Method for optimizing configuration of sub-module quantity of modular multilevel converter

Abstract

The application provides a sub-module quantity optimization configuration method of a modular multilevel converter, relates to the field of converters, and is applied to a segmented half-bridge-full-bridge hybrid modular multilevel converter including three-phase bridge arms, each phase bridge arm being composed of an upper bridge arm and a lower bridge arm, the upper bridge arm including cascaded half-bridge sections and full-bridge sections, the half-bridge section including at least two cascaded half-bridge type sub-modules, and the full-bridge section including at least two cascaded full-bridge type sub-modules, and the method comprises the following steps: constructing an amplitude function of an energy difference term of the half-bridge section and the full-bridge section; constructing an energy coupling degree function of the half-bridge section and the full-bridge section; constructing a bridge arm energy coordination function based on the amplitude function of the energy difference term of the half-bridge section and the full-bridge section and the energy coupling degree function; and determining the number of the half-bridge type sub-modules included in the half-bridge section and the number of the full-bridge type sub-modules included in the full-bridge section, so that the converter has the advantages of efficient and reliable support.

Description

Technical Field

[0001] This invention relates to the field of converters, and in particular to a method for optimizing the configuration of the number of sub-modules in a modular multilevel converter. Background Technology

[0002] With the large-scale integration of new energy sources into the power grid and the rapid growth in demand for long-distance, high-capacity power transmission, flexible DC transmission technology is gradually becoming an important component of new power systems due to its advantages such as controllable power, bidirectional transmission, and ease of grid connection. Modular multilevel converters (MMCs), as the core energy conversion unit of flexible DC transmission systems, are widely used in wind power, photovoltaics, grid interconnection, and power quality control due to their excellent output waveform, low voltage stress, and strong modular scalability. Currently, the mainstream structures of MMCs are mainly divided into three types: half-bridge, full-bridge, and hybrid. Half-bridge MMCs have a simple structure, low loss, and high efficiency, but they suffer from difficulties in DC fault protection and significant low-frequency harmonic components. Full-bridge MMCs possess excellent voltage regulation and fault ride-through capabilities, but due to the increased number of components and losses, the control strategy becomes more complex, leading to reduced system efficiency. Hybrid MMCs, by alternately configuring half-bridge and full-bridge submodules, balance efficiency and fault tolerance, improving the system's fault ride-through capability and operational flexibility.

[0003] However, in low-frequency, high-power flexible DC transmission scenarios, the operating characteristics of hybrid MMCs are significantly limited. The alternating module structure leads to uneven energy distribution in the bridge arms and amplified circulating currents. Frequent energy transfer between different modules makes it difficult to maintain system energy balance. Furthermore, the ratio of half-bridge to full-bridge submodules is usually selected empirically, lacking a unified mathematical model, resulting in significant differences in system energy balance and voltage support performance under different configurations. Asynchronous conduction characteristics between submodules and large capacitor voltage fluctuations necessitate complex voltage equalization control algorithms to maintain stable operation.

[0004] Therefore, a method for optimizing the number of submodules in a modular multilevel converter is needed to achieve efficient and reliable converter support. Summary of the Invention

[0005] This invention provides a method for optimizing the number of submodules in a modular multilevel converter. It is applied to a segmented half-bridge / full-bridge hybrid modular multilevel converter comprising three-phase arms, each phase arm consisting of an upper arm and a lower arm. The upper arm includes cascaded half-bridge and full-bridge sections. Each half-bridge section includes at least two cascaded half-bridge submodules, and each full-bridge section includes at least two cascaded full-bridge submodules. The method includes: constructing an amplitude function for the energy difference term of the half-bridge and full-bridge sections; constructing an energy coupling function for the half-bridge and full-bridge sections; constructing an arm energy coordination function based on the amplitude function and energy coupling function of the energy difference term of the half-bridge and full-bridge sections; and determining the number of half-bridge submodules included in the half-bridge section and the number of full-bridge submodules included in the full-bridge section based on the arm energy coordination function.

[0006] Furthermore, the amplitude functions of the energy difference terms for the half-bridge section and the full-bridge section are constructed, including: constructing the energy fluctuation function of the half-bridge section and the energy fluctuation function of the full-bridge section; and constructing the amplitude functions of the energy difference terms for the half-bridge section and the full-bridge section based on the energy fluctuation functions of the half-bridge section and the full-bridge section.

[0007] Furthermore, the energy fluctuation function of the half-bridge section is constructed, including: constructing the instantaneous energy storage function of the half-bridge submodule based on the equivalent capacitance and instantaneous capacitor voltage of the half-bridge submodule; constructing the instantaneous energy function of the half-bridge section based on the instantaneous energy storage function and quantity of the half-bridge submodule; and constructing the energy fluctuation function of the half-bridge section based on the instantaneous energy function of the half-bridge section.

[0008] Furthermore, energy coupling functions for the half-bridge and full-bridge sections are constructed, including: constructing an energy participation function for the half-bridge section based on the number of half-bridge submodules and their equivalent capacitance; constructing an energy participation function for the full-bridge section based on the number of full-bridge submodules and their equivalent capacitance; and constructing energy coupling functions for the half-bridge and full-bridge sections based on the energy participation functions of the half-bridge and full-bridge sections.

[0009] Furthermore, based on the following formulas, the energy coupling degree functions for the half-bridge section and the full-bridge section are:

[0010] ,

[0011] ,

[0012] ,

[0013] in, The energy coupling degree of the half-bridge section and the full-bridge section, For the energy participation of the half-bridge section, The energy participation rate of the entire bridge section, The number of half-bridge submodules, This is the equivalent capacitance of the half-bridge submodule. The number of full-bridge submodules, This is the equivalent capacitance of the full-bridge submodule.

[0014] Furthermore, based on the bridge arm energy coordination function, the number of half-bridge sub-modules included in the half-bridge section and the number of full-bridge sub-modules included in the full-bridge section are determined, including:

[0015] Based on the bridge arm energy coordination function, an objective function is constructed to evaluate different submodule quantity configuration schemes, wherein the objective function takes the maximum value of the bridge arm energy coordination function as the optimization objective.

[0016] Under the constraint of the total number of sub-modules of the bridge arm, Lagrange multipliers are introduced, and the equation to be solved is constructed based on the objective function;

[0017] The equation to be solved is solved to determine the number of half-bridge submodules and the number of full-bridge submodules.

[0018] Furthermore, the bridge arm energy cooperative function is:

[0019] ,

[0020] in, Let be the bridge arm energy coordination function. The number of half-bridge submodules, This represents the total number of half-bridge and full-bridge submodules in each phase arm.

[0021] Furthermore, the equation to be solved is:

[0022] ,

[0023] in, The equation to be solved is It is a Lagrange multiplier.

