Abstract:
A converter outputs reactive power and has three phase branches connected to a phase of an alternating voltage source. Each of the phase branches has a phase module with a series circuit containing two-pole sub modules. A voltage which is dropped across each phase module corresponds to the sum of the voltages which are dropped across the sub modules. The first and the third phase branch are electrically connected to one another by a magnetically coupled throttle pair. A control device controls phase module currents. The control device contains current controllers which are associated with the phase modules, a decoupling unit for decoupling control differences by a computational during control of the phase module currents, a coupling unit for correcting the decoupled computational actuating variables in accordance with the magnetic coupling of the first throttle pair to obtain corrected actuating voltages, and a control unit for driving the sub modules.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a converter for reactive power compensation and to a control method for such a converter. 
     Converters of this type are known from the prior art. Document WO 2013/087110 A1, for example, shows a converter with three phase branches connected together in a delta circuit. Each phase branch comprises a connection point for connecting to a phase of a three-phase alternating voltage supply network that has been assigned to it. The phase branches here are each connected through coupling inductances to the assigned phases of the alternating voltage supply network. Each phase branch comprises a phase module with a series circuit of two-pole sub modules, wherein the sub modules are formed as so-called full-bridge circuits. Each sub module accordingly comprises two series circuits of power semiconductor switching units, connected in parallel with one another, wherein each power semiconductor switching unit comprises a power semiconductor that can be switched off as well as a diode connected with it in an antiparallel manner. Each sub module further comprises an energy store connected in parallel with the two series circuits in the form of a capacitor. Each sub module can be driven in such a way that a voltage is dropped across its poles which either corresponds to the voltage of the capacitor, the voltage of the capacitor but with inverse polarity, or a zero voltage, i.e. a voltage with the value zero. By means of suitable drive of the sub modules it is thus possible to generate a step-shaped alternating voltage at the phase modules of the converter. The height of the step here corresponds to the capacitor voltage. The known converter further comprises a control device for controlling the currents in the phase branches of the converter, so that a desired inductive reactive power can be made available in the alternating voltage supply network. 
     In the known converter, each phase branch in the delta circuit is in addition directly connected to a further phase of the alternating voltage supply network, i.e. without a coupling inductance. If the connected alternating voltage supply network is, for example, grounded with low resistance, for example via a star point transformer, then this can have the result that in the event of a contact with ground inside the converter, short-circuit currents flow that can have the result of permanent damage to converter components. Even in the case of an alternating voltage supply network that is grounded with high resistance, the short-circuit currents from flashovers (resulting, for example, from soiling or loose, conductive material) can arise between the phase branches. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to propose a converter of the type described above in which the risk of damage as a result of short-circuit currents in the event of short-circuits inside the converter is minimized. 
     The object is achieved by a converter which, according to the invention, comprises a first, a second and a third phase branch, wherein the three phase branches can each be connected to an associated phase of an alternating voltage supply network, and are delta-connected to one another, wherein each of the phase branches comprises a phase module with a series circuit of two-pole sub modules, wherein each sub module comprises an energy store and at least one power semiconductor, and can be driven in such a way that at least one positive or at least one negative sub module voltage, or a voltage with the value zero, is dropped across the poles of the sub module, and a voltage dropped across each phase module corresponds to the sum of the sub module voltages dropped across its sub modules, wherein the second and the third phase branches are connected to the phases of the alternating voltage supply network assigned to them by means of coupling inductances, and the first and the second phase branches are electrically connected together by a magnetically coupled first choke pair comprising a first and a second end connection as well as a center connection, wherein the three connections of the first choke pair are connected in an arbitrary combination with the first phase branch, the second phase branch and with the phase of the alternating voltage supply network that is assigned to the first phase branch, wherein furthermore a control device is provided for control of phase module currents, comprising current controllers assigned to the phase modules, a decoupling unit arranged upstream of the current controllers for computationally decoupling control differences during control of the phase module currents in respect of the mutual dependence of the phase module currents on account of the magnetic coupling in the first choke pair, in such a way that decoupled computer actuating variables can be derived from the decoupled control differences by means of the current controller, a coupling unit which is arranged downstream of the current controllers for correcting the decoupled computer actuating variables in accordance with the magnetic coupling of the first choke pair so as to obtain corrected actuating voltages, and a control unit for driving the sub modules, so that the voltages which are dropped across the phase modules can be adjusted in such a way that they correspond to the corrected actuating voltages. 
