Multi-port hub-type hvdc converter topology with power flow control function and method

Abstract

The application provides a multi-port hub-type HVDC converter topology and method with power flow control function, comprising: parallel MMC and series cascaded H-bridge embedded flexible interconnection switch CHB-eFIS; the CHB-eFIS is embedded in the bridge arm loop of the parallel MMC, each phase of the CHB-eFIS is composed of a plurality of cascaded bridge arms in parallel, and the two ends of the parallel common point of the cascaded bridge arms are respectively connected with the bottom end of the upper bridge arm and the top end of the lower bridge arm of the parallel MMC; the differential mode output port of each cluster cascaded H-bridge bridge arm of the CHB-eFIS is used as an independent alternating current power flow regulation port; the CHB-eFIS is controllable for each feeder power flow by changing the series voltage between the feeders through the alternating current side output voltage of the CHB-eFIS, and the balance of the energy exchanged between the CHB-eFIS and the alternating current feeder is controlled through cooperation with the parallel MMC. The application meets the application requirements of the multi-voltage form, multi-port expansion, wide range regulation and modular design of the FIS of the AC / DC hybrid flexible interconnected power distribution network.

Description

Technical Field

[0001] This application relates to the fields of power grid power flow control, flexible interconnection technology, AC / DC hybrid power distribution technology, and power electronics technology. Specifically, it relates to a multi-port hub-type flexible DC converter topology and method with power flow control function. Background Technology

[0002] With the transformation of the energy structure and the advancement of the new infrastructure strategy, distributed new energy sources and new loads such as distributed photovoltaics, electric vehicle charging facilities, and data centers are showing a trend of large-scale integration into the distribution network. By the end of 2024, my country's cumulative installed capacity of distributed photovoltaics had reached 370 million kilowatts, the number of electric vehicles exceeded 18 million, and the number of charging infrastructures reached 10.24 million units. It is estimated that by 2030, the load capacity of data centers will reach 70-150 million kilowatts. The large-scale integration of these new sources and loads is profoundly changing the operating characteristics of the distribution network: distributed photovoltaics have led to the transformation of the distribution network from a passive unidirectional to an active bidirectional structure, and its random fluctuations have exacerbated the complexity of power flow and power quality issues; impulsive loads such as fast charging of electric vehicles place higher demands on power supply reliability, power quality, and load management. However, traditional radial distribution networks are mainly based on unidirectional power supply mode, relying on mechanical tie switches and primary equipment with limited regulation capabilities, making it difficult to adapt to the multidirectional power flow and dynamic balance requirements brought about by the high proportion of new sources and loads.

[0003] While microgrids, energy storage, and demand-side response technologies can partially alleviate the aforementioned problems, the inherent structure of the distribution network remains the fundamental constraint on its load-carrying capacity and operational flexibility. Flexible interconnection technology, by replacing traditional mechanical switches with power electronic converters, enables routine "soft connections" and precise power flow control between distribution network feeders, providing an effective way to improve the distribution network's capacity to accept new loads, operational flexibility, and power supply reliability. National policies have also explicitly proposed promoting the flexible transformation of distribution networks. Currently, flexible interconnection networks mainly exist in two forms: centralized and distributed. The former is suitable for AC feeder mutual support within the same / near area, but has a limited power supply radius; the latter supports remote interconnection and DC load access, but relies on high-cost DC distribution networks, resulting in poor economic efficiency. Therefore, developing a hybrid AC / DC flexible interconnection system that combines the advantages of both has become an important direction. This system integrates AC feeder mutual support and DC remote interconnection functions through a hybrid AC / DC flexible interconnection switch (FIS) hub, enabling power optimization configuration across a large spatial range and multiple time scales.

[0004] However, existing FIS hub stations mostly adopt multiple locally deployed back-to-back full-power converters (such as MMC) in combination, which have problems such as high equipment cost, large footprint, and low operating efficiency, making it difficult to promote on a large scale in suburban power distribution networks where land resources are scarce.

[0005] In the prior art, patent application publication number CN116131262A discloses a transformerless generalized unified power flow controller, method and system, including: a reactive power compensation module and a power flow regulation module. The reactive power compensation module includes a cascaded bridge inverter, and the power flow regulation module includes a multi-port single-phase MMC. The multi-port single-phase MMC is connected in series with the cascaded bridge inverter. The reactive power compensation module is a voltage source converter with bidirectional reactive power compensation function. It can absorb reactive power from the system and provide reactive power compensation to the system. The power flow regulation module realizes decoupled control of active power and reactive power of transmission lines, but it cannot meet the compatibility of multiple voltage modes and cannot directly support the compatible operation of DC grid or AC / DC hybrid system.

[0006] Therefore, there is an urgent need for a low-cost, compact flexible DC converter topology that can meet the core requirements of multi-voltage compatibility, flexible expansion of multiple ports, compact equipment and multi-functional integration, and solve key technical problems such as power coordination control, internal energy balance, fault protection and parameter design. Summary of the Invention

[0007] In view of one of the deficiencies in the prior art, the purpose of this application is to provide a multi-port hub-type flexible DC converter topology and method with power flow control function.

[0008] A first aspect of this application provides a multi-AC port hub-type flexible DC converter topology with unified power flow control function, including: parallel MMC and series cascaded H-bridge embedded flexible interconnection switch CHB-eFIS; The parallel MMC is a parallel circuit used to provide reactive power compensation and power supply to the medium-voltage DC bus; The CHB-eFIS is embedded in the bridge arm loop of the parallel MMC. Each phase of the CHB-eFIS is composed of multiple cascaded bridge arms connected in parallel. The two ends of the parallel common point of the cascaded bridge arms are respectively connected to the bottom end of the upper bridge arm and the top end of the lower bridge arm of the parallel MMC. The differential mode output ports of each cluster of cascaded H-bridge arms of the CHB-eFIS serve as independent AC power flow regulation ports for connecting to different AC feeders. The CHB-eFIS controls the power flow of each feeder by adjusting the series voltage between the feeders by changing its AC side output voltage, and the energy balance between the CHB-eFIS and the AC feeders is coordinated with the parallel MMC for regulation.

[0009] Optionally, the CHB-eFIS can linearly expand the number of AC power flow regulation ports by increasing the number of clusters of cascaded H-bridge arms, enabling flexible multi-port expansion; The CHB-eFIS uses its fundamental frequency differential mode output voltage to couple energy with the common mode current component in the parallel MMC, forming a collaborative energy balance channel between the series and parallel converters. The series-connected CHB-eFIS has four-quadrant operation capability and can bidirectionally exchange active and reactive power with AC lines.

[0010] A second aspect of this application provides a control method for a multi-AC port hub-type flexible DC converter topology with unified power flow control function, including: a multi-terminal power decoupling control method and a multi-level energy balance control method executed in parallel; The multi-terminal power decoupling control method calculates the differential mode output voltage reference value required by each cluster of cascaded H-bridge arms based on the active and reactive power commands of each AC port, and performs independent decoupling control on the power flow of each feeder. The multi-level energy balance control method includes series-parallel coordinated overall energy balance control and CHB-eFIS internal inter-cluster energy balance control. It adopts DC common-mode component injection to achieve overall energy balance and internal energy balance, and the injection of DC common-mode component does not affect AC line current and power flow regulation.

