MMC common mode voltage suppression method, device and system

Abstract

The invention discloses a method, device and system for suppressing common-mode voltage of MMC, belonging to the field of voltage converter, and the method comprises: constructing multiple groups of alternative switch vectors for MMC according to the modulation strategy adopted by MMC; respectively calculating the sum of the number of MMC submodules put into operation in the three-phase upper bridge arm or lower bridge arm under each group of alternative switch vectors, and dividing the alternative switch vectors with the sum of 3N / 2-1, 3N / 2, and 3N / 2+1 into categories I, II, and III respectively; calculating the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each group of alternative switch vectors; if the alternative switch vector corresponding to the minimum deviation is category II, the MMC is controlled according to the alternative switch vector corresponding to the minimum deviation, otherwise, according to whether the circulating current can be suppressed under the alternative switch vector corresponding to the minimum deviation, the corresponding alternative switch vector is selected to control the MMC. While suppressing the common-mode voltage, THD optimization and circulating current suppression are achieved, without adding additional power devices, and applicable to any topology.

Description

Technical Field

[0001] The present invention belongs to the field of voltage converters, and more specifically, relates to a method, device and system for suppressing common-mode voltage of MMC. Background Art

[0002] Modular multilevel converter (MMC) has broad application prospects in DC transmission, motor drive, and renewable energy integration due to its good modularity, flexible scalability, and good waveform quality. Conventional MMC modulation methods will generate common-mode voltage, thereby inducing common-mode current to further propagate and derive electromagnetic interference, threatening the safe operation of the converter itself. In photovoltaic systems, common-mode voltage will generate leakage current to damage photovoltaic panels; in motor systems, common-mode voltage will be converted into shaft voltage to endanger motor operation. At the same time, since the common-mode voltage is proportional to the sub-module voltage, with the increase in the rated voltage of power devices, especially the expansion of the application of SiC devices in MMC, the threat of common-mode voltage will increase further in the future.

[0003] The existing method of suppressing the common-mode voltage of MMC is mainly achieved by screening the zero common-mode voltage vector in the MMC switch vector. A major drawback of this method is that it will cause the total harmonic distortion (THD) of the MMC AC side to increase. In the future, as the device cut-off voltage increases and the number of sub-modules decreases, this defect will become more obvious, reducing the quality of the output waveform. At the same time, how to suppress the capacitor voltage fluctuation of the MMC sub-module is also a problem faced by MMC optimization. Summary of the invention

[0004] In view of the defects and improvement needs of the prior art, the present invention provides a method, device and system for suppressing common-mode voltage of MMC, which aims to achieve THD optimization and circulating current suppression while suppressing common-mode voltage.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present invention, a method for suppressing common-mode voltage of MMC is provided, comprising: S1, constructing multiple groups of alternative switch vectors for MMC according to the modulation strategy adopted by MMC, wherein the alternative switch vectors are used to characterize the number of MMC submodules put into operation in each bridge arm of MMC; S2, respectively calculating the sum of the number of MMC submodules put into operation in the three-phase upper bridge arm or the three-phase lower bridge arm under each group of alternative switch vectors, and classifying the alternative switch vectors with a sum of 3N / 2-1 into Class I, the alternative switch vectors with a sum of 3N / 2 into Class II, and the alternative switch vectors with a sum of 3N / 2+1 into Class III, where N is the number of MMC submodules in any bridge arm; S3, respectively calculating the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each group of alternative switch vectors; S4, if the alternative switch vector corresponding to the minimum deviation is Class II, controlling the MMC according to the alternative switch vector corresponding to the minimum deviation, otherwise, selecting the corresponding alternative switch vector to control the MMC according to whether the circulating current can be suppressed under the alternative switch vector corresponding to the minimum deviation.

[0006] Furthermore, when the alternative switching vector corresponding to the minimum deviation is Class I or Class III, if the alternative switching vector corresponding to the minimum deviation suppresses the circulating current, the MMC is controlled according to the alternative switching vector corresponding to the minimum deviation; otherwise, the MMC is controlled according to the alternative switching vector with the smallest deviation in Class II.