     In contrast to the known converter, the single coupling inductance arranged in the first phase branch is accordingly replaced by the first magnetically coupled choke pair that connects the first and the third phase branches electrically. Expressed otherwise, the coupling inductance forms the first choke pair together with a further inductance. The magnetic coupling between the two chokes of the first choke pair is taken into account in the control of the phase module currents by means of the proposed control device. 
     Through the proposed arrangement of the choke pair, the risk of short-circuit currents in the converter can advantageously be minimized. If a short-circuit occurs in the third phase branch with respect to the ground potential, the current will in any case be limited by the choke pair and the coupling inductance in the third phase branch. A direct connection between one phase of the alternating voltage supply network and the ground contact point, i.e. a current path that does not pass through any current-limiting inductances, is thus prevented. In this way damage to the components of the converter can be prevented. 
     The magnetically coupled choke pair can, for example, be realized in the form of two choke coils arranged spatially next to one another. The choke coils can be air-cored coils, but they can also comprise a common ferromagnetic core. The choke pair comprises three connections: a first and a second end connection as well as a center connection. 
     The center connection here is arranged at an electrical connection of the two choke coils. The three connections of the choke pair are assigned in any desired way to the first phase branch, the second phase branch and the phase of the alternating voltage supply network. For example, the center connection can be connected to the phase of the alternating voltage supply network that is assigned to the first phase branch, while the first end connection is connected to the first phase branch and the second end connection to the second phase branch. The two chokes of the choke pair can have a construction that is the same, but they can also have different constructions, for example they can comprise different numbers of windings. 
     The compact construction enabled by the magnetically coupled choke pair, or the spatial proximity of the two chokes of the choke pair, requires a comparatively small footprint, which reduces the cost of the converter as a whole. 
     A coupling of the currents in the phase branches results from the magnetic coupling between the two chokes of the first choke pair. In order to ensure a controlled acceptance of reactive power or output of reactive power, this dependence must be taken into account in the control of the phase module currents. According to a mathematical model of the control, the mesh and node equations in the converter yield the transformations between a computationally decoupled mathematical control system, including the associated equations, and a coupled mathematical control system. Appropriately, a system of equations in particular results, with coefficients that correspond to the magnetic coupling constants of the magnetic coupling in the choke pair and on the inductance values of the chokes and coupling inductances. In this connection, the decoupling unit is provided to computationally decouple from the mutually coupled control differences (since the system of equations is linear, the transformations can also be applied to the set point currents and the actual currents), i.e. to convert them into the mathematically decoupled system of currents. The decoupled control differences are supplied to the current controllers, which determine computer actuating variables from the control differences. The computer actuating variables can accordingly not be used to control the phase module currents, since they only represent an intermediate computational step. These actuating variables must be corrected in the next step in accordance with the magnetic couplings, in order to obtain the true actuating variables which are referred to as corrected actuating voltages. The mathematical transformations belonging to this emerge again from the state equations of the currents in the converter. The correction of the computer actuating variables is performed by the coupling unit. The corrected actuating voltages are appropriately first supplied to the control unit, which generates from them the control signals for the individual sub modules of the phase modules. 
