A converter module, a multi-ac port bridge arm multiplexing type modular multilevel converter and a control method thereof

Abstract

The present application relates to AC / DC power transmission system and power conversion technology field, especially to a kind of converter module, multi AC port bridge arm multiplexing type modular multilevel converter and its control method, converter module includes N+1 bridge arms, N+1 bridge arms include upper bridge arm, lower bridge arm and intermediate bridge arm group, intermediate bridge arm group includes N-1 intermediate bridge arms, N is greater than or equal to 2 positive integer;The connection point of every two adjacent bridge arms in the upper bridge arm, intermediate bridge arm group, lower bridge arm is led out an AC port, forms N AC ports;The end of upper bridge arm away from intermediate bridge arm and the end of lower bridge arm away from intermediate bridge arm are led out a DC port in common;Upper bridge arm and lower bridge arm are all composed of multiple half-bridge sub-modules cascaded, and intermediate bridge arm is composed of multiple full-bridge sub-modules cascaded.The present application can be flexibly expanded to realize any number of AC ports, while accessing multiple different voltage levels, different frequency AC voltage.

Description

Technical Field

[0001] This invention relates to the field of AC / DC power transmission systems and power conversion technology, and particularly to converter modules, multi-AC port bridge arm multiplexing modular multilevel converters and their control methods. Background Technology

[0002] With the rapid development of the offshore wind power industry, the development scope of the offshore wind power industry has gradually extended from near-shore to mid- and far-shore. Different transmission distances correspond to different power transmission technology paths: near-distance offshore wind power usually uses power frequency AC transmission to connect to the shore power system, mid-distance scenarios mostly use low frequency AC transmission technology, while deep-sea wind power relies on DC transmission to achieve efficient power transmission.

[0003] However, the existing power transmission system architecture has significant technical bottlenecks: for different types and parameters of power transmission, corresponding onshore substations, converter stations and transmission corridors need to be built separately. This not only occupies a large amount of land resources, resulting in low land utilization, but also causes the problem of redundant construction and insufficient utilization of transmission corridors, which greatly increases the system construction and operation and maintenance costs and seriously restricts the overall efficiency and economy of offshore wind power grid connection systems.

[0004] Therefore, developing a modular multilevel converter with a simple structure, strong scalability, and the ability to fully utilize the performance of submodule bridge arms and realize centralized access of multiple types and parameters of ports has become an urgent technical problem to be solved in the field of multi-port grid connection of offshore wind power. Summary of the Invention

[0005] This invention provides a converter module, a multi-AC port bridge arm multiplexed modular multilevel converter and its control method, which can effectively solve the problems in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A converter module includes N+1 bridge arms, wherein the N+1 bridge arms include an upper bridge arm, a lower bridge arm, and an intermediate bridge arm group, wherein the intermediate bridge arm group includes N-1 intermediate bridge arms, where N is a positive integer greater than or equal to 2. The upper bridge arm, the middle bridge arm group, and the lower bridge arm are connected in series. The middle bridge arms in the middle bridge arm group are connected in series. The first middle bridge arm in the middle bridge arm group is connected in series with the upper bridge arm. The last middle bridge arm in the middle bridge arm group is connected in series with the upper bridge arm. An AC port is led out from the connection point of every two adjacent bridge arms in the upper bridge arm, the middle bridge arm group, and the lower bridge arm, forming N AC ports. The end of the upper bridge arm furthest from the middle bridge arm and the end of the lower bridge arm furthest from the middle bridge arm share a common DC port. The upper and lower bridge arms are each composed of multiple cascaded half-bridge sub-modules, and the middle bridge arm is composed of multiple cascaded full-bridge sub-modules.

[0007] Furthermore, the half-bridge submodule adopts a half-bridge structure, which includes two power electronic switching devices with anti-parallel freewheeling diodes and a voltage-regulating capacitor. The two power electronic switching devices are connected in series, the voltage stabilizing capacitor is connected in parallel across the two power electronic switching devices, and the intermediate bridge arm is connected at the middle position of the two power electronic switching devices.