[0011] Optionally, the multi-terminal power decoupling control method specifically includes: Based on the AC differential-mode equivalent circuit, a dynamic mathematical model is established between the current of each line and the series-regulated voltage of CHB-eFIS. An outer-inner-loop dual-loop control architecture is adopted. The outer-loop controller generates corresponding d-axis and q-axis current reference values ​​based on the active and reactive power commands of the system scheduling. The inner-loop controller generates series equivalent voltage reference values ​​in a synchronous rotating coordinate system through PI control combined with coupling compensation and voltage feedforward. The series equivalent voltage reference value is converted into a three-phase voltage reference value through Park inverse transformation, and the differential mode output reference voltage of each cascaded H-bridge arm of the CHB-eFIS is determined to perform multi-terminal power decoupling control.

[0012] Optionally, the step of establishing a dynamic mathematical model between the current in each line and the series-regulated voltage of CHB-eFIS based on the AC differential-mode equivalent circuit includes: Based on the energy coupling relationship of the series-parallel converters injected with DC common mode components, an equivalent circuit of TLM-S2FIS is established, and a single-phase equivalent circuit is constructed according to the three-phase symmetry principle. Based on the superposition theorem, the single-phase equivalent circuit is decomposed into an AC differential-mode equivalent circuit and a DC common-mode equivalent circuit; One AC line in the CHB-eFIS is selected as the active power balancing line. Based on the AC differential mode equivalent circuit, the active and reactive power expressions of the AC line are established. Using the AC voltage vector of the balanced line as the orientation reference of the synchronously rotating dq coordinate system, the relationship between line power flow and current in the rotating coordinate system and the dynamic mathematical model of the system are established.

[0013] Optionally, when determining the differential mode output reference voltage of each cascaded H-bridge arm of the CHB-eFIS, a differential mode voltage reference value is specified for the arm connected to the active power balancing line, and the maximum value of the differential mode output reference voltage amplitude of all cascaded CHB-eFIS arms is minimized through optimization.

[0014] Optionally, the overall total energy control of the series-parallel coordination is as follows: by utilizing the common-mode energy interaction path between the CHB-eFIS and the MMC, based on DC common-mode component injection, a power balance constraint relationship between the CHB-eFIS and the MMC is established; The CHB-eFIS internal inter-cluster energy balance control is as follows: based on the DC common-mode voltage component, an independent DC bias voltage component is superimposed on each cluster cascaded H-bridge arm to independently adjust the DC circulating current flowing into the arm, compensate for the inter-cluster energy difference caused by power flow regulation, and maintain the balance of capacitor voltage of each cluster submodule.

[0015] Optionally, the power balance constraint relationship needs to take into account the deviation between the average value of the capacitor voltage of all sub-modules of the CHB-eFIS and the reference value of the capacitor voltage, and generate an overall DC common-mode voltage command through a PI regulator to control the DC common-mode output voltage of each cluster of cascaded bridge arms of the CHB-eFIS, and interact with the DC component in the current of the MMC bridge arm to achieve overall energy balance between the CHB-eFIS and the MMC.

[0016] Optionally, in the inter-cluster energy balance control, the DC bias voltage component superimposed on the arms of the cascaded H-bridge of the k-th cluster is determined by the deviation between the average value of the capacitor voltage of the cluster submodule and the total average value of the capacitor voltage of all cluster submodules, as well as the direction of the DC common-mode voltage component, and the injected DC circulating current is constrained inside the CHB-eFIS.

[0017] Optionally, when performing the overall energy balance control and internal energy equalization control, the direction of the DC component of the parallel MMC bridge arm current needs to be considered in order to determine the charging and discharging direction of energy interaction.

[0018] This application provides a multi-port hub-type flexible DC converter topology with power flow control function. It employs a parallel architecture based on MMCs (Multi-channel Controllers) and embeds the CHB-eFIS (Chain-Connected Flexible Interconnection Switch) of a series-cascaded H-bridge type into the MMC arm loop. By embedding the CHB-eFIS into the parallel MMC arm loop, dual coupling of the series-parallel converter in the AC loop and the internal arm loop is achieved. This frees the energy balance of the series converter from being limited by the AC output current of the MMC, thus liberating its adjustment freedom and significantly expanding the power flow control range. The use of CHB-eFIS in conjunction with the parallel MMC replaces the combination of multiple independent full-power MMCs in traditional solutions, significantly reducing equipment cost and footprint. Simultaneously, the modular design of the CHB-eFIS allows for flexible expansion of AC ports by increasing the number of cascaded arms, meeting the application requirements of AC / DC hybrid flexible interconnected distribution networks with multiple voltage configurations and multiple port access.

[0019] Other technical effects resulting from the additional features will be further illustrated in the corresponding embodiments. Attached Figure Description

[0020] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the topology evolution process of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application. Figure 2 This is a schematic diagram of a multi-port hub-type flexible DC converter topology with power flow control function in one embodiment of this application; Figure 3 This application provides the overall equivalent circuit and the AC differential mode and DC common mode equivalent circuits of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of the present application. Figure 4 This is a block diagram of the multi-port power decoupling control of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application; Figure 5 This is the CHB-eFIS and MMC collaborative energy interaction path of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application; Figure 6 This application defines the basic electrical quantities and energy balance common-mode electrical quantity coupling relationship of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of the present application. Figure 7 This is a block diagram of the overall energy balance control of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application; Figure 8This is a single-phase DC common-mode equivalent circuit diagram considering DC circulating current injection for a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application; Figure 9 This is a block diagram of the CHB-eFIS inter-cluster energy balance control of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application; Figure 10 The waveforms of phase A voltage and current of each AC line connected to a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application are shown. Figure 11 This is the equivalent series voltage waveform of the AC port of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application; Figure 12 The active and reactive power waveforms of each line connected to a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application are shown. Figure 13 The MMC and CHB-eFIS average capacitor voltage waveforms of a multi-port hub-type flexible DC converter with power flow control function in one embodiment of this application are shown. Figure 14 The above describes the capacitor voltage waveforms of each cluster arm of the CHB-eFIS, a multi-port hub-type flexible DC converter with power flow control function, in one embodiment of this application. Detailed Implementation

[0021] The present application will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present application, and these all fall within the protection scope of the present application. Parts not described in detail in the following embodiments can be implemented using existing technology.

[0022] In the description of the embodiments of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0023] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature.

[0024] In the description of the embodiments in this application, "multiple" means two or more, unless otherwise explicitly specified. In this application, unless otherwise explicitly specified and limited, the terms "installed," "connected," "linked," "fixed," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0025] The terms "comprising" and "having," and any variations thereof, in the embodiments of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such processes, methods, products, or devices.

[0026] In existing technologies, current FIS hub stations are mostly implemented using a combination of multiple locally deployed back-to-back full-power converters (such as MMCs), which suffers from problems such as high equipment costs, large footprint, and low operating efficiency, making it difficult to scale up in suburban power distribution networks where land resources are scarce. Based on the above problems, this application provides a multi-port hub-type flexible DC converter topology with power flow control function to solve the aforementioned issues.

