A method for MMC capacitor voltage balancing control based on dynamic grouping aggregation

Abstract

This invention relates to a dynamic grouping aggregation-based MMC capacitor voltage equalization control method, comprising: S1: at the beginning of each control cycle, determining the number of sub-modules to be deployed at the current control moment through a modulation strategy. N on Record the number of submodules deployed in the previous moment. N old S2: Collect the capacitor voltage of each submodule on the bridge arm at the current moment. U ci S3: Calculate the capacitor voltage fluctuation coefficient at the current moment. d S4: Use dynamic grouping aggregation to output the number of groups m and the allowable capacitor voltage fluctuation range. F min arrive F max S5: Fixed group M based on the number of groups and the allowable capacitor voltage fluctuation range. i S6: Traverse the submodule capacitor voltages and sort the submodule capacitor voltages into groups; S7: According to the direction of the bridge arm current, switch the corresponding submodules in sequence according to the grouping and sorting results; This invention can significantly reduce the number of switching submodules, thereby significantly reducing switching losses, improving system efficiency, and ensuring capacitor voltage stability.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a method for MMC capacitor voltage equalization control based on dynamic grouping and aggregation. Background Technology

[0002] In recent years, Modular Multilevel Converters (MMCs) have been widely used in many fields, such as high-voltage direct current (HVDC) transmission and high-power motor drives, due to their outstanding advantages such as high efficiency, low harmonic content, flexible structure, and easy expansion. With my country's further strengthening of policy support for green energy transformation and smart grid construction, the application of MMC technology in China has developed rapidly, especially in major projects such as large-scale offshore wind power grid connection, inter-regional grid interconnection, and ultra-high-voltage direct current (UHVDC) transmission, demonstrating significant technical advantages and engineering application value. However, in actual operation, MMCs still face two major technical challenges: submodule capacitor voltage balancing and excessively high control switching frequency. On the one hand, capacitor voltage imbalance directly leads to increased circulating current within the system, causing distortion of output voltage and current, and in severe cases, endangering the safety of power switching devices and even triggering cascading equipment failures, resulting in system instability. On the other hand, a high equivalent switching frequency directly leads to increased switching losses, thereby reducing the overall system efficiency and restricting its application in high-capacity, high-power-density scenarios.

[0003] Currently, the submodule capacitor voltage equalization control methods in MMC can be mainly divided into three categories: The first type uses an externally added equalization circuit, which, although simple in structure, significantly increases the hardware cost and complexity of the system. The second type is based on capacitor voltage sorting equalization control, which, although intuitive in principle and easy to implement, results in a large computational load when the number of submodules is large, leading to higher switching frequencies and larger capacitor voltage fluctuations. The third type introduces an independent voltage equalization and voltage regulation dual closed-loop controller for each submodule. While this method helps avoid switching losses, its controller parameter tuning process is complex. Summary of the Invention

[0004] This invention proposes a dynamic grouping and aggregation-based MMC capacitor voltage balancing control method, aiming to solve the problems of significant capacitor voltage fluctuations and excessive system switching losses caused by traditional voltage sorting-based balancing methods when the number of sub-modules is large. The core of this invention lies in using a dynamic grouping and aggregation method to significantly reduce the number of sub-module switching operations by sacrificing a small amount of capacitor voltage fluctuation while ensuring that capacitor voltage fluctuations always meet the requirements for safe system operation. This significantly reduces switching losses and improves system efficiency.

[0005] A method for equalizing MMC capacitor voltage based on dynamic grouping and aggregation includes:

[0006] S1: At the beginning of each control cycle, the number of sub-modules required for the current control moment is determined by the modulation strategy. N on Record the number of submodules deployed in the previous moment. N old ;

[0007] S2: Collect the capacitor voltage of each submodule on the bridge arm at the current moment. U ci ;

[0008] S3: Calculate the capacitor voltage fluctuation coefficient at the current moment. δ ;

[0009] S4: Use dynamic group aggregation to output the number of groups m and the allowable capacitor voltage fluctuation range. F min arrive F max ;

[0010] S5: Fixed group M based on the number of groups and the allowable capacitor voltage fluctuation range. i The boundary;

[0011] S6: Traverse the capacitor voltages of submodules and sort the capacitor voltages of submodules into groups;

[0012] S7: Based on the direction of the bridge arm current and the grouping results, switch the corresponding sub-modules in sequence;

[0013] Furthermore, S1 includes: calculating the number of MMC submodules that need to be deployed in each bridge arm at the current moment, based on the modulation strategy selected by the control system. N on Record the number of MMC submodules deployed at the previous moment. N old If the number of newly added sub-modules Δ N on =| N on - N old |=0, keep the on / off state of all submodules unchanged; if Δ N on If ≠0, proceed to step S2.