[0008] Furthermore, the full-bridge submodule adopts a full-bridge structure, which includes four power electronic switching devices with anti-parallel freewheeling diodes and a voltage-stabilizing capacitor. Two of the four power electronic switching devices are connected in series to form a first series branch, and the other two of the four power electronic switching devices are connected in series to form a second series branch. The first series branch and the second series branch are connected in parallel. The voltage stabilizing capacitor is connected in parallel at both ends of the first series branch or the second series branch. The upper bridge arm or the lower bridge arm is connected at the middle position of the two power electronic switching devices in the first series branch or the second series branch.

[0009] The present invention also provides a multi-port bridge arm multiplexing modular multilevel converter, which is a three-phase structure, wherein each phase of the three-phase structure is independently equipped with the converter module described above.

[0010] The present invention also provides a control method for a multi-port bridge arm multiplexed modular multilevel converter as described above, comprising: Define an allocation coefficient λi, where i = 1, 2, ..., N. Based on the allocation coefficient λi, establish a quantitative correlation between the current of the upper bridge arm and the current of each AC port; wherein, the allocation coefficient λi is the proportionality coefficient of the output current of the i-th AC port in the current of the upper bridge arm. For quantitative correlations, set constraints for stable system operation; Based on quantitative correlations and constraints, the average power formulas for the upper arm, lower arm, and N-1 intermediate arms are derived respectively. Based on the average power formula, a power balance equation system is constructed, and λi is obtained by solving the power balance equation system, where i=1,2,...,N, to determine the distribution relationship between the output currents of each AC port.

[0011] Furthermore, the established quantitative correlation between the current in the upper bridge arm and the current in each AC port is specifically as follows: The voltage calculation formulas for the upper bridge arm, each intermediate bridge arm, and the lower bridge arm are as follows: In the formula, u uj u lj Let u be the voltages of the upper and lower arms of phase j, respectively, where j = a, b, c; mjn Let U be the voltage of the nth intermediate bridge arm of phase j, where n = 1, 2, ..., N; dc The DC bus voltage, u jn Let be the output voltage of the nth AC port of phase j; This is the output voltage of the first AC port of phase j; Let be the output voltage of the (n+1)th AC port of phase j; Let N be the output voltage of the Nth AC port of phase j; The formulas for calculating the voltage and current at the AC port are as follows: In the formula, u jn Let i be the output voltage of the nth AC port of phase j; jn U is the output current of the nth AC port of phase j; jn I jn These are the output voltage and current amplitudes of the nth AC port of phase j, respectively, ω n Let θ be the angular velocity of the nth AC port. n φ n Let be the phase angle of the output voltage and output current of the nth AC port; The formulas for calculating the current in the upper arm, each intermediate arm, and the lower arm are as follows: In the formula, i uj i mjn i lj These represent the currents of the upper arm, the middle nth arm, and the lower arm of phase j, respectively; I dc For direct current; i zj i represents the circulating current component between bridge arms; ji λ is the output current of the i-th AC output port of phase j; i is the distribution coefficient of the output current of the i-th AC port in the upper bridge arm current.

[0012] Furthermore, the average power of the derived upper arm, middle arm, and lower arm is specifically as follows: Average power of upper arm in one cycle The calculation formula is as follows: In the formula, This is the DC bus voltage; It is direct current; The output voltage amplitude of the first AC port of phase j; Let be the output current amplitude of the i-th AC port of phase j; The distribution coefficient of the output current of the i-th AC port in the upper bridge arm current; To rearrange the intermediate angular velocities and The same quantity; The formula for calculating the average power of the nth intermediate bridge arm is: In the formula, M n1 To rearrange the intermediate angular velocities and The same quantity; M n2 To rearrange the intermediate angular velocities and The same quantity; Let be the output voltage amplitude of the (n+1)th AC port of phase j; Let be the phase angle of the output voltage at the i-th AC port; The formula for calculating the average power of the lower bridge arm is: In the formula, To rearrange the intermediate angular velocities and The same quantity; Let be the output voltage amplitude of the Nth AC port of phase j; Let be the output current amplitude of the i-th AC port of phase j; Let be the phase angle of the output voltage at the Nth AC port. Let be the phase angle of the output voltage of the i-th AC port.