[0024] Further, the equation to be solved is solved to determine the number of half-bridge sub-modules included in the half-bridge section and the number of full-bridge sub-modules included in the full-bridge section, including:

[0025] Based on the equation to be solved, the number of half-bridge submodules and the Lagrange multipliers are differentiated, and combined with the constraints, the number of half-bridge submodules included in the half-bridge section and the number of full-bridge submodules included in the full-bridge section are determined, wherein the constraints are as follows: , This represents the number of full-bridge submodules.

[0026] Furthermore, it also includes: verifying the number of half-bridge sub-modules included in the half-bridge section and the number of full-bridge sub-modules included in the full-bridge section by using the second derivative of the bridge arm energy coordination function with respect to the number of half-bridge sub-modules.

[0027] Compared with existing technologies, the method for optimizing the number of submodules in a modular multilevel converter provided by this invention has at least the following advantages:

[0028] By constructing the energy difference magnitude function and energy coupling degree function between the half-bridge and full-bridge sections, and based on this, a bridge arm energy coordination function is built to determine the number of submodules. This energy coordination-based configuration clarifies the energy flow direction between the main power region and the polarity adjustment region, resulting in a more balanced bridge arm voltage support relationship and reduced power crossover. After optimizing the submodule ratio, DC-side voltage fluctuations are reduced during low-frequency operation, power transmission and voltage output are more stable, and voltage stress accumulation is reduced during long-term operation, significantly improving the system's steady-state reliability and ensuring stable system operation. Attached Figure Description

[0029] This specification will be further described by way of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting; in these embodiments, the same reference numerals denote the same structures, wherein:

[0030] Figure 1 This is a schematic diagram of the structure of a segmented half-bridge-full-bridge hybrid modular multilevel converter, as shown in some embodiments of this specification.

[0031] Figure 2 This is a schematic diagram of the process for cross-segment coordinated control and voltage regulation optimization according to some embodiments of this specification;

[0032] Figure 3 This is a flowchart illustrating a method for optimizing the number of submodules in a modular multilevel converter, as shown in some embodiments of this specification.

[0033] Figure 4 The waveform diagram of the output voltage of a segmented half-bridge-full-bridge hybrid modular multilevel converter is shown in some embodiments of this specification.

[0034] Figure 5 The waveform diagram of the output current of a segmented half-bridge-full-bridge hybrid modular multilevel converter is shown in some embodiments of this specification.

[0035] Figure 6 The waveform diagrams are power waveforms of a segmented half-bridge-full-bridge hybrid modular multilevel converter as shown in some embodiments of this specification.

[0036] Figure 7 The waveform diagram of DC voltage based on a segmented half-bridge-full-bridge hybrid modular multilevel converter is shown in some embodiments of this specification.

[0037] Figure 8 This is a diagram of the bridge arm circulation waveform under the optimal quantity configuration conditions shown in some embodiments of this specification;

[0038] Figure 9 These are bridge arm circulation waveform diagrams shown in some embodiments of this specification when the optimal number configuration is not performed using this method;

[0039] Figure 10 This is a waveform diagram of the capacitor voltage of a submodule shown in some embodiments of this specification;

[0040] Figure 11 This is based on some embodiments shown in this specification. Figure 10 A magnified waveform of a portion of the image;

[0041] Figure 12 This is a waveform diagram of the capacitor voltage of a submodule under non-optimal configuration, as shown in some embodiments of this specification.

[0042] Figure 13 This is based on some embodiments shown in this specification. Figure 12 A magnified waveform of a portion of the waveform. Detailed Implementation

[0043] To more clearly illustrate the technical solutions of the embodiments in this specification, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are merely some examples or embodiments of this specification. For those skilled in the art, these drawings can be applied to other similar scenarios without creative effort. Unless obvious from the context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0044] Among existing flexible DC transmission systems and high-power conversion devices, the modular multilevel converter (MMC) is the most representative main topology. This structure forms a multilevel output by cascading multiple power submodules, offering advantages such as excellent output waveform, low voltage stress, and high modularity. Based on the topology and arrangement of the submodules, common MMCs mainly include three types: half-bridge, full-bridge, and hybrid.

[0045] (1) Half-bridge modular multilevel converter (HB-MMC):

[0046] A half-bridge modular multilevel converter consists of a bridge arm formed by two power switching devices and a capacitor connected in series. For example, in a three-phase six-arm configuration, each bridge arm is composed of several half-bridge submodules connected in series to achieve voltage output. This topology has advantages such as simple structure, low switching losses, and mature control strategies, and is widely used in high-voltage, high-current applications. Since each submodule only bears a portion of the total voltage, the number of system output levels can be flexibly expanded, and modular maintenance is convenient.

[0047] However, half-bridge submodules can only output unipolar voltage and lack polarity regulation capability on the DC side, making it unable to interrupt current during DC faults and resulting in poor system safety. Especially under low-frequency operating conditions, frequent energy exchange within the bridge arms leads to a prominent circulating current component, causing large fluctuations in the capacitor voltage of the half-bridge submodules, making it difficult to maintain energy balance, and affecting the stability and lifespan of the converter.

[0048] (2) Full-bridge modular multilevel converter (FB-MMC):

[0049] Each full-bridge submodule of the full-bridge modular multilevel converter consists of four power switching devices and one capacitor, enabling positive, negative, and zero-level outputs. This structure can actively cut off the current path during DC faults, achieving voltage reversal and fault isolation. It possesses excellent DC fault ride-through capability and voltage control capability, making it suitable for high-reliability, fault-tolerant power systems.

[0050] However, the number of switching devices and losses in a full-bridge modular multilevel converter are significantly higher than those in a half-bridge sub-module. The increased number of devices leads to higher switching frequencies, greater heat loss, and decreased system efficiency; it also increases control complexity and places higher demands on the real-time performance of modulation strategies and synchronization algorithms. Therefore, although full-bridge modular multilevel converters offer enhanced functionality, their higher cost and losses in high-power, low-frequency applications make large-scale adoption difficult.

[0051] (3) Alternating half-bridge-full-bridge hybrid modular multilevel converter (Hybrid-MMC):

[0052] The alternating half-bridge-full-bridge hybrid modular multilevel converter combines the high efficiency of a half-bridge converter with the fault tolerance of a full-bridge converter. In this structure, half-bridge and full-bridge submodules are arranged alternately in a certain proportion within each arm: the half-bridge submodules are used for main power transmission, while the full-bridge submodules are responsible for polarity regulation and DC-side voltage control. This hybrid topology, to a certain extent, simultaneously possesses the advantages of high efficiency and high reliability, and is one of the mainstream solutions for current flexible DC transmission systems.

