Nearest level approximation modulation method and system of MMC (Modular Multilevel Converter)

Abstract

The invention discloses a nearest level approximation modulation method and system for an MMC converter, belongs to the technical field of flexible power transmission and transformation, and aims to obtain the current of a bridge arm module and the capacitor voltage average value of each sub-module in the bridge arm module. And according to the current of the bridge arm module and the capacitance voltage average value of each sub-module in the bridge arm module, calculating the number of the sub-modules in each numerical value area range. And according to the number of the sub-modules in each numerical value area range, determining the sub-modules needing to be input. And after the input sub-module is determined, the capacitance voltage of the sub-module is further adjusted according to the current of the bridge arm module and the capacitance voltage value of each sub-module in the bridge arm module. In the whole modulation process, the current of the bridge arm module and the capacitance voltage value of each sub-module need to be continuously monitored, and the number of the input sub-modules and the capacitance voltage need to be continuously adjusted according to the monitoring result. Therefore, the method provided by the invention can solve the problem of unbalanced capacitor voltage in the prior art.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible power transmission and distribution, and relates to a nearest level modulation method and system of an MMC converter. BACKGROUND

[0002] In a modular multilevel converter (MMC), the nearest level modulation method controls the input and output of devices to make the waveform of an alternating current (AC) measurement approach a modulation wave. The key of this modulation strategy lies in formulating a reasonable input and output strategy to achieve accurate control of sub-modules. At present, the widely used nearest level modulation method is roughly divided into a carrier phase-shifted sinusoidal pulse modulation method (CPS-SPWM) and a nearest level modulation method (NLM).

[0003] The nearest level modulation method is an important modulation strategy in the field of power electronics, which is used to realize the MMC multilevel output and improve the efficiency and stability of power conversion. Among them, the NLM modulation method selects a suitable input and output strategy to make the output voltage waveform of the MMC as close as possible to the reference waveform. The advantage of this method is that the output voltage waveform has good quality and low harmonic content. For the MMC, the energy storage on the DC side is maintained by the series connection of the capacitor voltages of multiple sub-modules. When the energy changes, the capacitor voltage will inevitably fluctuate to some extent, causing the capacitor voltages of each sub-module to be unbalanced, which affects the normal operation of the MMC. SUMMARY

[0004] The purpose of the present application is to solve the problem of unbalanced capacitor voltage of each sub-module when the energy changes in the prior art, and to provide a nearest level modulation method and system of an MMC converter.

[0005] To achieve the above purpose, the following technical solutions are adopted:

[0006] The nearest level modulation method of the MMC converter provided by the present application comprises the following steps:

[0007] Obtain the current of the bridge arm module and the average value of the capacitor voltage of each sub-module in the bridge arm module;

[0008] According to the current of the bridge arm module and the average value of the capacitor voltage of each sub-module in the bridge arm module, obtain the number of sub-modules in each numerical value range;

[0009] Determine the input sub-module according to the number of sub-modules in each numerical value range;

[0010] According to the input sub-module, the current of the bridge arm module and the capacitor voltage value of each sub-module in the bridge arm module, obtain the average value of the capacitor voltage of the sub-module in the bridge arm module, and realize the nearest level modulation.

[0011] Preferably, the method for obtaining the average value of the capacitor voltage of each submodule in the bridge arm module is as follows:

[0012] The current direction of the bridge arm module and the capacitor voltage value of each submodule in the MMC converter are obtained, and the average value of the capacitor voltage of each submodule in the bridge arm module is obtained according to the current direction of the bridge arm module and the capacitor voltage value of each submodule.

[0013] Preferably, according to the current of the bridge arm module, the average value of the capacitor voltage of each submodule in the bridge arm module is divided into a plurality of numerical range ranges, and the number of submodules in each numerical range range is obtained.

[0014] Preferably, the number of submodules in each numerical range range includes the number of submodules put into each sampling period of the lower bridge arm and the number of submodules put into each sampling period of the upper bridge arm.

