Multi-level modulation

Abstract

A method for controlling a multi-level converter to generate a multi-level output signal includes: providing a carrier signal for each of a plurality of power converter modules; assigning a first number N of the plurality of power converter modules to a first group and at least one second number M of the plurality of power converter modules to at least one second group based on the at least one first variable parameter; controlling a switching state of the power converter modules to generate a first output signal based on a carrier signal provided for each of the first number N of power converter modules, which forms a first set of carrier signals; the switching state of the power converter modules is controlled to generate a second output signal based on a carrier signal provided for each of at least one second number M of power converter modules, which forms a second set of carrier signals, where the first set of carrier signals is phase shifted with respect to the second set of carrier signals by a set phase shift.

Description

Technical Field

[0001] The present invention relates to the technical field of multi-level modulation and more particularly to a multi-level converter and a method for controlling the multi-level converter. Background Art

[0002] A multilevel converter is a power conversion system capable of generating a multilevel (multi-level) voltage waveform from a number of DC input voltages. The multilevel voltage waveform has at least three levels. The number of levels of a multilevel converter can be defined as the number of step or constant voltage levels that can be generated by the multilevel converter between the output terminal and any arbitrary internal reference node.

[0003] For example, multilevel converters are described in J. Rodriguez et al., “Multilevel converters: An enabling technology for high-power applications”, Proceedings of the IEEE, vol. 97, no. 11, pp. 1786-1817, 2009.

[0004] Multilevel converters are widely used in many technical fields, especially in the field of power conversion, for example in motor vehicles or stationary energy storage systems. Other technical fields or applications may involve power grid applications, medical applications, etc.

[0005] For multilevel converters, there are many different topologies, such as diode clamped multilevel converters, flying capacitor multilevel converters, and cascaded multilevel converters, but combinations and variations thereof are also readily available. Some of these topologies, such as flying capacitor multilevel converters and cascaded multilevel converters, have a modular structure consisting of a plurality of power converter modules, also referred to as cells.

[0006] The output voltage of a multilevel converter is usually synthesized by a multilevel modulation technique. Several multilevel modulation techniques can be applied to multilevel converters. The modulation techniques used for multilevel converters can be classified into low switching frequency techniques and high switching frequency techniques according to the average switching frequency (switching frequency) at which they operate. An example of a high frequency switching technique is multi-carrier pulse width modulation (multi-carrier PWM). Examples of multi-carrier PWM are phase-shifted carrier PWM (PSCPWM) and level-shifted carrier PWM (LSC PWM), which are one of the most popular modulation techniques.

[0007] These modulation techniques can be easily implemented on a microcontroller, field programmable gate array (FPGA) or application specific integrated circuit (ASIC).These multi-carrier PWM techniques use a carrier waveform signal for each power converter module of the multi-level converter.

[0008] Phase-shifted carrier PWM (PSC PWM) evenly distributes the on-time of individual power converter modules of a multi-level converter and effectively increases the effective output switching rate of the multi-level converter, but does not allow active control of the on-time of individual power converter modules to balance them in the distribution of workload between the individual power converter modules.

[0009] Instead of evenly distributing the on-time of individual power converter modules, level-shifted carrier PWM (LSC PWM) utilizes a sorting-based algorithm and carrier waveform signals swapping of individual power converter modules of a multi-level converter as a balancing algorithm.

[0010] Therefore, each modulation technique has its own disadvantages and advantages, and the modulation technique applied strongly depends on the application field of the multilevel converter. Therefore, the balance in the distribution of the workload between the individual power converter modules of the multilevel converter strongly depends on the modulation technique applied.

[0011] Therefore, there is a need for an improved modulation method for a multilevel converter.

[0012] It is therefore an object of the present invention to provide an improved modulation method for a multilevel converter. Summary of the invention

[0013] The mentioned problems and disadvantages are solved by the subject-matter of the independent claims. Further preferred embodiments are defined in the dependent claims.

[0014] In a first aspect of the present invention, there is provided a method for controlling a multilevel converter to generate a multilevel output signal, the multilevel converter comprising a plurality of energy sources and a plurality of power converter modules, each power converter module comprising at least two switching elements (switching elements), the method comprising: providing a carrier signal to each of the plurality of power converter modules; allocating a first number N of power converter modules of the plurality of power converter modules to a first group of power converter modules based on at least one first variable parameter; Based on the at least one first variable parameter, at least one second number M of the plurality of power converter modules are allocated to at least one second group of power converter modules; based on the carrier signal provided for each of the first number N of the plurality of power converter modules, a switch state of each of the first number N of the plurality of power converter modules allocated to the first group is controlled to generate a first output signal; and Based on the carrier signal provided for each of the at least one second number M of power converter modules in the plurality of power converter modules, controlling a switch state of each of the at least one second number M of power converter modules in the plurality of power converter modules allocated to the at least one second group to generate a second output signal; Wherein, the carrier signal provided to each power converter module of a first number N of the multiple power converter modules forms a first group of carrier signals, and the carrier signal provided to each power converter module of at least one second number M of the multiple power converter modules forms a second group of carrier signals, wherein the first group of carrier signals is phase-shifted by a group phase shift relative to the second group of carrier signals.

[0015] In a second aspect of the present invention, a multilevel converter for generating a multilevel output signal is provided, the multilevel converter comprising a plurality of energy sources and a plurality of power converter modules, each power converter module comprising at least two switching elements, the multilevel converter comprising a device suitable for executing the steps of the method of the first aspect.