[0053] However, the alternating half-bridge-full-bridge hybrid modular multilevel converter still has the following shortcomings:

[0054] 1. Uneven energy distribution: The staggered arrangement makes the power flow path of the bridge arm complex, and energy is frequently transferred between different types of sub-modules, resulting in asymmetrical energy distribution and increased circulation in the bridge arm.

[0055] 2. Lack of theoretical basis for submodule quantity configuration: Currently, the number of half-bridge and full-bridge submodules in alternating half-bridge-full-bridge hybrid modular multilevel converters is mostly determined empirically, such as "half half-bridge and half full-bridge" or "full-bridge with a small amount of compensation," lacking a unified mathematical optimization criterion. Energy balance and fault tolerance performance vary significantly under different quantity combinations, and there is no precise calculation method to determine the optimal allocation ratio.

[0056] 3. Poor voltage balance: The conduction characteristics of half-bridge submodules and full-bridge submodules are different. The voltage changes of the submodule capacitors are not synchronized, making voltage equalization control complex and dynamic performance poor.

[0057] Therefore, although the existing alternating half-bridge-full-bridge hybrid modular multilevel converter achieves a performance trade-off in terms of functionality, the allocation of the number of its sub-modules still relies on experience and lacks a theoretical analysis method from the perspective of energy distribution. Especially under low-frequency operating conditions, the problems of energy coupling and circulating current amplification are more prominent, making it difficult to meet the requirements of high stability and high efficiency.

[0058] The method for optimizing the number of submodules in a modular multilevel converter is applied to a segmented half-bridge-full-bridge hybrid modular multilevel converter. Figure 1 This is a structural schematic diagram of a segmented half-bridge-full-bridge hybrid modular multilevel converter, as shown in some embodiments of this specification. Figure 1 As shown, the modular multilevel converter based on a segmented half-bridge-full-bridge hybrid design includes three-phase arms, each consisting of an upper arm and a lower arm. Energy conversion between the AC and DC sides is achieved through the coordinated operation of the upper and lower arms. The upper arm includes cascaded half-bridge and full-bridge sections. The half-bridge section includes at least two cascaded half-bridge submodules (e.g., HBS M1, HBS Mn), primarily responsible for main power transmission. The half-bridge submodules are simple in structure, low in loss, and high in efficiency, efficiently processing and transmitting the input electrical energy to meet the system's basic requirements for main power transmission. The full-bridge section includes at least two cascaded full-bridge submodules (e.g., FBS M1, FBS Mn), whose main functions are voltage polarity regulation and fault bypass. The full-bridge submodules can output positive, negative, and zero voltage levels, allowing for flexible adjustment of voltage polarity to adapt to different operating conditions. Figure 1 In this circuit, Idc is the DC current on the DC side, and Udc is the DC voltage on the DC side. The outputs of the upper and lower bridge arms are connected to the DC side to achieve energy interaction with the DC system. The midpoint of each phase bridge arm is connected to the AC system.

[0059] By controlling the switching states of the half-bridge and full-bridge submodules in the upper and lower bridge arms, the output voltage of the bridge arms is adjusted, thereby controlling the voltage and current on the AC side and achieving AC-DC conversion. Simultaneously, through a reasonable configuration of the number of submodules, the converter's performance can be optimized, problems such as uneven energy distribution and difficulty in suppressing circulating currents can be solved, and the system's energy balance and fault tolerance can be improved.

[0060] The segmented half-bridge-full-bridge hybrid modular multilevel converter achieves natural separation in its main power transmission and voltage polarity regulation functions, resulting in clearer energy transfer paths between different functional areas and fundamentally reducing energy coupling and circulating current excitation. This avoids the problems of uneven energy distribution and circulating current amplification caused by the crossover of power flow paths in bridge arms and frequent energy transfer between different types of modules, which are common in traditional alternating arrangement methods. This effectively improves the operational stability and reliability of the multilevel converter.

[0061] Assume the AC output voltage of the three-phase bridge arm is , , The phase current is , , The bridge arm current is denoted as... , The currents in the upper and lower bridge arms are defined as follows:

[0062] ,

[0063] ,

[0064] in, This is the DC current on the DC side.

[0065] The bridge arm output voltage can be expressed as:

[0066] ,

[0067] in, This is the output voltage of phase A bridge arm. and These are the instantaneous voltages of the upper and lower bridge arms, respectively.

[0068] The three-phase outputs satisfy a symmetrical relationship:

[0069] ,

[0070] Total power of bridge arm It can be written as:

[0071] ,

[0072] By the power conservation condition, we have:

[0073] ,

[0074] in, For the power of the bridge arm, This refers to the power on the DC side. This is the DC bus voltage. It is direct current.

[0075] The voltage equation for the bridge arm inductor is:

[0076] ,

[0077] in, The voltage across the bridge arm inductor. The sum of the voltages of all submodules. The equivalent resistance within the arm. For the bridge arm current, For bridge arm inductance, For time, For bridge arm current Regarding time The first derivative.

[0078] In conventional hybrid MMCs, submodules are arranged alternately in a "half-bridge—full-bridge—half-bridge—full-bridge" configuration. This alternating structure ensures voltage modulation linearity in industrial / high-frequency applications, but it presents two prominent problems during low-frequency operation:

[0079] 1. Uneven voltage stress distribution between different modules can easily cause capacitor voltage drift;

[0080] 2. The full-bridge submodule is in a redundant state for a long time in the non-polar section, which reduces energy utilization.

[0081] Time-domain analysis of the bridge arm voltage waveform and power flow reveals that, under low-frequency conditions, the output voltage of the bridge arm remains essentially constant in direction within a half-wave period, with circulating current primarily concentrated in the middle and early stages of the power transmission region. Therefore, continuing to use an alternating arrangement would lead to wasted voltage modulation resources and asymmetrical energy distribution across the bridge arms.

[0082] Based on the distribution characteristics of the power flux density function, it is derived that the power density of the bridge arm has a unimodal distribution along the length direction:

[0083] ,

[0084] in, Let x be the power flux density at position x along the length of the bridge arm; This represents the peak value of the power flux density; This refers to the submodule number; This represents the total number of submodules in the bridge arm.

[0085] Based on this physical law, the present invention divides the bridge arm into two functional sections: the half-bridge section undertakes the high-power transmission task and adopts a half-bridge sub-module to reduce losses; the full-bridge section undertakes the voltage polarity control and fault tolerance task and adopts a full-bridge sub-module to enhance controllability.

[0086] When the submodule is operating in AC rectification mode (such as AC frequency) The fundamental components of the voltage and current in each bridge arm can be approximated as follows:

[0087] ,

[0088] ,

[0089] in, This represents the instantaneous voltage value of the submodule. This represents the instantaneous value of the bridge arm current. , , These are the phase angles of the submodule voltage and the bridge arm current, respectively. This represents the amplitude of the fundamental component of the submodule voltage. This represents the amplitude of the fundamental component of the bridge arm current.

