CONVERTER AND METHOD FOR ITS OPERATION

Description

[0001] The invention relates to a method for controlling a converter and to a converter that can be operated accordingly.

[0002] The publication "Modular Multilevel Converter: An universal concept for HVDC networks and extended DC bus applications" (R. Marquardt, The 2010 International Power Electronics Conference, pages 502 to 507) discloses a method for operating a converter. The previously known converter has series circuits whose outer terminals form DC terminals of the converter. The series circuits each comprise two series-connected module series circuits, the electrical connections of which each form an AC terminal of the converter. The module series circuits each have at least two series-connected sub-modules, each with at least two switching elements and a capacitor. The converter is controlled by controlling the sub-modules.

[0003] From US 2017 / 271997 A1, a method for balancing switching module voltages of a modular multi-stage converter is known, in which the switching modules are divided into two subsets and operated in two different operating modes according to the division.

[0004] The invention is based on the objective of providing a converter which, in the case where no energy flow between the connection sides of the converter is desired, exhibits improved no-load behavior compared to conventional converters.

[0005] This problem is solved according to the invention by a converter with the features according to claim 1. Advantageous embodiments of the converter according to the invention are specified in the dependent claims.

[0006] According to the invention, the control device is designed to enable at least two idle operating modes, namely a first idle operating mode in which at least one of the sub-modules is in active operation, and a second idle operating mode in which all sub-modules are inactive, in particular the two switching elements of all sub-modules are switched off, and the control device is further designed to set the second idle operating mode when idle operation is desired, as long as the state of charge of the or a predetermined group of energy storage devices of the inverter meets a predetermined charging condition, and otherwise sets the first idle operating mode.

[0007] A significant advantage of the inverter according to the invention is that, in the case of idle operation, it enables lower energy consumption than conventional inverters due to its two idle operating modes. In the second idle operating mode, all submodules are inactive, in particular their switching elements are switched off, and no current is flowing. The second idle operating mode is preferably activated whenever the charge level of the energy storage devices permits; the first idle operating mode, which is less energy-efficient due to current flow, can thus be limited to time slots when the first idle operating mode is required to recharge the energy storage devices.The first idle operating mode can therefore only be activated, for example, if the energy storage devices have been discharged beyond a predetermined level during the second idle operating mode and recharging of the energy storage devices is to take place - in the first idle operating mode.

[0008] According to the invention, the control device is designed such that it switches from the second idle operating mode to the first idle operating mode if, during the second idle operating mode, the state of charge or the voltage of at least one of the energy storage devices of the at least one series-connected module reaches or falls below a predetermined minimum state of charge or a predetermined minimum module voltage.

[0009] If the inverter has several module series connections, an advantageous variant provides that the control device performs the switching from the second no-load operating mode to the first no-load operating mode individually for each module series connection, namely when or as soon as, during the second no-load operating mode, the state of charge or the voltage of at least one of the energy storage devices of the respective module series connection under consideration reaches or falls below a predetermined minimum state of charge or a predetermined minimum voltage.

[0010] In another variant, considered even more advantageous, it is provided that – in the case that the inverter has several module series connections – the control device performs the switching from the second no-load operating mode to the first no-load operating mode for all module series connections, i.e. across inverters or across module series connections, if or as soon as, during the second no-load operating mode, the state of charge or the voltage of at least one of the energy storage devices reaches or falls below a predetermined minimum state of charge or a predetermined minimum voltage.

[0011] It is also considered advantageous if the control unit switches from the second no-load operating mode to the first no-load operating mode when, during the second no-load operating mode, the voltage at least at one module series connection reaches or falls below a predetermined minimum voltage. If the inverter has several module series connections, the voltage-dependent switching – as already explained – can be performed individually for each module series connection or across the entire inverter.

[0012] In the event of switching across inverters, preferably all module series circuits are switched as soon as at least one of the module series circuits reaches or falls below the specified minimum voltage.

[0013] Furthermore, it is advantageous if the control unit switches from the first no-load operating mode to the second no-load operating mode as soon as the voltages at the energy storage devices of the at least one series module are high enough that, after switching to the second no-load operating mode, the voltage applied to the at least one series module reaches or exceeds a predetermined target voltage. If the inverter has several series module circuits, it is advantageous if the control unit only switches from the first no-load operating mode to the second no-load operating mode when, after switching to the second no-load operating mode, the voltages applied to all series module circuits reach or exceed the predetermined target voltage.

