Method and system for preselecting switching states for a multi-level converter

Description

[0001] The present invention relates to a method for switching control of a multilevel converter, in particular for real-time control thereof. Furthermore, a system for implementing this switching control is claimed.

[0002] To obtain an alternating voltage of a predetermined frequency from a direct current, input and output voltages in conventional power electronics are switched between a few, usually two or three, levels using a few power switches in order to achieve the desired average value. In contrast, modern multilevel converters operate according to a scheme that generates the alternating voltage through a dynamically interchangeable configuration of energy storage devices, such as capacitors or energy cells, connected via a large number of electronic switches. Significantly more electronic switches, such as power semiconductor switches, are used than in conventional power electronics, from which a large number of realizable overall switching states and associated output voltages of the multilevel converter can be created in very fine gradations or levels.

[0003] In principle, a specific design of a multilevel converter can be defined based on a single module, for example, as described in the applicant's publication DE 10 2015 112 512 A1. Depending on the circuit arrangement of the switching elements comprised by a single module, all switching states relating to the interconnection of the energy storage device also comprised can be achieved independently of additional connected, structurally identical individual modules.

[0004] The US publication US 2017 / 0123014 A1 discloses a dynamic change in the connectivity of individual battery components. This interconnects a plurality of energy storage devices.

[0005] The Chinese publication CN 105429495 A describes a real-time switching between series and parallel connection of adjacent modules of a modular multilevel converter. This can significantly reduce switching losses.

[0006] US 2014 / 0028266 A1 discloses a modular multilevel converter device comprising a control block for battery modules. A selection of the interconnection of battery modules is made depending on the requested power.

[0007] A fundamental problem with controlling modular multilevel converters is the high number of degrees of freedom, i.e., the multitude of different switching or module states that initially produce the same voltage. The totality of all module states determines the output voltage of the multilevel converter. For the multilevel converter to operate, a switching control unit, a so-called scheduler, must determine and actively control the states of all modules at all times. Such state determination and assignment to all switches of all modules occurs at a clock frequency between 1 kHz and 1 MHz, corresponding to 1,000 to 1 million times per second.

[0008] A real-time optimization method would be advantageous here, which determines a mathematical optimization based on the current state of all energy storage devices in the multilevel converter and / or other conceivable constraints, such as uniform loading of the energy storage devices. For a true mathematical optimization, all possible switching states—for example, with 10 MMSPC-type modules with at least five switching states per module—would have to be calculated and weighed against each other, amounting to a total of 5^10, ​​or almost 10^7, or 10 million possible switching states per time step. Then, a certain number of time steps into the future would have to be calculated to find the best solution. With M time steps, this would be (5^10)^M alternatives. A calculation for just two time steps, with approximately 10^21 alternatives, would far exceed the scope of a high-performance computer (10^15 operations per second).Therefore, none of the existing methods is currently capable of handling this enormous computational effort. Heuristic methods, on the other hand, can – depending on their complexity – find solutions online, but these will always be at a certain distance from an optimal overall switching state.

[0009] Against this background, it is an object of the present invention to provide a method for controlling the switching states of all switches of a multilevel converter, which method calculates online, i.e., during real-time operation of the multilevel converter, a respective optimal overall switching state of the multilevel converter for a respective voltage requirement according to predetermined constraints and provides this information to a controller of the multilevel converter. Furthermore, an associated system is to be presented.

[0010] To achieve the above-mentioned object, a method according to claim 1 for switching control of a multilevel converter is proposed, in which the multilevel converter has a plurality of modules, in which a respective module of the plurality of modules has at least one connection on a first side and at least one connection on a second side, at least two controllable switches and at least one energy store. In one embodiment, in a first connection between the at least one connection on the first side and the at least one connection on the second side, the at least one energy store is arranged in series with a first of the at least two controllable switches, and in a second connection, a second of the at least two controllable switches is arranged between the at least one connection on the first side and the at least one connection on the second side.This embodiment allows the energy storage device to be connected in series between the first and second terminals of the respective module of the plurality of modules, or it can be bypassed, or an electrical line can be completely interrupted. In a further embodiment, the respective module of 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 arranged directly between the at least two terminals of the first side or the second side, a first of the at least two controllable switches is arranged between a first terminal on the first side and a first terminal on the second side, and a second of the at least two controllable switches is arranged between a second terminal on the first side and a second terminal on the second side.This embodiment allows the energy storage device of the respective module of the plurality of modules to be connected in parallel to other modules of the plurality of modules when the at least two switches are closed, wherein series connection or bypassing of the energy storage device is still possible through other switching states. Each module of the plurality of modules has a respective module switching state through respective switch positions of the at least two controllable switches. The switching control system creates an overall switching state from respective module switching states of the plurality of modules. The current charge state of all energy storage devices of the multilevel converter is continuously provided to the switching control system. The switching control system is divided into an offline part and an online part.In the offline part, a plurality of offline switching tables are calculated in a continuous sequence by an optimizer, and for calculating a respective offline switching table of the plurality of offline switching tables, a respective cost function is minimized according to at least one respectively predefined offline optimization criterion for evaluating the overall switching state. In the online part, an online switching table is selected from the plurality of offline switching tables in a continuous sequence according to a predefined online selection criterion, a respective voltage level is assigned to a respective voltage request by a modulator for each time step in real time, a respective overall switching state is determined for the respective voltage level by a scheduler based on the selected online switching table, and this is passed on to all modules.