     According to an advantageous form of embodiment of the invention, a second magnetically coupled choke pair is provided, formed from the coupling inductance assigned to the second phase branch and from a further inductance, and connects the first and the second phase branches together, wherein the second choke pair has a first and a second end connection and a center connection, wherein the three connections of the second choke pair are joined in an arbitrary combination with the first phase branch, the second phase branch, or with the phase of the alternating voltage supply network that is assigned to the second phase branch. A third magnetically coupled choke pair is furthermore provided, formed from the coupling inductance assigned to the third phase branch and from an additional inductance, and connects the second and the third phase branches together, wherein the third choke pair has a first and a second end connection and a center connection, wherein the three connections of the third choke pair are joined in an arbitrary combination with the second phase branch, the third phase branch, or with the phase of the alternating voltage supply network that is assigned to the third phase branch. The decoupling unit is here additionally configured to computationally decouple the control differences in respect of the mutual dependence of the phase module currents resulting from the magnetic coupling in the second and third choke pairs, wherein the coupling unit is configured for correction of the decoupled computer actuating variables obtained by means of the current controller from the decoupled control differences in accordance with the magnetic coupling in the first, second and third choke pairs so as to obtain the corrected actuating voltages. 
     In other words, the first and the second phase branches are thus also connected together by a magnetically coupled second choke pair, so that the individual coupling inductance in the second phase branch constitutes one of the chokes of the second choke pair. The second and the third phase branches are also connected together by a magnetically coupled third choke pair, so that the individual coupling inductance in the third phase branch constitutes one of the chokes of the third choke pair. 
     In this way all the dependencies between the currents resulting from the magnetic coupling between the chokes of the three choke pairs are taken into account in the control of the phase module currents in the three phase branches of the converter. Controlled operation of the converter would not be possible without taking these dependencies into account. 
     The minimization of the damage from short-circuit currents in all three phase modules of the converter is particularly advantageous in this form of embodiment, since each short-circuit current is limited by at least two inductances of the choke pairs. 
     The sub modules are preferably constructed as full-bridge circuits, wherein the power semiconductor switches of the sub modules are, for example, bipolar transistors with insulated gate electrodes (IGBT) or metal oxide semiconductor field-effect transistors (MOSFET). In this case, the sub module voltages correspond to the energy storage voltages dropped across the energy stores of the sub modules. 
     It is, however, also possible for the sub modules to be designed differently, in order to match the converter better to the particular application. Sub modules are also possible in this connection that have a structure that differs from a full bridge. In particular, sub modules can be used at the poles of which a plurality of positive and/or a plurality of negative voltage values of the sub module voltage can be set through appropriate drive, for example a first and a second positive and/or a first and a second negative voltage value. Examples of such sub modules are described in the article “Novel DC-AC Converter Topology for Multilevel Battery Energy Storage Systems” by M. Gommeringer et. al, PCIM Europe 2013. The voltage dropped across the phase module corresponds to the total of the sub module voltages set at the sub modules. 
     Preferably the energy stores of the sub modules are storage capacitors, particularly preferably high-performance storage capacitors, in order to satisfy better the high requirements of the high-voltage application. 
     According to an advantageous form of embodiment of the invention, the control device further comprises a control unit connected to the decoupling unit for driving the sub modules by means of pulse-width modulation (PWM). Any appropriate PWM method, or the method known from the document WO 2008/086760 A1, can be used as the control method. The control unit can be implemented in the form of so-called Module Management Systems (MMS), wherein each of the three phase modules is assigned to one MMS. 
     Preferably the converter comprises measuring apparatus such as voltage and/or current transducers for detecting the phase module currents and the connection voltages. The current transducers can, for example, be arranged in one or a plurality of the phase branches. Capacitive voltage transducers can, furthermore, also be provided in order to measure the connection voltages. 
     According to a further preferred form of embodiment, the control device of the converter further comprises a signal processing unit with at least one mean value generator, at least one difference generator, at least one voltage controller, at least one frequency generator and at least one adder. The signal processing unit of the mean value generator is here configured to generate a mean value of the energy store voltages of the assigned phase module, wherein the difference generator is connected on the input side to an output of the mean value generator, and is configured to generate a control difference from the mean value of the energy store voltages and a predetermined DC voltage set point value; the voltage regulator is connected on the input side to an output of the difference generator, and is configured to generate a real set point current value from the control difference; the frequency generator is connected on the input side to an output of the voltage controller, and is configured to generate a sinusoidal real set point current from the real set point current value, and the adder is connected on the input side to an output of the frequency generator, and is configured to generate the set point current for the assigned phase module from the real set point current and a predetermined reactive set point current. 