[0090] Each bridge arm's sub-module consists of capacitors. When combined with switching devices, its voltage can be expressed as:

[0091] ,

[0092] in, This is the switching function for the k-th module. This is the capacitor voltage of the module.

[0093] The total voltage of the bridge arm is:

[0094] ,

[0095] in, This represents the total voltage of the bridge arm. and The number of half-bridge and full-bridge sub-modules for the bridge arm, respectively, to meet the requirements. , Let be the capacitor voltage of the kth half-bridge submodule of the bridge arm. This represents the capacitor voltage of the kth full-bridge submodule of the bridge arm.

[0096] In the half-bridge section, the half-bridge submodule mainly undertakes the functions of energy transmission and voltage composition. Therefore, a unified carrier phase-shifted PWM and a centralized capacitor voltage sorting mechanism are adopted to treat this section as a power output unit with consistent dynamic characteristics, focusing on ensuring the balance of capacitor voltage distribution and the continuity of energy transmission.

[0097] In the full-bridge section, the full-bridge submodule has independent polarity adjustment and voltage regulation capabilities. To this end, an independent voltage regulation link is constructed in this section, including polarity adjustment drive, energy balance regulation, and voltage amplitude optimization control, so that the full-bridge section can serve as a voltage regulation and energy buffer unit for the bridge arm, and realize fine control of the overall output voltage quality and energy distribution.

[0098] The second and fourth harmonic energy terms in the bridge arm circulation exhibit different distribution patterns in the half-bridge and full-bridge sections. Therefore, energy variations in the half-bridge and full-bridge sections are modeled and compensated separately. Based on an energy coordination function, an optimal energy regulation law across sections is constructed to minimize energy interaction between the two sections during the dynamic process, thereby reducing the overall circulation amplitude and ensuring stable system operation under low-frequency, high-power conditions.

[0099] like Figure 2 As shown, this converter introduces a cross-segment coordinated control mechanism at the bridge arm level: during system operation, based on the bridge arm energy distribution, average capacitor voltage, and inter-segment voltage deviation, the full-bridge segment is prioritized for voltage regulation and energy balancing operations, while the half-bridge segment maintains the stability of main power transmission and minimizes its dynamic participation. This zoned coordinated control method effectively reduces inter-segment energy coupling, improves the overall flexibility and response consistency of voltage regulation, and ensures smoother power flow within the bridge arm. During the control execution phase, half-bridge section control and full-bridge section control are carried out in parallel. In half-bridge section control, carrier phase shifting is first performed to adjust the signal phase, and then capacitor voltage is managed through centralized capacitor voltage sorting to ensure stable energy transmission before implementing half-bridge section circulating current suppression to reduce circulating current. Full-bridge section control constructs an independent modulation link to achieve flexible modulation, completes voltage regulation and energy transmission, and has fault support capabilities. Subsequently, full-bridge section circulating current suppression and second harmonic energy compensation are carried out. After the two section controls are completed, section-specific circulating current suppression and second harmonic energy compensation are performed for overall optimization. The system enters the online optimization and dynamic adjustment phase during operation. Based on real-time operating parameters, the control strategy and parameters are dynamically adjusted to adapt to changes. At the same time, real-time status monitoring is performed to obtain key parameters such as bridge arm energy distribution. Based on the monitoring results, it is determined whether the optimal state has been reached. If not, the control parameters are updated and the monitoring and judgment process is re-entered to form a closed-loop optimization. If the optimal state has been reached, the current configuration is maintained to ensure continuous and stable system operation.

[0100] Specifically, from the perspective of the basic principles of system operation, the energy distribution of the bridge arms, the average capacitor voltage, and the voltage deviation between sections are key parameters reflecting the system's operating status. The energy distribution of the bridge arms directly reflects the magnitude and proportion of energy carried by each section, the average capacitor voltage reflects the average level of energy stored in the capacitors, and the voltage deviation between sections characterizes the degree of voltage difference between different sections. The dynamic changes of these parameters are interrelated and jointly affect the overall performance of the system.

[0101] During actual system operation, the cross-segment coordination control mechanism monitors the aforementioned key parameters in real time and with precision. Based on the real-time system status reflected by these parameters, the mechanism analyzes and makes decisions, prioritizing the scheduling of the full-bridge segment to perform voltage regulation and energy balancing operations. Due to its unique structure, the full-bridge segment possesses a wider voltage regulation range and more flexible polarity adjustment capabilities. When voltage fluctuations or uneven energy distribution occur in the system, the full-bridge segment can respond quickly, effectively and finely regulating the voltage by adjusting its switching state and output voltage to achieve reasonable energy distribution and balance. For example, when a sudden change in system load causes a voltage drop in a certain segment, the full-bridge segment can promptly increase its output voltage to replenish energy to that segment and maintain system voltage stability.

[0102] Meanwhile, under the cross-segment coordinated control mechanism, the half-bridge section primarily bears the crucial responsibility of maintaining the stability of main power transmission. Main power transmission is the core function of the system, and its stability directly affects the system's reliability and efficiency. By maintaining a stable operating state, the half-bridge section ensures continuous and smooth main power transmission, providing reliable power support to the load. To minimize interference with main power transmission, the half-bridge section minimizes its own dynamic involvement. This means that during system operation, the half-bridge section will not frequently switch states or make significant voltage adjustments, but rather maintain a relatively stable operating mode to avoid affecting the quality of main power transmission due to its own instability.

[0103] The zoned coordinated control approach offers several significant advantages. Firstly, it is remarkably effective in reducing energy coupling between zones. In traditional control methods, energy interactions between zones are close, and the flow of energy between zones lacks effective coordination and control, easily leading to uneven energy distribution and localized energy accumulation. However, the cross-zone coordinated control mechanism, by clearly defining the responsibilities and division of labor of each zone, allows full-bridge and half-bridge zones to leverage their respective strengths, reducing energy interaction and interference between zones, thereby effectively lowering the degree of energy coupling between them. For example, the full-bridge zone focuses on voltage regulation and energy balance, avoiding energy conflicts with the half-bridge zone due to excessive participation in main power transmission, allowing for a more rational distribution and flow of energy among the zones.

[0104] This control method significantly improves the flexibility and consistency of overall voltage regulation. The priority scheduling of the full-bridge section enables it to quickly respond to various voltage regulation needs, adjusting voltage levels promptly and accurately whether dealing with sudden load changes or voltage fluctuations caused by system faults. Simultaneously, the stable operation of the half-bridge section provides a solid foundation for main power transmission, ensuring that voltage regulation in the full-bridge section does not excessively impact main power transmission. During collaborative operation, each section maintains a high degree of response consistency, ensuring that voltage regulation can be completed quickly and effectively under various complex operating conditions, thus improving the system's dynamic performance and stability.

