Grid-connected converter cluster model prediction dual-mode control method and system

Abstract

The invention belongs to the technical field of renewable energy source grid connection, and provides a grid-connected converter cluster model prediction dual-mode control method and a grid-connected converter cluster model prediction dual-mode control system, which overcome the influence of line impedance difference by introducing a distributed compensation mechanism into a traditional control architecture, realize accurate reactive power equalization among converters in a network construction mode, and improve the reliability of the system. The hidden danger of power oscillation is eliminated; limited control set model prediction control is adopted to replace a traditional PI controller to serve as a unified inner ring, the smoothness and safety of the mode switching process are effectively improved, a complete cluster control framework is constructed by integrating collaborative switching, power coordination and rapid inner ring control, and the operation stability of the system under wide-range power grid impedance changes is comprehensively improved.

Description

A Model Predictive Dual-Mode Control Method and System for Grid-Connected Converter Clusters Technical Field

[0001] This invention belongs to the field of renewable energy grid connection technology, specifically relating to a model prediction dual-mode control method and system for grid-connected converter clusters. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] As the proportion of renewable energy generation, such as wind and solar power, in the power system continues to increase, their operational stability is crucial to grid security. Currently, the mainstream control strategies for grid-connected converters include grid-following (GFL) control and grid-forming (GFM) control. With the increasing penetration rate of intermittent renewable energy sources such as wind and solar power, the equivalent impedance of the grid exhibits wide-range fluctuations, and the system short-circuit ratio decreases significantly, creating a weak or even extremely weak grid operating environment. This problem intensifies the dynamic interaction between grid-connected converters and the grid, sharply increasing the risk of system instability and posing a severe challenge to maintaining the stable and efficient operation of converter clusters. Therefore, it is urgent to study advanced converter control methods that can adapt to large fluctuations in grid impedance to ensure the safe and stable operation of grid-connected systems with a high proportion of renewable energy.

[0004] Currently, the industry's strategies for improving converter stability mainly follow two technical paths. The first path focuses on optimization within a single control mode. For example, for grid-based control, it widens the stability boundary under small-signal models by reshaping the phase-locked loop or introducing active damping; or for grid-based control, it designs transient energy dissipation methods under large disturbances based on Lyapunov stability theory.

[0005] The second approach is a grid impedance adaptive dual-mode switching strategy that combines grid tracking and grid construction. The core of this approach lies in real-time online identification of the grid impedance or short-circuit ratio (e.g., by injecting disturbance signals or utilizing the system's inherent harmonics and employing algorithms such as recursive least squares for estimation), and then dynamically switching the operating mode based on the identification results: when the system short-circuit ratio is high and the grid is determined to be strong, the converter operates in grid tracking mode to pursue optimal power point tracking accuracy; when the short-circuit ratio is lower than a set threshold and the grid is determined to be weak, it switches to grid construction mode to provide voltage and frequency support for the system. This strategy achieves a preliminary match between the operating mode and grid conditions.

[0006] However, when dealing with multi-converter parallel cluster systems, existing solutions are entirely based on single-machine control. Their design only considers the interaction between a single converter and the grid, lacking coordinated planning for "grid switching" at the multi-machine system level. When multiple independently operating dual-mode control converters are connected in parallel, their judgments of the grid state and mode decisions are decentralized and asynchronous. This lack of coordinated autonomous behavior is itself a potential cause of system instability. Furthermore, existing solutions lack effective cluster power coordination strategies. Especially after switching to grid-connected mode, traditional droop control struggles to achieve precise reactive power distribution due to differences in line parameters, affecting the system's steady-state performance and voltage quality. The traditional PI controllers commonly used in their inner loops are limited by bandwidth, resulting in insufficient response speed and anti-interference capabilities during the dynamic process of mode switching. Summary of the Invention

[0007] To address the aforementioned problems, this invention proposes a model predictive dual-mode control method and system for grid-connected converter clusters. By using finite control set model predictive control as a unified inner loop and combining it with a cluster reactive power sharing mechanism, this invention effectively solves the challenges of coordinated control and power allocation during dual-mode switching in multi-machine systems, significantly improving the dynamic performance and operational stability of the system.