[0015] Preferably, the method for obtaining the number of submodules put into each sampling period of the lower bridge arm is as follows:

[0016]

[0017] Wherein, N pj is the number of submodules put into each sampling period of the lower bridge arm, U c is the average voltage of the capacitor of the submodule, N is the number of bridge arm submodules, round() is the rounding function, and u sj is the voltage of the modulation wave.

[0018] Preferably, the method for obtaining the number of submodules put into each sampling period of the upper bridge arm is as follows:

[0019]

[0020] Wherein, N nj is the number of submodules put into each sampling period of the upper bridge arm, U c is the average voltage of the capacitor of the submodule, N is the number of bridge arm submodules, round() is the rounding function, and u sj is the voltage of the modulation wave.

[0021] Preferably, the capacitor voltage of each submodule in the bridge arm module is detected multiple times, and the average value of the capacitor voltage of each submodule is calculated according to the capacitor voltage value of the submodule detected multiple times.

[0022] The application provides an MMC converter nearest level approximation modulation system, which comprises:

[0023] A first data acquisition module is configured to acquire the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module.

[0024] The second data acquisition module is configured to acquire the number of sub-modules in each numerical region range according to the current of the bridge arm module and the average value of the capacitor voltage of each sub-module in the bridge arm module;

[0025] The sub-module determination module is configured to determine the input sub-module according to the number of sub-modules in each numerical region range.

[0026] The data processing module is configured to obtain the average value of the capacitor voltage of the sub-module in the bridge arm module according to the input sub-module, the current of the bridge arm module and the capacitor voltage value of each sub-module in the bridge arm module, and realize the nearest level approximation modulation.

[0027] A computer device comprises a memory and a processor, the memory stores a computer program, and the processor realizes the steps of the nearest level approximation modulation method of the MMC converter when executing the computer program.

[0028] A computer readable storage medium stores a computer program, and the computer program realizes the steps of the nearest level approximation modulation method of the MMC converter when executed by a processor.

[0029] Compared with the prior art, the present application has the following beneficial effects:

[0030] The nearest level modulation method of the MMC converter provided by the application first obtains the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module. This is the basis for understanding the current state of the system. According to the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module, the number of submodules in each numerical region range is calculated. This step is to determine the number of submodules that need to be put in or cut out under different voltage levels. According to the number of submodules in each numerical region range, the submodules that need to be put in are determined. Here, the capacitor voltage sorting algorithm is usually combined to preferentially select the submodules with capacitor voltage close to the average value for putting in, so as to achieve the balance of voltage. After the submodules to be put in are determined, the capacitor voltage of the submodules is further adjusted according to the current of the bridge arm module and the capacitor voltage value of each submodule in the bridge arm module. This is usually achieved by adjusting the on and off time of the switching tube to control the charging and discharging process of the submodule, so as to achieve the purpose of balancing the capacitor voltage. During the whole modulation process, the current of the bridge arm module and the capacitor voltage value of each submodule need to be continuously monitored, and the number of submodules to be put in and the capacitor voltage are continuously adjusted according to the monitoring results. This is a dynamic process that responds to the changes in the state of the system in real time. In order to better achieve the balance of the capacitor voltage, the capacitor voltage sorting algorithm is usually used. The algorithm can sort the capacitor voltage values of each submodule and preferentially put in or cut out the submodules with voltage values close to the average value. This can effectively reduce the voltage imbalance and improve the stability and efficiency of the system. Therefore, the method provided by the application can solve the problems existing in the prior art.

[0031] The nearest level modulation system of the MMC converter provided by the application realizes nearest level modulation by dividing the system into a first data acquisition module, a second data acquisition module, a submodule determination module and a data processing module. The modular idea makes each module independent of each other, facilitating unified management of each module. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0033] Figure 1 The nearest level modulation method flow chart of the MMC converter of the application.

