Modular multilevel converter current control method based on feedforward analysis

Abstract

The application provides a modular multilevel converter current control method based on feedforward analysis. Based on the deviation of the real-time monitoring of the DC bus voltage from the reference value, a first waveform signal is dynamically generated to limit the fluctuation range of the DC side voltage. The inter-phase circulating current voltage is balanced by collecting the actual value of the bridge arm sub-module capacitor voltage, calculating the dynamic deviation of the actual value from the rated value, and generating a second waveform signal to balance the circulating current distribution of each phase bridge arm. Based on the feedforward analysis, the circulating current is suppressed by comparing the theoretical modeling with the actual circulating current component in real time, and a third waveform signal is generated to compensate for the high-frequency circulating current interference. In the modulation wave generation process, the switching state of the sub-module is accurately controlled by integrating various aspects of information, so that the system improves the operation effect under complex working conditions, and through the combination of closed-loop adjustment and feedforward compensation, the real-time and predictability of the control are considered, thereby improving the effectiveness of the control.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrical control, and in particular to a modular multilevel converter current control method based on feedforward analysis. BACKGROUND

[0002] Modular Multilevel Converter (MMC) adopts modular design, is easy to expand, has the advantages of low harmonic, high efficiency, small loss, good scalability, etc., and has an important role in high-voltage direct-current transmission and flexible alternating-current transmission. As the core of the normal operation of MMC, the bridge arm current tracking control is directly related to the stability of the system and the quality of the output power.

[0003] However, there are multiple bridge arms in the MMC system, and there is a complex coupling relationship between these bridge arms, making the independent control of single bridge arm current more complex, and the accurate tracking control of bridge arm current very difficult. The circulating current exists between the DC bus and the upper and lower bridge arms of the MMC, and the circulating current will cause additional loss and imbalance of the sub-module capacitor voltage, thereby affecting the efficiency and stability of the system. Therefore, a modular multilevel converter current control method based on feedforward analysis is needed to achieve accurate tracking control of bridge arm current and effectively suppress the adverse effects of circulating current. SUMMARY

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

[0005] In a first aspect, the present application provides a modular multilevel converter current control method based on feedforward analysis, which is used for a target flexible interconnection system. The target flexible interconnection system includes two groups of modular multilevel converters (MMCs) connected back-to-back through a DC bus and two groups of three-phase AC systems corresponding to the MMCs. Each MMC converter adopts a three-phase six-bridge arm structure, each phase includes upper and lower bridge arms, and each bridge arm includes a plurality of 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 after being connected, and the three-phase output ports of each three-phase AC system are connected to each other.

[0006] The method includes:

[0007] According to the DC bus voltage stabilizing flow, a first waveform signal is determined;

[0008] According to the circulating current inter-phase voltage balancing flow, a second waveform signal is determined;

[0009] According to the circulating current suppression flow based on feedforward analysis, a third waveform signal is determined;

[0010] According to the first waveform signal, the second waveform signal and the third waveform signal, a target waveform signal is obtained, and a switching signal of each of the sub-modules is generated according to the target waveform signal, so as to control each of the sub-modules to operate according to a preset mode.

[0011] The target waveform signal is used to indicate an operating voltage corresponding to each of the sub-modules.

[0012] As an optional implementation, the first waveform signal is determined according to a direct-current bus voltage stabilization process, and the process comprises:

[0013] A direct-current side voltage is obtained in real time, and a d-axis current reference value is obtained by a first proportional integral link after the direct-current side voltage is subtracted from a preset direct-current voltage reference value;

[0014] An alternating-current current of each phase of the MMC is obtained in real time, and a d-axis current actual value and a q-axis current actual value are obtained by abc / dq conversion;

[0015] A d-axis voltage reference value is obtained by a second proportional integral link after the d-axis current actual value is subtracted from the d-axis current reference value, and a q-axis voltage reference value is obtained by a third proportional integral link after the q-axis current actual value is subtracted from a preset q-axis current reference value;

[0016] The d-axis current reference value and the q-axis voltage reference value are converted into a corresponding phase voltage reference value of each phase by dq / abc conversion, as the first waveform signal.

[0017] As an optional implementation, the second waveform signal is determined according to a circulating current inter-phase voltage equalization process, and the process comprises:

[0018] A capacitor voltage actual value of each phase bridge arm sub-module of the MMC is obtained in real time, and a capacitor voltage average value is obtained by calculation, and a circulating current reference value of each phase bridge arm is obtained by a fourth proportional integral link after the capacitor voltage average value is subtracted from a capacitor voltage rated value;

[0019] A current actual value of an upper bridge arm and a lower bridge arm of each phase of the MMC is obtained in real time, and a bridge arm circulating current actual value of each phase is obtained by calculation;

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

[0021] As an optional implementation, the third waveform signal is determined according to a circulating current suppression process based on feedforward analysis, and the process comprises:

[0022] acquire discrete current values of upper bridge arms and lower bridge arms of each phase, and according to capacitor values of sub-modules and a sampling period, integrate the discrete current values of the upper bridge arms of each phase to obtain upper bridge arm sub-module capacitor voltage theoretical values, and integrate the discrete current values of the lower bridge arms of each phase to obtain lower bridge arm sub-module capacitor voltage theoretical values;

[0023] calculate a root mean square of the upper bridge arm sub-module capacitor voltage theoretical values in the sampling period to obtain an upper bridge arm sub-module capacitor voltage effective value in the sampling period, and calculate a root mean square of the lower bridge arm sub-module capacitor voltage theoretical values in the sampling period to obtain a lower bridge arm sub-module capacitor voltage effective value in the sampling period;

[0024] determine switching functions of each sub-module, steady-state operation switching function conditions, and phase switching functions corresponding to each phase, calculate upper bridge arm switching functions and lower bridge arm switching functions of each phase, and according to the upper bridge arm switching functions of each phase, calculate average input times of upper bridge arm sub-modules of each phase, and according to the lower bridge arm switching functions of each phase, calculate average input times of lower bridge arm sub-modules of each phase;

[0025] according to the upper bridge arm sub-module capacitor voltage effective value, the lower bridge arm sub-module capacitor voltage effective value, the upper bridge arm sub-module average input time, the lower bridge arm sub-module average input time, and the voltage across the DC side of each phase, calculate a circulating current double-frequency effective value;

[0026] acquire actual voltage values across upper bridge arm inductors and actual voltage values across lower bridge arm inductors, according to the actual voltage values across the upper bridge arm inductors and the actual voltage values across the lower bridge arm inductors of each phase, calculate an inductor voltage actual value, and filter the actual values of the upper and lower bridge arm inductor voltages to obtain an inductor voltage double-frequency component, and obtain an inductor voltage double-frequency effective value;

[0027] if the circulating current double-frequency effective value is equal to the inductor voltage double-frequency effective value, output a preset circulating current suppression compensation value as the third waveform signal, and if the circulating current double-frequency effective value is less than the inductor voltage double-frequency effective value, according to the circulating current double-frequency effective value and the inductor voltage double-frequency effective value, calculate a circulating current suppression compensation value as the third waveform signal.

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

[0029] determine a waveform coefficient according to the number of all sub-modules in the MMC;

[0030] According to the direct current voltage reference value, the first waveform signal and the waveform coefficient, a first calculation item is calculated;

[0031] According to the second waveform signal, a second calculation item and a third calculation item are determined;

[0032] According to the third waveform signal, a fourth calculation item is determined;

[0033] According to the first calculation item, the second calculation item, the third calculation item and the fourth calculation item, a modulation wave signal corresponding to each of the sub-modules is calculated.