[0016] In a third aspect of the present invention, there is provided a computer program comprising instructions which, when executed by a computer, cause the computer to perform the steps of the method of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the present invention will now be described with reference to the accompanying drawings, which are provided for a better understanding of the inventive concept but should not be considered as limiting the present invention. In the accompanying drawings:

[0018] Figure 1 is a schematic block diagram of a multi-level converter according to an embodiment of the present invention;

[0019] Figure 2 is a schematic representation of a power converter module of a multilevel converter according to an embodiment of the present invention;

[0020] Figure 3 shows a phase-shifted carrier PWM according to a conventional solution;

[0021] Figure 4 A modulation technique according to an embodiment of the present invention is shown;

[0022] Figure 5 A modulation technique according to an embodiment of the present invention is shown;

[0023] Figure 6 is a flow chart showing the steps of a method for controlling a multilevel converter according to an embodiment of the present invention;

[0024] Figure 7 is a block diagram of an applied modulation technique according to the present invention;

[0025] Figure 8 A modulation technique according to an embodiment of the present invention is shown;

[0026] Fig. 9 A block diagram of a multi-level converter according to an embodiment of the present invention is shown;

[0027] Fig.10 A block diagram of a multi-level converter according to an embodiment of the present invention is shown;

[0028] Fig.11 shows the waveforms of the required voltage, output voltage and control signal of the multi-level converter in one embodiment of the present invention; DETAILED DESCRIPTION

[0029] Figure 1 A schematic block diagram of a multilevel converter 1 according to an embodiment of the present invention is shown. The multilevel converter 1 includes multiple energy sources 11 and multiple power converter modules 10_1, 10_2, 10_3, ... 10_m. Each power converter module 10_i (i = 1 ....m) includes at least two switching elements 12. Each of the at least two switching elements 12 can be a solid-state switching element, such as a two-terminal solid-state switch or a three-terminal solid-state switch. Examples of two-terminal solid-state switches include, but are not limited to, PIN (positive-intrinsic-negative) diodes and Schottky diodes. Examples of three-terminal solid-state switches include, but are not limited to, MOSFETs, JFETs, IGBTs, BJTs, and thyristors.

[0030] It should be understood that although Figure 1Each energy source 11 is shown to be connected to a corresponding one of power converter modules 10_1, 10_2, 10_3, ... 10_m, but this is not restrictive, and other connection configurations are also possible.Energy source 11 can be any kind of energy source, such as battery, photovoltaic cell, fuel cell, electromechanical power converter or capacitive energy source.Any energy source in these energy sources can also be equipped with one or more converter stages, such as rectifier, dc-dc converter (for example, for the maximum power point tracking of photovoltaic system) etc. It should also be understood that the present invention is not limited to the specific topology of multilevel converter 1.For example, power converter modules 10_1, 10_2, 10_3, ..., 10_m can be connected as string, and a plurality of such strings can be arranged in multilevel converter 1.Each string can have input terminal and output terminal.Each string can represent the phase or arm of multilevel converter 1.

[0031] Figure 2 In the schematic representation of the power converter module 10_i of the multilevel converter 1 according to an embodiment of the present invention is shown. In this embodiment of the present invention, each switch element 12 can be regarded as a set of several solid-state transistors and diodes. However, this is not restrictive, and each switch element can be represented by a single solid-state transistor or a diode. The energy source 11 can be a single energy source or a set of several energy sources or an energy source equipped with one or more converter levels. Arrow 13 shows the output voltage of the power converter module 10_i.

[0032] In a multilevel converter 1, depending on how the individual power converter modules 10_1, 10_2, 10_3, ..., 10_m of the multilevel converter 1 are connected together, the distribution of energy conversion (workload or load) between different power converter modules 10_1, 10_2, 10_3, ..., 10_m can be controlled. For the distribution of energy conversion, and therefore for the distribution of the workload (load) between power converter modules 10_1, 10_2, 10_3, ..., 10_m, at least the following factors work: i) the modulation technique of the power converter modules 10_1, 10_2, 10_3, ..., 10_m to be applied to the multilevel converter, which includes determining how many power converter modules 10_1, 10_2, 10_3, ..., 10_m will participate in energy conversion, which in turn depends on the required voltage level number of the output voltage required by the multilevel converter, and ii) scheduling the single power converter modules 10_1, 10_2, 10_3, ..., 10_m of the multilevel converter for energy conversion, which includes determining which power converter modules will participate in the distribution of energy conversion (workload or load). Therefore, the scheduling controls the distribution of the workload (load) between the single power converter modules 10_1, 10_2, 10_3, ..., 10_m of the multilevel converter.

[0033] As mentioned above, the popular examples of modulation techniques are phase shift carrier PWM (PSC PWM) and level shift carrier PWM (LSC PWM). In PSC PWM and LSC PWM, a carrier waveform signal is provided to each power converter module. Hereinafter, the expressions of "carrier waveform signal" and "carrier signal" may be used interchangeably.

[0034] Level shift carrier PWM can be simply implemented on one or more microcontrollers and can use multiple PWM peripherals. In level shift carrier PWM, the carrier waveform signal for a single power converter module is arranged to have different amplitude levels. The carrier waveform signal is arranged in a vertically shifted manner. Except for the first and last carrier waveform signals, each carrier waveform signal is set between two voltage levels.

[0035] However, as mentioned above, level shift carrier PWM (LSC PWM) can not evenly distribute the on-time of single power converter module, this is because at least in the control cycle, only one module is responsible for modulation so as to promote the generation (synthesis) of required (required) output voltage, and other modules are bypassed. In addition, the power converter module of series connection that carrier waveform signal is lower than reference signal also promotes the generation of required (required) output voltage. In order to solve the uneven distribution of the on-time of single power converter module, level shift carrier PWM utilizes the algorithm based on sorting and carrier waveform signal exchange of the single power converter module of multi-level converter as balancing algorithm.

[0036] On the other hand, phase-shifted carrier PWM evenly distributes the on-time of a single power converter module within a control period. In phase-shifted carrier PWM, a phase shift is introduced between carrier waveform signals for a single power converter module to generate phase-shifted switching between the single power converter modules.

[0037] In phase-shifted carrier PWM, all power converter modules contribute to the generation (synthesis) of the required (needed) output voltage, and there is a phase shift between their carrier signals. Figure 3 An example of a phase-shifted carrier PWM according to conventional applications is shown in FIG. A carrier signal 100 of a single power converter module is shown in the upper graph. The carrier signals are phase-shifted relative to each other. A provided reference signal R is also shown. The required (needed) output voltage is shown in the lower graph. The horizontal axis of each graph represents time. The unit may be, for example, ms or s. The vertical axis represents voltage. The unit may be V or mV. Figure 3 Each of the carrier signals shown in the upper graph in FIG. 1 has an asymmetrical waveform. More specifically, the waveform is a sawtooth waveform. However, the carrier signal may also have a symmetrical waveform.

[0038] Phase-shifted carrier PWM evenly distributes the on-time of a single power converter module within a control cycle. However, phase-shifted carrier PWM does not provide active control of the on-time of a single power converter module within a control cycle. On the other hand, level-shifted carrier PWM provides active control of the on-time of a single power converter module in a cycle, however, does not provide an effective switching rate for high output.

