A method and system for grid-forming flexible switching predictive control of a grid-connected converter

Abstract

The application discloses a kind of method and system of grid-connected converter's follow network flexible switching prediction control, short-circuit ratio is calculated according to the impedance data of acquired power grid, if short-circuit ratio is less than switching threshold, it indicates that power grid intensity drops, at this time, the prediction control reference current of switching process is calculated by the adaptive switching function of the form of first switching function and the current reference value under the mode of following network and network construction, and grid-connected converter is switched from the mode of following network to the mode of network construction;Otherwise, it indicates that power grid intensity increases, the prediction control reference current of switching process is calculated by the adaptive switching function of the form of second switching function and the current reference value under the mode of following network and network construction, and grid-connected converter is switched from the mode of network construction to the mode of following network.Therefore, the follow network flexible switching prediction control method based on adaptive switching function guarantees that grid-connected converter module is quickly and flexibly switched, and can effectively improve the stability and grid-connected output efficiency of converter in new energy grid-connected power generation system.

Description

Technical Field

[0001] This invention relates to the technical field of converter switching, and in particular to a method and system for predictive control of grid-connected converters with flexible grid switching. Background Technology

[0002] Traditional grid-connected converters mostly follow the grid-following mode, which adjusts active and reactive power output based on the grid phase, making it particularly suitable for strong grid environments. However, in scenarios with high penetration of renewable energy into the grid, its stability margin decreases significantly, potentially triggering harmonic resonance or even system instability, posing a challenge to the stable and efficient operation of renewable energy grid-connected power generation systems. In contrast, the grid-connecting mode adjusts output power by directly manipulating the phase of the output voltage vector, exhibiting more stable characteristics under weak grid conditions, but may be less stable in strong grids. Given the respective advantages and limitations of these two grid-connection modes, current research has proposed a dual-mode control strategy that combines their complementary strengths. This strategy monitors grid impedance and calculates the short-circuit ratio (SCR), flexibly switching the grid-connection mode when the SCR reaches a preset switching threshold.

[0003] However, traditional control schemes employ a dual-loop cascaded PI controller architecture in both grid-fed and grid-connected modes. In grid-fed mode, based on a given power reference value and instantaneous power calculations, the output current reference value is obtained through a PI regulator. This, combined with instantaneous current measurements, undergoes PI regulation, decoupling, and feedforward processing before being used to generate a switching control signal via PWM modulation. In grid-connected mode, the output voltage reference value is first calculated based on given voltage and angular frequency reference values, combined with droop characteristics. The output current reference value is then obtained through a PI regulator, and subsequent control is similar to that in grid-fed mode.

[0004] For switching between grid-connected and grid-connected modes, traditional methods construct different switching boundaries and use hysteresis control to avoid frequent switching caused by grid strength fluctuations, thereby ensuring system stability. However, this cascaded mechanism based on dual-loop PI controllers requires a total of six PI controllers, making parameter design and selection complex. Furthermore, the nested cascaded structure may lead to slow dynamic response, making it difficult to meet the requirements of rapid switching.

[0005] Specifically, when grid strength weakens and a switch from grid-following mode to grid-connecting mode is required, the system needs to quickly enter grid-connecting mode to support the grid and prevent collapse. However, the excessively long dynamic response time of PI regulation may lead to system instability due to untimely switching. Conversely, when grid strength strengthens and a switch back from grid-connecting mode is made to achieve maximum power output as quickly as possible, a faster switching speed is needed, and the slow response of PI regulation will affect economic efficiency. Furthermore, direct mode switching via switching or hysteresis results in high switching stress, which can easily trigger system inrush current and oscillations, thereby affecting system stability. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method and system for predictive control of grid-connected converters with flexible switching, which can effectively suppress system inrush current oscillation instability and can design switching functions in a targeted manner to achieve autonomous control of switching speed and switching mode.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for predictive control of grid-connected converters with flexible grid switching includes the following steps:

[0009] Acquire the detected grid impedance data, and calculate the short-circuit ratio based on the grid impedance data;

[0010] If the short-circuit ratio is less than the switching threshold, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​under grid-connected and grid-connected modes, and the grid-connected converter is switched from grid-connected mode to grid-connected mode. The adaptive switching function is in the form of the first switching function.

[0011] Otherwise, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​under grid-connected and grid-connected modes, and the grid-connected converter is switched from grid-connected mode to grid-connected mode. The adaptive switching function is in the form of the second switching function.