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

[0035] Half of the direct current voltage reference value is subtracted from the corresponding phase voltage reference value of each phase, and then multiplied by the waveform coefficient to obtain the first calculation item;

[0036] In addition, the second calculation item comprises the opposite number of the bridge arm circulating direct current reference value corresponding to each phase, the third calculation item comprises the opposite number of the bridge arm circulating double-frequency alternating current reference value, and the fourth calculation item comprises the circulating current suppression compensation value;

[0037] In addition, the calculation method of the modulation wave signal comprises:

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

[0039] As an optional implementation, the generation of the switching signal of each of the sub-modules according to the target waveform signal comprises:

[0040] According to the modulation wave signal corresponding to each of the sub-modules, the number of sub-modules to be put into operation is determined;

[0041] According to the difference between the capacitor voltage and the rated voltage of each of the sub-modules, each of the sub-modules is sorted to determine the target sub-module group and the operation sequence of each of the sub-modules in the target sub-module group.

[0042] In a second aspect, the application provides a modular multilevel converter current control device based on feed-forward analysis, which is used in a target flexible interconnection system, the target flexible interconnection system comprising two groups of modular multilevel converters (MMCs) connected back-to-back through 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 upper and lower bridge arms, 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 to connect corresponding three-phase AC systems, and the three-phase output ports of each of the three-phase AC systems 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 determination module is further configured to determine a second waveform signal according to a circulating current inter-phase voltage equalization process;

[0046] The determination module is further configured to determine a third waveform signal according to a circulating current suppression process based on feed-forward analysis;

[0047] A processing module configured to process a target waveform signal according to the first waveform signal, the second waveform signal and the third waveform signal, and generate a switching signal of each of the sub-modules according to the target waveform signal to control each of the sub-modules to operate according to a preset mode;

[0048] The target waveform signal is used to indicate an operating voltage corresponding to each of the sub-modules.

[0049] The specific mode in which the determination module determines the third waveform signal according to the circulating current suppression process based on feed-forward analysis comprises:

[0050] Discrete current values of upper bridge arms and discrete current values of lower bridge arms of each phase are obtained, and the discrete current values of the upper bridge arms of each phase are integrated to obtain upper bridge arm sub-module capacitor voltage theoretical values according to sub-module capacitor values and a sampling period, and the discrete current values of the lower bridge arms of each phase are integrated to obtain lower bridge arm sub-module capacitor voltage theoretical values according to the sub-module capacitor values and the sampling period;

[0051] The root mean square of the upper bridge arm sub-module capacitor voltage theoretical values in the sampling period is calculated to obtain an effective value of the upper bridge arm sub-module capacitor voltage in the sampling period, and the root mean square of the lower bridge arm sub-module capacitor voltage theoretical values in the sampling period is calculated to obtain an effective value of the lower bridge arm sub-module capacitor voltage in the sampling period;

[0052] Determine the switching function of each sub-module, the steady-state operation switching function condition and the corresponding phase switching function of each phase, calculate the upper arm switching function and the lower arm switching function of each phase, and calculate the average input frequency of the upper arm sub-module of each phase according to the upper arm switching function of each phase, and calculate the average input frequency of the lower arm sub-module of each phase according to the lower arm switching function of each phase;

[0053] According to the effective value of the upper arm sub-module capacitor voltage, the effective value of the lower arm sub-module capacitor voltage, the average input frequency of the upper arm sub-module, the average input frequency of the lower arm sub-module, and the voltage across the DC side of each phase, the effective value of the circulating current double-frequency component is calculated.

[0054] Obtain the actual value of the voltage across the upper arm inductor of each phase and the actual value of the voltage across the lower arm inductor, calculate the actual value of the upper and lower arm inductor voltage according to the actual value of the voltage across the upper arm inductor and the actual value of the voltage across the lower arm inductor of each phase, and filter the actual value of the upper and lower arm inductor voltage to obtain the inductor voltage double-frequency component, and obtain the effective value of the inductor voltage double-frequency component.

[0055] If the effective value of the circulating current double-frequency component is equal to the effective value of the inductor voltage double-frequency component, output a preset value of the circulating current suppression compensation value as the third waveform signal, if the effective value of the circulating current double-frequency component is less than the effective value of the inductor voltage double-frequency component, calculate the circulating current suppression compensation value according to the effective value of the circulating current double-frequency component and the effective value of the inductor voltage double-frequency component, and output the circulating current suppression compensation value as the third waveform signal.

[0056] 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 the computer readable instructions are executed by the one or more processors to perform the steps of the method of the first aspect.

[0057] In a fourth aspect, the present application provides a storage medium, wherein the storage medium stores computer readable instructions, and the computer readable instructions are executed by one or more processors to make the one or more processors perform the steps of the method of the first aspect.

[0058] From the above technical solutions, it can be seen that the embodiments of the present application have the following advantages:

[0059] Based on any of the above embodiments, the corresponding method of the present application improves the dynamic response capability and operation stability of the flexible interconnection system through a cooperative control mechanism. In a specific application scenario, the deviation of the DC side voltage from the reference value is monitored in real time based on the DC bus voltage stabilization process, a first waveform signal is dynamically generated through a proportional-integral element to limit the stable fluctuation of the DC side voltage within a preset range, and the risk of system collapse caused by voltage instability is avoided. The circulating current inter-phase voltage equalization process calculates the dynamic deviation of the actual value of the bridge arm sub-module capacitor voltage 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 by comparing the theoretical model with the actual circulating current component in real time, significantly reducing the additional loss caused by the coupling effect between the bridge arms. In the modulation wave generation process, the information corresponding to the above signals is integrated to accurately control the switching state of the sub-module, enabling the system to achieve fast current tracking and power quality optimization under complex operating conditions. Through the combination of closed-loop regulation and feedforward compensation, the real-time and predictive nature of the control is taken into account, providing an important foundation for the reliable operation of high-voltage and high-capacity converters. BRIEF DESCRIPTION OF DRAWINGS

[0060] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, a brief introduction will be given below to the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0061] Figure 1 An application scenario diagram of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the present application is shown in the figure.

[0062] Figure 2 A flowchart of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the present application is shown in the figure.

[0063] Figure 3 A block diagram of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the present application is shown in the figure.

[0064] Figure 4 An effect diagram of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the present application is shown in the figure.

[0065] Figure 5 An effect diagram of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the present application is shown in the figure.

[0066] Figure 6 An effect diagram of a modular multilevel converter current control method based on feedforward analysis provided for an embodiment of the present application is shown in FIG. 1.

[0067] Figure 7 An effect diagram of a modular multilevel converter current control method based on feedforward analysis provided for an embodiment of the present application is shown in FIG. 1.

[0068] Figure 8 An effect diagram of a modular multilevel converter current control method based on feedforward analysis provided for an embodiment of the present application is shown in FIG. 1.

[0069] Figure 9 An internal structure diagram of a computer device provided for an embodiment of the present application is shown in FIG. 1. DETAILED DESCRIPTION

[0070] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0071] With the development of power systems towards high voltage and large capacity, the traditional converter faces many challenges in high voltage and large capacity applications, such as high switching loss, large harmonic content, etc. With the increasing demand for efficient, reliable and highly scalable converters, MMC emerges as the times require. MMC adopts modular design, is easy to expand, has low harmonic, high efficiency, small loss and good scalability, and has important significance in high voltage direct current transmission and flexible alternating current transmission fields. Nowadays, the control technology research on MMC, especially in the aspects of bridge arm current tracking and circulating current suppression strategy, has urgent practicality and importance.

[0072] As the core of the normal operation of MMC, the bridge arm current tracking control is directly related to the stability of the system and the quality of the output power. However, there are multiple bridge arms in the MMC system, and there is a complex coupling relationship between these bridge arms, which makes the independent control of single bridge arm current more complex, and the accurate tracking control of bridge arm current becomes very difficult. The circulating current of MMC exists between the DC bus and the upper and lower bridge arms, and the circulating current will cause additional loss and imbalance of the sub-module capacitor voltage, thereby affecting the efficiency and stability of the system. It is necessary to quickly realize the accurate tracking control of the bridge arm current and effectively suppress the adverse effects of circulating current.