[0039] In both level-shift carrier PWM and phase-shift carrier PWM, for each power converter module 10_i, the corresponding carrier waveform signal is compared with the corresponding reference signal R, to generate a control signal for controlling the switch state of at least two switch elements 12 in the corresponding power converter module 10_i. The reference signal can be a common reference signal for all power converter modules 10_1, 10_2, 10_3, ..., 10_m, or a reference signal can be provided for each power converter module 10_i. In this way, the switch state of the corresponding power converter module 10_i is controlled. In other words, the switch state of each power converter module 10_i is controlled based on the comparison between the reference signal R and the corresponding carrier waveform signal, and therefore the scheduling (when the power converter module 10_i is placed in the on-state) of each power converter module 10_i is controlled. Typically, when the reference signal is higher than the carrier signal, the control signal controls the corresponding power converter module to be placed in the on-state.

[0040] However, conventional phase-shift carrier PWM generates noise under module switching rate and its harmonics, and this noise can fall into the resonance of output filter and cause the stability problem of output signal.For example, if 8 power converter modules and the module switching rate of each power converter module are 10kHz, the output signal may present noise at 80kHz.If reference signal (rather than providing common reference signal) is provided for each power converter module, noise may occur at 10kHz (from the subharmonic of 80kHz).In addition, as mentioned above, in phase-shift carrier PWM, it is impossible to dynamically control the switch state of single power converter module in control cycle.

[0041] The present invention provides a novel modulation technique, which is different from conventional phase-shift carrier PWM and conventional level-shift carrier PWM, and provides phase-shift operation, which increases the effective switching rate and provides balancing capability. In addition, the modulation technique provided does not introduce additional distortion in the output signal and its harmonic components.

[0042] Modulation technique according to the present invention is based on carrier signal grouping, therefore based on the grouping of single power converter module." grouping " can be regarded as single power converter module is distributed to at least two groups.In a preferred embodiment of the present invention, each group can comprise two or more power converter modules.However, it is enough that at least one power converter module is distributed to the first group and at least two power converter modules are distributed to an other (second) group.

[0043] In one embodiment of the present invention, such grouping of power converter modules and thus of carrier signals is performed in Figure 4 In Figure 4 The bottom of FIG. 1 shows the output voltage required by the multilevel converter 1. As an example, in this embodiment of the present invention, Figure 4 In FIG. 1 , the carrier signals and thus the power converter modules are grouped into four groups G1 , G2 , G3 , G4 , however, this is not mandatory, since the number of groups according to the modulation technique of the present invention is at least two groups, as described above. In addition, Figure 4 In the figure, it is shown that each group includes three power converter modules, however, this is not mandatory. It is sufficient for at least one group to have two or more than two power converter modules. Other one or more groups can have one or more than one power converter module respectively.

[0044] also, Figure 4 1 shows that the carrier signal has an asymmetrical waveform, and more specifically a sawtooth waveform. However, this is not mandatory, and the carrier signal may have a waveform similar to Figure 4 The waveform shown may be asymmetrical, or may have a symmetrical waveform, such as a triangular waveform. The carrier signal group is uniformly phase shifted by a group phase shift The vertical dashed lines are guides to the eye for visualizing and illustratively representing the group phase shifts between different groups of carrier signals. Likewise, the horizontal axis of each graph represents time. The unit may be, for example, ms or s. The vertical axis represents voltage. The unit may be V or mV. Figure 4 It is also shown in that within each group, the carrier signals are also phase shifted relative to each other.

[0045] In some embodiments of the present invention, the same reference signal R can be used for all groups G1, G2, G3, G4. In other embodiments of the present invention, multiple reference signals can be generated, and a reference signal is provided for each group G1, G2, G3, G4. As will be described in further detail below, the reference signal R provided for each group G1, G2, G3, G4 is divided into multiple reference signal components, more specifically, divided so that a reference signal component is provided to each power converter module assigned to the group.

[0046] The carrier signal in each group G1, G2, G3, G4 is phase shifted so that the falling edge and rising edge of the control signal generated for the corresponding power converter module in each group G1, G2, G3, G4 are aligned. For each power converter module 10_i, a corresponding control signal is generated based on a comparison of a corresponding reference signal component and a corresponding carrier signal. The reference signal component can be determined based on the modulation index of the power converter module.

[0047] Providing a phase shift between the carrier signals in each group G1, G2, G3, G4 provides the freedom to set different modulation indices (mod_i) for different power converter modules in the group. In order to keep the modulation index (mod) required in the multilevel converter 1 unchanged, the sum of all modulation indices in the group must be equal to the required modulation index (mod): Wherein j∈{1,…,n} is the group index and i∈{1,…,m} is the power converter module index in the corresponding group, n is the number of groups, and m is the number of power converter modules assigned to the group. Any mathematical operation can be used for the modulation index modification of a single power converter module in the group, preferably addition, multiplication or subtraction. The modulation index modification for a single specific power converter module can be regarded as a modification of the desired modulation index, thereby deriving the modulation index for a single power converter module.

[0048] Conventionally, the required modulation index is defined as the ratio of the required output voltage Vo to the maximum output voltage Vmax: mod=Vo / Vmax.

[0049] In a two-level converter, the maximum voltage is equal to the dc link voltage. In a multi-level converter, the maximum voltage is the sum of all module voltages in the string.

[0050] Unlike the conventional method, in the embodiment of the present invention, the maximum voltage Vmax is limited to the sum of the average voltages of each group. Therefore, the maximum voltage Vmax changes with the change of the number of groups, and therefore, considering the same value of the required output voltage Vo, the modulation index also changes.

[0051] When forming groups, the required output voltage Vo must be considered. In order to achieve the maximum output voltage Vmax, the number of groups should be maximized.

[0052] This concept in one embodiment of the present invention is Figure 5In the figure, in this embodiment of the present invention, three groups of power converter modules are formed. In other words, power converter modules, all or only a part of them, are assigned to three groups G1, G2, G3. The first group G1 includes a single power converter module, and the second group G2 and the third group G3 include two power converter modules respectively. Conventionally, when phase-shifted carrier PWM is applied, three output signals (represented by solid lines) can be generated by three power converter modules (each power converter module is in a conducting state in a time period T equal to one-third of the switching cycle TSW), so as to generate a desired output signal for a switching cycle Tsw of a desired output signal Tsw by their addition.

