Modular multilevel converter comprising a control unit having a modulation member

Abstract

The invention relates to a modular multilevel voltage converter (10) comprising three arms (a, b, c), each comprising an upper half-arm and a lower half-arm, wherein each half-arm comprises a chain of individually controllable submodules of formula (I), and wherein the converter further comprises a control unit (30) comprising at least one AC control module (40), a DC control module (52), and a modulation member (58) that is configured to deliver modulation signal setpoints for controlling the control members of the submodules of the upper and lower half-arms, wherein the modulation signal setpoints are delivered on the basis of voltages across the terminals of the capacitors of the submodules of the upper and lower half-arms and on the basis of modulation voltage setpoints applied to the upper and lower half-arms, and wherein the modulation voltage setpoints depend on a modulation voltage alternating component setpoint delivered by the AC control module, a modulation voltage common-mode component setpoint delivered by the DC control module, and a modulation voltage homopolar common-mode component setpoint, which depends on the voltages across the terminals of the capacitors of the submodules of the upper and lower half-arms.

Description

[0001] Description Title of the invention: Multi-level modular converter comprising a control unit having a modulation member

[0002] Technical Field

[0003] The present invention relates to the technical field of modular multi-level voltage converters for converting an alternating voltage into a direct voltage and vice versa. These modular multi-level voltage converters, which will subsequently be referred to as MMC converters for "Modular Multilevel Converter" in English, are particularly suitable for installation in high voltage direct current (HVDC) power supply installations.

[0004] MMC converters traditionally comprise several arms connected in parallel to each other. Each arm comprises an upper half-arm and a lower half-arm. Each of the half-arms comprises a chain of controllable sub-modules connected in series, said sub-modules each comprising a capacitor. By controlling said sub-modules to insert or not their capacitors into the associated chain of sub-modules, it is possible to adjust the voltage of the half-arm in which this chain of sub-modules is connected.

[0005] Each upper and lower half-arm of the MMC converter can be modeled by an equivalent capacitor, having a voltage v^j across its terminals, where I and u are indices associated with the upper or lower half-arms and j is an index associated with the corresponding arm.

[0006] The insertion of the sub-modules into the half-arms is controlled by means of modulation members generating modulation signals, for controlling the control members of the sub-modules of said upper and lower half-arms. To each half-arm of the MMC converter is applied a modulation voltage, which is a function of a modulation index specific to said half-arm and of the voltage across the capacitors of the sub-modules of this half-arm. This modulation voltage can be adjusted by controlling the number of sub-modules inserted into the corresponding half-arm.

[0007] The present invention relates more specifically to the control of such MMC converters, equipped with a control unit. Traditionally, voltage source converters have eleven state variables, namely five current variables and six voltage variables, which must be controlled. The choice of the method for controlling these state variables, and more generally of the converter, is decisive in maintaining the stability of the MMC converter and the DC and AC networks to which it is connected.

[0008] Prior art

[0009] There are mainly two known methods for controlling MMC converters, namely an energy-based control method, or "Energy-Based Control (EBC)" in English, and a non-energy-based control method, "Non-Energy-Based Control (NEBC)" in English. The so-called energy-based control method is based on the particular energy behavior of the MMC converter. The so-called non-energy-based control method is based on the traditional control scheme of voltage source converters, called VSC converters. Both methods are implemented by means of adapted voltage converter control units, depending on the control requirements.

[0010] Traditionally, the modulation voltage applied to each of the half-arms of an MMC converter can be decomposed into a common-mode component of modulation voltage, usually representing a DC offset, and an AC component of modulation voltage.

[0011] In the so-called non-energy-based (NEBC) control methods of an MMC converter, six modulation signals are generated, namely from three modulation voltage common mode component setpoints and three modulation voltage AC component setpoints, each being associated with one of the converter arms. The modulation voltage common mode component setpoints are set to a value of v dc / 2, where v dc is the voltage of the direct current power supply network.

[0012] As a result, the modulation signal setpoint m“- 1* for an upper or lower half arm of an arm j is written: u,l» V mj m,- = -

[0013] 7 Vdc where is the modulation voltage setpoint of the half-arm, u and I are indices indicating whether it is an upper or lower half-arm and j is the index associated with the arm.

[0014] It is noted that in this so-called non-energy-based control method, the modulation signal does not take into account the voltages across the capacitors of the sub-modules v^j x , x being the number of the submodule among the submodules in series in the half-arm, nor of the tensions across the equivalent capacitors of the half-arms. However, these voltages vÿ across the equivalent capacitors of each of the half-arms are not constant and are likely to oscillate at any time. Consequently, this so-called non-energy-based control method does not provide compensation for the oscillations of the voltages across the equivalent capacitors and their impact on the modulation voltages applied to the half-arms. This control method is therefore referred to as uncompensated modulation. A disadvantage of this control method is that the oscillation of the half-arm capacitor voltages generates significant internal common-mode currents circulating within the voltage converter. These internal currents are highly harmonic, which results in high power losses and can lead to instability of the voltage converter.As a result, this control method requires the use of an additional controller called a circulating current suppression controller (CCSC) to suppress internal common mode current harmonics.

[0015] This circulating current suppression controller, however, only allows specific harmonics to be processed. Given the value of v dc / 2 imposed on the modulation voltage common mode component setpoints by this control method, the modulation signal has an offset of 1 Zz, which results in the systematic insertion in each half-arm of half of the capacitors of the sub-modules of each half-arm, as an offset, whatever their state of charge.

[0016] Furthermore, this control method only controls four of the eleven state variables of the voltage converter. The other state variables are not explicitly controlled, so their asymptotic stability must be ensured by selecting only a few adjustable parameters of the controller. This method therefore has the disadvantage that a large number of state variables are not explicitly controlled, and are prone to instability.

[0017] Conversely, the so-called energy-based control (EBC) method aims to compensate for voltage oscillations across the equivalent capacitors of the half-arms. According to this method, the modulation signal setpoint m^' 1 * for a half arm of an arm j is determined as follows: where v^j is the modulation voltage setpoint of the half-arm, j is the index associated with the arm, u and I are indices indicating whether it is an upper or lower half-arm, and is the voltage of the equivalent capacitor of the corresponding half-arm, equal to the sum of the voltages v^j x half-arm capacitors.

[0018] In this method, the modulation signal takes into account the voltages across the capacitors of the sub-modules. This control method is called compensated modulation. It prevents the circulation of internal common-mode currents by precisely controlling the modulation voltages applied to each half-arm.

[0019] A disadvantage of this so-called energy-based control method is that it involves controlling all eleven state variables of the voltage converter. It also does not allow for natural energy balancing between the half-arms.

[0020] Furthermore, this control is particularly complex and expensive in terms of computing resources since it involves controlling all eleven state variables of the converter but also measuring the voltage of the capacitors of each of the sub-modules and implementing these measurements in the control loops.