[0013] Furthermore, the power balance equations for the constructed upper arm, middle arm, and lower arm must satisfy: In the formula, The average power of the upper bridge arm over one cycle; The average power of the nth intermediate bridge arm; This represents the average power of the lower bridge arm.

[0014] Furthermore, the value of the allocation coefficient λi is determined by the bridge arm circuit parameters of the converter and the port operating conditions. When the operating conditions are constant, the allocation coefficient λi is a fixed value.

[0015] Furthermore, the constraint condition is to control the circulating current between the upper arm, each intermediate arm, and the lower arm within the same phase to be zero, and the average power of the upper arm, each intermediate arm, and the lower arm within one operating cycle to be zero.

[0016] The technical solution of this invention can achieve the following technical effects: This invention employs a modular multilevel topology structure with N+1 bridge arms connected in series and AC ports directly led out at the connection points of adjacent bridge arms. This allows for flexible expansion to realize any number of AC ports without the need to configure a separate converter unit for each new AC port. It can simultaneously connect to multiple AC voltages of different voltage levels and frequencies, making it suitable for multi-source aggregation networking in high-voltage transmission or medium- and low-voltage power distribution fields. It can also directly adjust the power flow direction between multiple different AC buses. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 For multi-port bridge arm multiplexing modular multilevel converter topology; Figure 2 Operating principle diagram of a multi-port bridge arm multiplexing modular multilevel converter; Figure 3 It is a three-AC port bridge arm multiplexed modular multilevel converter topology; Figure 4 Simulation results for the DC port and the first AC port; Figure 5 The simulation results are for the second AC port; Figure 6 The simulation results are for the third AC port. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] Unless otherwise defined, 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. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0021] Example 1: This application proposes a converter module, including N+1 bridge arms, the N+1 bridge arms including an upper bridge arm, a lower bridge arm and an intermediate bridge arm group, the intermediate bridge arm group including N-1 intermediate bridge arms, and N is a positive integer greater than or equal to 2; The upper bridge arm, the middle bridge arm group, and the lower bridge arm are connected in series. The middle bridge arms in the middle bridge arm group are connected in series. The first middle bridge arm in the middle bridge arm group is connected in series with the upper bridge arm. The last middle bridge arm in the middle bridge arm group is connected in series with the upper bridge arm. An AC port is led out from the connection point of every two adjacent bridge arms in the upper bridge arm, the middle bridge arm group, and the lower bridge arm, forming N AC ports. The end of the upper bridge arm furthest from the middle bridge arm and the end of the lower bridge arm furthest from the middle bridge arm share a common DC port. The upper and lower bridge arms are each composed of multiple cascaded half-bridge sub-modules, while the middle bridge arm is composed of multiple cascaded full-bridge sub-modules.

[0022] like Figure 1 As shown, this solution proposes a multiplexing topology design with N AC ports corresponding to N+1 bridge arms. When N AC ports need to be connected, only the upper bridge arm, lower bridge arm, and N-1 intermediate bridge arms need to be configured. One AC port is led out between every two adjacent bridge arms, forming N AC ports. Simultaneously, one DC port is directly led out from the terminals of the upper and lower bridge arms. This topology does not require additional converters; the arbitrary expansion of AC ports can be achieved simply by linearly increasing the number of intermediate bridge arms. It can flexibly adapt to the aggregation and networking needs in high-voltage transmission or medium- and low-voltage power distribution fields, effectively regulating the power flow direction between multiple different AC buses.

