Optimal modulation method and system for large-capacity direct current interconnected power conversion cluster

Abstract

The application discloses a kind of optimal modulation methods and systems for large-capacity direct-current interconnected electric energy conversion cluster, belong to power electronic conversion and new energy technology field, this method includes: the coupling mechanism of pulse current in cluster inside is analyzed at direct-current side, construct global optimization model with minimum direct-current bus current ripple as target;Collect the modulation degree of each converter in large-capacity direct-current interconnected electric energy conversion cluster at current time and cluster circuit parameter data;According to the data obtained, gradient descent algorithm is used to solve the model, and the optimal switch pulse sequence combination of converter cluster at the current sampling time is obtained;According to the modulation degree data of sampling time, adjust the modulation degree value in global optimization model, calculate the optimal pulse sequence in the modulation degree deviation range, to control the modulation of each converter switch pulse.The application can realize the significant suppression of direct-current side current ripple, avoid the dependence on large-capacity passive filter device.

Description

Technical Field

[0001] This invention belongs to the field of power electronic conversion and new energy technology, and in particular relates to an optimal modulation method and system for large-capacity DC interconnected power conversion clusters. Background Technology

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

[0003] With the rapid development of DC / AC / DC hybrid microgrids, large-capacity DC interconnected power conversion clusters have gradually become the core equipment for energy collection and interconnection. These clusters consist of multiple high-power conversion units connected in parallel, and energy collection and transmission are achieved through a common DC bus. Compared to single-unit converters, these clusters have stronger power carrying capacity and higher system redundancy, effectively reducing transmission line losses, improving voltage regulation flexibility, and enhancing the system's anti-interference capability. Therefore, they are widely regarded as the core equipment form for future DC / AC / DC hybrid microgrids and large-scale renewable energy power plants.

[0004] However, due to the switching losses and thermal limits of power devices, large-capacity conversion units (such as high-power converters) typically need to operate at low switching frequencies for extended periods. Under these conditions, the switching pulses of each converter unit superimpose on the DC side, leading to a significant amplification of the bus feed current ripple. On the one hand, the increased ripple subjects DC filter devices and power supply equipment to higher current stress, shortening their service life and increasing system costs. On the other hand, excessive DC current ripple reduces system energy transfer efficiency and may induce voltage fluctuations and operational instability. Therefore, effectively suppressing bus feed current ripple at the cluster scale has become a key technical bottleneck restricting the safe and economical operation of DC / AC-DC hybrid microgrids.

[0005] To suppress bus feed current ripple, the approach focuses on the modulation stage of the power converter, and currently there are two main solutions:

[0006] (1) Traditional carrier modulation and space vector modulation: These methods have a clear structure and are easy to implement in engineering. However, they are prone to generating a large number of low-frequency harmonic components at low switching frequencies. The DC side ripple suppression capability is limited, and often requires additional large-capacity passive filter devices for compensation, which leads to an increase in system size and cost.

[0007] (2) Programming modulation: By optimizing the pulse angle configuration to weaken specific harmonics, the output waveform can be improved. However, most existing studies focus on AC harmonic optimization of single-machine converters, neglecting the ripple effect of DC bus current. Moreover, they have not made full use of the cooperative degree of freedom of the cluster system, making it difficult to achieve DC current ripple suppression for large-capacity DC power conversion clusters. Summary of the Invention

[0008] To address the shortcomings of the existing technologies, this invention provides an optimal modulation method and system for large-capacity DC-DC interconnected power conversion clusters. By performing global modeling and optimization at the cluster scale and fully utilizing the additional control degrees of freedom brought about by the parallel operation of the cluster, the optimal switching pulse sequence is solved and generated, thereby achieving significant suppression of DC-side current ripple, avoiding dependence on large-capacity passive filtering devices, and effectively improving the system's operating efficiency, stability, and economy.

[0009] In a first aspect, the present invention provides an optimal modulation method for large-capacity DC interconnected power conversion clusters.

[0010] An optimal modulation method for large-capacity DC-DC interconnected power conversion clusters includes:

[0011] The coupling mechanism of pulse current on the DC side within the cluster is analyzed, and a global optimization model is constructed with the goal of minimizing DC bus current ripple.