[0021] Furthermore, since this control method provides for controlling all eleven state variables of the voltage converter, the MMC converter controlled according to this so-called energy-based method tends to keep its state variables stable, without considering the state of the DC power supply network to which it is connected. Furthermore, with this control method, the voltage of each of the arms is essentially determined by a voltage v dccalled "feed forward", which is applied to each arm of the voltage converter. Therefore, the voltage converter naturally rejects disturbances appearing on the DC network and does not contribute to limiting or countering these disturbances. Unlike so-called non-energy-based control, the internal energy of the converter is not used here to contribute to stabilizing the DC power supply network. As a result, the DC network is all the more sensitive to disturbances and therefore loses stability. In other words, this control method prevents the inherent contribution of the voltage converter to the stability of the DC network, due to the explicit control of all internal state variables, which fundamentally rejects disturbances.

[0022] Statement of the invention

[0023] An aim of the present invention is to propose a modular multi-level voltage converter which overcomes the aforementioned drawbacks.

[0024] To this end, the invention relates to a multi-level modular voltage converter, making it possible to convert an alternating voltage into a direct voltage and vice versa, comprising a so-called direct part intended to be connected to a direct electrical supply network and a so-called alternating part intended to be connected to an alternating electrical supply network, the converter comprising three arms, each arm comprising an upper half-arm and a lower half-arm, each half-arm comprising a chain of sub-modules individually controllable by a control member specific to each sub-module and each sub-module comprising a capacitor, the control member of each sub-module being able to take at least a first state in which the capacitor is inserted into the corresponding chain of sub-modules and a second state in which the capacitor is not inserted into said chain of sub-modules,each half-arm having a modulation voltage which is a function of the number of capacitors inserted in the corresponding chain of sub-modules, the converter further comprising a converter control unit comprising at least: an AC control module configured to deliver a modulation voltage AC component setpoint; a DC control module configured to deliver a modulation voltage common mode component setpoint; a modulation member configured to deliver modulation signal setpoints, for controlling the control members of the sub-modules of said upper and lower half-arms,from voltages across the capacitors of the sub-modules of the upper and lower half-arms and from modulation voltage setpoints applied to the upper and lower half-arms which are a function of said modulation voltage alternating component setpoint delivered by the alternating control module, of said modulation voltage common mode component setpoint delivered by the continuous control module, and of a modulation voltage common mode homopolar component setpoint which depends on said voltages across the capacitors of the sub-modules of the upper and lower half-arms.,

[0025] The modular multi-level voltage converter (MMC) according to the invention is particularly suitable for connection in a high-voltage direct current (HVDC) power supply installation, between a direct current power supply network and an alternating current power supply network.

[0026] The voltage converter is advantageously a voltage source converter, called a VSC converter for “Voltage Source Converter” in English.

[0027] The modulation voltage applied to each of the half-arms of the MMC converter can advantageously be decomposed into at least one common mode modulation voltage component and one alternating modulation voltage component.

[0028] Preferably, each half-arm of the converter can be modeled by a modeled voltage source associated in parallel with an equivalent capacitor having an equivalent voltage at its terminals. This equivalent voltage advantageously corresponds to the sum of the voltages of the capacitors of said half-arm. The inverse of the capacitance of the equivalent capacitor is advantageously equal to the sum of the inverses of the capacitances of the capacitors of the half-arm.

[0029] The modulation device receives as input the said voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms and the said modulation voltage setpoints applied to the upper and lower half-arms.

[0030] In a non-limiting manner, the value of said voltages at the terminals of the capacitors of the sub-modules can be measured or determined by calculation. The modulation device is configured to deliver modulation signal setpoints for the upper half-arms and modulation signal setpoints for the lower half-arms.

[0031] Preferably, the modulation member receives as input three modulation voltage setpoints applied to each of the three upper half-arms and three modulation voltage setpoints applied to each of the three lower half-arms. Preferably, the modulation member is configured to deliver three modulation signal setpoints for controlling the control members of the sub-modules of the three upper half-arms and three modulation signal setpoints for controlling the control members of the sub-modules of the three lower half-arms.

[0032] Preferably, for each half-arm, the modulation member is configured to determine a modulation signal, for controlling the control members of the sub-modules of said half-arm, from voltages at the terminals of the capacitors of the sub-modules of this half-arm and from modulation voltage setpoints applied to said half-arm.

[0033] The modulation signal instructions are used to generate modulation signals. The modulation signals are then used to control the sub-module control units, in order to insert or not the corresponding capacitors in the half-arms and thus adjust the modulation voltage.

[0034] The modulation signals are preferably indices which can take a decimal value between 0 and 1. A modulation index of value 0 corresponds to the insertion of no capacitor in the associated half-arm, while a modulation signal of value 1 corresponds to the insertion of all the capacitors in the half-arm.

[0035] Preferably, the number of capacitors to be inserted in a half-arm is obtained by multiplying the value of the modulation signal associated with the half-arm by the number of sub-modules of the half-arm.

[0036] The voltage converter preferably comprises a balancing module configured to receive said modulation signal setpoints and to apply a balancing algorithm to them. Such an algorithm is also called BCA, for "Balancing Control Algorithm" in English.

[0037] The present invention provides that the setpoints of the modulation signals depend on the voltages across the capacitors of the sub-modules of the half-arms of the converter, which can be measured or determined by calculation. In other words, the modulation device receives as input said voltages across the capacitors of the sub-modules of the half-arms. The modulation therefore takes these voltages into account, so that compensation for the oscillations of the voltages across the equivalent capacitors of the half-arms is achieved. The converter according to the invention makes it possible to achieve compensated modulation. One advantage is to limit the circulation of internal common-mode currents within the converter, thus reducing power losses within the voltage converter.An additional advantage is to avoid the use of a circulating current suppression controller (CCSC) used in the prior art to suppress internal common mode current harmonics.

[0038] The converter according to the invention also makes it possible to decouple the dynamics on the AC and DC sides.

[0039] Preferably, the common mode homopolar component of modulation voltage is equal to: where v^ a , v nb and v^ c are the common mode components of modulation voltage associated with each of the arms a, b and c of the voltage converter.

[0040] Unlike converters implementing so-called energy-based (EBC) and non-energy-based (NEBC) control techniques, the converter according to the invention provides for determining the modulation voltage setpoints applied to the half-arms so that they depend on a modulation voltage common mode homopolar component setpoint which depends on said voltages across the capacitors of the sub-modules of the upper and lower half-arms.

[0041] According to the invention, said common mode homopolar component setpoint of modulation voltage is not fixed at a value of v dc / 2, where v dc is the voltage of the DC power supply network. As a result, the internal DC voltage of the converter takes an initial value which is a function of the said voltages at the terminals of the capacitors of the sub-modules.