[0023] In this invention, half-bridge submodules are used for the upper and lower bridge arms, which not only meet the requirements for stable DC port output and basic commutation, but also effectively control hardware costs. The middle bridge arm uses a full-bridge submodule, which can flexibly adapt to the voltage level and frequency adjustment requirements of multiple AC ports, greatly improving the system's operational flexibility. The differentiated submodule configuration fully leverages the cost advantages of half-bridge submodules and the adjustment advantages of full-bridge submodules, avoiding the overuse of full-bridge submodules and achieving the optimal balance between performance and cost.

[0024] Furthermore, the half-bridge submodule adopts a half-bridge structure, which includes two power electronic switching devices with anti-parallel freewheeling diodes and a voltage-regulating capacitor. Two power electronic switching devices are connected in series, a voltage stabilizing capacitor is connected in parallel across the two power electronic switching devices, and the middle bridge arm is connected in the middle of the two power electronic switching devices.

[0025] Furthermore, the full-bridge submodule adopts a full-bridge structure, which includes four power electronic switching devices with anti-parallel freewheeling diodes and a voltage-regulating capacitor. Two of the four power electronic switching devices are connected in series to form a first series branch, and the other two power electronic switching devices are connected in series to form a second series branch. The first series branch and the second series branch are connected in parallel. A voltage stabilizing capacitor is connected in parallel across the two ends of the first series branch or the second series branch. The upper or lower bridge arm is connected to the middle position of the two power electronic switching devices in the first series branch or the second series branch.

[0026] The present invention also provides a multi-port bridge arm multiplexing modular multilevel converter. The multi-port bridge arm multiplexing modular multilevel converter is a three-phase structure. Each phase in the three-phase structure is independently equipped with a converter module, which together constitutes a three-phase converter unit to adapt to the multi-port power interaction requirements of a three-phase AC power grid.

[0027] Based on the above topology and configuration, this invention enables centralized access to any number of AC ports of different voltage levels and frequencies and one DC port. It can simultaneously accommodate multiple AC voltages such as power frequency AC, low frequency AC, and high frequency AC, as well as high voltage DC voltage. There is no need to build separate converter stations or substations for different types of voltages. It realizes the dual value of site reuse and corridor intensification, greatly improves the economy of the power system and the utilization rate of land resources, and provides an efficient solution for centralized networking of multiple types of new energy.

[0028] Example 2: The present invention also provides a control method for a multi-port bridge arm multiplexed modular multilevel converter, comprising: Step 1: Define the allocation coefficient λi, where i = 1, 2, ..., N. Based on the defined allocation coefficient λi, establish a quantitative correlation between the current of the upper bridge arm and the current of each AC port. Here, the allocation coefficient λi is the proportionality coefficient of the output current of the i-th AC port in the upper bridge arm current. Preferably, the value of the allocation coefficient λi is determined by the circuit parameters of the upper bridge arm, each intermediate bridge arm, and the lower bridge arm within the same phase of the converter, as well as the operating conditions of each AC port. When the operating conditions are constant, the allocation coefficient λi is a fixed value. The defined allocation coefficient λi is specific to a single AC port. Each λi can be flexibly adjusted independently according to the voltage level and frequency of the corresponding port, allowing N heterogeneous AC ports to be centrally connected through the same converter, while also being compatible with one high-voltage DC port, without the need to configure separate converters or interface modules for different ports.