[0012] To solve the above-mentioned technical problems, another technical solution adopted by the present invention is as follows:

[0013] A system for predictive control of grid-connected converter with flexible switching of the grid includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the various steps of the above-described method for predictive control of grid-connected converter with flexible switching of the grid.

[0014] The beneficial effects of the present invention are as follows: calculate the short-circuit ratio according to the obtained grid impedance data. If the short-circuit ratio is less than the switching threshold, it indicates that the grid strength has decreased. At this time, calculate the predicted control reference current for the switching process through the adaptive switching function in the form of the first switching function and the current reference values in the grid-following and grid-forming modes, and switch the grid-connected converter from the grid-following mode to the grid-forming mode; otherwise, it indicates that the grid strength has increased, calculate the predicted control reference current for the switching process through the adaptive switching function in the form of the second switching function and the current reference values in the grid-following and grid-forming modes, and switch the grid-connected converter from the grid-forming mode to the grid-following mode. Therefore, the grid-following and grid-forming flexible switching predictive control method based on the adaptive switching function ensures the fast and flexible switching of the grid-connected converter module, takes into account the system stability and grid-connected output efficiency, and can effectively improve the stability of the converter and the grid-connected power quality in the new energy grid-connected power generation system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a flowchart of a method for grid-following and grid-forming flexible switching predictive control of a grid-connected converter according to an embodiment of the present invention;

[0016] Figure 2 is a schematic diagram of a system for grid-following and grid-forming flexible switching predictive control of a grid-connected converter according to an embodiment of the present invention;

[0017] Figure 3 is a grid-following predictive control structure diagram according to an embodiment of the present invention;

[0018] Figure 4 is a grid-forming predictive control structure diagram according to an embodiment of the present invention;

[0019] Figure 5 is a grid-following and grid-forming mode switching control diagram according to an embodiment of the present invention;

[0020] Label description:

[0021] 1. A system for grid-following and grid-forming flexible switching predictive control of a grid-connected converter; 2. Memory; 3. Processor. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] To describe the technical content, achieved objectives and effects of the present invention in detail, the following is described in conjunction with the embodiments and with reference to the accompanying drawings.

[0023] Please refer to Figure 1 , an embodiment of the present invention provides a method for grid-following and grid-forming flexible switching predictive control of a grid-connected converter, including the steps of:

[0024] Obtain the detected grid impedance data, and calculate the short-circuit ratio according to the grid impedance data;

[0025] If the short-circuit ratio is less than the switching threshold, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​under grid-connected and grid-connected modes, and the grid-connected converter is switched from grid-connected mode to grid-connected mode. The adaptive switching function is in the form of the first switching function.

[0026] Otherwise, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​under grid-connected and grid-connected modes, and the grid-connected converter is switched from grid-connected mode to grid-connected mode. The adaptive switching function is in the form of the second switching function.

[0027] As described above, the beneficial effects of this invention are as follows: Based on the acquired grid impedance data, the short-circuit ratio is calculated. If the short-circuit ratio is less than the switching threshold, it indicates a decrease in grid strength. In this case, the predictive control reference current for the switching process is calculated using an adaptive switching function of the first switching function form and current reference values ​​under grid-following and grid-connected modes, switching the grid-connected converter from grid-following mode to grid-connected mode. Otherwise, it indicates an increase in grid strength. The predictive control reference current for the switching process is calculated using an adaptive switching function of the second switching function form and current reference values ​​under grid-following and grid-connected modes, switching the grid-connected converter from grid-connected mode to grid-following mode. Therefore, the grid-connected converter module switching prediction control method based on the adaptive switching function ensures rapid and flexible switching, balancing system stability and grid-connected output efficiency, and effectively improves the stability of the converter and the grid-connected power quality in new energy grid-connected power generation systems.

[0028] Furthermore, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​in the following and grid-connected modes, including:

[0029] Setting the calculation method for the reference current:

[0030]

[0031] In the formula, f sw This represents an adaptive switching function. and These represent the reference values ​​for the α-axis and β-axis currents in the wire mesh mode. and These are the reference values ​​for the current along the α and β axes in the network mode.

[0032] As described above, when the switching conditions are met, the reference current for predictive control can be calculated based on the current reference values ​​of the α-axis and β-axis in the following mode and the network mode. An adaptive switching function is used to achieve a fast and flexible switching process.

[0033] Furthermore, the adaptive switching function is in the form of a first switching function, including:

[0034] In the first switching function form, the adaptive switching function f sw for:

[0035]

[0036] In the formula, t represents the switching time and a represents the adjustable switching speed. The switching is completed when t = 6 / a.

