Modularized multi-level converter current control method based on feedforward analysis

Abstract

The invention provides a modularized multi-level converter current control method based on feedforward analysis. The deviation between the direct-current side voltage and the reference value is stably monitored in real time based on the direct-current bus voltage, a first waveform signal is dynamically generated, and the direct-current side voltage fluctuation range is limited. According to the circulating current interphase voltage sharing, the dynamic deviation between the actual value and the rated value is calculated by collecting the capacitor voltage actual value of the bridge arm sub-module, and a second waveform signal is generated to balance the circulating current distribution of each phase of bridge arm. The loop current suppression based on feedforward analysis generates a third waveform signal to compensate for high-frequency loop current interference through real-time comparison of theoretical modeling and actual loop current components. In the modulation wave generation process, information in all aspects is integrated to accurately control the on-off state of the sub-module, so that the operation effect of the system is improved under the complex working condition, and the real-time performance and predictability of control are considered through combination of closed-loop adjustment and feed-forward compensation, so that the effectiveness of control is improved.

Description

Technical Field

[0001] The present application relates to the field of electrical control technology, and in particular to a modular multi-level converter current control method based on feedforward analysis. Background Art

[0002] Modular multilevel converters (MMCs) feature a modular design that's easily expandable, offering advantages such as low harmonics, high efficiency, minimal losses, and excellent scalability. They play a crucial role in HVDC and FAC transmission. As the core of MMC operation, arm current tracking control is directly impacted by system stability and output power quality.

[0003] However, the complex coupling relationships between multiple bridge arms in an MMC system complicate independent control of the current in a single arm and make precise tracking of the arm current very difficult. Circulating currents in the MMC, which exist between the DC bus and the upper and lower bridge arms, can lead to additional losses and voltage imbalances in the submodule capacitors, thus impacting system efficiency and stability. Therefore, a modular multilevel converter current control method based on feedforward analysis is needed to achieve precise tracking of the arm currents and effectively mitigate the adverse effects of circulating currents. Summary of the Invention

[0004] The purpose of this application is to solve at least one of the above-mentioned technical defects, especially the technical defect of poor bridge arm current tracking control effect in the prior art.

[0005] In a first aspect, the present application provides a modular multilevel converter current control method based on feedforward analysis, the method being used in a target flexible interconnected system, the target flexible interconnected system comprising two groups of modular multilevel converters MMC connected back-to-back via a DC bus and two groups of three-phase AC systems corresponding to the MMCs, each of the MMC converters adopting a three-phase six-bridge arm structure, each phase comprising an upper and lower bridge arm, each bridge arm comprising a plurality of cascaded half-bridge sub-modules and a bridge arm inductor, the upper and lower bridge arms of each phase being connected and then connected to the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system being connected to each other;

[0006] The method comprises:

[0007] Determining a first waveform signal according to a DC bus voltage stabilization process;

[0008] Determine a second waveform signal according to a circulating current inter-phase voltage balancing process;

[0009] determining a third waveform signal according to a circulating current suppression process based on feedforward analysis;

[0010] Processing the first waveform signal, the second waveform signal, and the third waveform signal to obtain a target waveform signal, and generating a switch signal for each of the submodules according to the target waveform signal to control each of the submodules to operate according to a preset mode;

[0011] The target waveform signal is used to indicate the operating voltage corresponding to each of the submodules.

[0012] As an optional implementation manner, determining the first waveform signal according to the DC bus voltage stabilization process includes:

[0013] The voltage across the DC side is obtained in real time, and after subtracting it from a preset DC voltage reference value, a d-axis current reference value is obtained through a first proportional integral link;

[0014] Acquire the AC current of each phase of the MMC in real time, and obtain the actual value of the d-axis current and the actual value of the q-axis current through abc / dq conversion;

[0015] Subtracting the d-axis current actual value from the d-axis current reference value to obtain a d-axis voltage reference value through a second proportional-integral link, and subtracting the q-axis current actual value from a preset q-axis current reference value to obtain a q-axis voltage reference value through a third proportional-integral link;

[0016] The d-axis current reference value and the q-axis voltage reference value are converted into corresponding phase voltage reference values as first waveform signals through dq / abc conversion.

[0017] As an optional implementation manner, determining the second waveform signal according to the circulating current inter-phase voltage balancing process includes:

[0018] Acquire the actual value of the capacitor voltage of each phase bridge arm submodule of the MMC in real time, calculate and obtain the average value of the capacitor voltage, subtract the average value of the capacitor voltage from the rated value of the capacitor voltage, and then obtain the reference value of the circulating current of each phase bridge arm through the fourth proportional integral link;

[0019] Obtain the actual current value of the upper and lower bridge arms of each phase of the MMC in real time, and calculate the actual value of the bridge arm circulating current of each phase;

[0020] The actual value of the bridge arm circulating current of each phase is subtracted from the bridge arm circulating current reference value, and the DC reference value of the bridge arm circulating current corresponding to each phase is obtained through the fifth proportional integral link, and the double frequency AC reference value of the bridge arm circulating current corresponding to each phase is obtained through the resonance control link.

[0021] As an optional implementation manner, determining the third waveform signal according to the circulating current suppression process based on feedforward analysis includes:

[0022] Obtain the upper bridge arm discrete current value and the lower bridge arm discrete current value of each phase, and integrate the upper bridge arm discrete current value of each phase according to the submodule capacitance value and the sampling period to obtain the upper bridge arm submodule capacitance voltage theoretical value, and integrate the lower bridge arm discrete current value of each phase to obtain the lower bridge arm submodule capacitance voltage theoretical value;

[0023] Calculating the root mean square of the theoretical value of the capacitor voltage of the upper bridge arm submodule during the sampling period to obtain the effective value of the capacitor voltage of the upper bridge arm submodule during the sampling period, and calculating the root mean square of the theoretical value of the capacitor voltage of the lower bridge arm submodule during the sampling period to obtain the effective value of the capacitor voltage of the lower bridge arm submodule during the sampling period;

[0024] Determine the switching function, steady-state operation switching function conditions, and corresponding phase switching functions of each submodule, calculate the upper bridge arm switching function and the lower bridge arm switching function of each phase, and calculate the average number of times the upper bridge arm submodule of each phase is put into operation based on the upper bridge arm switching function of each phase, and calculate the average number of times the lower bridge arm submodule of each phase is put into operation based on the lower bridge arm switching function of each phase;

[0025] Calculate the effective value of the double frequency of the circulating current according to the effective value of the capacitor voltage of the upper bridge arm submodule of each phase, the effective value of the capacitor voltage of the lower bridge arm submodule, the average number of times the upper bridge arm submodule is put into operation, the average number of times the lower bridge arm submodule is put into operation, and the voltage across the DC side;

[0026] Obtaining the actual voltage value across the upper bridge arm inductor and the actual voltage value across the lower bridge arm inductor of each phase, calculating the actual value of the inductor voltage based on the actual voltage value across the upper bridge arm inductor and the actual voltage value across the lower bridge arm inductor of each phase, and filtering the upper and lower bridge arm inductor voltages and the corresponding actual values to obtain a double frequency component of the inductor voltage, thereby obtaining an effective value of the double frequency of the inductor voltage;

[0027] If the effective value of the double frequency of the circulating current is equal to the effective value of the double frequency of the inductor voltage, a circulating current suppression compensation value of a preset value is output as the third target signal; if the effective value of the double frequency of the circulating current is less than the effective value of the double frequency of the inductor voltage, a circulating current suppression compensation value is calculated based on the effective value of the double frequency of the circulating current and the effective value of the double frequency of the inductor voltage, and is used as the third target signal.