[0073] The power distribution network flexible interconnection system current tracking control strategy based on the feedforward analytical method adopts a frequency division current control strategy to decouple and control the bridge arm current components, realizes independent tracking of each frequency band current, thereby realizing fast and accurate control of the current components, and can improve the output power quality; the feedforward analytical method based on the feedforward analytical method is simple, and at the same time, it does not increase the loss of the system, that is, the circulation can be suppressed, the reliability of the system can be improved, and the stability and long-term reliability of the system under various working conditions are ensured.

[0074] Please refer to Figure 1 , Figure 1 The application scenario diagram of the application scene of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the application is used to illustrate the technical concept of the application according to the structure of the target flexible interconnection system.

[0075] The target flexible interconnection system includes two groups of modular multilevel converters MMC connected back-to-back through a DC bus and two groups of three-phase AC systems corresponding to the MMC, each MMC converter adopts a three-phase six-bridge arm structure, each phase includes upper and lower bridge arms, each bridge arm includes a plurality of cascaded half-bridge sub-modules and a bridge arm inductor, the upper and lower bridge arms of each phase are connected to connect the corresponding three-phase AC system, and the three-phase output ports of each three-phase AC system are connected to each other.

[0076] Specifically, as Figure 1 indicated, the power distribution network flexible interconnection system is composed of a set of back-to-back MMC converters, and the back-to-back MMC converter is composed of two groups of three-phase MMC systems, a DC side and two three-phase AC power systems.

[0077] The two groups of three-phase MMC systems have the same structure and are of a three-phase six-bridge arm structure, and the left three-phase MMC system is taken as an example, wherein each phase is connected by upper and lower bridge arms with the same structure, and the connection points of each bridge arm are respectively referred to as a1, b1 and c1; each bridge arm is composed of M cascaded half-bridge sub-modules and 1 bridge arm inductor Lb_ij in series connection, wherein i represents the phase connected with the three-phase AC power system, i is A1, B1 and 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 each upper bridge arm are connected together and referred to as a P node; the lower end points of each lower bridge arm are connected together and referred to as an N node; and the back-to-back MMC converter is connected together by the two groups of three-phase MMC systems through the DC side.

[0078] The input ports A1, B1 and C1 of the three-phase alternating current power system 1 of the flexible interconnection system of the power distribution network are connected with the left bridge arm connection points a1, b1 and c1 of the back-to-back MMC converter, the three-phase output ports of the three-phase alternating current power system 1 are connected together and are referred to as T1 node; the right bridge arm connection points a2, b2 and c2 of the back-to-back MMC converter are connected with the input ports A2, B2 and C2 of the three-phase alternating current power system 1, and the three-phase output ports of the three-phase alternating current power system 2 are connected together and are referred to as T2 node;

[0079] The application decouples the bridge arm current component according to the frequency, adopts a frequency division current tracking control strategy, realizes independent tracking of the current of each frequency band, thereby realizing fast and accurate control of the current component, and can improve the power quality of the output; the system circulating current is analyzed by using the analytical method, the circulating current suppression compensation value is obtained, and the circulating current suppression control strategy based on the feedforward analytical method is adopted, so that the system can realize the suppression of circulating current and will not increase the loss, and the reliability and stability of the system can be improved.

[0080] Specifically, the application provides a topology structure of the MMC-based flexible interconnection system of the power distribution network, and proposes a frequency division current tracking control strategy for the topology structure, so that the bridge arm current tracking speed is faster and the accuracy is higher, and the control structure is simple, without the need to increase an additional circulating current suppression controller, thereby saving the cost, and the circulating current suppression strategy based on the feedforward analytical method can better suppress the circulating current, reduce the circulating current, reduce the system loss, and enhance the reliability and stability of the system.

[0081] In summary, the technical concept of the application lies in that the dynamic response capability and operation stability of the flexible interconnection system are improved through the cooperative control mechanism. In a specific application scenario, the deviation of the DC side voltage from the reference value is monitored in real time based on the DC bus voltage stabilization process, a first waveform signal is dynamically generated through a proportional integral link to limit the stable fluctuation of the DC side voltage within a preset range, and the risk of system collapse caused by voltage instability is avoided. The circulating current inter-phase voltage equalization process acquires the actual value of the bridge arm sub-module capacitor voltage, calculates the dynamic deviation of the actual value from the rated value, and generates a second waveform signal to balance the circulating current distribution of each phase bridge arm, effectively preventing the overvoltage or efficiency reduction of the device caused by capacitor voltage imbalance. At the same time, the circulating current suppression process based on the feedforward analysis compares the theoretical modeling with the actual circulating current component in real time, generates a third waveform signal to compensate for the high-frequency circulating current interference, and significantly reduces the additional loss caused by the coupling effect between the bridge arms. In the modulation wave generation process, the switching state of the sub-module is accurately controlled by comprehensively considering the information corresponding to the above-mentioned signals, so that the system realizes fast tracking of the current and optimization of the power quality under complex working conditions, and through the combination of closed-loop adjustment and feedforward compensation, the real-time performance and predictability of the control are taken into account, thereby providing an important foundation for the reliable operation of the high-voltage large-capacity converter.

[0082] The method provided by the application is described in detail below according to corresponding embodiments in some practical application scenarios.

[0083] Please refer to Figure 2 , Figure 2 The flowchart of the modular multilevel converter current control method based on feedforward analysis provided by an embodiment of the application is applied to the target flexible interconnection system as shown in Figure 1 .

[0084] As shown in Figure 2 , the method comprises:

[0085] S101, determining a first waveform signal according to a DC bus voltage stabilization process;

[0086] S102, determining a second waveform signal according to a circulating current phase-to-phase voltage equalization process;

[0087] S103, determining a third waveform signal according to a circulating current suppression process based on feedforward analysis;

[0088] S104, processing a target waveform signal according to the first waveform signal, the second waveform signal and the third waveform signal, and generating a switching signal of each sub-module according to the target waveform signal to control each sub-module to operate according to a preset mode;

[0089] The target waveform signal is used to indicate the operating voltage corresponding to each sub-module.

[0090] In this embodiment, the dynamic response capability and operating stability of the flexible interconnection system are improved through the cooperative control mechanism. In a specific application scenario, the deviation of the DC side voltage from the reference value is monitored in real time based on the DC bus voltage stabilization process, the first waveform signal is dynamically generated through the proportional integral link to limit the stable fluctuation of the DC side voltage within a preset range, and the risk of system collapse caused by voltage instability is avoided. The circulating current phase-to-phase voltage equalization process calculates the dynamic deviation of the actual value of the bridge arm sub-module capacitor voltage from the rated value by collecting the actual value, and generates the second waveform signal to balance the circulating current distribution of each phase bridge arm, effectively preventing the overvoltage or efficiency reduction of the device caused by unbalanced capacitor voltage. At the same time, the circulating current suppression process based on feedforward analysis generates the third waveform signal to compensate for the high-frequency circulating current interference by comparing the theoretical modeling with the actual circulating current component in real time, which significantly reduces the additional loss caused by the coupling effect between the bridge arms. In the modulation wave generation process, the switching state of the sub-module is accurately controlled by comprehensively considering the information corresponding to the above-mentioned signals, so that the system can realize rapid tracking of the current and optimization of the power quality under complex working conditions. Through the combination of closed-loop regulation and feedforward compensation, the real-time and predictability of the control are taken into account, which provides an important foundation for the reliable operation of the high-voltage and large-capacity converter.

