Method and system for preselecting switching states for a multilevel converter

Abstract

A method for switching a multi-level converter is disclosed. The multi-level converter has a plurality of modules. The switch control forms a total switch state from the switch states of the plurality of modules. The switch control continuously provides the current power of all energy storage devices of the multi-level converter. The switch control is divided into an offline part and an online part. In the offline part, an optimizer continuously and sequentially calculates a plurality of offline switch tables and calculates corresponding offline switch tables in the plurality of offline switch tables. In the online part, an online switch table is continuously and sequentially selected from the plurality of offline switch tables according to a specified online selection criterion. For each time step, a modulator allocates a corresponding voltage level to a corresponding voltage request in real time. For the corresponding voltage level, a scheduler obtains a corresponding total switch state based on the selected online switch table and forwards it to all modules. A system for implementing such switch control is also disclosed.

Description

Technical Field

[0001] The present invention relates to a method for switching control of a multi-level converter, in particular a method for real-time control of the multi-level converter. In addition, protection is claimed for a system for implementing such switching control. Background Art

[0002] In order to obtain an AC voltage of a predetermined frequency from a DC voltage, the input voltage and the output voltage are switched between a few (mostly two or three) levels in conventional power electronics by means of a few power switches in order to obtain the desired magnitude on average. In contrast, modern multilevel converters work according to the following scheme: the AC voltage is generated by a dynamically variable configuration of energy storage devices (e.g. capacitors or energy cells) connected via a plurality of electronic switches. In this case, substantially more electronic switches (e.g. power semiconductor switches) can be used than in conventional power electronics, whereby a plurality of achievable total switching states and the output voltage of the multilevel converter associated therewith can be formed in very finely divided steps or levels.

[0003] In theory, the corresponding construction of a multilevel converter can be defined with the aid of individual modules, as is shown, for example, in the document DE 10 2015 112 512 A1 of the applicant. Depending on the circuit arrangement of the switching elements contained in the individual modules, all switching states associated with the connection of the energy stores also contained can be realized independently of additionally connected, identically constructed individual modules.

[0004] US 2017 / 0123014 A1 discloses the dynamic change of the connectivity of each battery assembly, thereby connecting multiple energy storage devices to each other.

[0005] Chinese document CN 105429495 A describes the conversion between adjacent modules of a modular multilevel converter in real time in series or in parallel, whereby the switching losses can be greatly reduced.

[0006] Document US 2014 / 0028266 A1 discloses a modular multilevel converter device, which includes a control block for a battery module. The interconnection of the battery modules is selected according to the requested power.

[0007] The basic problem of controlling modular multilevel converters is the high number of degrees of freedom, that is to say, first of all, a large number of different switch states or module states of the same voltage are formed. The totality of these module states determines the output voltage of the multilevel converter. For the operation of the multilevel converter, the control device of the circuit control (the so-called scheduler) must determine and actively control the states of all modules at each time point. Such state acquisition and distribution of all switches of all modules is carried out at a sampling frequency between 1kHz and 1MHz, corresponding to 1000 to 1 million times per second.

[0008] Here, a real-time optimization method may be advantageous, which obtains a mathematical optimization based on the current state of all energy storage devices of the multilevel converter and / or other conceivable auxiliary conditions (such as uniform loading of the energy storage devices). In order to truly optimize mathematically, all possible switching states must theoretically be calculated and weighed against each other, for example, in the case of 10 MMSPC-type modules with at least five switching states per module, there are a total of 5^10 and therefore close to 10^7 (equal to 10 million) possible switching states per time step. Then a certain number of future time steps must be calculated in order to find the best solution associated with them. In the case of M time steps, there may be (5^10)^M possibilities. The calculation of just two time steps already has about 10^21 possibilities, which is far beyond the scope of high-performance computers (10^15 operations per second). Therefore, none of the existing methods to date can handle this huge computational workload. In contrast, although heuristic methods can be solved online (depending on the complexity), these solutions are always still a certain distance from the optimal total switching state. Summary of the invention

[0009] In this context, the object of the present invention is to provide a method for controlling the switching states of all switches of a multilevel converter, which method calculates the corresponding optimal total switching state of the multilevel converter according to specified auxiliary conditions for the corresponding voltage request online (that is, in real time when the multilevel converter is running) and provides control of the multilevel converter. In addition, a related system will be introduced.