[0028] As an optional implementation manner, the target waveform signal includes a modulated wave signal, and the processing to obtain the target waveform signal according to the first waveform signal, the second waveform signal, and the third waveform signal includes:

[0029] Determining a waveform factor according to the number of all submodules in the MMC;

[0030] Calculating a first calculation item according to the DC voltage reference value, the first waveform signal, and the waveform coefficient;

[0031] determining a second calculation item and a third calculation item according to the second waveform signal;

[0032] determining a fourth calculation item according to the third waveform signal;

[0033] According to the first calculation item, the second calculation item, the third calculation item and the fourth calculation item, a modulated wave signal corresponding to each of the submodules is calculated and obtained.

[0034] As an optional implementation manner, the calculation method of the first calculation item includes:

[0035] After obtaining the difference between half of the DC voltage reference value and each corresponding phase voltage reference value, the difference is multiplied by the waveform coefficient to obtain the first calculation item;

[0036] Furthermore, the second calculation item includes the inverse of the bridge arm circulating current DC reference value corresponding to each phase, the third calculation item includes the inverse of the bridge arm circulating current double frequency AC reference value, and the fourth calculation item includes the circulating current suppression compensation value;

[0037] And, the calculation method of the modulated wave signal includes:

[0038] The first calculation item, the second calculation item, the third calculation item and the fourth calculation item are summed to obtain the modulated wave signal.

[0039] As an optional implementation manner, generating a switch signal of each submodule according to the target waveform signal includes:

[0040] Determining the number of submodules to be put into use according to the modulation wave signals corresponding to the submodules;

[0041] The submodules are sorted according to the difference between the capacitor voltage corresponding to each submodule and the rated voltage, and a target submodule group and a sequence of inputting the submodules in the target submodule group are determined.

[0042] In a second aspect, the present application provides a modular multilevel converter current control device based on feedforward analysis, the device being used in a target flexible interconnected system, the target flexible interconnected system comprising two groups of modular multilevel converters MMC connected back-to-back via a DC bus and two groups of three-phase AC systems corresponding to the MMCs, each of the MMC converters adopting a three-phase six-bridge arm structure, each phase including an upper and lower bridge arm, each bridge arm comprising a plurality of cascaded half-bridge sub-modules and a bridge arm inductor, the upper and lower bridge arms of each phase being connected and then connected to the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system being connected to each other;

[0043] The device comprises:

[0044] A determination module, configured to determine a first waveform signal according to a DC bus voltage stabilization process;

[0045] The determining module is further configured to determine a second waveform signal according to a circulating current inter-phase voltage balancing process;

[0046] The determining module is further configured to determine a third waveform signal according to a circulating current suppression process based on feedforward analysis;

[0047] a processing module, configured to obtain a target waveform signal based on the first waveform signal, the second waveform signal, and the third waveform signal, and generate a switch signal for each of the submodules based on the target waveform signal, so as to control each of the submodules to operate according to a preset mode;

[0048] The target waveform signal is used to indicate the operating voltage corresponding to each of the submodules.

[0049] In a third aspect, the present application provides a computer device comprising one or more processors and a memory, wherein the memory stores computer-readable instructions, and when the computer-readable instructions are executed by the one or more processors, the steps of the method described in the first aspect are performed.

[0050] In a fourth aspect, the present application provides a storage medium storing computer-readable instructions. When the computer-readable instructions are executed by one or more processors, the one or more processors execute the steps of the method described in the first aspect.

[0051] It can be seen from the above technical solutions that the embodiments of the present application have the following advantages:

[0052] Based on any of the above embodiments, the corresponding method of the present application improves the dynamic response capability and operational stability of the flexible interconnected system through a collaborative control mechanism. In a specific application scenario, the deviation between the DC side voltage and the reference value is monitored in real time based on the DC bus voltage stabilization process, and a first waveform signal is dynamically generated through a proportional integral link to limit the DC side voltage to fluctuate stably within a preset range, thereby avoiding the risk of system collapse due to voltage instability. The circulating current inter-phase voltage equalization process collects the actual value of the capacitor voltage of the bridge arm submodule, calculates its dynamic deviation from the rated value, and generates a second waveform signal to balance the circulating current distribution of each phase bridge arm, effectively preventing the device overvoltage or efficiency reduction caused by capacitor voltage imbalance. At the same time, the circulating current suppression process based on feedforward analysis generates a third waveform signal to compensate for high-frequency circulating current interference through real-time comparison of theoretical modeling and actual circulating current components, significantly reducing the additional loss caused by the coupling effect between bridge arms. In the modulation wave generation process, the information corresponding to the above-mentioned signals is integrated to accurately control the switching status of the sub-modules, enabling the system to achieve rapid current tracking and optimize power quality under complex working conditions. By combining closed-loop regulation with feedforward compensation, both real-time and predictive control are taken into account, providing an important foundation for the reliable operation of high-voltage and large-capacity converters. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0054] Figure 1 A schematic diagram of an application scenario of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0055] Figure 2 A schematic flow chart of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0056] Figure 3 This is an overall block diagram of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0057] Figure 4 A schematic diagram showing the effect of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0058] Figure 5 A schematic diagram showing the effect of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0059] Figure 6 A schematic diagram showing the effect of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0060] Figure 7 A schematic diagram showing the effect of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0061] Figure 8 A schematic diagram showing the effect of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application;

[0062] Figure 9 This is a diagram of the internal structure of a computer device provided in an embodiment of the present application. DETAILED DESCRIPTION

[0063] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0064] As power systems move toward higher voltages and larger capacities, traditional converters face numerous challenges in these applications, such as high switching losses and high harmonic content. This has led to the emergence of Multi-Mode Converters (MMCs), driven by the growing demand for efficient, reliable, and scalable converters. MMCs utilize a modular design, are easily scalable, and offer advantages such as low harmonics, high efficiency, low losses, and excellent scalability. These advantages are particularly significant in the fields of high-voltage direct current (HVDC) and flexible alternating current (FAC) transmission. Research on MMC control technologies, particularly in bridge arm current tracking and circulating current suppression strategies, is of urgent practical importance.

[0065] As the core of MMC's normal operation, bridge arm current tracking and control are directly related to system stability and output power quality. However, the presence of multiple bridge arms in an MMC system, coupled with complex coupling relationships, complicates independent control of a single arm's current and makes precise tracking and control of the arm's current extremely difficult. Circulating current in the MMC, between the DC bus and the upper and lower bridge arms, causes additional losses and voltage imbalances in the submodule capacitors, thus impacting system efficiency and stability. Accurate tracking and control of the bridge arm currents must be achieved quickly and effectively to mitigate their adverse effects.