[0027] This application comprehensively considers the application requirements of AC / DC hybrid flexible interconnection schemes for FIS (Flexible Interconnection System) in terms of multi-voltage configurations, multi-port expansion, low-cost compact design, and multi-directional flexible power flow control. Based on the design concept of "series voltage regulation, parallel support, and coordinated balance," and using MMC (Multi-Mechanical Controlled Transformer) as the basic parallel architecture, a cascaded H-bridge is embedded as a series FIS within the MMC bridge arm circuit. This results in a low-cost, compact design with flexible multi-port power flow adjustment and unified power flow control, forming a multi-AC port hub-type flexible DC converter topology (Transformer-Less Multiport Series-Shunt-coupled FIS, TLM-S). 2 FIS).

[0028] Reference Figure 1 and Figure 2As shown in one embodiment of this application, a multi-port hub-type flexible DC converter topology with power flow control function includes: parallel MMC and series cascaded H-bridge embedded flexible interconnection switch CHB-eFIS.

[0029] The parallel MMC is a parallel circuit used to provide reactive power compensation and power supply to the medium-voltage DC bus. The CHB-eFIS is embedded in the bridge arm loop of the parallel MMC. Each phase of the CHB-eFIS consists of multiple cascaded bridge arms connected in parallel. The two ends of the parallel common point of the cascaded bridge arms are connected to the bottom end of the upper bridge arm and the top end of the lower bridge arm of the parallel MMC, respectively. The differential mode output port of each cluster of cascaded H-bridge arms of the CHB-eFIS serves as an independent AC power flow regulation port for connecting to different AC feeders. Among them, CHB-eFIS controls the power flow of each feeder by changing the series voltage between the feeders by changing its AC side output voltage, and the energy balance between CHB-eFIS and AC feeders is coordinated with the parallel MMC for regulation.

[0030] The above embodiments of this application, through CHB eFIS is embedded in the MMC bridge arm loop to form a series-parallel cooperative structure, which can utilize CHB eFIS independently regulates the power flow of each AC feeder, while using MMC to achieve reactive power support and DC bus power supply. Energy balance is completed collaboratively by series and parallel units. It eliminates the need for independent transformers and multiple full-power converters, significantly reducing equipment costs and floor space. It has the advantages of flexible expansion with multiple ports, wide power flow control range, and stable and reliable operation, and can better adapt to the needs of multi-source load access and flexible interconnection in AC / DC hybrid distribution networks.

[0031] Specifically, TLM-S 2 FIS Design Concept and Evolution Figure 1 As shown: First, referring to the basic principles and topology of T-UPFC, and considering the requirements of medium-voltage DC ports, the parallel-side converter is designed as an MMC, and the series and parallel converters operate independently, such as... Figure 1 (a) A basic single-feeder series-parallel coupled topology, in which the parallel MMC converter and the series converter each need to achieve energy balance, thus limiting their adjustment freedom and making it impossible to achieve active-reactive decoupling control at each AC port. Therefore, by shifting the series converter to the AC output side of the parallel MMC, an energy coupling channel between the series and parallel converters is constructed, extending to multi-feeder access conditions. Figure 1(b) Multi-feeder basic topology, in which parallel MMC converters can collaboratively achieve energy balance of series converters, liberating the adjustment freedom of series converters and enabling active-reactive decoupling control of each AC port, as well as flexible power interaction between AC and DC. However, since the energy balance of the series converters relies entirely on the current of the parallel MMC AC output lines, when the MMC does not provide reactive power and the DC-side power interaction is small, the entire topology is difficult to operate stably, thus limiting its power flow control range. Furthermore, embedding the series converters into the MMC arm loop expands the internal energy coupling path to obtain... Figure 1 (c) Improved topology: The series converter is coupled not only to the parallel MMC converter in the AC loop, but also to its internal bridge arm loop. The coordinated energy balance between the series and parallel converters can be achieved by the AC current or the circulating current within the MMC. The control capability and adjustment range of the series converter are no longer constrained by energy balance, thus achieving wide-range operation. Based on this, the power submodule of the series converter is modularly refined, resulting in a single-phase device-level topology as follows: Figure 1 (d) Finally, considering that the series voltage required to achieve full-range power flow regulation of the distribution lines under the 10kV / 35kV voltage level of the medium-voltage AC distribution network is between 0.1 pu and 0.2 pu, the series converter adopts a cascaded H-bridge structure, and finally forms a TLM-S 2 FIS topology, such as Figure 1 As shown in (e).

[0032] It should be noted that adjusting the series voltage of the feeder components includes: changing its AC side output voltage or changing the fundamental frequency differential mode output voltage of each cluster of cascaded bridge arms; both are the same quantity. The fundamental frequency differential mode output voltage is named from the perspective of the upper and lower bridge arms of CHB-eFIS; the AC side output voltage is to consider the effect of regulating AC power flow.

[0033] In some specific embodiments of this application, the CHB-eFIS can linearly expand the number of AC power flow regulation ports by increasing the number of clusters of cascaded H-bridge arms, enabling flexible multi-port expansion; the CHB-eFIS uses its fundamental frequency differential mode output voltage to couple energy with the common mode current component in the parallel MMC, forming a collaborative energy balance channel between series and parallel converters; the series CHB-eFIS has four-quadrant operation capability, bidirectionally exchanging active and reactive power with the AC line.

[0034] Specifically, refer to Figure 2 As shown, TLM-S 2The FIS comprises two parts: a parallel MMC and a cascaded H-bridge embedded-FIS (CHB-eFIS). The CHB-eFIS is a series coupling circuit between AC feeders. By changing its AC output voltage, the series voltage between feeders can be adjusted to achieve controllable power flow for each feeder. Furthermore, the CHB-eFIS has four-quadrant operation capability; the energy exchanged between the device and the feeders can be balanced through coordinated control with the MMC, thus maintaining stable operation. The MMC, as a parallel circuit, provides reactive power compensation for the system and also supplies power to the medium-voltage DC bus. Therefore, compared to commonly used FIS solutions, the TLM-S... 2 By using a collaborative design of partial-power series converters and full-power parallel converters, FIS replaces multiple independent full-power MMCs, which not only significantly reduces the cost and footprint of FIS, but also meets the application requirements of AC / DC hybrid flexible interconnected distribution networks for FIS's multi-voltage configuration, multi-port expansion, wide-range regulation, and modular design.

[0035] Based on the same inventive concept, another embodiment of this application also provides a control method for the above-mentioned multi-port hub-type flexible DC converter topology with power flow control function, including a multi-port power decoupling control method and a multi-level energy balance control method executed in parallel.

[0036] The multi-terminal power decoupling control method calculates the differential mode output voltage reference value required by each cluster of cascaded H-bridge arms based on the active and reactive power commands of each AC port, and performs independent decoupling control on the power flow of each feeder. The multi-level energy balance control method includes series-parallel coordinated overall energy balance control and CHB-eFIS internal inter-cluster energy balance control. It adopts DC common-mode component injection to achieve overall energy balance and internal energy balance, and the injection of DC common-mode component does not affect AC line current and power flow regulation.