     The (alternating) set point current for each phase module is thus composed of two components: the real set point current and the reactive set point current. The intake of a real power by the converter serves here to compensate for an unwanted voltage drop in the energy stores of the sub modules. The voltage controller can, for example, be a suitable linear controller. 
     The signal processing unit is preferably part of a digital data processing installation. If the individual energy stores of the sub modules are of different designs, the energy store voltages can also be compared with different assigned DC voltage set point values, in order to control the energy intake. It is also possible to replace the simple mean value generation in the mean value generator by a calculation adapted according to the sub modules being used, for example a weighted mean value generation. 
     The frequency generator creates an alternating current magnitude from the predetermined DC voltage set point value which is constant or which changes slowly over time, the phase angle of which is equal to the phase angle of the voltage in the alternating voltage supply network. The outputs of the signal processing units are appropriately connected in each case to the input of a current controller of the assigned phase module. 
     It is a further object of the invention to propose a control method for the converter described above which enables controlled output of reactive power. 
     The object is achieved by a control method in which, according to the invention, a set point current is predetermined for each phase module, control differences that are formed from the predetermined set point currents and the measured phase module currents, are computationally decoupled with reference to the mutual dependence of the phase module currents on account of the magnetic coupling in the first choke pair by means of a decoupling unit, so that current controllers assigned to the phase modules and arranged downstream of the decoupling unit each derive decoupled computer actuating variables from the decoupled control differences, and the decoupled computer actuating variables are corrected by means of a coupling unit arranged downstream from the current controllers in accordance with the magnetic coupling of the first choke pair to obtain corrected actuating voltages, wherein the sub modules of the phase modules are driven by means of a control unit in such a way that the voltages dropped across the phase modules correspond to the corrected actuating voltages. 
     A controlled operation of the converter according to the invention can be ensured by means of the proposed control according to the invention, so that reactive current compensation is possible in the connected alternating voltage supply network. 
     A preferred form of embodiment of the control method can particularly be applied to a converter in which a second magnetically coupled choke pair is provided, formed from the coupling inductance assigned to the second phase branch and from a further inductance, and connects the first and the second phase branches together, wherein the second choke pair has a first and a second end connection and a center connection, wherein the three connections of the second choke pair are joined in an arbitrary combination with the first phase branch, the second phase branch, or with the phase of the alternating voltage supply network that is assigned to the second phase branch, and a third magnetically coupled choke pair is provided, formed from the coupling inductance assigned to the third phase branch and from an additional inductance, and connects the second and the third phase branches together, wherein the third choke pair has a first and a second end connection and a center connection, wherein the three connections of the third choke pair are joined in an arbitrary combination with the second phase branch, the third phase branch, or with the phase of the alternating voltage supply network that is assigned to the third phase branch, wherein the control differences are computationally decoupled with reference to the mutual dependence of the phase module currents on account of the magnetic coupling in the first, second and third choke pairs by means of the decoupling unit, so that current controllers assigned to the phase modules and arranged downstream of the decoupling unit each derive decoupled computer actuating variables from the decoupled control differences, and the decoupled computer actuating variables are corrected by means of a coupling unit arranged downstream from the current controllers in accordance with the magnetic coupling of the first, second and third choke pairs to obtain corrected actuating voltages. The coupling inductances in the second and third phase branches are accordingly here included in the magnetically coupled second and third choke pairs. In this way the risk of short-circuit currents is minimized in all phase branches of the converter. 
