A modulation method, system, device and medium for common-mode voltage suppression of modular multilevel converters

Abstract

The application discloses a modulation method, system, device and medium for common-mode voltage suppression of a modular multilevel converter, relates to the field of common-mode voltage suppression, and comprises the following steps: receiving a three-phase modulation signal, and determining the number of optional inputs of each phase bridge arm sub-module; constructing a sub-module input combination according to the number of optional inputs of each phase bridge arm sub-module, and calculating the total input number of the bridge arm sub-modules corresponding to each combination; taking the equal total number of sub-modules of the upper bridge arms and the lower bridge arms of the three phases as a constraint condition, and screening a target combination from the sub-module input combination, which satisfies a constant total input number relationship; and determining the number of sub-modules actually input by each phase bridge arm according to the target combination. The application reduces the amplitude of the common-mode voltage on the alternating current side, reduces electromagnetic interference, electrical insulation stress and additional loss problems caused by the common-mode voltage, simultaneously does not need to increase an additional hardware suppression circuit, and is favorable for reducing the system structure complexity and operation cost.

Description

Technical Field

[0001] This invention relates to the field of common-mode voltage suppression technology, and in particular to a modulation method, system, device and medium for common-mode voltage suppression in modular multilevel converters. Background Technology

[0002] In existing modular multilevel converter modulation control strategies, the number of three-phase bridge arm submodules deployed is difficult to maintain strictly symmetrically under certain operating conditions, which can easily lead to the generation of common-mode voltage components on the AC side of the converter. When there is a deviation in the sum of the voltages of the upper and lower bridge arms of the three phases, the common-mode voltage will be coupled and transmitted through the parasitic capacitance between the transformer windings, causing distortion of the transformer excitation current, resulting in increased iron and copper losses, reduced system operating efficiency, and potentially causing local overheating.

[0003] In existing technologies, the impact of common-mode voltage is usually reduced by adding filtering devices, optimizing structural layout, or introducing additional suppression circuits. However, these methods often increase the complexity of the system structure, increase system cost, and may bring additional power loss, making it difficult to achieve effective suppression of common-mode voltage while ensuring system efficiency and control performance.

[0004] Therefore, how to effectively suppress the common-mode voltage of modular multilevel converters from the modulation control strategy level without significantly increasing the system hardware complexity or reducing the system operating efficiency has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] In view of the above-mentioned problems, the present invention provides a modulation method, system, device and medium for common-mode voltage suppression of modular multilevel converters.

[0006] Therefore, the problem to be solved by this invention is: how to effectively suppress the common-mode voltage of a modular multilevel converter from the modulation control strategy level without significantly increasing the system hardware complexity or reducing the system operating efficiency.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a modulation method for common-mode voltage suppression in a modular multilevel converter, comprising: receiving a three-phase modulation signal; determining the candidate number of sub-modules to be put into operation for each phase arm; constructing sub-module input combinations based on the candidate number of sub-modules to be put into operation for each phase arm, and calculating the total number of sub-modules to be put into operation for each combination; selecting target combinations from the sub-module input combinations that satisfy the constant relationship of the total number of inputs, with the constraint that the total number of conducting sub-modules in the upper and lower three-phase arms is equal; and determining the actual number of sub-modules to be put into operation for each phase arm based on the target combinations.

[0008] As a preferred embodiment of the modulation method for common-mode voltage suppression in a modular multilevel converter according to the present invention, the step of determining the candidate number of each phase bridge arm submodule includes: obtaining the reference modulation voltage value of each phase based on the three-phase modulation signal; performing a rounding operation on the reference modulation voltage value of each phase to obtain the basic number of corresponding bridge arm submodules; and determining two adjacent numbers of corresponding phase bridge arm submodules as candidate numbers based on the deviation relationship between the reference modulation voltage value and the basic number of numbers, wherein the two adjacent numbers are the rounded-down value and the rounded-up value corresponding to the basic number of numbers, respectively.

[0009] As a preferred embodiment of the modulation method for common-mode voltage suppression of a modular multilevel converter as described in this invention, the step of constructing a submodule input combination based on the candidate input quantity of each phase bridge arm submodule includes: using two candidate input quantities of each phase bridge arm submodule as a candidate set; performing combination enumeration on the candidate input quantities of each phase bridge arm submodule to construct a submodule input combination.

[0010] As a preferred embodiment of the modulation method for common-mode voltage suppression of a modular multilevel converter as described in this invention, wherein: the sub-module input combination is formed by selecting one candidate input quantity combination from each phase bridge arm sub-module candidate input quantity in its respective candidate set; the total input quantity of the bridge arm sub-module is obtained by summing the input quantities of the three-phase bridge arm sub-modules corresponding to each group of sub-module input combinations.