[0037] The embodiments described above in this application, through multi-terminal power decoupling control, accurately calculate the differential-mode output voltage reference value required by each cluster of cascaded H-bridge arms based on the active and reactive power commands of each AC port, thereby achieving independent and decoupled regulation of power flow in multiple feedback lines. Furthermore, through multi-level energy balance control, using DC common-mode component injection, the overall energy balance between series and parallel converters and the internal energy balance among clusters of cascaded H-bridges are collaboratively achieved. The injected DC common-mode component only participates in energy interaction and does not affect the AC line current or the predetermined power flow regulation. This ensures flexible, independent, and precise control of multi-port power flow, and fundamentally solves the energy self-balancing problem of series-parallel structures under wide operating conditions through the common-mode energy channel, ensuring the stability and balance of capacitor voltages in each sub-module within the device. This provides stable, efficient, and highly reliable operation assurance for low-cost, compact, multi-port flexible DC converters.

[0038] Among them, the multi-terminal power decoupling control method is used for power flow control of AC lines, while the multi-level energy balance control method is used to maintain the energy balance of the device itself.

[0039] In some specific embodiments of this application, the multi-terminal power decoupling control method specifically includes: Based on the AC differential-mode equivalent circuit, a dynamic mathematical model is established between the current of each line and the series-regulated voltage of the CHB-eFIS. An outer-loop-inner-loop dual-loop control architecture is adopted. The outer-loop controller generates corresponding d-axis and q-axis current reference values ​​according to the active and reactive power commands of the system scheduling. The inner-loop controller generates the series equivalent voltage reference value in the synchronous rotating coordinate system through PI control combined with coupling compensation and voltage feedforward. The series equivalent voltage reference value is converted into a three-phase voltage reference value through Park inverse transformation to determine the differential-mode output reference voltage of each cascaded H-bridge arm of the CHB-eFIS, and multi-terminal power decoupling control is performed.

[0040] Specifically, firstly, based on the AC differential-mode equivalent circuit, a dynamic mathematical model is established between the current of each line and the series-regulated voltage of the cascaded H-bridge embedded flexible interconnected switch (CHB-eFIS). On this basis, an outer-inner-loop dual-loop control architecture is adopted. The outer-loop controller generates corresponding d-axis and q-axis current reference values ​​through power-current conversion based on the active and reactive power commands issued by the system dispatch. The inner-loop controller uses a proportional-integral (PI) regulator to ensure steady-state accuracy and dynamic response, and introduces a current-feedback-based coupling compensation term to achieve decoupling control of the d-axis and q-axis currents. Simultaneously, the detected line node voltage is introduced as a feedforward term into the control law to enhance the suppression of external voltage disturbances, thereby generating a series equivalent voltage reference value in a synchronous rotating coordinate system. Finally, this series equivalent voltage reference value is converted into a three-phase voltage reference value through Park inverse transformation, and combined with an optimized allocation strategy, the differential-mode output reference voltage of each cascaded H-bridge arm of the CHB-eFIS is determined, completing multi-terminal power decoupling control.

[0041] The embodiments described above in this application achieve reliable conversion of power commands to current commands through a dual-loop control architecture. Combined with coupling compensation and voltage feedforward, they effectively eliminate dq-axis current coupling and suppress the influence of grid voltage disturbances. After coordinate transformation and voltage distribution, they can accurately output the differential mode reference voltage of each bridge arm, thereby realizing independent decoupling control of multi-port power flow, improving system regulation accuracy, dynamic response speed and anti-disturbance capability, and ensuring stable and efficient operation of multi-port flexible interconnection devices.

[0042] like Figure 1-2 TLM-S in the illustrated embodiment 2 The topology of the FIS (Flexible Interconnection System) consists of multiple cascaded arms connected in parallel. Their common parallel connection point is connected to the bottom of the upper arm and the top of the lower arm of the parallel MMC (Multi-Level Connector), forming an embedded structure. The differential mode output port of each cluster of cascaded arms serves as the AC power flow regulation port. The number of AC ports can be flexibly expanded by increasing the number of cascaded arms. Based on the working principle of series-regulated flexible interconnection devices that flexibly adjust power flow distribution by changing the series voltage between AC ports, for CHB-eFIS, the series voltage between AC lines can be adjusted by changing the fundamental frequency differential mode output voltage of each cluster of cascaded arms, thus achieving power flow regulation of the connected multi-region AC power grid.

[0043] against Figure 2 Explain the meaning of the variables appearing in the text. Assume that CHB-eFIS interconnects N AC lines, such as Figure 2 As shown. Where u ka i ka (k=1,2,…,N) represents the voltage and current of phase a of the power grid for the k-th line; u pa and u naThe voltages of the upper and lower arms of phase a of the MMC; i pa and i na The upper and lower arm currents of phase a of the MMC; u pjk and u njk Let i be the upper and lower arm voltages of the k-th cluster cascaded bridge arm of the j-phase (j=a,b,c) CHB-eFIS; pjk and i njk For the upper and lower arm currents of the k-th cluster cascaded bridge arm of the (j-phase) CHB-eFIS; U dc and I dc These represent the DC side voltage and current.

[0044] This application achieves the ability to energy couple each frequency component of the common-mode output voltage of the CHB-eFIS with the corresponding frequency component of the common-mode current of the MMC by embedding the series CHB-eFIS within the parallel MMC structure.

[0045] In some specific embodiments of this application, a dynamic mathematical model is established based on the AC differential-mode equivalent circuit, relating the current in each line to the series-regulated voltage of the CHB-eFIS, including: Based on the energy coupling relationship of series-parallel converters with DC common-mode component injection, a TLM-S is established. 2 The FIS equivalent circuit is constructed based on the three-phase symmetry principle, and the single-phase equivalent circuit is decomposed into an AC differential-mode equivalent circuit and a DC common-mode equivalent circuit based on the superposition theorem. An AC line in CHB-eFIS is selected as the active power balancing line, and the active and reactive power expressions of the AC line are established based on the AC differential-mode equivalent circuit. The AC voltage vector of the balancing line is used as the orientation reference of the synchronous rotating dq coordinate system to establish the relationship between the line power flow and the current in the rotating coordinate system and the dynamic mathematical model of the system.

[0046] Specifically, this application establishes an equivalent circuit for the energy coupling relationship of series-parallel converters based on DC common-mode component injection. Based on the three-phase symmetry principle, a single-phase equivalent circuit for CHB-eFIS and MMC is constructed, such as... Figure 3 As shown in (a). Where, u dj and U j,dc These are the fundamental frequency AC and DC components of the j-phase bridge arm voltage of the MMC, respectively, representing the differential-mode component and the common-mode component; i pj and i nj The current in the upper and lower arms of phase j of the MMC; u djk and U jk,dcThese represent the fundamental frequency AC and DC components of the arm voltage of the j-phase k-th cluster cascaded bridge arm in the CHB-eFIS, respectively. The fundamental frequency AC component is a differential mode component, thus exhibiting a series voltage component between AC lines, enabling power flow regulation of the AC lines. The DC component is a common mode component, forming an energy coupling relationship between the CHB-eFIS and the MMC DC current, and between the cascaded bridge arms, thereby realizing TLM-S 2 FIS serial-parallel coordinated energy balance and internal energy equilibrium. pjk and i njk Let i be the upper and lower arm currents of the j-th cluster cascaded bridge arm of CHB-eFIS; jk Let be the phase current of the k-th AC line.

