connected to the potential node between the two power semiconductor switches that are capable of being switched on and off. Depending on the switching state of the two actuable power semiconductors, either the capacitor voltage, present at the storage capacitor or a zero voltage can be generated at the two output terminals of the sub module. As a result of the series circuit of the sub modules within the power semiconductor valve, what is known as a DC voltage impressing multi-stage converter is provided, wherein the height of the voltage stages is determined by the height of the respective capacitor voltage. Multi-stage or multi-point converters of this sort have the advantage over the two-stage or three-stage converters with central capacitor banks that high discharge currents are avoided in the event of a short-circuit on the DC voltage side of the converter. In addition to this, the expense required to filter upper harmonics of multi-stage converters is less than that required for two-point or three-point converters. 
     Appropriate topologies are meanwhile employed industrially for HVDC. One of the advantages of this topology—as is known from the document cited above—lies in its strictly modular design. 
     However, in particular for constructing spatially extended, branched HVDC networks, the secure and fast handling of possible faults in the HVDC network has not been satisfactorily solved. Corresponding, spatially extended, branched HVDC networks will in future be required, amongst other things, for large offshore wind farms and for the use of large solar power stations in remote desert regions. It must in particular be possible to handle short-circuits in the HVDC network. 
     Favorable mechanical switches for the extremely high DC voltages, able to switch high fault currents under load, are not available due to fundamental physical problems. The technically achievable switch-off times and the switching over-voltages of mechanical switches are also troublesome. In accordance with the prior art, therefore, mechanical switches for these applications can only be realized favorably as zero-load (zero-current) isolators. 
     A direct substitution of mechanical power switches by electronic DC power switches in the high-voltage field is extremely expensive. The additional conduction losses of the semiconductors also speak against it. For this reason, what are known as hybrid HVDC switches have been developed and publicized, containing additional mechanical switches for the purpose of avoiding and/or reducing the conduction losses. This measure, however, again impairs the achievable switch-off times due to the mechanical switches. 
       FIG. 2  shows a schematic illustration of an example of the interior circuitry of a sub module as is known from the prior art. The sub module  1  illustrated in  FIG. 2  differs from the embodiment known from DE 10 103 031 A1 in that it has an additional thyristor  8 . In the event of a fault, this has the purpose of relieving the parallel freewheeling diode  71  of unacceptably high current surges. For this purpose, the thyristor  8  must be triggered in the event of a fault. The sub module  1  of  FIG. 2  contains, as further components, two controllable electronic switches  73 ,  74  in a known arrangement, consisting of IGBTs with a high reverse voltage, associated anti-parallel freewheeling diodes  71 ,  72 , and an energy store  6 , which is embodied as a unipolar storage capacitor. 
     When the terminal current ix has a polarity opposite to the technical current direction drawn in  FIG. 2 , the sub module  1  of  FIG. 2  cannot absorb any energy, regardless of the actuated switching state. This fact is very disadvantageous in the event of a fault. This applies in general for the population with sub modules that can only generate one polarity of the terminal voltage Ux. 
     The use of what are known as H-bridges (full bridges) as sub modules is obvious to the expert—and is known from a variety of publications (see, for example, the document DE 102 17 889 A1). These can generate an appropriate opposing voltage for any polarity of the terminal current, i.e. can absorb energy. This offers the following advantages:
         the currents on the DC side and on the three-phase side can be electronically switched off and/or limited by the converter itself in the event of network faults—in particular in the event of short-circuits in the DC network.   the achievable switch-off times are short in comparison with the switch-off times of mechanical switches or of hybrid HVDC switches   a higher AC voltage can be achieved in normal operation, so allowing a design with a somewhat higher secondary voltage in the network transformer (and consequently a smaller AC current). This is a valuable degree of freedom of the dimensioning.       

     However, the fact that the conduction power loss of the sub modules at the same current is doubled is extremely disadvantageous. This is of considerable commercial significance, in particular in the energy supply field, as a result of the continuous operation at high powers. In terms of functionality during normal operation, however, the new degree of freedom of dimensioning for smaller AC currents is valuable. 