[0066] The current tracking control strategy of the MMC distribution network flexible interconnection system based on the feedforward analytical method uses a frequency-division current control strategy to decouple the bridge arm current components, realize independent tracking of currents in each frequency band, and thus achieve fast and precise control of current components, which can improve the output power quality; the circulating current suppression control based on the feedforward analytical method is simple and can achieve circulating current suppression without increasing system losses, which can improve system reliability and ensure the stability and long-term reliability of the system under various working conditions.

[0067] See also Figure 1 , Figure 1 A schematic diagram of an application scenario of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application is used to illustrate the technical concept of the present application according to the structure of a target flexible interconnection system.

[0068] The target flexible interconnection system includes two groups of modular multilevel converters (MMCs) connected back-to-back via DC busbars and two groups of three-phase AC systems corresponding to the MMCs. Each of the MMC converters adopts a three-phase six-bridge-arm structure, each phase including an upper and lower bridge arm, each bridge arm including multiple cascaded half-bridge sub-modules and a bridge arm inductor. The upper and lower bridge arms of each phase are connected to the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system are connected to each other.

[0069] Specifically, such as Figure 1 As shown, the distribution network flexible interconnection system consists of a set of back-to-back MMC converters, and the back-to-back MMC converters consist of two groups of three-phase MMC systems, a DC side, and two three-phase AC power systems.

[0070] The two groups of three-phase MMC systems have the same structure, both of which are three-phase six-bridge arm structures. Taking the three-phase MMC system on the left as an example, each phase is composed of upper and lower bridge arms with the same structure, where the connection points of each bridge arm are called a1, b1, and c1 respectively; each bridge arm is composed of M cascaded half-bridge sub-modules and 1 bridge arm inductor Lb_ij in series, where i represents the phase connected to the three-phase AC power system, i is A1, B1, C1, j represents the upper bridge arm or the lower bridge arm, 1 represents the upper bridge arm, and 2 represents the lower bridge arm; the upper end points of the upper bridge arm of each phase are connected together, called the P node; the lower end points of the lower bridge arm of each phase are connected together, called the N node; the back-to-back MMC converter is composed of two groups of three-phase MMC systems connected together through the DC side.

[0071] The input ports A1, B1, and C1 of the three-phase AC power system 1 of the flexible interconnected distribution network system are connected to the left-side bridge arm connection points a1, b1, and c1 of the back-to-back MMC converter, and the three-phase output ports of the three-phase AC power system 1 are connected together, which is called the T1 node; the right-side bridge arm connection points a2, b2, and c2 of the back-to-back MMC converter are connected to the input ports A2, B2, and C2 of the three-phase AC power system 1, and the three-phase output ports of the three-phase AC power system 2 are connected together, which is called the T2 node;

[0072] This application decouples the bridge arm current components according to frequency, and adopts a frequency-divided current tracking control strategy to achieve independent tracking of the current in each frequency band, thereby achieving fast and precise control of the current components, which can improve the output power quality; by using an analytical method to analyze the system circulating current, the circulating current suppression compensation value is obtained, and a circulating current suppression control strategy based on the feedforward analytical method is adopted. Under this strategy, the system can suppress the circulating current without increasing losses, which can improve the reliability and stability of the system.

[0073] Specifically, this application provides a topological structure of a flexible interconnected distribution network system of MMC based on the feedforward analytical method, and proposes a frequency-divided current tracking control strategy for this topological structure, which has the advantages of faster bridge arm current tracking speed and higher accuracy, and a simple control structure. There is no need to add an additional circulating current suppression controller, which saves costs. The circulating current suppression strategy based on the feedforward analytical method can better suppress circulating current. Reducing circulating current can reduce system losses and enhance system reliability and stability.

[0074] In summary, the technical concept of the present application is to improve the dynamic response capability and operational stability of the flexible interconnected system through a collaborative control mechanism. In a specific application scenario, the deviation between the DC side voltage and the reference value is monitored in real time based on the DC bus voltage stabilization process, and a first waveform signal is dynamically generated through a proportional integral link to limit the DC side voltage to fluctuate stably within a preset range, thereby avoiding the risk of system collapse due to voltage instability. The circulating current inter-phase voltage equalization process collects the actual value of the capacitor voltage of the bridge arm submodule, calculates its dynamic deviation from the rated value, and generates a second waveform signal to balance the circulating current distribution of each phase bridge arm, effectively preventing device overvoltage or efficiency reduction caused by capacitor voltage imbalance. At the same time, the circulating current suppression process based on feedforward analysis generates a third waveform signal to compensate for high-frequency circulating current interference through real-time comparison of theoretical modeling and actual circulating current components, significantly reducing the additional loss caused by the coupling effect between bridge arms. In the modulation wave generation process, the information corresponding to the above-mentioned signals is integrated to accurately control the switching status of the sub-modules, enabling the system to achieve rapid current tracking and optimize power quality under complex working conditions. By combining closed-loop regulation with feedforward compensation, both real-time and predictive control are taken into account, providing an important foundation for the reliable operation of high-voltage and large-capacity converters.

[0075] The method provided in this application is described in detail below based on corresponding implementation methods in some actual application scenarios.

[0076] See also Figure 2 , Figure 2 A schematic flow chart of a modular multi-level converter current control method based on feedforward analysis provided in one embodiment of the present application, wherein the method is applied to Figure 1 The target flexible interconnection system shown;

[0077] like Figure 2 As shown, the method includes:

[0078] S101, determining a first waveform signal according to a DC bus voltage stabilization process;

[0079] S102: Determine a second waveform signal according to a circulating current interphase voltage balancing process;

[0080] S103, determining a third waveform signal according to a circulating current suppression process based on feedforward analysis;

[0081] S104: Processing the first waveform signal, the second waveform signal, and the third waveform signal to obtain a target waveform signal, and generating a switch signal for each submodule based on the target waveform signal to control each submodule to operate according to a preset mode;

[0082] The target waveform signal is used to indicate the operating voltage corresponding to each of the submodules.

[0083] In this embodiment, the dynamic response capability and operational stability of the flexible interconnected system are improved through a collaborative control mechanism. In a specific application scenario, the deviation between the DC side voltage and the reference value is monitored in real time based on the DC bus voltage stabilization process, and a first waveform signal is dynamically generated through a proportional integral link to limit the DC side voltage to fluctuate stably within a preset range, thereby avoiding the risk of system collapse due to voltage instability. The circulating current interphase voltage equalization process collects the actual value of the capacitor voltage of the bridge arm submodule, calculates its dynamic deviation from the rated value, and generates a second waveform signal to balance the circulating current distribution of each phase bridge arm, effectively preventing device overvoltage or efficiency reduction caused by capacitor voltage imbalance. At the same time, the circulating current suppression process based on feedforward analysis generates a third waveform signal to compensate for high-frequency circulating current interference through real-time comparison of theoretical modeling and actual circulating current components, significantly reducing the additional loss caused by the coupling effect between bridge arms. In the modulation wave generation process, the information corresponding to the above-mentioned signals is integrated to accurately control the switching status of the sub-modules, enabling the system to achieve rapid current tracking and optimize power quality under complex working conditions. By combining closed-loop regulation with feedforward compensation, both real-time and predictive control are taken into account, providing an important foundation for the reliable operation of high-voltage and large-capacity converters.