[0034] Figure 2 The NLM modulation principle diagram of the application.

[0035] Figure 3The NLM modulation output phase voltage waveform of the application.

[0036] Figure 4 The nearest level modulation system diagram of the MMC converter of the application.

[0037] Figure 5 The structural schematic diagram of an electronic device of the application. DETAILED DESCRIPTION

[0038] The application will be further described in detail below with reference to the accompanying drawings:

[0039] The nearest level modulation method of the MMC converter of the application, as shown in the figure, comprises the following steps: Figure 1

[0040] S1, obtaining the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module;

[0041] The method for obtaining the average value of the capacitor voltage of each submodule in the bridge arm module is as follows:

[0042] Obtaining the current direction of the bridge arm module and the capacitor voltage value of each submodule in the MMC converter, and obtaining the average value of the capacitor voltage of each submodule in the bridge arm module according to the current direction of the bridge arm module and the capacitor voltage value of each submodule.

[0043] S2, obtaining the number of submodules in each numerical range according to the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module;

[0044] According to the current of the bridge arm module, the average value of the capacitor voltage of each submodule in the bridge arm module is divided into several numerical range, and the number of submodules in each numerical range is obtained.

[0045] The number of submodules in each numerical range includes the number of submodules put into each sampling period of the lower bridge arm and the number of submodules put into each sampling period of the upper bridge arm.

[0046] The method for obtaining the number of submodules put into each sampling period of the lower bridge arm is as follows:

[0047]

[0048] Wherein, N pj is the number of submodules put into each sampling period of the lower bridge arm, U c is the average voltage of the capacitor of the submodule, N is the number of bridge arm submodules, round() is the rounding function, and u sj is the modulation wave voltage.

[0049] ​The method for obtaining the number of sub-modules put in each sampling period of the upper bridge arm is as follows:

[0050]

[0051] Wherein, N nj is the number of sub-modules put in each sampling period of the upper bridge arm, U c is the average voltage of the capacitor of the sub-module, N is the number of sub-modules of the bridge arm, round() is the rounding function, and u sj is the voltage of the modulation wave.

[0052] S3, determining the sub-modules put in according to the number of sub-modules in each numerical region range;

[0053] S4, obtaining the average value of the capacitor voltage of the sub-modules in the bridge arm module according to the sub-modules put in, the current of the bridge arm module and the capacitor voltage values of the sub-modules in the bridge arm module, and realizing the nearest level approximation modulation.

[0054] The capacitor voltage of each sub-module in the bridge arm module is detected for multiple times, and the average value of the capacitor voltage of each sub-module is calculated according to the capacitor voltage values of the sub-modules detected for multiple times.

[0055] Embodiment 2

[0056] The nearest level approximation modulation system of the MMC converter provided by the application comprises: Figure 4 as shown in the figure, which comprises:

[0057] A first data acquisition module is used for acquiring the current of the bridge arm module and the average value of the capacitor voltage of each sub-module in the bridge arm module.

[0058] A second data acquisition module is used for obtaining the number of sub-modules in each numerical region range according to the current of the bridge arm module and the average value of the capacitor voltage of each sub-module in the bridge arm module.

[0059] A sub-module determination module is used for determining the sub-modules put in according to the number of sub-modules in each numerical region range.

[0060] A data processing module is used for obtaining the average value of the capacitor voltage of the sub-modules in the bridge arm module according to the sub-modules put in, the current of the bridge arm module and the capacitor voltage values of the sub-modules in the bridge arm module, and realizing the nearest level approximation modulation.

[0061] Embodiment 3

[0062] Please refer to Figure 5As shown, the present application also provides an electronic device 100 for the nearest level approximation modulation method of MMC converter; the electronic device 100 comprises a memory 101, at least one processor 102, a computer program 103 stored in the memory 101 and executable on the at least one processor 102, and at least one communication bus 104.

