A method for dynamic balancing control of MMC capacitor voltage based on K-means algorithm

Abstract

The application relates to a K-means algorithm-based MMC capacitor voltage dynamic balancing control method, and the steps comprise the following: S1: at the beginning of each control cycle, the number of sub-modules required to be input in the current control cycle is determined through a modulation strategy; S2: the capacitor voltage of each sub-module on the bridge arm at the current moment is collected; S3: the K-means algorithm is used to cluster and group the capacitor voltage of the sub-module; S4: according to the bridge arm current direction, corresponding operations are respectively performed; S5: the clusters are sorted according to the size of the cluster centers; S6: according to the bridge arm current direction, the corresponding sub-modules are sequentially input according to the sorting result of the clusters; compared with a traditional direct voltage balancing method based on capacitor voltage sorting, the application has the advantages of reducing the switching frequency and significantly reducing the capacitor voltage fluctuation rate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a MMC capacitor voltage dynamic balancing control method based on a K-means algorithm. BACKGROUND

[0002] In recent years, a modular multilevel converter (MMC) is widely applied to high-voltage direct-current transmission, high-power motor driving and other fields due to its advantages of high efficiency, low harmonic, flexible expansion and the like. With China increasing support for green energy transformation and smart grid construction in planning, the application of the MMC in China has developed rapidly, and the MMC has shown important technical advantages in large-scale offshore wind power access, cross-regional power grid interconnection and ultra-high voltage direct-current transmission (UHVDC) and the like. However, in actual application, the problem of dynamic balancing of sub-module capacitor voltage still exists, and unbalanced capacitor voltage can lead to unstable operation of the MMC system, damage of switching devices, deterioration of power quality, overload failure of modules, and even serious equipment damage and system paralysis.

[0003] MMC sub-module capacitor voltage balancing control methods mainly include three categories: the first category is to realize balancing of sub-module capacitor voltage by adding an external voltage balancing circuit, which is simple but increases the hardware cost and complexity of the system; the second category is a direct voltage balancing method based on capacitor voltage sorting, which is simple in principle and easy to implement, but in the case of large calculation amount, high switching frequency and large number of sub-modules, can lead to large capacitor voltage fluctuation; and the third category is to add two additional voltage closed-loop controllers of independent voltage balancing control and voltage stabilization control to the MMC sub-module, which avoids switching loss, but it is difficult to set the parameters of the controller, especially when the number of sub-modules is large. SUMMARY

[0004] The application provides a MMC capacitor voltage dynamic balancing control method based on a K-means algorithm, which aims to solve the problems of large capacitor voltage fluctuation and switching loss caused by the second category of direct voltage balancing method based on MMC sub-module capacitor voltage sorting when the number of sub-modules is large. The method can not only reduce the number of sub-module switching in the MMC operation process to a certain extent and reduce the overall switching frequency, but also reduce the capacitor voltage fluctuation when the number of sub-modules is large.

[0005] A MMC capacitor voltage dynamic balancing control method based on a K-means algorithm includes the following steps:

[0006] S1: At the beginning of each control period, the number of sub-modules required to be put into the current control period is determined through a modulation strategy;

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

[0008] S3: Cluster the submodule capacitor voltage using the K-means algorithm;

[0009] S4: According to the direction of the bridge arm current, respectively, perform the corresponding operation;

[0010] S5: Sort the clusters according to the size of the cluster centroid;

[0011] S6: According to the direction of the bridge arm current, according to the sorting result of the cluster, in turn, put into the corresponding submodule;

[0012] Further, S1 includes the following steps: according to the modulation strategy selected by the control system, calculate the number of MMC submodules N required by each bridge arm at a certain time on Each bridge arm contains N submodules, if N on =N, then all MMC submodules on the bridge arm are put into use; if N on =0, then all MMC submodules on the bridge arm are cut off; if 0 on <N, then enter step S2.

[0013] Further, S3 includes the following steps: clustering the submodule capacitor voltage U ci This data set using K-means algorithm to divide into several clusters.

[0014] Further, the K-means algorithm includes: first initialization: select k initial cluster centroids, uniformly distributed around the rated voltage of the submodule capacitor, for the submodule capacitor voltage U ci , calculate its distance from each cluster centroid, and assign it to the nearest cluster, recalculate each cluster centroid, repeat the assignment and update steps until a specified number of iterations d is reached.

[0015] Further, the distance in it uses Euclidean distance as the distance measure, the calculation formula is as follows:

[0016]

[0017] Where U ci is the submodule capacitor voltage, C i is the centroid of the i-th cluster.

[0018] Further, for the recalculation of each cluster centroid in it, the new cluster centroid is the mean of all capacitor voltages in the cluster, the calculation formula is as follows:

[0019]

[0020] wherein S i is the number of capacitor voltage data points in the i-th cluster.

[0021] Further, S4 comprises: letting the bridge arm current i arm have the same direction as in Figure 1 , the bridge arm current i arm has a positive direction from the high voltage end to the low voltage end, and when i arm > 0, if the number of data points in the cluster S with the smallest centroid is S a much larger than the required number of sub-modules N on , then S3 is performed again for the cluster S; otherwise, S5 is performed; and when i arm < 0, if the number of data points in the cluster B with the largest centroid is B a much larger than the required number of sub-modules N on , then S3 is performed again for the cluster B; otherwise, S5 is performed.