[0091] As an optional implementation, the first waveform signal is determined according to the direct-current bus voltage stabilization process, comprising:

[0092] The direct-current side voltage is acquired in real time, and a d-axis current reference value is obtained by subtracting a preset direct-current voltage reference value from the direct-current side voltage through a first proportional integral link;

[0093] The alternating-current currents of the MMC are acquired in real time, and a d-axis current actual value and a q-axis current actual value are obtained through abc / dq transformation;

[0094] The d-axis current actual value is subtracted from the d-axis current reference value to obtain a d-axis voltage reference value through a second proportional integral link, and the q-axis current actual value is subtracted from a preset q-axis current reference value to obtain a q-axis voltage reference value through a third proportional integral link;

[0095] The d-axis current reference value and the q-axis voltage reference value are converted into corresponding phase voltage reference values of each phase through dq / abc transformation, as the first waveform signal.

[0096] The embodiment realizes the coordinated control of the direct-current side voltage and the alternating-current side current through the hierarchical control architecture, including voltage outer loop control and current inner loop control, and the data processing process in the control involves mutual transformation of the dq axis and the abc three-phase. In the voltage outer loop control, the direct-current side voltage deviation is detected in real time, and the d-axis current reference value is generated by using the proportional integral link, so that the steady-state deviation is gradually eliminated through the cumulative effect of the error, and the long-term stability of the direct-current bus voltage is ensured. In the current inner loop control, the alternating-current current is converted into the d-axis component for closed-loop tracking, the d-axis voltage reference value is dynamically adjusted through the multi-stage proportional integral link, and then the phase voltage modulation signal is converted. This process decouples the voltage regulation and the current tracking into independent control dimensions, avoids the conflict of single control target, and simplifies the multi-variable coupling problem through coordinate transformation. In addition, the series design of the multi-stage control link enhances the anti-interference ability of the system to load mutation or power grid disturbance, and through the adaptive adjustment of the dynamic reference value, the adaptability of the converter in a wide operating condition range is significantly improved, which provides a basis for fine control of the flexible interconnection system.

[0097] As an optional implementation, the second waveform signal is determined according to the inter-loop current inter-phase voltage equalization process, comprising:

[0098] The capacitor voltage actual values of the MMC phase bridge arm sub-modules are acquired in real time, and the capacitor voltage average value is calculated and obtained, and each phase bridge arm inter-loop current reference value is obtained by subtracting the capacitor voltage average value from the capacitor voltage rated value through a fourth proportional integral link;

[0099] Real-time acquisition of the actual value of the upper and lower bridge arm current of each phase of the MMC, and calculation of the actual value of the bridge arm circulating current of each phase;

[0100] Subtracting the actual value of the bridge arm circulating current of each phase from the reference value of the bridge arm circulating current, obtaining the DC reference value of the bridge arm circulating current corresponding to each phase through a fifth proportional integral link, and obtaining the two-frequency AC reference value of the bridge arm circulating current corresponding to each phase through a resonance control link.

[0101] The present embodiment combines proportional integral and resonance control, solving the problems of response lag and poor frequency selectivity of traditional circulating current suppression methods. By real-time acquisition of the bridge arm sub-module capacitor voltage, calculation of its dynamic average value and comparison with the rated value, the proportional integral link is used to generate a circulating current reference value, actively adjusting the circulating current distribution of each phase bridge arm, and realizing the rapid balancing of the capacitor voltage. Further, by extracting the instantaneous deviation of the actual circulating current and the reference value, and introducing a resonance controller to specifically suppress the two-frequency component, the influence of high-frequency circulating current on system stability is effectively eliminated. This double-loop control structure not only retains the strong tracking ability of the proportional integral link to the DC component, but also precisely compensates the AC interference of a specific frequency band through resonance control, significantly reducing the additional loss and voltage fluctuation caused by circulating current. At the same time, through real-time decomposition and reconstruction of the bridge arm current, the long-term stability of the sub-module capacitor voltage under dynamic load is ensured, and the effectiveness of current control is improved, thereby improving the operation effect of the converter.

[0102] As an optional embodiment, the third waveform signal is determined according to a circulating current suppression process based on feedforward analysis, including:

[0103] Acquiring discrete current values of the upper and lower bridge arms of each phase, and performing integral processing on the discrete current values of the upper bridge arms of each phase to obtain upper bridge arm sub-module capacitor voltage theoretical values, and performing integral processing on the discrete current values of the lower bridge arms of each phase to obtain lower bridge arm sub-module capacitor voltage theoretical values, according to the sub-module capacitor values and the sampling period;

[0104] Calculating the root mean square of the upper bridge arm sub-module capacitor voltage theoretical values within the sampling period to obtain the effective value of the upper bridge arm sub-module capacitor voltage within the sampling period, and calculating the root mean square of the lower bridge arm sub-module capacitor voltage theoretical values within the sampling period to obtain the effective value of the lower bridge arm sub-module capacitor voltage within the sampling period;

[0105] Determining the switching function of each sub-module, the steady-state running switching function condition, and the phase switching function corresponding to each phase, calculating the upper and lower bridge arm switching functions of each phase, and calculating the average input frequency of the upper bridge arm sub-modules of each phase according to the upper bridge arm switching function of each phase, and calculating the average input frequency of the lower bridge arm sub-modules of each phase according to the lower bridge arm switching function of each phase;

[0106] According to the RMS of the capacitor voltage of the upper bridge arm sub-module, the RMS of the capacitor voltage of the lower bridge arm sub-module, the average input times of the upper bridge arm sub-module, the average input times of the lower bridge arm sub-module, and the voltage across the DC side, a RMS of a circulating current double-frequency component is calculated and obtained;

[0107] The actual value of the voltage across the inductor of the upper bridge arm and the actual value of the voltage across the inductor of the lower bridge arm are obtained, and according to the actual value of the voltage across the inductor of the upper bridge arm and the actual value of the voltage across the inductor of the lower bridge arm, the actual value of the voltage across the inductor of the upper bridge arm and the actual value of the voltage across the inductor of the lower bridge arm are calculated and obtained, and the actual value of the voltage across the inductor of the upper bridge arm and the actual value of the voltage across the inductor of the lower bridge arm are filtered to obtain a double-frequency component of the inductor voltage, and a RMS of the double-frequency component of the inductor voltage is obtained;

[0108] If the RMS of the circulating current double-frequency component is equal to the RMS of the double-frequency component of the inductor voltage, a preset value of a circulating current suppression compensation value is output as the third waveform signal, and if the RMS of the circulating current double-frequency component is less than the RMS of the double-frequency component of the inductor voltage, a circulating current suppression compensation value is calculated and obtained according to the RMS of the circulating current double-frequency component and the RMS of the double-frequency component of the inductor voltage, as the third waveform signal.

[0109] The present embodiment realizes active intervention of circulating current suppression and closed-loop correction of residual error through a feedforward analysis and dynamic prediction mechanism. First, the theoretical RMS of the sub-module capacitor voltage is derived according to a discretization method, and the amplitude characteristics of the circulating current double-frequency component are dynamically predicted in combination with a switching function to construct an accurate feedforward model of circulating current interference. By comparing the theoretical prediction value with the actual detection value, a circulating current suppression compensation signal is dynamically generated, and when the two match, a preset compensation amount is directly output to maintain a steady state. When there is a deviation, the error is dynamically corrected by real-time calculation of a compensation coefficient. This strategy combines the prediction advantage of the feedforward model with the error correction ability of the feedback control, predicting the circulating current trend in advance and eliminating the influence of model error or external disturbance through closed-loop compensation. In addition, by optimizing the switching strategy through statistical sub-module input times, electromagnetic interference and device loss caused by high-frequency switching can be reduced, providing further protection for long-term reliable operation of the system.

[0110] As an optional embodiment, the target waveform signal includes a modulation wave signal, and the target waveform signal is obtained by processing the first waveform signal, the second waveform signal, and the third waveform signal.