[0010] In order to achieve the above-mentioned object, a method for switching control of a multi-level converter is proposed, wherein the multi-level converter has a plurality of modules, wherein a corresponding module in the plurality of modules has: at least one terminal on the first side and at least one terminal on the second side, at least two controllable switches and at least one energy storage. In one embodiment, the at least one energy storage is arranged in series with the first controllable switch of the at least two controllable switches in the first connection between the at least one terminal on the first side and the at least one terminal on the second side, and the second controllable switch of the at least two controllable switches is arranged in the second connection between the at least one terminal on the first side and the at least one terminal on the second side. Through this embodiment, the energy storage can be connected in series between the first terminal and the second terminal of the corresponding module in the plurality of modules, or the energy storage can be bypassed, or the wire can be completely interrupted. In another embodiment, the corresponding module in the plurality of modules has at least two terminals on the first side and at least two terminals on the second side. The at least one energy storage device is directly arranged between the at least two terminals of the first side or the second side, the first controllable switch of the at least two controllable switches is arranged between the first terminal on the first side and the first terminal on the second side, and the second controllable switch of the at least two controllable switches is arranged between the second terminal on the first side and the second terminal on the second side. Through this embodiment, the energy storage device of the corresponding module in the plurality of modules can be connected in parallel with other modules in the plurality of modules when the at least two switches are in the closed state, wherein the series connection of the energy storage device or bypassing of the energy storage device can also be realized through other switch states. Each module in the plurality of modules has a corresponding module switch state by means of the respective switch positions of the at least two controllable switches. The total switch state is formed by the switch states of the individual modules in the plurality of modules by means of the switch control. The current power of all energy storage devices of the multi-level converter is continuously provided to the switch control. The switch control is divided into an offline part and an online part. In the offline part, the optimizer continuously and sequentially calculates a plurality of offline switch tables, and minimizes a corresponding cost function according to at least one corresponding specified offline optimization criterion so as to calculate a corresponding offline switch table in the plurality of offline switch tables for evaluating a total switch state. In the online part, an online switch table is continuously and sequentially selected from the plurality of offline switch tables according to a specified online selection criterion; for each time step, the modulator allocates a corresponding voltage level to a corresponding voltage request in real time; for a corresponding voltage level, the scheduler obtains a corresponding total switch state based on the selected online switch table and forwards it to all modules.

[0011] A central multilevel converter on which the method according to the invention can be advantageously performed is a modular multilevel converter MMSPC, which is described by SM Goetz, AV Peterchev and T. Weyh in "Modular Multilevel Converter With Series and Parallel Module Connectivity: Topology and Control," IEEE Transactions on Power Electronics, vol. 30, no. 1, pp. 203-215, Jan. 2015 and also in the documents US 9,502,960 B2 and DE 10 2016 112250 A1. Such modular multilevel converters are distinguished by the presence of additional parallel states compared to conventional modular multilevel converters (such as those described by R. Marquardt in document DE 101 030301 A1 and to which the present invention is likewise applicable), whereby the MMSPC can generate almost any electrical series-parallel circuit configuration of module-integrated energy storage devices in a string and can dynamically change this circuit configuration. In addition, bypass states, so-called bypasses, can also be used in most cases in order to bypass the energy storage devices of the modules. In general, in the case of modular multilevel converters, the module switching states (i.e. the switching states of the switches of the individual modules) can be clearly described according to the following list: parallel, series, bypass, disconnected. One application is a modular multiphase multilevel converter, in which a plurality of modules are respectively arranged in a string, wherein the respective string then provides a respective phase of a multiphase AC voltage.