[0014] It is advantageous if, in the first idle operating mode, at least one switching element of at least one of the sub-modules is switched on, and the energy storage of the associated sub-module is connected or bypassed by the action of the at least one switched-on switching element, and the electrical voltages of the connected or bypassed sub-modules determine the module series connection voltage applied to the at least one module series connection and ensure idle operation. Particularly preferred is the fact that all sub-modules are switched on or active in the first idle operating mode.

[0015] It is advantageous if, in the second idle operating mode, due to switched-off switching elements, the energy storage devices of all sub-modules are neither switched on nor bridged, and the at least one module series connection voltage has an idle voltage that is determined or at least partly determined by the sum voltage of all sub-modules.

[0016] The control unit switches – in the case of a single module series connection or in the case of multiple module series connections, in each of the module series connections – from the first idle operating mode to the second idle operating mode and vice versa, preferably in such a way that the sub-modules as a whole, or at least in the respective module series connection, are not switched simultaneously, but rather staggered or offset one after the other, in order to avoid a sudden blocking or unblocking of the inverter.

[0017] In other words, the inverter is preferably not abruptly blocked or unblocked, or switched between no-load operating modes, in order to avoid voltage transients, as these could cause EMC problems and stress the insulation with parasitic capacitive currents, which in turn could lead to damage to the inverter's hardware.

[0018] Preferably, the time derivative of the module series connection voltages is limited to a predetermined maximum value by switching the sub-modules, i.e., blocking or unblocking them, in a time-staggered manner, as described.

[0019] It is also advantageous if the switching of the idle operating modes, in particular from the first to the second idle operating mode, only takes place after the control device has previously reduced the current through each of the module series circuits to zero or at least approximately to zero by prior control of the sub-modules.

[0020] Preferably, in the second idle operating mode, at least one module series connection (or each of the module series connections in the case of several module series connections) is insulating with respect to current flow between the first connection side and the second connection side due to the magnitude of the module series connection voltage applied to it.

[0021] The first connection side is preferably an AC voltage side, which includes at least one AC voltage connection from which an alternating current can be supplied or drawn.

[0022] The second connection side is preferably a DC voltage side comprising two DC voltage terminals from which a DC current can be supplied or drawn.

[0023] The at least one module series connection (or each of the module series connections in the case of several module series connections) is preferably connected between one of the two DC voltage connections and the AC voltage connections of the first connection side.

[0024] When idle operation is desired, the control unit preferably sets the first idle operating mode in the case of at least one module series connection when the sum voltage of the voltages of the energy storage devices of the sub-modules reaches or falls below a predetermined minimum voltage.

[0025] If the inverter has several module series connections, it is advantageous if the control device - when no-load operation is desired - sets the first no-load operating mode for all module series connections when the sum voltage from the voltages of the energy storage units of at least one of the module series connections reaches or falls below a predetermined minimum voltage.

[0026] Alternatively, switching from idle mode to the first idle mode can also be done individually for each module in series, i.e., whenever the sum of the voltages of the energy storage units of the sub-modules of the respective module in series reaches or falls below a predetermined minimum voltage.

[0027] The control unit calculates the specified minimum voltage, preferably by taking into account the current operating state of the inverter.

[0028] It is particularly advantageous if the minimum voltage corresponds to a voltage sum or at least depends on a voltage sum determined by summation, namely from half the magnitude of the DC voltage that lies or should lie between the two DC voltage terminals and the magnitude of the voltage amplitude of the AC voltage applied to the AC voltage terminal or between the AC voltage terminals.

[0029] Preferably, at least one series circuit with two module series circuits is connected between the two DC voltage terminals, the center terminal of which forms the AC voltage terminal or one of the AC voltage terminals.

[0030] It is also particularly advantageous if the converter comprises at least two, preferably three, parallel series circuits, each comprising two module series circuits and whose outer terminals each form one of the DC voltage terminals of the converter arrangement, and each of the center terminals between two series-connected module series circuits forms an AC voltage terminal of the converter.

[0031] The submodule, or at least one of the submodules, is preferably an electrical half-bridge module.

[0032] The submodules, or at least one of the submodules, is preferably an electrical full bridge module.

[0033] The sub-modules preferably have switching transistors as switching elements, to which a freewheeling diode is connected in parallel (or antiparallel) in each case.