[0012] Collect the modulation index and cluster circuit parameter data of each converter in the high-capacity DC interconnected power conversion cluster at the current moment;

[0013] Based on the acquired data, the gradient descent algorithm is used to solve the model, and the optimal combination of switching pulse sequences of the converter cluster at the current sampling time is obtained.

[0014] Based on the modulation data at the sampling time, the modulation value in the global optimization model is adjusted, and the optimal pulse sequence within the modulation deviation range is calculated to control the modulation of the switching pulses of each converter.

[0015] A further technical solution, the construction of the global optimization model, includes:

[0016] Based on the law of conservation of energy, the relationship between the DC-side current and the AC-side current of each converter in the cluster is derived.

[0017] Based on Fourier series theory, the correlation between the converter switching pulse sequence and AC voltage harmonics is analyzed.

[0018] Based on the AC grid-connected frequency domain model of a single converter, the nth harmonic current of the AC grid-connected system under nth harmonic voltage excitation is analyzed, and the relationship between harmonic current and harmonic voltage is obtained.

[0019] The three types of relationships derived from the integrated analysis transform the optimization of the total DC bus current ripple into the optimization of the harmonic components of the grid-connected AC current of each AC unit, and construct a global optimization model with the goal of minimizing the DC bus current ripple.

[0020] A further technical solution, the global optimization model, is:

[0021] ;

[0022] in, This represents the DC current of the j-th unit on the DC side. This represents the nth harmonic current of the xth phase in the j-th unit on the AC side; EF represents the DC side voltage; EF represents the degree of optimization for DC bus current ripple. Indicates the first j The first converter i The switching pulse timing; Indicates the first j The modulation index of each unit, i.e., the desired AC voltage fundamental per unit value; Indicates the first j The weighting coefficients of each converter are used to customize the proportion of its total DC bus ripple optimization to the AC side current optimization of each unit.

[0023] Secondly, the present invention provides an optimal modulation system for large-capacity DC interconnected power conversion clusters.

[0024] An optimal modulation system for large-capacity DC-DC interconnected power conversion clusters includes:

[0025] The model building module is used to analyze the coupling mechanism of pulse current on the DC side within the cluster and build a global optimization model with the goal of minimizing DC bus current ripple.

[0026] The data acquisition module is used to collect the modulation index and cluster circuit parameter data of each converter in the high-capacity DC interconnected power conversion cluster at the current moment.

[0027] The optimal pulse sequence solution module is used to solve the model using the gradient descent algorithm based on the acquired data, and obtain the optimal combination of switching pulse sequences of the converter cluster at the current sampling time.

[0028] The optimal pulse width extension module is used to adjust the modulation value in the global optimization model based on the modulation data at the sampling time, and calculate the optimal pulse sequence within the modulation deviation range to control the modulation of the switching pulses of each converter.

[0029] Thirdly, the present invention also provides an electronic device, comprising: a memory for storing executable instructions; and a processor for implementing the above-described optimal modulation method for large-capacity DC-DC interconnected power conversion clusters when executing the executable instructions stored in the memory.

[0030] Fourthly, the present invention also provides a computer-readable storage medium storing executable instructions for causing a processor to execute the executable instructions to implement the above-described optimal modulation method for large-capacity DC-DC interconnected power conversion clusters.

[0031] Fifthly, the present invention also provides a computer program product comprising executable instructions stored in a computer-readable storage medium; wherein, when the processor of an electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the above-mentioned optimal modulation method for large-capacity DC interconnect power conversion clusters is implemented.

[0032] The above one or more technical solutions have the following beneficial effects:

[0033] 1. This invention proposes an optimal modulation method and system for large-capacity DC-DC interconnected power conversion clusters. By analyzing the coupling mechanism of pulse currents on the DC side within the cluster, a global optimization model is constructed at the cluster scale with the goal of minimizing DC bus feed current ripple. The method fully utilizes the additional control degrees of freedom provided by the parallel operation of the cluster to solve for and generate the optimal switching pulse sequence, achieving significant suppression of DC-side current ripple at the system level and avoiding ripple amplification caused by the superposition of switching pulses from various converter units on the DC side. This method can significantly reduce DC-side current ripple at the system level without requiring large-capacity passive filters for ripple compensation, greatly reducing system size and cost. It also reduces power losses associated with passive filters, improving system operating efficiency, stability, and economy. This provides strong technical support for the promotion and application of large-scale new energy grid connection and DC interconnection systems. By deploying this method, the grid-friendliness of power electronic power conversion equipment can be improved, the size and capacity of DC filters can be reduced, cost reduction and efficiency improvement of new energy converter equipment can be promoted, and the development of related industrial chains can be driven.