[0011] A central multilevel converter on which the method according to the invention can be advantageously carried out is the modular multilevel converter MMSPC, described by SM Goetz, AV Peterchev and T. Weyh, "Modular Multilevel Converter With Series and Parallel Module Connectivity: Topology and Control," in IEEE Transactions on Power Electronics, vol. 30, no. 1, pp. 203-215, Jan. 2015 and further in the documents US 9,502,960 B2 and DE 10 2016 112 250 A1. This is distinguished from conventional modular multilevel converters, for example as described by R. Marquardt in the document DE 101 03 0301 A1, and to which the invention is also applicable, by the existence of an additional parallel state, whereby an MMSPC in a string can generate almost any electrical serial-parallel circuit configuration of module-integrated energy storage devices and can change these dynamically.Furthermore, a bypass state is usually available to bypass a module's energy storage. In general, a module switching state, i.e., the switching states of the individual module's switches, can be clearly described according to the following list: parallel, series, bypass, off. One application is a modular multiphase multilevel converter, in which several modules are arranged in a string, with each string then providing a respective phase of a multiphase AC voltage.

[0012] A respective switching table, also known as a look-up table, assigns a combination of switch positions in the respective modules to each of the voltage levels to be represented at the output of the multilevel converter with N modules (usually 2N+1). The respective switch positions within a respective module are collectively referred to as a respective module switching state. Essentially, the respective module switching state of a respective module always concerns how the energy storage device contained in the respective module is connected to the other energy storage devices of the multilevel converter. From the online switching table, the scheduler searches for an overall switching state for a voltage level quantized and transferred to it by the modulator; this overall switching state is defined or can be defined by the respective module switching states.For example, for the MMSPC mentioned above, for a module with eight semiconductor switches (S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , S 8 ), arranged on two sides with two half-bridges on each side, with energy storage located at the input terminals and connected together to form a high-side connection and a low-side connection, a total of five module switching states are defined, for which the eight switches assume the switch positions listed in Table 1. The module switching states are: "s+" for serial connection with positive polarity, "s-" for serial connection with negative polarity, "p" for parallel connection, "b+" for bypass with positive polarity and "b-" for bypass with negative polarity. The positive orNegative polarity: which polarity of the two connections on the first side of the module is connected to the two connections on the second side of the module. In the case of a bypass, or module bypass, this is equivalent to connecting the high-side connection or the low-side connection of the switches arranged in half-bridges of the example module. Table 1: Module switching states s+, s-, p, b+, b- and respective switch positions of the eight switches S1 to S8 of the example module of an MMSPC described in the text. S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 s+ THERE IS OFF THERE IS OFF OFF THERE IS OFF THERE IS s- OFF THERE IS OFF THERE IS THERE IS OFF THERE IS OFF P THERE IS OFF OFF THERE IS THERE IS OFF OFF THERE IS b+ THERE IS OFF THERE IS OFF THERE IS OFF THERE IS OFF b- OFF THERE IS OFF THERE IS OFF THERE IS OFF THERE IS

[0013] Between a first extreme with a real-time capable scheduler and only a single look-up table, from which the next module switching state (of all modules) can be read out with a fixed assignment depending on a number of conditions, and a second extreme with full optimization, i.e. testing of all switching state alternatives and evaluation of the best switching states according to predetermined optimization criteria, the method according to the invention offers a mixture of the two extreme cases, which can also be viewed as a hybrid solution. In the offline part, an optimization also takes place, which can run at a fixed speed ratio to the online part, but does not have to, i.e. can be asynchronous. However, this optimization now generates a number of alternative switching tables with several alternative module switching states. This means that the online part can select from these alternatives in real time with only minimal computing effort.