[0012] The corresponding switch table (also referred to as a lookup table) assigns a combination of switch positions in the individual modules to each of the voltage levels (in the case of N modules, in most cases (2N+1)) to be displayed at the output of the multilevel converter, wherein the individual switch positions within the corresponding module are collectively referred to as the corresponding module switch state. The corresponding module switch state of the corresponding module is basically always about how the energy storage included in the corresponding module is connected to other energy storages of the multilevel converter. For the voltage level quantized by the modulator and transmitted to the scheduler, the scheduler searches the online switch table for the total switch state defined or definable by the individual module switch states. Thus, for example, for the MMSPC mentioned above, the corresponding module has eight semiconductor switches (S1, S2, S3, S4, S5, S6, S7, S8), which are arranged in two sides, each side has two half bridges, energy storages at the input terminals and are connected together to form a high side or high voltage side connection and a low side or low voltage side connection. In this module, five module switch states are specified, and these eight switches take the switch positions listed in Table 1 for these five module switch states. The module switch states are: "s+" means series connection with positive polarity, "s-" means series connection with negative polarity, "p" means parallel connection, "b+" means bypass with positive polarity and "b-" means bypass with negative polarity. Here, positive polarity or negative polarity indicates with which polarity two terminals on the first side of the module are connected to two terminals on the second side of the module. In the case of bypass or module bypassing, this is equivalent to the connection of the high-side connection or low-side connection via the switches arranged in the half bridge of the example module.

[0013] Table 1: Module switch states s+, s-, p, b+, b- and the individual switch positions of the eight switches S1 to S8 of an example module of the MMSPC described herein.

[0014]

[0015] The method according to the invention provides a mixture of these two extremes (which can also be regarded as a hybrid solution) between the first extreme with a real-time capable scheduler and only a single lookup table (from which the next module switching state (of all modules) can be read out according to a plurality of conditions in the case of a fixed allocation) and the second extreme that is fully optimized, that is, all switching state possibilities are tested and the best switching state is evaluated according to a specified optimization criterion. In addition, an optimization is also performed in the offline part, which can be run at a fixed speed ratio relative to the online part, but does not have to be so, that is, it can be asynchronous. However, this optimization now generates a plurality of alternative switch tables with a plurality of alternative module switching states. Therefore, a selection can be made among these alternatives in real time in the online part with little computational effort.

[0016] In one embodiment of the method according to the invention, a correspondingly specified online selection criterion and / or a correspondingly specified offline optimization criterion is selected at least from the following list for evaluating the total switching state by means of a cost function: minimum current load of the corresponding energy storage, uniform discharge of all energy storages of the multilevel converter, minimum forward conduction losses, minimum energy storage losses, minimum total losses. For example, if uniform discharge of all energy storages is selected as the online selection criterion so that the discharge of the individual modules approaches the discharge state averaged over all modules, this is achieved within the short-term average value. In contrast, in the offline part with a non-real-time capable optimizer, uniform discharge of all energy storages that are the same but now selected as the offline optimization criterion can only be achieved within the long-term average value. Another example is the above-mentioned minimum current load of the corresponding energy storage, which should be as low as possible, but in particular should be less than one second in terms of duration, preferably even only a multiple of 100 ms or less.

[0017] In another embodiment of the method according to the invention, an integer number M of offline switch tables having similar cost function minimization values ​​are selected from a plurality of offline switch tables, thereby achieving that the provided alternatives tend to represent random selections having the same or similar good cost function values.

[0018] In another embodiment of the method according to the invention, the corresponding offline switching tables from the plurality of offline switching tables provided by the optimizer are optimized according to corresponding different offline optimization criteria, thereby achieving that the corresponding selection among the plurality of offline switching tables provided has optimized the corresponding target.