[0029] Reference Figure 2 The schematic diagram of the topology operation established a quantitative correlation between the current in the upper arm and the current in each AC port, as follows: The formulas for calculating the voltage of the upper arm, each intermediate arm, and the lower arm are as follows: In the formula, u uj u lj Let u be the voltages of the upper and lower arms of phase j, respectively, where j = a, b, c; mjn Let U be the voltage of the nth intermediate bridge arm of phase j, where n = 1, 2, ..., N; dc The DC bus voltage, u jn Let be the output voltage of the nth AC port of phase j; This is the output voltage of the first AC port of phase j; Let be the output voltage of the (n+1)th AC port of phase j; Let N be the output voltage of the Nth AC port of phase j; The formulas for calculating the voltage and current at the AC port are as follows: In the formula, u jn Let i be the output voltage of the nth AC port of phase j; jn U is the output current of the nth AC port of phase j; jn I jn Let ω be the output voltage and current amplitude of the nth AC port of phase j. n Let θ be the angular velocity of the nth AC port. n φ n Let be the phase angle of the output voltage and output current of the nth AC port; The formulas for calculating the current in the upper arm, each intermediate arm, and the lower arm are as follows: In the formula, i uj i mjn i lj These represent the currents of the upper arm, the middle nth arm, and the lower arm of phase j, respectively; I dc For direct current; i zj i represents the circulating current component between bridge arms; ji λ is the output current of the i-th AC output port of phase j; i is the distribution coefficient of the output current of the i-th AC port in the upper bridge arm current.

[0030] By quantitatively correlating port current with bridge arm current, the energy demand of different types of ports can be precisely allocated to the corresponding bridge arm through λi, realizing intensive networking of power frequency AC, low frequency AC, high frequency AC, and high voltage DC, significantly reducing the number of converter stations and site occupation, reducing the construction cost of transmission corridors, and improving land resource utilization, especially suitable for scenarios such as centralized access of new energy and interconnection of multi-voltage level grids.

[0031] Based on the quantitative control logic of λi, the contribution ratio of each AC port to the bridge arm current can be accurately quantified, avoiding current distribution imbalance caused by inter-port coupling, ensuring dynamic stability during multi-port collaborative operation, and maintaining overall system stability by quickly adjusting λi even if the operating conditions of a certain port change abruptly.

[0032] Step 2: Based on the quantitative correlation, set the constraints for stable system operation; the constraints are to control the circulating current between the upper arm, each intermediate arm and the lower arm in the same phase to be zero, and the average power of the upper arm, each intermediate arm and the lower arm in one operating cycle to be zero.

[0033] By setting a constraint that the circulating current between the upper bridge arm, each intermediate bridge arm, and the lower bridge arm is zero, the unwanted circulating current between the bridge arms is directly suppressed, reducing power loss and improving the converter operating efficiency. By setting a constraint that the average power of the upper bridge arm, each intermediate bridge arm, and the lower bridge arm is zero within one operating cycle, the problem of submodule capacitor overvoltage and device damage caused by continuous energy accumulation in a single bridge arm can be avoided from the root, ensuring the long-term stable operation of the system.

[0034] Step 3: Based on the quantitative correlation and constraints, derive the average power formulas for the upper arm, lower arm, and N-1 intermediate arms respectively. The average power of the upper arm, middle arm, and lower arm derived in this step is as follows: Average power of upper arm in one cycle The calculation formula is as follows: In the formula, This is the DC bus voltage; It is direct current; The output voltage amplitude of the first AC port of phase j; Let be the output current amplitude of the i-th AC port of phase j; The distribution coefficient of the output current of the i-th AC port in the upper bridge arm current; To rearrange the intermediate angular velocities and The same quantity; The formula for calculating the average power of the nth intermediate bridge arm is: In the formula, M n1 To rearrange the intermediate angular velocities and The same quantity, M n2 To rearrange the intermediate angular velocities and The same quantity; Let be the output voltage amplitude of the (n+1)th AC port of phase j; Let be the phase angle of the output voltage at the i-th AC port; The formula for calculating the average power of the lower bridge arm is: In the formula, To rearrange the intermediate angular velocities and The same quantity; Let be the output voltage amplitude of the Nth AC port of phase j; Let be the output current amplitude of the i-th AC port of phase j; Let be the phase angle of the output voltage at the Nth AC port; Let be the phase angle of the i-th AC output voltage.