[0047] It should be noted that the equivalent circuit of the entire series-parallel flexible interconnection switch topology can be found in [reference needed]. Figure 3 ,in Figure 3 In the diagram, (a) is the overall equivalent circuit, and (b) and (c) are the differential-mode and common-mode equivalent circuits decomposed based on the superposition theorem.

[0048] Based on the superposition theorem Figure 3 The single-phase equivalent circuit shown in (a) can be further decomposed into Figure 3 The AC differential-mode equivalent circuit shown in (b) and Figure 3 The DC common-mode equivalent circuit is shown in (c). Figure 3 In (b), i oj This is the equivalent differential-mode output current of the MMC; in Figure 3 In (c) I cj I is the DC component of the j-phase bridge arm current of the MMC. cjk Let be the DC component of the single-phase MMC arm current of the k-th cluster of phase j in CHB-eFIS.

[0049] because Figure 3 The DC common-mode equivalent circuit shown in (c) does not affect the AC line current, therefore the following discussion focuses on... Figure 3 (b) The AC differential-mode equivalent circuit shown is analyzed to demonstrate the feasibility of CHB-eFIS power flow control. Considering TLM-S 2 The three-phase symmetry of the FIS topology will be simplified by omitting the subscript j. AC line 1 is selected as the active power balancing line of the system, based on... Figure 3 (b) N-port TLM-S 2 The AC differential-mode equivalent circuit of the FIS topology, the active and reactive power of AC line k can be expressed as: (1) in, and The first The actual active and reactive power transmitted on each AC line, with the superscript conj indicating the conjugate of the vector. , For the natural active and reactive power flow of line k. For CHB-eFIS in the first line and the second Controllable series adjustable voltage phasors injected between the lines For the first The node voltage phasors of the line. The node voltage phasor of the first line, which serves as the active power balance reference. This is the inherent series voltage phasor corresponding to the line reactance voltage drop in the circuit containing the first line. For the first The equivalent reactance of an AC line, in the denominator It is the imaginary unit. And ΔP k and ΔQ k For the series equivalent output voltage ΔU d1k A controllable power flow that can be adjusted is represented as: (2) in, and After injecting series regulated voltage into CHB-eFIS, respectively, at the... The active power regulation and reactive power regulation (i.e., controllable power flow components) caused on each AC line. For the first The node voltage amplitude of the line, The amplitude of the series regulated voltage injected by CHB-eFIS between line 1 and line k. For the first The phase angle of the line node voltage, To inject series regulated voltage phase angle, For the first The equivalent reactance of an AC line.

[0050] Therefore, CHB-eFIS can independently regulate the active and reactive power of lines 2 to n according to the power flow control command. The main MMC will play the role of supporting the medium-voltage DC side voltage and reactive power of the whole device. According to the analysis of the control degree of freedom, the main MMC can regulate the medium-voltage DC side voltage or active power, and can also further regulate the reactive power of line 1.

[0051] Considering the existence of system losses, it is impossible to further regulate the active power of all lines. Due to TLM-S 2The capacitors inside the FIS only act as energy buffers and cannot continuously absorb or release energy. Therefore, line 1 serves as a balancing line, and the active power on this line will be automatically adjusted according to the system's energy balance requirements, i.e.: (3) In the formula, P dc This refers to the active power on the medium-voltage DC side. ( P represents the actual active power of all AC lines except the balanced line. Loss This refers to the system's operational costs.

[0052] To achieve modeling, a method is adopted that uses the AC voltage vector of line 1 as the orientation reference of the synchronously rotating dq coordinate system, thereby establishing the relationship between line power flow and current in the rotating coordinate system: (4) In the formula, and The first The actual active and reactive power transmitted by each AC line; , The first The line terminal voltages rotate synchronously. - coordinate system Axial components and Axial components; , The first The current in the line is the same - coordinate system Axial components and Axial components.

[0053] This equation shows that power flow management can be achieved by regulating the line current. According to Kirchhoff's voltage law, the port voltage of line 1... (subscript) (Corresponding to three phases) to the line k port voltage The resulting loop satisfies the following equation: (5) in, and These are the instantaneous values ​​of the three-phase terminal voltages of line 1 and line k, respectively. and These are the instantaneous values ​​of the three-phase current for the corresponding lines. and These are the equivalent resistance and equivalent inductance of line 1 and line k, respectively. The instantaneous value of the three-phase series regulated voltage injected by CHB-eFIS between line 1 and line k; Based on this, Kirchhoff's voltage law and Park transformation are used to rigorously derive the dynamic differential equations of the entire system in the stationary coordinate system (abc) and the rotating coordinate system (dq), as shown in equation (6).

[0054] (6) In the formula, and The series regulated voltage injected by CHB-eFIS between line 1 and line k is shown in the synchronous rotating coordinate system. shaft and Axial components; and The voltages at line 1 and line k are respectively shaft and Axial components; and These are the corresponding line currents. shaft and Axial components; This is the fundamental angular frequency of the power grid.

[0055] Applying a Laplace transform to equation (6), we obtain the fundamental frequency domain equations for the series regulation voltage, line voltage, and line current of MMC-CHB-eFIS in the synchronous rotating coordinate system: (7) in, For the Laplace operator (complex frequency variable), this equation shows that the current response of any line is jointly determined by the AC voltage at its connecting nodes and the corresponding CHB-eFIS regulation voltage. Further analysis of this mathematical model reveals a core control challenge: the components of the current in each line on the d-axis and q-axis are dynamically coupled, and their response characteristics are affected by voltage disturbances at the nodes on the AC grid side.

[0056] To address the multi-line power flow regulation requirements of the MMC-CHB-eFIS system, this application designs an outer-inner-loop dual-loop control architecture: the outer loop controller calculates and generates corresponding d-axis and q-axis current reference values ​​based on the active and reactive power commands issued by the system dispatch, as shown in Equation (8); the inner loop controller drives the actual line current to accurately and quickly track the above reference values ​​by rapidly adjusting the series output voltage of CHB-eFIS.

[0057] (8) in, and The first The AC line current rotates synchronously. - coordinate system shaft and Shaft current command value (reference value); and The system dispatching order is issued to the first The active power command and reactive power command for each line.

[0058] To address the two core challenges revealed by the mathematical model—dynamic coupling of d- and q-axis currents and the impact of AC node voltage disturbances—the inner loop controller design was specifically enhanced. Specifically, the inner loop employs a PI controller to ensure steady-state accuracy and dynamic response. Furthermore, by introducing a current-feedback-based coupling compensation term, effective decoupling control of the d- and q-axis currents is achieved. Simultaneously, the detected line node voltage is introduced as a feedforward term into the control law, significantly enhancing the system's ability to suppress external voltage disturbances. Thus, the expression for the series equivalent voltage reference value required for power flow regulation is obtained, as shown in (9).