[0053] In this embodiment of the present invention, utilize five power converter modules that are assigned to three groups of G1, G2, G3 to realize identical output voltage.One power converter module is assigned to the first group G1, and is in conducting state in the time period T equaling one-third of the switching period Tsw.Each power converter module assigned to the second group G2 is in conducting state in the half of the time period T.Equally, time period T equals one-third of the switching period Tsw.This is illustrated by dotted line and dash-dotted line respectively.In the third group G3, a power converter module assigned to the third group G3 is in conducting state in one-third of time period T, and another power converter module is in conducting state in two-thirds of the remaining time period of time period T or time period T.This is illustrated by dotted line and dash-dotted line respectively.Equally, time period T equals one-third of the switching period Tsw.

[0054] It should be noted that for illustrative purposes, the dashed and dot-dashed lines do not coincide with the solid lines.

[0055] Therefore, the output signal of each group of groups G1, G2 and G3 is phase-shifted relative to each other by the group phase shift. The group phase shift is equal to the time period T. In addition, when applying conventional phase-shift carrier PWM, the output signal of each group in groups G1, G2 and G3 is phase-shifted relative to each other in the same manner as the output signal of the three power converter modules, and each of the three power converter modules is in a conducting state in a time period T equal to one-third of the switching period Tsw.

[0056] In each group in the group that is assigned a plurality of power converter modules, in this embodiment, be the second group G2 and the 3rd group G3, when a power converter module is turned off, another power converter module is connected simultaneously.The amount of time that each power converter module is connected in each group is determined by the reference signal component provided to each power converter module.Therefore, by adjusting (tailoring, customization) corresponding reference signal component, the amount of time that each power converter module is connected in each group can be adjusted.Because in each group, when a power converter module is turned off, another power converter module is connected simultaneously, therefore generating the unified (uniform, consistent) output signal of each group.

[0057] As from Figure 5 It is obvious that the sum of the modulation index of the power converter module assigned to each in the second group G2 and the third group G3 is equal to the required modulation index. In other words, for each power converter module assigned to each in the second group G2 and the third group G3, the required modulation index has been modified. This modification can be regarded as distributing the required modulation index between the power converter modules assigned to the group. In this embodiment, owing to only distributing a power converter module in the first group, the required modulation index corresponds to the modulation index of the power converter module assigned to the first group.

[0058] The allocation of power converter modules to different groups and the degree of modification of the required modulation index for determining the modulation index of each power converter module within each group can be based on any one or a combination of the temperature, current, voltage, health state, required output voltage and state of charge of each of the multiple energy sources.

[0059] Figure 6 is a flow chart showing the steps of a method for controlling a multilevel converter 1 according to an embodiment of the present invention.

[0060] According to the method for controlling a multilevel converter 1 according to an embodiment of the present invention, the method comprises, in step S100, providing a carrier signal to each of the plurality of power converter modules. As mentioned above, the carrier signal may have an asymmetric waveform or a symmetric waveform. An example of an asymmetric waveform is a sawtooth waveform, e.g. Figure 4 As shown. An example of a symmetrical waveform is a triangular waveform. Each carrier signal may have the same waveform. In the following, it will be assumed that the carrier signal has the following waveform: Figure 4 A sawtooth waveform is shown, however, this is not limiting.

[0061] The method further includes, in step S200 , allocating a first number N of power converter modules from the plurality of power converter modules to a first group G1 of power converter modules based on at least one first variable parameter.

[0062] The at least one first variable parameter may be based on any one or a combination of the temperature, current, voltage, health state, required output voltage and state of charge of each of the multiple energy sources. Additionally, or optionally, the at least one first variable parameter may be based on the required output voltage of the multilevel converter 1. The determination of the at least one first variable parameter may depend on the application environment of the multilevel converter 1 and the specific application or function of the multilevel converter 1. An example of an application environment is a battery system in a vehicle, such as an electric vehicle. An example of a specific function may include the charging of a battery system or the discharging of a battery system (providing energy to one or more components of a vehicle). For example, when the multilevel converter 1 is applied to a battery system of a vehicle, such as an electric vehicle, the battery system is used for discharging (providing energy to one or more components of an electric vehicle), and the at least one first variable parameter may be based on the temperature and / or state of charge of each of the multiple energy sources.

[0063] The method further comprises, in step S300, allocating at least one second number M of the plurality of power converter modules to at least one second group G2 of power converter modules based on the at least one first variable parameter. The term "at least one second group" should be understood as "at least one additional group".

[0064] The method also includes, in step S400, controlling the switching state of each of the first number N power converter modules among the multiple power converter modules allocated to the first group G1 based on the carrier signal provided to each of the first number N power converter modules among the multiple power converter modules to generate a first output signal, and in step S500, controlling the switching state of each of the at least one second number M power converter modules among the multiple power converter modules allocated to the at least one second group G2 based on the carrier signal provided to each of the at least one second number M power converter modules among the multiple power converter modules to generate a second output signal.

[0065] The carrier signal provided to each power converter module of the first number N of the plurality of power converter modules forms a first group of carrier signals, and the carrier signal provided to each power converter module of the at least one second number M of the plurality of power converter modules forms a second group of carrier signals. The first group of carrier signals is phase-shifted by a group phase shift relative to the second group of carrier signals.

[0066] The group phase shift can be calculated using the following equation where j∈{1,…,n}, where n is the number of groups, and T SW is the switching cycle.

[0067] In other words, the group phase shift The switching period T SW ) and the number of groups of power converter modules.

[0068] As mentioned above, Figure 4 An embodiment of the present invention is shown, in which four groups are formed by twelve power converter modules. In the shown embodiment, a first number N (N=3) power converter modules of the plurality of power converter modules are assigned to the first group G1, a second number M (M=3) power converter modules of the plurality of power converter modules are assigned to the second group G2, a third number K (K=3) power converter modules of the plurality of power converter modules are assigned to the third group G3 and a fourth number L (L=3) power converter modules of the plurality of power converter modules are assigned to the fourth group G4. However, this is not restrictive. It is sufficient to form at least two groups, and in at least one group, the number of power converter modules is 2 or more than two. Therefore, in an embodiment of the present invention, N is 1 or greater than 1, and M is 2 or greater than 2. In addition, the number of groups does not have to be as Figure 4 In the embodiment shown, it is four. However, the maximum number of the group of power converter modules is less than the number of the level of the required multi-level output signal of the multi-level converter 1. In addition, it is not necessary to assign all power converter modules of the multi-level converter 1 to a specific group. In addition, even if the power converter module is assigned to a certain group, the modulation index of the power converter module can also be 0.