     Preferably the corrected actuating voltages each have a disturbing voltage impressed upon them, wherein the disturbing voltages are determined depending on connection voltages dropped across the phase branches, and the corrected actuating voltages onto which the disturbing voltages have been impressed are supplied to the control unit. The disturbing voltages are determined by means of suitable measuring equipment. They correspond to the voltages that are driven into the individual phase branches of the converter by the currents from the alternating voltage supply network. For that reason, the disturbing voltages, according to the definition used here, have the opposite arithmetic sign to the voltages at the phase modules of the converter. Preferably the actuating voltages are converted by means of pulse-width modulation (PWM) into control signals for driving the assigned sub modules. 
     According to a further preferred form of embodiment of the control method, the set point currents are each composed of a real set point current and a reactive set point current, wherein the real set point currents are determined depending on a mean value of the voltages dropped across the sub modules of the associated phase module and of a predetermined DC voltage set point value. 
     The invention is explained below in relation to exemplary embodiments of the converter according to the invention illustrated in  FIGS. 1-6 . 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a schematic representation of a first exemplary embodiment of a converter according to the invention. 
         FIG. 2  shows a schematic representation of an exemplary embodiment of a sub module of the converter according to the invention. 
         FIG. 3  shows a schematic representation of an exemplary embodiment of a magnetically coupled choke pair. 
         FIG. 4  shows an exemplary embodiment of a control device of the converter according to the invention. 
         FIG. 5  shows a schematic representation of a second exemplary embodiment of a converter according to the invention. 
         FIG. 6  shows a schematic representation of a third exemplary embodiment of a converter according to the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In detail, an exemplary embodiment of a converter  10  according to the invention is illustrated in  FIG. 1 . The converter  10  comprises a first phase branch  1 , a second phase branch  2  and a third phase branch  3 . The first phase branch  1  comprises a connection point  4  for connecting to an alternating voltage supply network. The alternating voltage supply network is not illustrated graphically in  FIG. 1 . Accordingly, the phase branches  2  and  3  each comprise the connection points  5  and  6 , which connect each of the phase branches  2 ,  3  to the phases of the alternating voltage supply network assigned to them. 
     The first phase branch  1  comprises a phase module  7 . The phase module  7  comprises a series circuit of sub modules  8 . In general, the phase module  7  comprises an arbitrary number of phase modules  8 , where in  FIG. 1  only four sub modules  8  are illustrated graphically. The number m of sub modules  8  depends on the voltages to be generated in the sub module  7 . The phase branches  2 ,  3  also comprise phase modules  9 ,  11  respectively. The phase modules  9 ,  11  also each comprise a series circuit of sub modules  8 . In the exemplary embodiment of the converter  10  illustrated in  FIG. 1 , the phase modules  7 ,  9 ,  11  comprise the same number of sub modules  8 . It is, however, also possible that the number of sub modules  8  in the phase modules of the individual phase branches  1 ,  2 ,  3  is different. In the exemplary embodiment of the converter  10  illustrated, the sub modules  8  have the same construction. Here again it is possible for the sub modules  8  to be of different designs. 
     The phase module currents are measured by means of measuring apparatus  12 ,  13 ,  14 . 
     The voltages between the connection points  4 ,  5 ,  6 , which are identified in  FIG. 1  with UAB, UBC and UCA respectively, are measured by means of suitable measuring apparatus such as voltage transducers  15 ,  16  and  17 . 
     The first phase branch  1  and the third phase branch  3  are connected together by a magnetically coupled first choke pair  22 , which comprises end connections  23  and  24  along with a center connection  25 . The end connection  23  is connected to the third phase branch  3 , and the second end connection  24  to the first phase branch  1 . The center connection  25  of the choke pair  22  is connected to the connection point  4  and, through that, to the phase of the alternating voltage supply network that is assigned to the first phase branch  1 . 
     The first phase branch  1  and the second phase branch  2  are connected to one another by a magnetically coupled second choke pair  18 . The second choke pair  18  has two end connections  19  and  20  respectively, wherein the end connection  19  is connected to the first phase branch  1 , and the second end connection  20  is connected to the second phase branch  2 . The second choke pair  18  further has a center connection  21 , which can be connected to the phase of the alternating voltage supply network assigned to the second phase branch  2 . 
