Method and system for network-following flexible switching prediction control of grid-connected converter

Abstract

The invention discloses a network-following flexible switching prediction control method and system for a grid-connected converter, and the method comprises the steps: calculating a short-circuit ratio according to obtained power grid impedance data, and if the short-circuit ratio is smaller than a switching threshold value, indicating that the power grid strength is reduced, and otherwise, indicating that the power grid strength is not reduced. At the moment, a prediction control reference current in the switching process is calculated through an adaptive switching function in a first switching function form and a current reference value in a grid following mode and a grid construction mode, and the grid-connected converter is switched from the grid following mode to the grid construction mode; and otherwise, indicating that the power grid intensity is increased, calculating a prediction control reference current in a switching process through an adaptive switching function in a second switching function form and a current reference value in a grid following mode and a grid construction mode, and switching the grid-connected converter from the grid construction mode to the grid following mode. Therefore, according to the adaptive switching function-based network-following flexible switching prediction control method, rapid and flexible switching of the grid-connected converter module is ensured, and the stability and the grid-connected output efficiency of the converter in the new energy grid-connected power generation system can be effectively improved.

Description

Technical Field

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

[0002] Most traditional grid-connected converters follow the grid-following mode, which adjusts the active and reactive power output according to the grid phase and is particularly suitable for strong grid environments. However, in the scenario where new energy is connected to the grid at a high penetration rate, its stability margin decreases significantly, which may trigger harmonic resonance and even system instability, posing a challenge to the stable and efficient operation of new energy grid-connected power generation systems. In contrast, the grid-building mode adjusts the output power by directly manipulating the phase of the output voltage vector, showing a more stable characteristic under weak grid conditions, but may appear unstable in strong grids. In view of the respective advantages and limitations of these two grid-connected modes, current research has proposed a dual-mode control strategy that combines the complementarity of the two. This strategy monitors the grid impedance and calculates the short circuit ratio (SCR), and flexibly switches the grid-connected mode when the short circuit ratio reaches the preset switching threshold.

[0003] However, the traditional control scheme adopts a dual-loop cascaded PI controller architecture in both the grid-following and grid-building modes. In the grid-following mode, the output current reference value is obtained through the PI regulator based on the given power reference value and the instantaneous power calculation, and then combined with the instantaneous current measurement value, after PI regulation, decoupling, feedforward and other processing, the switch control signal is generated using PWM modulation. In the grid-building mode, the output voltage reference value is first calculated based on the given voltage reference value and angular frequency reference value, combined with the droop characteristics, and then the output current reference value is obtained through the PI regulator. The subsequent control is similar to the grid-following mode.

[0004] For the switching between the grid-following and grid-building modes, the traditional method constructs different switching boundaries and uses hysteresis control to avoid frequent switching caused by grid strength fluctuations, thereby ensuring system stability. However, this dual-loop PI-based cascade mechanism requires a total of 6 PI regulators, and the parameter design and selection process is complex. In addition, the nested cascade structure may also lead to slow dynamic response, making it difficult to meet the needs of fast switching.

[0005] Specifically, when the strength of the power grid weakens and it is necessary to switch from the grid-following mode to the grid-building mode, the system needs to quickly enter the grid-building state to support the power grid and prevent collapse. However, the dynamic response time of PI regulation is too long, which may cause system instability due to untimely switching. On the contrary, when the strength of the power grid is enhanced and the grid-building mode is switched back to the grid-following mode, a faster switching speed is required in order to output the maximum power as soon as possible, and the slow response of PI regulation will affect the economy. In addition, the mode switching is performed directly through switches or hysteresis loops, and the switching stress is large, which is easy to cause system inrush and oscillation, thereby affecting the stability of the system. 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 flexible switching of grid-connected converters, which can effectively suppress system inrush current oscillation instability, and can specifically design switching functions to achieve autonomous control of switching speed and switching mode.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0008] A method for predictive control of flexible switching of a grid-connected converter, comprising the steps of:

[0009] Acquiring detected grid impedance data, and calculating the short circuit ratio according to the grid impedance data;

[0010] If the short-circuit ratio is less than the switching threshold, a predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-following mode to the grid-forming mode, wherein the adaptive switching function is in the form of a first switching function;

[0011] Otherwise, the predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes to switch the grid-connected converter from the grid-forming mode to the grid-following mode, and the adaptive switching function is in the form of a second switching function.