[0042] As a result, unlike the so-called non-energy-based control method, the modulation signal does not have an offset of 1 Zz, so that this control does not provide for systematically inserting half of the capacitors of the sub-modules of each arm. One interest is in particular to reduce the oscillations associated with the voltages at the terminals of the equivalent capacitors of the half-arms.

[0043] Moreover, unlike the so-called energy-based control method, with this invention, each arm does not depend on a "feed forward" signal v dc at its terminals. Also, the voltage converter absorbs, and therefore does not reject, the disturbances appearing on the DC network and it can therefore contribute to stabilizing the DC network.

[0044] The AC control module manages the power exchanged between the voltage converter and the AC network connected to it. The DC control module balances the energies of the half-arms of the voltage converter.

[0045] Preferably, the voltages across the capacitors of the sub-modules of the upper and lower half-arms are determined, for example by calculation or by measurement, in real time.

[0046] Preferably, said common mode homopolar component setpoint of modulation voltage depends solely on said voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms. In other words, it advantageously does not depend on any other quantity which would be associated with the voltage converter, the DC network or the AC network.

[0047] Advantageously, each half-arm can be modeled by a modeled voltage source associated in parallel with an equivalent capacitor having an equivalent voltage at its terminals, said modulation member being configured to determine said modulation signal setpoints as a function of the equivalent voltages considered at the terminals of the equivalent capacitors of said upper and lower half-arms.

[0048] Preferably, said common mode homopolar component setpoint of modulation voltage is determined as a function of the sums of the voltages across the capacitors of the sub-modules of each of the upper half-arms and as a function of the sums of the voltages across the capacitors of the sub-modules of each of the lower half-arms.

[0049] More preferably, said common mode homopolar component setpoint of modulation voltage is a function of the average of said sums of the voltages across the capacitors of the sub-modules of each of the upper and lower half-arms. An advantage is that the voltage converter presents said average of the sums of the voltages across the capacitors as internal DC voltage.

[0050] Advantageously, said common mode homopolar component setpoint of modulation voltage v^*0 is equal to: where a, b and c are indices designating each of the arms of the voltage converter, are the sums of the voltages across the capacitors of the sub-modules of each of the upper half-arms and v c l jare the sums of the voltages across the capacitors of the sub-modules of each of the lower half-arms. In other words, initially, the converter sets its internal DC voltage to the average value, divided by two, of the sum of the voltages across the converters. Consequently, the same common-mode voltage equal to said common-mode zero-sequence component of modulation voltage v^*0 is applied to each half-arm. This results in an absence of voltage difference between the arms, resulting in an absence of circulation of internal currents in the voltage converter.

[0051] Furthermore, this formulation of the said common mode homopolar component instruction of modulation voltage v^o makes it possible to precisely reproduce the behavior of a capacitor whose size is equal to the size of an equivalent physical capacitor.

[0052] Preferably, said AC control module is configured to control the alternating currents between the arms of the voltage converter and the AC network to which the voltage converter is connected.

[0053] Advantageously, said continuous control module is configured to control the internal common mode currents of the converter. These internal common mode currents are the currents flowing within the half-arms of each arm.

[0054] Advantageously, the AC control module is configured to express said modulation voltage AC component setpoint in a first reference frame, and the control unit further comprises a reference frame changing member configured to deliver a modulation voltage AC component setpoint expressed in a reference frame abc in which it has three components, each being associated with one of the arms of the voltage converter, from said modulation voltage AC component setpoint expressed in the first reference frame provided by the AC control module. It is understood that said modulation voltage setpoints applied to the upper and lower half-arms are a function of said modulation voltage AC component setpoint expressed in the reference frame abc.

[0055] The reference change member is advantageously connected to the output of the AC control module. Preferably, the reference change member receives as input the AC modulation voltage component setpoint expressed in the first reference, where it advantageously has two components.

[0056] Preferably, the first reference is a two-phase reference. In other words, said alternating voltage modulation component setpoint delivered by the alternating control module advantageously has two components.

[0057] It is understood that the alternating voltage component instruction of modulation expressed in the reference abc is an expression of said alternating voltage component instruction of modulation in a three-phase reference, where each of the components is associated with one of the arms of the voltage converter. In other words, the indices a, b and c each refer to one of the three arms of the voltage converter.

[0058] Said alternating modulation voltage component setpoint expressed in the abc reference frame is then used to determine the modulation voltage setpoints applied to the upper and lower half-arms.

[0059] Advantageously, said first reference frame is a rotating reference frame dq, the reference frame changing member being configured to express said modulation voltage alternating component setpoint in said reference frame abc by applying an inverse Park transformation to said modulation voltage alternating component setpoint expressed in the reference frame dq.

[0060] Advantageously, the control unit further comprises a reference frame change module configured to deliver a modulation voltage common mode component setpoint expressed in a reference frame abc where it has three components, each being associated with one of the arms of the voltage converter, from said modulation voltage common mode component setpoint delivered by the continuous control module and from said modulation voltage common mode homopolar component setpoint. In other words, the reference frame change module receives as input the modulation voltage common mode component setpoint delivered by the continuous control module as well as said modulation voltage common mode homopolar component setpoint.

[0061] It is understood that said modulation voltage setpoints applied to the upper and lower half-arms are a function of said modulation voltage common mode component setpoint expressed in the abc reference frame.

[0062] Said modulation voltage common mode component setpoint delivered by the control module is preferably expressed in an initial reference frame. Preferably, said initial reference frame is a two-phase reference frame. In other words, said modulation voltage common mode component setpoint delivered by the continuous control module advantageously has two components.

[0063] Preferably, said modulation voltage common mode component setpoint delivered by the continuous control module is expressed in a reference frame a(3, the reference frame change module being configured to express said modulation voltage common mode component in the reference frame abc by applying an inverse Clarke transformation to said modulation voltage common mode component setpoint delivered by the continuous control module and to said modulation voltage common mode homopolar component setpoint.

[0064] Preferably, the control unit further comprises a variable change module configured to carry out a variable change in order to determine said modulation voltage setpoints applied to the upper and lower half-arms from said modulation voltage common mode component setpoint expressed in the abc reference frame and said modulation voltage alternating component setpoint expressed in the abc reference frame.

[0065] The variable change module receives as input said modulation voltage common mode component setpoint expressed in the abc reference frame and said modulation voltage alternating component setpoint expressed in the abc reference frame. Preferably, the variable change module is connected as input to said reference frame change module and to said reference frame change member. Preferably, the variable change module is also connected as output to said modulation member. The variable change module delivers modulation voltage setpoints applied to the upper and lower half-arms which can be used directly by the modulation member to generate the modulation signals.

[0066] Advantageously, said AC control module is configured to determine said AC modulation voltage component setpoint from a current setpoint of the AC power supply network.

