Fractional level MMC sub-module classification modulation method and system

Abstract

The invention belongs to the technical field of power electronics, and provides a fractional level MMC sub-module classification modulation method and system, and the method comprises the steps: carrying out the initial distribution of the number of input sub-modules according to the number of equivalent sub-modules to be input and the principle that only A-type sub-modules are input as much as possible when integer levels are output, and B-type equivalent sub-modules are input when fractional levels are output; taking the capacitor voltage of the sub-module as a control object, adopting a proportional-integral controller and combining the direction of the direct current side current to dynamically adjust the input number, and if the adjusted input number is not matched with the number of equivalent sub-modules to be input, correcting the input number to obtain a final input number; the two types of sub-modules are arranged in an ascending order or a descending order according to the respective capacitor voltages, and the switching state of the sub-modules is determined according to the respective final input number. On the premise that the level multiplication effect is not affected and the energy balance of the sub-modules is ensured, the two types of sub-modules can be independently sorted and switched according to the respective capacitor voltage and the input number, and the method has the advantages of being clear in principle and low in switching loss.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a fractional-level MMC submodule classification modulation method and system. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] The number of AC output voltage levels in a modular multilevel converter (MMC) is linearly proportional to the number of bridge arm submodules. When the number of submodules is small, the number of AC output levels in the MMC is low and may not meet voltage quality requirements. In order to solve the problem of low output voltage quality of the minority-level module MMC, the invention with patent number "ZL202010738177.7" and titled "A half-level MMC topology structure and its modulation method" discloses an MMC topology structure and its modulation method that can double the number of output levels; the document "Wang Y, Wang C, Tao J, et al. Topology and modulation of fractional-level MMC for DC distributiongrids[J]. IEEE Transactions on Industry Applications, 2023, 59(3): 3615-3626." named the MMC equipped with sub-modules of different rated capacitance voltages as fractional-level MMC, and the corresponding modulation method as fractional-level modulation; the document "Wang Chen, Yu Xinyang, Wang Yi, et al. An improved fractional-level modulation strategy suitable for fractional-level MMC[J]. High Voltage Technology, 2024, 50(8): 3680-3692." proposed an improved modulation strategy that can significantly reduce the switching loss of the fractional-level MMC based on the original modulation method.

[0004] For fractional-level MMC, existing modulation methods all use the concept of equivalent submodules, arranging both conventional and fractional-level submodules in the same sequence. This results in a complex submodule sorting and switching process. While maintaining the level multiplication effect, how to independently sort and switch the two submodules of fractional-level MMC and ensure energy balance between them is a question worthy of further research. Summary of the Invention

[0005] To address the above problems, the present invention proposes a fractional-level MMC sub-module classification modulation method and system. Without affecting the level multiplication effect and ensuring the energy balance of the sub-modules, the conventional level sub-modules and fractional level sub-modules can be independently sorted and switched. It has the advantages of clear principles and low switching loss.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A first aspect of the present invention provides a fractional-level MMC submodule classification modulation method, comprising the following steps:

[0008] Based on the number of equivalent submodules that should be put into use in each bridge arm of the fractional-level MMC, and the principle of only putting in use type A submodules when outputting integer levels and only putting in use type B equivalent submodules when outputting fractional levels, the number of put-in-use type A submodules and type B equivalent submodules is initially allocated;

[0009] Based on the initial number of submodules put into operation, the capacitor voltage of type A submodule and type B equivalent submodule is used as the control object. The proportional-integral controller is used in combination with the direction of the DC side current to dynamically adjust the number of submodules put into operation.

[0010] After dynamic adjustment, the number of type A submodules and the number of equivalent type B submodules put into operation may not match the number of equivalent submodules that should be put into operation in each bridge arm. Therefore, the number of submodules put into operation needs to be corrected to obtain the final number of submodules put into operation.

[0011] The A-type submodules and the B-type equivalent submodules are arranged in ascending or descending order according to their respective capacitor voltage magnitudes and bridge arm current directions, and the input and output states of each submodule are determined according to their respective final input numbers.

[0012] As a further implementation, the conventional level submodule is also called A-type submodule, the fractional level submodule is also called B-type submodule, and the single submodule contained in each B-type submodule is called an equivalent submodule. Taking the upper bridge arm of phase A as an example, the number of equivalent submodules N that should be put into this bridge arm is epa 、The initial investment number N of type A SM pa_A_init and the initial number N of equivalent submodules of type B pa_eB_init They are

[0013]

[0014] N pa_eB_init =N epa -k·N pa_A_init

[0015] Among them, u * pais the reference voltage of the upper bridge arm of phase A, U cn is the rated capacitance voltage of the conventional level submodule, k is the level multiplication rate, N1 is the number of conventional level submodules contained in each bridge arm, round is the rounding function, and floor is the flooring function.