[0012] In order to solve the above technical problems, another technical solution adopted by the present invention is:

[0013] A system for predictive control of flexible switching of a grid-connected converter with a grid-connected network comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, each step of the above-mentioned method for predictive control of flexible switching of a grid-connected converter with a grid-connected network is implemented.

[0014] The beneficial effects of the present invention are: the short-circuit ratio is calculated according to the acquired grid impedance data. If the short-circuit ratio is less than the switching threshold, it means that the strength of the grid decreases. At this time, the predictive control reference current of the switching process is calculated by the adaptive switching function in the form of the first switching function and the current reference value under the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-following mode to the grid-forming mode; otherwise, it means that the strength of the grid increases. The predictive control reference current of the switching process is calculated by the adaptive switching function in the form of the second switching function and the current reference value under the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-forming mode to the grid-following mode. Therefore, the predictive control method for flexible switching of the grid-following and grid-forming based on the adaptive switching function ensures that the grid-connected converter module switches quickly and flexibly, takes into account the system stability and the 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 It is a flow chart of a method for predictive control of flexible switching of a grid-connected converter according to an embodiment of the present invention;

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

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

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

[0019] Figure 5 This is a control diagram of the switching mode of the follow-up network according to an embodiment of the present invention;

[0020] Description of labels:

[0021] 1. A system for predictive control of flexible switching of grid-connected converters; 2. Memory; 3. Processor. DETAILED DESCRIPTION

[0022] In order to explain the technical content, achieved objectives and effects of the present invention in detail, the following is an explanation in combination with the implementation modes and the accompanying drawings.

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

[0024] Acquiring detected grid impedance data, and calculating the short circuit ratio according to the grid impedance data;

[0025] If the short-circuit ratio is less than the switching threshold, a predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-following mode to the grid-forming mode, wherein the adaptive switching function is in the form of a first switching function;

[0026] Otherwise, the predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes to switch the grid-connected converter from the grid-forming mode to the grid-following mode, and the adaptive switching function is in the form of a second switching function.

[0027] From the above description, it can be seen that the beneficial effects of the present invention are: the short-circuit ratio is calculated according to the acquired grid impedance data. If the short-circuit ratio is less than the switching threshold, it means that the strength of the grid decreases. At this time, the predictive control reference current of the switching process is calculated by the adaptive switching function in the form of the first switching function and the current reference value under the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-following mode to the grid-forming mode; otherwise, it means that the strength of the grid increases. The predictive control reference current of the switching process is calculated by the adaptive switching function in the form of the second switching function and the current reference value under the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-forming mode to the grid-following mode. Therefore, the predictive control method for flexible switching of the grid-following and grid-forming based on the adaptive switching function ensures that the grid-connected converter module switches quickly and flexibly, 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.

[0028] Furthermore, the predictive control reference current of the switching process is calculated by using the adaptive switching function and the current reference values ​​in the following and building modes, including:

[0029] Set the calculation method of reference current:

[0030]

[0031] In the formula, f sw represents the adaptive switching function, and Indicates the current reference value of the α-axis and β-axis in the grid-following mode, and are the current reference values ​​of the α-axis and β-axis in the network mode.

[0032] From the above description, it can be seen that when the switching conditions are met, the reference current of the predictive control can be calculated according to the current reference values ​​of the α-axis and β-axis in the following mode and the building mode, wherein an adaptive switching function is used to achieve a fast and flexible switching process.

[0033] Further, 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] Wherein, t represents the switching time, a represents the adjustable switching speed, and the switching is completed when t=6 / a.

[0037] It can be seen from the above description that by using the hyperbolic tangent function as the switching function, it is further ensured that fast and flexible switching can be achieved.

[0038] Further, 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] Wherein, t represents the switching time, a represents the adjustable switching speed, and the switching is completed when t=6 / a.

[0042] From the above description, it can be seen that the form of the switching function can be adaptively adjusted under different switching modes. By specifically designing the switching function, the switching speed and switching mode can be autonomously controlled, so as to provide a simple, effective and engineering-friendly network switching control method.

[0043] Further, calculating the short circuit ratio according to the grid impedance data includes:

[0044] Calculate the short circuit ratio of SCR:

[0045]

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

[0047] From the above description, it can be known that the short-circuit ratio is calculated by detecting the impedance of the power grid, and in this way, the strength of the power grid can be judged by the size 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 a grid-connected converter with a grid-connected network, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements each step of the above-mentioned method for predictive control of flexible switching of a grid-connected converter with a grid-connected network when executing the computer program.