     A converter with the generic sub module is known from document DE 10 2008 057 288 A1. A potential isolation diode as well as, optionally, a damping resistor, are arranged there in each case in the emitter connecting branch and in the collector connecting branch of the connecting means. The potential isolation diodes are arranged such that the switching branch of the connecting means connects a cathode of the potential isolation diode of the emitter connecting branch to an anode of the potential isolation diode of the collector connecting branch. Through the design of the connecting means, it is possible to achieve, with suitable actuation of the power semiconductor switching units, that a flow of current between the two connecting terminals of the sub module must always take place by way of at least one energy store. Regardless of the polarization of the terminal current, the energy store concerned in each case always develops an opposing voltage that allows the flow of current to decay more quickly. It has been found to be disadvantageous with this solution that negative terminal voltages cannot be generated for both polarities of the terminal current ix. As a result, the additional degree of freedom of dimensioning for smaller AC currents cannot be realized. This disadvantage is also exhibited by the known arrangement of a converter with an HVDC switch immediately following on the DC side of the converter. 
     There therefore continues to be a high demand for a technically more favorable realization of the sub modules than is possible with cascaded full bridges. 
     The object of the present invention is to propose a sub module and a converter of the type mentioned at the outset in which the semiconductor power loss of the sub modules in normal operation is reduced, the number of controllable semiconductor switches is limited, and a uniform population of the sub modules with semiconductors is permitted. 
     On the basis of the sub module mentioned at the outset, the invention achieves the object in that at least one power semiconductor switching unit is arranged in the emitter connecting branch or the collector connecting branch of the connecting means of the sub module according to the invention. 
     On the basis of the converter mentioned at the outset, the invention achieves the object in that each sub module of the converter according to the invention is a sub module according to the present invention. 
     Advantageously, the sub module according to the invention permits the desired handling of faults, and in normal operation replaces a series circuit of two full bridges through the possibility of generating negative terminal voltages of the sub module. 
     Depending on the configuration of the invention, the relevant improvements are as follows:
         a reduction in the total semiconductor power loss of the sub modules in normal operation.   a limit on the number of controllable semiconductor switches (including IGBTs) and of the total semiconductor area.   retention of the possibility of populating the sub module with semiconductors of uniform reverse voltage and structure.       

     The first two points represent a significant advance in comparison with the known use of cascaded full bridges. The last point is equivalent to the use of full bridges. Its significance arises in that only a few semiconductor switches are suitable for the extremely high voltages and powers. At present, these are IGBT transistors with a high reverse voltage, or IGCTs, and in future will also include SiC semiconductors. A uniform population makes it possible to employ only those semiconductors that are most suitable and of the highest performance in each case. 
     The power semiconductor switching units can be realized as semiconductor switches each with associated antiparallel diodes, or as reverse-conducting semiconductor switches. The required reverse voltage of all the power semiconductor switching units is oriented to the maximum voltage of the energy stores which take the form, for example, of unipolar storage capacitors. Preferably, the reverse voltage is the same for all the power semiconductor switching units. 
     The invention in particular includes a configuration of the sub module in which at least one power semiconductor switching unit is arranged in the emitter connecting branch and at least one potential isolation diode is arranged in the collector connecting branch. 
     An embodiment is accordingly also possible in which the at least one potential isolation diode is arranged in the emitter connecting branch and the at least one power semiconductor switch is arranged in the collector connecting branch. 
     The at least one potential isolation diode here serves to maintain a voltage difference between the first and second subunits of the sub module. 
     According to a preferred embodiment of the invention, at least one power semiconductor switching unit is provided both in the emitter connecting branch and at least one also in the collector connecting branch of the connecting means. This embodiment has the advantage that negative voltages can be generated at the connecting terminals of the sub module, corresponding to the voltages of the energy stores of the subunits. 
     The switching branch can, for example, connect an emitter of the power semiconductor switching unit of the connecting means arranged in the collector connecting branch to a collector of the power semiconductor switching unit of the connecting means arranged in the emitter connecting branch. 
     The switching unit of the switching branch can be realized as a mechanical switching unit, as a semiconductor switch or as a power semiconductor switching unit. 