[0059] (9) Obtain the series equivalent voltage reference value in the synchronous rotating coordinate system. and Next, the values ​​need to be transformed back to the abc coordinate system using the inverse Park transform to obtain the corresponding three-phase voltage reference values. , and .

[0060] Furthermore, when determining the differential mode output reference voltage of each cascaded H-bridge arm of CHB-eFIS, the differential mode voltage reference value of the arm connected to the active power balancing line is specified, and the maximum value of the differential mode output reference voltage amplitude of all CHB-eFIS cascaded arms is minimized through optimization.

[0061] Specifically, in order to determine the differential mode output reference voltage of each cascaded bridge arm of CHB-eFIS, it is also necessary to specify the differential mode voltage reference value of the bridge arm connected to line 1, and the specific relationship is described by equation (10).

[0062] (10) in, For CHB-eFIS cluster( The differential-mode output voltage phasor of the cascaded H-bridge arm (i.e., the reference value of the series regulated voltage injected into the corresponding AC line by the bridge arm). The differential-mode output voltage reference phasor of the first cluster of bridge arms connected to the active power balancing line (line 1); The series regulated voltage phasor injected by CHB-eFIS between line 1 and line k.

[0063] To fully utilize the adjustment potential of each submodule, the maximum value of the differential mode output reference voltage amplitude of all CHB-eFIS cascaded bridge arms should be minimized through optimization, that is, to satisfy the optimization objective of equation (11).

[0064] (11) Based on the above steps, a complete line power flow control strategy can be constructed, and its overall structure is as follows: Figure 4 As shown.

[0065] When the MMC operates in rectification or inverter mode, the embedded CHB-eFIS can effectively regulate power flow, thus initially verifying the TLM-S... 2 The feasibility of bidirectional power flow control using FIS. When AC line current flows through the CHB-eFIS port, it causes energy interaction between the CHB-eFIS and the line. Taking a single phase as an example, the interaction power P... eFIS_out for: (12) in, The total active power exchanged between CHB-eFIS and each AC line through the fundamental frequency differential mode output voltage (i.e., the algebraic sum of the differential mode channel interaction power of each cluster of cascaded H-bridge arms). For the first The fundamental frequency differential mode output voltage amplitude of the arms of a clustered H-bridge. For the first The current amplitude of the AC line, For the first The phase angle of the differential mode output voltage. For the first The phase angle of the current in an AC line.

[0066] Therefore, while achieving multi-terminal power decoupling control, it is also necessary to maintain the TLM-S to ensure the stable operation of the device. 2 The FIS device maintains overall energy balance and internal energy equilibrium.

[0067] Reference Figure 5 As shown, according to TLM-S 2 In the series-parallel structure of FIS, there are two main energy interaction paths between CHB-eFIS and MMC: one is the differential-mode energy interaction path based on the differential-mode output voltage of CHB-eFIS and the differential-mode current of MMC, such as... Figure 5 As shown in (a); the second is the common-mode energy interaction path between the common-mode output voltage and the common-mode current of the MMC based on CHB-eFIS, as shown in (a). Figure 5As shown in (b). The energy balance method based on the differential mode circuit has certain limitations. That is, under certain operating conditions, when the differential mode output current of the MMC is very small, a large differential mode voltage is required to achieve the overall energy balance of CHB-eFIS, which will lead to exceeding the linear modulation region of CHB-eFIS.

[0068] Therefore, in order to achieve TLM-S 2 Wide-range operation of FIS requires exploring energy balance methods based on common-mode loops. Figure 5 As shown in (b) of the common-mode energy interaction path, the common-mode output voltage components of the CHB-eFIS have the ability to couple energy with the corresponding frequency components of the MMC's common-mode current at each frequency. This depends on whether it is coupled with the TLM-S... 2 The original basic electrical quantities of FIS have a coupling relationship, dividing the common-mode components of each frequency into two categories: 1) coupled common-mode components; 2) independent common-mode components. Due to TLM-S 2 The bridge arm voltages and currents of the MMC section and CHB-eFIS section of FIS contain DC and fundamental frequency components. The coupled common-mode components contain DC and fundamental frequency components, while the independent common-mode components contain second harmonic and higher frequency components, such as... Figure 6 As shown.

[0069] If TLM-S is achieved by injecting coupled common-mode components... 2 FIS energy balancing, due to the coupling relationship between the energy balancing electrical quantity and the original electrical quantity, on the one hand, can utilize some of the original electrical quantity to achieve device energy balancing, reducing the introduced additional components, and can coordinate and optimize the design of the energy balancing electrical quantity and the original electrical quantity to further reduce the impact of additional components on the original system design; but on the other hand, the coupling relationship of electrical quantities increases the complexity of decoupling control strategy design.

[0070] If TLM-S is achieved by injecting independent common-mode components 2 FIS energy balance, due to the decoupling relationship between the energy balance electrical quantity and the original electrical quantity, on the one hand, the energy balance control and the original basic control have a natural decoupling relationship, and the control strategy design is simple; but on the other hand, the different frequency components and the original electrical quantity have a linear superposition relationship, which has a significant impact on the parameter design of the original system.

[0071] This application employs a common-mode component injection method to achieve coordinated energy balance between series and parallel converters. Taking DC common-mode component injection as an example, it proposes a TLM-S... 2 FIS serial-parallel coordinated energy balance and internal energy equilibrium control method.

[0072] In some specific embodiments of this application, the overall total energy control of the series-parallel coordination is as follows: by utilizing the common-mode energy interaction path between CHB-eFIS and MMC, a power balance constraint relationship between CHB-eFIS and MMC is established based on DC common-mode component injection.

[0073] Furthermore, the power balance constraint relationship needs to take into account the deviation between the average value of the capacitor voltage of all sub-modules of CHB-eFIS and the reference value of the capacitor voltage, and generate an overall DC common-mode voltage command through the PI regulator to control the DC common-mode output voltage of each cluster of cascaded bridge arms of CHB-eFIS, and interact with the DC component in the MMC bridge arm current to achieve overall energy balance between CHB-eFIS and the MMC.