[0014] In one embodiment of the method according to the invention, the respective predefined online selection criterion and / or the respective predefined offline optimization criterion for evaluating the overall switching state by the cost function is selected at least from the following list: minimum current load of a respective energy storage device, uniform discharge of all energy storage devices of the multilevel converter, minimal conduction losses, minimal losses in the energy storage devices, minimal overall losses. For example, if a uniform discharge of all energy storage devices is selected as an online selection criterion in order to design the discharge of an individual module close to the discharge state averaged across all modules, this occurs in a short-term average.In contrast, in the offline part, with the non-real-time optimizer, only a long-term average can be achieved for the same uniform discharge of all energy storage devices, which is now selected as the offline optimization criterion. Another example is the aforementioned minimum current load of a respective energy storage device, which should be as low as possible, but in terms of duration, should be below one second, and even better, a multiple of 100 ms or even less.

[0015] It is conceivable that an integer number of M offline switching tables with a similar minimization value of the cost function is provided from the majority of offline switching tables. This ensures that the provided alternatives represent a random selection with the same or similarly good cost function values.

[0016] It is conceivable that a respective offline shift table provided by the optimizer from the plurality of offline shift tables is optimized for a respective different offline optimization criterion. This ensures that a respective selection from the plurality of provided offline shift tables already optimizes a respective goal.

[0017] According to the invention, the online switching table is selected again from the plurality of online switching tables at each time step.

[0018] It is conceivable that the online selection criterion for selecting the online switching table from the plurality of online switching tables could be different from the respective offline optimization criterion for calculating the plurality of offline switching tables. The respective alternative online switching tables could, for example, be identified in such a way that the optima from the objective functions of the asynchronous offline part, e.g., the best integer K, are subjected to a further evaluation with respect to a further objective, and from these, an integer L smaller than K is selected. This further objective could, for example, differ from one another with respect to the current load of individual modules. However, it would also be conceivable for the further objective to simply be a complementary objective to the first selected objective, e.g., the modules could be classified as having a very high discharge current load in the offline optimization criterion, but as having a very below-average load in the further objective.

[0019] According to the invention, a number N of switching vectors corresponding to the number N of modules is calculated for a given offline optimization criterion. A respective switching vector of the number N of switching vectors indicates optimal switch positions for a respective module of the number N of modules in accordance with the given offline optimization criterion. A respective switching vector of the number N of these switching vectors corresponds to a respective offline switching table of a number N of the plurality of offline switching tables, wherein the number N of the plurality of offline switching tables spans a complete vector space.

[0020] It is conceivable that the online switching table is provided by a linear combination of the number N of the plurality of offline switching tables, distributed in proportions of time steps. A respective linear coefficient is calculated according to the specified online selection criterion, and a scalar value of the respective linear coefficient corresponds to a respective proportion of time steps in a temporal progression of several time steps. However, such a method does not ensure that the weighting of the respective number N of the plurality of offline switching tables in the linear combination is equal for balancing, for example, the current load. Accordingly, another solution can select the alternatives in such a way that equal use of each alternative leads, on average over time, to a relatively balanced current load on the individual energy storage devices.

[0021] In a further embodiment of the method according to the invention, after a selection of an online switching table, the module switching states assigned to the individual modules are permuted to the next module of a module string at each further time step.

[0022] Alternatives to an existing module configuration can also be generated systematically. Since such a process is very fast and can guarantee a finite execution time, it can also take place in the online part. Accordingly, as an alternative solution approach, the previous asynchronous (slow / non-real-time capable) part of the scheduler can be used and only the real-time capable part is modified. In this case, the number of inter-module connections in which module switching state (e.g., 3 s+, 1 p, 1 b+) is extracted from an existing overall switching state, whereby position information for each module switching state (i.e., which module switching state is present at which inter-module connection) is forgotten. The alternatives are then generated by redistributing the module switching states to inter-module connections. The online part now initially uses, for example, the module switching state entered in the look-up table for the current conditions.This can be marked after use (either with a flag or a counter). If the scheduler wants to use this module switching state again before updating the table (or more than a certain number of times, or every other use), this is detected by the marking or the counter, and the module switching state is varied by reordering. This advantageously allows the module memory load to be distributed very quickly.

[0023] Furthermore, it can always be enforced that a best solution, e.g. with a minimum value of the cost function, is always part of the majority of online switching tables.