[0034] The invention further relates to a method for operating an inverter equipped with a first connection side and a second connection side, wherein the inverter has at least one series module connection with two electrically connected sub-modules and a control device for controlling the sub-modules, wherein the sub-modules each comprise an energy storage device and at least two switching elements, of which at least one switching element is switched on during active operation of the sub-module, and wherein the control device sets a predetermined energy flow between the two connection sides by controlling the switching elements or causes an idle state in which there is no energy flow between the two connection sides by controlling the switching elements.According to the invention, the control device enables at least two idle operating modes, namely a first idle operating mode in which at least one of the sub-modules is in active operation, and a second idle operating mode in which all sub-modules are inactive, in particular the at least two switching elements of each sub-module are switched off, and the control device sets the second idle operating mode when idle operation is desired, as long as the state of charge of the or a predetermined group of energy storage devices of the inverter meets a predetermined charging condition, and otherwise sets the first idle operating mode.

[0035] Regarding the advantages of the method according to the invention and regarding advantageous embodiments of the method according to the invention, reference is made to the above statements in connection with the converter according to the invention.

[0036] It is advantageous if, during operation in the first idle mode, all sub-modules are actively operated, i.e., at least one of the switching elements is switched on in each of the sub-modules.

[0037] In the first idle operating mode, the control unit preferably controls the sub-modules in such a way that charging currents flow through the sub-modules and the energy storage of the sub-modules is charged.

[0038] Switching off the sub-modules for the purpose of switching from the first idle operating mode to the second idle operating mode is preferably carried out in a staggered manner to avoid large current and / or voltage jumps.

[0039] Switching off the sub-modules for the purpose of switching from the first idle operating mode to the second idle operating mode preferably only takes place after the current through the module series connection(s) has been set to zero or at least approximately zero by the control unit, i.e., in other words, the aforementioned charging currents for charging the energy storage of the sub-modules have been switched off beforehand.

[0040] Switching on the sub-modules for the purpose of switching from the second idle operating mode to the first idle operating mode is preferably staggered over time – preferably staggered for each module in series – in order to avoid large current and / or voltage jumps. In other words, preferably not all sub-modules are switched on simultaneously.

[0041] The invention is explained in more detail below with reference to exemplary embodiments; these show, by way of example, Figure 1 shows an embodiment of an inverter according to the invention, Figure 2 shows an embodiment of a partial module used to form module series circuits in the inverter according to Figure 1 Figure 3 shows a further embodiment of a sub-module that can be used to form module series circuits in the inverter according to Figure 1 Figure 4 shows an example of the operation of the control unit of the inverter 10 according to Figure 1 Using a flowchart and Figure 5, an example of the voltage time course at one of the module series circuits of the inverter according to Figure 1 .

[0042] For the sake of clarity, the same reference symbols are always used in the figures for identical or comparable components.

[0043] The Figure 1Figure 1 shows an inverter 10 which has three AC voltage terminals L1, L2 and L3 on its first terminal side AS1, at each of which an AC current can be fed into or drawn from the inverter 10. On a second terminal side AS2 there are two DC voltage terminals at which a DC current Idc can be fed into or drawn from the inverter 10; these are shown in the Figure 1 marked with the reference symbols L+ and L-.

[0044] The DC voltage between the DC terminals L+ and L- is designated Udc. The AC voltage between the AC terminals L1, L2, and L3 is designated Uac.

[0045] The inverter 10 has three series circuits R1, R2, and R3, the outer terminals of which form the DC voltage terminals L+ and L- of the inverter 10. The series circuits R1, R2, and R3 each comprise two series-connected module series circuits TS.

[0046] Each of the module series circuits TS comprises at least two sub-modules SM connected in series, each of which includes at least two switching elements and a capacitor. Exemplary embodiments of suitable sub-modules SM are described below in connection with the Figures 2 and 3 explained.

[0047] The inverter 10 has a control unit 20, which is suitable for controlling the submodules SM and thus for controlling the module series circuits TS. For this purpose, the control unit 20 has a computing unit 21 and a memory 22. A control program module SPM, which determines the operation of the computing unit 21, is stored in the memory 22.

[0048] A possible operating procedure for the converter 10 is described further below in connection with the Figure 4 explained in more detail.

[0049] The Figure 2 Figure 1 shows an embodiment of a submodule SM comprising two switching elements S in the form of transistors, each with a diode D connected in parallel (or antiparallel), and a capacitor C as an energy storage device. These components form a half-bridge circuit, which is controlled by the control unit 20 by activating the switching elements S. Figure 1- enables purely or exclusively unipolar operation of the capacitor C. The transistors and the diodes D connected in parallel can be prefabricated components, as indicated graphically by boxes in the figures; the transistors and the diodes D connected in parallel can, for example, be IGBT devices.