     According to an exemplary embodiment of the invention, the switching unit in the switching branch of the connecting means is a power semiconductor switching unit. The emitter of the power semiconductor switching unit arranged in the collector connecting branch is connected to the collector of the power semiconductor switching unit of the switching branch, and the emitter of the power semiconductor switching unit of the switching branch is connected to the collector of the power semiconductor switching unit arranged in the emitter connecting branch. 
     In any case, it is advantageous for the switching unit to be selected such that the power loss arising in it during normal operation of the sub module is as low as possible. 
     Depending on the topology of the sub module, a switching state of the power semiconductor switching units of the sub module can be defined in which the sub module absorbs energy regardless of the current direction. Preferably, the sub module absorbs energy regardless of the current direction in a switching state in which all the power semiconductor switching units are in their interrupting state. If, accordingly, all the power semiconductor switching units are placed into their interrupting state, the sub module can advantageously develop an opposing voltage for decay of the current in the event of a fault, regardless of the current direction. According to the invention, this allows a high short-circuit current to be handled without additional external switches. It is ensured in the context of the invention that high short-circuit currents can be avoided quickly, reliably and effectively in both directions by the converter itself. Additional switches, for example in the DC voltage circuit that is connected to the converter, or else semiconductor switches connected in parallel to a power semiconductor of the sub module, are superfluous in the context of the invention. In the event of a fault, the sub modules absorb the energy released almost exclusively, so that this is fully absorbed. The absorption of energy has an opposing voltage as a result, and can be measured in a defined manner through the dimensioning of the capacitors. Unfavorably high voltages can be avoided through this. In addition, the controlled charging of an energy store is not necessary to restart the converter. The converter according to the invention is, rather, able to restart its normal operation at any time following an electronic switch-off. 
     Further usable switching states that generate opposing voltage are given in association with the exemplary embodiment illustrated in  FIG. 6 . Each of these is highlighted in that one or both of the arithmetic signs (cf. Wc 1 , Wc 2  in  FIG. 6 ) is/are positive, so indicating energy absorption of the energy stores concerned. 
     Preferably, the power semiconductor switching units are reverse-conducting power semiconductor switches that can be switched on and off. 
     The power semiconductor switching units can each also comprise a power semiconductor that can be switched on and off, with which a freewheeling diode is connected in parallel but with the opposite polarity. 
     According to one exemplary embodiment of the invention, each energy store of the sub module is a unipolar storage capacitor. 
     According to a further embodiment of the invention, the connecting means comprise a second switching branch that connects the emitter switching branch to the collector switching branch, and in which a power semiconductor switching unit is arranged. The power semiconductor switching unit arranged in the first switching branch is here connected in parallel with the power semiconductor switching unit arranged in the second switching branch. This embodiment yields the advantage of a reduced conduction power loss. In addition, connecting lines arranged between the subunits are not critical in terms of their length and stray inductance. This allows both of the partial units of the sub module that are connected to one another by the connecting lines to be structurally and spatially separate, so giving rise to significant advantages for the industrial series production and for servicing. 
     Embodiments of the sub modules according to the invention that replace three or more cascaded full bridges can in principle also be realized. The connecting means can be fitted with three or more switching branches for this purpose, in which further power semiconductor switching units, energy stores or other components can be arranged. Under some circumstances, however, the relative advantages of such embodiments can wane in comparison with the embodiments described above. 
     It can be established that the conduction power loss can be reduced in comparison with cascaded full bridges in general by a factor of between 0.5 and 0.8, depending on the embodiment of the sub module and depending on the characteristic semiconductor conduction curve. 
     This is explained with reference to the following exemplary characteristic conduction curves. If all the power semiconductors exhibit a purely ohmic characteristic conduction curve in both current directions—as would be the case with appropriately actuated field effect transistors—then the following conduction power loss would apply for two conventional, cascaded full bridges: 
         P   P =( I   XRMS ) 2 ·4· R   0  
 
     where I XRMS  represents the effective value of the branch current, and R 0  represents the conduction resistance per power semiconductor. The following applies to the exemplary embodiment according to  FIG. 3 : 
         P   P ′=( I   XRMS ) 2 ·3 R   0  
 
     wherein the required semiconductor area is additionally reduced to ⅞. With an equal total semiconductor area, the conduction resistance per power semiconductor can be reduced to R 0 ′=⅞·R 0 , so that the power loss is even smaller. 