[0011] As a preferred embodiment of the modulation method for common-mode voltage suppression of a modular multilevel converter according to the present invention, the method for selecting a target combination that satisfies a constant total number of inputs from the sub-module input combinations includes: determining the quantity deviation category of the corresponding sub-module input combination based on the total number of inputs of the bridge arm sub-modules; determining the corresponding candidate sub-module input combination selection range according to the quantity deviation category; and selecting sub-module input combinations that satisfy a preset constant total number of inputs of bridge arm sub-modules within the candidate sub-module input combination selection range, and using them as the target combination.

[0012] As a preferred embodiment of the modulation method for common-mode voltage suppression of a modular multilevel converter according to the present invention, wherein: the quantity deviation category is used to characterize the deviation of the total number of bridge arm sub-modules corresponding to each sub-module input combination from the preset benchmark total number of inputs; the preset benchmark total number of inputs is the target total number of bridge arm sub-modules in each phase, used to characterize the total number of bridge arm sub-modules in operation when the total number of in operation sub-modules in the upper and lower three-phase bridge arms is equal; the constant relationship of the total number of inputs is: the total number of bridge arm sub-modules corresponding to the selected sub-module input combination is equal to the preset benchmark total number of inputs.

[0013] As a preferred embodiment of the modulation method for common-mode voltage suppression of a modular multilevel converter according to the present invention, the quantity deviation category includes at least three categories: the total number of corresponding bridge arm sub-modules is equal to the preset reference total number, lower than the preset reference total number, and higher than the preset reference total number; after determining the actual number of sub-modules in each phase bridge arm according to the target combination, the sub-module control signal is output to drive the modular multilevel converter to operate.

[0014] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a modulation system for common-mode voltage suppression of a modular multilevel converter, comprising: a candidate input quantity calculation module, an input combination construction module, and a screening module; the candidate input quantity calculation module is used to receive a three-phase modulation signal and determine the candidate input quantity of each phase bridge arm sub-module; the input combination construction module constructs sub-module input combinations based on the candidate input quantities of each phase bridge arm sub-module and calculates the total input quantity of bridge arm sub-modules corresponding to each combination; the screening module is used to select target combinations that satisfy the constant relationship of total input quantity from the sub-module input combinations, with the constraint that the total number of conducting sub-modules of the three-phase upper and lower bridge arms is equal, and determine the actual input quantity of sub-modules of each phase bridge arm based on the target combinations.

[0015] A computer device includes a memory and a processor, the memory storing a computer program, the processor executing the computer program to implement the steps of a modulation method for common-mode voltage suppression of a modular multilevel converter as described above.

[0016] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of a modulation method for common-mode voltage suppression of a modular multilevel converter as described above.

[0017] The beneficial effects of this invention are as follows: This invention reduces the amplitude of the common-mode voltage on the AC side, thereby reducing electromagnetic interference, electrical insulation stress, and additional losses caused by the common-mode voltage. At the same time, it eliminates the need for additional hardware suppression circuits, which helps to reduce the complexity of the system structure and operating costs. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1This is a flowchart of a modulation method for common-mode voltage suppression in a modular multilevel converter, as described in Example 1.

[0020] Figure 2 This is an experimental diagram of a modulation method for common-mode voltage suppression in a modular multilevel converter, as shown in Example 2. Detailed Implementation

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0022] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0023] Example 1, referring to Figure 1 This is the first embodiment of the present invention, which provides a modulation method for common-mode voltage suppression in a modular multilevel converter, comprising: S1: Receive three-phase modulation signals and determine the number of candidate modules to be put into operation for each phase bridge arm.

[0024] S2: Construct submodule input combinations based on the alternative input quantities of each phase bridge arm submodule, and calculate the total input quantity of bridge arm submodules corresponding to each combination.

[0025] S3: Using the constraint that the total number of conducting sub-modules of the three-phase upper arm and lower arm is equal, select the target combination from the sub-module input combination that satisfies the constant relationship of the total input quantity.

[0026] S4: Determine the actual number of sub-modules to be deployed in each phase arm based on the target combination.

[0027] It should be noted that under certain operating conditions, it is difficult to maintain a consistent number of three-phase upper and lower bridge arm submodules, which can easily lead to the generation of common-mode voltage, resulting in increased transformer losses and accelerated insulation aging. At the same time, existing technologies often reduce the impact of common-mode voltage by adding filtering devices or additional suppression circuits, which not only increases the system hardware complexity and operating costs, but may also bring additional power losses.