     The second and the third phase branches  2 ,  3  are furthermore connected through a third choke pair  26 . The magnetically coupled third choke pair  26  has end terminals  27  and  28  and a center connection  29 . The connection  27  is here to be connected to the second phase branch  2 , the connection  28  to the third phase branch  3 , and the connection  29  to the connection point  6  to the phase of the alternating voltage supply network assigned to the third phase branch. 
     The converter  10  further comprises a control device  31  only shown here schematically, which is configured to control the phase module currents by means of control outputs  311 , control outputs  312  and control outputs  313 . 
     The construction of the sub module  8  is considered in more detail in  FIG. 2 . The sub module  8  is constructed as a two-terminal network, wherein the two poles of the sub module  8  are identified in  FIG. 2  with X 1  and X 2 . The sub module  8  of  FIG. 2  is designed as a so-called full-bridge circuit or H-bridge circuit. It comprises two series circuits of power semiconductor switching units  81 , each of which consists of a parallel interconnection of a power semiconductor switch that can be switched off and of a diode connected with it in antiparallel. The sub module  8  further comprises an energy store  82 , which is implemented in the exemplary embodiment illustrated in  FIG. 2  as a storage capacitor. The storage capacitor here is connected in parallel with the two series circuits with the power semiconductor switching units  81 . Through suitable drive of the sub modules via the control outputs  311 ,  312 ,  313  of the control device  31 , which is not illustrated in  FIG. 2 , the sub modules  8  can each be driven in such a way that a sub module voltage is dropped at the two terminals X 1  and X 2  of the sub modules which is equal to the voltage dropped across the capacitor  82 , the voltage across the capacitor  82  but with the inverse polarity, or to a voltage with the value zero. With this, in a temporal sequence, a voltage curve can be generated at each of the phase modules  7 ,  8 ,  9 , which corresponds to a stepped alternating voltage. 
       FIG. 3  shows a schematic illustration of one of the magnetically coupled choke pairs  18 ,  22 ,  26  of the converter  10  of  FIG. 1 . In detail, the first choke pair  22  is illustrated in  FIG. 3 . In the exemplary embodiment of the converter  10  shown here, the second and third choke pairs  18  and  26  have the same construction as the choke pair  22 . It is also, however, possible for the choke pairs  22 ,  18 ,  26  to be of different designs. The magnetically coupled first choke pair  22  comprises a first end connection  23 , a second end connection  24  and a center connection  25 . The choke pair  22  further comprises a first choke  221  along with a second choke  222 . The two chokes  221  and  222  are arranged in close spatial proximity to one another. In this way a magnetic coupling arises between the two chokes  221 ,  222 , and is not to be neglected in the control of the converter  10  of  FIG. 1 , in contrast to chokes that are not coupled. The two chokes  221 ,  222  are wound around a common core  223  that is manufactured of a ferromagnetic material. It should be pointed out in this connection, that air coupling of the two chokes  221 ,  223  is also possible. 