[0008] According to some embodiments, the present invention adopts the following technical solution: a dual-mode control method for predicting grid-connected converter cluster models, comprising the following steps: performing inner-loop control of converter current: establishing a discrete converter model for prediction, transforming the predicted current to a dq rotating coordinate system through Park transformation, traversing all possible switching states of the converter, calculating the corresponding predicted current for each state, calculating the cost function based on the predicted current, selecting the switching state that minimizes the cost function value, and applying it to the next control cycle; performing follow / grid mode switching control: acquiring the voltage and current signals of the common connection point, determining the grid strength, and when the grid strength is lower than a preset threshold, switching at least some converters to grid-connected mode; when the grid strength rises to a higher threshold, switching at least some converters to follow mode, and after mode switching, synchronizing the converter internal state with the target mode; performing reactive power compensation and voltage recovery control in grid-connected mode: comparing the reactive power output of each converter with the cluster average value, generating a compensation signal to eliminate distribution errors; generating a voltage recovery signal based on the deviation between the voltage of each node and the cluster average value to maintain the system voltage within the allowable range.

[0009] As an alternative implementation method, the converter current inner loop control process is performed, which is applicable to both grid-following control and grid-connected control modes.

[0010] As an alternative implementation, the process of establishing a discrete converter model for prediction includes: calculating the predicted current at the next sampling time based on the inductor voltage equation in the αβ stationary coordinate system. i1 αβ (k+1) represent the components of the converter output measured at the predicted next time step (time k+1) in the α and β coordinates, respectively, i1 αβ (k) represents the components of the converter output measured at time k in the α and β coordinates, respectively. f L is the equivalent series resistance of the filter inductor. f T is the value of the filter inductance. s For the sampling period of the control system, u αβ (k) represents the components of the converter-side output voltage measured at time k in the α and β coordinates, respectively. c αβ (k) represents the components of the grid-side voltage or point of common coupling voltage measured at time k in the α and β coordinates.

[0011] As an alternative implementation, the process of calculating the cost function based on the predicted current includes: the cost function is: ,in, This is the reference value for the d-axis current on the grid side. This is the predicted value of the d-axis current on the grid side. This is the reference value for the q-axis current on the grid side. This is the predicted value of the q-axis current on the grid side.

[0012] As an alternative implementation, the process of obtaining the voltage and current signals at the point of common coupling (PCC) and determining the grid strength includes obtaining the PCC voltage and current signals, calculating the system's equivalent short-circuit ratio, and using this as a criterion for grid strength. The calculation formula is as follows: Among them, S sc For the system short-circuit capacity, P rated This refers to the rated power of the converter cluster.

[0013] As an alternative implementation, after mode switching, the process of synchronizing the converter's internal state with the target mode includes phase reference pre-synchronization, voltage reference pre-synchronization, and current reference pre-synchronization. When the switching signal changes from grid-following type to grid-building type, the system's inner-loop reference signal changes from the output signal of the grid-following controller to the output signal of the grid-building controller after pre-synchronization. When the switching signal changes from grid-building type to grid-following type, the system's inner-loop reference signal changes from the output signal of the grid-building controller to the output signal of the grid-following controller after pre-synchronization, thus achieving shock-free mode switching.

[0014] As a further limitation, the phase reference pre-synchronization is achieved by adjusting the angular frequency of the virtual synchronizer, and the pre-synchronization phase is obtained by integrating the angular velocity: ;ω GFM θ is the original angular frequency of the virtual synchronous generator. grid θ represents the actual phase of the grid voltage. GFM For the original phase of the virtual synchronous generator, ω GFM_pre θ is the angular frequency of the virtual synchronous generator after pre-synchronization adjustment. GFM_pre This refers to the phase of the virtual synchronous generator after pre-synchronization.

[0015] As a further defined implementation, the voltage reference pre-synchronization stage is as follows: ;u oGFM u is the original output voltage reference value of the virtual synchronous generator. ogrid The actual measured value of the grid voltage, u oGFMpre The output voltage reference value is the pre-synchronized value; the current reference pre-synchronization circuit is as follows: i2 dq For the dq-axis component of the grid-side current, u o dq Let u be the dq-axis component of the grid voltage. oGFMpre dq For the dq-axis components of the pre-synchronization voltage, ω o i is the rated angular frequency. 1GFMpre dq The dq-axis component of the inner loop current reference quantity after pre-synchronization.

[0016] As an alternative implementation method, the process of reactive power compensation and voltage recovery control in grid-connected mode includes: the output voltage reference value of each converter is composed of the traditional droop characteristic and the compensation signal. U oi Let be the output voltage of the i-th converter. U oi n is the output voltage reference of the converter. i Let Q be the reactive power droop factor of the i-th converter. i For the output of each converter, δ u It is a composite compensation signal;

[0017] δ qc For reactive power sharing signal, δ ur For voltage recovery signal, Q k For the output of each converter, E k Let n be the output voltage of each converter, and n be the number of converters.