[0034] 2. Compared with existing programming modulation methods that can only optimize AC harmonics at the single-machine level and cannot take into account the control of the common convergence point (PCC), this invention focuses on the collaborative control of the trunking system. It realizes information sharing and collaborative modulation of each converter through the central control unit, giving full play to the collaborative advantages of the trunking system, realizing the control of the common convergence point (PCC), and further improving the ripple suppression effect and system operation stability.

[0035] 3. By extending the optimal pulse width, this invention prepares the optimal pulse sequence that can be used by each converter within a wide operating range, enabling the system to adapt to changes in the modulation scheme under actual operating conditions and ensuring good ripple suppression under different operating conditions, thus further improving the practicality and reliability of this method.

[0036] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0037] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0038] Figure 1 This is a flowchart of the optimal modulation method for large-capacity DC-DC interconnected power conversion clusters proposed in this embodiment of the invention;

[0039] Figure 2 This is a schematic diagram of the connection of the AC / DC hybrid microgrid as described in the embodiments of the present invention;

[0040] Figure 3 This is a schematic diagram of the switching pulse sequence of a three-level power converter in an embodiment of the present invention;

[0041] Figure 4 This is a schematic diagram of the grid-connected frequency domain model connection of the power converter in an embodiment of the present invention. Detailed Implementation

[0042] It should be noted that the following detailed descriptions are exemplary and are intended only to describe specific embodiments and to provide further explanation of the invention, and are not intended to limit the scope of exemplary embodiments of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0043] Example 1

[0044] For DC current ripple suppression in large-capacity DC-connected power conversion clusters, traditional carrier modulation and space vector modulation methods tend to generate a large number of low-order components at low switching frequencies, making it difficult to effectively suppress DC bus ripple. This often necessitates the configuration of large-capacity passive filters, significantly increasing system size and cost. While existing programmable modulation methods can reduce AC harmonics at the single-machine level, they fail to consider the ripple effect of DC bus current and do not fully utilize the cooperative degrees of freedom of the cluster system. Considering the difficulty of existing methods in effectively suppressing DC current ripple, this embodiment proposes an optimal modulation method for large-capacity DC-connected power conversion clusters. This method involves global modeling and optimization at the cluster scale, fully utilizing the additional control degrees of freedom provided by the parallel system to perform cooperative power quality optimization modulation, achieving significant suppression of DC-side current ripple.

[0045] The optimal modulation method for large-capacity DC-DC interconnected power conversion clusters proposed in this embodiment is based on the following: a global optimization model is constructed with the goal of minimizing DC bus feed current ripple. Utilizing the additional control degrees of freedom provided by the cluster, the optimal switching pulse sequence is calculated, thereby significantly reducing DC-side current ripple at the system level. For example... Figure 1 As shown, the method specifically includes the following steps:

[0046] Step S1: Analyze the coupling mechanism of pulse current on the DC side within the cluster and construct a global optimization model with the goal of minimizing DC bus current ripple.

[0047] Step S2: Collect the modulation index and cluster circuit parameter data of each converter in the high-capacity DC-DC interconnected power conversion cluster at the current moment;

[0048] Step S3: Based on the acquired data, the gradient descent algorithm is used to solve the model to obtain the optimal switching pulse sequence combination of the converter cluster at the current sampling time;

[0049] Step S4: Based on the modulation data at the sampling time, adjust the modulation value in the global optimization model and calculate the optimal pulse sequence within the modulation deviation range to control the modulation of the switching pulses of each converter.

[0050] For example Figure 2 Taking the three-level interconnected converter cluster of the AC / DC hybrid microgrid as an example, this paper provides a more detailed introduction to the optimal modulation method from two aspects: the analysis of the DC bus ripple coupling mechanism and the detailed deployment scheme of the algorithm.