[0105] The cross-segment coordinated control mechanism ensures smoother power flow within the bridge arm. By rationally allocating tasks between the full-bridge and half-bridge segments, the power transmission path is optimized, reducing power fluctuations and impacts. When performing voltage regulation and energy balancing operations, the full-bridge segment fully considers the impact on power flow within the bridge arm, adopting a smooth adjustment method to avoid excessive stress on internal components due to drastic power changes. The stable operation of the half-bridge segment further ensures the continuity and smoothness of power flow, making the power flow within the bridge arm more orderly and stable, thus improving system reliability and service life.

[0106] Through the hierarchical design of modulation and voltage management within the above-mentioned sections, the control objectives of the half-bridge section and the full-bridge section are effectively decoupled, significantly reducing energy coupling between sections and improving the stability and accuracy of the overall voltage management of the bridge arm.

[0107] Combined again Figure 1 Therefore, the bridge arm output voltage can be written as:

[0108] ,

[0109] in, For the bridge arm output voltage, The output voltage of the half-bridge section of the bridge arm. This is the output voltage of the entire bridge section of the bridge arm.

[0110] The two sections of the bridge arm play different energy roles during operation: the half-bridge section is responsible for main power output, while the full-bridge section is responsible for polarity regulation and energy balance. To achieve stable operation, the power balance conditions for each section must be met:

[0111] ,

[0112] in: , This refers to the output current of the half-bridge section of the bridge arm. This represents the output current of the entire bridge section of the bridge arm.

[0113] Assuming the current in the bridge arms is uniformly distributed, the energy balance equation can be obtained:

[0114] ,

[0115] Combination The distribution constraints of the bridge arm voltages can be obtained:

[0116] ,

[0117] At this time, the total voltage of the bridge arm for:

[0118] ,

[0119] Therefore, it can be concluded that bridge arm voltage balancing depends on and The matching relationship.

[0120] Based on the above analysis, this invention proposes a method for optimizing the configuration of the number of sub-modules in a modular multilevel converter.

[0121] Figure 3 This is a flowchart illustrating a method for optimizing the number of submodules in a modular multilevel converter, as shown in some embodiments of this specification. Figure 3 As shown, the method for optimizing the number of submodules in a modular multilevel converter can include the following steps:

[0122] Step 310: Construct the magnitude function of the energy difference term for the half-bridge section and the full-bridge section.

[0123] Specifically, it includes:

[0124] Construct the energy fluctuation function for the half-bridge section and the energy fluctuation function for the full-bridge section;

[0125] Based on the energy fluctuation function of the half-bridge section and the energy fluctuation function of the full-bridge section, the amplitude function of the energy difference term of the half-bridge section and the full-bridge section is constructed.

[0126] In some embodiments, constructing the energy fluctuation function for the half-bridge segment includes:

[0127] Based on the equivalent capacitance and instantaneous capacitance voltage of the half-bridge submodule, the instantaneous energy storage function of the half-bridge submodule is constructed.

[0128] Based on the instantaneous energy storage function and quantity of the half-bridge submodules, the instantaneous energy function of the half-bridge section is constructed;

[0129] Based on the instantaneous energy function of the half-bridge section, an energy fluctuation function of the half-bridge section is constructed.

[0130] Specifically, the instantaneous energy storage function of the half-bridge submodule can be:

[0131] ,

[0132] in, For instantaneous energy storage of the half-bridge submodule, Let be the equivalent capacitance of the i-th half-bridge submodule. Let be the instantaneous capacitor voltage of the i-th half-bridge submodule.

[0133] The instantaneous energy function of the half-bridge section can be:

[0134] ,

[0135] in, For the instantaneous energy of the half-bridge section, This represents the number of half-bridge submodules.

[0136] The energy fluctuation function of the half-bridge section can be:

[0137] ,

[0138] in, For energy fluctuations in the half-bridge section, This represents the average instantaneous energy of the half-bridge section.

[0139] The method for constructing the energy fluctuation function of the full bridge section is similar to that for constructing the energy fluctuation function of the half bridge section, and will not be repeated here.

[0140] Under operating conditions, the main source of energy disturbance in the bridge arm is the second harmonic component, and its energy fluctuation can be expressed as:

[0141] ,

[0142] in, This refers to the energy fluctuation amplitude in the half-bridge section. The phase of the second harmonic component. ω is the fundamental angular frequency.

[0143] The energy fluctuation amplitude of the half-bridge section can be determined based on the above formula.

[0144] The method for calculating the energy fluctuation amplitude of the full bridge section is similar to that for calculating the energy fluctuation amplitude of the half bridge section, and will not be repeated here.

[0145] If the energy fluctuations in the half-bridge section and the full-bridge section are not synchronized, additional circulation will occur:

[0146] ,

[0147] in, For additional circulation, For the energy fluctuations of the entire bridge section, It means "in direct proportion" or "proportional to".

[0148] Therefore, the magnitude of the energy difference term between the half-bridge section and the full-bridge section is a key indicator for suppressing the bridge arm circulation.

[0149] The magnitude function of the energy difference term between the half-bridge section and the full-bridge section can be:

[0150] ,

[0151] in, This represents the magnitude of the energy difference term between the half-bridge section and the full-bridge section. This refers to the energy fluctuation amplitude in the half-bridge section. This represents the energy fluctuation amplitude across the entire bridge section.

[0152] Step 320: Construct the energy coupling function for the half-bridge section and the full-bridge section.

[0153] Specifically, it includes:

[0154] Based on the number of half-bridge submodules and the equivalent capacitance, the energy participation function of the half-bridge section is constructed.

[0155] Based on the number of full-bridge submodules and the equivalent capacitance, the energy participation function of the full-bridge section is constructed.

[0156] Based on the energy participation function of the half-bridge section and the energy participation function of the full-bridge section, energy coupling function of the half-bridge section and the full-bridge section is constructed.

[0157] Energy participation is used to characterize the effective contribution of each section to the energy balance of the bridge arm.

[0158] In some embodiments, the energy coupling function for the half-bridge section and the full-bridge section can be:

[0159] ,

[0160] ,

[0161] ,

[0162] in, The energy coupling degree of the half-bridge section and the full-bridge section, For the energy participation of the half-bridge section, The energy participation rate of the entire bridge section, The number of half-bridge submodules, This is the equivalent capacitance of the half-bridge submodule. The number of full-bridge submodules, This is the equivalent capacitance of the full-bridge submodule.

[0163] The energy coupling degree of the half-bridge section and the full-bridge section is used to characterize the effectiveness of the inter-section participation in the energy regulation of the bridge arm. Its value reflects the participation characteristics of different sections in the energy interaction process of the bridge arm, thereby affecting the overall dynamic energy distribution state of the bridge arm.