[0007] Furthermore, when the alternative switching vector corresponding to the minimum deviation is of Class I or Class III, if the direction of the circulating current under the alternative switching vector corresponding to the minimum deviation is opposite to the changing direction of the circulating current, the circulating current can be suppressed, otherwise, the circulating current cannot be suppressed.

[0008] Furthermore, the modulation strategy is an NLM modulation strategy, and the number of MMC submodules put into use in the j-phase bridge arm is [n j,ref ] or [n j,ref ]+1, j=A, B, C, 8 groups of candidate switch vectors are constructed in S1;

[0009]

[0010]

[0011]

[0012] Among them, n j,ref is the theoretical value of the number of MMC submodules in the j-phase bridge arm, m is the modulation ratio, ω0 is the fundamental frequency, is the initial phase angle, and t is the time.

[0013] Furthermore, the deviation is:

[0014]

[0015]

[0016]

[0017] Where, ΔV x is the deviation, n jo,ref is the target modulation value of phase j on the AC side of the MMC, n jo,x is the actual output value of phase j on the AC side of the MMC, n jn,ref 、n jp,ref are the target values ​​of the number of upper and lower bridge arm MMC submodules in the j-phase MMC, n jn,x 、n jp,x They are the actual values ​​of the number of upper and lower bridge arm MMC sub-modules in the j-phase of the MMC, j=A, B, C.

[0018] Furthermore, in S4, the MMC submodules in each bridge arm are sorted respectively, and the corresponding MMC submodules are put into operation or removed in real time according to the finally determined candidate switch vectors and the sorting result.

[0019] According to another aspect of the present invention, a common-mode voltage suppression device for MMC is provided, comprising: a construction module, for constructing multiple groups of alternative switch vectors for the MMC according to the modulation strategy adopted by the MMC, wherein the alternative switch vectors are used to characterize the number of MMC submodules put into operation in each bridge arm of the MMC; a division module, for respectively calculating the sum of the number of MMC submodules put into operation in the three-phase upper bridge arm or the three-phase lower bridge arm under each group of alternative switch vectors, and classifying the alternative switch vectors with a sum of 3N / 2-1 into Class I, the alternative switch vectors with a sum of 3N / 2 into Class II, and the alternative switch vectors with a sum of 3N / 2+1 into Class III, where N is the number of MMC submodules in any bridge arm; a calculation module, for respectively calculating the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each group of alternative switch vectors; and a selection and control module, for controlling the MMC according to the alternative switch vector corresponding to the minimum deviation if the alternative switch vector corresponding to the minimum deviation is Class II, otherwise, selecting the corresponding alternative switch vector to control the MMC according to whether the circulating current can be suppressed under the alternative switch vector corresponding to the minimum deviation.

[0020] According to another aspect of the present invention, there is provided an MMC common-mode voltage suppression system, comprising: an MMC and a control unit, wherein each bridge arm of the MMC comprises N MMC sub-modules connected in series, and the control unit is used to execute the MMC common-mode voltage suppression method as described above to control the MMC.

[0021] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0022] (1) A method for suppressing the common-mode voltage of MMC is proposed. Based on the NLM modulation strategy, in addition to selecting the switch combination with the sum of the bridge arm submodules being N, the switch combinations with the sum of the switches being N-1 and N+1 are also designed. The selection of the N-1, N, and N+1 switch combinations is determined by the circulating current state. More switch vectors are utilized to achieve the suppression of the common-mode voltage of MMC while avoiding the problem of increased AC current THD caused by the traditional method, thereby optimizing the output power quality.

[0023] (2) When the circulating current is greater than 0, the N+1 switch combination is selected, and when the circulating current is less than 0, the N-1 switch combination is selected, so that the change in the number of sub-modules put into operation on the DC side is conducive to the suppression of the circulating current, thereby reducing the voltage fluctuation of the capacitor of the MMC sub-module, which is conducive to reducing the volume of the MMC and realizing the lightweight device; at the same time, since the input and output of the three-phase upper bridge arm and the input and output of the three-phase lower bridge arm are always consistent, the common mode voltage suppression effect is maintained;