[0037] As described above, using the hyperbolic tangent function as the switching function further ensures that fast and flexible switching can be achieved.

[0038] Furthermore, the adaptive switching function is in the form of a second switching function, including:

[0039] In the second switching function form, the adaptive switching function f sw for:

[0040]

[0041] In the formula, t represents the switching time and a represents the adjustable switching speed. The switching is completed when t = 6 / a.

[0042] As described above, by adaptively adjusting the form of the switching function under different switching modes, the switching speed and switching mode can be autonomously controlled through targeted design of the switching function, so as to provide a simple, effective and engineering-friendly tracking network switching control method.

[0043] Further, the short-circuit ratio is calculated based on the grid impedance data, including:

[0044] Calculate the short-circuit ratio (SCR):

[0045]

[0046] In the formula, S DG S represents the rated capacity of a distributed generation system connected to the power grid. SC U represents the short-circuit capacity of the power grid. g Z represents the rated effective value of the grid voltage. g This represents the magnitude of the power grid impedance.

[0047] As described above, the short-circuit ratio can be calculated by detecting the grid impedance, and the strength of the grid can be judged by the magnitude of the short-circuit ratio.

[0048] Please refer to Figure 2Another embodiment of the present invention provides a system for predictive control of flexible switching of grid-connected converters, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the various steps of the above-described method for predictive control of flexible switching of grid-connected converters.

[0049] The method and system for predictive control of grid-connected converter flexible switching in accordance with the above-mentioned invention are applicable to effectively suppressing system inrush current oscillation instability and achieving autonomous controllability of switching speed and switching mode. The following is a description of specific implementation methods:

[0050] Please refer to Figure 1 Embodiment 1 of the present invention is as follows:

[0051] First, in this embodiment, the grid-connected converter's grid-connected predictive control structure is as follows: Figure 3 As shown. Among them, and The grid output current command values ​​for the d-axis and q-axis are respectively obtained after dq / αβ transformation. and These are the grid output current command values ​​for the α-axis and β-axis, respectively, θ GFL Real-time phase of the grid voltage acquired by the PLL. Grid voltage, grid-connected current, and output current measurements obtained from the measuring device. After abc / αβ transformation, the measured values ​​of grid voltage, grid-connected current, and output current in the αβ coordinate system are obtained. in, i 1,α [k] and i 1,β [k] represents the output current measurements along the α and β axes, respectively. The state variables for the next time step can be calculated using a predictive model. Using cost function Calculate the output gate drive signal S corresponding to the minimum cost function. abc .

[0052] Networked predictive control structures such as Figure 4 As shown. Where, P ref and Q ref Given active and reactive power values, θ GFM The phase output by the droop controller. and These represent the output voltage command values ​​for the d-axis and q-axis of the droop controller, respectively. These are then processed by a PI regulator to obtain the gridded output current command values ​​for the d-axis and q-axis. and After dq / αβ transformation, the meshing output current command values ​​for the α-axis and β-axis are obtained. and Grid voltage, grid-connected current, and output current measurements in the αβ coordinate system The state variables for the next time step are calculated using the prediction model. Using cost function Calculate the output gate drive signal S corresponding to the minimum cost function. abc Different control signals result in different cost functions J. The gate drive signal that minimizes the cost function J is selected in the next control cycle. During mode switching, it can quickly reach a steady state within a few control cycles, achieving rapid adaptive tracking and network switching.

[0053] Therefore, please refer to Figure 1 and Figure 5 This embodiment provides a method for predictive control of grid-connected converters with flexible grid switching, including the following steps:

[0054] S1. Obtain the detected grid impedance data and calculate the short-circuit ratio based on the grid impedance data.

[0055] For details, please refer to Figure 5 , where v dc Indicates the DC bus voltage. and S represents the current command values ​​for the α-axis and β-axis. abc For the output gate drive signal, S1, S2, S3, S4, S5, and S6 represent the drive signals for the six switching transistors of the grid-connected converter, respectively. f and L g These are the converter-side inductance and the grid-side inductance. g,a v g,b v g,c Let i represent the grid voltage values ​​for phases a, b, and c, respectively. g,a i g,b i g,c Let i represent the grid-connected current values ​​for phases a, b, and c, respectively. 1,a i 1,b i 1,c These are the output current values ​​for phase a, phase b, and phase c, respectively.