[0164] Step 330: Construct the bridge arm energy coordination function based on the magnitude function of the energy difference term and the energy coupling degree function of the half-bridge section and the full-bridge section.

[0165] In a hybrid MMC, the energy distribution of the bridge arms is jointly determined by the half-bridge and full-bridge submodules. The energy imbalance between the half-bridge and full-bridge sections can be addressed through the energy difference term. The energy exchange capacity between the half-bridge section and the full-bridge section can be characterized by the degree of energy coupling. describe.

[0166] Energy difference term This is closely related to the allocation relationship between the number of half-bridge and full-bridge submodules. When only a single type of submodule is configured in the bridge arm, there is a lack of effective energy exchange channels between sections, resulting in low energy coupling. Limited by constraints, the energy difference term is difficult to effectively regulate; when half-bridge and full-bridge submodules participate in the operation of the bridge arm simultaneously, an energy regulation path can be formed within the bridge arm involving both types of submodules, increasing the energy coupling degree. The energy difference term between sections is determined by the combined effect of the number of the two types of sub-modules. It has an impact.

[0167] Furthermore, the bridge arm of the segmented half-bridge-full-bridge hybrid modular multilevel converter is composed of both half-bridge and full-bridge submodules. The energy regulation process within the bridge arm is not completed independently by a single type of submodule, but rather relies on the synergistic effect of both types of submodules. When only one type of submodule is used in the bridge arm, the control path and degree of freedom for energy regulation are limited. When both half-bridge and full-bridge submodules exist simultaneously, an energy regulation path involving both types of submodules can be formed within the bridge arm, thereby improving the bridge arm's adaptability to energy fluctuations.

[0168] Therefore, the magnitude of the bridge arm energy coordination depends on the number of half-bridge and full-bridge submodules simultaneously participating in energy regulation within the bridge arm. If the number of either type of submodule is zero, the bridge arm does not possess the ability to coordinate regulation between the two types of submodules; if the number of both types of submodules is not zero, there exists an energy regulation path within the bridge arm in which both types of submodules participate, and its number increases with the increase in the number of the two types of submodules.

[0169] Based on the above principles, to quantitatively characterize the variation law of bridge arm energy synergy under different submodule configuration schemes, this method expresses bridge arm energy synergy as a function that is simultaneously related to the number of half-bridge submodules and the number of full-bridge submodules. Considering that the synergistic regulation capability is only formed when both types of submodules participate simultaneously, and that the synergistic regulation scale is closely related to the combined effect of the number of both types of submodules, this method uses the product of the number of half-bridge submodules and the number of full-bridge submodules to characterize bridge arm energy synergy, and constructs the following bridge arm energy synergy function:

[0170] ,

[0171] in, Let be the bridge arm energy coordination function. The number of half-bridge submodules, This represents the total number of half-bridge and full-bridge submodules in each phase arm. ,in, The number of full-bridge submodules is represented by this function. The aforementioned arm energy coordination function allows for a unified evaluation of arm energy coordination under different half-bridge and full-bridge submodule configurations, providing a basis for subsequent optimization of submodule numbers. Compared to functions that only relate to the number of a single type of submodule, this arm energy coordination function avoids overestimating the coordinated adjustment capability simply by increasing the number of a particular type of submodule, thus better aligning with the mechanism of arm energy regulation.

[0172] Step 340: Based on the bridge arm energy coordination function, determine the number of half-bridge sub-modules included in the half-bridge section and the number of full-bridge sub-modules included in the full-bridge section.

[0173] Specifically, it includes:

[0174] Based on the bridge arm energy coordination function, an objective function is constructed to evaluate different submodule quantity configuration schemes, wherein the objective function takes the maximum value of the bridge arm energy coordination function as the optimization objective.

[0175] Under the constraint of the total number of sub-modules of the bridge arm, Lagrange multipliers are introduced, and the equation to be solved is constructed based on the objective function;

[0176] Solve the equations to be solved to determine the number of half-bridge and full-bridge submodules.

[0177] The equation to be solved is:

[0178] ,

[0179] in, The equation to be solved is It is a Lagrange multiplier.

[0180] In some embodiments, solving the equation to be solved and determining the number of half-bridge submodules included in the half-bridge section and the number of full-bridge submodules included in the full-bridge section includes:

[0181] Based on the equation to be solved, the number of half-bridge submodules and the Lagrange multipliers are differentiated. Combined with the constraints, the number of half-bridge submodules in the half-bridge section and the number of full-bridge submodules in the full-bridge section are determined. The constraints are as follows: , Number of full-bridge submodules:

[0182] ,

[0183] The bridge arm includes a total of [number of sub-modules]. The determination is based on the rated voltage of the DC bus. Reference value (or allowable voltage range) for capacitor voltage of a single submodule The ratio is determined. , In order to Round up.

[0184] In some embodiments, it also includes:

[0185] By using the second derivative of the arm energy coordination function with respect to the number of half-bridge submodules, we can verify the number of half-bridge submodules in the half-bridge section and the number of full-bridge submodules in the full-bridge section. Specifically, if the second derivative is negative, it means that the function exhibits convex characteristics at this point, that is, the module number configuration obtained by the previous solution method corresponds to the maximum point of the function, which can ensure that the arm energy coordination effect is optimal at this time. The determined number of half-bridge and full-bridge submodules can enable the system to achieve a better state in terms of energy distribution and circulating current suppression, ensuring the stable and efficient operation of the low-frequency high-power flexible DC transmission system.

[0186] In traditional alternating hybrid topologies, energy flow paths are complex, easily leading to uneven energy distribution and circulating current excitation. This method constructs amplitude functions and energy coupling functions for the energy difference terms in the half-bridge and full-bridge sections, and optimizes the number of submodules based on these functions. This makes the main power transmission and voltage regulation paths electrically independent, significantly improving the energy distribution in the bridge arms, making the power flow more orderly, effectively suppressing circulating currents in the bridge arms, and enhancing the system's stability and power balance characteristics. For example, under low-frequency, high-power operating conditions, it can reduce energy losses caused by circulating currents and improve system efficiency.

[0187] Current hybrid topology designs rely on experience to determine the number of half-bridge and full-bridge submodules, lacking a unified mathematical basis and making it difficult to balance energy balance and fault tolerance. This method transforms the submodule configuration from empirical design to theoretical calculation. By constructing a bridge arm energy coordination function and solving for its extrema, the optimal half-bridge and full-bridge submodule configurations are obtained, providing a reproducible and scalable design principle for segmented hybrid topologies. This ensures that the system achieves an optimal balance between energy equilibrium and polarity control, reducing capacitor voltage ripple and circulating current. This method can be used to configure the number of modules in flexible DC transmission projects of varying scales, improving the scientific rigor and reliability of the design.