[0014] Furthermore, S3 includes: the capacitor voltage fluctuation coefficient at the current moment. δ The calculation formula is as follows:

[0015]

[0016] Where Δ Umax It is the maximum deviation of the submodule capacitor voltage from its rated value. U cref This is the rated voltage of the submodule capacitor.

[0017] Furthermore, S4 includes: for dynamic grouping aggregation such as Figure 3 Determine the A parameter of the PI element: proportional gain K PA Integral constant K iA ; PI element B parameter: proportional gain K PB Integral constant K iB When the capacitor voltage fluctuation coefficient δ Reference value of volatility coefficient δ cref When the two values ​​are different, the deviation value is input to PI circuit A, and the output is the number of groups m; the deviation value is input to PI circuit B, and the output is the allowable capacitor voltage fluctuation range. F min arrive F max .

[0018] Furthermore, S5 includes: the submodule capacitor voltage is greater than... F max Divided into a group M max The voltage of the submodule capacitor is less than F min Divided into a group M min The submodule capacitor voltage is greater than F min and less than F max Divide into m groups on average, which means fixing the boundaries of m+2 groups;

[0019] Among them, M i It is the first of the dynamic grouping i Group.

[0020] Furthermore, S6 includes: grouping and sorting the submodule capacitor voltages: assigning the submodule capacitor voltages to corresponding groups, with no strict sorting within each group.

[0021] Furthermore, S7 includes: controlling the bridge arm current. i arm The direction of flow is from the high-pressure end to the low-pressure end, and... Figure 1 The same in the middle, when i arm Greater than 0 and Δ N on When the value is greater than 0, select Δ sequentially from the capacitor voltage value group to the voltage value group. N on An uninvested submodule is invested; when iarm Greater than 0 and Δ N on When the value is less than 0, select |Δ sequentially from the largest group of capacitor voltage values ​​to the smallest group of voltage values. N on |A submodule is removed; when i arm Less than 0 and Δ N on When the value is greater than 0, select Δ sequentially from the largest group of capacitor voltage values ​​to the smallest group of voltage values. N on An uninvested submodule is invested; when i arm Less than 0 and Δ N on When the value is less than 0, select |Δ sequentially from the capacitor voltage value group to the voltage value group. N on | Each input submodule is removed; the selection principle for the group is entirely based on random selection.

[0022] The invention is characterized by fully utilizing the flexible grouping advantage of the dynamic grouping aggregation method. Through dual PI links A and B, the capacitor voltage fluctuation coefficient is compared with the target fluctuation coefficient reference value, and the grouping number m and the allowable capacitor voltage fluctuation range are dynamically adjusted. F min arrive F max This allows for dynamic grouping of submodule capacitor voltages without sorting within each group. Based on this, and combined with the previous switching state, submodule switching decisions are made with the goal of minimizing switching actions. While ensuring that capacitor voltage fluctuations always meet the requirements for safe system operation, a small amount of capacitor voltage fluctuation is sacrificed to significantly reduce the number of submodule switching operations, thereby significantly reducing switching losses and improving system efficiency. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following is a brief introduction to the drawings used in the prior art and embodiments. The following drawings are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a model diagram of the modular multilevel converter system of the present invention;

[0025] Figure 2 This is a schematic diagram of the capacitor voltage equalization control method of the present invention;

[0026] Figure 3 This is a schematic diagram of the dynamic grouping and aggregation process of the present invention;

[0027] Figure 4 The waveform of the capacitor voltage after stabilization of the upper bridge arm submodule of phase A of a modular multilevel converter using the traditional bubbling method;

[0028] Figure 5 The diagram shows the trigger pulses for the IGBTs in the upper bridge arm submodule of the A phase of a modular multilevel converter using the traditional bubbling method.

[0029] Figure 6 This is a waveform diagram of the capacitor voltage after stabilization of the upper bridge arm submodule of phase A of the modular multilevel converter of the present invention.

[0030] Figure 7 This is a diagram showing the IGBT trigger pulse of the A-phase upper bridge arm submodule of the modular multilevel converter of the present invention. Detailed Implementation

[0031] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0032] This application provides a dynamic grouping aggregation-based MMC capacitor voltage balancing control method, applicable to the balancing control of MMC submodule capacitor voltages. This method, while ensuring that capacitor voltage fluctuations always meet the system's safe operation requirements, sacrifices a small amount of capacitor voltage fluctuation, significantly reducing the number of submodule switching operations, thereby significantly reducing switching losses and improving system efficiency.