[0024] Furthermore, a system according to claim 3 is claimed, comprising a switching controller for a multilevel converter and a multilevel converter, in which the multilevel converter has a plurality of modules. A respective module of the plurality of modules has at least one connection on a first side and at least one connection on a second side, at least two controllable switches, and at least one energy store. In a first embodiment, in a first connection between the at least one connection on the first side and the at least one connection on the second side, the at least one energy store is arranged in series with a first of the at least two controllable switches, and in a second connection, a second of the at least two controllable switches is arranged between the at least one connection on the first side and the at least one connection on the second side.This configuration allows the energy storage device to be connected in series between the first and second connections of the respective module of the plurality of modules, or it can be bypassed, or an electrical line can be completely interrupted. In a second configuration, the module of the plurality of modules has at least two connections on the first side and at least two connections on the second side. The at least one energy storage device is arranged directly between the at least two connections on the first side or the second side, and a first of the at least two controllable switches is arranged between a first connection on the first side and a first connection on the second side. A second of the at least two controllable switches is arranged between a second connection on the first side and a second connection on the second side.Each module of the plurality of modules has a respective module switching state due to respective switch positions of the at least two controllable switches. The switching controller forms an overall switching state from respective module switching states of the plurality of modules. The system comprises a modulator and a scheduler, and the switching controller is continuously provided with a current charge state of all energy storage devices of the multilevel converter. The switching controller, which is divided into an offline part and an online part, is configured to calculate a plurality of offline switching tables in continuous sequence in the offline part using an optimizer and to minimize a respective cost function according to at least one respectively predetermined offline optimization criterion for evaluating the overall switching state in order to calculate a respective offline switching table of the plurality of offline switching tables.The switching control is further configured in the online part to select an online switching table from the plurality of offline switching tables in a continuous sequence according to a predetermined online selection criterion, to assign a respective voltage level to a respective voltage request by a modulator for each time step in real time, and to determine a respective overall switching state for the respective voltage level by a scheduler based on the selected online switching table and to pass it on to all modules.

[0025] In an embodiment of the system according to the invention, the system is designed to carry out a method according to the invention.

[0026] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0027] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0028] Figure 1 shows a schematic representation of an online and offline part of a switching control according to an embodiment of the method according to the invention.

[0029] In Figure 1A schematic representation of 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 a switching control of a modular multilevel converter according to an embodiment of the method according to the invention is shown. In the offline part 2, several solutions for a respective cost function are calculated. For this purpose, those switch positions that can implement a respective possible module switching state 21 and a voltage difference 22 to be switched are queried in a table with all available overall switching states 20. As a result 23, for example, six possible module switching states are output here that can implement the specified conditions 21, 22. These are input into a calculation of a cost function 24 according to a respective offline optimization criterion, wherein, for example, a current direction 25 of a string with the respective modules is also included here.Therefore, in the example of six found module switching states 23, a total of two times six, i.e. twelve values ​​26 of the cost function 24 are output. From these twelve values ​​26, for example, those module switching states 28 are determined which have the smallest cost function value 27 among the values ​​26 (second based on the current direction). These module switching states 28 for the two current directions form the offline switching tables 29 calculated in the offline part 2, which are calculated, for example, for every possible constellation under predetermined conditions 21, 22 or a subselection thereof and thus provide a plurality of offline switching tables 29. In the online part 1, the plurality of provided offline switching tables 29 can now be accessed with significantly less computing effort. A modulator 10 outputs for a respectively requested or output overall switching state 14, e.g.Predetermined by a sinusoidal reference signal, a specific voltage level to be output by the modular multilevel converter is provided, which is formed from a current module switching state 11, at a predetermined voltage difference 12 and current direction 13 according to an online switching table from the plurality of online switching tables 15. The respective online switching table from the plurality of online switching tables can be selected according to an online selection criterion.