[0019] In yet another embodiment of the method according to the present invention, an online switching table is newly selected from the plurality of online switching tables for each time step.

[0020] In yet another embodiment of the method according to the invention, a criterion different from the corresponding offline optimization criterion for calculating the plurality of offline switching tables is selected as the online selection criterion for selecting the online switching table from the plurality of online switching tables. For example, the corresponding alternative online switching table can be identified in such a way that the optimal value of the objective function from the asynchronous offline part (e.g. the best integer K) is again subjected to an evaluation with respect to a further objective, and an integer L less than K is selected therefrom. This further objective can differ from one another, for example, in terms of the current load of the individual modules. However, it is also conceivable that the further objective is only a complementary objective to the first selected objective, for example, the module is subjected to a very high discharge current load in the offline optimization criterion, while the load subjected thereto in the further objective is significantly below the average.

[0021] In yet another embodiment of the method according to the present invention, N switching vectors corresponding to the N modules are calculated for a specified offline optimization criterion. With the help of corresponding switching vectors in the N switching vectors, optimal switch positions are respectively indicated for corresponding modules in the N modules according to the specified offline optimization criterion. The corresponding switching vectors in the N switching vectors correspond to corresponding offline switching tables in N offline switching tables in the plurality of offline switching tables, wherein the N offline switching tables in the plurality of offline switching tables span the complete vector space.

[0022] In yet another further embodiment of the method according to the invention, the online switch table is provided by means of a linear combination of N offline switch tables distributed in proportion to the time steps from the plurality of offline switch tables. The corresponding linear coefficients are calculated according to a specified online selection criterion, and the scalar value of the corresponding linear coefficients corresponds to the corresponding proportion of the time steps in the time history of the plurality of time steps. However, in this method it cannot be ensured that the weights for compensating, for example, the current load of the corresponding N offline switch tables in the linear combination of the plurality of offline switch tables are the same. Accordingly, another solution can select the alternatives in such a way that the use of each alternative for the same duration on average over time results in a relatively balanced current load on the individual energy storage devices.

[0023] In yet another embodiment of the method according to the invention, after the online switching table has been selected, the module switching state respectively assigned to the individual modules is replaced in each further time step to the next module of the module string.

[0024] Alternatives to existing module configurations can also be generated systematically. Because this process is very fast and can ensure limited execution time, the process can also be carried out in the online part. Accordingly, as an alternative solution, the existing asynchronous (slow / non-real-time) part of the scheduler can also be used, and only the real-time part can be modified. Here, how many module-to-module connections are in what module switch state (for example, 3 in s+, 1 in p, and 1 in b+) are extracted from the existing total switch state, wherein the position information of each module switch state is ignored (that is, what module switch state exists at which module-to-module connection now). Alternatives are now generated by redistributing the module switch state to the module-to-module connection. The online part now uses, for example, the module switch state recorded in the lookup table for the current conditions at the beginning. The module switch state can be marked (by a flag or a counter) after it is used. If the scheduler wants to use this module switch state again before the table is updated (or more than a certain number of times, or every other time), this is identified by a flag or a counter, and the module switch state is changed by reclassification. In this way, the module memory load can be distributed very quickly.

[0025] Furthermore, it may be always enforced that the best solution (eg, having the smallest cost function value) is always part of the plurality of online switch tables.