[0024] (3) The MMC common mode voltage suppression method that can achieve THD optimization and circulating current suppression does not require changing the device structure of the MMC and is applicable to any submodule and system topology; in addition, the method does not cause changes in the MMC output, and is therefore compatible with any MMC system layer control;

[0025] (4) In view of the situation where the cut-off voltage of devices will gradually increase in the future, compared with the traditional method, the advantages of the MMC common-mode voltage suppression method that can achieve THD optimization and circulating current suppression will be more obvious, with long-term value and rich application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A flowchart of a method for suppressing common-mode voltage of MMC provided by an embodiment of the present invention;

[0027] Figure 2 A diagram of the implementation process of the MMC common mode voltage suppression method provided by an embodiment of the present invention;

[0028] Figure 3 A schematic diagram of an MMC topology and common mode voltage provided by an embodiment of the present invention;

[0029] Figure 4 A schematic diagram of MMC switch vector grouping provided by an embodiment of the present invention;

[0030] Figure 5 A schematic diagram of an MMC common mode voltage suppression method based on an NLM modulation strategy and rounding up and down in an embodiment of the present invention;

[0031] Figure 6A schematic diagram of the nearest switching vector and the required modulation vector of the three categories I, II and III provided in an embodiment of the present invention;

[0032] Figure 7 A schematic diagram comparing the MMC common-mode voltage suppression method provided by an embodiment of the present invention, a method without common-mode voltage suppression, and a method with a traditional common-mode voltage suppression;

[0033] Figure 8 A block diagram of an MMC common-mode voltage suppression device provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] In the present invention, the terms "first", "second", etc. (if any) in the present invention and the accompanying drawings are used to distinguish similar objects but not necessarily to describe a specific order or sequence.

[0036] Figure 1 Flow chart of the MMC common mode voltage suppression method provided by the embodiment of the present invention. Figure 1 , combined with Figure 2-Figure 7 , the MMC common mode voltage suppression method in this embodiment is described in detail, and the method includes operations S1 to S4.

[0037] Operation S1: construct multiple groups of candidate switching vectors for the MMC according to the modulation strategy adopted by the MMC. The candidate switching vectors are used to represent the number of MMC submodules put into operation in each bridge arm of the MMC.

[0038] In this embodiment, the MMC topology is as follows Figure 3 See Figure 3 The MMC includes a plurality of bridge arms, each of which (i.e., the upper bridge arm or the lower bridge arm of each phase) is provided with N series-connected MMC submodules SM1, SM2, ..., SM N , the structure of each MMC submodule is as follows Figure 3 The middle half-bridge submodule is shown in FIG. N is a set value, which can be determined according to the application scenario of the MMC.

[0039] Preferably, in this embodiment, the modulation strategy adopted by the MMC is the Nearest Level Modulation (NLM) strategy. Under the NLM modulation strategy, the number of MMC submodules in the j-phase bridge arm is [nj,ref ] or [n j,ref ]+1, j=A, B, C, where [] is a rounding operation, such as Figure 5 shown.

[0040]

[0041]

[0042]

[0043] Among them, n j,ref is the theoretical value of the number of MMC submodules in the j-phase bridge arm, m is the modulation ratio, ω0 is the fundamental frequency, is the initial phase angle, and t is the time.

[0044] Therefore, in operation S1, 8 groups of candidate switch vectors are constructed, which are: {[n A,ref ],[n B,ref ],[n C,ref ]}、{[n A,ref ],[n B,ref ]+1,[n C,ref ]}、{[n A,ref ],[n B,ref ],[n C,ref ]+1}, {[n A,ref ],[n B,ref ]+1,[n C,ref ]+1}, {[n A,ref ]+1,[n B,ref ],[n C,ref ]}、{[n A,ref ]+1,[n B,ref ]+1,[n C,ref ]}、{[n A,ref ]+1,[n B,ref ],[n C,ref ]+1}, {[n A,ref ]+1,[n B,ref ]+1,[n C,ref ]+1}. The three parameters in the candidate switch vector represent the number of MMC submodules put into the A, B, and C phase bridge arms respectively. The number of MMC submodules put into the upper and lower bridge arms of the same phase is the same.