Claims

1. Method for a switching control of a multilevel converter, wherein the multilevel converter comprises a plurality of modules, wherein each module of the plurality of modules comprises at least one port on a first side and at least one port 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 port on the first side and the at least one port on the second side, the at least one energy storage device is arranged in series with a first of the at least two controllable switches, and in a second connection between the at least one port on the first side and the at least one port on the second side, a second of the at least two controllable switches is arranged, wherein each module of the plurality of modules comprises a respective module switching state (21) by means of the respective switch positions of the at least two controllable switches, wherein an overall switching state (14, 20) is formed from respective module switching states (21) of the plurality of modules by means of the switching control, wherein a current charging state of all energy storage devices of the multilevel converter is continuously provided to the switching controller, wherein the switching controller is divided into an offline part (2) and an online part (1), wherein, in the offline part (2), • a plurality of alternative offline switching tables (29) is calculated by an optimiser in a continuous sequence, and • a respective cost function (24) according to at least one respective predetermined offline optimisation criterion for an assessment of the overall switching state (20) is minimised for the calculation of a respective offline switching table of the plurality of alternative offline switching tables (29), wherein in the online part (1), • an online switching table is selected from the plurality of alternative offline switching tables (29) in a continuous sequence according to a predetermined online selection criterion, • for each time step in real time, a respective voltage request is assigned a respective voltage level by a modulator (10), • a respective overall switching state (14) is determined for the respective voltage level by a scheduler based on the selected online switching table and passed on to all modules, wherein the online switching table (15) is selected from the plurality of alternative offline switching tables (29) in real time at each time step, wherein a number N of switching vectors corresponding to the number N of modules are calculated for a predetermined offline optimisation criterion, wherein, by means of a respective switching vector of the number N of switching vectors for a respective module of the number N of modules, optimal switch positions are shown according to the predetermined offline optimisation criterion, wherein a respective switching vector corresponds to the number N of these switching vectors of a respective offline switching table of a number N of the plurality of offline switching tables (29) and the number N of the plurality of offline switching tables (29) spans a full vector space, wherein the overall switching state is determined based on the selected online switching table, and thereby the respective predetermined online selection criterion and / or the respective predetermined offline optimisation criterion for the assessment of the overall switching state (14) is selected by means of the cost function at least from the following list: minimum current load of a respective energy storage device, equal discharge of all energy storage devices of the multilevel converter, minimum conduction losses, minimum losses in the energy storage device, minimum total losses, as a result of which the same usage of each alternative over an average time leads to a relatively balanced current load of the individual energy storage devices.

2. Method according to one of the preceding claims, characterised in that, after a selection of an online switching table from the plurality of online switching tables (15), the respective module switching states associated with the individual modules are respectively permuted to the next module of a module strand at each further time step.

3. System comprising a switching control for a multilevel converter and a multilevel converter, wherein the multilevel converter comprises a plurality of modules, wherein each module of the plurality of modules comprises at least one port on a first side and at least one port 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 port on the first side and the at least one port on the second side, the at least one energy storage device is arranged in series with a first of the at least two controllable switches, and in a second connection between the at least one port on the first side and the at least one port on the second side, a second of the at least two controllable switches is arranged, wherein each module of the plurality of modules comprises a respective module switching state (21) by means of the respective switch positions of the at least two controllable switches, wherein an overall switching state (14, 20) is formed from respective module switching states (21) of the plurality of modules by means of the switching control, wherein the system further comprises a modulator (10) and a schedule and a current charging state of all energy storage devices of the multilevel converter is to be continuously provided to the switching controller, wherein the switching controller divided into an offline part (2) and an online part (1) is configured to, in the offline part (2), • calculate a plurality of alternative offline switching tables (29) by means of an optimiser in a continuous sequence, and • a respective cost function (24) according to at least one respective predetermined offline optimisation criterion for an assessment of the overall switching state (20) is to be minimised for the calculation of a respective offline switching table of the plurality of alternative offline switching tables (29), and in the online part (1), • an online switching table is to be selected from the plurality of alternative offline switching tables (29) in a continuous sequence according to a predetermined online selection criterion, • a respective voltage level is to be assigned for each time step in real time of a respective voltage request by a modulator (10), and • a respective overall switching state (14) is to be determined based on the selected online switching table using a scheduler and is to be passed on to all modules, wherein the switching control is additionally configured to select the online switching table (15) from the plurality of alternative offline switching tables (29) in real time at each time step, wherein a number N of switching vectors corresponding to the number N of modules are to be calculated for a predetermined offline optimisation criterion, wherein, by means of a respective switching vector of the number N of switching vectors for a respective module of the number N of modules, optimal switch positions are to be shown according to the predetermined offline optimisation criterion, wherein a respective switching vector corresponds to the number N of these switching vectors of a respective offline switching table of a number N of the plurality of offline switching tables (29) and the number N of the plurality of offline switching tables (29) spans a full vector space, the overall switching state is to be determined based on the selected online switching table, and thereby the respective predetermined online selection criterion and / or the respective predetermined offline optimisation criterion for the assessment of the overall switching state is selected by means of the cost function at least from the following list: minimum current load of a respective energy storage device, equal discharge of all energy storage devices of the multilevel converter, minimum conduction losses, minimum losses in the energy storage device, minimum total losses, as a result of which the same usage of each alternative over an average time leads to a relatively balanced current load of the individual energy storage devices.

4. System according to claim 3, wherein the system is designed to carry out an inventive method according to claim 2.