[0074] Specifically, as can be seen from equation (12), TLM-S 2 During FIS power flow control, the series-connected CHB-eFIS interacts with the AC line, requiring energy balance through coordinated control with the parallel MMC. The interaction power at the power flow regulation port of the CHB-eFIS should be balanced with the interaction power at the energy balance port. Therefore: (13) in, The total active power exchanged between CHB-eFIS and MMC via common-mode output voltage; For the first DC common-mode output voltage of the arms of a clustered H-bridge P represents the DC component of the common-mode current flowing through the bridge arm of this cluster. eFIS_diff The fundamental frequency differential mode output voltage of each cluster of cascaded bridge arms in CHB-eFIS is determined by the AC line current and is constrained by AC power flow control requirements, so it cannot be used for energy balance. Therefore, as shown in equation (13), the energy interaction between CHB-eFIS and MMC can be controlled by adjusting the DC common mode output voltage of each cluster of cascaded bridge arms in CHB-eFIS. First, under the premise of considering only the overall energy balance of CHB-eFIS, it is assumed that the DC common mode output voltage of each cluster of cascaded bridge arms is equal, that is, it satisfies: (14) Combining equations (13) and (14), the power balance constraint relationship of CHB-eFIS can be expressed as: (15) in Let U be the command value of the DC common-mode output voltage of each cluster cascaded bridge arm. As can be seen from equation (15), by adjusting the overall DC common-mode output voltage U of each cluster cascaded bridge arm in CHB-eFIS, com,dcInteracting with the DC component of the MMC bridge arm current enables the overall energy balance of CHB-eFIS. The overall energy balance of CHB-eFIS, as described previously in the TLM-S... 2 The FIS series-parallel coordinated energy balance control achieves energy balance for the entire topology; both are the same control mechanism. The former is from the perspective of the series converter (CHB-eFIS) itself, while the latter is from the perspective of the coordinated operation between the series and parallel converters. Based on the control objectives and control variables described above, the control block diagram for the overall energy balance of CHB-eFIS is as follows: Figure 7 As shown.

[0075] in, This is the reference value for the capacitor voltage of the CHB-eFIS submodule; This is the arithmetic mean of the capacitor voltages of the CHB-eFIS submodules. Furthermore, when performing overall energy balance control of the CHB-eFIS, the direction of the DC current, i.e., the charging and discharging direction, also needs to be considered.

[0076] In some specific embodiments of this application, the energy balance control between clusters within CHB-eFIS is as follows: based on the DC common-mode voltage component, an independent DC bias voltage component is superimposed on each cluster cascaded H-bridge arm to independently adjust the DC circulating current flowing into that arm, compensate for the energy difference between clusters caused by power flow regulation, and maintain the balance of capacitor voltages of each cluster submodule.

[0077] Furthermore, in the inter-cluster energy balance control, the DC bias voltage component superimposed on the arms of the cascaded H-bridge of the k-th cluster is determined by the deviation between the average value of the capacitor voltage of the sub-module of the cluster and the total average value of the capacitor voltage of all sub-modules of the cluster, as well as the direction of the DC common-mode voltage component, and the injected DC circulating current is constrained inside the CHB-eFIS.

[0078] Because the current in each AC line differs from the fundamental frequency differential mode output voltage of each cluster of cascaded bridge arms in CHB-eFIS, energy imbalances will occur between the cascaded bridge arms. If left uncontrolled, this will lead to voltage imbalance in the submodule capacitors. The energy differences between the cascaded bridge arms are mainly caused by power flow regulation. The interaction power between each cluster of cascaded bridge arms and its corresponding AC line in CHB-eFIS is as follows: (16) As shown in equation (16), the interaction power between each cluster cascaded bridge arm and the AC system is determined by its fundamental frequency AC differential mode output component and the current of the connected AC line, thus causing energy imbalance among the cluster cascaded bridge arms. Without control, this will lead to voltage imbalance in the submodule capacitors. Therefore, based on the overall energy balance of CHB-eFIS, an additional DC common mode voltage component, denoted as ΔU, is superimposed on the overall DC common mode output voltage of each cluster cascaded bridge arm.ck The DC circulating current flowing into each cluster of cascaded bridge arms is redistributed to achieve energy balance among the clusters of cascaded bridge arms in CHB-eFIS. The bridge arm voltage of each cluster of cascaded bridge arms can be expressed as: (17) Similarly, the common-mode current of each cluster of cascaded bridge arms in CHB-eFIS consists of a naturally distributed component and a controlled component, i.e. (18) in, To control the energy balance among clusters in the first... The additional controlled DC common-mode current regulation introduced into the cluster.

[0079] To further investigate the impact of inter-cluster circulating current regulation of the DC common-mode voltage on the DC circulating current of each cluster cascaded bridge arm, based on the above analysis, then... Figure 3 The DC common-mode equivalent circuit shown in (c) can be drawn in detail as follows: Figure 8 .

[0080] exist Figure 8 middle, I k (k=1,2,…,N) represents the virtual mesh current. Based on KVL and mesh current analysis, and combined with the relationship between mesh current and actual current, the inter-cluster circulating current-regulated DC common-mode voltage and controlled DC common-mode current of each cluster cascaded bridge arm in CHB-eFIS satisfy the following relationship: (19) As shown in equation (19), by adjusting the inter-cluster circulating current of each cluster cascaded bridge arm in CHB-eFIS to regulate the DC common-mode voltage, the controlled DC common-mode current of that cluster can be independently and decoupledly controlled, thereby achieving the redistribution of the DC circulating current flowing into each cluster cascaded bridge arm. Based on the above analysis, the average change in the fundamental frequency period of the energy of each cluster cascaded bridge arm in CHB-eFIS after the redistribution of the circulating current is: (20) As can be seen from equation (20), by adjusting the DC common-mode voltage through the inter-cluster circulating current, the energy interaction between the cascaded bridge arms of each cluster can be controlled, thereby achieving energy balance within the CHB-eFIS.

[0081] Based on the control objective and control variables, the control block diagram for inter-cluster energy balance in CHB-eFIS can be obtained as follows: Figure 9 As shown.

[0082] in, (k=2,3,…,n) represents the average capacitor voltage of the submodule in the CHB-eFIS cascaded bridge arm of the k-th cluster. Furthermore, when performing energy balance control between CHB-eFIS clusters, the direction of the injected DC common-mode voltage also needs to be controlled by overall energy balance. To ensure that the injected DC circulating current exists only within the CHB-eFIS, the following conditions must be met: (twenty one) The preferred features in the above embodiments can be used individually in any embodiment, or in any combination thereof, provided they do not conflict with each other. Furthermore, parts not described in detail in the embodiments can be implemented using existing technologies.

[0083] The following examples and comparative examples will be used to further illustrate this application in order to better understand the above-mentioned technical solutions. It should be understood that the following are only some examples and are not intended to limit this application.

[0084] Application Example 1: To further verify TLM-S 2 The feasibility and effectiveness of FIS multi-port power flow flexible decoupling control are demonstrated by taking the system parameters of the actual Foshan Sanshui flexible interconnection project as an example. See Table 2 for details. A 10kV / 3MW three-port TLM-S was built on the MATLAB / Simulink platform. 2 FIS simulation verification model.

[0085] The simulation operation is divided into two stages: uncontrolled power flow and controlled power flow. The specific operation is designed as follows: Stage 1 (natural power flow): The rectified power of the MMC converter station is 3MW. At this time, CHB-eFIS is in bypass state, and the AC line power flow is in a natural distribution state. Since the impedance or voltage of the three AC lines is inconsistent, the current of the three AC lines is inconsistent, as shown in the interval [0~0.3s]. Stage 2 (controlled power flow): The active power of the AC lines connected to the three AC ports is evenly distributed and the reactive power is zero by adjusting the equivalent series voltage output of CHB-eFIS port, as shown in the interval [0.3~0.6s].