[0051] In step S1, the coupling mechanism of DC bus ripple is analyzed, and the optimization of the total DC bus current ripple is transformed into the optimization of the harmonic components of the grid-connected AC current of each unit on the AC side, and the corresponding global optimization model is constructed.

[0052] Specifically, firstly, based on the law of conservation of energy, the relationship between the DC-side current and the AC-side current of each power converter (or simply converter) in the cluster is derived. Specifically, according to the law of conservation of energy, the DC-side power of a power converter is equal to its AC-side power, that is:

[0053] ;

[0054] In the above formula, This is the DC bus voltage. For the bus current of a single converter, , , These are the three-phase AC voltages of the converter, and , , This refers to the three-phase AC grid-connected current of the converter. Furthermore, the DC bus current... This can be represented by alternating current, that is:

[0055] ;

[0056] For a large-capacity DC power conversion cluster, the total current at the DC collection point is the sum of the DC currents of each converter unit. Therefore, its total current can be expressed as:

[0057] ;

[0058] In the above formula, This is the total DC current. For unit numbering, The total number of converter clusters, For the first j The DC bus current of the converter.

[0059] Furthermore, since the DC side of the aforementioned power conversion cluster is connected in parallel to the same DC power grid, and the AC side is also connected to the same AC power grid, the DC voltage of each unit is... Approximately equal, the three-phase AC voltages of each unit are... , , They are also approximately equal; therefore, the derivation of the total DC bus feed current can be further simplified to:

[0060] ;

[0061] In the above formula, This indicates the phase sequence of the three phases in the alternating current. express x AC voltage of phase, Representation unit j of x The phase of the AC grid-connected current.

[0062] Based on the above derivation, it can be concluded that the analysis of DC bus ripple can be indirectly achieved by analyzing AC side ripple. Furthermore, due to the DC bus voltage... With AC power grid voltage Since the amplitude cannot be adjusted from the existing system setpoint, the optimization of the total DC bus current ripple can be transformed into:

[0063] .

[0064] It should be noted that, since the three phases of the AC measurement are symmetrical and balanced under rated operating conditions, for ease of understanding, the subsequent derivation will use one phase as an example for explanation.

[0065] Secondly, targeting Optimization is then performed. Specifically, according to Fourier series theory, any periodic signal satisfying the Dirichlet conditions can be expanded into a series of sine functions of different frequencies, i.e.:

[0066] ;

[0067] In the above formula, T is the fundamental frequency period, and n represents the harmonic order of the decomposed frequency. a n , b n These are the Fourier coefficients. Therefore, for example, Figure 3 The following is a pulse sequence generated by a three-level converter. Its horizontal axis represents angle, and its vertical axis represents pulse sequence, which can be used to determine its harmonic composition, namely:

[0068] ;

[0069] In the above formula, Indicates frequency.

[0070] Due to the quarter-symmetric waveform (e.g.) Figure 3 (As shown) It possesses the inherent ability to eliminate even-order components and cosine quantities. Therefore, this invention adopts this symmetrical scheme, that is, based on the waveform of the quarter-symmetric switching pulse sequence, the relationship between its voltage harmonics and the switching pulse is determined, which can be expressed as:

[0071] ;

[0072] In the above formula, It's the order in which the time is switched. It is the number of switching times within a quarter of a cycle. The pulse number is represented by the first pulse. The switching pulse timing This indicates the DC side voltage.

[0073] Furthermore, based on the above formula, the relationship between the pulse sequence and voltage harmonics can be accurately established as follows:

[0074] ;

[0075] In the above formula, v n This is a harmonic voltage.

[0076] Then, based on an AC grid-connected system with a single converter, the relationship between harmonic current and harmonic voltage is analyzed. Specifically, for example... Figure 4The AC grid-connected frequency domain model of a single power converter is shown, and the AC grid-connected system is analyzed. n Under subharmonic voltage excitation n The second harmonic current can be expressed as:

[0077] ;

[0078] In the above formula, For harmonic orders, and Let L be the system fundamental frequency, L be the inductance of the AC filter, and R be its parasitic resistance. Since the parasitic inductance is much smaller than the filter reactance, it can be approximately ignored.