[0028] Therefore, in response to the above problems, such as Figure 1As shown, through steps S1-S4, discrete candidate construction is carried out for the number of bridge arm sub-modules to be put into operation during the modulation control process. A constant constraint on the total number of bridge arm sub-modules to be put into operation is introduced in multiple sub-module combinations. The candidate combinations are screened and optimized to keep the overall three-phase upper and lower bridge arm conducting sub-modules coordinated and consistent, thereby avoiding the generation of common mode voltage.

[0029] Example 2, refer to Figure 2 This is the second embodiment of the present invention, which differs from the first embodiment in that: a modulation method for common-mode voltage suppression of a modular multilevel converter further includes, in step S1, determining the number of candidate modules for each phase bridge arm submodule, which includes the following steps A1-A3: A1: Obtain the reference modulation voltage values ​​for each phase based on the three-phase modulation signals; A2: Round the reference modulation voltage value of each phase to obtain the basic number of corresponding bridge arm sub-modules; A3: Based on the deviation relationship between the reference modulation voltage value and the basic input quantity, determine the two adjacent input quantities of the corresponding phase bridge arm submodule as the candidate input quantities, where the two adjacent input quantities are the floor value and floor value corresponding to the basic input quantity, respectively.

[0030] Specifically, taking a five-level modular multilevel converter (MMC) with four sub-modules as an example, since the output voltages of the upper and lower bridge arms must be symmetrical (same magnitude, opposite direction), taking any phase as an example, assuming the continuous modulation signal is... , For MMC converters The actual signal output by the upper or lower bridge arm is represented as: in, [] indicates rounding, and [] indicates integer rounding. ; The floor value corresponding to the basic investment amount. This is the rounded-up value corresponding to the basic investment amount.

[0031] To further explain, the generation of the bridge arm voltage signal includes: the vector form of the following bridge arm modulation signal. for, in, , and These are the modulation signals for the lower bridge arms of the three phases a, b, and c, respectively, representing the actual number of sub-modules engaged in each phase.

[0032] When the modulation signal is a three-phase sinusoidal voltage, we can obtain: in, The modulation ratio, The initial phase angle, For the initial phase, This represents the number of submodules.

[0033] Furthermore, in step S2, constructing the submodule input combination based on the candidate input quantity of each phase bridge arm submodule includes the following steps B1-B2: B1: The candidate set is the two alternative deployment quantities of each phase bridge arm submodule.

[0034] B2: Combine and enumerate the number of candidate inputs for each phase bridge arm submodule to construct the input combination of submodules.

[0035] The submodule input combination is formed by selecting one candidate input quantity from each phase bridge arm submodule's candidate input quantity in its respective candidate set.

[0036] The total number of bridge arm submodules deployed is obtained by summing the number of three-phase bridge arm submodules deployed for each group of submodules, expressed as follows: , , .

[0037] Specifically, each phase is based on , With two different numbers of submodules as alternative methods, there are a total of 8 possible combinations for the abc three-phase configuration, as shown in Table 1: Table 1. List of Alternative Combinations

[0038] Furthermore, in step S3, selecting target combinations from the sub-module input combinations that satisfy the constant relationship of total input quantity includes the following steps C1-C3: C1: Determine the quantity deviation category of the corresponding sub-module input combination based on the total number of bridge arm sub-modules; C2: Determine the range of candidate sub-modules to be combined and screened based on the category of quantity deviation; C3: Within the candidate submodule input combination screening range, select submodule input combinations that satisfy the preset relationship of constant total input quantity of bridge arm submodules, and use them as target combinations.

[0039] To further explain, the quantity deviation category is used to characterize the deviation of the total number of bridge arm sub-modules invested in each sub-module investment combination from the preset benchmark total number of investments; the preset benchmark total number of investments is the target total number of bridge arm sub-modules to be turned on for each phase, and is used to characterize the total number of bridge arm sub-modules to be invested when the total number of the three-phase upper and lower bridge arm conducting sub-modules is equal.

[0040] The total investment quantity is constant because the total investment quantity of the bridge arm sub-modules corresponding to the selected sub-module investment combination is equal to the preset baseline total investment quantity.

[0041] The quantity deviation categories include at least three categories: the total number of corresponding bridge arm sub-modules is equal to the preset benchmark total number of modules, is lower than the preset benchmark total number of modules, and is higher than the preset benchmark total number of modules.