[0084] As an optional implementation manner, determining the first waveform signal according to the DC bus voltage stabilization process includes:

[0085] The voltage across the DC side is obtained in real time, and after subtracting it from a preset DC voltage reference value, a d-axis current reference value is obtained through a first proportional integral link;

[0086] Acquire the AC current of each phase of the MMC in real time, and obtain the actual value of the d-axis current and the actual value of the q-axis current through abc / dq conversion;

[0087] Subtracting the d-axis current actual value from the d-axis current reference value to obtain a d-axis voltage reference value through a second proportional-integral link, and subtracting the q-axis current actual value from a preset q-axis current reference value to obtain a q-axis voltage reference value through a third proportional-integral link;

[0088] The d-axis current reference value and the q-axis voltage reference value are converted into corresponding phase voltage reference values as first waveform signals through dq / abc conversion.

[0089] This implementation achieves coordinated control of DC-side voltage and AC-side current through a hierarchical control architecture. This architecture comprises an outer voltage loop and an inner current loop. The data processing involved involves the conversion of the D / Q axis and the three phases, ABC. In the outer voltage loop, DC-side voltage deviations are detected in real time, and a proportional-integral (PI) phase is used to generate a shaft current reference. This cumulative error gradually eliminates steady-state deviations, ensuring long-term DC bus voltage stability. In the inner current loop, the AC current is converted into shaft components for closed-loop tracking. The shaft voltage reference is dynamically adjusted using a multi-stage PI phase, and then back-transformed into a phase voltage modulation signal. This process decouples voltage regulation and current tracking into independent control dimensions, avoiding conflicts between single control objectives and simplifying multivariable coupling through coordinate transformation. Furthermore, the cascaded design of the multi-stage control loop enhances the system's immunity to sudden load changes or grid disturbances. Furthermore, the adaptive adjustment of the dynamic reference value significantly improves the converter's adaptability over a wide range of operating conditions, providing a foundation for refined control of flexible interconnected systems.

[0090] As an optional implementation manner, determining the second waveform signal according to the circulating current inter-phase voltage balancing process includes:

[0091] Acquire the actual value of the capacitor voltage of each phase bridge arm submodule of the MMC in real time, calculate and obtain the average value of the capacitor voltage, subtract the average value of the capacitor voltage from the rated value of the capacitor voltage, and then obtain the reference value of the circulating current of each phase bridge arm through the fourth proportional integral link;

[0092] Obtain the actual current value of the upper and lower bridge arms of each phase of the MMC in real time, and calculate the actual value of the bridge arm circulating current of each phase;

[0093] The actual value of the bridge arm circulating current of each phase is subtracted from the bridge arm circulating current reference value, and the DC reference value of the bridge arm circulating current corresponding to each phase is obtained through the fifth proportional integral link, and the double frequency AC reference value of the bridge arm circulating current corresponding to each phase is obtained through the resonance control link.

[0094] This implementation combines proportional-integral and resonant control to solve the problems of response lag and poor frequency band selectivity of traditional circulating current suppression methods. By real-time acquisition of the bridge arm submodule capacitor voltage, calculating its dynamic average value and comparing it with the rated value, using the proportional-integral link to generate a circulating current reference value, and actively adjusting the circulating current distribution of each phase bridge arm, rapid balancing of the capacitor voltage is achieved. Furthermore, by extracting the instantaneous deviation between the actual circulating current and the reference value, and introducing a resonant controller to specifically suppress the double frequency component, the impact of high-frequency circulating current on system stability is effectively eliminated. This dual-loop control structure not only retains the strong tracking capability of the proportional-integral link for the DC component, but also accurately compensates for AC interference in a specific frequency band through resonant control, significantly reducing the additional loss and voltage fluctuation caused by the circulating current. At the same time, through the real-time decomposition and reconstruction of the bridge arm current, the long-term stability of the submodule capacitor voltage under dynamic load is ensured, the effectiveness of current control is improved, and thus the operation effect of the converter is improved.

[0095] As an optional implementation manner, determining the third waveform signal according to the circulating current suppression process based on feedforward analysis includes:

[0096] Obtain the upper bridge arm discrete current value and the lower bridge arm discrete current value of each phase, and integrate the upper bridge arm discrete current value of each phase according to the submodule capacitance value and the sampling period to obtain the upper bridge arm submodule capacitance voltage theoretical value, and integrate the lower bridge arm discrete current value of each phase to obtain the lower bridge arm submodule capacitance voltage theoretical value;

[0097] Calculating the root mean square of the theoretical value of the capacitor voltage of the upper bridge arm submodule during the sampling period to obtain the effective value of the capacitor voltage of the upper bridge arm submodule during the sampling period, and calculating the root mean square of the theoretical value of the capacitor voltage of the lower bridge arm submodule during the sampling period to obtain the effective value of the capacitor voltage of the lower bridge arm submodule during the sampling period;

[0098] Determine the switching function, steady-state operation switching function conditions, and corresponding phase switching functions of each submodule, calculate the upper bridge arm switching function and the lower bridge arm switching function of each phase, and calculate the average number of times the upper bridge arm submodule of each phase is put into operation based on the upper bridge arm switching function of each phase, and calculate the average number of times the lower bridge arm submodule of each phase is put into operation based on the lower bridge arm switching function of each phase;

[0099] Calculate the effective value of the double frequency of the circulating current according to the effective value of the capacitor voltage of the upper bridge arm submodule of each phase, the effective value of the capacitor voltage of the lower bridge arm submodule, the average number of times the upper bridge arm submodule is put into operation, the average number of times the lower bridge arm submodule is put into operation, and the voltage across the DC side;

[0100] Obtaining the actual voltage value across the upper bridge arm inductor and the actual voltage value across the lower bridge arm inductor of each phase, calculating the upper and lower bridge arm inductor voltages and corresponding actual values based on the actual voltage value across the upper bridge arm inductor and the actual voltage value across the lower bridge arm inductor of each phase, and filtering the upper and lower bridge arm inductor voltages and corresponding actual values to obtain a double frequency component of the inductor voltage, thereby obtaining an effective value of the double frequency of the inductor voltage;

[0101] If the effective value of the double frequency of the circulating current is equal to the effective value of the double frequency of the inductor voltage, a circulating current suppression compensation value of a preset value is output as the third target signal; if the effective value of the double frequency of the circulating current is less than the effective value of the double frequency of the inductor voltage, a circulating current suppression compensation value is calculated based on the effective value of the double frequency of the circulating current and the effective value of the double frequency of the inductor voltage, and is used as the third target signal.