[0035] It should be noted that the parameters i and n involved in the above formulas all take values ​​from 1, 2, ..., N; however, they correspond to different numerical meanings of technical parameters in different formulas. By clarifying the types and application scenarios of their respective technical parameters, it is ensured that the calculation formulas for bridge arm voltage and current can accurately support the establishment of a quantitative correlation between the allocation coefficient λi and the port and bridge arm.

[0036] This step is based on the quantitative correlation between the ports and the upper bridge arm, incorporating the quantitative correspondence between the current and voltage of each bridge arm and the currents of N AC ports and 1 DC port. Simultaneously, the dual constraints of zero circulating current and zero average power of the bridge arm are used as the boundary conditions for solving the equations, achieving the fusion of correlation and constraint conditions to form a mathematical solution system. This system ensures that the solution process for λi is a collaborative process of achieving precise energy allocation across multiple ports while satisfying system stability constraints, rather than satisfying a single objective in isolation, thus avoiding contradictions in the solution results. Furthermore, the structured design of the equations makes the solution process for λi reproducible and verifiable, further guaranteeing the reliability and consistency of the results.

[0037] Step 4: Based on the average power formula, construct a power balance equation set and solve for λi (i=1,2,...,N) to determine the distribution relationship between the output currents of each AC port. Ensure that all bridge arms meet the constraints, ultimately achieving stable energy transfer between N AC ports and 1 DC port.

[0038] The power balance equations for the upper arm, middle arm, and lower arm must satisfy the following: In the formula, The average power of the upper bridge arm over one cycle; The average power of the nth intermediate bridge arm; This represents the average power of the lower bridge arm.

[0039] Based on the above formula, we can solve for λ1, λ2, ..., λ. N The specific values ​​are used to determine the distribution relationship between the currents at multiple AC ports of the converter under specific operating conditions.

[0040] For the upper and lower bridge arms, the contribution ratio of the corresponding port current is adjusted by λi, so that the half-bridge sub-module only undertakes the basic commutation and DC port stable output functions, making full use of the advantages of the simple structure and low cost of the half-bridge module to reduce the overall hardware cost. For the middle bridge arm, the voltage / frequency adjustment requirements of different AC ports can be adapted through the flexible adjustment of λi, giving full play to the wide adjustment range and high flexibility of the full bridge submodule, and ensuring the realization of functions in multi-port heterogeneous scenarios; The control method does not require the addition of complex auxiliary control modules or circulating current suppression circuits. Multi-objective control can be achieved simply by solving and adjusting the core parameter λi. The control logic is simple, reducing control complexity and operating losses, and further optimizing the system economy.

[0041] In the control method, the number of allocation coefficients λi is completely synchronized with the number of AC ports N. When an additional AC port is needed, only the corresponding port λi needs to be added, and the existing closed-loop logic of establishing association, setting constraints, solving parameters, and implementing control can be substituted in, without the need to reconstruct the control framework. Whether it is the interconnection of multiple AC buses in high-voltage power transmission scenarios or the centralized access of distributed power sources in medium and low-voltage power distribution scenarios, this method can quickly adapt to different scenario requirements by adjusting the number of ports N and the corresponding allocation coefficients λi, which can greatly improve the versatility and engineering application value of the converter and reduce the cost of subsequent expansion or transformation.

[0042] In specific implementation, taking the connection of 3 AC ports as an example, i.e., N=3, the topology is as follows: Figure 3As shown, a corresponding simulation system model was built in MATLAB / Simulink. A constant DC voltage source was connected to the DC side, and the AC side controlled to output three AC voltages with different amplitudes and frequencies, simultaneously connecting three loads of different power. The DC port voltage level was 100kV. The first AC port voltage amplitude was 10kV with a frequency of 50Hz, connected to a 100kW active load; the second AC port voltage amplitude was 5kV with a frequency of 17Hz, connected to a 50kW active load; and the third AC port voltage amplitude was 8kV with a frequency of 100Hz, connected to an 80kW active load.