[0063] The memory 101 can be used to store the computer program 103, and the processor 102 realizes the steps of the nearest level approximation modulation method of MMC converter by running or executing the computer program stored in the memory 101 and calling the data stored in the memory 101. The memory 101 can mainly comprise a program storage area and a data storage area, wherein the program storage area can store an operating system, at least one application program required by a function (such as a sound playing function, an image playing function, etc.), etc.; and the data storage area can store data (such as audio data) created according to the use of the electronic device 100, etc. In addition, the memory 101 can comprise a non-volatile memory, such as a hard disk, a memory, a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, at least one disk storage device, a flash memory device, or other non-volatile solid-state storage device.

[0064] The at least one processor 102 can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. The processor 102 can be a microprocessor or any conventional processor, etc. The processor 102 is the control center of the electronic device 100, and connects all parts of the electronic device 100 through various interfaces and lines.

[0065] The memory 101 in the electronic device 100 stores a plurality of instructions to realize a nearest level approximation modulation method of MMC converter, and the processor 102 can execute the plurality of instructions to realize:

[0066] Obtaining the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module;

[0067] According to the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module, the number of submodules in each numerical range is obtained;

[0068] According to the number of submodules in each numerical range, the submodules to be put in are determined;

[0069] According to the submodules to be put in, the current of the bridge arm module and the capacitor voltage value of each submodule in the bridge arm module, the average value of the capacitor voltage of the submodules in the bridge arm module is obtained, and the nearest level approximation modulation is realized.

[0070] Embodiment 4

[0071] The modules / units integrated in the electronic device 100, if realized in the form of a software function unit and sold or used as an independent product, can be stored in a computer-readable storage medium. Based on such understanding, all or part of the processes in the above-mentioned embodiment methods can also be completed by a computer program instructing related hardware, and the computer program can be stored in a computer-readable storage medium. When the processor executes the computer program, the steps of each method embodiment described above can be implemented. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or some intermediate forms. The computer-readable medium can include any entity or device that can carry the computer program code, recording medium, U disk, mobile hard disk, magnetic disk, optical disk, computer memory, and read-only memory (ROM).

[0072] Those skilled in the art will appreciate that embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer usable program code.

[0073] The present application is described with reference to flowcharts and / or block diagrams according to the method, device (system), and computer program product of the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram, and the combination of the flows and / or blocks in the flowchart and / or block diagram can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device produce a means for implementing the functions described in the flowchart and / or block diagram.Figure 1 apparatuses that perform the functions recited in block Figure 1

[0074] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the Figure 1 apparatuses that perform the functions recited in block Figure 1

[0075] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the Figure 1 apparatuses that perform the functions recited in block Figure 1

[0076] Therefore, the present application proposes a nearest level modulation method of MMC converter, voltage approximation modulation is a staircase modulation, due to its good dynamic performance and simple and convenient control mode, the most widely used in the field of flexible DC power transmission, its control principle is in the form of sinusoidal modulation wave, by controlling the number of sub-modules in the upper and lower arms, the instantaneous value of the output voltage is as close as possible to the sine waveform. This modulation method is often used in the scene of more levels, higher voltage, compared with other modulation methods, MLM modulation output waveform is closer to the sine wave with the increase of the number of sub-modules, can effectively reduce the harmonic content, greatly reduces the requirement of filter, improves the MMC AC output characteristics, is a relatively ideal modulation method of MMC converter. NLM modulation principle is shown in Figure 2 , wherein Udc is the DC line voltage, Uc is the average voltage of the capacitor of the sub-module. From Figure 2 , it can be seen that the sinusoidal modulation wave gradually increases from 0, the sub-module input and removal mode of the upper and lower arms is complementary, the total number of sub-modules input by the upper and lower arms remains unchanged, so as to ensure that the output differential mode voltage of each phase unit can well track the sinusoidal modulation waveform. According to Figure 3 , when N = 12, the MMC three-phase voltage waveform adopts NLM modulation, from the figure, it can be seen that each phase voltage contains 13 levels, which well tracks the modulation sine waveform. Obviously, when N is larger, the NLM approximation effect is better, and the harmonic content is lower.