[0111] A waveform coefficient is determined according to the number of all sub-modules in the MMC;

[0112] A first calculation item is calculated and obtained according to the DC voltage reference value, the first waveform signal, and the waveform coefficient;

[0113] determining a second calculation item and a third calculation item according to the second waveform signal;

[0114] determining a fourth calculation item according to the third waveform signal;

[0115] calculating a modulation wave signal corresponding to each of the sub-modules according to the first calculation item, the second calculation item, the third calculation item and the fourth calculation item.

[0116] The embodiment significantly improves the generation accuracy and dynamic adaptability of the modulation wave signal through the multi-parameter fusion and hierarchical compensation mechanism. In the modulation wave calculation, the first calculation item divides the DC side voltage equally to each phase and dynamically couples with the AC side voltage reference value, ensuring real-time matching of the modulation wave amplitude and the system operating condition. The second and third calculation items respectively compensate for the circulating current 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, avoiding the increase in control complexity caused by multi-variable coupling, and achieving the coordinated optimization of each control target through linear superposition. The finally generated modulation wave signal has high precision and strong robustness, and can accurately guide the switching timing of the sub-modules, providing a high-quality reference signal for current tracking control.

[0117] As an optional embodiment, the calculation method of the first calculation item includes:

[0118] differencing half of the DC voltage reference value from the phase voltage reference value corresponding to each phase, and multiplying the waveform coefficient to obtain the first calculation item;

[0119] The second calculation item includes the opposite number of the bridge arm circulating current DC reference value corresponding to each phase, the third calculation item includes the opposite number of the bridge arm circulating current twice frequency AC reference value, and the fourth calculation item includes the circulating current suppression compensation value.

[0120] The calculation method of the modulation wave signal includes:

[0121] summing the first calculation item, the second calculation item, the third calculation item and the fourth calculation item as the modulation wave signal.

[0122] In the embodiment, the first calculation term relates the DC voltage to the AC reference value through the waveform coefficient, ensuring that the modulation wave fundamental component strictly matches the system demand, the second and third calculation terms cancel the interference of the circulating DC and high-frequency component on the modulation wave in a symmetrical form, and the precise compensation of the circulating current suppression is realized through the positive and negative superposition mechanism, and the fourth calculation term dynamically corrects the error introduced by the nonlinear characteristics of the system based on the feedforward model. This design converts the complex physical relationship into intuitive algebraic operation, reduces the resource consumption of real-time calculation, enhances the adjustability of the control system through the explainability of the parameters, ensures the continuity and stability of the modulation wave signal under dynamic working conditions, and provides a data basis for efficient switching of the sub-modules.

[0123] As an optional embodiment, the generating of the switching signal of each sub-module according to the target waveform signal comprises:

[0124] determining the number of sub-modules to be put into according to the modulation wave signal corresponding to each sub-module;

[0125] sorting each sub-module according to the difference between the capacitor voltage and the rated voltage of each sub-module, determining the target sub-module group and the input sequence of each sub-module in the target sub-module group.

[0126] The embodiment realizes the balanced use and life optimization of the sub-modules through the state monitoring and intelligent sorting strategy. After dynamically determining the number of sub-modules to be put into based on the modulation wave signal, the capacitor voltage of each sub-module is collected in real time and compared with the rated value, and the sub-module with voltage close to the rated value is preferentially put into, while the sub-module with large voltage deviation is temporarily used. This strategy makes the capacitor voltage of all sub-modules tend to be consistent in long-term operation through the dynamic sorting mechanism, avoids the accelerated aging of some sub-modules due to long-term overload, and reduces the generation of circulating current and harmonics through voltage balancing. At the same time, the optimization of the switching sequence significantly reduces the frequency and amplitude of the switching action, reduces the thermal stress and electromagnetic interference caused by frequent switching of power devices, improves the reliability and economy of the converter under the premise of ensuring the real-time performance of the system.

[0127] Please refer to Figure 3 , Figure 3 The overall control block diagram corresponding to the modular multilevel converter current control method based on parameter identification provided by an embodiment of the application is used to illustrate an example of the combination of each embodiment.

[0128] In Figure 3In the block diagram shown, the method corresponding to the embodiment of the application is presented in a modular control flow, including a DC bus voltage stabilization module based on voltage outer loop and current inner loop, a circulating current inter-phase voltage equalization module based on PI and PR control, a circulating current suppression module based on feedforward analytical method, and a modulation wave calculation module. The three-phase alternating current power system 1 is taken as an example to illustrate the current control strategy.

[0129] Specifically, the DC bus voltage stabilization module based on voltage outer loop and current inner loop includes the following steps:

[0130] (1) Real-time detection of the DC side voltage Udc, subtraction from the DC side voltage reference value Udc_ref, and the difference between the two is brought into the first stage PI controller to obtain the reference value id1_ref of the d-axis component of the three-phase alternating current power system 1 current, and the reference value iq1_ref of the q-axis component of the three-phase alternating current power system 1 current is given by scheduling instruction.

[0131] (2) Real-time detection of the three-phase alternating current power system 1 phase current ia1, ib1, ic1, and the actual values id1 and iq1 of the d-axis and q-axis components of the three-phase alternating current power system 1 current are obtained by abc / dq transformation.

[0132] (3) The actual value id1 of the d-axis component of the three-phase alternating current power system 1 current is subtracted from the reference value id1_ref, and the difference between the two is brought into the second stage PI controller to obtain the reference value ud1_ref of the d-axis component of the three-phase alternating current power system 1 voltage; similarly, the reference value uq1_ref of the q-axis component of the three-phase alternating current power system 1 voltage is obtained by the third stage PI controller.

[0133] (4) The first part of the three-phase alternating current power system 1 voltage modulation wave reference value ua_ref, ub_ref, uc_ref is obtained by dq / abc transformation of the reference value ud1_ref of the d-axis component and the reference value uq1_ref of the q-axis component of the three-phase alternating current power system 1 voltage.

[0134] The circulating current inter-phase voltage equalization module based on PI and PR control includes the following steps:

[0135] (1) Real-time detection of the actual value of the capacitor voltage of the i-th phase bridge arm sub-module of the three-phase MMC system, and calculation of the average value Uc_iave thereof;

[0136] (2) The average value Uc_iave of the capacitor voltage of the i-th phase bridge arm sub-module is subtracted from the rated value Uc of the sub-module capacitor voltage, and the difference between the two is brought into the fourth stage PI controller to obtain the reference value iciri_ref of the i-th phase bridge arm circulating current:

[0137]

[0138] Wherein, Kp4 and Ki4 are the proportional coefficient and integral coefficient of the fourth level PI controller respectively;

[0139] (3) Real-time detection of the actual value ii1 and ii2 of the upper and lower bridge arm current of the i phase, and calculation of the actual value iciri of the i phase bridge arm circulating current:

[0140]

[0141] (4) The actual value iciri of the i phase bridge arm circulating current and the reference value iciri_ref are subtracted, and the difference value of the two is sent into the fifth level PI controller and the PR controller respectively, wherein the resonance frequency of the PR controller is 2 times frequency, and the reference value uciri_dcref of the i phase bridge arm circulating current DC component and the reference value uciri_acref of the 2 times frequency AC component are obtained respectively:

[0142]

[0143]

[0144] Wherein, 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 resonance coefficient of the PR controller; w0 is the resonance frequency, equal to 2*wg1; wg1 is the frequency of the three-phase alternating current power system 1;

[0145] The circulating current suppression module based on the feedforward analytical method comprises the following execution steps:

[0146] (1) The discrete-time mathematical model of MMC can be derived, and the upper and lower bridge arm current values ii1 and ii2 can be represented as:

[0147]

[0148] Wherein, Iciri is the current value of the DC circulating current, which can be obtained dynamically by scheduling; Ii is the effective value of the three-phase alternating current power system 1 phase current, φ is the power factor angle of the power grid, Ts is the sampling period, and k is the sampling time.