[0079] Finally, based on the relationships obtained from the above analysis, optimizing the DC bus current ripple can be equivalent to optimizing the assumed total AC current. The optimization of the total DC current ripple is not possible because the basic AC grid-connected current is given by the power command and cannot be adjusted. Therefore, the optimization of the total DC current ripple will be converted into optimization of the AC grid-connected AC current of each unit on the AC side. The optimization of harmonic components, namely:

[0080] .

[0081] Based on the above derivation, the timing of the switching pulse can be established. AC current to the unit grid Then to the unit With cluster DC feed current The optimized transmission relationship can then be established. Furthermore, a global optimization problem can be constructed to suppress the total DC feed current ripple of the AC / DC interconnected power conversion cluster, while simultaneously optimizing its AC unit grid-connected current. The overall optimization equation, with the goal of minimizing the DC bus current ripple, can be described as follows:

[0082] ;

[0083] In the above formula, This represents the DC current of the j-th unit on the DC side. This represents the nth harmonic current of the xth phase in the j-th unit on the AC side; Represents the DC side voltage; EF (Evaluation Function) represents the degree of optimization for the DC bus current ripple. Indicates the first j The first converter i The switching pulse timing; Indicates the first j The modulation index of each unit, i.e., the desired AC voltage fundamental per unit value; Indicates the first jThe weighting coefficients of each converter are used to customize the proportion of its total DC bus ripple optimization to the AC side current optimization of each unit.

[0084] Furthermore, based on the above model, the scheme is deployed through a unified central control unit. The deployment process can be divided into: 1) global information collection; 2) ideal operating point determination; 3) optimal pulse width extension and information distribution, i.e.:

[0085] In step S2, global information is collected. Through a unified central control unit, the steady-state operating information (i.e., modulation index) of each converter in the DC-DC power conversion cluster at the current moment is collected. And circuit parameters: AC filter inductance L, parasitic resistance R.

[0086] In step S3, the sampling point pulse sequence is solved. Specifically, based on the optimization problem constructed in step S1, the gradient descent algorithm is used to solve it. That is, the current gradient is calculated based on the initial iteration point (which can be randomly selected), and then the iteration point is updated according to the set step size. The gradient is then calculated again, and this process is repeated iteratively until convergence. This solution yields the optimal combination of switching pulse sequences for the converter cluster at the measurement time. In other words, each converter can suppress the DC grid feed current ripple and optimize the AC grid connection quality of each unit by operating according to this switching pulse sequence.

[0087] It should be noted that step S3 above only obtains the optimal pulse sequence combination at the instant of sampling. However, under actual operating conditions (even under steady-state conditions), the modulation index of the converter system is not fixed. Therefore, it is necessary to extend the solved pulse sequence to obtain the optimal pulse sequence within a larger modulation index range, i.e.:

[0088] Step S4, Extend the optimal pulse width: Based on the modulation data collected at the sampling time Further adjust the value of the modulation index in the optimization problem. The optimal pulse sequence within the modulation deviation range is calculated, and the optimal pulse sequence usable by each converter in a wide operating range is prepared from this, forming an optimal pulse sequence lookup table (LUT).

[0089] Here, B refers to the modulation deviation value corresponding to the range of the lookup table that needs to be extended. It can be set according to the fluctuation range of the modulation in steady state. Taking the modulation at the ideal operating point as an example, if B is set to 0.1, the extended lookup table will cover the range of 0.6-0.8.

[0090] In practice, since the gradient descent method is used in step S3 to solve the problem, it is necessary to set an initial iteration point. The value of this iteration point directly affects the convergence of the algorithm. Therefore, it is usually necessary to randomly set a large number of initial iteration points for calculation to ensure the optimality of the result. In this embodiment, the ideal working point is calculated first through step S3, and then the extension is performed based on the ideal working point in step S4. In this way, a large number of initial iteration points are only needed in the calculation of the first ideal working point, so as to provide a good initial point for the subsequent LUT extension. Then, in the subsequent LUT extension stage, only the optimal solution obtained in the previous solution needs to be used as the initial iteration point for the next calculation. This can effectively reduce the amount of computation while ensuring the optimization effect.

[0091] Finally, the unified central control unit sends the optimal pulse sequence lookup table (LUT) to the local controllers of each converter through the existing communication link. The LUT contains a series of optimal pulse sequences corresponding to different modulation schemes. Based on the actual operating conditions, each controller selects the final optimal pulse sequence according to the LUT and modulates the switching pulse sequence. Based on the modulated pulse sequence, the current ripple of the DC bus of the power group can be effectively suppressed.