[0045] Operation S2, respectively calculates the sum of the number of MMC sub-modules put into operation in the three-phase upper bridge arm or the three-phase lower bridge arm under each group of alternative switching vectors, and divides the alternative switching vectors whose sum is 3N / 2-1 into Class I, the alternative switching vectors whose sum is 3N / 2 into Class II, and the alternative switching vectors whose sum is 3N / 2+1 into Class III, where N is the number of MMC sub-modules in any bridge arm.

[0046] See also Figure 4 For any set of candidate switching vectors, taking the upper bridge arm as an example, the number of MMC submodules put into the three-phase upper bridge arm is added, and the addition results are 3N / 2-1, 3N / 2, and 3N / 2+1, corresponding to Class I, Class II, and Class III respectively. Therefore, the 8 sets of candidate switching vectors constructed in operation S1 are divided into three groups.

[0047] Operation S3 is to calculate the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each group of candidate switching vectors.

[0048] According to an embodiment of the present invention, the calculated deviation is:

[0049]

[0050]

[0051]

[0052] Where, ΔV x is the deviation, n jo,ref is the target modulation value of phase j on the AC side of the MMC, n jo,x is the actual output value of phase j on the AC side of the MMC, n jn,ref 、n jp,ref are the target values ​​of the number of upper and lower bridge arm MMC submodules in the j-phase MMC, n jn,x 、n jp,x They are the actual values ​​of the number of upper and lower bridge arm MMC sub-modules in the j-phase of the MMC, j=A, B, C.

[0053] Operation S4, if the alternative switching vector corresponding to the minimum deviation is of type II, the MMC is controlled according to the alternative switching vector corresponding to the minimum deviation; otherwise, the corresponding alternative switching vector is selected to control the MMC according to whether the circulating current can be suppressed under the alternative switching vector corresponding to the minimum deviation.

[0054] According to an embodiment of the present invention, when the alternative switching vector corresponding to the minimum deviation is Class I or Class III, if the alternative switching vector corresponding to the minimum deviation suppresses the circulating current, the MMC is controlled according to the alternative switching vector corresponding to the minimum deviation; otherwise, the MMC is controlled according to the alternative switching vector with the smallest deviation in Class II.

[0055] According to an embodiment of the present invention, when the alternative switching vector corresponding to the minimum deviation is Class I or Class III, if the direction of the circulating current under the alternative switching vector corresponding to the minimum deviation is opposite to the direction of change of the circulating current, the circulating current can be suppressed, otherwise, the circulating current cannot be suppressed.

[0056] Specifically, in this embodiment, the candidate switching vectors with the smallest deviation in the category are selected from the three categories respectively, and are respectively recorded as candidate switching vector I, candidate switching vector II, and candidate switching vector III. The relationship between these three candidate switching vectors and the target modulation vector is as follows: Figure 6 shown.

[0057] In this embodiment, the switching mode of the second type of candidate switching vector will not affect the circulation amount amplitude, the switching mode of the first type of candidate switching vector will increase the specific two-phase circulation amount, and the switching mode of the third type of candidate switching vector will reduce the specific two-phase circulation amount. Based on this, after selecting the above three candidate switching vectors, the following is performed: Figure 2 The method shown determines the final selected backup switching vector.

[0058] See also Figure 2 , if the alternative switching vector II is the smallest, that is, the alternative switching vector corresponding to the minimum deviation is of type II, the MMC is directly controlled according to the alternative switching vector II.

[0059] If the candidate switching vector I is the smallest, that is, the candidate switching vector corresponding to the minimum deviation is of type I, the circulating current of each phase will change in real time over time, and further judgment is required based on the circulating current: when the circulating current changes i cir_j When it is negative, the direction of the circulating current change is opposite to the circulating current direction, which plays a role in suppressing the circulating current. The MMC can be directly controlled according to the alternative switching vector I. cir_j When it is positive, the direction of circulation change is the same as the circulation direction, which will cause the circulation current to increase. The MMC should be controlled according to the alternative switching vector II.