[0018] A grid-connected converter cluster model prediction dual-mode control system includes: a converter current inner loop control module, configured to establish a discrete converter model for prediction, transform the predicted current to a dq rotating coordinate system through Park transformation, traverse all possible switching states of the converter, calculate the corresponding predicted current for each state, calculate a cost function based on the predicted current, select the switching state that minimizes the cost function value, and apply it to the next control cycle; a follow / grid mode switching control module, configured to acquire voltage and current signals at the point of common coupling, determine the grid strength, and switch at least some converters to grid-connected mode when the grid strength is lower than a preset threshold; when the grid strength rises to a higher threshold, switch at least some converters to follow-grid mode, and perform synchronous adjustment of the converter internal state with the target mode after mode switching; and a reactive power compensation and voltage recovery control module in grid-connected mode, configured to compare the reactive power output of each converter with the cluster average value, generate a compensation signal to eliminate distribution errors, and generate a voltage recovery signal based on the deviation between the voltage of each node and the cluster average value to maintain the system voltage within the allowable range.

[0019] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention establishes a cluster collaborative decision-making mechanism to ensure that all converters perform synchronous mode switching based on a unified grid state assessment, fundamentally avoiding the risk of system instability caused by decentralized decision-making; simultaneously, it designs a cluster power coordination strategy deeply coupled with dual-mode operation, overcoming the influence of line impedance differences by introducing a distributed compensation mechanism into the traditional control architecture, achieving precise reactive power sharing among converters in the grid configuration mode, thereby eliminating the hidden danger of power oscillation; furthermore, it adopts finite control set model predictive control to replace the traditional PI controller as a unified inner loop, utilizing its fast dynamic response and precise constraint handling capabilities to effectively improve the smoothness and safety of the mode switching process. Finally, by integrating collaborative switching, power coordination, and fast inner loop control, a complete cluster control architecture is constructed, comprehensively improving the system's operational stability under a wide range of grid impedance variations.

[0020] This invention enables cooperative mode switching of converter clusters, improving system stability; solves the problem of difficult reactive power distribution in network mode, achieving precise reactive power allocation; adopts model predictive control as a unified inner loop, improving the dynamic performance of the system; and constructs a complete cluster control architecture, enhancing the system's adaptability and robustness.

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0023] Figure 1 is a control block diagram of a traditional grid-connected converter; Figure 2 is a control block diagram of a traditional grid-connected converter; Figure 3 is a topology diagram of a grid-connected converter cluster circuit according to an embodiment; Figure 4 is a flowchart of the finite set model predictive control principle according to an embodiment; Figure 5 is a logic block diagram of pre-synchronization and mode switching signals according to an embodiment; Figure 6 is a block diagram of an improved reactive power sharing and voltage recovery control according to an embodiment. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0025] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0026] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0027] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0028] Example 1: As described in the background section, with the increasing penetration of renewable energy in power systems, power generation clusters consisting of multiple grid-connected converters are becoming increasingly common. Under weak grid conditions with large fluctuations in grid impedance, the stable operation of such clusters faces severe challenges: First, traditional grid-following control modes based on phase-locked loops are prone to instability under weak grid conditions; second, it is difficult to achieve precise reactive power sharing among multiple converters, leading to circulating currents and system oscillations; third, existing control methods lack adaptive mode switching capabilities when grid conditions change, resulting in slow dynamic response.

[0029] To address the aforementioned issues, this embodiment proposes a grid-connected converter cluster model predictive dual-mode control method oriented towards active grid support. By using finite control set model predictive control as a unified inner loop and combining it with a cluster reactive power sharing mechanism, this method effectively solves the challenges of coordinated control and power allocation during dual-mode switching in multi-machine systems, significantly improving the dynamic performance and operational stability of the system.

[0030] The following is a detailed introduction.

[0031] Figures 1 and 2 show the converter models and control equations for grid-connected and grid-connected modes, respectively. The grid-connected converter is connected to the grid via an LC filter. Based on the power calculation formula in dq coordinates, we can obtain the instantaneous power injected by the converter to the point of common coupling (PCC): (1) For grid-connected control mode, the controller synchronizes with the power grid via a phase-locked loop and acts as a controlled current source. Its dq-axis current reference value is directly calculated from the active and reactive power commands: (2) P in the above formula and Q These are the reference values ​​for the active and reactive power outputs of the converter, i. d and i q These are the reference values ​​for the inner current loop. In particular, the phase-locked loop can achieve precise synchronization in steady state, making u... c q =0, the above expression can be simplified to: (3) After passing through the current inner loop control based on model predictive control, the grid-connected converter can be controlled.