[0092] The above methods can significantly reduce the DC-side current ripple level, thereby reducing the reliance on large-capacity DC filter devices, improving the operating efficiency and equipment economy of DC / AC-DC hybrid microgrids, and providing reliable technical support for the large-scale integration of new energy sources into the grid.

[0093] Example 2

[0094] This embodiment provides an optimal modulation system for large-capacity DC-DC interconnected power conversion clusters, including:

[0095] The model building module is used to analyze the coupling mechanism of pulse current on the DC side within the cluster and build a global optimization model with the goal of minimizing DC bus current ripple.

[0096] The data acquisition module is used to collect the modulation index and cluster circuit parameter data of each converter in the high-capacity DC interconnected power conversion cluster at the current moment.

[0097] The optimal pulse sequence solution module is used to solve the model using the gradient descent algorithm based on the acquired data, and obtain the optimal combination of switching pulse sequences of the converter cluster at the current sampling time.

[0098] The optimal pulse width extension module is used to adjust the modulation value in the global optimization model based on the modulation data at the sampling time, and calculate the optimal pulse sequence within the modulation deviation range to control the modulation of the switching pulses of each converter.

[0099] Example 3

[0100] This embodiment provides an electronic device, including: a memory for storing executable instructions; and a processor for executing the executable instructions stored in the memory to implement the method provided in this embodiment.

[0101] Example 4

[0102] This embodiment also provides a computer-readable storage medium storing executable instructions, which, when executed by a processor, will cause the processor to execute the method described above in this embodiment.

[0103] Example 5

[0104] This embodiment provides a computer program product including executable instructions, which are computer instructions; the executable instructions are stored in a computer-readable storage medium. When the processor of an electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the electronic device performs the method described in this embodiment.

[0105] The steps involved in Embodiments 2 to 5 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0106] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computer devices. Optionally, they can be implemented using computer-executable program code, thereby allowing them to be stored in a storage device for execution by a computer device, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. The present invention is not limited to any particular combination of hardware and software.

[0107] The above description is only a preferred embodiment of the present invention. Although the specific implementation of the present invention has been described in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that, based on the technical solution of the present invention, various modifications or variations that can be made by those skilled in the art without creative effort are still within the scope of protection of the present invention.

Claims

1. An optimal modulation method for a high-capacity direct current interconnected power conversion cluster, characterized in that, The application relates to a method for optimizing pulse width modulation of a large-capacity direct-current interconnected power conversion cluster. The method comprises the following steps: Collecting modulation degrees of each converter in the large-capacity direct-current interconnected power conversion cluster at the current moment and collecting circuit parameter data of the cluster; According to the obtained data, a gradient descent algorithm is used to solve the model to obtain an optimal switch pulse sequence combination of the converter cluster at the current sampling moment; According to the modulation degree data of the sampling moment, the modulation degree value in the global optimization model is adjusted, and the optimal pulse sequence in the modulation degree deviation range is calculated to control the modulation of the switch pulse of each converter. The global optimization model is as follows: ； In the above formula, represents the direct current of the first j unit on the direct current side, represents the alternating current of the first j unit of the first phase, x n harmonic current; EF represents the optimization degree for the direct current bus current ripple; represents the direct current voltage on the direct current side; represents the switching time of the first j converter of the first i switching pulse; represents the modulation degree of the first j unit, that is, the expected alternating voltage fundamental unit value; represents the weight coefficient of the first j converter, which is used to customize the proportion of the total direct current bus ripple optimization and the alternating current optimization of each unit thereof.​ 2. The optimal modulation method for a high-capacity DC interconnected power conversion cluster according to claim 1, wherein, The global optimization model is constructed by the following steps: Based on the law of conservation of energy, the relationship between the direct-current side current of each converter in the cluster and the alternating-current side current is derived; According to the Fourier series theory, the correlation between the switch pulse sequence of the converter and the alternating-current voltage harmonic is analyzed; According to the alternating-current grid-connected frequency domain model of a single converter, the n-th harmonic current of the alternating-current grid-connected system under the excitation of the n-th harmonic voltage is analyzed, and the relationship between the harmonic current and the harmonic voltage is obtained; The three types of relationships derived are integrated to convert the optimization of the total direct-current bus current ripple into the optimization of the harmonic components of the alternating-current side grid-connected alternating-current of each unit, and a global optimization model with the minimum direct-current bus current ripple as the target is constructed.