[0102] This implementation achieves active intervention in circulating current suppression and closed-loop correction of residual errors through feedforward analysis and dynamic prediction mechanisms. First, the theoretical effective value of the submodule capacitor voltage is derived using discretization techniques. The amplitude characteristics of the circulating current's second-harmonic frequency component are dynamically predicted using switching functions to construct an accurate feedforward model for circulating current interference. By comparing the theoretical predictions with the actual measured values, a circulating current suppression compensation signal is dynamically generated. When the two match, a preset compensation amount is directly output to maintain steady state. If a deviation exists, the error is dynamically corrected by calculating the compensation coefficient in real time. This strategy combines the predictive advantages of the feedforward model with the error-correcting capabilities of feedback control, enabling both early prediction of circulating current trends and, through closed-loop compensation, eliminating the effects of model errors or external disturbances. Furthermore, optimizing the switching strategy by counting the number of submodule activations can reduce electromagnetic interference and component losses caused by high-frequency switching, further ensuring the long-term reliable operation of the system.

[0103] As an optional implementation manner, the target waveform signal includes a modulated wave signal, and the processing to obtain the target waveform signal according to the first waveform signal, the second waveform signal, and the third waveform signal includes:

[0104] Determining a waveform factor according to the number of all submodules in the MMC;

[0105] Calculating a first calculation item according to the DC voltage reference value, the first waveform signal, and the waveform coefficient;

[0106] determining a second calculation item and a third calculation item according to the second waveform signal;

[0107] determining a fourth calculation item according to the third waveform signal;

[0108] According to the first calculation item, the second calculation item, the third calculation item and the fourth calculation item, a modulated wave signal corresponding to each of the submodules is calculated and obtained.

[0109] This implementation significantly improves the generation accuracy and dynamic adaptability of the modulation wave signal through multi-parameter fusion and hierarchical compensation mechanism. In the modulation wave calculation, the first calculation item divides the DC side voltage equally into each phase and dynamically couples it with the AC side voltage reference value, ensuring the real-time matching of the modulation wave amplitude with the system operating conditions. The second and third calculation items respectively perform reverse compensation for the circulating DC and AC components, effectively offsetting the distortion effect of the circulating current on the modulation wave. The fourth calculation item further corrects the residual error through the feedforward analytical model. This hierarchical architecture decouples voltage control, circulating current suppression and dynamic compensation into independent calculation dimensions, which not only avoids the increase in control complexity caused by multi-variable coupling, but also realizes the coordinated optimization of various control objectives through linear superposition. The modulated wave signal finally generated has both high precision and strong robustness, can accurately guide the switching timing of sub-modules, and provides a high-quality reference signal for current tracking control.

[0110] As an optional implementation manner, the calculation method of the first calculation item includes:

[0111] After obtaining the difference between half of the DC voltage reference value and each corresponding phase voltage reference value, the difference is multiplied by the waveform coefficient to obtain the first calculation item;

[0112] Furthermore, the second calculation item includes the inverse of the bridge arm circulating current DC reference value corresponding to each phase, the third calculation item includes the inverse of the bridge arm circulating current double frequency AC reference value, and the fourth calculation item includes the circulating current suppression compensation value;

[0113] And, the calculation method of the modulated wave signal includes:

[0114] The first calculation item, the second calculation item, the third calculation item and the fourth calculation item are summed to obtain the modulated wave signal.

[0115] In this implementation, the first calculation item associates the DC voltage with the AC reference value through the waveform coefficient, ensuring that the fundamental component of the modulation wave strictly matches the system requirements. The second and third calculation items symmetrically offset the interference of the circulating DC and high-frequency components on the modulation wave, and achieve accurate compensation for circulating current suppression through the positive and negative superposition mechanism. The fourth calculation item dynamically corrects the error introduced by the nonlinear characteristics of the system based on the feedforward model. This design transforms complex physical relationships into intuitive algebraic operations, which not only reduces the resource consumption of real-time calculations, but also enhances the adjustability of the control system through the interpretability of parameters, ensuring the continuity and stability of the modulation wave signal under dynamic working conditions, and providing a data basis for the efficient switching of sub-modules.

[0116] As an optional implementation manner, generating a switch signal of each submodule according to the target waveform signal includes:

[0117] Determining the number of submodules to be put into use according to the modulation wave signals corresponding to the submodules;

[0118] The submodules are sorted according to the difference between the capacitor voltage corresponding to each submodule and the rated voltage, and a target submodule group and a sequence of inputting the submodules in the target submodule group are determined.

[0119] This implementation achieves balanced use and lifespan optimization of submodules through status monitoring and intelligent sorting strategies. After dynamically determining the number of submodules to be put into operation based on the modulated wave signal, the capacitor voltage of each submodule is collected in real time and compared with the rated value. Submodules with voltages close to the rated value are prioritized for use, while submodules with larger voltage deviations are temporarily suspended. This strategy, through a dynamic sorting mechanism, ensures that the capacitor voltages of all submodules tend to be consistent during long-term operation, avoiding the accelerated aging of some submodules due to long-term overload and reducing the generation of circulating currents and harmonics through voltage balancing. At the same time, the optimization of the switching sequence significantly reduces the frequency and amplitude of switching actions, reduces the thermal stress and electromagnetic interference caused by frequent switching of power devices, and improves the reliability and economy of the converter while ensuring the real-time performance of the system.

[0120] See also Figure 3 , Figure 3 This is an overall control block diagram corresponding to a modular multi-level converter current control method based on parameter identification provided in one embodiment of the present application, which is used to illustrate an example of combining various implementation methods according to actual application scenarios.

[0121] exist Figure 3In the block diagram shown, the method corresponding to the embodiment of the present application is presented in a modular control process, including a DC bus voltage stabilization module based on a voltage outer loop and a current inner loop, a circulating current phase-to-phase voltage equalization module based on PI and PR control, a circulating current suppression module based on a feedforward analytical method, and a modulation wave calculation module. The three-phase AC power system 1 adopts a current control strategy as an example for illustration.

[0122] Specifically, the DC bus voltage stabilization module based on the voltage outer loop and the current inner loop includes the following steps:

[0123] (1) Real-time detection of the voltage Udc at both ends of the DC side, subtracting it from the DC side voltage reference value Udc_ref, and feeding the difference between the two into the first-level PI controller to obtain the reference value id1_ref of the d-axis component of the current of the three-phase AC power system 1; the reference value iq1_ref of the q-axis component of the current of the three-phase AC power system 1 is given by the dispatch instruction;

[0124] (2) Real-time detection of the phase currents ia1, ib1, and ic1 of the three-phase AC power system 1, and the actual values id1 and iq1 of the d-axis and q-axis components of the current of the three-phase AC power system 1 are obtained through abc / dq transformation;

[0125] (3) The actual value id1 of the d-axis component of the current of the three-phase AC power system 1 is subtracted from the reference value id1_ref. The difference between the two is fed into the second-stage PI controller to obtain the reference value ud1_ref of the d-axis component of the voltage of the three-phase AC power system 1. Similarly, the reference value uq1_ref of the q-axis component of the voltage of the three-phase AC power system 1 is obtained through the third-stage PI controller.