[0043] Simulation results for the DC port and the first AC port are as follows: Figure 4 As shown, the voltage and current waveforms of the second and third AC ports are as follows: Figure 5 and Figure 6 As shown, the proposed topology can output voltages and currents with different amplitudes and frequencies, realizing the aggregation and networking of multiple types of AC and DC voltages.

[0044] Although this application has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of this application. Accordingly, this specification and drawings are merely exemplary illustrations of the application as defined herein, and are to be considered as covering any and all modifications, variations, combinations, or equivalents within the scope of this application. Clearly, those skilled in the art can make various alterations and modifications to this application without departing from its scope. Thus, if such modifications and modifications fall within the scope of this application and its equivalents, this application intends to include such modifications and modifications.

Claims

1. A converter module, characterized in that, It includes N+1 bridge arms, the N+1 bridge arms include an upper bridge arm, a lower bridge arm and an intermediate bridge arm group, the intermediate bridge arm group includes N-1 intermediate bridge arms, and N is a positive integer greater than or equal to 2; The upper bridge arm, the middle bridge arm group, and the lower bridge arm are connected in series. The middle bridge arms in the middle bridge arm group are connected in series. The first middle bridge arm in the middle bridge arm group is connected in series with the upper bridge arm. The last middle bridge arm in the middle bridge arm group is connected in series with the upper bridge arm. An AC port is led out from the connection point of every two adjacent bridge arms in the upper bridge arm, the middle bridge arm group, and the lower bridge arm, forming N AC ports. The end of the upper bridge arm furthest from the middle bridge arm and the end of the lower bridge arm furthest from the middle bridge arm share a common DC port. The upper and lower bridge arms are each composed of multiple cascaded half-bridge sub-modules, and the middle bridge arm is composed of multiple cascaded full-bridge sub-modules.

2. The converter module according to claim 1, characterized in that, The half-bridge submodule adopts a half-bridge structure, which includes two power electronic switching devices with anti-parallel freewheeling diodes and a voltage-regulating capacitor. The two power electronic switching devices are connected in series, the voltage stabilizing capacitor is connected in parallel across the two power electronic switching devices, and the intermediate bridge arm is connected at the middle position of the two power electronic switching devices.

3. The converter module according to claim 1, characterized in that, The full-bridge submodule adopts a full-bridge structure, which includes four power electronic switching devices with anti-parallel freewheeling diodes and a voltage-stabilizing capacitor. Two of the four power electronic switching devices are connected in series to form a first series branch, and the other two of the four power electronic switching devices are connected in series to form a second series branch. The first series branch and the second series branch are connected in parallel. The voltage stabilizing capacitor is connected in parallel at both ends of the first series branch or the second series branch. The upper bridge arm or the lower bridge arm is connected at the middle position of the two power electronic switching devices in the first series circuit or the second series circuit.

4. A multi-port bridge arm multiplexing modular multilevel converter, characterized in that, The multi-port bridge arm multiplexed modular multilevel converter is a three-phase structure, in which each phase is independently equipped with a converter module as described in any one of claims 1-3.

5. A control method applied to the multi-AC port bridge arm multiplexing modular multilevel converter as described in claim 4, characterized in that, include: Define an allocation coefficient λi, where i = 1, 2, ..., N. Based on the allocation coefficient λi, establish a quantitative correlation between the current of the upper bridge arm and the current of each AC port; wherein, the allocation coefficient λi is the proportionality coefficient of the output current of the i-th AC port in the current of the upper bridge arm. For quantitative correlations, set constraints for stable system operation; Based on quantitative correlations and constraints, the average power formulas for the upper arm, lower arm, and N-1 intermediate arms are derived respectively. Based on the average power formula, a power balance equation system is constructed, and λi is obtained by solving the power balance equation system, where i=1,2,...,N, to determine the distribution relationship between the output currents of each AC port.