[0050] In active operation of the submodule SM, one of the two switching elements S is switched on and the other switching element S is switched off. Is this in the Figure 2 If the upper switching element S is on and the lower switching element S is off, then the capacitor C is connected between the two terminals A1 and A2 and is electrically active; if this is the case in the Figure 2 With the upper switching element off and the lower switching element on, the capacitor C is bridged and separated from the two terminals A1 and A2 by the diodes.

[0051] In passive operation of the sub-module SM, both switching elements S are switched off.

[0052] The Figure 3 Figure 1 shows an embodiment of a submodule SM comprising four switching elements S in the form of transistors, each with a diode connected in parallel, and a capacitor C as an energy storage device. These components form a full bridge circuit which, by controlling the switching elements S – by the control unit 20 shown in Figure 1 – enables bipolar operation of the capacitor C.

[0053] In the active operation of the sub-module SM according to Figure 3 Two of the four switching elements S are switched on and the other two are switched off. In passive operation of the submodule SM according to Figure 3 All four switching elements S are switched off.

[0054] The Figure 4 illustrates a preferred operating mode of the SPM control program module of the control unit 20 using a flowchart according to Figure 1 .

[0055] In an operating mode setting step 100, an input-side control signal ST determines whether the inverter 10 should be operated in a normal operating mode 110, in which a desired energy flow between the first connection side AS1 and the second connection side AS2 is set, or in an idle mode 120, in which an energy flow between the first connection side AS1 and the second connection side AS2 is blocked.

[0056] If the input control signal ST specifies that the inverter 10 is to be operated in normal operating mode 110, the control of the submodules TS takes place as is generally known and is described, for example, in the aforementioned literature reference.

[0057] If the input control signal ST specifies that the inverter 10 should be operated in no-load mode 120, then in a test step 121 it is checked whether the voltage Um applied to the module series circuits TS reaches or exceeds a minimum voltage Umin.

[0058] The minimum voltage preferably corresponds to a voltage sum determined by summation according to Umin = Udc / 2 + Uac , max where Uac,max denotes the magnitude of the voltage amplitude of the alternating voltage Uac applied between the AC voltage terminals L1, L2, L3.

[0059] If the condition is not met, i.e., Um ≤ Umin, then the energy storage devices of the submodules SM or the capacitors C are according to the Figures 2 and 3 not sufficiently charged and it switches to the first idle operating mode LL1 to charge the energy storage of the submodules SM or the capacitors C according to the Figures 2 and 3to charge until the condition Um ≥ Usoll is met; Usoll denotes a predetermined target voltage, which is preferably between 5% and 20% greater than the minimum voltage Umin.

[0060] If, after a certain charging time, the condition Um ≥ Usoll is met, then the energy storage devices of the submodules SM or the capacitors C are charged according to the Figures 2 and 3 sufficiently charged and it can be switched to the second idle operating mode LL2, in which the switches S of the submodules SM are switched off.

[0061] In the second idle operating mode LL2, the energy storage devices of the submodules SM or the capacitors C will operate according to the Figures 2 and 3 - despite the switched-off switching elements S - they discharge again, so that after a certain time the voltage Um applied to the module series circuits TS will reach and fall below the minimum voltage Umin again.

[0062] It is advantageous if the switching on and off of the sub-modules for the purpose of switching from the first idle operating mode to the second idle operating mode or vice versa is carried out in a staggered manner for each of the module series circuits TS.

[0063] The sub-modules SM are preferably switched off for the purpose of switching from the first idle operating mode LL1 to the second idle operating mode LL2 only after the current I through the module series circuits TS has been set to zero or at least approximately zero, i.e., the charging current for charging the energy storage devices or capacitors C of the sub-modules TS has been switched off again.

[0064] The Figure 5 This shows, by way of example, the switching between the idle operating modes LL1 and LL2 using the voltage curve of the voltage Um applied to one of the module series circuits TS over time t.

[0065] It can be found in the Figure 5 The concept of an automatic start-stop system is recognized. In the second idle operating mode LL2, the inverter 10 remains blocked for a large part of the time to minimize losses. Subsequently, depending on its own charge level and the dynamic requirements, it is briefly switched on again (first idle operating mode LL1) to recharge and / or restore a symmetrical energy distribution within the inverter 10.