[0069] The first output signal is phase-shifted relative to the second output signal by the group phase shift. The first output signal and the second output signal may comprise the same number of signal changes between the first amplitude level and the second amplitude level, however, this is not mandatory. In other words, the first output signal and the second output signal may be two equivalent signals that are phase-shifted relative to each other by the group phase shift.

[0070] The switching state of each power converter module 10_i and thus its scheduling (when to place the power converter module 10_i in the on state and thus switch from the non-conducting state or the parallel converter module configuration state or the bypass state to the on state) is controlled based on the comparison between the corresponding reference signal R and the corresponding carrier signal.

[0071] The power converter module in the conducting state actively promotes the generation of the required output signal, or in other words, promotes the distribution of the workload (the load in the multilevel converter 1). The power converter module in the non-conducting state or the converter module configuration state or the bypass state in parallel will not actively promote the generation of the required output signal, or in other words, will not actively promote the distribution of the workload (the load in the multilevel converter 1). For example, the output voltage of the power converter module in the non-conducting state can be 0V. It should be noted that each of the non-conducting state or the parallel converter module configuration state or the bypass state can also be referred to as the non-activated state. The conducting state can also be referred to as the activated state. The non-conducting state of the at least one power converter module 10_i does not necessarily mean that for at least one power converter module 10_i in the non-conducting state, each element of the power converter module 10_i is in the non-conducting state. For example, when the at least one power converter module 10_i is in a non-conductive state, at least one of the two switching elements 12 of the power converter module 10_i may be in a conductive state, thereby bypassing the energy source 11 connected to the power converter module 10_i.

[0072] Therefore, the method for controlling a multilevel converter 1 in an embodiment of the present invention may include further providing a first reference signal for controlling the switching state of each power converter module of a first number N of the multiple power converter modules and a second reference signal for controlling the switching state of each power converter module of at least one second number M of the multiple power converter modules.

[0073] In an embodiment of the present invention, the first reference signal and the second reference signal may be the same signal. In other words, in an embodiment of the present invention, the method may include providing a single reference signal (common reference signal) for each of the at least one first group and the at least one second group. By way of example, the single reference signal may be provided or generated by a central controller at a system level. In other embodiments of the present invention, a reference signal may be generated for each group. In other words, the first reference signal and the second reference signal may be different reference signals.

[0074] Based on the at least one first variable parameter, the second reference signal can be divided into M second reference signal components. For N greater than 1, the first reference signal can be divided into N first reference signal components based on the at least one first variable parameter. For N=1, the first reference signal can be used as a first reference signal component of a power converter module.

[0075] The reference signal component provided to each power converter module of the corresponding group can be determined based on the difference of the reference signal and the reference signal component provided for the group. The difference of the reference signal component can be the difference of the reference signal component and the reference signal provided for the group. Usually, the difference of the reference signal component is determined by the higher order entity in the system.

[0076] In one embodiment of the present invention, the following equation is satisfied: Where, as described above, j∈{1,…,n}, n is the number of groups and m is the number of power converter modules in the group. d is the common reference signal. In this equation, the common reference signal d is the same for each group, so d=d gj , d gj is the reference signal provided for the group with index j. gj,i is the reference signal component for the corresponding power converter module with index i in the corresponding group j. Δd i is the difference in the reference signal components of the corresponding power converter module with index i in group j.

[0077] It also applies to:

[0078] Therefore, by comparing the corresponding carrier signal with the corresponding reference signal component d gj,i The comparison is performed to control the switch state of each power converter module 10_i assigned to the at least one first group and the at least one second group. Based on the comparison, a control signal is generated to control the switch state of the corresponding power converter module 10_i.

[0079] In other words, controlling the switch state of each power converter module of the first number N power converter modules in the plurality of power converter modules to generate a first output signal includes: for each power converter module of the first number N power converter modules in the plurality of power converter modules, comparing the corresponding carrier waveform signal with the corresponding first reference signal component, and providing a control signal for controlling the switch state based on the comparison. Controlling the switch state of each power converter module of at least one second number M power converter modules in the plurality of power converter modules to generate a second output signal includes: for each power converter module of the second number M power converter modules in the plurality of power converter modules, comparing the corresponding carrier waveform signal with the corresponding second reference signal component, and providing a control signal for controlling the switch state based on the comparison.

[0080] For each power converter module 10_i, the control signal may be a signal, such as a voltage signal, applied to a corresponding terminal of at least one of the switching elements of the power converter module 10_i.

[0081] In one or more embodiments of the present invention, the sum of N first reference signal components can be equal to the first reference signal, and the sum of M second reference signal components can be equal to the second reference signal. In other words, for the reference signal components of the first number N power converter modules in the plurality of power converter modules for distribution to the first group, and for the reference signal components of the at least one second number M power converter modules in the plurality of power converter modules for distribution to the second group, as shown in equation (3) above, the following applies: and

[0082] In each of the groups to which more than two power converter modules are assigned, the corresponding carrier signals provided to the power converter modules are phase-shifted with respect to each other.

[0083] In general, for the sawtooth carrier signal described above, the following applies to the phase shift of the carrier signal for each group: is the phase shift of the ith power converter in the jth group, is the group phase shift of the jth group and T SW is the switching period of the required voltage signal from the multilevel converter 1. The smaller value of the index i is 2.

[0084] Therefore, in each group, the phase shift between the carrier signals depends on the group phase shift. Since the group phase shift is calculated based on equation (2), in each group, the phase shift between the carrier signals within each group depends on the number of groups and the cycle length of the desired output signal. In addition, the phase shift also depends on the reference signal component.

[0085] In an embodiment of the invention, wherein in at least one group only one power converter module is assigned to the group, the following applies:

[0086] In the above embodiment, wherein the second number M power converter modules are assigned to the second group G2, the carrier signals of each power converter module in the second number M power converter modules of the plurality of power converter modules are phase-shifted relative to each other. The carrier signals of each power converter module in the second number M power converter modules of the plurality of power converter modules are phase-shifted relative to each other, so that the rising edges of the control signals are phase-shifted relative to each other, and the falling edges of the control signals are phase-shifted relative to each other.