[0016] As a further implementation method, in order to ensure the energy balance of the two types of sub-modules, a proportional-integral controller is used to control the capacitor voltage of type A sub-module and type B equivalent sub-module to the rated value, and the variable obtained based on the proportional-integral controller output and the DC side current direction is used as the dynamic adjustment amount of the sub-module input number.

[0017] As a further implementation method, the control object and control target of the proportional-integral controller can be selected from the following three forms:

[0018] The control object is the DC component of the average value of the capacitor voltage of the A-type submodule, and the control target is the rated value of the capacitor voltage of the A-type submodule U cn ;

[0019] The control object is the DC component of the average value of the capacitor voltage of the B-type equivalent submodule, and the control target is the rated value of the capacitor voltage of the B-type equivalent submodule U cn / k;

[0020] The control object is the DC component of the average value of the capacitor voltage of the type A submodule and the DC component of the average value of the capacitor voltage of the type B equivalent submodule. The control target is the rated value of the capacitor voltage of the type A submodule U cn and B-type equivalent submodule capacitor voltage rating U cn / k.

[0021] As a further implementation method, the direction of the DC side current will affect the charging and discharging of the capacitor of the constant input sub-module. It is necessary to monitor the direction of the DC side current to correct the output of the proportional-integral controller, thereby obtaining a dynamic adjustment amount for the number of sub-modules put into operation.

[0022] As a further implementation method, the variable obtained based on the proportional-integral controller output and the DC side current direction can be used as the dynamic adjustment amount ΔN of the number of A-type submodules put into operation. pa_A , or as a dynamic adjustment of the number of B-type equivalent submodules ΔN pa_eB .

[0023] As a further implementation method, it is necessary to invest N number of A-type submodules after dynamic adjustment. pa_A_adj and the number of equivalent submodules of type B N pa_eB_adj For correction, let the final number of A-type submodules be N pa_A The final number of B-type equivalent submodules is N pa_eB, the expressions of which are

[0024]

[0025] where ΔN epa =N epa -(k·N pa_A_adj +N pa_eB_adj ), mod is a remainder function.

[0026] The second aspect of the present application provides a fractional level MMC sub-module classification modulation system, comprising:

[0027] The initial allocation module of the number of inputs is configured to: according to the number of equivalent sub-modules that each bridge arm of the fractional level MMC should input, and the principle that A-type sub-modules are preferentially input when outputting integer level and B-type equivalent sub-modules are input only when outputting fractional level, initially allocate the number of A-type sub-modules and B-type equivalent sub-modules to be input;

[0028] The dynamic adjustment module of the number of inputs is configured to: on the basis of the initial number of sub-modules to be input, take the capacitance voltage of the A-type sub-module and the B-type equivalent sub-module as a control object, adopt a proportional-integral controller and combine the direction of the direct current side current to dynamically adjust the number of sub-modules to be input;

[0029] The final correction module of the number of inputs is configured to: after dynamic adjustment, the number of A-type sub-modules to be input and the number of B-type equivalent sub-modules to be input may not match the number of equivalent sub-modules that each bridge arm should input, so the number of sub-modules to be input needs to be corrected to obtain the final number of sub-modules to be input;

[0030] The switching state determination module is configured to: the A-type sub-module and the B-type equivalent sub-module are arranged in ascending order or descending order according to the capacitance voltage of each and the direction of the bridge arm current respectively, and the input and removal state of each sub-module is determined according to the final number of each to be input.

[0031] The third aspect of the present application provides a medium having a program stored thereon, which, when executed by a processor, implements the steps of the fractional level MMC sub-module classification modulation method of the first aspect of the present application.

[0032] The fourth aspect of the present application provides an electronic device comprising a memory, a processor, and a program stored on the memory and executable on the processor, wherein the processor implements the steps of the fractional level MMC sub-module classification modulation method of the first aspect of the present application when executing the program.