[0066] The Figure 5 Figure 1 shows an exemplary implementation where the inverter 10 is reactivated every 30 seconds for 0.5 seconds, switching to the first no-load operating mode LL1 and recharged with a relatively small current. The choice of the lower voltage limit Umin is preferably defined by the dynamic requirements and the external terminal voltages.

[0067] In a preferred implementation, the inverter 10 is not abruptly blocked or unblocked to avoid voltage transients. These could cause EMC problems and stress the insulation with parasitic capacitive currents, potentially leading to hardware damage. Preferably, the dUm / dt for blocking and unblocking the submodules SM is set such that the aforementioned problems are minimized by staggered blocking / unblocking of the individual submodules SM.

[0068] It is also advantageous if the switching process for each inverter branch occurs without current, so that no or only small voltage fluctuations occur. Once all inverter branches are active, the recharging current is specified. The blocking process is performed in reverse order.

[0069] Although the invention has been further illustrated and described in detail by means of preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

Claims

1. A converter (10) with a first terminal side (AS1) and a second terminal side (AS2), - wherein the converter (10) has at least one module series connection (TS) with two submodules (SM) electrically connected in series and a control device (20) for driving the submodules (SM), - wherein the submodules (SM) each comprise an energy store and at least two switching elements (S), of which at least one switching element (S) is switched on during active operation of the submodule (SM), and - wherein the control device (20) is suitable to ensure a specified energy flow between the two terminal sides (AS1, AS2) by driving the switching elements (S) or to enable an open circuit, during which no energy flow is present between the two terminal sides (AS1, AS2), by driving the switching elements (S), wherein - the control device (20) is designed such that it enables at least two open-circuit operating types (LL1, LL2), namely - a first open-circuit operating type (LL1) in which at least one of the submodules (SM) is in active operation, and - a second open-circuit operating type (LL2) in which all submodules (SM) are inactive, and - the control device (20) is furthermore designed such that, upon an open circuit being desired, it sets the second open-circuit operating type (LL2), as long as the state of charge of the or a specified group of energy stores of the converter (10) satisfies a specified charging condition, and otherwise sets the first open-circuit operating type (LL1), characterised in that the control device (20) is designed such that it switches from the second open-circuit operating type (LL2) to the first open-circuit operating type (LL1) when, during the second open-circuit operating type (LL2), the state of charge or the voltage (Uc) of at least one of the energy stores of the at least one module series connection (TS) reaches or falls below a specified minimum state of charge or a specified module minimum voltage.

2. The converter (10) according to any one of the preceding claims, characterised in that the control device (20) is designed such that it switches from the second open-circuit operating type (LL2) to the first open-circuit operating type (LL1) when, during the second open-circuit operating type (LL2), the voltage (Um) at the at least one module series connection (TS) reaches or falls below a specified minimum voltage (Umin).

3. The converter (10) according to any one of the preceding claims, characterised in that the control device (20) is designed such that it switches from the first open-circuit operating type (LL1) to the second open-circuit operating type (LL2) as soon as the voltages (Uc) at the energy stores are so high that, after changing over to the second open-circuit operating type (LL2), the voltage (Um) applied to the at least one module series connection (TS) reaches or exceeds a specified target voltage (Usoll) which is greater than the minimum voltage (Umin).

4. The converter (10) according to any one of the preceding claims, characterised in that - in the first open-circuit operating type (LL1), at least one switching element (S) of at least one of the submodules (SM) is switched on and the energy store of the associated submodule (SM) is added or bridged by participation of the at least one switched-on switching element (S) and the electrical voltages of the added or bridged submodules (SM) determine the module series connection voltage (Um) applied to the at least one module series connection (TS) and ensure the open circuit, and - in the second open-circuit operating type (LL2), due to the switching elements (S) being switched off, the at least one module series connection (TS) generates an open-circuit voltage which is determined or at least co-determined by the sum voltage of the energy stores of all submodules (SM).

5. The converter (10) according to any one of the preceding claims, characterised in that in the second open-circuit operating type (LL2), the at least one module series connection (TS) - due to the level of the module series connection voltage applied to it and the blocking effect of diodes within its submodules (SM) - is insulating in terms of current flow between the first terminal side (AS1) and the second terminal side (AS2).

6. The converter (10) according to claim 5, characterised in that the first terminal side (AS1) is an alternating-voltage side comprising at least one alternating-voltage terminal (L1, L2, L3) at which an alternating current can be supplied or drawn.