[0067] Preferably, said current setpoint of the alternating current supply network is delivered by an external control loop.

[0068] Said current setpoint of the alternating current supply network is preferably expressed in a rotating reference dq. Preferably, said current setpoint of the alternating current supply network comprises a first component delivered by a module for controlling the active power of the voltage converter and a second component delivered by a module for controlling the reactive power of the voltage converter.

[0069] Preferably, said continuous control module is configured to determine said modulation voltage common mode component setpoint from a common mode current setpoint.

[0070] Preferably, said common mode current setpoint is delivered by an energy control loop. Preferably, said common mode current setpoint is expressed in a reference frame a(3.

[0071] Preferably, the modulation member is configured to determine said modulation signal setpoints for controlling the control members of the sub-modules of said upper and lower half-arms, according to where a, b and c are indices designating each of the arms of the voltage converter, u and I are indices associated respectively with the upper and lower half-arms, v^ a l bcare the sums of the voltages across the capacitors of each of the upper and lower half-arms and v^ l * bc are the modulation voltage instructions applied to the upper and lower half-arms.

[0072] In other words, each voltage v^bc corresponds to the sum of the voltages across the capacitors of the sub-modules of the given half-arm. The voltage v^ bc also corresponds to the equivalent voltage of the equivalent capacitor of the half-arm, according to the model in which the half-arm is modeled by a voltage source to which an equivalent capacitor is connected in parallel.

[0073] Here again, we see that the modulation signal setpoints are a function of the voltages across the capacitors of the converter half-arm sub-modules. It is useful to carry out a control to compensate for the oscillations in the voltages of the equivalent capacitors of the half-arms.

[0074] Preferably, the voltage converter is configured not to control the currents of the DC power supply network. The currents of the DC power supply network evolve freely and form an additional degree of freedom.

[0075] Preferably, said control unit is devoid of a module for controlling the currents of the continuous electrical supply network. It is understood that not all the state variables of the converter are then controlled.

[0076] Consequently, unlike so-called energy-based control methods, the modulation voltage common mode homopolar component setpoint does not depend on a voltage setpoint delivered by such a DC power supply network current control module.

[0077] One advantage is to leave the control of the DC power supply network currents open loop, so that the converter does not explicitly control these currents. Therefore, the voltage converter naturally exchanges its internal energy with the DC power supply network to which the voltage converter is connected. More precisely, the voltage converter absorbs energy from the DC network or delivers energy to said DC network depending on the evolution of the latter's voltage. For example, if the internal DC voltage of the voltage converter, which advantageously corresponds to the average of the voltages of the equivalent capacitors of the half-arms, is higher than the voltage of the power supply network, the voltage converter will supply energy to said DC network.Conversely, if the DC network voltage is higher than the internal DC voltage of the converter, the latter will absorb energy from the DC network. One advantage is that the internal energy of the converter is used to intrinsically and naturally contribute to countering disturbances that may occur on the DC power supply network to which the voltage converter is connected. The DC network is therefore stabilized.

[0078] Preferably, the control unit is devoid of a module for controlling the total energy of the voltage converter. Preferably, the control unit is devoid of a module for balancing the total energy of the voltage converter.

[0079] Preferably, said control unit further comprises an energy balance control module configured to ensure a balanced distribution of the internal energy of the converter between the arms of the voltage converter, as well as between the upper and lower half-arms of the voltage converter. The energy balance control module is configured to explicitly regulate the distribution of the internal energy of the converter, which is not possible with the non-energy-based control method of the prior art.

[0080] Said energy balance control module is advantageously connected to said continuous control module. Preferably, said energy balance control module is configured to provide current setpoints to said continuous control module. More preferably, said energy balance control module is configured to provide a common mode current setpoint to the continuous control module.

[0081] The invention further relates to a control unit of a multi-level modular voltage converter as described previously, comprising at least: an AC control module configured to deliver a modulation voltage AC component setpoint; a DC control module configured to deliver a modulation voltage common mode component setpoint;a modulation member configured to deliver modulation signal setpoints, for controlling the control members of the sub-modules of said upper and lower half-arms, from voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms and from modulation voltage setpoints applied to the upper and lower half-arms which are a function of said modulation voltage alternating component setpoint delivered by the alternating control module, of said modulation voltage common mode component setpoint delivered by the continuous control module, and of a modulation voltage common mode homopolar component setpoint which depends on said voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms.;

[0082] The invention also relates to a method for controlling a multi-level modular voltage converter, making it possible to convert an alternating voltage into a direct voltage and vice versa, the voltage converter comprising a so-called direct part intended to be connected to a direct electrical supply network and a so-called alternating part intended to be connected to an alternating electrical supply network, the converter comprising three arms, each arm comprising an upper half-arm and a lower half-arm, each half-arm comprising a chain of sub-modules individually controllable by a control member specific to each sub-module and each sub-module comprising a capacitor,the control member of each sub-module being able to take at least a first state in which the capacitor is inserted in the corresponding chain of sub-modules and a second state in which the capacitor is not inserted in said chain of sub-modules, each half-arm having a modulation voltage which is a function of the number of capacitors inserted in the corresponding chain of sub-modules, the method comprising the steps according to which: a modulation voltage alternating component setpoint is determined; a modulation voltage common mode component setpoint is determined; the voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms are determined; modulation signal setpoints are delivered, for the control of the control members of the sub-modules of said upper and lower half-arms,from said voltages across the capacitors of the sub-modules of the upper and lower half-arms and from modulation voltage setpoints applied to the upper and lower half-arms which are a function of said determined modulation voltage alternating component setpoint, of said determined modulation voltage common mode component setpoint, and of a modulation voltage common mode homopolar component setpoint which depends on said voltages across the capacitors of the sub-modules of the upper and lower half-arms.,

[0083] In a non-limiting manner, the determination of the voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms can be carried out by measuring or by calculating these voltages.

[0084] Preferably, said control method does not comprise a step of controlling the currents of the DC power supply network. Preferably, the currents of the DC power supply network are not controlled.

[0085] Brief description of the drawings

[0086] The invention will be better understood on reading the following description of an embodiment of the invention given by way of non-limiting example, with reference to the appended drawings, in which:

[0087] [Fig. 1]Figure 1 shows a voltage converter according to the invention;

[0088] [Fig. 2]Figure 2 shows a control unit of the voltage converter of Figure 1;

[0089] [Fig.3A to 3C] Figure 3A to 3C shows the evolution of the voltage of a DC network connecting two voltage converters in a first simulation where a control method called non-energy based according to the prior art is implemented (Figure 3A), in a second simulation where a control method called energy based according to the prior art is implemented (Figure 3B), and in a third simulation where the voltage converters are provided with a control unit according to the invention (Figure 3C);

[0090] [Fig. 4A to 4C] Figure 4A to 4C shows the evolution of the internal energy of the two voltage converters for said first simulation where a control method called non-energy based is implemented (Figure 4A), for said second simulation where a control method called energy based is implemented (Figure 4B), and for said third simulation where the voltage converters are equipped with a control unit according to the invention (Figure 4C); and

[0091] [Fig. 5A to 5C] Figure 5A to 5C shows the evolution of the voltage of the equivalent capacitors of the half-arms of the voltage converter for said first simulation where a control method called non-energy based is implemented (Figure 5A), for said second simulation where a control method called energy based is implemented (Figure 5B), and for said third simulation where the voltage converters are equipped with a control unit according to the invention (Figure 5C).