[0042] Specifically, the actual number of sub-modules deployed in the following bridge arm for, Because the three-phase modulation signal is symmetrical, =0; at the same time, because It can be known that It can take three values: 0, -1, and -2. The possible values ​​are 3n / 2-1, 3n / 2, and 3n / 2+1.

[0043] In this embodiment, let 3n / 2 be the preset baseline total input quantity, when When the value is 3n / 2-1, the quantity deviation category is that the total number of corresponding bridge arm sub-modules is lower than the preset benchmark total number of modules; when When the value is 3n / 2, the quantity deviation category is that the total number of corresponding bridge arm sub-modules invested is equal to the preset baseline total number of invested modules; when the value is... When the value is 3n / 2+1, the quantity deviation category is that the total number of corresponding bridge arm sub-modules invested is higher than the preset benchmark total number of invested.

[0044] To eliminate the common-mode voltage generated by the MMC converter, the number of submodules in the conducting state of the three lower arms must be equal to that of the three upper arms, that is, the total number must be kept at 3n / 2.

[0045] Under direct voltage output modulation, there are three actual number of conducting submodules: 3n / 2-1, 3n / 2, and 3n / 2+1. To eliminate common-mode voltage, it is necessary to turn off (3n / 2+1 case) or add a conducting submodule (3n / 2-1 case) to ensure that the total number of conducting submodules in the upper and lower bridge arms is equal to 3n / 2.

[0046] Therefore, based on the category of quantity deviation, and =0, -1, or -2, provided that the total number of conducting submodules in the upper and lower bridge arms equals 3n / 2, select the corresponding combination from Table 1 that meets the conditions, specifically including: such as If the result is 0, select a combination from groups numbered 1 and 2 where the sum of the number of three candidate modules equals 3n / 2; for example... The result is -1. From groups numbered 3, 4, and 5, select combinations where the sum of the number of three candidate modules equals 3n / 2. The result is -2. Select a combination from groups numbered 6, 7, and 8 where the sum of the number of candidate modules equals 3n / 2.

[0047] Furthermore, in step S4, after determining the actual number of sub-modules to be put into operation for each phase arm based on the target combination, the sub-module control signal is output to drive the modular multilevel converter to run.

[0048] To verify and illustrate the technical effects employed in this method, experimental results are compared using scientific methods to confirm the actual effectiveness of the method. Figure 2 As shown, for an 11-level MMC converter, when the DC bus voltage is 5000V, direct voltage output modulation is used before 0.25s, and the common-mode voltage is relatively large, close to ±600V; after 0.25s, after using the invented common-mode voltage suppression modulation method, the common-mode voltage drops to about ±50V.

[0049] Example 3, the third embodiment of the present invention, differs from the previous two embodiments in that: a modulation system for common-mode voltage suppression of a modular multilevel converter includes: a candidate input quantity calculation module, an input combination construction module, and a screening module; the candidate input quantity calculation module is used to receive three-phase modulation signals and determine the candidate input quantity of each phase bridge arm sub-module; the input combination construction module constructs sub-module input combinations based on the candidate input quantities of each phase bridge arm sub-module and calculates the total input quantity of bridge arm sub-modules corresponding to each combination; the screening module is used to select target combinations that satisfy the constant relationship of total input quantity from the sub-module input combinations, with the constraint that the total number of conducting sub-modules of the three-phase upper and lower bridge arms is equal, and determines the actual input quantity of sub-modules of each phase bridge arm based on the target combinations.

[0050] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, essentially, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0051] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a sequenced list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (such as a computer-based system, a processor-including system, or other system that can fetch and execute instructions from, an instruction execution system, apparatus, or device). For the purposes of this specification, "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transmit programs for use by, or in conjunction with, an instruction execution system, apparatus, or device.

[0052] More specific examples of computer-readable media (a non-exhaustive list) include: electrical connections (electronic devices) having one or more wires, portable computer disk drives (magnetic devices), random access memory (RAM), read-only memory (ROM), erasable and editable read-only memory (EPROM or flash memory), fiber optic devices, and portable optical disc read-only memory (CDROM). Furthermore, computer-readable media can even be paper or other suitable media on which the program can be printed, because the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpreting, or otherwise processing as necessary, and then stored in computer memory.