[0033] Compared with the prior art, the present application has the following beneficial effects:

[0034] The fractional level MMC sub-module classification modulation method of the application does not need to arrange the conventional sub-modules and the fractional level sub-modules into the same sequence based on the equivalent sub-module concept, avoids the complex sub-module sorting and switching process, and under the premise of not affecting the level multiplication effect and ensuring the sub-module energy balance, sequentially performs initial allocation, dynamic adjustment and final correction on the sub-module input number, so that the conventional level sub-modules and the fractional level sub-modules can be independently sorted and switched according to the respective capacitor voltage size and input number, compared with the prior art, has the advantages of clear principle and low switching loss.

[0035] Advantages of the additional aspects of the application will be partially given in the following description, partially will become obvious from the following description, or will be known by the practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0036] The drawings accompanying the specification of this application form a part thereof, serve to provide further understanding of the application, and together with the description of the exemplary embodiments of the application given below, serve to explain the application, and do not constitute an improper limitation of the application.

[0037] Figure 1 is a topological structure diagram of fractional level MMC;

[0038] Figure 2 is the initial input number allocation result of the sub-module;

[0039] Figure 3 is the overall control block diagram of the sub-module classification modulation method;

[0040] Figure 4 is a simulation structure diagram of fractional level MMC;

[0041] Figure 5 is the simulation result of the alternating output voltage and current;

[0042] Figure 6 is the simulation result of the sub-module capacitor voltage;

[0043] Figure 7 is the simulation result of the sub-module input number. DETAILED DESCRIPTION

[0044] The application will be further described below in combination with the drawings and embodiments.

[0045] It should be pointed out that the following detailed description is all exemplary, and aims to provide further description of the application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as generally understood by those skilled in the art to which the application belongs.

[0046] In the absence of conflict, the embodiments in this disclosure and the features in the embodiments can be combined with each other. It should be noted that the terms used here are only for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0047] Example 1

[0048] A first embodiment of the present invention provides a fractional-level MMC submodule classification modulation method, comprising the following steps:

[0049] Step 1: Based on the number of equivalent submodules that should be put into use in each bridge arm of the fractional-level MMC, and the principle of only putting in use type A submodules when outputting integer levels and only putting in use type B equivalent submodules when outputting fractional levels, initially allocate the number of type A submodules and type B equivalent submodules to be put into use;

[0050] Step 2: Based on the initial number of submodules, the capacitor voltages of type A submodules and equivalent type B submodules are used as control objects. A proportional-integral controller is used in combination with the direction of the DC side current to dynamically adjust the number of submodules.

[0051] Step 3: After dynamic adjustment, the number of type A submodules and the number of equivalent type B submodules in operation may not match the number of equivalent submodules that should be in operation in each bridge arm. Therefore, the number of submodules in operation needs to be corrected to obtain the final number of submodules in operation.

[0052] Step 4: Arrange the A-type submodules and the B-type equivalent submodules in ascending or descending order according to their respective capacitor voltages and bridge arm current directions, and determine the on / off status of each submodule according to their final on-board numbers.

[0053] A submodule classification modulation method of the present invention is based on a fractional level MMC topology structure including a conventional level submodule and a fractional level submodule, such as Figure 1 By replacing some conventional level submodules with fractional level submodules, fractional level MMC can be easily transformed from conventional MMC. Here, conventional level submodules and fractional level submodules are respectively referred to as type A submodules and type B submodules. The number of these two types of submodules in the bridge arm is N1 and N2 respectively. Figure 1 As shown, a B-type submodule contains k identical independent submodules, each of which is called an equivalent submodule, where k is the level multiplication rate; the capacitance value of the A-type submodule is C, and the rated capacitance voltage is U cn, the capacitor capacity of the equivalent sub-modules of the B type is k*C, and the rated capacitor voltage is U cn / k.

[0054] Taking the upper arm of the A phase of the fractional level MMC as an example, the number of the equivalent sub-modules N epa that should be put in can be expressed as

[0055]

[0056] In the formula, u * pa is the reference voltage of the upper arm of the A phase, and round is a rounding function.

[0057] Generally, the application expects to put in only the B type equivalent sub-modules when the output is a fractional level, and to put in only the A type sub-modules as much as possible when the output is an integer level. According to the above principle and the initial number of the sub-modules of formula (1), the initial number of the A type sub-modules N pa_A_init and the initial number of the B type equivalent sub-modules N pa_eB_init are obtained as

[0058]

[0059] N pa_eB_init =N epa -k*N pa_A_init (3)

[0060] In the formula, floor is a down rounding function. By substituting N1=4, N2=2 and k=2 into formula (2) and (3), Figure 2 the initial number of the A type sub-modules and the B type equivalent sub-modules is obtained.