[0033] As a typical embodiment of the present invention, a 23-level modular multilevel converter system is used as an example for illustration.

[0034] This invention uses the Matlab / Simulink simulation platform to simulate a modular multilevel converter. The system employs nearest-level approximation modulation and PIR circulating current control technology. In this simulation model, each arm of the modular multilevel converter contains 22 sub-modules, with no redundant sub-modules configured. The rated DC-side voltage is... U dc =11kV, the rated voltage of each submodule capacitor is U cref =500V, capacitor capacity C 0 = 7mF, bridge arm inductance is L 0 = 13.5mH, sampling frequency is 20kHz, control period is 50 μs The specific implementation is as follows:

[0035] According to S1 in the instruction manual: at the beginning of each control cycle, the number of MMC submodules that each bridge arm needs to be engaged at the current moment is calculated by approximating the nearest voltage level. N on Record the number of MMC submodules deployed at the previous moment. N old If the number of newly added sub-modules Δ N on =|Δ N on - N old |=0, keep the on / off state of all submodules unchanged; if Δ N on If ≠0, proceed to step S2.

[0036] According to S2: Collect the capacitor voltage of each submodule on the bridge arm at the current moment. U ci .

[0037] Based on S3: Calculate the capacitor voltage fluctuation coefficient at the current moment. δ For the capacitor voltage fluctuation coefficient at the current moment δ The calculation formula is as follows:

[0038]

[0039] Where Δ U max It is the maximum deviation of the submodule capacitor voltage from its rated value. U cref This is the rated voltage of the submodule capacitor.

[0040] According to S4: Using dynamic group aggregation, output the number of groups m and the allowable capacitor voltage fluctuation range. F min arrive F max ;

[0041] Specifically, in this example: for dynamic grouping aggregation such as Figure 3 Determine the A parameter of the PI element: proportional gain K PA =0.25, integration constant K iA =0.05; PI element B parameter: proportional gain K PB =0.15, integration constant K iB =0.03; when the capacitor voltage fluctuation coefficient δ= 6% and volatility coefficient reference value δ crefWhen the values ​​are not both 5%, the deviation between the two values ​​is input to PI circuit A, and the output group number m=6. When the deviation between the two values ​​is input to PI circuit B, the output allows capacitor voltage fluctuation range. F min -4% to F max =+4%; when the capacitor voltage fluctuation coefficient δ Greater than the reference value of volatility coefficient δ cref When the number of output groups m increases through PI circuit A, the allowable capacitor voltage fluctuation range decreases through PI circuit B, and vice versa.

[0042] According to S5: Group M is fixed based on the number of groups and the allowable capacitor voltage fluctuation range. i The boundary;

[0043] Specifically, the submodule capacitor voltage is greater than F max Divided into a group M max The voltage of the submodule capacitor is less than F min Divided into a group M min The submodule capacitor voltage is greater than F min and less than F max The capacitor voltage is divided into m groups on average, which means the boundaries of m+2 groups are fixed. When the allowable range of capacitor voltage fluctuation is reduced, the number of groups m increases, and the boundaries of each group within the allowable range of capacitor voltage fluctuation become narrower, achieving a more refined division of capacitor voltage. Combined with the sorting and switching strategy, the capacitor voltage fluctuation coefficient quickly approaches the fluctuation coefficient reference value, achieving rapid stabilization of capacitor voltage. When the allowable range of capacitor voltage fluctuation is increased, the number of groups m decreases, and the boundaries of each group within the allowable range of capacitor voltage fluctuation become wider, forming a relatively coarse capacitor voltage grouping. This reduces unnecessary switching actions, significantly reduces the number of switching operations of submodules, and thus significantly reduces switching losses and improves system efficiency.

[0044] Among them, M i It is the first of the dynamic grouping i Group.

[0045] S6: Traverse the capacitor voltages of submodules and sort the capacitor voltages of submodules into groups;

[0046] Specifically, the submodule capacitor voltages are grouped and sorted: the submodule capacitor voltages are assigned to corresponding groups, and the sorting within each group is not strict.

[0047] S7: Based on the direction of the bridge arm current and the grouping results, the corresponding sub-modules are sequentially deployed.

[0048] Specifically, S7 includes: controlling the bridge arm current. iarm The direction of flow is from the high-pressure end to the low-pressure end, and... Figure 1 The same in the middle, when i arm Greater than 0 and Δ N on When the value is greater than 0, select Δ sequentially from the capacitor voltage value group to the voltage value group. N on An uninvested submodule is invested; when i arm Greater than 0 and Δ N on When the value is less than 0, select |Δ sequentially from the largest group of capacitor voltage values ​​to the smallest group of voltage values. N on |A submodule is removed; when i arm Less than 0 and Δ N on When the value is greater than 0, select Δ sequentially from the largest group of capacitor voltage values ​​to the smallest group of voltage values. N on An uninvested submodule is invested; when i arm Less than 0 and Δ N on When the value is less than 0, select |Δ sequentially from the capacitor voltage value group to the voltage value group. N on | Each input submodule is removed; the selection principle for the group is entirely based on random selection.