[0092] Description of the embodiments

[0093] The invention relates to a modular multi-level voltage converter for converting an alternating voltage into a direct voltage and vice versa, as well as to a method for controlling such a converter.

[0094] Figure 1 illustrates a modular multilevel voltage converter 10 according to the invention, which is here a voltage source converter (VSC). This voltage converter 10 comprises a first DC terminal 12 and a second DC terminal 14, configured to be electrically connected to a DC power supply network, also called a DC network. The DC network has a DC voltage v dc at its terminals. The DC terminals 12, 14 constitute a DC part of the voltage converter. The voltage converter 10 further comprises three AC terminals 15, 16, 17 configured to be connected to an AC power supply network, also called an AC network. The AC terminals 15, 16, 17 constitute an AC part of the voltage converter.

[0095] In a known manner, the voltage converter 10 comprises a first arm a, a second arm b and a third arm c. The three arms a, b and c are connected in parallel to each other. Subsequently, the letters “a”, “b” and “c” will be associated with quantities, in particular voltage and current, as references to said arms.

[0096] Each arm a,b,c comprises an upper half-arm and a lower half-arm, indicated respectively by the subscripts "u" for upper and "I" for lower. Each half-arm connects a continuous terminal 12,14 to an alternating terminal 15,16,17. It should be noted that the terms "arm" and "half-arm" are translated into English as "leg" and "arm" respectively. Each half-arm comprises a chain of SM^- submodules connected in series in the half-arm, "x" being the number of the submodule among the submodules in series in the half-arm and "j" indicating the arm of the submodule.

[0097] Here, only three submodules SM' have been represented per half-arm. In practice, each lower or upper half-arm has a number N of submodules, ranging from a few tens to a few hundreds.

[0098] The constitution of a sub-module SM' is also illustrated in Figure 1. The sub-modules can be individually controlled by a control member T specific to each sub-module. The control member here consists of a first switching element T1 such as an insulated gate bipolar transistor (IGBT) connected between the input and output terminals of the sub-module, and a second switching element T2. The sub-modules SM^ further comprise a capacitor C connected in series with the first switching element T1. The capacitor C and the first switching element T1 are connected in parallel with the second switching element T2. Each sub-module SM' has a voltage v jx at its terminals while the capacitor C of a sub-module presents a voltage to its limits.

[0099] The sub-modules are controlled according to a sequence chosen to progressively vary the number of capacitors C inserted in each half-arm of the converter 10 so as to provide several voltage levels.

[0100] The control member T of each sub-module can take a first state in which the capacitor C is inserted in the corresponding chain of sub-modules and a second state in which the capacitor C is not inserted in said chain of sub-modules. In the first state the first and second switching elements T1, T2 are configured so as to connect the capacitor C in series in the chain of sub-modules. In a second state, the first and second switching elements T1, T2 are configured so as to short-circuit the capacitor C.

[0101] As illustrated in the lower right corner of Figure 1, each half-arm has a modulation voltage across its terminals which is a function of the number of capacitors C inserted in the chain of sub-modules of said half-arm. The sub-modules SM' are controlled according to a sequence chosen to progressively vary the number of capacitors C inserted in each half-arm of the converter 10 so as to adjust the modulation voltage Said modulation voltage can be decomposed into an alternating modulation voltage component and a common mode modulation voltage component and possibly a common mode zero sequence modulation voltage component.

[0102] Each half-arm is crossed by a current t x l .

[0103] As also illustrated in Figure 1, it is known that each half-arm, having a modulation voltage at its terminals, can be modeled by a modeled voltage source, having the same modulation voltage v^ zat its terminals, associated in parallel with an equivalent capacitor C ar m presenting at its terminals an equivalent voltage v^j and being crossed by a current This equivalent voltage v^ is equal to the sum of the voltages v“ z x across the capacitors. The modulation voltage v^ z is a function of the number of capacitors inserted in the half-arm, which is translated by a modulation index m“ ,z associated with the half arm.

[0104] The control of the control members T of the sub-modules of an upper or lower half-arm, and therefore the insertion of the capacitors C in such a half-arm, is carried out by generating a modulation signal

[0105] The voltage converter 10 further comprises a control unit 30 of the voltage converter. Such a control unit 30 of the voltage converter is illustrated in FIG. 2.

[0106] As illustrated in Figure 2, the control unit 30 of the voltage converter 10 according to the invention firstly comprises an external control loop 32 for controlling the powers of the converter. This external control loop 32 comprises a module for controlling the active power 34 of the converter and a module for controlling the reactive power 36 of the converter.

[0107] Together, said active and reactive power control modules 34, 36 are configured to deliver a current setpoint of the alternating electrical power supply network i * , expressed in a rotating reference frame dq. More precisely, the active power control module 34 is configured to deliver a component “d” of the current setpoint of the alternating electrical power supply network, denoted i * • The reactive power control module 36 is configured to deliver a component “q” of the current setpoint of the alternating electrical power supply network, denoted i“ c *.

[0108] The control unit 30 further comprises an alternating current control loop 38 comprising an alternating current control module 40, connected to said active and reactive power control modules 34, 36. The alternating current control module 40 receives as input said current setpoint from the alternating current supply network i * , tc * delivered by the latter. The AC control module 40 is configured to control and regulate the AC currents of the converter. According to the invention, the AC control module 40 is configured to deliver a modulation voltage AC component setpoint. This setpoint is here expressed in a first rotating two-phase reference frame dq, in which it has a first component iz^ c d and a second component vg*.

[0109] The alternating current control loop 38 further comprises a reference change member 42 connected to the alternating control module. Said reference change member 42 receives as input the two components v^ d And of said modulation voltage alternating component setpoint. The reference change member 42 is configured to apply an inverse Park transformation to said modulation voltage alternating component setpoint. It then delivers a modulation voltage alternating component setpoint v^ abc expressed in a three-phase reference frame abc. In this three-phase reference frame, said alternating component setpoint of modulation voltage expressed in the reference frame abc v^ abc has three components v *, v br v * r each being associated with one of the arms a, b, c of the voltage converter. In figure 2, these three components are symbolized by an arrow crossed by three lines.