[0149] (2) The theoretical value Uci1M_ref and Uci2M_ref of the upper and lower bridge arm capacitor voltage of each submodule can be obtained:

[0150]

[0151] Wherein, Csm is the capacitor value of the submodule.

[0152] (3) The theoretical effective value Uci1M_rms and Uci2M_rms of the capacitor voltage of each submodule of the upper and lower bridge arms in a cycle can be obtained:

[0153]

[0154] (4) The switching function of the mth submodule of the upper and lower bridge arms of the ith phase is set as

[0155]

[0156] wherein S is the working state of the submodule, S=1 represents that the submodule is put into operation, and S=0 represents that the submodule is cut off.

[0157] (5) When the MMC is in steady operation, all submodules need to satisfy:

[0158]

[0159] wherein ui is the voltage of one phase of the three-phase alternating current power system, which can be expressed as , and m is the modulation ratio.

[0160] (6) The switching function of the upper and lower bridge arms of the ith phase can be calculated as:

[0161]

[0162] wherein the switching function Si of the MMC of the ith phase is defined as:

[0163] (7) The average number of times of putting into operation of the submodule of the upper and lower bridge arms of the ith phase in a cycle can be obtained:

[0164]

[0165] (8) The effective value of the circulating current double frequency can be calculated as:

[0166]

[0167] (9) The actual value uLi1 and uLi2 of the voltage across the two bridge inductors in the bridge arm of the ith phase are detected in real time, the actual value uLi of the sum of the voltages across the two bridge inductors is calculated, the double frequency component uLi_2 is obtained by using a filter, and the effective value ULi_2rms of the double frequency component is obtained by using Fourier decomposition;

[0168] (10) The size relationship between Uciri_2rms and ULi_2rms is compared, if Uciri_2rms=ULi_2rms, no circulating current suppression compensation is needed, and if ULi_2rms>Uciri_2rms, circulating current suppression compensation is needed;

[0169]

[0170] The modulation wave calculation module integrates the above three aspects of information, and specifically includes the following two steps:

[0171] (1) Calculate the modulation wave ui1j_ref, ui2j_ref of the jth submodule voltage of the upper and lower bridge arms of the ith phase, respectively.

[0172]

[0173]

[0174] (2) The modulation wave of the jth submodule voltage of the upper and lower bridge arms of the ith phase adopts an NLM modulation strategy based on sorting to obtain the switching signals of each submodule of the upper and lower bridge arms.

[0175] The present application determines the corresponding exemplary parameter value scheme through actual simulation and experimental process. The voltage level of the target flexible interconnection system 1 and 2 is 10kV, the target flexible interconnection system power instruction can be obtained dynamically by scheduling, the DC side voltage is 20kV, the bridge arm inductance Lb_ij of the three-phase MMC system is 3mH, the number of each bridge arm half-bridge submodule M is 20, and the submodule 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, and wg1=2*50*π.

[0176] Figure 4 to Figure 8 The effect diagram of the corresponding parameter identification based modular multilevel converter current control method of the present application is used to illustrate the execution effect of the foregoing embodiments.

[0177] Wherein, Figure 4 to Figure 7 is the relevant waveform diagram of the system when using the PI and PR control circulating current interphase voltage sharing module, Figure 4 In the above, 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 In the above, the capacitor voltage of each submodule in the bridge arm fluctuates around the rated value, the maximum floating value does not exceed 1050V, the minimum value does not drop below 950V, and the fluctuation range is less than ±5%; Figure 6In the middle, 0.5s to 1s, without using the circulating current suppression module, the a1 phase bridge arm current is composed of a direct current component and an alternating current component, the direct current component is 68.44A, and the alternating current component is 156.22A; from 1s to 1.5s, due to the use of the circulating current suppression module based on the feedforward analytical method, the a1 phase bridge arm current alternating current component is reduced, which is consistent with the theoretical analysis; Figure 7 In the middle, the three-phase alternating current of the grid connection point is symmetrical, and the current peak value is 312.55A, which is consistent with the theoretical analysis, and it can be seen that the effect of the circulating current inter-phase voltage equalization module controlled by the PI and PR control is better.

[0178] Figure 8 For comparison of circulating current waveforms without using the circulating current suppression module and using the circulating current suppression module based on the feedforward analytical method, in the period of 0.5s to 1.5s, from 0.5s to 1s, the a1 phase circulating current without using the circulating current suppression module contains not only a direct current component, but also a large number of two-frequency harmonic fluctuations; from 1s to 1.5s, after suppression by the circulating current suppression module based on the feedforward analytical method, the amplitude of the a1 phase circulating current is greatly reduced, the a1 phase circulating current mainly contains a direct current component with a value of-68.44A and a small amount of high-order harmonics, and the circulating current suppression effect of the module is relatively rapid, which is consistent with the theoretical analysis.

[0179] The embodiment of the application also provides a modular multilevel converter current control device based on feedforward analysis, which is used for a target flexible interconnection system, the target flexible interconnection system comprising two groups of modular multilevel converters MMC connected back-to-back through a direct current bus and two groups of three-phase alternating current systems corresponding to the MMC, each of the MMC converters adopting a three-phase six-bridge-arm structure, each phase comprising upper and lower bridge arms, 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 to connect the corresponding three-phase alternating current system, and three-phase output ports of each of the three-phase alternating current systems being connected to each other;

[0180] The device comprises:

[0181] A determination module is configured to determine a first waveform signal according to a direct current bus voltage stabilization process;

[0182] The determination module is further configured to determine a second waveform signal according to a circulating current inter-phase voltage equalization process;

[0183] The determination module is further configured to determine a third waveform signal according to a circulating current suppression process based on feedforward analysis;

[0184] A processing module is configured to process a target waveform signal according to the first waveform signal, the second waveform signal and the third waveform signal, and generate a switching signal of each of the sub-modules according to the target waveform signal to control each of the sub-modules to be put into operation in a preset mode.

[0185] The target waveform signal is used to indicate the operating voltage corresponding to each of the sub-modules.

[0186] The specific manner in which the determination module determines the third waveform signal according to a loop current suppression procedure based on feedforward analysis comprises:

[0187] The discrete current values of the upper bridge arm and the discrete current values of the lower bridge arm of each phase are obtained, and the discrete current values of the upper bridge arm of each phase are integrated according to the sub-module capacitance value and the sampling period to obtain the theoretical value of the sub-module capacitance voltage of the upper bridge arm, and the discrete current values of the lower bridge arm of each phase are integrated to obtain the theoretical value of the sub-module capacitance voltage of the lower bridge arm.

[0188] The root mean square of the theoretical value of the sub-module capacitance voltage of the upper bridge arm in the sampling period is calculated to obtain the effective value of the sub-module capacitance voltage of the upper bridge arm in the sampling period, and the root mean square of the theoretical value of the sub-module capacitance voltage of the lower bridge arm in the sampling period is calculated to obtain the effective value of the sub-module capacitance voltage of the lower bridge arm in the sampling period.

[0189] The switching function of each sub-module, the steady-state operating switching function condition, and the phase switching function corresponding to each phase are determined, the switching function of the upper bridge arm and the switching function of the lower bridge arm of each phase are calculated, the average input frequency of the sub-module of the upper bridge arm of each phase is calculated according to the switching function of the upper bridge arm, and the average input frequency of the sub-module of the lower bridge arm of each phase is calculated according to the switching function of the lower bridge arm.

[0190] The effective value of the loop current double-frequency is calculated according to the effective value of the sub-module capacitance voltage of the upper bridge arm, the effective value of the sub-module capacitance voltage of the lower bridge arm, the average input frequency of the sub-module of the upper bridge arm, the average input frequency of the sub-module of the lower bridge arm, and the voltage across the DC side.