7. The converter (10) according to any one of the preceding claims, characterised in that the second terminal side (AS2) is a direct-voltage side comprising two direct-voltage terminals (L+, L-) at which a direct current (Idc) can be supplied or drawn.

8. The converter (10) according to any one of the preceding claims, characterised in that - the at least one module series connection (TS) is connected between one of the two direct-voltage terminals and the or one of the alternating-voltage terminals, and - the control device (20) - upon an open circuit being desired - sets the first open-circuit operating type (LL1) in the at least one module series connection (TS) if the sum voltage of the voltages of the submodules (SM) of the at least one module series connection (TS) reaches or falls below a specified minimum voltage (Umin).

9. The converter (10) according to claim 8, characterised in that the control device (20) is designed such that it calculates the specified minimum voltage (Umin) utilising the respective operating state of the converter (10).

10. The converter (10) according to any one of the preceding claims, characterised in that the minimum voltage (Umin) corresponds to a voltage sum or is at least dependent on a voltage sum which is identified by summing, more precisely - of half the amount of the direct voltage (Udc) which is applied or is at least intended to be applied between the two direct-voltage terminals (L+, L-), and - the amount of the voltage amplitude of the alternating voltage (Uac) applied to the alternating-voltage terminal.

11. The converter (10) according to any one of the preceding claims, characterised in that at least one series connection having two module series connections (TS) is connected between the two direct-voltage terminals, the centre terminal of which forms the or one of the alternating-voltage terminals.

12. The converter (10) according to any one of the preceding claims, characterised in that - the converter (10) comprises at least two series connections (R1, R2, R3) connected in parallel, each of which comprises two module series connections (TS) and whose outer terminals each form one of the direct-voltage terminals of the converter (10), and - each of the centre terminals between two module series connections (TS) located in series forms an alternating-voltage terminal of the converter (10).

13. A method for operating a converter (10) equipped with a first terminal side (AS1) and a second terminal side (AS2), - wherein the converter (10) has at least one module series connection (TS) with two submodules (SM) electrically connected in series and a control device (20) for driving the submodules (SM), - wherein the submodules (SM) each comprise an energy store and at least two switching elements (S), of which at least one switching element (S) is switched on during active operation of the submodule (SM), and - wherein the control device (20) sets a specified energy flow between the two terminal sides (AS1, AS2) by driving the switching elements (S) or elicits an open circuit, during which no energy flow is present between the two terminal sides (AS1, AS2), by driving the switching elements (S), wherein - the control device (20) enables at least two open-circuit operating types (LL1, LL2), namely - a first open-circuit operating type (LL1) in which at least one of the submodules (SM) is in active operation, and - a second open-circuit operating type (LL2) in which all submodules (SM) are inactive, and - the control device (20), upon an open circuit being desired, sets the second open-circuit operating type (LL2), as long as the state of charge of the or a specified group of energy stores of the converter (10) satisfies a specified charging condition, and otherwise sets the first open-circuit operating type (LL1), characterised in that the control device (20) switches from the second open-circuit operating type (LL2) to the first open-circuit operating type (LL1) when, during the second open-circuit operating type (LL2), the state of charge or the voltage (Uc) of at least one of the energy stores of the at least one module series connection (TS) reaches or falls below a specified minimum state of charge or a specified module minimum voltage.

14. The method according to claim 13, characterised in that - during the first open-circuit operating type (LL1), during ongoing operation, all submodules (SM) are operated in active operation, in particular each with at least one switching element (S) switched on, and / or - in the first open-circuit operating type (LL1), the submodules (SM) are driven such that charging currents flow through the submodules (SM) and the energy stores of the submodules (SM) are charged, and / or - switching off the submodules (SM) for the purpose of changing over from the first open-circuit operating type (LL1) to the second open-circuit operating type (LL2) is performed in a timestaggered manner, and / or - switching off the submodules (SM) for the purpose of changing over from the first open-circuit operating type (LL1) to the second open-circuit operating type (LL2) preferably only occurs after the current (I) through the at least one module series connection (TS) has been set to zero or at least approximately zero, that is, in particular, the charging currents mentioned for charging the energy stores of the submodules (SM) have been turned off, and / or - switching on the submodules (SM) for the purpose of changing over from the second open-circuit operating type (LL2) to the first open-circuit operating type (LL1) occurs in a timestaggered manner.