[0026] In addition, a system for switching a multilevel converter is claimed, in which the multilevel converter has a plurality of modules. A corresponding module among the plurality of modules has: at least one terminal on the first side and at least one terminal on the second side, at least two controllable switches and at least one energy storage device. In a first design scheme, the at least one energy storage device is arranged in series with a first controllable switch among the at least two controllable switches in a first connection between the at least one terminal on the first side and the at least one terminal on the second side, and a second controllable switch among the at least two controllable switches is arranged in a second connection between the at least one terminal on the first side and the at least one terminal on the second side. Through this design scheme, the energy storage device can be connected in series between the first terminal and the second terminal of the corresponding module among the plurality of modules, or the energy storage device can be bypassed, or the wire can be completely interrupted. In a second design scheme, the module among the plurality of modules has at least two terminals on the first side and at least two terminals on the second side. The at least one energy storage is directly arranged between the at least two terminals of the first side or the second side, and the first controllable switch of the at least two controllable switches is arranged between the first terminal on the first side and the first terminal on the second side. The second controllable switch of the at least two controllable switches is arranged between the second terminal on the first side and the second terminal on the second side. By means of the respective switch positions of the at least two controllable switches, each module of the plurality of modules has a corresponding module switch state, and the total switch state is formed according to the respective module switch states of the plurality of modules by means of the switch control. The system includes a modulator and a scheduler, and the current power of all energy storages of the multi-level converter is continuously provided to the switch control. The switch control divided into an offline part and an online part is configured such that: in the offline part, the optimizer continuously and sequentially calculates a plurality of offline switch tables, and minimizes the corresponding cost function according to at least one corresponding specified offline optimization criterion so as to calculate the corresponding offline switch table of the plurality of offline switch tables for evaluating the total switch state. The switch control is also configured to: in the online part, continuously and sequentially select an online switch table from the multiple offline switch tables according to specified online selection criteria; for each time step, the modulator allocates a corresponding voltage level to a corresponding voltage request in real time; and for the corresponding voltage level, the scheduler obtains a corresponding total switch state based on the selected online switch table and forwards it to all modules.

[0027] In one embodiment of the system according to the invention, the system is designed to carry out the method according to the invention.

[0028] Further advantages and embodiments of the invention are apparent from the description and the drawings.

[0029] In general, the present invention discloses the following technical solutions 1 and 10, and the following 2-9 and 11 are preferred technical solutions of the present invention:

[0030] 1. A method for switching a multilevel converter, wherein the multilevel converter has a plurality of modules, wherein each of the plurality of modules has: at least one terminal on a first side and at least one terminal on a second side, at least two controllable switches and at least one energy storage device, wherein in a first connection between the at least one terminal on the first side and the at least one terminal on the second side, the at least one energy storage device is arranged in series with a first controllable switch of the at least two controllable switches, and in a second connection between the at least one terminal on the first side and the at least one terminal on the second side, a second controllable switch of the at least two controllable switches is arranged, wherein by means of a respective switching position of the at least two controllable switches, each of the plurality of modules has a corresponding module switching state, wherein by means of the switch control, a total switch state is formed from the individual module switching states of the plurality of modules, wherein the switch control is continuously provided with the current charge of all energy storage devices of the multilevel converter, wherein the switch control is divided into an offline part (2) and an online part (1),

[0031] In the offline part (2),

[0032] The optimizer continuously calculates multiple offline switch tables (29) one after another, and

[0033] minimizing a corresponding cost function (24) according to at least one corresponding specified offline optimization criterion in order to calculate a corresponding offline switching table of the plurality of offline switching tables (29) for evaluating the overall switching state,

[0034] In the online part (1),

[0035] Continuously and sequentially selecting an online switch table from the plurality of offline switch tables (15) according to a specified online selection criterion,

[0036] For each time step, the modulator (10) assigns the corresponding voltage level to the corresponding voltage request in real time,

[0037] For the corresponding voltage level, the scheduler obtains the corresponding total switch state (14) based on the selected online switch table and forwards it to all modules.

[0038] 2. The method according to item 1 above, wherein at least a corresponding specified online selection criterion and / or a corresponding specified offline optimization criterion are selected from the following list to evaluate the total switching state through the cost function: minimum current load of the corresponding energy storage device, uniform discharge of all energy storage devices of the multi-level converter, minimum forward conduction loss, minimum energy storage loss, and minimum total loss.