[0087] In the above embodiment, where N is 2 or greater than 2, the carrier signals of each of the first number N of the plurality of power converter modules are phase-shifted relative to each other. The carrier signals of each of the first number N of the plurality of power converter modules are phase-shifted relative to each other so that the rising edges of the control signals are phase-shifted relative to each other and the falling edges of the control signals are phase-shifted relative to each other.

[0088] It should be noted that it is not mandatory that within each group, the carrier signals are shifted by the same phase shift. In other words, for example, for the case when five carrier signals are allocated in a certain group, there are four phase shifts for the group. All four phase shifts may be the same or different from each other, or at least two may be different from each other.

[0089] It is apparent from equation (6) that in the first group, the phase shift depends on the cycle length of the desired output signal and the first reference signal component. In the second group, the phase shift depends on the cycle length of the desired output signal and the second reference signal component. In other words, in each group, the phase shift between the carrier signals depends on the cycle length of the desired output signal and the second reference signal component.

[0090] In each control signal, each change of the waveform from the first amplitude level to the second amplitude level defines a rising edge of each control signal, and each change of the waveform from the second amplitude level to the first amplitude level defines a falling edge of each control signal.

[0091] The carrier signals in each group are phase-shifted relative to each other, so that the rising edge and the falling edge of the corresponding control signal are aligned.Thus, owing to obtain the same output signal from each group, a unified switch pattern is generated.In addition, in this way, it is possible to avoid generating harmonics with the power converter module switching rate.The expression of " switch pattern " can be regarded as the set of the switch states of the power converter module assigned to this group in the cycle of the required output signal.

[0092] In other words, in a preferred embodiment of the present invention, within each of the first group and at least one of the second groups, the carrier signals are phase-shifted relative to each other, and except for the first and last carrier waveform signals, the rising and falling edges of the control signals for the power converter modules of the group are aligned.

[0093] Figure 7 The block diagram of four groups of power converter modules using the above modulation technology in one embodiment of the present invention is shown, wherein each group includes three power converter modules. Figure 7 In the figure, it is shown that each of the multiple power converter modules in each group of four groups provides a carrier signal 50. For different power converter modules of different groups, the carrier signal is separated. The first carrier signal group 50-1 represents the carrier signal for distributing to the power converter module of the first group, the second carrier signal group 50-2 represents the carrier signal for distributing to the power converter module of the second group, the third carrier signal group 50-3 represents the carrier signal for distributing to the power converter module of the third group, and the fourth carrier signal group 50-4 represents the carrier signal for distributing to the power converter module of the fourth group.

[0094] like Figure 7 As can be seen, the carrier signal group phase shift 51 of the power converter module of each group in the second group, the third group and the fourth group is +Tsw / 4 (group phase shift). In other words, the second carrier signal group 50-2 is phase shifted by +Tsw / 4 relative to the first carrier signal group 50-1, the third carrier signal group 50-3 is phase shifted by +Tsw / 4 relative to the second carrier signal group 50-2, and the fourth carrier signal group 50-4 is phase shifted by +Tsw / 4 relative to the third carrier signal group 50-3.

[0095] In the control diagram, the modulation index 60 of the required output signal and the modulation index modification 70 of each power converter module assigned to the corresponding group are also provided. As mentioned above, any mathematical operation can be used for the modulation index modification of the single power converter module in each group, such as addition, subtraction or multiplication. Based on the required modulation index and the modulation index modification, the reference signal component 55 of each power converter module is determined in block 52. The reference signal 55 of each power converter module is sent to the corresponding comparator 54. Alternatively, the reference signal 55 also can be sent to block 53, which will be described in further detail below.

[0096] In block 53, for each group, for each power converter module assigned to this group, use equation (6) to calculate the phase shift between the carrier signal in this group.The carrier signal 56 of the phase shift assigned to each power converter module of this group is sent to comparator 54.Therefore, for each group, 3 carrier signals 56 are sent to corresponding comparator 54.

[0097] In comparator 54 , a reference signal for each power converter module is compared with a carrier signal for each power converter module, and based on the comparison, a control signal 57 is generated for controlling the switching state of each power converter module.

[0098] The reference signal component d for the corresponding power converter module 10_i i Usually the equation is satisfied: 0≤d i ≤1…(8)

[0099] Figure 8 1 shows the carrier waveform signal and the reference component signal in the embodiment of the present invention, wherein three power converter modules are allocated in the first group G1, one power converter module is allocated in the second group G2, three power converter modules are allocated in the third group G3, and two power converter modules are allocated in the fourth group G4. The corresponding reference component signal (indicated by R in the figure) is also shown. gj,i , where i∈{1,…m}, where m is the number of power converter modules assigned to the group.

[0100] The reference signal component of the corresponding power converter module can be determined using the following equation: Where, d is the duty cycle required by the multilevel converter 1, d i is the duty cycle (reference signal component) of the power converter module 10_i, n group,j is the number of power converter modules assigned to the group, and Δd iis the duty cycle difference of the power converter module 10_i. It should be understood that throughout the description of the embodiments of the present invention, "duty cycle" and "reference signal" are used interchangeably.

[0101] Fig. 9 In the block diagram of the multilevel converter according to an embodiment of the present invention is shown. In this embodiment of the present invention, two groups of power converter modules are formed. Two power converter modules 10_1, 10_3 are assigned to the first group, and a power converter module 10_2 is assigned to the second group. The power converter modules are connected as strings, and the output voltage 87 of the power converter modules 10_1, 10_2 and 10_3 is the voltage between the first power converter module and the last power converter module in the string. The block diagram also includes a control unit 80, a group unit 81 and a modulator unit 82, which are used to perform the steps of the method for controlling the multilevel converter 1.

[0102] The control unit 80, the group unit 81 and the modulator unit 82 can be functional units or logic units within one or more hardware units. In other embodiments of the present invention, they can be separate hardware units. In any case, at least one processing unit can perform the functions of each or even all of the units. The at least one processing unit can be any suitable unit or include any suitable unit that can perform computer processing. The processing unit can include any one of a microprocessor unit (MCU), a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC), a complex programmable logic device (CPLD) or a combination thereof. The processing unit can include a storage unit, such as a memory (not shown in the figure). Alternatively, or additionally, one or more memories can be provided in addition to the at least one processing unit. The one or more memories can store different information required for processing by the at least one processing unit.