[0061] In order to ensure the energy balance of the A type sub-modules and the B type equivalent sub-modules, the application adopts a proportional-integral controller to control the capacitor voltage of the sub-modules, and discloses three feasible implementation manners, namely:

[0062] (1) the control object is the direct current component of the average value u cpa_A_avg of the A type sub-module capacitor voltage, and the control target is the rated value U cn of the A type sub-module capacitor voltage;

[0063] (2) the control object is the direct current component of the average value u cpa_eB_avg of the B type equivalent sub-module capacitor voltage, and the control target is the rated value U cn / k of the B type equivalent sub-module capacitor voltage;

[0064] (3) the control object is the direct current component of the average value u cpa_A_avg of the A type sub-module capacitor voltage and the average value u cpa_eB_avgThe DC component of the control target is the rated value of the capacitor voltage of the A-type submodule U cn and B-type equivalent submodule capacitor voltage rating U cn / k.

[0065] The average value of the capacitor voltage of the submodule is obtained by collecting the capacitor voltage of the submodule, summing it and averaging it, and the DC component is obtained by passing it through a low-pass filter.

[0066] If the submodule is in a constant-on state, its capacitor energy storage will be determined by the DC component of the bridge arm current. Figure 1 The DC side current I dc The direction is positive, when I dc >0, the capacitor voltage of the submodule in constant input will increase due to charging; when I dc <0, the capacitor voltage of the submodule that is constantly put into operation will decrease due to discharge. dc The output of the proportional-integral controller is corrected in the direction of

[0067] The present invention will be based on the proportional-integral controller output and I dc The variables obtained in the direction are used as dynamic adjustment quantities for the number of submodules input. Two feasible implementation methods are disclosed, namely:

[0068] (1) Based on the proportional-integral controller output and I dc The variable obtained in the direction is used as the dynamic adjustment value ΔN of the number of A-type submodules pa_A ;

[0069] (2) Based on the proportional-integral controller output and I dc The variable obtained in the direction is used as the dynamic adjustment value ΔN of the number of B-type equivalent sub-modules pa_eB .

[0070] After dynamic adjustment, the number of A-type submodules N pa_A_adj and the number of inputs N of type B equivalent submodules pa_eB_adj The present invention corrects the number of submodules to be put into use, and the final number of A-type submodules put into use after correction is N. pa_A The final number of equivalent submodules of type B is N pa_eB Can be expressed as

[0071]

[0072] Where ΔN epa =N epa -(k·N pa_A_adj +N pa_eB_adj ), mod is the remainder function.

[0073] After obtaining the final number of submodules put into operation, the switching status of the two types of submodules can be determined independently:

[0074] (1) For type A submodule, according to the capacitor voltage u of type A submodule cpa_A_i (i=1、2、…、N1) and the bridge arm current i pa Arrange the A-type submodules in ascending or descending order, the first N pa_A A-type submodules are put into use and the rest of the A-type submodules are removed;

[0075] (2) For the B-type equivalent submodule, according to the capacitance voltage u of the B-type equivalent submodule cpa_eB_i (i=1、2、…、k·N2) and the bridge arm current i pa Arrange the B-type equivalent submodules in ascending or descending order, and the first N pa_eB Type B equivalent submodules are put into use and the remaining Type B equivalent submodules are removed.

[0076] Let the control object of the proportional-integral controller be the average value of the capacitor voltage of type A submodule u cpa_A_avg The DC component and the average value of the B-type equivalent submodule capacitor voltage u cpa_eB_avg The DC component of the control target is the rated value of the capacitor voltage of the A-type submodule U cn and B-type equivalent submodule capacitor voltage rating U cn / k, based on the proportional-integral controller output and I dc The variable obtained in the direction is used as the dynamic adjustment value ΔN of the number of A-type submodules pa_A , Figure 3 The overall control block diagram of the fractional level MMC submodule classification modulation method disclosed in the present invention is given, wherein "LPF" stands for low-pass filter, "PI" stands for proportional-integral controller, and "sgn" is the sign function.

[0077] In order to verify the effectiveness of the invented fractional level MMC submodule classification modulation method, a simulation platform was built based on the electromagnetic transient simulation platform. Figure 4 The fractional-level MMC simulation model shown in FIG. 1 has specific simulation parameters as shown in Table 1 below.