[0053] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented in combination with any of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0054] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A modulation method for common-mode voltage suppression in a modular multilevel converter, characterized in that: include, Receive three-phase modulation signals and determine the number of candidate modules to be put into operation for each phase bridge arm; Construct submodule deployment combinations based on the alternative deployment quantities of each phase bridge arm submodule, and calculate the total deployment quantity of bridge arm submodules corresponding to each combination; With the constraint that the total number of conducting sub-modules of the three-phase upper arm and lower arm is equal, a target combination that satisfies the constant relationship of the total number of inputs is selected from the input combinations of the sub-modules. The actual number of sub-modules deployed in each phase arm is determined based on the target combination.

2. The modulation method for common-mode voltage suppression in a modular multilevel converter as described in claim 1, characterized in that, The determination of the alternative deployment quantity for each phase bridge arm submodule includes: Based on the three-phase modulation signal, the reference modulation voltage value of each phase is obtained respectively; The basic number of bridge arm submodules is obtained by rounding down the reference modulation voltage values ​​of each phase. Based on the deviation relationship between the reference modulation voltage value and the basic input quantity, two adjacent input quantities of the corresponding phase bridge arm submodule are determined as candidate input quantities, wherein the two adjacent input quantities are the floor value and floor value corresponding to the basic input quantity, respectively.

3. The modulation method for common-mode voltage suppression in a modular multilevel converter as described in claim 2, characterized in that, The construction of submodule input combinations based on the alternative input quantities of each phase bridge arm submodule includes: The candidate set is defined by the two alternative deployment quantities of each phase bridge arm submodule; The number of candidate inputs for each phase bridge arm submodule is combined and enumerated to construct the input combination of submodules.

4. The modulation method for common-mode voltage suppression in a modular multilevel converter as described in claim 3, characterized in that, The submodule input combination is formed by selecting one candidate input quantity from each phase bridge arm submodule's candidate set and combining them. The total number of bridge arm sub-modules is obtained by summing the number of three-phase bridge arm sub-modules corresponding to each group of sub-modules.

5. The modulation method for common-mode voltage suppression in a modular multilevel converter as described in claim 4, characterized in that, Selecting target combinations from the sub-module input combinations that satisfy a constant total input quantity includes: Determine the quantity deviation category of the corresponding sub-module input combination based on the total number of bridge arm sub-modules; The range of candidate sub-modules to be combined and screened is determined based on the aforementioned quantity deviation category. Within the candidate submodule input combination screening range, submodule input combinations that satisfy the preset relationship of a constant total input quantity of bridge arm submodules are selected as target combinations.

6. The modulation method for common-mode voltage suppression in a modular multilevel converter as described in claim 5, characterized in that, The quantity deviation category is used to characterize the deviation of the total number of bridge arm sub-modules corresponding to each sub-module input combination from the preset benchmark total number of inputs. The preset benchmark total number of inputs is the target total number of conduction sub-modules of each phase bridge arm, which is used to characterize the total number of input bridge arm modules when the total number of conduction sub-modules of the three-phase upper bridge arm and lower bridge arm is equal. The constant relationship of the total investment quantity is as follows: the total investment quantity of the bridge arm sub-modules corresponding to the selected sub-module investment combination is equal to the preset benchmark total investment quantity.

7. The modulation method for common-mode voltage suppression in a modular multilevel converter as described in claim 6, characterized in that, The quantity deviation categories include at least three categories: the total number of corresponding bridge arm sub-modules invested is equal to the preset benchmark total number of invested, lower than the preset benchmark total number of invested, and higher than the preset benchmark total number of invested; After determining the actual number of sub-modules in each phase arm based on the target combination, the sub-module control signal is output to drive the modular multilevel converter to operate.

8. A modulation method system for common-mode voltage suppression in modular multilevel converters, employing the modulation method for common-mode voltage suppression in modular multilevel converters as described in any one of claims 1 to 7, characterized in that, include: The module includes a candidate input quantity calculation module, an input combination construction module, and a screening module. The alternative input quantity calculation module is used to receive the three-phase modulation signal and determine the alternative input quantity of each phase bridge arm sub-module. The input combination construction module constructs sub-module input combinations based on the alternative input quantities of each phase bridge arm sub-module, and calculates the total input quantity of bridge arm sub-modules corresponding to each combination. The screening module is used to select a target combination from the sub-module input combinations that satisfies the constant relationship of the total input quantity, with the constraint that the total number of conducting sub-modules of the three-phase upper arm and lower arm are equal, and to determine the actual number of sub-modules put into each phase arm based on the target combination.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: When the processor executes the computer program, it implements the steps of a modulation method for common-mode voltage suppression of a modular multilevel converter as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by the processor, it implements the steps of the modulation method for common-mode voltage suppression of a modular multilevel converter as described in any one of claims 1 to 7.

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