[0022] Further, S6 comprises: letting the bridge arm current i arm have the same direction as in Figure 1 , when i arm > 0, starting from the cluster with the smaller centroid, N on sub-modules are selected to be put in, and the selection is performed cluster by cluster, and if the remaining number of sub-modules is insufficient to meet the requirement of the next cluster, then the capacitor with the largest distance from the centroid is selected to be extracted from the current cluster; and when i arm < 0, starting from the cluster with the larger centroid, N on sub-modules are selected to be put in, and the selection is performed cluster by cluster, and if the remaining number of sub-modules is insufficient to meet the requirement of the next cluster, then the capacitor with the largest distance from the centroid is selected to be extracted from the current cluster.

[0023] The application has the advantages that the K-means algorithm is used to cluster and sort the capacitor voltages of the sub-modules, the voltage values in each cluster are concentrated, and the sorting is not required, only the centroid of each cluster is compared, and then the clusters are sorted, so that the traditional sorting and multiple actions are avoided, the number of sub-module switching is reduced, the switching frequency is reduced, and the loss is reduced; the parameters k and d in the algorithm are adjusted, and the number N on of sub-modules is calculated according to the modulation strategy, so that the dynamic adjustment is realized; in addition, the capacitor voltage fluctuation rate is also reduced. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required to be used in the prior art and embodiments, and the following drawings are only some embodiments of the present application, and other drawings can be obtained by those of ordinary skill in the art without any creative effort on the premise of the drawings.

[0025] Figure 1 A modular multilevel converter system model diagram of the present application;

[0026] Figure 2 A capacitor voltage dynamic balancing control method flowchart of the present application;

[0027] Figure 3 A capacitor voltage waveform diagram of the upper bridge arm sub-module of the A phase of the modular multilevel converter of the present application after stabilization;

[0028] Figure 4 An IGBT trigger pulse diagram of the upper bridge arm sub-module of the A phase of the modular multilevel converter of the present application. DETAILED DESCRIPTION

[0029] In order to make the application purpose, features and advantages of the present application more obvious and easy to understand, the following will combine the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application, it should be pointed out that the following detailed description is all exemplary, and is intended to provide further description of the present application. Unless otherwise specified, all other embodiments obtained by those of ordinary skill in the art on the premise of the embodiments in the present application without any creative effort belong to the scope of protection of the present application.

[0030] The present application embodiment provides a kind of MMC capacitor voltage dynamic balancing control method based on K-means algorithm, it is applicable to the equalization control of MMC sub-module capacitor voltage.This method can reduce the number of sub-module switching in MMC operation process to a certain extent, reduce overall switching frequency, and effectively reduce capacitor voltage fluctuation in the case where sub-module number is more.At the same time, this method also supports dynamic adjustment algorithm to realize more efficient equalization control.

[0031] As a typical embodiment of the present application, forty-nine level modular multilevel converter system is taken as an example to illustrate.

[0032] The present application uses Matlab / Simulink simulation platform to simulate modular multilevel converter, and the system uses nearest level approximation modulation.In the simulation model, each bridge arm of the modular multilevel converter contains 48 sub-modules, and no redundant sub-modules are configured, and the rated voltage of DC side is U dc = 48kV, and the rated voltage of each sub-module capacitor is U cn=1kV, capacitance C0 = 670μF, the initial value of the capacitance voltage of each submodule of the bridge arm is distributed within ±30%U cn , the rated capacitance of the capacitor is distributed within ±30% C0, the bridge arm inductance is L0 = 10mH, the sampling frequency is 10kHz, and the control period is 100μs. The specific implementation is as follows:

[0033] According to S1 in the manual, at the beginning of each control cycle, the number of submodules N required for the current control cycle is determined by the nearest level approximation. on ;

[0034] According to S2, the capacitor voltage U of each submodule on the bridge arm at the current moment is collected ci ;

[0035] According to S3, the submodule capacitor voltage U ci This data set is clustered using the K-means algorithm to separate several clusters. The K-means algorithm includes the following steps: First, initialization: select three initial cluster centroids, which are 950, 1000, and 1050 respectively. The value of k must be greater than or equal to 1. The larger the k value, the smaller the capacitance fluctuation rate, but the number of switching times will increase accordingly. The selection of the k value usually needs to be reasonably set according to the number of system submodules; then, for the submodule capacitance voltage U ci , calculate its distance to each cluster centroid and assign it to the cluster with the closest distance, recalculate each cluster centroid, and repeat the assignment and update steps until the specified number of iterations d = 3 is reached.

[0036] Specifically, the Euclidean distance is used as the distance metric, and the calculation formula is as follows:

[0037]

[0038] Among them U ci is the submodule capacitor voltage, C i is the centroid of the ith cluster.