[0078] Table 1 Fractional level MMC simulation parameters

[0079]

[0080] Figure 5 The fractional-level MMC AC output voltage u is given diffa and the output current i aThe number of output voltage levels is 21, meaning the fractional-level MMC achieves a 21-level output voltage staircase wave with only eight A-type submodules and four B-type equivalent submodules per bridge arm. This significantly multiplies the level and greatly improves the quality of the output voltage and current.

[0081] Figure 6 The capacitor voltage u of the A-type submodule is given cpa_A and B-type equivalent submodule u cpa_eB The capacitor voltage of the A-type submodule is stable at a rated value of 2kV, and the capacitor voltage of the B-type equivalent submodule is stable at a rated value of 1kV. The energy balance between the two types of submodules is good.

[0082] Figure 7 Given the number of A-type submodules N pa_A and the number of equivalent submodules of type B N pa_eB The simulation results are shown in Figure 2. Among them, N pa_A_init and N pa_eB_init The dotted lines represent the initial allocation numbers of the two types of submodules. It can be seen that the actual number of submodules invested is mostly consistent with the initial allocation number. The inconsistency is mainly caused by the energy balance requirements of the submodules.

[0083] Based on the above verification results, it can be seen that the invented fractional-level MMC sub-module classification modulation method is feasible and has good operating effect.

[0084] Example 2

[0085] A second embodiment of the present invention provides a fractional-level MMC submodule classification modulation system, including:

[0086] The module for initially allocating the number of input submodules is configured to initially allocate the number of input submodules of type A and type B based on the number of equivalent submodules that should be input in each bridge arm of the fractional-level MMC and the principle of only inputting type A submodules when outputting integer levels and only inputting type B equivalent submodules when outputting fractional levels.

[0087] The module for dynamically adjusting the number of submodules put into operation is configured to dynamically adjust the number of submodules put into operation based on the initial number of submodules put into operation, taking the capacitor voltages of the type A submodules and the equivalent type B submodules as the control objects, using a proportional-integral controller and taking into account the direction of the DC side current;

[0088] The final input number correction module is configured as follows: after dynamic adjustment, the number of type A submodules and the number of equivalent type B submodules may not match the number of equivalent submodules that should be input in each bridge arm, so the number of submodules input needs to be corrected to obtain the final number of submodules input;

[0089] The switching state determination module is configured as follows: type A submodules and type B equivalent submodules are arranged in ascending or descending order according to their respective capacitor voltages and bridge arm current directions, and the switching state of each submodule is determined according to their respective final switching numbers.

[0090] The more detailed steps are the same as those in Example 1 and will not be repeated here.

[0091] Example 3

[0092] A third embodiment of the present invention provides a medium on which a program is stored. When the program is executed by a processor, the steps of the fractional-level MMC sub-module classification modulation method described in the first embodiment of the present invention are implemented.

[0093] The more detailed steps are the same as those in Example 1 and will not be repeated here.

[0094] Example 4

[0095] Embodiment 4 of the present invention provides an electronic device, including a memory, a processor, and a program stored in the memory and executable on the processor. When the processor executes the program, the steps in the fractional-level MMC sub-module classification modulation method described in embodiment 1 of the present invention are implemented.

[0096] The more detailed steps are the same as those in Example 1 and will not be repeated here.

[0097] The foregoing description is merely a preferred embodiment of the present disclosure and is not intended to limit the present disclosure. Those skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present disclosure shall be included within the scope of protection of the present disclosure.

Claims

1. A fractional level MMC submodule classification modulation method, characterized in that: Based on the number of equivalent submodules that should be put into use in each bridge arm of the fractional-level MMC, and the principle of only putting in use type A submodules when outputting integer levels and only putting in use type B equivalent submodules when outputting fractional levels, the number of put-in-use type A submodules and type B equivalent submodules is initially allocated; Based on the initial number of submodules put into operation, the capacitor voltage of type A submodule and type B equivalent submodule is used as the control object. The proportional-integral controller is used in combination with the direction of the DC side current to dynamically adjust the number of submodules put into operation. After dynamic adjustment, the number of type A submodules and the number of equivalent type B submodules put into operation may not match the number of equivalent submodules that should be put into operation in each bridge arm. Therefore, the number of submodules put into operation needs to be corrected to obtain the final number of submodules put into operation. The A-type submodules and the B-type equivalent submodules are arranged in ascending or descending order according to their respective capacitor voltage magnitudes and bridge arm current directions, and the input and output states of each submodule are determined according to their respective final input numbers.