[0056] In this embodiment, the converter output mode is controlled by a grid-based flexible switching strategy using only a flexible adjustment cost function and a reference. For a single-circuit DC feed, the effective short-circuit ratio (SCR) index for evaluating the relative strength of the AC / DC system is as follows: Strong – SCR greater than 3.0; Medium – SCR between 2.0 and 3.0; Weak – SCR less than 2.0. The grid strength is determined by calculating the SCR through grid impedance detection. Methods for detecting grid impedance include injecting non-characteristic subharmonics and injecting wide-frequency signals, etc., and the specific processes will not be detailed here.

[0057] The formula for calculating the short-circuit ratio (SCR) is:

[0058]

[0059] In the formula, S DG S represents the rated capacity of a distributed generation system connected to the power grid. SC U represents the short-circuit capacity of the power grid. g Z represents the rated effective value of the grid voltage. g This represents the magnitude of the power grid impedance.

[0060] S2. If the short-circuit ratio is less than the switching threshold, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​under the grid-connected and grid-connected modes, and the grid-connected converter is switched from the grid-connected mode to the grid-connected mode. The adaptive switching function is in the form of the first switching function.

[0061] In this embodiment, the switching threshold is 2. When SCR < 2, the converter switches from grid-connected mode to grid-connected mode; when SCR > 2, it switches from grid-connected mode to grid-connected mode.

[0062] When the switching conditions are met, set the calculation method for the reference current:

[0063]

[0064] In the formula, f sw This represents an adaptive switching function. and These represent the reference values ​​for the α-axis and β-axis currents in the wire mesh mode. and These are the reference values ​​for the current along the α and β axes in the network mode.

[0065] Here, the design of the switching function can refer to the sliding membrane observer. For a control variable that jumps from 0 to 1, a sign function, saturation function, continuous function, arctangent function, hyperbolic tangent function, etc., can be used. This invention selects the hyperbolic tangent function as the switching function:

[0066]

[0067] Where x = a·t, a is the adjustable switching speed, and t represents the switching time. When the following mode needs to switch to the constructing mode, let... Set t to zero. The switch can be completed in a timely manner.

[0068] S3. Otherwise, the predictive control reference current for the switching process is calculated using the adaptive switching function and the current reference values ​​under the grid-connected and grid-connected modes, and the grid-connected converter is switched from the grid-connected mode to the grid-connected mode. The adaptive switching function is in the form of the second switching function.

[0069] In this embodiment, when the mesh-building mode needs to switch to the mesh-following mode, let Set t to zero. The switch can be completed in a timely manner.

[0070] Compared with the traditional tail-structure network switching method, this algorithm can effectively suppress system inrush oscillation instability and can design switching functions in a targeted manner to achieve autonomous control of switching speed and switching mode. It is a simple, effective and engineering-friendly tail-structure network switching control method.

[0071] Under strong power grid conditions, the converter operates in grid-following mode, regulating active and reactive power through current adjustment based on grid voltage orientation. By employing a phase-locked loop to observe the grid voltage phase, it maximizes the utilization of renewable energy and ensures high power quality. Under weak power grid conditions, the converter operates in grid-connected mode, regulating output power by controlling the output voltage vector phase, exhibiting better stability.

[0072] After obtaining the reference current, a cost function is needed to ensure flexible current switching. For example, when switching from grid connection to grid connection, the initial cost function is the one for grid connection:

[0073]

[0074] It became:

[0075]

[0076] in, In f sw After changing from 0 to 1, the switching function becomes the one under the network structure:

[0077]

[0078] Therefore, this embodiment establishes a converter control model based on predictive control, determines the switching boundary based on grid strength, and performs grid-connected switching based on adaptive switching function. This realizes the working mechanism of the converter flexible switching control mode when the grid strength changes, which can effectively improve the stability of the converter and the grid-connected power quality in the new energy grid-connected power generation system.

[0079] Please refer to Figure 2 Embodiment two of the present invention is as follows:

[0080] A system 1 for predictive control of grid-connected converter with flexible switching to the grid includes a memory 2, a processor 3, and a computer program stored in the memory 2 and executable on the processor 3. When the processor 3 executes the computer program, it implements the various steps of the method for predictive control of grid-connected converter with flexible switching to the grid according to Embodiment 1.