[0049] Depend on Figure 4 and Figure 5 As can be seen from the implementation of the traditional bubble method for capacitor voltage control, the capacitor voltage fluctuation coefficient... δ The percentage is 1.8%, and the average switching frequency of a single IGBT is 3.5kHz; [The remaining text appears to be incomplete and requires further context.] Figure 6 and Figure 7 As can be seen from the implementation of capacitor voltage control using the method of the present invention, the capacitor voltage fluctuation coefficient is... δ The voltage fluctuation coefficient is 2.5%, and the average switching frequency of a single IGBT is reduced to 88Hz. In comparison, it can be seen that after adopting the method of this invention, the capacitor voltage fluctuation coefficient is significantly reduced. δ Although it rose slightly (by 0.7 percentage points), it is still far below the threshold allowed for safe system operation. δ (less than 5%), and the average switching frequency of a single IGBT has been reduced by orders of magnitude.

[0050] Therefore, the MMC capacitor voltage equalization control method based on dynamic grouping and aggregation proposed in this invention, while ensuring that capacitor voltage fluctuations always meet the requirements for safe system operation, sacrifices a small amount of capacitor voltage fluctuations and significantly reduces the number of switching operations of sub-modules, thereby significantly reducing switching losses and improving system efficiency.

Claims

1. A method for equalizing MMC capacitor voltage based on dynamic grouping and aggregation, characterized in that, include: S1: At the beginning of each control cycle, the number of sub-modules required for the current control moment is determined by the modulation strategy. N on Record the number of submodules deployed in the previous moment. N old ; S2: Collect the capacitor voltage of each submodule on the bridge arm at the current moment. U ci ; S3: Calculate the capacitor voltage fluctuation coefficient at the current moment. δ S3 includes: the capacitor voltage fluctuation coefficient at the current moment. δ The calculation formula is as follows: Where Δ U max It is the maximum deviation of the submodule capacitor voltage from its rated value. U cref This is the rated voltage of the submodule capacitor; S4: Use dynamic group aggregation to output the number of groups m and the allowable capacitor voltage fluctuation range. F min arrive F max ; S4 includes: for dynamic grouping aggregation: determining the A parameter of the PI stage: proportional gain K. PA Integral constant K iA ; PI element B parameter: proportional gain K PB Integral constant K iB When the capacitor voltage fluctuation coefficient δ Reference value of volatility coefficient δ cref When the two values ​​are different, the deviation value is input to PI circuit A, and the output is the number of groups m; the deviation value is input to PI circuit B, and the output is the allowable capacitor voltage fluctuation range. F min arrive F max ; S5: Fixed group M based on the number of groups and the allowable capacitor voltage fluctuation range. i The boundary; S5 includes: the submodule capacitor voltage is greater than F max Divided into a group M max The submodule capacitor voltage is less than F min Divided into a group M min The submodule capacitor voltage is greater than F min and less than F max Divide into m groups on average, which means fixing the boundaries of m+2 groups; Among them, M i It is the first of the dynamic grouping i Group; S6: Traverse the capacitor voltages of submodules and sort the capacitor voltages of submodules into groups; S6 includes: grouping and sorting the capacitor voltages of the sub-modules: assigning the capacitor voltages of the sub-modules to corresponding groups, without strictly sorting them by size within each group; S7: Based on the direction of the bridge arm current and the grouping results, switch the corresponding sub-modules in sequence; S7 includes: controlling the bridge arm current. i arm The positive direction is from the high-pressure end to the low-pressure end, when i arm Greater than 0 and Δ N on When the value is greater than 0, select Δ sequentially from the capacitor voltage value group to the voltage value group. N on An uninvested submodule is invested; when i arm Greater than 0 and Δ N on When the value is less than 0, select |Δ sequentially from the largest group of capacitor voltage values ​​to the smallest group of voltage values. N on |A submodule is removed; when i arm Less than 0 and Δ N on When the value is greater than 0, select Δ sequentially from the largest group of capacitor voltage values ​​to the smallest group of voltage values. N on An uninvested submodule is invested; when i arm Less than 0 and Δ N on When the value is less than 0, select |Δ sequentially from the capacitor voltage value group to the voltage value group. N on | Each input submodule is removed; the selection principle for the group is entirely based on random selection.

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