[0032] For grid-type control mode, phase-locked loops are not used to orient the grid voltage to achieve power angle tracking. Instead, virtual synchronous machine technology is used to simulate the rotor motion equation of a synchronous generator to establish the frequency and phase of the converter output voltage. The replication of the output voltage is regulated through reactive power-voltage droop control, as shown in the following expression: (4) Where J is the virtual inertia, D is the damping coefficient, and K p and K q These are the active and reactive power droop coefficients, respectively. The resulting voltage amplitude E and phase θ serve as reference commands for the voltage outer loop and the converter phase angle.

[0033] Compared to the grid control mode, the grid control mode adds a voltage outer loop, using the voltage signal generated by the droop control as the input reference command to generate the reference command for the current inner loop.

[0034] The converter current inner-loop control framework based on model predictive control employs finite control set model predictive control as a unified inner-loop current controller, applicable to both grid-following and grid-connected control modes. First, a discretized converter model for prediction is established. In the αβ stationary coordinate system, based on the inductor voltage equation, the predicted current at the next sampling time can be derived as: (5)i1αβ (k+1) represent the components of the converter output measured at the predicted next time step (time k+1) in the α and β coordinates, respectively, i1 αβ (k) represents the components of the converter output measured at time k in the α and β coordinates, respectively. f L is the equivalent series resistance of the filter inductor. f T is the value of the filter inductance. s For the sampling period of the control system, u αβ (k) represents the components of the converter-side output voltage measured at time k in the α and β coordinates, respectively. c αβ (k) represents the components of the grid-side voltage (or point of common coupling voltage) measured at time k in the α and β coordinates.

[0035] By using the Park transformation, the predicted current is converted to the dq rotating coordinate system to obtain i1. dq (k+1), then the controller conveniently calculates the predicted current for each of the eight possible switching states of the converter, and evaluates it using the following cost function: (6) Finally, the switching state that minimizes the cost function g is selected and directly applied to the next control cycle. This method eliminates the need for a traditional PWM modulator, achieving fast and accurate current tracking. Figure 4 also illustrates in detail all the steps involved in implementing the inner-loop predictive control of the current.

[0036] The adaptive grid impedance-based mode switching strategy coordinates the synchronous operation mode switching of the entire converter cluster by real-time assessment of grid strength. First, the system calculates the equivalent short-circuit ratio (SCR) by real-time monitoring of the voltage and current signals at the point of common coupling, using this as a criterion for grid strength. The calculation formula is as follows: (7) Wherein, S sc For the system short-circuit capacity, P rated This refers to the rated power of the converter cluster.

[0037] For a single-unit converter, when the calculated SCR is lower than a preset threshold, the cluster control unit generates a unified "switch to grid-connected mode" command; when the SCR rises to a higher threshold, it generates a "switch to grid-following mode" command. The logic for switching commands for multi-unit converters is similar (taking a three-converter unit as an example): (8) Upon receiving the switching command, the system initiates a pre-synchronization process. This process synchronizes the converter's internal state with the target mode through the following adjustments: phase reference pre-synchronization is achieved by adjusting the angular frequency of the virtual synchronizer, and the pre-synchronization phase is obtained by integrating the angular velocity. (9)ω GFMθ is the original angular frequency of the virtual synchronous generator. grid The actual phase of the grid voltage (measured via PLL), θ GFM For the original phase of the virtual synchronous generator, ω GFM_pre θ is the angular frequency of the virtual synchronous generator after pre-synchronization adjustment. GFM_pre This refers to the phase of the virtual synchronous generator after pre-synchronization.

[0038] The voltage reference pre-synchronization process is as follows: (10)u oGFM u is the original output voltage reference value of the virtual synchronous generator. ogrid The actual measured value of the grid voltage, u oGFMpre This is the reference value for the output voltage after pre-synchronization.

[0039] The current reference pre-synchronization element is as follows: (11)i2 dq For the dq-axis component of the grid-side current, u o dq Let u be the dq-axis component of the grid voltage. oGFMpre dq For the dq-axis components of the pre-synchronization voltage, ω o i is the rated angular frequency. 1GFMpre dq The dq-axis component of the inner loop current reference quantity after pre-synchronization.