[0126] (4) The reference values ua_ref, ub_ref, and uc_ref of the first component of the voltage modulation wave of the three-phase AC power system 1 are obtained by performing dq / abc changes on the reference value ud1_ref of the d-axis component of the voltage of the three-phase AC power system 1 and the reference value uq1_ref of the q-axis component.

[0127] The circulating phase-to-phase pressure equalization module based on PI and PR control includes the following steps:

[0128] (1) Detect the actual value of the capacitor voltage of the i-th phase bridge arm submodule of the three-phase MMC system in real time and calculate its average value Uc_iave;

[0129] (2) The average value Uc_iave of the capacitor voltage of the i-th phase bridge arm submodule is subtracted from the rated value Uc of the submodule capacitor voltage. The difference between the two is brought into the fourth-level PI controller to obtain the reference value iciri_ref of the circulating current of the i-th phase bridge arm:

[0130]

[0131] Among them, Kp4 and Ki4 are the proportional coefficient and integral coefficient of the fourth-level PI controller respectively;

[0132] (3) Detect the actual current values ii1 and ii2 of the upper and lower bridge arms of phase i in real time, and calculate the actual value of the circulating current of the bridge arm of phase i, iciri:

[0133]

[0134] (4) The actual value iciri of the i-th phase bridge arm circulating current is subtracted from the reference value iciri_ref. The difference between the two is sent to the fifth-stage PI controller and PR controller respectively. The resonant frequency of the PR controller is 2 times the frequency. The reference value uciri_dcref of the DC component of the i-th phase bridge arm circulating current and the reference value uciri_acref of the 2 times the frequency AC component are obtained respectively:

[0135]

[0136]

[0137] Where Kp5 and Ki5 are the proportional coefficient and integral coefficient of the fifth-level PI controller, respectively; Kpr and Krr are the proportional coefficient and resonant coefficient of the PR controller, respectively; w0 is the resonant frequency, which is equal to 2*wg1; wg1 is the frequency of the three-phase AC power system 1;

[0138] The circulating current suppression module based on the feedforward analytical method specifically includes the following execution steps:

[0139] (1) The discrete-time mathematical model of the MMC can be derived, and the upper and lower arm current values ii1 and ii2 can be expressed as:

[0140]

[0141] Among them, Iciri is the current value of the DC loop, and the dynamic value can be obtained through scheduling; Ii is the effective value of the current of one phase of the three-phase AC power system, φ is the power factor angle of the power grid, Ts is the sampling period, and k is the sampling time.

[0142] (2) Calculate the theoretical values of the capacitor voltage of each submodule of the upper and lower bridge arms, Uci1M_ref and Uci2M_ref, and you can get:

[0143]

[0144] Wherein, Csm is the submodule capacitance value.

[0145] (3) Calculate the theoretical effective values of the capacitor voltage of each submodule of the upper and lower bridge arms in one cycle, Uci1M_rms and Uci2M_rms, and you can get:

[0146]

[0147] (4) Assume that the switching function of the mth submodule of the upper and lower bridge arms of the i-th phase is

[0148]

[0149] Among them, S is the working status of the submodule, S=1 means the submodule is put into use, and S=0 means the submodule is removed.

[0150] (5) When the MMC is running in steady state, all submodules need to meet the following requirements:

[0151]

[0152] Where ui is the phase 1 voltage of the three-phase AC power system, which can be expressed as , m is the modulation ratio.

[0153] (6) The switching functions of the upper and lower bridge arms of the i-th phase can be calculated as:

[0154]

[0155] Among them, the switching function Si of the MMC phase i is defined as:

[0156] (7) Calculate the average number of times the submodules of the upper and lower bridge arms of the i-th phase are put into operation in one cycle:

[0157]

[0158] (8) The effective value of the second harmonic frequency of the circulating current can be calculated as:

[0159]

[0160] (9) Real-time detection of the actual voltage values uLi1 and uLi2 across the two bridge arm inductors in the i-th phase bridge arm, calculation of the actual value uLi of the sum of the two bridge arm inductor voltages, use of a filter to obtain the double frequency component uLi_2, and use of Fourier decomposition to obtain the effective value ULi_2rms of its double frequency component;

[0161] (10) Compare the size relationship between Uciri_2rms and ULi_2rms. If Uciri_2rms=ULi_2rms, then no circulating current suppression compensation is required; if ULi_2rms>Uciri_2rms, then circulating current suppression compensation is required;

[0162]

[0163] The modulation wave calculation module, based on the above three information, specifically includes the following two steps:

[0164] (1) Calculate the modulation waves ui1j_ref and ui2j_ref of the voltage of the jth submodule of the upper and lower bridge arms of the i-th phase respectively:

[0165]

[0166]

[0167] (2) The modulation wave of the voltage of the jth submodule of the upper and lower bridge arms of the i-th phase adopts the NLM modulation strategy based on the sorting method to obtain the switching signals of each submodule of the upper and lower bridge arms.

[0168] This application determines the corresponding exemplary parameter value scheme through actual simulation and experimental processes. The voltage levels of the target flexible interconnected systems 1 and 2 are both 10kV, the target flexible interconnected system power instructions can obtain dynamic values through scheduling, the DC side voltage is 20kV, the bridge arm inductance Lb_ij of the three-phase MMC system is 3mH, the number of half-bridge sub-modules in each bridge arm M is 20, and the sub-module capacitor voltage rating Uc is 1000V. The reference values of the coefficients of each control link can be Kp1=0.5, Ki1=15; Kp2=3, Ki2=100; Kp3=3, Ki3=100; Kp4=0.1, Ki4=5; Kp5=2, Ki5=10; Kpr=0.1, Krr=0.01, wg1=2*50*π.

[0169] Figures 4 to 8 1 is a schematic diagram showing the effect of the modular multi-level converter current control method based on parameter identification corresponding to the present application, which is used to illustrate the execution effect of the aforementioned embodiment.

[0170] in, Figures 4 to 7 The relevant waveform diagram of the system when the circulating phase equalizing module is controlled by PI and PR is shown. Figure 4 In the middle, from 0.5s to 1.5s, the waveform of the DC side voltage fluctuates around 20kV, which is consistent with the DC side voltage reference value; Figure 5 The capacitor voltages of each submodule in the bridge arm fluctuate around the rated value, with the maximum floating value not exceeding 1050V and the minimum value not falling below 950V, and the fluctuation range being less than ±5%; Figure 6From 0.5s to 1s, without the circulating current suppression module, the A1 phase arm current is composed of a combination of DC and AC components, with the DC component being 68.44A and the AC component being 156.22A. From 1s to 1.5s, due to the use of the circulating current suppression module based on the feedforward analytical method, the AC component of the A1 phase arm current is reduced, which is consistent with the theoretical analysis. Figure 7 In the figure, the three-phase AC at the grid connection point is symmetrical, and the current peak is 312.55A, which is consistent with the theoretical analysis. It can be seen that the effect is better when the circulating current inter-phase voltage balancing module with PI and PR control is used.