[0060] If the candidate switching vector III is the smallest, that is, the candidate switching vector corresponding to the minimum deviation is of type III, the circulating current of each phase will change in real time over time, and further judgment is required based on the circulating current: when the circulating current changes i cir_j When it is positive, the direction of circulation change is opposite to the circulation direction, which plays a role in suppressing circulation. The MMC can be directly controlled according to the alternative switch vector III. cir_j When it is negative, the circulating current changes in the same direction as the circulating current, which will cause the circulating current to increase. The MMC should be controlled according to the alternative switching vector II.

[0061] According to an embodiment of the present invention, in operation S4, controlling the MMC according to the finally determined candidate switching vector includes: sorting the MMC submodules in each bridge arm respectively, and according to the finally determined candidate switching vector and the sorting result, putting the corresponding MMC submodule into operation or removing it in real time.

[0062] See also Figure 7, showing the control effect comparison diagram of the MMC common mode voltage suppression method of this embodiment, without adding common mode voltage suppression, and adding the traditional common mode voltage suppression method. Figure 7 It can be seen that compared with the other two methods, the MMC common-mode voltage suppression method in this embodiment greatly optimizes THD on the basis of achieving common-mode voltage suppression, and has a certain ability to suppress circulating current, reducing the voltage fluctuation of the MMC submodule capacitor. In addition, this method does not require the addition of additional power devices, is applicable to any topology, and has a wide range of applicable scenarios.

[0063] Figure 8 A block diagram of an MMC common mode voltage suppression device provided by an embodiment of the present invention. Figure 8 The MMC common mode voltage suppression device 800 includes a construction module 810 , a division module 820 , a calculation module 830 , and a selection and control module 840 .

[0064] The construction module 810, for example, executes operation S1 to construct multiple groups of candidate switching vectors for the MMC according to the modulation strategy adopted by the MMC, where the candidate switching vectors are used to represent the number of MMC submodules put into use in each bridge arm of the MMC.

[0065] The division module 820, for example, performs operation S2, which is used to respectively calculate the sum of the number of MMC sub-modules put into operation in the three-phase upper bridge arm or the three-phase lower bridge arm under each group of alternative switching vectors, and divide the alternative switching vectors with a sum of 3N / 2-1 into Class I, the alternative switching vectors with a sum of 3N / 2 into Class II, and the alternative switching vectors with a sum of 3N / 2+1 into Class III, where N is the number of MMC sub-modules in any bridge arm.

[0066] The calculation module 830, for example, performs operation S3 to respectively calculate the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each group of candidate switching vectors.

[0067] The selection and control module 840, for example, performs operation S4, which is used to control the MMC according to the alternative switching vector corresponding to the minimum deviation if the alternative switching vector corresponding to the minimum deviation is of Class II; otherwise, according to whether the circulating current can be suppressed under the alternative switching vector corresponding to the minimum deviation, select the corresponding alternative switching vector to control the MMC.

[0068] The MMC common mode voltage suppression device 800 is used to perform the above Figure 1-Figure 7 The MMC common mode voltage suppression method in the embodiment shown. For details not covered in this embodiment, please refer to the aforementioned Figure 1-Figure 7 The MMC common mode voltage suppression method in the illustrated embodiment will not be described in detail here.

[0069] The embodiment of the present invention also provides an MMC common mode voltage suppression system, including an MMC and a control unit. Each bridge arm of the MMC includes N MMC sub-modules connected in series, such as Figure 3 The control unit is used to execute the above Figure 1-Figure 7 The MMC common mode voltage suppression method in the illustrated embodiment is used to control the MMC.

[0070] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for suppressing common mode voltage of MMC, characterized in that: include: S1, constructing multiple groups of candidate switching vectors for the MMC according to the modulation strategy adopted by the MMC, wherein the candidate switching vectors are used to represent the number of MMC submodules put into use in each bridge arm of the MMC; S2, respectively calculate the sum of the number of MMC submodules in the three-phase upper bridge arm or the three-phase lower bridge arm under each group of alternative switch vectors, and classify the alternative switch vectors with a sum of 3N / 2-1 into Class I, the alternative switch vectors with a sum of 3N / 2 into Class II, and the alternative switch vectors with a sum of 3N / 2+1 into Class III, where N is the number of MMC submodules in any bridge arm; S3, respectively calculating the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each set of alternative switching vectors; S4, if the alternative switching vector corresponding to the minimum deviation is of type II, the MMC is controlled according to the alternative switching vector corresponding to the minimum deviation; otherwise, the corresponding alternative switching vector is selected to control the MMC according to whether the circulating current can be suppressed under the alternative switching vector corresponding to the minimum deviation; The modulation strategy is the NLM modulation strategy, and the number of MMC submodules put into use in the j-phase bridge arm is [n j,ref ] or [n j,ref ]+1, j=A, B, C, 8 groups of candidate switch vectors are constructed in S1; Among them, n j,ref is the theoretical value of the number of MMC submodules in the j-phase bridge arm, m is the modulation ratio, ω0 is the fundamental frequency, is the initial phase angle, and t is the time.