[0040] When the switching signal is switched from the tracking network type to the network type, the switching signal controller will change the system inner loop reference signal from the output i of the tracking network controller. 1GFL θ PLL Switching to the pre-synchronized network controller output i 1GFMpre θ GFMpre This achieves a shockless mode switching. The same mechanism is used in the reverse switching process to ensure that it is consistent with the grid status before the switch, thus achieving a truly seamless mode switching.

[0041] To achieve accurate reactive power distribution and voltage stability of converter clusters in grid-connected mode, considering the power compensation mechanism of reactive power compensation and voltage recovery, this invention adopts a composite compensation strategy. The output voltage reference value of each converter is composed of traditional droop characteristics and compensation signals. (12)U oi Let be the output voltage of the i-th converter. U oi n is the output voltage reference of the converter. i Let Q be the reactive power droop factor of the i-th converter. i For the output of each converter, δ u It is a composite compensation signal.

[0042] (13)δ qc For reactive power sharing signal, δ ur For voltage recovery signal, Q k For the output of each converter, E k Let n be the output voltage of each converter, and n be the number of converters.

[0043] This control structure compares the reactive power output Q of each converter in real time. i The compensation signal δ is generated by combining the cluster average value. qc To eliminate allocation errors; and simultaneously monitor the voltage E of each node. i The deviation from the cluster average, expressed by δ ur The system voltage is maintained within the allowable range. The synergistic effect of the two compensation terms ensures accurate reactive power distribution while avoiding voltage instability caused by overcompensation.

[0044] Example 2: A grid-connected converter cluster model prediction dual-mode control system includes: a converter current inner loop control module, configured to establish a discrete converter model for prediction, transform the predicted current to a dq rotating coordinate system through Park transformation, traverse all possible switching states of the converter, calculate the corresponding predicted current for each state, calculate a cost function based on the predicted current, select the switching state that minimizes the cost function value, and apply it to the next control cycle; a follow / grid mode switching control module, configured to acquire the voltage and current signals of the common coupling point, determine the grid strength, and switch at least some converters to grid mode when the grid strength is lower than a preset threshold; when the grid strength rises to a higher threshold, switch at least some converters to follow mode, and perform synchronous adjustment of the converter internal state with the target mode after mode switching; and a reactive power compensation and voltage recovery control module in grid mode, configured to compare the reactive power output of each converter with the cluster average value, generate a compensation signal to eliminate distribution errors, and generate a voltage recovery signal based on the deviation between the voltage of each node and the cluster average value to maintain the system voltage within the allowable range.

[0045] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0046] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in one or more blocks of the flowchart illustrations and / or one or more blocks of the block diagrams.

[0047] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means that implement the functions specified in one or more flowcharts and / or one or more block diagrams.

[0048] These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, such that the instructions, which execute on the computer or other programmable apparatus, provide steps for implementing the functions specified in one or more flowcharts and / or one or more block diagrams.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art without creative effort within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A model predictive dual-mode control method for grid-connected converter clusters, characterized in that, Includes the following steps: Inner-loop control of converter current: Establish a discretized converter model for prediction. Transform the predicted current to the dq rotating coordinate system using Park transformation. Traverse all possible switching states of the converter, calculate the corresponding predicted current for each state, and calculate the cost function based on the predicted current. Select the switching state that minimizes the cost function and apply it to the next control cycle. Follow / grid mode switching control: Obtain the voltage and current signals of the point of common coupling to determine the grid strength. When the grid strength is lower than a preset threshold, switch at least some converters to grid-connected mode. When the grid strength rises to a higher threshold, switch at least some converters to follow-grid mode. After mode switching, synchronize the converter's internal state with the target mode. Reactive power compensation and voltage recovery control in network mode: Compare the reactive power output of each converter with the average value of the cluster to generate a compensation signal to eliminate distribution errors; generate a voltage recovery signal based on the deviation of the voltage of each node from the average value of the cluster to maintain the system voltage within the allowable range.

2. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, The converter current inner loop control process is applicable to both grid-following control and grid-connected control modes.

3. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, The process of establishing the discrete model of the converter for prediction includes: in the αβ stationary coordinate system, based on the inductor voltage equation, calculating the predicted current at the next sampling time as follows: i1 αβ (k+1) represent the components of the converter output measured at the predicted next time step (time k+1) in the α and β coordinates, respectively, i1 αβ (k) represents the components of the converter output measured at time k in the α and β coordinates, respectively. f L is the equivalent series resistance of the filter inductor. f T is the value of the filter inductance. s For the sampling period of the control system, u αβ (k) represents the components of the converter-side output voltage measured at time k in the α and β coordinates, respectively. c αβ (k) represents the components of the grid-side voltage or point of common coupling voltage measured at time k in the α and β coordinates.

4. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, The process of calculating the cost function based on the predicted current includes: The cost function is: ,in, This is the reference value for the d-axis current on the grid side. This is the predicted value of the d-axis current on the grid side. This is the reference value for the q-axis current on the grid side. This is the predicted value of the q-axis current on the grid side.

5. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, The process of obtaining voltage and current signals at the point of common coupling (PCC) to determine grid strength includes obtaining the PCC voltage and current signals, calculating the system's equivalent short-circuit ratio (EMR), and using this as a criterion for grid strength. The calculation formula is as follows: Among them, S sc For the system short-circuit capacity, P rated This refers to the rated power of the converter cluster.

6. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, After mode switching, the process of synchronizing the converter's internal state with the target mode includes phase reference pre-synchronization, voltage reference pre-synchronization, and current reference pre-synchronization. When the switching signal changes from grid-following type to grid-building type, the system's inner loop reference signal changes from the output signal of the grid-following controller to the output signal of the grid-building controller after pre-synchronization. When the switching signal changes from grid-building type to grid-following type, the system's inner loop reference signal changes from the output signal of the grid-building controller to the output signal of the grid-following controller after pre-synchronization, thus achieving shock-free mode switching.

7. The grid-connected converter cluster model predictive dual-mode control method as described in claim 6, characterized in that, The phase reference pre-synchronization is achieved by adjusting the angular frequency of the virtual synchronizer, and the pre-synchronization phase is obtained by integrating the angular velocity: ;ω GFM θ is the original angular frequency of the virtual synchronous generator. grid θ represents the actual phase of the grid voltage. GFM For the original phase of the virtual synchronous generator, ω GFM_pre θ is the angular frequency of the virtual synchronous generator after pre-synchronization adjustment. GFM_pre This refers to the phase of the virtual synchronous generator after pre-synchronization.

8. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, The voltage reference pre-synchronization link is as follows: ;u oGFM u is the original output voltage reference value of the virtual synchronous generator. ogrid The actual measured value of the grid voltage, u oGFMpre The output voltage reference value is the pre-synchronized value; the current reference pre-synchronization circuit is as follows: i2 dq For the dq-axis component of the grid-side current, u o dq Let u be the dq-axis component of the grid voltage. oGFMpre dq For the dq-axis components of the pre-synchronization voltage, ω o i is the rated angular frequency. 1GFMpre dq The dq-axis component of the inner loop current reference quantity after pre-synchronization.

9. The grid-connected converter cluster model predictive dual-mode control method as described in claim 1, characterized in that, The process of reactive power compensation and voltage recovery control in grid-connected mode includes: the output voltage reference value of each converter is composed of the traditional droop characteristic and the compensation signal. U oi Let be the output voltage of the i-th converter. U oi n is the output voltage reference of the converter. i Let Q be the reactive power droop factor of the i-th converter. i For the output of each converter, δ u It is a composite compensation signal; δ qc For reactive power sharing signal, δ ur For voltage recovery signal, Q k For the output of each converter, E k Let n be the output voltage of each converter, and n be the number of converters.

10. A grid-connected converter cluster model predictive dual-mode control system, characterized in that, include: The converter current inner loop control module is configured to establish a discretized model of the converter for prediction, transform the predicted current to the dq rotating coordinate system through Park transformation, traverse all possible switching states of the converter, calculate the corresponding predicted current for each state, calculate the cost function based on the predicted current, select the switching state that minimizes the cost function value, and apply it to the next control cycle. The grid connection / grid connection mode switching control module is configured to acquire the voltage and current signals of the common coupling point, determine the grid strength, and switch at least a portion of the converters to grid connection mode when the grid strength is lower than a preset threshold; when the grid strength rises to a higher threshold, at least a portion of the converters are switched to grid connection mode. After the mode switch, the internal state of the converter is synchronized with the target mode. The reactive power compensation and voltage recovery control module in the grid mode is configured to compare the reactive power output of each converter with the average value of the cluster and generate a compensation signal to eliminate distribution errors. Based on the deviation of each node's voltage from the cluster average, a voltage recovery signal is generated to maintain the system voltage within the allowable range.

Images