       FIG. 4  shows in detail the construction of an exemplary embodiment of a signal processing unit  32  of the control device  31  of the converter  10  from the exemplary embodiment of  FIG. 1 . For the sake of better understanding of the structure, the individual components of the signal processing unit  32  are divided in the form of blocks  100  to  1000 . The control steps executing in the blocks  100  to  500  occur in parallel for the three phase modules  7 ,  8 ,  9  of the first, second and third phase branches  1 ,  2 ,  3 . To avoid repetitions, these control steps are only described in detail below for the phase module  7  of the first phase branch  1 . The voltages UDC 1  to UDCN of the energy store  16  of the sub modules  8  of the phase module  7  are added in a mean value generator  100 , and a mean value of these voltages is formed by dividing by the number N of the sub modules in the phase module. The difference between the mean value of the voltages determined in block  100  is compared in a difference generator  200  with a DC voltage set point value UDCREF and supplied to a voltage controller  201  that supplies a real set point current value to the output of the difference generator  200 . The real set point current value determined is converted in a frequency generator  300  to an alternating current variable, in that the real set point current is generated from the real set point current value. The real set point current is an alternating current variable whose phase corresponds to the phase of the network voltage in the alternating voltage supply network. A predetermined reactive set point current irefAC for the phase module  7  is added by means of an adder  400  to the real set point current. It must be noted here that the predetermined reactive set point currents for the two other phase modules  8 ,  9 , which are identified in  FIG. 4  with irefBC and irefCA respectively, differ in general in their value and their temporal course from irefAB. The set point current calculated from the real set point current and the reactive set point current is compared in a unit  500  with a phase module current iAB measured by means of the current measuring unit  11 , so forming a control difference eAB. The control differences eBC and eCA are correspondingly generated for the two phase modules  8 ,  9 . 
     The control differences eAB, eBC and eCA are supplied to a decoupling unit  600 . By means of the decoupling unit, the control differences are computationally decoupled, wherein the decoupling is taken into account in respect of the mutual dependence of the phase module currents resulting from the magnetic coupling in the choke pairs. The decoupling can be described mathematically as follows according to an exemplary embodiment. If x=(iconvAB, iconvBC, iconvCA) represents a vector of the phase module currents, and u=(uAB-uconvAB, uBC-uconvBC, uCA-uconvCA) represents a vector of the differences between the connection voltages and the voltages dropped across the phase modules, then the time derivative dx/dt of the phase module currents can be described by the equation
 
 dx/dt−M 1* x+M 2* u  
 
     The 3×3 matrices M 1  and M 2  in the above equation take the mutual dependence of the phase module currents resulting from the magnetic coupling in the choke pairs into account. Through a transformation T, the matrix M 1  can be converted into diagonal form, so that the mutual dependencies of the phase module currents are computationally resolved, which is referred to here as decoupling. Due to the linearity of the system of mathematical equations, the transformation T can also be applied to the control differences eAB, eBC, eCA. In this case, decoupled control differences e′AB, e′BC, e′CA result from the decoupling. The decoupled control differences e′AB, e′BC, e′CA are supplied to the current controllers  700  that are arranged downstream of the decoupling unit  600 . The current controllers  700  determine decoupled computer actuating variables u′AB, u′BC, u′CA from these. A coupling unit  800  arranged downstream of the current controllers  700  then calculates corrected actuating voltages ustellAB, ustellBC, ustellCA from the decoupled actuating variables, wherein the magnetic coupling in the choke pairs is taken into account again by the correction. 
     The decoupling can, for example, be carried out as follows:
 
 e′AB =(− eAB+ 2* eBC−eCA )/3,
 
 e′BC =(− eAB−eBC+ 2* eCA )/3,
 
 e′CA =( eAB+eBC+eCA )/3.
 
     The following then applies to the coupling:
 
ustell AB =( M−L )* u′AB +( M−L )* u′BC +( L+ 2* M )* u′CA,  
 
ustell BC =( L−M )* u′AB +( L+ 2* M )* u′CA,  
 
ustell CA =( L−M )* u′BC +( L+ 2* M )* u′CA,  
 
where M represents the magnetic interaction between the chokes of one of the choke pairs, and L represents the value of inductance of the chokes of the choke pairs. M and L here are the same for all the choke pairs, because the choke pairs in this exemplary embodiment are implemented identically, which is not, however, in general necessary.
 
     According to the exemplary embodiment illustrated in  FIGS. 1 to 4 , the corrected actuating voltages ustellAB, ustellBC, ustellCA have measured disturbing voltages impressed upon them in the units  900 , which in this exemplary embodiment correspond to measured connection voltages uAB, uBC and uCA. Phase module voltages uconvAB, uconvBC, uconvCA that are to be set are obtained through this second correction. These are forwarded to the control unit  1000 , wherein the control unit  1000  comprises so-called Module Management Systems (MMS). By means of the MMS, the phase module voltages that are to be set are converted into control signals for the sub modules  8 . The control signals are supplied to the sub modules by means of the control outputs  311 ,  312 ,  313 . 