2. The MMC common mode voltage suppression method according to claim 1, characterized in that: When the alternative switching vector corresponding to the minimum deviation is of Class I or Class III, if the circulating current is suppressed under the alternative switching vector corresponding to the minimum deviation, the MMC is controlled according to the alternative switching vector corresponding to the minimum deviation; otherwise, the MMC is controlled according to the alternative switching vector with the minimum deviation in Class II.

3. The MMC common mode voltage suppression method according to claim 2, characterized in that: When the alternative switching vector corresponding to the minimum deviation is of Class I or Class III, if the direction of the circulating current under the alternative switching vector corresponding to the minimum deviation is opposite to the changing direction of the circulating current, the circulating current can be suppressed, otherwise, the circulating current cannot be suppressed.

4. The MMC common mode voltage suppression method according to any one of claims 1 to 3, characterized in that: The deviation is: Where, ΔV x is the deviation, n jo,ref is the target modulation value of phase j on the AC side of the MMC, n jo,x is the actual output value of phase j on the AC side of the MMC, n jn,ref 、n jp,ref are the target values ​​of the number of upper and lower bridge arm MMC submodules in the j-phase MMC, n jn,x 、n jp,x They are the actual values ​​of the number of upper and lower bridge arm MMC sub-modules in the j-phase of the MMC, j=A, B, C.

5. The MMC common mode voltage suppression method according to claim 1, characterized in that: In S4, the MMC submodules in each bridge arm are sorted respectively, and the corresponding MMC submodules are put into operation or removed in real time according to the finally determined candidate switch vectors and the sorting result.

6. An MMC common mode voltage suppression device, characterized in that: include: A construction module, used for constructing multiple groups of candidate switch vectors for the MMC according to the modulation strategy adopted by the MMC, wherein the candidate switch vectors are used to represent the number of MMC submodules put into use in each bridge arm of the MMC; A division module is used to calculate the sum of the number of MMC submodules put into operation in the three-phase upper bridge arm or the three-phase lower bridge arm under each group of alternative switch vectors, and divide the alternative switch vectors with a sum of 3N / 2-1 into Class I, the alternative switch vectors with a sum of 3N / 2 into Class II, and the alternative switch vectors with a sum of 3N / 2+1 into Class III, where N is the number of MMC submodules in any bridge arm; A calculation module, used for respectively calculating the deviation of the actual output vector of the MMC AC side relative to the target modulation vector under each group of alternative switching vectors; A selection and control module, for controlling the MMC according to the alternative switching vector corresponding to the minimum deviation if the alternative switching vector corresponding to the minimum deviation is of type II, otherwise, selecting the corresponding alternative switching vector to control the MMC according to whether the circulating current can be suppressed under the alternative switching vector corresponding to the minimum deviation; The modulation strategy is the NLM modulation strategy, and the number of MMC submodules put into use in the j-phase bridge arm is [n j,ref ]or]n j,ref [+1, j=A, B, C, 8 groups of candidate switch vectors are constructed in the construction module; Among them, n j,ref is the theoretical value of the number of MMC submodules in the j-phase bridge arm, m is the modulation ratio, ω0 is the fundamental frequency, is the initial phase angle, and t is the time.

7. An MMC common mode voltage suppression system, characterized in that: include: An MMC and a control unit, wherein each bridge arm of the MMC comprises N MMC sub-modules connected in series, and the control unit is used to execute the MMC common-mode voltage suppression method according to any one of claims 1 to 5 to control the MMC.

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