[0039] Specifically, the centroid of each cluster is recalculated, and the new centroid is the mean of all capacitor voltages in the cluster, calculated as follows:

[0040]

[0041] Among them S i is the number of capacitor voltage data points in the i-th cluster.

[0042] According to S4: let the bridge arm current i arm Direction and Figure 1 The same as in the figure, the bridge arm current i arm The positive direction is from the high pressure end to the low pressure end. armWhen it is greater than 0, if the number of data points S in the cluster S with the smallest centroid is a Far greater than the number of submodules N required on , then execute S3 again for cluster S, and the submodule capacitor voltage U in cluster S ci This data set is clustered by K-means algorithm to divide into several clusters; otherwise, S5 is executed to sort the clusters according to the size of the centroid of each cluster; when i arm When it is less than 0, if the number of data points in the cluster B with the largest centroid is B a Far greater than the number of submodules N required on , then execute S3 again for cluster B; otherwise, execute S5.

[0043] Specifically, it can be selected as 5 times or 10 times according to the number of submodules. In the example, due to the small number of modules, 5 times can be selected, that is, S a >5N on Can be considered as far greater than.

[0044] According to S6: let the bridge arm current i arm Direction and Figure 1 Same as in, when i arm When it is greater than 0, select N clusters starting from the cluster with the smaller centroid. on Sub-modules are put into use and selected cluster by cluster. If the remaining number of available modules is not enough to meet the needs of the next cluster, capacitors farther from the centroid are selected from the current cluster to charge these sub-module capacitors. arm When it is less than 0, N clusters are selected in sequence starting from the cluster with the larger centroid. on Sub-modules are put into use and selected cluster by cluster. If the remaining number of available modules is not enough to meet the needs of the next cluster, capacitors farther away from the centroid are preferentially selected from the current cluster to discharge the capacitors of these sub-modules.

[0045] Depend on Figure 3 It can be seen that under the implementation of the above capacitor voltage control method, the capacitor voltage control effect is good, and the maximum fluctuation rate ε = 1.4% can meet the demand. Figure 4 It can be seen that under this capacitor voltage control method, the trigger pulses of the switching device are relatively sparse, and the average switching frequency of a single IGBT is 1.2 kHz, which reduces the switching frequency.

[0046] Therefore, the MMC capacitor voltage dynamic balancing control method based on the K-means algorithm proposed in the present invention can reduce the switching frequency, effectively reduce the capacitor voltage fluctuation rate, and improve the capacitor voltage balancing effect compared with the traditional direct voltage balancing method based on capacitor voltage sorting, further verifying the accuracy of the proposed method.

Claims

1.A method for dynamic balancing control of MMC capacitor voltage based on K-means algorithm, characterized in that, Comprising: S1: at the beginning of each control cycle, determine the number of sub-modules required for the current control cycle by modulation strategy; S2: collect the capacitor voltage of each submodule on the bridge arm at the current time U ci ; S3: using K-means algorithm to collect the sub-module capacitor voltage clustering clustering; the K-means algorithm in the step S3 includes: first initialization: selecting k An initial cluster center, uniformly distributed around the rated voltage of the sub-module capacitor, for the sub-module capacitor voltage U ci , calculate its distance from each cluster center, and assign it to the nearest cluster, recalculate each cluster center, repeat the assignment and update steps until a specified number of iterations d ; for the distance in it, the Euclidean distance is used as the distance measure, and the calculation formula is as follows: wherein U ci is a sub-module capacitor voltage, C i is a first i cluster centroid; For the case where each cluster center is recalculated, the new cluster center is the mean of all the capacitance voltages within the cluster, and the calculation formula is as follows: wherein S i is the number of capacitance voltage data points in the ith cluster; S4: according to the bridge arm current direction, respectively, the corresponding operation is executed; the step S4 makes the bridge arm current i arm The positive direction is from the high voltage end to the low voltage end, and the negative direction is from the low voltage end to the high voltage end. i arm When greater than 0, if the number of data points in the cluster S with the smallest centroid S a Is much larger than the number of sub-modules required to be input N on , then execute S3 again on the cluster S; otherwise, execute S5; when i arm Less than 0, if the number of data points in the cluster B with the largest centroid B a Is much larger than the number of sub-modules required to be input N on , then execute S3 again on the cluster B; otherwise, execute S5; S5: sort the clusters according to the size of the cluster centers; S6: according to the bridge arm current direction, according to the sorting result of the cluster, the corresponding sub-module is put in turn; the step S6 makes the bridge arm current i arm The positive direction is from the high voltage end to the low voltage end, and the negative direction is from the low voltage end to the high voltage end. i arm When greater than 0, start from the cluster with smaller centroid and select N 𝑜𝑛 Sub-modules are put in turn, and selection is performed cluster by cluster. If the remaining number of put-in is insufficient to meet the demand of the next cluster, the capacitor farthest from the centroid is preferentially extracted from the current cluster; when i arm When less than 0, start from the cluster with larger centroid and select N 𝑜𝑛 Sub-modules are put in turn, and selection is performed cluster by cluster. If the remaining number of put-in is insufficient to meet the demand of the next cluster, the capacitor farthest from the centroid is preferentially extracted from the current cluster.

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