[0039] 3. The method according to any one of 1-2 above, wherein an integer number M of offline switch tables having similar minimization values ​​(27) of the cost function (24) are selected from the plurality of offline switch tables (29).

[0040] 4. The method according to 1 or 2 above, wherein a corresponding offline switch table among the multiple offline switch tables (29) provided by the optimizer has been optimized according to corresponding different offline optimization criteria.

[0041] 5. The method according to any one of 1 to 4 above, wherein an online switch table is newly selected from the plurality of online switch tables (15) for each time step.

[0042] 6. A method according to any one of 1 to 5 above, wherein a criterion different from the corresponding offline optimization criterion used to calculate the multiple offline switch tables (29) is selected as an online selection criterion for selecting an online switch table from the multiple online switch tables (15).

[0043] 7. The method according to any one of 1 to 6 above, wherein N switch vectors corresponding to the N modules are calculated according to a specified offline optimization criterion, wherein with the help of corresponding switch vectors in the N switch vectors, optimal switch positions are respectively indicated for corresponding modules in the N modules according to the specified offline optimization criterion, wherein the corresponding switch vectors in the N switch vectors correspond to corresponding offline switch tables in N offline switch tables in the plurality of offline switch tables (29), and the N offline switch tables in the plurality of offline switch tables (29) span a complete vector space.

[0044] 8. The method according to item 7 above, wherein an online switch table (15) is provided by a linear combination of N offline switch tables distributed in proportion to the time step in the multiple offline switch tables (29), wherein a corresponding linear coefficient is calculated according to the specified online selection criterion, and a scalar value of the corresponding linear coefficient corresponds to a corresponding proportion of the time step in the time history of the multiple time steps.

[0045] 9. A method according to any one of 1 to 8 above, wherein after selecting an online switch table from the multiple online switch tables (15), the module switch states correspondingly assigned to each module are replaced to the next module in the module string in each additional time step.

[0046] 10. A system for switching control of a multilevel converter, wherein the multilevel converter has a plurality of modules, wherein each of the plurality of modules has: at least one terminal on a first side and at least one terminal on a second side, at least two controllable switches and at least one energy storage, wherein in a first connection between the at least one terminal on the first side and the at least one terminal on the second side, the at least one energy storage is arranged in series with a first controllable switch of the at least two controllable switches, and in a second connection between the at least one terminal on the first side and the at least one terminal on the second side, a second controllable switch of the at least two controllable switches is arranged, wherein each of the plurality of modules has a corresponding module switching state by means of a respective switch position of the at least two controllable switches, wherein a total switching state is formed by means of the switch control from the respective module switching states of the plurality of modules, wherein the system comprises a modulator (10) and a scheduler, and the current charge of all energy storages of the multilevel converter is continuously provided to the switch control, wherein the switch control divided into an offline part (2) and an online part (1) is configured to,

[0047] In the offline part (2),

[0048] The optimizer continuously calculates multiple offline switch tables (29) one after another, and

[0049] minimizing a corresponding cost function (24) according to at least one corresponding specified offline optimization criterion in order to calculate a corresponding offline switching table of the plurality of offline switching tables (29) for evaluating the overall switching state, and

[0050] In the online part (1),

[0051] Continuously and sequentially selecting an online switch table from the plurality of offline switch tables (15) according to a specified online selection criterion,

[0052] For each time step, the modulator (10) assigns a corresponding voltage level to the corresponding voltage request in real time, and

[0053] For the corresponding voltage level, the scheduler obtains the corresponding total switch state (14) based on the selected online switch table and forwards it to all modules.

[0054] 11. The system according to item 10 above, wherein the system is configured to execute the methods according to items 2 to 9 above.