2. A fractional level MMC submodule classification modulation method as described in claim 1, characterized in that: The conventional level submodule is also called A-type submodule, the fractional level submodule is also called B-type submodule, and the single submodule contained in each B-type submodule is called an equivalent submodule. Taking the upper bridge arm of phase A as an example, the number of equivalent submodules N that should be put into this bridge arm is epa 、The initial investment number N of type A SM pa_A_init and the initial number N of equivalent submodules of type B pa_eB_init They are N pa_eB_init =N epa -k·N pa_A_init in, is the reference voltage of the upper bridge arm of phase A, U cn is the rated capacitance voltage of the conventional level submodule, k is the level multiplication rate, N1 is the number of conventional level submodules contained in each bridge arm, round is the rounding function, and floor is the flooring function.

3. A fractional level MMC submodule classification modulation method as described in claim 1, characterized in that: In order to ensure the energy balance of the two types of sub-modules, a proportional-integral controller is used to control the capacitor voltage of type A sub-module and type B equivalent sub-module to the rated value, and the variable obtained based on the proportional-integral controller output and the DC side current direction is used as the dynamic adjustment variable for the number of sub-modules put into operation.

4. A fractional level MMC submodule classification modulation method as described in claim 3, characterized in that: The control object and control target of the proportional-integral controller can be selected from the following three forms: The control object is the DC component of the average value of the capacitor voltage of the A-type submodule, and the control target is the rated value of the capacitor voltage of the A-type submodule U cn ; The control object is the DC component of the average value of the capacitor voltage of the B-type equivalent submodule, and the control target is the rated value of the capacitor voltage of the B-type equivalent submodule U cn / k; The control object is the DC component of the average value of the capacitor voltage of the type A submodule and the DC component of the average value of the capacitor voltage of the type B equivalent submodule. The control target is the rated value of the capacitor voltage of the type A submodule U cn and B-type equivalent submodule capacitor voltage rating U cn / k.

5. A fractional level MMC submodule classification modulation method as described in claim 3, characterized in that: The direction of the DC side current will affect the charging and discharging of the capacitor of the constant input sub-module. It is necessary to monitor the direction of the DC side current to correct the output of the proportional-integral controller, thereby obtaining the dynamic adjustment amount of the number of sub-modules put into operation.

6. A fractional level MMC submodule classification modulation method as claimed in claim 3, characterized in that: The variable obtained based on the proportional-integral controller output and the DC side current direction can be used as the dynamic adjustment amount ΔN of the number of A-type submodules put into operation pa_A , or as a dynamic adjustment of the number of B-type equivalent submodules ΔN pa_eB .

7. A fractional level MMC submodule classification modulation method as claimed in claim 1, characterized in that: The number N of A-type submodules that need to be invested after dynamic adjustment pa_A_adj and the number of equivalent submodules of type B N pa_eB_adj For correction, let the final number of A-type submodules be N pa_A The final number of B-type equivalent submodules is N pa_eB , and their expressions are Where ΔN epa =N epa -(k·N pa_A_adj +N pa_eB_adj ), mod is the remainder function.

8. A fractional level MMC submodule classification modulation system, characterized in that: include: The module for initially allocating the number of input submodules is configured to initially allocate the number of input submodules of type A and type B based on the number of equivalent submodules that should be input in each bridge arm of the fractional-level MMC and the principle of only inputting type A submodules when outputting integer levels and only inputting type B equivalent submodules when outputting fractional levels. The module for dynamically adjusting the number of submodules put into operation is configured to dynamically adjust the number of submodules put into operation based on the initial number of submodules put into operation, taking the capacitor voltages of the type A submodules and the equivalent type B submodules as the control objects, using a proportional-integral controller and taking into account the direction of the DC side current; The final input number correction module is configured as follows: after dynamic adjustment, the number of type A submodules and the number of equivalent type B submodules may not match the number of equivalent submodules that should be input in each bridge arm, so the number of submodules input needs to be corrected to obtain the final number of submodules input; The switching state determination module is configured as follows: type A submodules and type B equivalent submodules are arranged in ascending or descending order according to their respective capacitor voltages and bridge arm current directions, and the switching state of each submodule is determined according to their respective final switching numbers.

9. A medium having a program stored thereon, characterized in that: When the program is executed by a processor, the steps of the fractional-level MMC submodule classification modulation method according to any one of claims 1 to 7 are implemented.

10. An electronic device comprising a memory, a processor, and a program stored in the memory and executable on the processor, wherein: When the processor executes the program, the steps in the fractional-level MMC submodule classification modulation method according to any one of claims 1 to 7 are implemented.

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