    
    
     
       The invention is explained in more detail below with reference to  FIGS. 1 to 6 . 
         FIG. 1  shows a schematic representation of an exemplary embodiment of a multi-stage converter; 
         FIG. 2  shows a sub module from the prior art; 
         FIG. 3  shows a schematic representation of a first exemplary embodiment of a sub module according to the invention; 
         FIG. 4  shows a schematic representation of a second exemplary embodiment of the sub module according to the invention; 
         FIG. 5  shows a schematic representation of a third exemplary embodiment of the sub module according to the invention; 
         FIG. 6  shows a tabular summary of switching states of the sub module according to the invention. 
     
    
    
     In detail,  FIG. 1  shows a converter  10 , wherein the converter  10  is designed as a multi-stage converter. The converter  10  comprises three AC voltage terminals L 1 , L 2 , L 3  for connecting to a three-phase AC voltage network. The converter  10  furthermore comprises DC voltage terminals  104 ,  105 ,  106 ,  107 ,  108  and  109  for connecting to a positive pole terminal  102  and a negative pole terminal  103 . 
     The positive pole terminal  102  and the negative pole terminal  103  can be connected to a positive and negative pole respectively of a DC voltage network, not illustrated in  FIG. 1 . 
     The AC voltage terminals L 1 , L 2 , L 3  can each be connected to a secondary winding of a transformer. The primary winding of the transformer is connected to an AC voltage network, not illustrated in  FIG. 1 . The direct electrical connection to the AC voltage network, for example with the intermediate connection of a coil or choke or of a capacitive component, is also possible in the context of the invention. 
     Power semiconductor valves  101  extend between each one of the DC voltage terminals  104 ,  105 ,  106 ,  107 ,  108 ,  109  and one of the AC voltage terminals L 1 , L 2 , L 3 . Each of the power semiconductor valves  101  comprises a series circuit of sub modules  1 . 
     Each power semiconductor valve  101  moreover has a choke  5 . 
     Each of the two-pole sub modules  1 , which have identical designs in the embodiment illustrated in  FIG. 1 , comprises two current-carrying terminals X 1  and X 2 . 
     In the exemplary embodiment illustrated in  FIG. 1 , the converter  10  is part of an HVDC installation, and serves to connect AC voltage networks via a high voltage direct current network. The converter  10  is constructed in order to transfer high electrical powers between the AC voltage networks. The converter  10  can, however, also be part of a reactive power compensation/network stabilization plant, such as for example what is known as a FACTS installation. Further applications of the converter  10 , such as for example in drive technology, are moreover conceivable. 
     The basic structure of one embodiment of a sub module  1  according to the invention is illustrated in  FIG. 3 . 
     The sub module  1  comprises a first subunit  2  as well as a second subunit  3 , each of which is indicated by a broken line for the purposes of illustration. The first subunit  2  and the second subunit  3  have the same structure. 
     The first subunit  2  comprises a first series circuit of power semiconductor switching units  22  and  23 , which, in the variant embodiment shown, each comprise an IGBT  221  and  231  respectively as a power semiconductor that can be switched on and off, and in each case a freewheeling diode  222  and  232  respectively. The freewheeling diodes  222 ,  232  are connected in parallel but with opposite polarity with the respectively assigned IGBT  221 ,  231 . The two IGBTs  221 ,  231  are oriented in the same sense as one another, and thus have the same forward conduction direction. The potential node between the power semiconductor switching units  22 ,  23  is connected to a first connecting terminal X 2 . The series circuit of the two power semiconductor switching units  22  and  23  is connected in parallel with a first energy store that is realized as a capacitor  21 . A voltage UC 1  is dropped across the capacitor  21 . 
     The second subunit  3  comprises a first series circuit of power semiconductor switching units  32  and  33 , which each comprise an IGBT  321  and  331  respectively as a power semiconductor that can be switched on and off, and in each case a freewheeling diode  322  and  332  respectively. 
     The freewheeling diodes  322 ,  332  are connected in parallel but with opposite polarity with the respectively assigned IGBT  321 ,  331 . The two IGBTs  321 ,  331  are oriented in the same sense as one another, and thus have the same forward conduction direction. The potential node between the power semiconductor switching units  32 ,  33  is connected to a second connecting terminal X 1 . The series circuit of the two power semiconductor switching units  32  and  33  is connected in parallel with a first energy store that is realized as a capacitor  31 . A voltage UC 2  is dropped across the capacitor  31 . 