[0188] In traditional alternating arrangement methods, the differences in conduction characteristics and energy variation patterns between half-bridge and full-bridge submodules lead to asynchronous and large fluctuations in submodule capacitor voltage changes. This invention optimizes the number of submodules through a segmented structure and coordinated design, enabling similar submodules to operate in a concentrated manner, possessing consistent electrical characteristics and charging / discharging patterns, thus achieving a synchronized capacitor voltage change trend. Simultaneously, by determining the optimal module ratio through an energy coordination function, the participation of each segment in bridge arm energy regulation becomes more rational, further enhancing capacitor voltage synchronization, significantly reducing voltage fluctuation amplitude, simplifying voltage equalization control, achieving stable bridge arm voltage output, improving long-term system reliability, and providing a robust voltage support foundation for mixed module operation under high-power conditions. For example, in long-term operating low-frequency high-power flexible DC transmission systems, it can effectively reduce the risk of faults caused by voltage fluctuations and ensure the safe and stable operation of the system.

[0189] The following examples illustrate the beneficial effects of the method for optimizing the number of submodules in a modular multilevel converter.

[0190] Calculations show that the optimal structure is obtained when N=12. , .

[0191] Figure 4 The MMC output voltage waveform is shown. The figure illustrates the optimal configuration. and Under optimal conditions, the voltage waveform becomes more stable, with significantly reduced fluctuations.

[0192] The results demonstrate that by optimizing the number of submodules, the configuration of both half-bridge and full-bridge submodules has been scientifically optimized, ensuring a more balanced participation of the two types of submodules in energy transfer. In particular, by optimizing the number of submodules, the modular multilevel converter can achieve an optimal balance between voltage support and energy transfer, significantly improving the stability of voltage control.

[0193] Figure 5The simulation results demonstrate the variation of the MMC output current waveform under optimal configuration. The simulation results show that with optimized submodule number configuration (…),… and Under these conditions, the current waveform becomes smoother, the harmonic content is significantly reduced, and the current peak value decreases. This indicates that optimizing the number of submodules, by adjusting the ratio of half-bridge to full-bridge submodules, makes the participation of the half-bridge and full-bridge sections in energy transmission more balanced, thereby reducing current fluctuations and harmonics that may occur under unreasonable configuration.

[0194] Furthermore, by optimizing the number of half-bridge and full-bridge submodules, the hybrid modular multilevel converter can achieve better decoupling in control, making the half-bridge section and the full-bridge section independent in energy transmission, reducing high-frequency harmonics and current fluctuations caused by unreasonable module configuration, thereby improving the overall current stability of the system.

[0195] By combining the optimization of the number of submodules with a segmented layout, the modular multilevel converter is more efficient and stable in the energy transmission process. The overall current waveform is effectively optimized, improving the current stability of the system and reducing energy loss and harmonic interference.

[0196] Figure 6 The output power waveform of the MMC is shown. Figure 6 The display shows that, in optimal configuration and The power waveform is smoother and less volatile, improving power stability. The half-bridge section primarily handles power transmission, while the full-bridge section is responsible for voltage regulation and energy buffering. Because each section performs its specific function, energy stability is significantly enhanced during power transmission.

[0197] Figure 7 The changes in the DC-side voltage waveform are shown. Under the optimal configuration, the DC voltage fluctuation is small and remains stable, verifying that the stability of the DC voltage is significantly improved under this configuration. The half-bridge section focuses on power output, while the full-bridge section is responsible for voltage regulation and polarity control, making the voltage management of the entire system more coordinated and stable, avoiding fluctuations caused by the mixing of power and voltage regulation in the same section.

[0198] Figure 8 and Figure 9 The changes in the arm circulation are shown under the same circulation suppression control strategy. Figure 9 The image shows the bridge arm circulation waveform when the optimal number configuration was not achieved using this method. Figure 8 The image shows the bridge arm circulating waveform under the optimal configuration condition obtained using this method. It can be seen that under the optimal configuration... , Under the condition that the amplitude of the bridge arm circulation is significantly smaller than that under the non-computational configuration, it is evident that the proposed submodule number optimization configuration method can effectively improve the energy distribution of the bridge arm under the same control strategy, thereby enhancing the circulation suppression effect.

[0199] Figure 10 Demonstrates optimal configuration , The waveform of the capacitor voltage in the bridge arm submodule is shown in the figure. The waveform indicates that the capacitor voltage is stable with minimal fluctuations. Figure 11 for Figure 10 The magnified view shows the detailed changes in the capacitor voltage under the optimal configuration. The magnified portion of the waveform reveals the fluctuations in the capacitor voltage over short time intervals. It is clearly visible that under this configuration, the capacitor voltage fluctuations are smaller, indicating better voltage stability and smoother system operation.

[0200] Figure 12 The waveform of the bridge arm submodule capacitor voltage is shown under non-optimal configuration. Figure 10 In contrast, the capacitor voltage waveform fluctuates more, indicating that the system has significant capacitor voltage fluctuations under this configuration. Figure 13 for Figure 12 The magnified view shows the detailed changes in the capacitor voltage. From Figure 13 As can be seen, under non-optimal configuration, the capacitor voltage fluctuation amplitude increases significantly and the voltage stability is poor. This indicates that an unreasonable configuration of the number of submodules will lead to large fluctuations in the system capacitor voltage, affecting voltage regulation and system stability.

[0201] By comparison Figure 11 and Figure 13 In optimal configuration , The capacitor voltage fluctuation under the optimal configuration is significantly smaller, and the system voltage stability is significantly improved. In contrast, the capacitor voltage fluctuation under the non-optimal configuration is larger, indicating that the configuration cannot achieve the optimal balance of energy distribution, resulting in capacitor voltage instability. This verifies the effectiveness of the proposed converter and method in reducing capacitor voltage ripple and improving system energy transmission stability.

[0202] The simulation results under different configurations are shown in Table 1.

[0203]

[0204] in, : RMS value of circulating current. Optimal configuration Below, the circulation amplitude is significantly reduced. : The maximum value of capacitor voltage ripple. Under optimal configuration, the maximum value of capacitor voltage ripple is minimized, indicating a more balanced energy distribution. : The root mean square value of the energy fluctuation difference in the section. Under the optimal configuration, the root mean square value of the energy fluctuation difference in the section is minimized, further verifying the effectiveness of optimizing the number of submodules.

[0205] In summary, the method for optimizing the number of submodules in a modular multilevel converter provided in this specific embodiment, based on the principle of bridge arm energy coordination and segmented hybrid topology design, brings significant benefits in several aspects:

[0206] 1. Optimize energy distribution and enhance system stability.