[0055] It goes without saying that the features mentioned above and those still to be explained below can be used not only in the respectively stated combination but also in other combinations or alone, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 The schematic diagram shows an online part and an offline part of a switch control according to one embodiment of the method of the present invention. DETAILED DESCRIPTION

[0057] exist Figure 1 Schematically shows an online part 1 (also referred to as a high-speed loop) and an offline part 2 (also referred to as a low-speed loop) of the switch control performed in a modular multilevel converter according to an embodiment of the method of the present invention. In the offline part 2, multiple solutions are calculated for the corresponding cost function. For this purpose, a table 20 with all available total switch states is queried for those switch positions that can achieve the corresponding possible module switch states 21 and the voltage difference 22 to be switched. Here, for example, six possible module switch states are output as results 23, which can achieve the specified conditions 21, 22. These module switch states are used as inputs for the calculation of the cost function 24 according to the corresponding offline optimization criterion, wherein, for example, the current direction 25 of the string with the corresponding module is also included here. Therefore, in the example where six module switch states 23 are found, the total of two times six values ​​of the cost function 24 is output, that is, twelve values ​​26. For example, among these twelve values ​​26, those module switch states 28 (in pairs according to the current direction) with the smallest cost function value 27 among the values ​​26 are determined. These module switching states 28 for both current directions form an offline switching table 29 calculated in the offline part 2, which is calculated, for example, for each possible situation under the specified conditions 21, 22 or a sub-selection thereof, and thus a plurality of offline switching tables 29 are provided. In the online part 1, the plurality of provided offline switching tables 29 can now be accessed with a significantly lower computational effort. The modulator 10 specifies a specific voltage level to be output by the modular multilevel converter for a corresponding requested or output total switching state 14 (specified, for example, by a sinusoidal reference signal), which is formed from the current module switching state 11 according to one online switching table from a plurality of online switching tables 15 under the specified voltage difference 12 and current direction 13. The corresponding online switching table can be selected from the plurality of online switching tables according to an online selection criterion.

Claims

1. A method for switching a multilevel converter, wherein the multilevel converter has a plurality of modules, wherein each of the plurality of modules has: at least one terminal on a first side and at least one terminal on a second side, at least two controllable switches and at least one energy storage device, wherein in a first connection between the at least one terminal on the first side and the at least one terminal on the second side, the at least one energy storage device is arranged in series with a first controllable switch of the at least two controllable switches, and in a second connection between the at least one terminal on the first side and the at least one terminal on the second side, a second controllable switch of the at least two controllable switches is arranged, wherein by means of a respective switching position of the at least two controllable switches, each of the plurality of modules has a corresponding module switching state, wherein by means of the switch control, a total switch state is formed from the individual module switching states of the plurality of modules, wherein the switch control is continuously provided with the current charge of all energy storage devices of the multilevel converter, wherein the switch control is divided into an offline part (2) and an online part (1), In the offline part (2), The optimizer continuously calculates multiple offline switch tables (29) one after another, and minimizing a corresponding cost function (24) according to at least one corresponding specified offline optimization criterion in order to calculate a corresponding offline switching table of the plurality of offline switching tables (29) for evaluating the overall switching state, In the online part (1), Continuously and sequentially selecting an online switch table from the plurality of offline switch tables (29) according to a specified online selection criterion, For each time step, the modulator (10) assigns the corresponding voltage level to the corresponding voltage request in real time, For the corresponding voltage level, the scheduler obtains the corresponding total switch state (14) based on the selected online switch table and forwards it to all modules; wherein a corresponding offline switch table among the plurality of offline switch tables (29) provided by the optimizer has been optimized according to corresponding different offline optimization criteria; N switch vectors corresponding to the N modules are calculated according to a specified offline optimization criterion, wherein with the help of corresponding switch vectors in the N switch vectors, optimal switch positions are respectively indicated for corresponding modules in the N modules according to the specified offline optimization criterion, wherein the corresponding switch vectors in the N switch vectors correspond to corresponding offline switch tables in N offline switch tables in the multiple offline switch tables (29), and the N offline switch tables in the multiple offline switch tables (29) span a complete vector space.