[0103] The control unit 80 may be arranged to receive the required output voltage 83 of the multilevel converter 1. The control unit 80 may be arranged to send a signal 85 carrying information about the required output voltage to the modulator unit 82. The information may be information about the amplitude of the required voltage signal, the number of levels of the required voltage signal, etc. The modulator unit 82 may be arranged to generate a control signal for each control power converter module in the switch state. The group unit 81 may be arranged to perform the distribution of the power converter module to the corresponding first group and the second group based on the first variable parameter. For this distribution, the group unit 81 may be arranged to use the information 84 received from the control unit 80. The information may be information about any one of the temperature, current, voltage, health state, required output voltage and charge state of each of the multiple energy sources in each of the multiple power converter modules. Additionally, or optionally, the information may be information about the required output voltage of the multilevel converter 1. The reference voltage may be understood as a measured physical value, and the required output voltage may be understood as a reference value as a virtual value required by the control unit 80. Group unit 81 can be configured to determine at least one first variable parameter based on this information, for performing this distribution. Group unit 81 can also be configured to determine the phase shift between the carrier signal in group phase shift and each group. In this embodiment, because a power converter module 10_2 is assigned to the second group, the phase shift of a power converter module 10_2 corresponds to the group phase shift. Group unit 81 can be arranged to send information 86 about the phase shift between the carrier signal in group phase shift and each group to modulator unit 82. Modulator unit 82 can be arranged to send control signal for each power converter module in described multiple power converter modules.

[0104] Fig.10 Another embodiment of the present invention is shown. In this embodiment of the present invention, three power converter modules 10_1, 10_3 and 10_4 are assigned to the first group, one power converter module 10_2 is assigned to the second group, two power converter modules 10_5 and 10_6 are assigned to the third group, and one power converter module 10_7 is assigned to the fourth group. The functions of the control unit 80, the group unit 81 and the modulator unit 82 are similar to those of the reference Fig. 9 The same as in the described embodiment. Fig.10 In the embodiment, no information about the determination of the first variable parameter is sent from the control unit 80 to the group unit 81. This setting is optional. This step can be performed for each control cycle or for multiple control cycles. In other words, the determination of the first variable parameter can be dynamic or semi-permanent.

[0105] exist Fig.11FIG. 1 shows the waveforms of the required input voltage and the required output voltage of the multilevel converter 1 in one embodiment of the present invention. In this embodiment of the present invention, two groups of power converter modules 10_1, 10_2, 10_3 are shown in FIG. Fig. 9 The embodiment described in detail is formed as shown in the following. The number of output voltage levels required is 1.5.

[0106] In the upper part, reference signal components M1R, M2R, M3R and carrier signals M1C, M2C, M3C for power converter modules 10_1, 10_2, 10_3 are shown. In the lower diagram, the required output voltage of the multilevel converter is shown. In the middle, control signals M1C, M2C, M3C for each power converter module are shown, so as to be in the on-state for controlling the power converter module. As mentioned above, based on the comparison of the corresponding reference signal component and the corresponding carrier signal of the corresponding power converter module, a control signal for controlling each power converter module to be in the on-state is generated.

[0107] It is obvious from the upper diagram that when the reference signal for a corresponding power converter module 10_1, 10_2, 10_3 is above the carrier signal, a control signal is issued to control the corresponding power converter module to be in a conducting state. Therefore, the control signal can present a rising edge. When the reference signal for a corresponding power converter module 10_1, 10_2, 10_3 is lower than the carrier signal, a control signal is issued to control the corresponding power converter module to switch from the conducting state to a non-conducting state, a parallel configuration state, or one of the bypass states. Therefore, the control signal can present a falling edge.

[0108] exist Fig.11 1 shows that the falling edge of the control signal for power converter module 10_1 is aligned with the rising edge of the control signal for power converter module 10_3. It should be understood that the first power converter module in the group (the power converter module with the highest reference signal) will not have the rising edge of the control signal aligned with anything, and the last power converter module in the group (the power converter module with the lowest reference signal) will not have the falling edge aligned with anything.

[0109] In this embodiment of the invention, the control unit 80 receives information about the required voltage of the multilevel converter and determines the duty cycle of the multilevel converter based on this information. It further determines T SW. In this embodiment, the duty cycle d is 0.75. The control unit 80 further determines the duty cycle difference of the corresponding power converter module. In this embodiment, the difference of the corresponding power converter module is 0.1, -0.1 and 0. The control unit provides this information to the group unit 81. The group unit performs the allocation of the power converter module to the first group or the second group. The group control unit 81 determines the duty cycle of the corresponding power converter module based on equation 9. In other words, determine the following:

[0110] Therefore, when expressed in V, the corresponding reference signals are: 0.475 V for power converter module 10_1 , 0.75 V for power converter module 10_2 , and 0.275 V for power converter module 10_3 .

[0111] The group phase shift is determined based on equation (2) above.

[0112] Therefore, in this embodiment, the following is determined:

[0113] The corresponding power converter module phase shifts within each group are determined based on equation (6) for group 1 and equation (7) for group 2.

[0114] Therefore, in this embodiment, the following is determined:

[0115] The group unit 81 sends the difference in duty cycle of the corresponding power converter modules and the phase shift of the carrier signal within each group to the modulator unit 82. Subsequently, the modulator unit 82 sends a corresponding control signal.

[0116] Furthermore, once the reference signal component Δd of each power converter module is determined iThe difference, a first number N power converter modules in the plurality of power converter modules are allocated to the first group, and at least one second number M power converter modules in the plurality of power converter modules are allocated to at least one second group, and the phase shift of the single power converter module in each group is calculated, then for the entire generation of the required output voltage of the multilevel converter, the value and the allocation can be maintained, or the value and the allocation can be recalculated, and the allocation can be performed again. For example, when a multilevel converter is used in a solar panel, as a possible exemplary environment, it may be advantageous to recalculate the value and perform the allocation again according to the change of the state of charge of the energy source. For example, during a time period of a day, the energy source that is more exposed to sunlight during this time period of the day can be used more heavily, while during other time periods of the day, other energy sources that are more exposed to sunlight during other time periods of the day can be used more.