[0207] With the optimal submodule configuration, half-bridge submodules are concentrated in the main power region, while full-bridge submodules are concentrated in the polarity regulation region. The main power transmission and voltage regulation paths are electrically independent, further optimizing energy distribution. Simultaneously, by optimizing the ratio of half-bridge to full-bridge submodules, the balance of bridge arm voltage support is improved. This significantly reduces DC-side voltage fluctuations during low-frequency operation, ensuring stable power transmission and voltage output. Over long periods, it reduces voltage stress accumulation, improves system steady-state reliability, and thus enhances overall stability.

[0208] 2. Simplify the control structure and improve modulation and response performance.

[0209] Similar submodules are grouped together, allowing the use of a unified carrier and modulation signal. This reduces control coupling between modules, decreases the number of control loops and parameters, and simplifies the control system. Optimized module count combined with a segmented structural design maintains high modulation accuracy and rapid dynamic response during low-frequency operation, improving system stability and control sensitivity, and ensuring fast and accurate response under various operating conditions.

[0210] 3. Suppress low-frequency circulating current and improve output power quality

[0211] The optimization of the number of submodules reduces energy crossover between different functional areas, weakens the second and fourth harmonic circulating current excitation sources during low-frequency operation, reduces circulating current amplitude, smooths bridge arm current waveforms, and reduces voltage waveform distortion. When the system is connected to the grid or transmitted over long distances, it can maintain high voltage waveform quality, reduce harmonic interference, reduce dependence on filtering devices, improve power quality and stability under low-frequency conditions, and thus improve system efficiency and reliability.

[0212] 4. Reduce capacitor voltage fluctuations and improve voltage balance.

[0213] The optimized number of submodules ensures consistent electrical characteristics and energy trends during operation, leading to more synchronized capacitor voltage changes, reduced voltage deviations between submodules, and decreased frequency of voltage equalization control adjustments. The resulting smoother capacitor voltage distribution in steady state provides voltage support for long-term stable operation, effectively improving system voltage balance and long-term operational stability.

[0214] Finally, it should be understood that the embodiments described in this specification are merely illustrative of the principles of the embodiments described herein. Other variations may also fall within the scope of this specification. Therefore, alternative configurations of the embodiments described herein are intended to be consistent with the teachings of this specification, rather than as examples or limitations. Accordingly, the embodiments described herein are not limited to those explicitly introduced and described herein.

Claims

1. A method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter, characterized in that, The method applied to a segmented half-bridge-full-bridge hybrid modular multilevel converter includes three-phase arms, each phase arm consisting of an upper arm and a lower arm. The upper arm includes cascaded half-bridge sections and full-bridge sections. The half-bridge section includes at least two cascaded half-bridge submodules, and the full-bridge section includes at least two cascaded full-bridge submodules. The method includes: Based on the energy fluctuation function of the half-bridge section and the energy fluctuation function of the full-bridge section, the amplitude function of the energy difference term of the half-bridge section and the full-bridge section is constructed. The energy fluctuation function of the half-bridge section is related to the equivalent capacitance, instantaneous capacitance voltage and quantity of the half-bridge sub-module. Based on the number and equivalent capacitance of half-bridge submodules and the number and equivalent capacitance of full-bridge submodules, energy coupling degree functions for half-bridge and full-bridge sections are constructed. Based on the magnitude function of the energy difference term and the energy coupling degree function of the half-bridge section and the full-bridge section, a bridge arm energy coordination function is constructed. Based on the bridge arm energy coordination function, the number of half-bridge sub-modules included in the half-bridge section and the number of full-bridge sub-modules included in the full-bridge section are determined.

2. The method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 1, characterized in that, Constructing the energy fluctuation function for the half-bridge section includes: Based on the equivalent capacitance and instantaneous capacitance voltage of the half-bridge submodule, the instantaneous energy storage function of the half-bridge submodule is constructed. Based on the instantaneous energy storage function and quantity of the half-bridge submodules, the instantaneous energy function of the half-bridge section is constructed; Based on the instantaneous energy function of the half-bridge section, an energy fluctuation function of the half-bridge section is constructed.

3. A method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 1 or 2, characterized in that, Construct the energy coupling degree functions for the half-bridge section and the full-bridge section, including: Based on the number of half-bridge submodules and the equivalent capacitance, the energy participation function of the half-bridge section is constructed. Based on the number of full-bridge submodules and the equivalent capacitance, the energy participation function of the full-bridge section is constructed. Based on the energy participation function of the half-bridge section and the energy participation function of the full-bridge section, energy coupling function of the half-bridge section and the full-bridge section is constructed.

4. The method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 3, characterized in that, The energy coupling functions for the half-bridge section and the full-bridge section are: ， ， ， in, The energy coupling degree of the half-bridge section and the full-bridge section, For the energy participation of the half-bridge section, The energy participation rate of the entire bridge section, The number of half-bridge submodules, This is the equivalent capacitance of the half-bridge submodule. The number of full-bridge submodules, This is the equivalent capacitance of the full-bridge submodule.

5. A method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 1 or 2, characterized in that, Based on the bridge arm energy coordination function, the number of half-bridge sub-modules included in the half-bridge section and the number of full-bridge sub-modules included in the full-bridge section are determined, including: Based on the bridge arm energy coordination function, an objective function is constructed to evaluate different submodule quantity configuration schemes, wherein the objective function takes the maximum value of the bridge arm energy coordination function as the optimization objective. Under the constraint of the total number of sub-modules of the bridge arm, Lagrange multipliers are introduced, and the equation to be solved is constructed based on the objective function; The equation to be solved is solved to determine the number of half-bridge submodules and the number of full-bridge submodules.

6. The method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 5, characterized in that, The bridge arm energy cooperation function is: ， in, Let be the bridge arm energy coordination function. The number of half-bridge submodules, This represents the total number of half-bridge and full-bridge submodules in each phase arm.

7. The method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 6, characterized in that, The equation to be solved is: ， in, The equation to be solved is... It is a Lagrange multiplier.

8. The method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 6, characterized in that, Solving the equation to be solved, and determining the number of half-bridge submodules included in the half-bridge section and the number of full-bridge submodules included in the full-bridge section, includes: Based on the equation to be solved, the number of half-bridge submodules and the Lagrange multipliers are differentiated, and combined with the constraints, the number of half-bridge submodules included in the half-bridge section and the number of full-bridge submodules included in the full-bridge section are determined, wherein the constraints are as follows: , This represents the number of full-bridge submodules.

9. The method for optimizing the number of submodules in a segmented half-bridge-full-bridge hybrid modular multilevel converter according to claim 8, characterized in that, Also includes: The number of half-bridge submodules included in the half-bridge section and the number of full-bridge submodules included in the full-bridge section are verified by using the second derivative of the bridge arm energy coordination function with respect to the number of half-bridge submodules.

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