[0191] The actual value of the voltage across the inductance of the upper bridge arm and the actual value of the voltage across the inductance of the lower bridge arm of each phase are obtained, the actual value of the voltage across the inductance of the upper bridge arm and the actual value of the voltage across the inductance of the lower bridge arm of each phase are calculated to obtain the actual value of the voltage across the inductance of the upper and lower bridge arms, and the actual value of the voltage across the inductance of the upper and lower bridge arms is filtered to obtain the double-frequency component of the inductance voltage, and the effective value of the double-frequency component of the inductance voltage is obtained.

[0192] If the effective value of the loop current double-frequency is equal to the effective value of the double-frequency component of the inductance voltage, a preset value of the loop current suppression compensation value is output as the third waveform signal, and if the effective value of the loop current double-frequency is less than the effective value of the double-frequency component of the inductance voltage, the effective value of the loop current double-frequency and the effective value of the double-frequency component of the inductance voltage are calculated to obtain the loop current suppression compensation value as the third waveform signal.

[0193] In this embodiment, the dynamic response capability and operation stability of the flexible interconnection system are improved through the cooperative control mechanism. In a specific application scenario, the deviation of the DC side voltage from the reference value is monitored in real time based on the DC bus voltage stabilization process, a first waveform signal is dynamically generated through a proportional integral element, the DC side voltage is limited to stably fluctuate within a preset range, and the risk of system collapse caused by voltage instability is avoided. The circulating current inter-phase voltage equalization process calculates the dynamic deviation of the actual value of the bridge arm sub-module capacitor voltage 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 compares the theoretical modeling with the actual circulating current component in real time, generates a third waveform signal to compensate for high-frequency circulating current interference, and significantly reduces the additional loss caused by the coupling effect between the bridge arms. In the modulation wave generation process, the information corresponding to the above-mentioned signals is integrated, and the switching state of the sub-module is accurately controlled, so that the system realizes rapid tracking of the current and optimization of the power quality under complex working conditions. Through the combination of closed-loop regulation and feedforward compensation, the real-time and predictive nature of the control is taken into account, providing an important foundation for the reliable operation of high-voltage and large-capacity converters.

[0194] On the device side, other embodiments can be implemented based on a determination module and a processing module, and specific reference can be made to the related description of the method side, which will not be repeated here.

[0195] It should be noted that the division of each module of the above device is only a logical functional division, and all or part of it can be integrated into a physical entity, or physically separated. These modules can all be implemented in the form of software called by a processing element; they can all be implemented in the form of hardware; 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 form of program code in the memory of the above device, and the function of the above determination module can be called and executed by a processing element of the above device. The implementation of other modules is similar. In addition, all or part of these modules can be integrated together or independently implemented. The processing element here can be an integrated circuit with signal processing capability. In the implementation process, each step of the above method or each module can be completed by the integrated logic circuit of hardware or the instruction of software in the processing element.

[0196] Schematically, as Figure 9 shown, Figure 9 an internal structure schematic diagram of a computer device provided by an embodiment of the present application, the computer device 300 can be provided as a server. Referring to Figure 9The computer device 300 comprises a processing assembly 302, further comprising one or more processors, and a memory resource represented by the memory 301 for storing instructions, such as application programs, executable by the processing assembly 302. The application programs stored in the memory 301 can comprise one or more than one module each corresponding to a set of instructions. In addition, the processing assembly 302 is configured to execute the instructions to perform the method of any of the embodiments described above.

[0197] The computer device 300 can further comprise a power supply assembly 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 can operate based on an operating system stored in the memory 301, such as Windows Server TM, Mac OS X TM, Unix TM, Linux TM, Free BSD TM or the like.

[0198] Those skilled in the art can understand that Figure 9 The structure shown in the figure is only a block diagram of part of the structure related to the scheme of the present application, and does not constitute a limitation on the computer device to which the scheme of the present application is applied. The specific computer device can comprise more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0199] The embodiment of the present application provides a storage medium, the storage medium stores computer readable instructions, and the computer readable instructions are executed by one or more processors to make the one or more processors execute the method provided in any of the embodiments.

[0200] Finally, it should also be noted that in this document, the relationship terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device. Without more limitations, the element defined by the statement "including a" does not exclude the presence of another identical element in the process, method, article or device including the element.

[0201] The various embodiments described in this specification are intended to be combinable unless otherwise indicated herein. The various embodiments described in this specification are described in the progressions noted, with each embodiment emphasizing different aspects over others, and the various embodiments can be combined as desired, with reference to each other as appropriate.

[0202] The above description of disclosed embodiments is intended to be illustrative and not restrictive. Many modifications of these embodiments by one having ordinary skill in the art, using the principles and novel features disclosed herein, will be within the scope of the application. The scope of the application, therefore, is not to be limited to the above described embodiments but is to be accorded the widest scope consistent with the principles and novel features described herein.

Claims

1. A feed-forward resolution based modular multilevel converter current control method, characterized in that, The method is used for a target flexible interconnection system, the target flexible interconnection system comprising two groups of modular multilevel converters (MMC) connected back-to-back through a DC bus and two groups of three-phase AC systems corresponding to the MMC, each MMC converter adopting a three-phase six-bridge-arm structure, each phase comprising upper and lower bridge arms, 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 to connect a corresponding three-phase AC system, and three-phase output ports of each three-phase AC system being connected to each other; The method comprises: determining a first waveform signal according to a DC bus voltage stabilization process; determining a second waveform signal according to a circulating current inter-phase voltage equalization process; determining a third waveform signal according to a circulating current suppression process based on feedforward analysis; processing to obtain a target waveform signal according to the first waveform signal, the second waveform signal and the third waveform signal, and generating a switching signal of each sub-module according to the target waveform signal to control each sub-module to be put into operation according to a preset mode; wherein the target waveform signal is used to indicate an operating voltage corresponding to each sub-module; wherein the determination of the third waveform signal according to the circulating current suppression process based on feedforward analysis comprises: obtaining discrete current values of upper bridge arms and discrete current values of lower bridge arms of each phase, and performing integral processing on the discrete current values of the upper bridge arms of each phase to obtain upper bridge arm sub-module capacitor voltage theoretical values according to sub-module capacitor values and a sampling period, and performing integral processing on the discrete current values of the lower bridge arms of each phase to obtain lower bridge arm sub-module capacitor voltage theoretical values according to the sub-module capacitor values and the sampling period; calculating a root mean square of the upper bridge arm sub-module capacitor voltage theoretical values in the sampling period to obtain an effective value of the upper bridge arm sub-module capacitor voltage in the sampling period, and calculating a root mean square of the lower bridge arm sub-module capacitor voltage theoretical values in the sampling period to obtain an effective value of the lower bridge arm sub-module capacitor voltage in the sampling period; determining a switching function of each sub-module, a steady-state operation switching function condition and a phase switching function corresponding to each phase, calculating to obtain upper bridge arm switching functions and lower bridge arm switching functions of each phase, and calculating to obtain an average number of times of putting into operation of upper bridge arm sub-modules of each phase according to the upper bridge arm switching functions of each phase, and calculating to obtain an average number of times of putting into operation of lower bridge arm sub-modules of each phase according to the lower bridge arm switching functions of each phase; calculating to obtain a circulating current double-frequency effective value according to the effective values of the upper bridge arm sub-module capacitor voltage, the effective values of the lower bridge arm sub-module capacitor voltage, the average number of times of putting into operation of the upper bridge arm sub-modules, the average number of times of putting into operation of the lower bridge arm sub-modules and a voltage across the DC side; obtaining actual values of voltages across upper bridge arm inductors and actual values of voltages across lower bridge arm inductors of each phase, calculating to obtain actual values of upper and lower bridge arm inductor voltages according to the actual values of the voltages across the upper bridge arm inductors and the actual values of the voltages across the lower bridge arm inductors of each phase, and performing filtering processing on the actual values of the upper and lower bridge arm inductor voltages to obtain inductor voltage double-frequency components and obtain an inductor voltage double-frequency effective value. If the ring current double frequency effective value is equal to the inductance voltage double frequency effective value, outputting a preset value of the ring current suppression compensation value as the third waveform signal; if the ring current double frequency effective value is less than the inductance voltage double frequency effective value, calculating a ring current suppression compensation value according to the ring current double frequency effective value and the inductance voltage double frequency effective value as the third waveform signal.