6. The control method for a multi-AC port bridge arm multiplexing modular multilevel converter according to claim 5, characterized in that, The established quantitative correlation between the current in the upper bridge arm and the current in each AC port is as follows: The voltage calculation formulas for the upper arm, each intermediate arm, and the lower arm are as follows: In the formula, u uj u lj Let u be the voltages of the upper and lower arms of phase j, respectively, where j = a, b, c; mjn Let U be the voltage of the nth intermediate bridge arm of phase j, where n = 1, 2, ..., N; dc The DC bus voltage, u jn Let be the output voltage of the nth AC port of phase j; This is the output voltage of the first AC port of phase j; Let be the output voltage of the (n+1)th AC port of phase j; Let N be the output voltage of the Nth AC port of phase j; The formulas for calculating the voltage and current at the AC port are as follows: In the formula, u jn Let i be the output voltage of the nth AC port of phase j; jn U is the output current of the nth AC port of phase j; jn I jn These are the output voltage and current amplitudes of the nth AC port of phase j, respectively, ω n Let θ be the angular velocity of the nth AC port. n φ n Let be the phase angle of the output voltage and output current of the nth AC port; The formulas for calculating the current in the upper arm, each intermediate arm, and the lower arm are as follows: In the formula, i uj i mjn i lj These represent the currents of the upper arm, the middle nth arm, and the lower arm of phase j, respectively; I dc For direct current; i zj This refers to the circulating current component between bridge arms; i ji λ is the output current of the i-th AC output port of phase j; i is the distribution coefficient of the output current of the i-th AC port in the upper bridge arm current.

7. The control method for a multi-AC port bridge arm multiplexing modular multilevel converter according to claim 6, characterized in that, The average power of the upper arm, middle arm, and lower arm derived therefrom is as follows: Average power of upper arm in one cycle The calculation formula is as follows: In the formula, This is the DC bus voltage; It is direct current; The output voltage amplitude of the first AC port of phase j; Let be the output current amplitude of the i-th AC port of phase j; The distribution coefficient of the output current of the i-th AC port in the upper bridge arm current; To rearrange the intermediate angular velocities and The same quantity; The formula for calculating the average power of the nth intermediate bridge arm is: In the formula, M n1 To rearrange the intermediate angular velocities and The same quantity, M n2 To rearrange the intermediate angular velocities and The same quantity; Let be the output voltage amplitude of the (n+1)th AC port of phase j; Let be the phase angle of the output voltage at the i-th AC port; The formula for calculating the average power of the lower bridge arm is: In the formula, To rearrange the intermediate angular velocities and The same quantity; Let be the output voltage amplitude of the Nth AC port of phase j; Let be the output current amplitude of the i-th AC port of phase j; Let be the phase angle of the output voltage at the Nth AC port. Let be the phase angle of the output voltage of the i-th AC port.

8. The control method for a multi-AC port bridge arm multiplexing modular multilevel converter according to claim 7, characterized in that, The power balance equations for the constructed upper arm, middle arm, and lower arm must satisfy: In the formula, The average power of the upper bridge arm over one cycle; The average power of the nth intermediate bridge arm; This represents the average power of the lower bridge arm.

9. The control method for a multi-AC port bridge arm multiplexing modular multilevel converter according to claim 5, characterized in that, The value of the allocation coefficient λi is determined by the circuit parameters of the upper arm, each intermediate arm and the lower arm in the same phase of the converter, as well as the operating conditions of each AC port. When the operating conditions are constant, the allocation coefficient λi is a constant value.

10. The control method for a multi-AC port bridge arm multiplexing modular multilevel converter according to claim 5, characterized in that, The constraint condition is that the circulating current between the upper arm, each intermediate arm, and the lower arm in the same phase is zero, and the average power of the upper arm, each intermediate arm, and the lower arm in one operating cycle is zero.

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