[0081] In summary, the present invention provides a method and system for predictive control of flexible switching of a grid-connected converter with grid connection. Based on the acquired grid impedance data, the short-circuit ratio is calculated. If the short-circuit ratio is less than the switching threshold, it indicates a decrease in grid strength. In this case, a predictive control reference current for the switching process is calculated using an adaptive switching function of the first switching function form and current reference values ​​under both grid-connected and grid-connected modes, switching the grid-connected converter from grid-connected mode to grid-connected mode. Otherwise, it indicates an increase in grid strength. A predictive control reference current for the switching process is calculated using an adaptive switching function of the second switching function form and current reference values ​​under both grid-connected and grid-connected modes, switching the grid-connected converter from grid-connected mode to grid-connected mode. Therefore, the predictive control method for flexible switching of the grid-connected converter based on the adaptive switching function ensures rapid and flexible switching of the grid-connected converter module, balancing system stability and grid-connected output efficiency, and effectively improving the stability of the converter and the quality of grid-connected power in new energy grid-connected power generation systems.

[0082] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for grid-forming grid flexibility switching predictive control of a grid-connected converter, characterized in that, The method comprises the steps of: obtaining detected grid impedance data, and calculating a short-circuit ratio according to the grid impedance data; if the short-circuit ratio is less than a switching threshold, calculating a predictive control reference current of a switching process by an adaptive switching function and current reference values in a grid-following mode and a grid-forming mode, and switching the grid-connected converter from the grid-following mode to the grid-forming mode, the adaptive switching function being in a first switching function form; otherwise, calculating the predictive control reference current of the switching process by the adaptive switching function and the current reference values in the grid-following mode and the grid-forming mode, and switching the grid-connected converter from the grid-forming mode to the grid-following mode, the adaptive switching function being in a second switching function form; calculating the predictive control reference current of the switching process by the adaptive switching function and the current reference values in the grid-following mode and the grid-forming mode, comprising: setting a calculation mode of the reference current; ， ； In the formula, f sw represents an adaptive switching function, and represents the current reference value of the α-axis and the β-axis under the grid-connected mode, and is the current reference value of the α-axis and the β-axis under the grid-connected mode. the adaptive switching function being in the first switching function form, comprising: In the first form of the switching function, the adaptive switching function f sw is: ； wherein t represents a switching time, and a represents an adjustable switching speed, and the switching is completed when t = 6 / a; the adaptive switching function being in the second switching function form, comprising: In the second form of the switching function, the adaptive switching function f sw is: ； wherein t represents a switching time, and a represents an adjustable switching speed, and the switching is completed when t = 6 / a.

2. The method of claim 1, wherein, calculating a short-circuit ratio according to the grid impedance data, comprising: calculating a short-circuit ratio SCR: ； wherein S DG denotes the rated capacity of the distributed generation system to be incorporated into the power grid, S SC denotes the short circuit capacity of the power grid, U g denotes the rated effective value of the voltage of the power grid, Z g denotes the impedance modulus of the power grid.

3. A system for predictive control of grid-connected converters with flexible grid switching, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, the processor implements the following steps when executing the computer program: obtaining detected grid impedance data, and calculating a short-circuit ratio according to the grid impedance data; if the short-circuit ratio is less than a switching threshold, calculating a predictive control reference current of a switching process by an adaptive switching function and current reference values in a grid-following mode and a grid-forming mode, and switching the grid-connected converter from the grid-following mode to the grid-forming mode, the adaptive switching function being in a first switching function form; otherwise, calculating the predictive control reference current of the switching process by the adaptive switching function and the current reference values in the grid-following mode and the grid-forming mode, and switching the grid-connected converter from the grid-forming mode to the grid-following mode, the adaptive switching function being in a second switching function form; calculating the predictive control reference current of the switching process by the adaptive switching function and the current reference values in the grid-following mode and the grid-forming mode, comprising: setting a calculation mode of the reference current; ， ； In the formula, f sw represents an adaptive switching function, and represents the current reference value of the α-axis and the β-axis in the grid-following mode, and is the current reference value of the α-axis and the β-axis in the grid-forming mode. the adaptive switching function being in the first switching function form, comprising: In the first form of the switching function, the adaptive switching function f sw is: ； wherein t represents a switching time, and a represents an adjustable switching speed, and the switching is completed when t = 6 / a; the adaptive switching function being in the second switching function form, comprising: In the second form of the switching function, the adaptive switching function f sw is: ； wherein t represents a switching time, and a represents an adjustable switching speed, and the switching is completed when t = 6 / a.

4. The system of claim 3, wherein, calculating a short-circuit ratio according to the grid impedance data, comprising: calculating a short-circuit ratio SCR: ； wherein S DG denotes the rated capacity of the distributed generation system to be incorporated into the power grid, S SC denotes the short circuit capacity of the power grid, U g denotes the grid voltage rated effective value, Z g denotes the grid impedance modulus.

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