[0110] The external control loop 32 and the alternating current control loop 38 provide control of the alternating currents and voltages of the voltage converter 10.

[0111] Still in Figure 2, it can also be seen that the control unit 30 further comprises an energy control loop 44. This energy control loop comprises an energy balance control module 45 of the voltage converter 10. This energy balance control module 45 is configured to ensure a balanced distribution of the internal energy of the converter, stored in the capacitors C of the sub-modules, between the upper and lower half-arms of the voltage converter. Unlike the control units according to the prior art, implementing a non-energy-based control method, this energy balance control module 45 is configured to explicitly regulate the distribution of the internal energy of the voltage converter 10.

[0112] The energy balance control module 45 here comprises a so-called vertical control member 46, configured to ensure a balanced distribution of the internal energy of the converter between the sub-modules of the same arm, and a so-called horizontal control member 48, configured to ensure a balanced distribution of the internal energy between the arms a, b, c.

[0113] Said energy balance control module 45 delivers a common mode current setpoint i^, which is here expressed in a fixed two-phase reference frame a, where it has a first component and a second component t .

[0114] The control unit 30 further comprises a DC current control loop 50. This DC current control loop 50 comprises a DC control module 52 configured to control the internal common mode currents of the converter. These internal common mode currents are DC currents. The DC control module 52 is connected to said energy balance control module 45 and therefore receives said common mode current setpoint as input. The continuous control module 52 delivers a common mode component setpoint of modulation voltage v x ma p . This modulation voltage common mode component instruction is expressed in a fixed two-phase initial reference frame a, so that it includes a first component v m x a and a second component v x mp .

[0115] The DC control loop 50 further comprises a reference change module 54. This reference change module 54 is connected to the DC control module 52. The reference change module 54 receives as input said common mode component setpoint of modulation voltage v x ma p delivered by the continuous control module 52, as well as a common mode homopolar component of modulation voltage v x m0 .

[0116] The reference change module 54 is configured to deliver a modulation voltage common mode component setpoint v mabc expressed in the abc frame, presenting three components v m x ai Vmb' v mc, each being associated with one of the arms a, b, c of the voltage converter. In Figure 2, these three components are symbolized by an arrow crossed by three lines.

[0117] According to the invention, said common mode homopolar component of modulation voltage v x m0 is a function of tensions at the terminals of the capacitors C of the sub-modules SM" of the upper half-arms and as a function of voltages v C l jx at the terminals of the capacitors C of the SM sub-modules x l j lower half-arms. In a non-limiting manner, these tensions and v C l jx are obtained here by measurement.

[0118] Unlike known methods of controlling a converter known as energy-based and known methods of controlling a converter known as non-energy-based, the present invention provides that said homopolar component does not depend on the direct voltage v dc at the terminals of the DC power supply network. Here it depends only on the voltages v“ yx and v C ljx at the terminals of the capacitors C of the SM sub-modules xj lower and upper half arms.

[0119] More precisely, said common mode homopolar component of modulation voltage v x m0 is determined as a function of the sum v“ a measured voltages v“ ax at the terminals of the capacitors C of the upper half-arm of the first arm a, of the sum measured voltages Vc bx at the terminals of the capacitors C of the upper half-arm of the second arm b, of the sum v“ c measured voltages v“ cx at the terminals of the capacitors C of the upper half-arm of the third arm c, of the sum v l Ca measured voltages v l Cax at the terminals of the capacitors C of the lower half-arm of the first arm a, of the sum v l cb measured voltages v l cbxat the terminals of the capacitors C of the lower half-arm of the second arm b, of the sum v l Cc measured voltages v l Ccx across the capacitors C of the lower half-arm of the third arm c.

[0120] It is further noted that the control unit 30 does not have a module for controlling the currents of the DC power supply network. It is therefore intended not to control these currents of the DC power supply network, which evolve freely. In other words, the control of the currents of the DC power supply network is maintained in open loop, so that the voltage converter does not explicitly control these currents. Therefore, the voltage converter naturally exchanges its internal energy with the DC power supply network to which the voltage converter is connected. The internal energy of the voltage converter is thus used to contribute intrinsically and naturally to countering disturbances that may occur on the DC power supply network.

[0121] In Figure 2, the three sums of the voltages across the capacitors C of the three upper half-arms are noted v“ai)C and are represented by an arrow crossed by three lines. Similarly, the said three sums of the voltages across the capacitors C of the three lower half-arms are noted v l Cabc and are represented by an arrow crossed out with three lines.

[0122] Each of the said sums v^ bc voltages across the capacitors C of the sub-modules of the upper and lower half-arms also correspond to the equivalent voltage of the equivalent capacitor Carm modeling the corresponding half-arm.

[0123] In this non-limiting example, said common mode homopolar component of modulation voltage v z m0 is determined as a function of the average of said sums of the voltages at the terminals of the capacitors C of the sub-modules of the upper and lower half-arms, according to the expression: where V are the sums of the voltages across the capacitors of the sub-modules of each of the upper half-arms and v l Cj are the sums of the voltages across the capacitors of the sub-modules of each of the lower half-arms.

[0124] The energy control loop 44 and the direct current control loop 50 ensure the control of the direct currents and voltages of the voltage converter 10.

[0125] The control unit 30 also comprises a variable change module 56 to which the reference change member 42 and the reference change module 54 are connected. The variable change module 56 receives as input said alternating modulation voltage component instruction expressed in the reference abc v^bc et said common mode component instruction of modulation voltage expressed in the reference abc v z mabc. The variable change module 56 is configured to deliver three modulation voltage setpoints applied to the upper half-arms v^ abc as well as three modulation voltage instructions applied to the lower half-arms v' mabc .

[0126] The control unit 30 further comprises a modulation member 58 connected to said variable change module 56. The modulation member is configured to determine modulation signals m“ bc for each of the three upper half-arms and modulation signals m l * bc for each of the three lower half-arms.

[0127] More precisely, according to the invention, for each half-arm, the modulation member 58 is configured to determine a modulation signal m“' bcl for controlling the control organs of the sub-modules of said half-arm, from the voltages at the terminals of the capacitors C of each of the sub-modules of this half-arm and from the modulation voltage setpoint applied to said half-arm v^ l * bc . Consequently, the modulation therefore takes into account these voltages at the terminals of the capacitors of the sub-modules, so that compensation for the oscillations of the voltages of the equivalent capacitors of the half-arms is carried out during the control, which makes it possible to limit or even eliminate the circulation of internal currents, thus reducing losses. For each half-arm, the modulation member 58 is here configured to calculate the corresponding modulation signal according to the equation: where, we recall, v^ a l hc is the sum of the tensions v^ x , measured here, at the terminals of the capacitors C of the sub-modules of the corresponding half-arm.