2. The method according to claim 1, wherein at least a correspondingly specified online selection criterion and / or a correspondingly specified offline optimization criterion is selected from the following list to evaluate the total switching state via the cost function: minimum current load of the corresponding energy storage device, uniform discharge of all energy storage devices of the multi-level converter, minimum forward conduction losses, minimum energy storage losses, minimum total losses.

3. A method according to any one of the preceding claims, wherein provision is made for selecting an integer number M of offline switch tables from the plurality of offline switch tables (29) having the same minimization value (27) of the cost function (24).

4. The method according to claim 1 or 2, wherein an online switching table is newly selected from the plurality of online switching tables (15) for each time step.

5. The method according to claim 1 or 2, wherein a criterion different from the corresponding offline optimization criterion used for calculating the plurality of offline switching tables (29) is selected as an online selection criterion for selecting an online switching table from the plurality of online switching tables (15).

6. The method according to claim 1, wherein the online switch table (15) is provided by a linear combination of N offline switch tables in the plurality of offline switch tables (29) distributed in proportion to the time step, wherein: The corresponding linear coefficient is calculated according to the specified online selection criterion, and the scalar value of the corresponding linear coefficient corresponds to the corresponding proportion of the time step in the time history of the plurality of time steps.

7. The method according to claim 1 or 2, wherein after selecting an online switching table from the plurality of online switching tables (15), the module switching states respectively assigned to each module are replaced to the next module of the module string in each additional time step.

8. A system for switching control of a multilevel converter, wherein the multilevel converter has a plurality of modules, wherein each of the plurality of modules has: at least one terminal on a first side and at least one terminal on a second side, at least two controllable switches and at least one energy storage device, wherein in a first connection between the at least one terminal on the first side and the at least one terminal on the second side, the at least one energy storage device is arranged in series with a first controllable switch of the at least two controllable switches, and in a second connection between the at least one terminal on the first side and the at least one terminal on the second side, a second controllable switch of the at least two controllable switches is arranged, wherein each of the plurality of modules has a corresponding module switching state by means of a respective switch position of the at least two controllable switches, wherein a total switching state is formed by means of the switch control by means of the respective module switching states of the plurality of modules, wherein the system comprises a modulator (10) and a scheduler, and the current charge of all energy storage devices of the multilevel converter is continuously provided to the switch control device, wherein the switch control device, which is divided into an offline part (2) and an online part (1), is configured to: In the offline part (2), The optimizer continuously calculates multiple offline switch tables (29) one after another, and minimizing a corresponding cost function (24) according to at least one corresponding specified offline optimization criterion in order to calculate a corresponding offline switching table of the plurality of offline switching tables (29) for evaluating the overall switching state, and In the online part (1), Continuously and sequentially selecting an online switch table from the plurality of offline switch tables (29) according to a specified online selection criterion, For each time step, the modulator (10) assigns a corresponding voltage level to the corresponding voltage request in real time, and For the corresponding voltage level, the scheduler obtains the corresponding total switch state (14) based on the selected online switch table and forwards it to all modules; wherein a corresponding offline switch table among the plurality of offline switch tables (29) provided by the optimizer has been optimized according to corresponding different offline optimization criteria; N switch vectors corresponding to the N modules are calculated according to a specified offline optimization criterion, wherein with the help of corresponding switch vectors in the N switch vectors, optimal switch positions are respectively indicated for corresponding modules in the N modules according to the specified offline optimization criterion, wherein the corresponding switch vectors in the N switch vectors correspond to corresponding offline switch tables in N offline switch tables in the multiple offline switch tables (29), and the N offline switch tables in the multiple offline switch tables (29) span a complete vector space.

9. The system according to claim 8, wherein: The system is configured to perform the method according to any one of claims 2 to 7.

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