     The subunits  2  and  3  are linked to one another via connecting means  4 . The connecting means  4  are surrounded by a broken line in  FIG. 3  for the purposes of illustration. The connecting means  4  comprise an emitter connecting branch  41  and a collector connecting branch  42 . 
     The emitter connecting branch  41  connects an emitter of the IGBT  231  to an emitter of the IGBT  331 . A power semiconductor switching unit  46  is arranged in the emitter connecting branch  41 . The power semiconductor switching unit  46  comprises an IGBT  461  and a diode  462  connected in parallel with it but with the opposite polarity. 
     The collector connecting branch  42  connects a collector of the IGBT  221  to the collector of the IGBT  321 . A power semiconductor switching unit  45  is arranged in the collector connecting branch  42 . The power semiconductor switching unit  45  comprises an IGBT  451  and a diode  452  connected in parallel with it but with the opposite polarity. 
     The emitter connecting branch  41  is connected to the collector connecting branch  42  via a switching branch  43 . 
     A switching unit is arranged in the switching branch  43  which, according to the exemplary embodiment illustrated in  FIG. 3 , is designed as a power semiconductor switching unit  44 . The power semiconductor switching unit  44  comprises an IGBT  441  and a diode  442  connected in parallel with it but with the opposite polarity. The switching branch  43  connects the emitter of the IGBT  451  to the collector of the IGBT  461 . 
     The manner in which the circuit of the sub module  1  according to the invention operates is to be explained in more detail below with reference to the table illustrated in  FIG. 6 ; the table in  FIG. 6  summarizes the switching states of the sub module  1  that are preferably used. 
     The first column of the table in  FIG. 6  contains the serial number assigned to a switching state; the second column contains the information regarding the current direction/polarity of the terminal current ix; the third through ninth columns each reveal a state of the individual IGBTs, with the number 1 for “switched on” and 0 for “interrupting”, wherein each IGBT can be identified with reference to the associated numerical identifier from  FIG. 3 ; the tenth column contains the terminal voltage UX associated with the respective switching state; columns WC 1  and WC 2  are to make clear whether the storage capacitors  21  and  31  are absorbing or releasing energy, wherein +1 represents the absorption and −1 represents the release of energy. 
     It can be seen from the table in  FIG. 6  that a positive voltage UX is always generated at the connecting terminals X 1  and X 2  in switching states  2 ,  3  and  4 . This is true regardless of the direction of the terminal current. Thus, for example, the capacitor voltage UC 1  or the capacitor voltage UC 2 , or else the sum of the two capacitor voltages UC 1 +UC 2 , can be generated at the connecting terminals. 
     In switching state  5 , all the IGBTs  231 ,  221 ,  331 ,  321 ,  441 ,  451 ,  461  are in their interrupting state, so that the flow of current through the IGBTs  231 ,  221 ,  331 ,  321 ,  441 ,  451 ,  461  is interrupted. In this switching state, the terminal voltage UX generates an opposing voltage, regardless of the polarity of the terminal current ix, so that the sub module  1  absorbs energy. 
     When the current direction is negative (current flowing in a direction opposite to the direction of the arrow identified by ix), an autonomous balancing of the capacitor voltages UC 1  and UC 2  means that, approximately, UX=−(UC 1 +UC 2 )/2. When the current direction is positive (current flowing in the direction of the arrow identified by ix), a positive opposing voltage UX=UC 1 +UC 2  is developed. It is advantageous here that the current that occurs in this switching state is passed through both capacitors, since a lower over-voltage then occurs at them than if only one capacitor were to absorb the energy. 