[0171] Figure 8 The circulating current waveforms are compared between the case where the circulating current suppression module is not used and the case where the circulating current suppression module is used based on the feedforward analytical method. In the time period from 0.5s to 1.5s, the circulating current of phase A1 without the circulating current suppression module from 0.5 to 1s obviously contains not only a DC component, but also a DC component and a large number of second-harmonic fluctuations. After the circulating current suppression module based on the feedforward analytical method is used from 1 to 1.5s, the amplitude of the circulating current of phase A1 drops significantly. The circulating current of phase A1 mainly contains a DC component with a value of -68.44A and a small amount of high-order harmonics. The circulating current suppression effect of this module is relatively rapid, which is consistent with the theoretical analysis.

[0172] The present application also provides a modular multilevel converter current control device based on feedforward analysis, the device being used in a target flexible interconnected system, the target flexible interconnected system comprising two groups of modular multilevel converters (MMCs) connected back-to-back via DC busbars and two groups of three-phase AC systems corresponding to the MMCs, each of the MMC converters adopting a three-phase six-bridge-arm structure, each phase comprising an upper and lower bridge arm, each bridge arm comprising a plurality of cascaded half-bridge sub-modules and a bridge arm inductor, the upper and lower bridge arms of each phase being connected and then connected to the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system being connected to each other;

[0173] The device comprises:

[0174] A determination module, configured to determine a first waveform signal according to a DC bus voltage stabilization process;

[0175] The determining module is further configured to determine a second waveform signal according to a circulating current inter-phase voltage balancing process;

[0176] The determining module is further configured to determine a third waveform signal according to a circulating current suppression process based on feedforward analysis;

[0177] a processing module, configured to obtain a target waveform signal based on the first waveform signal, the second waveform signal, and the third waveform signal, and generate a switch signal for each of the submodules based on the target waveform signal, so as to control each of the submodules to operate according to a preset mode;

[0178] The target waveform signal is used to indicate the operating voltage corresponding to each of the submodules.

[0179] In this embodiment, the dynamic response capability and operational stability of the flexible interconnected system are improved through a collaborative control mechanism. In a specific application scenario, the deviation between the DC side voltage and the reference value is monitored in real time based on the DC bus voltage stabilization process, and a first waveform signal is dynamically generated through a proportional integral link to limit the DC side voltage to fluctuate stably within a preset range, thereby avoiding the risk of system collapse due to voltage instability. The circulating current interphase voltage equalization process collects the actual value of the capacitor voltage of the bridge arm submodule, calculates its dynamic deviation from the rated value, and generates a second waveform signal to balance the circulating current distribution of each phase bridge arm, effectively preventing device overvoltage or efficiency reduction caused by capacitor voltage imbalance. At the same time, the circulating current suppression process based on feedforward analysis generates a third waveform signal to compensate for high-frequency circulating current interference through real-time comparison of theoretical modeling and actual circulating current components, significantly reducing the additional loss caused by the coupling effect between bridge arms. In the modulation wave generation process, the information corresponding to the above-mentioned signals is integrated to accurately control the switching status of the sub-modules, enabling the system to achieve rapid current tracking and optimize power quality under complex working conditions. By combining closed-loop regulation with feedforward compensation, both real-time and predictive control are taken into account, providing an important foundation for the reliable operation of high-voltage and large-capacity converters.

[0180] On the device side, other implementation methods can be implemented based on the determination module and the processing module. For details, please refer to the relevant description on the method side, which will not be repeated here.

[0181] It should be noted that it should be understood that the division of the various modules of the above device is merely a division of logical functions. In actual implementation, they can be fully or partially integrated into one physical entity, or they can be physically separated. Moreover, these modules can all be implemented in the form of software called by a processing element; or they can all be implemented in the form of hardware; or some modules can be implemented in the form of software called by a processing element, and some modules can be implemented in the form of hardware. For example, the processing module can be a separately established processing element, or it can be integrated into a chip of the above device. In addition, it can also be stored in the memory of the above device in the form of program code, and called by a processing element of the above device to perform the functions of the above-mentioned module. The implementation of other modules is similar. In addition, these modules can all or partly be integrated together, or they can be implemented independently. The processing element here can be an integrated circuit with signal processing capabilities. In the implementation process, each step of the above method or each of the above modules can be completed by the hardware integrated logic circuit in the processor element or by instructions in the form of software.

[0182] Schematically, as Figure 9 As shown, Figure 9This is a schematic diagram of the internal structure of a computer device provided in an embodiment of the present application. The computer device 300 can be provided as a server. Figure 9 Computer device 300 includes a processing component 302, which further includes one or more processors, and a memory resource represented by memory 301 for storing instructions executable by processing component 302, such as an application. The application stored in memory 301 may include one or more modules, each corresponding to a set of instructions. In addition, processing component 302 is configured to execute the instructions to perform the method of any of the above embodiments.

[0183] The computer device 300 may further include a power supply component 303 configured to perform power management of the computer device 300, a wired or wireless network interface 304 configured to connect the computer device 300 to a network, and an input / output (I / O) interface 305. The computer device 300 may operate based on an operating system stored in the memory 301, such as Windows Server™, Mac OS X™, Unix™, Linux™, Free BSD™, or the like.

[0184] Those skilled in the art will understand that Figure 9 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than shown in the figure, or combine certain components, or have a different component arrangement.

[0185] An embodiment of the present application provides a storage medium storing computer-readable instructions. When the computer-readable instructions are executed by one or more processors, the one or more processors execute a method as provided in any embodiment.

[0186] Finally, it should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article, or device comprising the element.

[0187] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The various embodiments can be combined as needed, and the same or similar parts can be referenced to each other.

[0188] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A modular multi-level converter current control method based on feedforward analysis, characterized in that: The method is used for a target flexible interconnected system, which includes two groups of modular multilevel converters (MMCs) connected back-to-back via DC busbars and two groups of three-phase AC systems corresponding to the MMCs. Each of the MMC converters adopts a three-phase six-bridge-arm structure, each phase including an upper and lower bridge arm, each bridge arm including multiple cascaded half-bridge sub-modules and a bridge arm inductor. The upper and lower bridge arms of each phase are connected and then connected to the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system are connected to each other. The method comprises: Determining a first waveform signal according to a DC bus voltage stabilization process; Determine a second waveform signal according to a circulating current inter-phase voltage balancing process; determining a third waveform signal according to a circulating current suppression process based on feedforward analysis; Processing the first waveform signal, the second waveform signal, and the third waveform signal to obtain a target waveform signal, and generating a switch signal for each of the submodules according to the target waveform signal to control each of the submodules to operate according to a preset mode; The target waveform signal is used to indicate the operating voltage corresponding to each of the submodules.