[0128] The voltage converter may also include a balancing module configured to receive the modulation signal setpoints and to apply a balancing algorithm to them. Such an algorithm is also called BCA, for "Balancing Control Algorithm" in English.

[0129] We will now detail the results of three simulations to highlight the advantages of the voltage converter equipped with a control unit according to the invention. For these three simulations, a DC power supply network is connected between two multi-level modular voltage converters MMC1 and MMC2. The first voltage converter is subjected to changes in active power setpoints while the second voltage converter is responsible for regulating the voltage of the DC power supply network.

[0130] For the first simulation, the two voltage converters MMC1, MMC2 comprise a control unit for controlling them according to a prior art method called non-energy based (NEBC). For the second simulation, the two voltage converters comprise a control unit for controlling them according to a prior art method called energy based (EBC). Finally, according to the third simulation, the two voltage converters are voltage converters according to the invention, comprising a control unit according to the invention.

[0131] In [Fig. 3A to 3C], the evolution of the voltage v is shown dcof the DC network for these three simulations. Figure 4A to 4C shows the evolution of the internal energy of the two voltage converters for the three simulations. Figure 5A to 5C shows the evolution of the voltage of the equivalent capacitors of the half-arms of the voltage converter for the three simulations.

[0132] Referring to [Fig. 3A to 3C], we see that at t = 0.5 seconds, the active power supplied by the first voltage converter to the DC network is suddenly increased. This results in an increase in the energy of the DC network, resulting, for each of the simulations, in an increase in the voltage v dc of the DC network. Conversely, at time t=ls, the network voltage v dc DC suddenly drops, due to a decrease in the active power supplied by the first voltage converter to the DC network.

[0133] In Figure 3A, corresponding to the so-called non-energy-based control, we observe that the increase in voltage v dc of the DC network is accompanied by distortions. These distortions are due to the large internal common mode currents circulating within the converter controlled using this NEBC method. Indeed, this method only controls four of the eleven state variables of the MMC converter. This generates large voltage differences between the arms of the converter, which are the origin of these internal common mode currents.

[0134] This increase in voltage v dc of the DC network remains contained. Similarly, as can be seen in Figure 3C, in the case of the third simulation implementing the voltage converters according to the invention, the voltage v dc of the DC network increases at time t=0.5 seconds, but remains contained.

[0135] On the contrary, in Figure 3B corresponding to a so-called energy-based control, the voltage v dc of the DC network suddenly increases significantly and excessively.

[0136] To the extent that the control unit 30 of the voltage converters 10 according to the invention does not comprise a module for controlling the currents of the direct current power supply network, these voltage converters naturally exchange their internal energy with the direct current power supply network and contribute to its stability.

[0137] This can be observed in Figure-4C showing the evolution of the internal energy W mmcvoltage converters 10 according to the invention. It can be seen that the voltage converter 10 absorbs energy from the DC network or delivers energy to said DC network depending on the evolution of the voltage of the latter. For example, if the internal DC voltage of the voltage converter, i.e. the average of the voltages of the equivalent capacitors of the half-arms, is higher than the voltage of the power supply network, the voltage converter will supply energy to said DC network. Conversely, if the voltage of the DC network is higher than the voltage of the converter, the latter will absorb energy from said DC network. The behavior here is similar to that observed in figure- 4A for a so-called non-energy-based control.

[0138] On the contrary, we see in Figure 4B that with the EBC control method, the internal energy of the converter does not change when the DC network voltage changes. This is due to the fact that this method plans to control the eleven state variables of the voltage converter, without considering the state of the DC power supply network to which it is connected. Therefore, the disturbances are rejected and the internal energy of the voltage converter is not used to help limit or counter these disturbances on the DC network.

[0139] Figures 5A, 5B and 5C illustrate the evolution of the voltages v a l bcequivalent capacitors of the half-arms of the voltage converters respectively with a so-called non-energy-based control according to the prior art, with a so-called energy-based control according to the prior art and finally with a control carried out by means of the control unit of the voltage converter according to the invention.

[0140] In Figure 5C, corresponding to the control carried out by means of the control unit according to the invention, the average energy of the equivalent capacitors of the six half-arms of the voltage converter increases at t=0.5s, in order to absorb part of the energy of the DC network, so as to counter the increase in voltage v dc, and therefore the disturbance appearing on said DC network. Similarly at t=ls, the average energy of the equivalent capacitors of the six half-arms of the voltage converter decreases, in order to inject energy into the DC network and thus counter the voltage drop of the latter. The voltage converter according to the invention therefore naturally contributes to stabilizing the DC network. The behavior at t=0.5s and at t=ls is similar to that of the voltage converter controlled according to a method known as based on an N EBC technique of the prior art and which is illustrated in figure-5A.

[0141] In Figure-5A illustrating the N EBC control, we see that throughout the simulation, the internal energy of the voltage converter is not controlled, but that it evolves only according to the DC network voltage. The six equivalent capacitors of the six half-arms have the same average energy value, which reflects the impossibility of individually controlling the energy of each half-arm.

[0142] Furthermore, at t=1.5s, as can be seen in Figure 5C, the equivalent capacitors of the six half-arms of the voltage converter according to the invention have different average internal energy values, which reflects the possibility for the operator to control the average energy of each half-arm individually. This controllability is also offered by the so-called energy-based control, as illustrated in Figure 5B, where the behavior at t=1.5s is identical.

[0143] In Figure-5B, we see that with an EBC type control, the average energy of the equivalent capacitors of the six half-arms remains unchanged at t=0.5s and at t=ls, despite the disturbances appearing on the DC network. This invariance of the average energy of the six half-arms throughout the simulation, despite the disturbance occurring on the DC network, demonstrates the possibility of explicitly regulating the internal energy of the six half-arms.

[0144] The voltage converter according to the invention therefore makes it possible to counter disturbances occurring on the DC network, as is the case for NEBC type control and also offers the possibility of individually controlling the average energy of the equivalent capacitor of each half-arm, as is the case for so-called EBC control.

Claims

Claims 1. Multi-level modular voltage converter (10), for converting an alternating voltage into a direct voltage and vice versa, comprising a so-called direct part (12, 14) intended to be connected to a direct electrical supply network and a so-called alternating part (15, 16, 17) intended to be connected to an alternating electrical supply network, the converter comprising three arms (a, b, c), each arm comprising an upper half-arm and a lower half-arm, each half-arm comprising a chain of sub-modules (SM^') individually controllable by a control member (T) specific to each sub-module and each sub-module comprising a capacitor (C), the control member of each sub-module being able to take at least a first state in which the capacitor is inserted into the corresponding chain of sub-modules and a second state in which the capacitor is not inserted into said chain of sub-modules,each half-arm having a modulation voltage which is a function of the number of capacitors inserted in the corresponding chain of sub-modules, the converter further comprising a control unit (30) of the converter comprising at least: an AC control module (40) configured to deliver a modulation voltage AC component setpoint; a DC control module (52) configured to deliver a modulation voltage common mode component setpoint; a modulation member (58) configured to deliver modulation signal setpoints, for controlling the control members of the sub-modules of said upper and lower half-arms,from voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms and from modulation voltage setpoints applied to the upper and lower half-arms which are a function of said modulation voltage AC component setpoint delivered by the AC control module, of said modulation voltage common mode component setpoint delivered by, the continuous control module, and a common mode homopolar component setpoint of modulation voltage which depends on said voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms.