     A second exemplary embodiment of a converter  101  according to the invention is illustrated in  FIG. 5 . Parts that are the same or similar are given the same reference codes in  FIGS. 1 and 5 . To avoid repetitions, only the differences between the exemplary forms of embodiment of  FIG. 1  and  FIG. 5  will be considered in the description of  FIG. 5 . The sub modules  8  of the converter  20  also have the same construction, and correspond to the sub modules  8  of  FIG. 2 . This also applies correspondingly to a third exemplary embodiment of a converter  102  according to the invention illustrated in  FIG. 6 . 
       FIGS. 5 and 6  illustrate in particular two further possibilities for the arrangement of the choke pairs  18 ,  22  and  26 . 
     In  FIG. 5 , the end connection  24  of the first choke pair  22  is connected to the connection point  4 . The center connection  25  is connected to the third phase branch  3 . The end connection  23  is connected to the first phase branch  1 . The end connection  20  of the second choke pair  18  is connected to the connection point  5 . The center connection  21  is connected to the first phase branch  1 . The end connection  19  is connected to the second phase branch  2 . The end connection  28  of the third choke pair  26  is connected to the connection point  6 . The center connection  29  is connected to the second phase branch  2 . The end connection  27  is connected to the third phase branch  3 . 
     In  FIG. 6 , the end connection  23  of the first choke pair  22  is connected to the connection point  4 . The center connection  25  is connected to the first phase branch  1 . The end connection  24  is connected to the third phase branch  3 . The end connection  19  of the second choke pair  18  is connected to the connection point  5 . The center connection  21  is connected to the second phase branch  2 . The end connection  20  is connected to the first phase branch  1 . The end connection  27  of the third choke pair  26  is connected to the connection point  6 . The center connection  29  is connected to the third phase branch  3 . The end connection  26  is connected to the second phase branch  2 . 
     The converters  101 ,  102  of  FIGS. 5 and 6  are controlled correspondingly by means of the control device  31  illustrated in  FIG. 4 . The decoupling and coupling described there differ, however, through the use of different transformations M 1 , M 2 , T. The transformations here take the different coupling in the choke pairs into account. 
     LIST OF REFERENCE CODES 
     
         
           1 ,  2 ,  3  Phase branch 
           4 ,  5 ,  6  Connection point 
           7 ,  9 ,  11  Phase module 
           8  Sub module 
           81  Power semiconductor switching unit 
           82  Capacitor 
           10  Converter 
           12 ,  13 ,  14  Measuring apparatus 
           15 ,  16 ,  17  Measuring apparatus 
           18 ,  22 ,  26  Choke pair 
           19 ,  20 ,  21  Connection 
           23 ,  24 ,  25  Connection 
           27 ,  28 ,  29  Connection 
           221 ,  222  Winding 
           223  Coil core 
           31  Control device 
           311 ,  312 ,  313  Control outputs 
           32  Signal processing unit 
           100  Mean value generator 
           200  Difference generator 
           300  Frequency generator 
           400  Adder 
           500  Unit 
           600  Decoupling unit 
           700  Current controller 
           800  Coupling unit 
           900  Unit 
           1000  Control unit 
         X 1 , X 2  Connection of the sub module 
         UDC 1  . . . UDCN Energy storage voltage 
         UDCREF DC voltage set point value 
         irefAB, irefBC, irefCA Reactive set point current 
         iAB, iBC, iCA Phase module current 
         eAB, eBC, eCA Control difference 
         e′AB, e′BC, e′CA Decoupled control difference 
         u′AB, u′BC, u′CA Computer actuating variable 
         ustellAB, ustellBC, ustellCA Corrected actuating voltage 
         uconvAB, uconvBC, uconvCA Voltage to be set 
         MMS Module Management System