[0086] Table 1 Power Flow Conditions for Each Line Table 2 Simulation Parameters Figures 10-12 This demonstrates that the embedded CHB-eFIS can effectively regulate power flow, validating the TLM-S... 2 Feasibility of FIS multi-terminal power decoupling control method. Figure 13The waveforms show the average capacitor voltages of the MMC and CHB-eFIS modules. During the power flow regulation of CHB-eFIS, the average capacitor voltages of the CHB-eFIS and MMC submodules remain constant, verifying the feasibility of coordinated energy balance control between CHB-eFIS and MMC. Figure 14 The waveforms of the bridge arm capacitors in phase a of CHB-eFIS are shown. During the simulation, the capacitor voltages of the cascaded bridge arm submodules are consistent, verifying the feasibility of internal energy balance control.

[0087] Simulation results show that the proposed multi-AC port hub-type flexible DC converter topology with unified power flow control function and its control method can achieve multi-terminal power flow decoupling control and internal stable operation, and have the feasibility of application in AC / DC hybrid flexible interconnection scenarios.

[0088] The foregoing has described some specific embodiments of this application. It should be understood that this application is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the substantive content of this application. The above-described preferred features can be used in any combination without conflict.

Claims

1. A multi-port hub-type flexible DC converter topology with power flow control function, characterized in that... ,include: CHB-eFIS is a parallel MMC and a series cascaded H-bridge embedded flexible interconnect switch. The parallel MMC is a parallel circuit used to provide reactive power compensation and power supply to the medium-voltage DC bus; The CHB-eFIS is embedded in the bridge arm loop of the parallel MMC. Each phase of the CHB-eFIS is composed of multiple cascaded bridge arms connected in parallel. The two ends of the parallel common point of the cascaded bridge arms are respectively connected to the bottom end of the upper bridge arm and the top end of the lower bridge arm of the parallel MMC. The differential mode output ports of each cluster of cascaded H-bridge arms of the CHB-eFIS serve as independent AC power flow regulation ports for connecting to different AC feeders. The CHB-eFIS controls the power flow of each feeder by adjusting the series voltage between the feeders by changing its AC side output voltage, and the energy balance between the CHB-eFIS and the AC feeders is coordinated with the parallel MMC for regulation.

2. The multi-port hub-type flexible DC converter topology with power flow control function according to claim 1, characterized in that, The CHB-eFIS can linearly expand the number of AC power flow regulation ports by increasing the number of clusters of cascaded H-bridge arms, enabling flexible multi-port expansion; The CHB-eFIS uses its fundamental frequency differential mode output voltage to couple energy with the common mode current component in the parallel MMC, forming a collaborative energy balance channel between the series and parallel converters. The series-connected CHB-eFIS has four-quadrant operation capability and can bidirectionally exchange active and reactive power with AC lines.

3. A control method applied to the multi-port hub-type flexible DC converter topology with power flow control function as described in any one of claims 1-2, characterized in that, This includes parallel execution of multi-terminal power decoupling control methods and multi-level energy balance control methods; The multi-terminal power decoupling control method calculates the differential mode output voltage reference value required by each cluster of cascaded H-bridge arms based on the active and reactive power commands of each AC port, and performs independent decoupling control on the power flow of each feeder. The multi-level energy balance control method includes series-parallel coordinated overall energy balance control and CHB-eFIS internal inter-cluster energy balance control. It adopts DC common-mode component injection to achieve overall energy balance and internal energy balance, and the injection of DC common-mode component does not affect AC line current and power flow regulation.

4. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 3, characterized in that, The multi-terminal power decoupling control method specifically includes: Based on the AC differential-mode equivalent circuit, a dynamic mathematical model is established between the current of each line and the series-regulated voltage of CHB-eFIS. An outer-inner-loop dual-loop control architecture is adopted. The outer-loop controller generates corresponding d-axis and q-axis current reference values ​​based on the active and reactive power commands of the system scheduling. The inner-loop controller generates series equivalent voltage reference values ​​in a synchronous rotating coordinate system through PI control combined with coupling compensation and voltage feedforward. The series equivalent voltage reference value is converted into a three-phase voltage reference value through Park inverse transformation, and the differential mode output reference voltage of each cascaded H-bridge arm of the CHB-eFIS is determined to perform multi-terminal power decoupling control.

5. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 4, characterized in that, The dynamic mathematical model established based on the AC differential-mode equivalent circuit and the CHB-eFIS series regulated voltage includes: Based on the energy coupling relationship of series-parallel converters with DC common-mode component injection, a TLM-S is established. 2 The FIS equivalent circuit is constructed based on the three-phase symmetry principle to create a single-phase equivalent circuit. Based on the superposition theorem, the single-phase equivalent circuit is decomposed into an AC differential-mode equivalent circuit and a DC common-mode equivalent circuit; One AC line in the CHB-eFIS is selected as the active power balancing line. Based on the AC differential mode equivalent circuit, the active and reactive power expressions of the AC line are established. Using the AC voltage vector of the balanced line as the orientation reference of the synchronously rotating dq coordinate system, the relationship between line power flow and current in the rotating coordinate system and the dynamic mathematical model of the system are established.

6. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 5, characterized in that, When determining the differential mode output reference voltage of each cascaded H-bridge arm of the CHB-eFIS, a differential mode voltage reference value is specified for the arm connected to the active power balancing line, and the maximum value of the differential mode output reference voltage amplitude of all cascaded CHB-eFIS arms is minimized through optimization.

7. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 3, characterized in that, The overall total energy control of the series-parallel coordinated system is as follows: by utilizing the common-mode energy interaction path between the CHB-eFIS and the MMC, and based on DC common-mode component injection, a power balance constraint relationship between the CHB-eFIS and the MMC is established. The CHB-eFIS internal inter-cluster energy balance control is as follows: based on the DC common-mode voltage component, an independent DC bias voltage component is superimposed on each cluster cascaded H-bridge arm to independently adjust the DC circulating current flowing into the arm, compensate for the inter-cluster energy difference caused by power flow regulation, and maintain the balance of capacitor voltage of each cluster submodule.

8. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 7, characterized in that, The power balance constraint relationship needs to take into account the deviation between the average value of the capacitor voltage of all sub-modules of CHB-eFIS and the reference value of the capacitor voltage, and generate an overall DC common-mode voltage command through the PI regulator to control the DC common-mode output voltage of each cluster of cascaded bridge arms of CHB-eFIS, and interact with the DC component in the current of the MMC bridge arm to achieve the overall energy balance between CHB-eFIS and MMC.

9. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 7, characterized in that, In the inter-cluster energy balance control, the DC bias voltage component superimposed on the arms of the cascaded H-bridge of the k-th cluster is determined by the deviation between the average value of the capacitor voltage of the sub-module of the cluster and the total average value of the capacitor voltage of all sub-modules of the cluster, as well as the direction of the DC common-mode voltage component, and the injected DC circulating current is constrained inside the CHB-eFIS.

10. The control method for a multi-port hub-type flexible DC converter topology with power flow control function according to claim 7, characterized in that, When performing the overall energy balance control and internal energy equalization control, the direction of the DC component of the parallel MMC bridge arm current must be considered to determine the charging and discharging direction of energy interaction.

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