[0117] In one embodiment of the present invention, a multilevel converter 1 is provided, which includes multiple energy sources 11 and multiple power converter modules 10_1, 10_2, 10_3, ..., 10_m, each power converter module includes at least two switching elements, and the multilevel converter 1 includes a device suitable for executing the steps of the method of one or more of the above embodiments.

[0118] In addition, in one embodiment of the present invention, a computer program including instructions is provided, and when a computer executes the program, the instructions cause the computer to perform the steps of the method of one or more embodiments described above. Such instructions may be stored in one or more memories.

[0119] Therefore, according to the present invention, a modulation technique for a multilevel converter is provided, which allows phase shift operation that effectively increases the effective output switching rate. The modulation technique provided simultaneously introduces a balancing capability. In addition, the modulation technique provided does not introduce additional distortion at the module switching rate and its harmonics.

[0120] Although detailed embodiments have been described, these are only provided for a better understanding of the invention as defined by the independent claims and should not be considered as limiting.

Claims

1. A method for controlling a multilevel converter to generate a multilevel output signal, the multilevel converter comprising a plurality of energy sources and a plurality of power converter modules, each power converter module comprising at least two switching elements, the method comprising: providing a carrier signal to each of the plurality of power converter modules; allocating a first number N of the plurality of power converter modules to a first group of power converter modules based on at least one first variable parameter; allocating at least one second number M of the plurality of power converter modules to at least one second group of power converter modules based on the at least one first variable parameter; Based on the carrier signal provided to each of the first number N of the plurality of power converter modules, controlling a switch state of each of the first number N of the plurality of power converter modules allocated to the first group of power converter modules to generate a first output signal; as well as Based on the carrier signal provided for each of the at least one second number M of power converter modules in the plurality of power converter modules, controlling a switch state of each of the at least one second number M of power converter modules in the plurality of power converter modules allocated to the at least one second group of power converter modules to generate a second output signal; Wherein, the carrier signal provided to each power converter module of the first number N power converter modules among the multiple power converter modules forms a first group of carrier signals, and the carrier signal provided to each power converter module of the at least one second number M power converter modules among the multiple power converter modules forms a second group of carrier signals, wherein the first group of carrier signals is phase-shifted by a group phase shift relative to the second group of carrier signals.

2. The method according to claim 1, wherein: N is 1 or greater than 1, wherein M is 2 or greater than 2, wherein the maximum number of groups of power converter modules is less than the number of levels of the required multi-level output signal.

3. The method according to any one of claims 1 or 2, wherein: The first output signal is phase-shifted relative to the second output signal by the set of phase shifts, wherein preferably the first output signal and the second output signal comprise a same number of signal changes between a first amplitude level and a second amplitude level.

4. The method according to any one of claims 1 to 3, further comprising: A first reference signal is provided for controlling the switching state of each power converter module of the first number N of the multiple power converter modules and a second reference signal is provided for controlling the switching state of each power converter module of the at least one second number M of the multiple power converter modules.

5. The method according to claim 4 further includes dividing the second reference signal into M second reference signal components based on the at least one first variable parameter, and also includes, for N greater than 1, dividing the first reference signal into N first reference signal components based on the at least one first variable parameter, and for N=1, using the first reference signal as the first reference signal component of a power converter module.

6. The method according to any one of claims 1 to 5, wherein: The at least one first variable parameter is based on any of: a temperature, a current, a voltage, a state of health, a requested output voltage, and a state of charge of each of the plurality of energy sources.

7. A method according to any one of the preceding claims, wherein: Controlling a switch state of each of the first number N of the plurality of power converter modules to generate a first output signal comprises: for each of the first number N of the plurality of power converter modules, comparing a corresponding carrier waveform signal with a corresponding first reference signal component, and providing a control signal for controlling the switch state based on the comparison; Wherein, controlling the switching state of each power converter module of at least one second number M power converter modules among the multiple power converter modules to generate a second output signal includes: for each power converter module of the second number M power converter modules among the multiple power converter modules, comparing the corresponding carrier waveform signal with the corresponding second reference signal component, and providing a control signal for controlling the switching state based on the comparison.

8. The method according to any one of claims 5 to 7, wherein: The sum of the N first reference signal components is equal to the first reference signal, wherein the sum of the M second reference signal components is equal to the second reference signal.

9. The method according to any one of claims 4 to 8, wherein: The first reference signal and the second reference signal are the same reference signal.

10. A method according to any one of the preceding claims, wherein: Each of the carrier signals has the same waveform, wherein the waveform is a symmetrical waveform or an asymmetrical waveform.

11. A method according to any one of the preceding claims, wherein: The carrier signals of each of the second number M of the plurality of power converter modules are phase shifted relative to each other by a second phase shift; and Wherein, for N equal to 2 or greater than 2, the carrier signals of each of the first number N of the plurality of power converter modules are phase shifted relative to each other.

12. The method according to claim 11, wherein: In each of the control signals, each change of the waveform from a first amplitude level to a second amplitude level defines a rising edge of each of the control signals, wherein each change of the waveform from the second amplitude level to the first amplitude level defines a falling edge of each of the control signals; wherein, for N being equal to 2 or greater than 2, the carrier signals of each of the first number N of the plurality of power converter modules are phase-shifted relative to each other, so that the rising edges of the control signals are phase-shifted relative to each other, and the falling edges of the control signals are phase-shifted relative to each other, wherein, preferably, the phase shift depends on the cycle length of the desired output signal and the first reference signal component; And wherein, the carrier signals of each of the second number M of power converter modules among the multiple power converter modules are phase-shifted relative to each other, so that the rising edges of the control signals are phase-shifted relative to each other, and the falling edges of the control signals are phase-shifted relative to each other, wherein, preferably, the phase shift depends on the cycle length of the desired output signal and the second reference signal component.

13. The method according to claim 12, wherein: Within each of the first group of power converter modules and the at least one second group of power converter modules, the carrier signals are phase shifted relative to each other to align the rising and falling edges of the control signals for the groups of power converter modules.

14. A method according to any one of the preceding claims, wherein: The group phase shift depends on the cycle length of the desired output signal and the number of groups of power converter modules.

15. A multilevel converter for generating a multilevel output signal, the multilevel converter comprising a plurality of energy sources and a plurality of power converter modules, each power converter module comprising at least two switching elements, the multilevel converter comprising a device suitable for performing the steps in the method according to claim 1.

16. A computer program comprising instructions which, when a computer executes said program, cause said computer to perform the steps of the method according to claim 1.

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