[0077] ​​​The above merely describes the preferred embodiments of the present application, and is not used to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A nearest level approximation modulation method of an MMC converter, characterized in that, The method comprises the following steps: obtaining the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module; obtaining the number of submodules in each numerical range according to the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module; determining the input submodules according to the number of submodules in each numerical range; obtaining the average value of the capacitor voltage of the submodules in the bridge arm module according to the input submodules, the current of the bridge arm module and the capacitor voltage of each submodule in the bridge arm module, and realizing the nearest level approximation modulation.

2. The nearest level approximation modulation method of the MMC converter according to claim 1, characterized in that, The method for obtaining the average value of the capacitor voltage of each submodule in the bridge arm module is as follows: obtaining the current direction of the bridge arm module and the capacitor voltage of each submodule in the MMC converter, and obtaining the average value of the capacitor voltage of each submodule in the bridge arm module according to the current direction of the bridge arm module and the capacitor voltage of each submodule.

3. The nearest level approximation modulation method of the MMC converter according to claim 1, characterized in that, According to the current of the bridge arm module, the average value of the capacitor voltage of each submodule in the bridge arm module is divided into several numerical ranges, and the number of submodules in each numerical range is obtained.

4. The nearest level approximation modulation method of the MMC converter according to claim 3, characterized in that, The number of submodules in each numerical range includes the number of input submodules in each sampling period of the lower bridge arm and the number of input submodules in each sampling period of the upper bridge arm.

5. The nearest level approximation modulation method of the MMC converter according to claim 4, characterized in that, The method for obtaining the number of input submodules in each sampling period of the lower bridge arm is as follows: Wherein, N pj is the number of sub-modules put into each sampling period of the lower bridge arm, U c is the average voltage of the capacitor of the sub-module, N is the number of sub-modules of the bridge arm, round() is the rounding function, u sj is the modulation wave order voltage.

6. The nearest level approximation modulation method of the MMC converter according to claim 4, characterized in that, The method for obtaining the number of input submodules in each sampling period of the upper bridge arm is as follows: Wherein, N nj The number of sub-modules put into each sampling period of the upper bridge arm, U c The average voltage of the sub-module capacitor, N is the number of bridge arm sub-modules, round() is the rounding function, u sj The modulation wave order voltage.

7. The nearest level approximation modulation method of the MMC converter according to claim 1, wherein, The capacitor voltage of each submodule in the bridge arm module is detected multiple times, and the average value of the capacitor voltage of each submodule is calculated according to the capacitor voltage values of the submodules detected multiple times.

8. A nearest level approximation modulation system of an MMC converter, characterized in that, It comprises: a first data acquisition module for obtaining the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module; a second data acquisition module for obtaining the number of submodules in each numerical range according to the current of the bridge arm module and the average value of the capacitor voltage of each submodule in the bridge arm module; a submodule determination module for determining the input submodules according to the number of submodules in each numerical range; a data processing module for obtaining the average value of the capacitor voltage of the submodules in the bridge arm module according to the input submodules, the current of the bridge arm module and the capacitor voltage of each submodule in the bridge arm module, and realizing the nearest level approximation modulation. 9.A computer device, comprising a memory and a processor, wherein the memory stores a computer program, and the computer device is configured to perform the method according to any one of claims 1-8 when the computer program is executed by the processor. The processor executes the computer program to realize the steps of the nearest level approximation modulation method of the MMC converter in any one of claims 1 to 7.

10. A computer-readable storage medium storing a computer program, the computer program comprising instructions that, when executed by a computer, cause the computer to perform the method of any one of claims 1 to 9. The computer program is executed by the processor to realize the steps of the nearest level approximation modulation method of the MMC converter in any one of claims 1 to 7.

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