2. The method according to claim 1, characterized in that The step of determining the first waveform signal according to the DC bus voltage stabilization process includes: The voltage across the DC side is obtained in real time, and after subtracting it from a preset DC voltage reference value, a d-axis current reference value is obtained through a first proportional integral link; Acquire the AC current of each phase of the MMC in real time, and obtain the actual value of the d-axis current and the actual value of the q-axis current through abc / dq conversion; Subtracting the d-axis current actual value from the d-axis current reference value to obtain a d-axis voltage reference value through a second proportional-integral link, and subtracting the q-axis current actual value from a preset q-axis current reference value to obtain a q-axis voltage reference value through a third proportional-integral link; The d-axis current reference value and the q-axis voltage reference value are converted into corresponding phase voltage reference values as first waveform signals through dq / abc conversion.

3. The method according to claim 2, characterized in that The determining of the second waveform signal according to the circulating phase-to-phase voltage balancing process includes: Acquire the actual value of the capacitor voltage of each phase bridge arm submodule of the MMC in real time, calculate and obtain the average value of the capacitor voltage, subtract the average value of the capacitor voltage from the rated value of the capacitor voltage, and then obtain the reference value of the circulating current of each phase bridge arm through the fourth proportional integral link; Obtain the actual current value of the upper and lower bridge arms of each phase of the MMC in real time, and calculate the actual value of the bridge arm circulating current of each phase; The actual value of the bridge arm circulating current of each phase is subtracted from the bridge arm circulating current reference value, and the DC reference value of the bridge arm circulating current corresponding to each phase is obtained through the fifth proportional integral link, and the double frequency AC reference value of the bridge arm circulating current corresponding to each phase is obtained through the resonance control link.

4. The method according to claim 3, characterized in that The determining of the third waveform signal according to the circulating current suppression process based on feedforward analysis includes: Obtain the upper bridge arm discrete current value and the lower bridge arm discrete current value of each phase, and integrate the upper bridge arm discrete current value of each phase according to the submodule capacitance value and the sampling period to obtain the upper bridge arm submodule capacitance voltage theoretical value, and integrate the lower bridge arm discrete current value of each phase to obtain the lower bridge arm submodule capacitance voltage theoretical value; Calculate the root mean square of the theoretical value of the capacitor voltage of the upper bridge arm submodule during the sampling period to obtain the effective value of the capacitor voltage of the upper bridge arm submodule during the sampling period, and calculate the root mean square of the theoretical value of the capacitor voltage of the lower bridge arm submodule during the sampling period to obtain the effective value of the capacitor voltage of the lower bridge arm submodule during the sampling period; Determine the switching function, steady-state operation switching function conditions, and corresponding phase switching functions of each submodule, calculate the upper bridge arm switching function and the lower bridge arm switching function of each phase, and calculate the average number of times the upper bridge arm submodule of each phase is put into operation based on the upper bridge arm switching function of each phase, and calculate the average number of times the lower bridge arm submodule of each phase is put into operation based on the lower bridge arm switching function of each phase; Calculate the effective value of the double frequency of the circulating current according to the effective value of the capacitor voltage of the upper bridge arm submodule of each phase, the effective value of the capacitor voltage of the lower bridge arm submodule, the average number of times the upper bridge arm submodule is put into operation, the average number of times the lower bridge arm submodule is put into operation, and the voltage across the DC side; Obtaining the actual voltage value across the upper bridge arm inductor and the actual voltage value across the lower bridge arm inductor of each phase, calculating the upper and lower bridge arm inductor voltages and corresponding actual values based on the actual voltage value across the upper bridge arm inductor and the actual voltage value across the lower bridge arm inductor of each phase, and filtering the upper and lower bridge arm inductor voltages and corresponding actual values to obtain a double frequency component of the inductor voltage, thereby obtaining an effective value of the double frequency of the inductor voltage; If the effective value of the double frequency of the circulating current is equal to the effective value of the double frequency of the inductor voltage, a circulating current suppression compensation value of a preset value is output as the third target signal; if the effective value of the double frequency of the circulating current is less than the effective value of the double frequency of the inductor voltage, a circulating current suppression compensation value is calculated based on the effective value of the double frequency of the circulating current and the effective value of the double frequency of the inductor voltage, and is used as the third target signal.

5. The method according to claim 4, characterized in that The target waveform signal includes a modulated wave signal, and the processing to obtain the target waveform signal according to the first waveform signal, the second waveform signal, and the third waveform signal includes: Determining a waveform factor according to the number of all submodules in the MMC; Calculating a first calculation item according to the DC voltage reference value, the first waveform signal, and the waveform coefficient; determining a second calculation item and a third calculation item according to the second waveform signal; determining a fourth calculation item according to the third waveform signal; According to the first calculation item, the second calculation item, the third calculation item and the fourth calculation item, a modulated wave signal corresponding to each of the submodules is calculated and obtained.

6. The method according to claim 5, characterized in that The calculation method of the first calculation item includes: After obtaining the difference between half of the DC voltage reference value and each corresponding phase voltage reference value, the difference is multiplied by the waveform coefficient to obtain the first calculation item; Furthermore, the second calculation item includes the inverse of the bridge arm circulating current DC reference value corresponding to each phase, the third calculation item includes the inverse of the bridge arm circulating current double frequency AC reference value, and the fourth calculation item includes the circulating current suppression compensation value; And, the calculation method of the modulated wave signal includes: The first calculation item, the second calculation item, the third calculation item and the fourth calculation item are summed to obtain the modulated wave signal.

7. The method according to claim 5, characterized in that Generating a switch signal for each of the submodules according to the target waveform signal includes: Determining the number of submodules to be put into use according to the modulation wave signals corresponding to the submodules; The submodules are sorted according to the difference between the capacitor voltage corresponding to each submodule and the rated voltage, and a target submodule group and a sequence of inputting the submodules in the target submodule group are determined.

8. A modular multi-level converter current control device based on feedforward analysis, characterized in that: The device is used in a target flexible interconnection system, which includes two groups of modular multilevel converters (MMCs) connected back-to-back via DC busbars and two groups of three-phase AC systems corresponding to the MMCs. Each of the MMC converters adopts a three-phase six-bridge arm structure, each phase includes an upper and lower bridge arm, each bridge arm includes multiple cascaded half-bridge sub-modules and a bridge arm inductor, and the upper and lower bridge arms of each phase are connected to the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system are connected to each other. The device comprises: A determination module, configured to determine a first waveform signal according to a DC bus voltage stabilization process; The determining module is further configured to determine a second waveform signal according to a circulating current inter-phase voltage balancing process; The determining module is further configured to determine a third waveform signal according to a circulating current suppression process based on feedforward analysis; a processing module, configured to obtain a target waveform signal based on the first waveform signal, the second waveform signal, and the third waveform signal, and generate a switch signal for each of the submodules based on the target waveform signal, so as to control each of the submodules to operate according to a preset mode; The target waveform signal is used to indicate the operating voltage corresponding to each of the submodules.

9. A computer device, characterized in that: The method comprises one or more processors and a memory, wherein the memory stores computer-readable instructions, and when the computer-readable instructions are executed by the one or more processors, the steps of the method according to any one of claims 1 to 7 are performed.

10. A storage medium, characterized in that: The storage medium stores computer-readable instructions, which, when executed by one or more processors, enable the one or more processors to perform the steps of the method according to any one of claims 1 to 7.

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