3. The optimal modulation method for the high-capacity DC interconnected power conversion cluster of claim 2, wherein, The derivation process of the relationship between the direct-current side current of each converter in the cluster and the alternating-current side current is as follows: According to the law of conservation of energy, the direct-current side power of the converter is equal to the alternating-current side power, which is expressed as: ； Further, the expression of the direct-current bus current through the alternating-current current is obtained, which is as follows: ； For the large-capacity direct-current interconnected power conversion cluster, the total current of the direct-current collection point is the sum of the direct-current currents of each conversion unit, and the direct-current voltages of each unit are approximately equal, and the three-phase alternating-current voltages of each unit are approximately equal, so the total direct-current current is expressed as: ； In the above formula, is the DC bus voltage, is the bus current of a single converter, , , are the three-phase AC voltages of the converter, , , are the three-phase AC grid currents of the converter; represents the DC bus current; is the total DC current, is the unit number, is the total number of converter clusters, is the DC bus current of the j th converter; represents the phase sequence of the AC three-phase, represents the AC voltage of the x phase, represents the AC grid current of the j phase of the x unit.

4. The optimal modulation method for a high-capacity DC interconnected power conversion cluster according to claim 2, wherein, The analysis process of the correlation between the switch pulse sequence of the converter and the alternating-current voltage harmonic is as follows: According to the Fourier series theory, the harmonic composition of the switch pulse sequence generated by the three-level converter is determined; The relationship between the switch pulse and the voltage harmonic is determined by using the quarter-symmetrical switch pulse sequence waveform, and the relationship between the pulse sequence and the voltage harmonic is established, which is as follows: ； In the above formula, n represents the harmonic order of decomposition, a n , b n is the Fourier coefficient, v n is the harmonic voltage.

5. The optimal modulation method for the high-capacity DC interconnected power conversion cluster of claim 2, wherein, In the alternating-current grid-connected frequency domain model of a single converter, the n-th harmonic current expression of the system under the excitation of the n-th harmonic voltage is as follows: ； In the above formula, v n is the harmonic voltage, is the harmonic order, and is the system fundamental frequency, L is the inductance value of the AC filter, and R is the parasitic resistance value thereof.

6. An optimal modulation system for a high-capacity DC interconnected power conversion cluster, which executes the optimal modulation method for a high-capacity DC interconnected power conversion cluster according to any one of claims 1 to 5, characterized in that, The method comprises the following steps: The model construction module is used for analyzing the coupling mechanism of the pulse current in the cluster on the direct-current side, and constructing a global optimization model with the minimum direct-current bus current ripple as the target; The data acquisition module is used for collecting the modulation degrees of each converter in the large-capacity direct-current interconnected power conversion cluster at the current moment and collecting the circuit parameter data of the cluster; The optimal pulse sequence solving module is used for solving the model by using a gradient descent algorithm according to the obtained data to obtain an optimal switch pulse sequence combination of the converter cluster at the current sampling moment; The optimal pulse width extension module is used for adjusting the modulation degree value in the global optimization model according to the modulation degree data of the sampling moment, calculating the optimal pulse sequence in the modulation degree deviation range, and controlling the modulation of the switch pulse of each converter.

7. An electronic device, comprising: The method comprises the following steps: a memory for storing executable instructions; a processor for executing the executable instructions stored in the memory to implement the optimal modulation method for a high-capacity DC interconnected power conversion cluster according to any one of claims 1-5.

8. A computer-readable storage medium, characterized in that, executable instructions stored in the memory for causing the processor to execute the executable instructions to implement the optimal modulation method for a high-capacity DC interconnected power conversion cluster according to any one of claims 1-5.

9. A computer program product, characterised in that, The computer program product includes executable instructions stored in a computer-readable storage medium; When the processor of the electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the optimal modulation method for a high-capacity DC interconnected power conversion cluster according to any one of claims 1-5 is implemented.

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