     Switching state  5  can be used in the event of a fault for full current decay. If all the sub modules  1  are placed into this switching state, the branch currents of the converter  10 , and consequently also the currents on the AC voltage side and the DC voltage side, are brought down to a value of zero very quickly as a result of the total of the opposing voltages of all the series-connected sub modules  1 . The speed of this current decay results from the above-mentioned opposing voltage and from the total inductances present in the electric circuits. In the case of the illustrated exemplary embodiment, this can typically lie in the order of magnitude of a few milliseconds. The dead time before the current decay starts depends largely on the response time of the switching unit  44 . If a power semiconductor switching unit is used for the switching unit  44 , this dead time is negligible. The dead time is then primarily a result of the slowness of the various measuring sensors and current converters with whose aid a fault is recognized. This delay in this measurement value acquisition is at present typically in the range of a few tens of microseconds. 
     It should be noted that the first four switching states can also be realized using two cascaded sub modules from the prior art according to  FIG. 2 . The first five of the switching states can be realized using the sub module designed according to document DE 10 2009 057 288 A1. 
     In switching state  6 , a negative terminal voltage UX of the sub module  1  is generated whatever the current direction. 
     In switching state  7 , a negative terminal voltage UX of the sub module  1  is also generated whatever the current direction. 
     Additional (redundant) switching states  8  and  9  are moreover possible, which can be used for a more even distribution of the conduction losses when UX=0. 
       FIG. 4  illustrates a second exemplary embodiment of the sub module  1  according to the invention. Parts in  FIGS. 3, 4 and 5  that are identical and equivalent are here given the same reference signs in each case. For the avoidance of repetitions, only the differences between the individual embodiments will therefore be considered in more detail below. 
     The sub module  1  according to  FIG. 4  differs from the embodiment of  FIG. 3  in that the connecting means  4  in  FIG. 4  comprise two switching branches  431  and  432 . Each of the switching branches comprises a power semiconductor switching unit  44 . 
     Connecting lines  91  and  92  are arranged between the switching branches  431  and  432  as parts of the emitter and collector connecting branch  41 ,  42  respectively. The particular advantage of the embodiment of  FIG. 4  is that the length and the stray inductance of the connecting lines  91 ,  92  are not critical for the overall performance of the sub module  1 . The connecting lines can thus have a length that is adapted to the particular application. A structurally and spatially separate or adapted construction of the sub module  1  can be of great advantage for production and for servicing. 
       FIG. 5  shows a schematic representation of a third embodiment of the sub module  1  according to the invention. The emitter connecting branch of the connecting means  4  here comprises two connecting lines  92  as well as two power semiconductor switching units  45 . The collector connecting branch  42  of the connecting means  4  also comprises two connecting lines  91  as well as two power semiconductor switching units  46 . 
     The connecting means  4  furthermore comprise four switching branches  431 ,  432 ,  433  and  434 , wherein a power semiconductor switching unit  44  is arranged in each switching branch. The connecting means  4  furthermore comprise an energy storage branch  11  in which a third energy store  12  is arranged which, in the present example, is designed as a unipolar storage capacitor, across which the voltage UC 3  is dropped. 
     TABLE OF REFERENCE SIGNS 
     
         
         
           
               1  Sub module 
               2  First subunit 
               21  First energy store 
               22 ,  23  Power semiconductor switching unit 
               221 ,  231  Power semiconductor 
               222 ,  232  Freewheeling diode 
               3  Second subunit 
               31  Second energy store 
               32 ,  33  Power semiconductor switching unit 
               321 ,  331  Power semiconductor 
               322 ,  332  Freewheeling diode 
               4  Connecting means 
               41  Emitter connecting branch 
               42  Collector connecting branch 
               43 ,  431 ,  432  Switching branch 
               433 ,  434  Switching branch 
               44  Switching unit 
               45 ,  46  Power semiconductor switching unit 
               441 ,  451 ,  461  Power semiconductor 
               442 ,  452 ,  462  Freewheeling diode 
               5  Choke 
               6  Energy store 
               7  Power semiconductor switching unit 
               71 ,  72  Freewheeling diode 
               73 ,  74  Electronic switch 
               8  Thyristor 
               91 ,  92  Connecting line 
               10  Converter 
               101  Power semiconductor valve 
               102  Positive pole terminal 
               103  Negative pole terminal 
               104 ,  105 ,  106  DC voltage terminal 
               107 ,  108 ,  109  DC voltage terminal 
               11  Energy storage branch 
               12  Third energy store 
             L 1 , L 2 , L 3  AC voltage terminal 
             X 1  Second connecting terminal 
             X 2  First connecting terminal