2. The method of claim 1, wherein, The first waveform signal is determined according to the DC bus voltage stabilization process, and the second waveform signal is determined according to the ring current inter-phase voltage equalization process. The d-axis current reference value is obtained by subtracting the preset DC voltage reference value from the DC side voltage and then passing through a first proportional integral link; The d-axis current actual value and the q-axis current actual value are obtained by converting the AC currents of the MMC phases through abc / dq conversion; The d-axis voltage reference value is obtained by subtracting the d-axis current actual value from the d-axis current reference value and then passing through a second proportional integral link, and the q-axis voltage reference value is obtained by subtracting the q-axis current actual value from the preset q-axis current reference value and then passing through a third proportional integral link; The d-axis current reference value and the q-axis voltage reference value are converted into phase voltage reference values corresponding to each phase through dq / abc conversion, serving as the first waveform signal.

3. The method of claim 2, wherein, The second waveform signal is determined according to the ring current inter-phase voltage equalization process, and the third waveform signal is determined according to the ring current suppression process. The capacitor voltage average value is obtained by real-time acquisition of the capacitor voltage actual value of each phase bridge arm sub-module of the MMC, and the bridge arm ring current reference value is obtained by subtracting the capacitor voltage average value from the capacitor voltage rated value and then passing through a fourth proportional integral link; The bridge arm ring current actual value of each phase is obtained by real-time acquisition of the upper and lower bridge arm current actual values of each phase of the MMC; The bridge arm ring current DC reference value corresponding to each phase is obtained by subtracting the bridge arm ring current actual value from the bridge arm ring current reference value and then passing through a fifth proportional integral link, and the bridge arm ring current double frequency AC reference value corresponding to each phase is obtained through a resonance control link.

4. The method of claim 3, wherein, The target waveform signal includes a modulation wave signal, and the target waveform signal is obtained by processing the first waveform signal, the second waveform signal and the third waveform signal. The waveform coefficient is determined according to the number of all sub-modules in the MMC; The first calculation item is obtained by calculation according to the DC voltage reference value, the first waveform signal and the waveform coefficient; The second calculation item and the third calculation item are determined according to the second waveform signal; The fourth calculation item is determined according to the third waveform signal; The modulation wave signal corresponding to each sub-module is obtained by calculation according to the first calculation item, the second calculation item, the third calculation item and the fourth calculation item.

5. The method of claim 4, wherein, The first calculation item is obtained by calculation according to the DC voltage reference value, the first waveform signal and the waveform coefficient; The second calculation item includes the opposite number of the bridge arm ring current DC reference value corresponding to each phase, the third calculation item includes the opposite number of the bridge arm ring current double frequency AC reference value, and the fourth calculation item includes the ring current suppression compensation value. ​ And, the calculation manner of the modulation wave signal comprises: Summing the first calculation item, the second calculation item, the third calculation item and the fourth calculation item as the modulation wave signal.

6. The method of claim 4, wherein, The generation of the switch signal of each sub-module according to the target waveform signal comprises: Determining the number of sub-modules to be put in according to the modulation wave signal corresponding to each sub-module; According to the difference between the capacitor voltage and the rated voltage of each sub-module, the sub-modules are sorted to determine the target sub-module group and the input sequence of each sub-module in the target sub-module group.

7. A feed-forward analytical based modular multilevel converter current control apparatus, characterized by, The device is used for a target flexible interconnection system, the target flexible interconnection system comprising two groups of modular multilevel converters (MMC) connected back-to-back through a direct current bus and two groups of three-phase alternating current systems corresponding to the MMC, each MMC converter adopting a three-phase six-bridge-arm structure, each phase comprising upper and lower bridge arms, 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 to connect the corresponding three-phase alternating current system, and the three-phase output ports of each three-phase alternating current system being connected to each other. The device comprises: A determination module configured to determine a first waveform signal according to a direct current bus voltage stabilization process; The determination module is further configured to determine a second waveform signal according to a circulating current inter-phase voltage equalization process; The determination 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 process a target waveform signal according to the first waveform signal, the second waveform signal and the third waveform signal, and generate a switch signal of each sub-module according to the target waveform signal to control each sub-module to be put into operation in a preset manner; The target waveform signal is used to indicate the operating voltage corresponding to each sub-module; The specific manner of determining the third waveform signal according to the circulating current suppression process based on feedforward analysis comprises: Obtaining discrete current values of the upper bridge arms and the lower bridge arms of each phase, and integrating the discrete current values of the upper bridge arms of each phase to obtain upper bridge arm sub-module capacitor voltage theoretical values according to sub-module capacitor values and a sampling period, and integrating the discrete current values of the lower bridge arms of each phase to obtain lower bridge arm sub-module capacitor voltage theoretical values according to the sub-module capacitor values and the sampling period; Calculating the root mean square of the upper bridge arm sub-module capacitor voltage theoretical values in the sampling period to obtain the effective value of the upper bridge arm sub-module capacitor voltage in the sampling period, and calculating the root mean square of the lower bridge arm sub-module capacitor voltage theoretical values in the sampling period to obtain the effective value of the lower bridge arm sub-module capacitor voltage in the sampling period; Determining the switch function of each sub-module, the steady-state operating switch function condition and the phase switch function corresponding to each phase, calculating the upper bridge arm switch function and the lower bridge arm switch function of each phase, and calculating the average number of times of putting in the upper bridge arm sub-modules of each phase according to the upper bridge arm switch function of each phase, and calculating the average number of times of putting in the lower bridge arm sub-modules of each phase according to the lower bridge arm switch function of each phase. According to the RMS of the capacitor voltage of the upper bridge arm sub-module, the RMS of the capacitor voltage of the lower bridge arm sub-module, the average input times of the upper bridge arm sub-module, the average input times of the lower bridge arm sub-module, and the voltage across the DC side, the RMS of the circulating current double frequency is obtained; The actual value of the voltage across the inductor of the upper bridge arm and the actual value of the voltage across the inductor of the lower bridge arm are obtained, and according to the actual value of the voltage across the inductor of the upper bridge arm and the actual value of the voltage across the inductor of the lower bridge arm, the actual value of the voltage across the inductor of the upper and lower bridge arms is obtained, and the actual value of the voltage across the inductor of the upper and lower bridge arms is filtered to obtain the double frequency component of the inductor voltage, and the RMS of the double frequency component of the inductor voltage is obtained. If the RMS of the circulating current double frequency is equal to the RMS of the double frequency component of the inductor voltage, a preset value of the circulating current suppression compensation value is output as the third waveform signal, and if the RMS of the circulating current double frequency is less than the RMS of the double frequency component of the inductor voltage, the circulating current suppression compensation value is obtained according to the RMS of the circulating current double frequency and the RMS of the double frequency component of the inductor voltage, and the third waveform signal is obtained.

8. A computer device, comprising: The storage medium stores computer readable instructions, and the computer readable instructions are executed by one or more processors to make the one or more processors execute the steps of the method of any one of claims 1-6.

9. A storage medium, characterized by The storage medium stores computer readable instructions, and the computer readable instructions are executed by one or more processors to make the one or more processors execute the steps of the method of any one of claims 1-6.

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