2. Voltage converter according to claim 1, wherein said modulation voltage common mode homopolar component setpoint is determined as a function of the sums of the voltages across the capacitors of the sub-modules of each of the upper half-arms and according to the sums of the voltages across the capacitors of the sub-modules of each of the lower half-arms.

3. Voltage converter according to claim 2, wherein said common mode homopolar component setpoint of modulation voltage is a function of the average of said sums of the voltages across the capacitors of the sub-modules of each of the upper and lower half-arms.

4. Voltage converter according to claim 3, wherein said modulation voltage common mode homopolar component setpoint is equal to: where a, b and c are indices designating each of the arms of the voltage converter, are the sums of the voltages across the capacitors of the sub-modules of each of the upper half-arms and v c l jare the sums of the voltages across the capacitors of the sub-modules of each of the lower half-arms.

5. Voltage converter according to any one of claims 1 to 4, wherein said AC control module (40) is configured to control the AC currents of the converter.

6. A voltage converter according to any one of claims 1 to 5, wherein said continuous control module (52) is configured to control the internal common mode currents of the converter.

7. Voltage converter according to any one of claims 1 to 6, in which the AC control module (40) is configured to express said AC modulation voltage component setpoint in a first reference frame, and in which the control unit further comprises a reference frame changing member (42) configured to deliver an AC modulation voltage component setpoint expressed in a reference frame abc in which it has three components, each being associated with one of the arms of the voltage converter, from said AC modulation voltage component setpoint expressed in the first reference frame provided by the AC control module.

8. Converter according to claim 7, in which said first reference frame is a rotating reference frame dq, the reference frame changing member (42) being configured to express said modulation voltage alternating component setpoint in said reference frame abc by applying an inverse Park transformation to said modulation voltage alternating component setpoint expressed in the reference frame dq.

9. Voltage converter according to any one of claims 1 to 8, wherein the control unit (30) further comprises a reference frame change module (54) configured to deliver a modulation voltage common mode component setpoint expressed in a reference frame abc where it has three components, each being associated with one of the arms of the voltage converter, from said modulation voltage common mode component setpoint delivered by the continuous control module and from said modulation voltage common mode homopolar component setpoint.

10. Voltage converter according to claim 9, wherein said modulation voltage common mode component setpoint delivered by the continuous control module is expressed in a reference frame a, the reference frame change module (54) being configured to express said modulation voltage common mode component in said reference frame abc by applying a transformation Clarke inverse to said modulation voltage common mode component setpoint delivered by the continuous control module and to said modulation voltage common mode zero sequence component setpoint.

11. Voltage converter according to claim 7 or 8 in combination with claim 9 or 10, wherein the control unit (30) further comprises a variable change module (56) configured to perform a variable change in order to determine said modulation voltage setpoints applied to the upper and lower half-arms from said modulation voltage common mode component setpoint expressed in the abc reference frame and said modulation voltage alternating component setpoint expressed in the abc reference frame.

12. Voltage converter according to any one of claims 1 to 11, wherein said AC control module (40) is configured to determine said modulation voltage AC component setpoint from a current setpoint of the AC power supply network.

13. Voltage converter according to any one of claims 1 to 12, wherein said continuous control module (52) is configured to determine said modulation voltage common mode component setpoint from a common mode current setpoint.

14. Voltage converter according to any one of claims 1 to 13, wherein the modulation member (58) is configured to determine said modulation signal setpoints. for the control of the control organs of the sub-modules of said upper and lower half-arms, according to the equation: where a, b and c are indices designating each of the arms of the voltage converter, u and I are indices associated respectively with the upper and lower half-arms, v^ a l bc are the sums of the voltages across the capacitors of each of the upper and lower half arms and v^l * bc are the modulation voltage instructions applied to the upper and lower half-arms.

15. Voltage converter according to any one of claims 1 to 14, wherein said control unit (30) is devoid of a module for controlling the currents of the continuous electrical supply network.

16. A voltage converter according to any one of claims 1 to 15, wherein said control unit (30) further comprises an energy balance control module (45) configured to ensure a balanced distribution of the internal energy of the converter between the arms of the voltage converter, as well as between the upper and lower half-arms of the voltage converter.

17. Control unit (30) of a multilevel modular voltage converter (10) according to any one of claims 1 to 16, comprising at least: an AC control module (40) configured to deliver a modulation voltage AC component setpoint; a DC control module (52) configured to deliver a modulation voltage common mode component setpoint;a modulation member (58) configured to deliver modulation signal setpoints, for controlling the control members of the sub-modules of said upper and lower half-arms, from voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms and from modulation voltage setpoints applied to the upper and lower half-arms which are a function of said modulation voltage alternating component setpoint delivered by the alternating control module, of said modulation voltage common mode component setpoint delivered by the continuous control module, and of a modulation voltage common mode homopolar component setpoint which depends; of said voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms.

18. Method for controlling a multi-level modular voltage converter (10), making it possible to convert an alternating voltage into a direct voltage and vice versa, the voltage converter comprising a so-called direct part intended to be connected to a direct electrical supply network and a so-called alternating part intended to be connected to an alternating electrical supply network, the converter comprising three arms, each arm comprising an upper half-arm and a lower half-arm, each half-arm comprising a chain of sub-modules individually controllable by a control member specific to each sub-module and each sub-module comprising a capacitor, the control member of each sub-module being able to take at least a first state in which the capacitor is inserted into the corresponding chain of sub-modules and a second state in which the capacitor is not inserted into said chain of sub-modules,each half-arm having a modulation voltage which is a function of the number of capacitors inserted in the corresponding chain of sub-modules, the method comprising the steps of: determining an AC component setpoint of modulation voltage; determining a common mode component setpoint of modulation voltage; determining the voltages across the capacitors of the sub-modules of the upper and lower half-arms; delivering modulation signal setpoints, for controlling the control members of the sub-modules of said upper and lower half-arms, from said voltages across the capacitors of the sub-modules of the upper and lower half-arms and from modulation voltage setpoints applied to the upper and lower half-arms which are a function of said determined AC component setpoint of modulation voltage,of said determined modulation voltage common mode component setpoint, and of a component setpoint, common mode homopolar modulation voltage which depends on said voltages at the terminals of the capacitors of the sub-modules of the upper and lower half-arms.

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