[0049] The method and system for predictive control of flexible switching of a grid-connected converter according to the present invention are suitable for effectively suppressing the inrush current oscillation instability of the system and realizing autonomous control of the switching speed and switching mode. The following is an explanation through specific implementation methods:

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

[0051] First, the grid-following predictive control structure of the grid-connected converter in this embodiment is as follows: Figure 3 As shown. Among them, and They are the grid-following output current command values ​​of the d-axis and q-axis respectively, which are obtained through dq / αβ transformation. and are the grid-following output current command values ​​of the α-axis and the β-axis, θ GFL The real-time phase of the grid voltage obtained by PLL. The grid voltage, grid current, and output current measurement values ​​obtained by the measuring device After abc / αβ transformation, the grid voltage, grid-connected current and output current measurement values ​​in the αβ coordinate system are obtained. in, i 1,α [k] and i 1,β [k] are the output current measurement values ​​of the α-axis and β-axis respectively. The state variable at the next moment can be calculated using the prediction model. Using the cost function Calculate the output gate drive signal S corresponding to the minimum cost function abc .

[0052] The network-based predictive control structure is as follows Figure 4 As shown. Among them, P ref and Q ref is the given value of active power and reactive power, θ GFM is the phase of the droop controller output, and They represent the output voltage command values ​​of the d-axis and q-axis output of the droop controller, and the network output current command values ​​of the d-axis and q-axis are obtained through the PI regulator. and After dq / αβ transformation, the grid output current command values ​​of α-axis and β-axis are obtained. and Grid voltage, grid current, and output current measurement values ​​in the αβ coordinate system Through the prediction model, the state variables at the next moment are calculated Using the cost function Calculate the output gate drive signal S corresponding to the minimum cost function abc Different control signals lead to different cost functions J, so that the gate drive signal with the smallest cost function J is selected in the next control cycle. When the mode is switched, the steady state can be quickly reached within a few control cycles, realizing fast adaptive grid switching.

[0053] Therefore, please refer to Figure 1 and Figure 5 In this embodiment, a method for predictive control of flexible switching of a grid-connected converter is provided, comprising the steps of:

[0054] S1. Acquire detected grid impedance data, and calculate the short-circuit ratio according to the grid impedance data.

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

[0056] In this embodiment, only the flexible adjustment cost function and reference are used to control the flexible switching strategy of the inverter output mode. For the single-loop DC feed-in situation, the effective short-circuit ratio (SCR) index for evaluating the relative strength of the AC / DC system is: strong - SCR greater than 3.0; medium - SCR between 2.0 and 3.0; weak - SCR less than 2.0. The SCR is calculated by detecting the grid impedance to determine the strength of the grid. The method for detecting the grid impedance includes injecting non-characteristic subharmonics, injecting wide-frequency signals, etc. The specific process will not be described in detail here.

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

[0058]

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

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

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

[0062] When the switching condition is reached, set the reference current calculation method:

[0063]

[0064] In the formula, f sw represents the adaptive switching function, and Indicates the current reference value of the α-axis and β-axis in the grid-following mode, and are the current reference values ​​of the α-axis and β-axis in the network mode.

[0065] Here, the design of the switching function can refer to the synovial observer. For a control quantity that jumps from 0 to 1, a sign function, a saturation function, a continuous function, an inverse tangent function, a hyperbolic tangent function, etc. can be used to implement it. The present invention selects the hyperbolic tangent function as the switching function:

[0066]

[0067] Where x = a·t, a is the adjustable switching speed, and t is the switching time. When the network following mode needs to be switched to the network building mode, let t is set to zero, Complete the switch instantly.

[0068] S3. Otherwise, the predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-forming mode to the grid-following mode, wherein the adaptive switching function is in the form of a second switching function.

[0069] In this embodiment, when the network building mode needs to be switched to the network following mode, t is set to zero, Complete the switch instantly.

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

[0071] In a strong power grid, the converter works in a grid-following mode. By adjusting the active and reactive power based on the grid voltage-oriented current and using a phase-locked loop to observe the grid voltage phase, the utilization rate of new energy can be maximized and a high power quality can be guaranteed. In a weak power grid, the converter works in a grid-building mode. The output power is adjusted by controlling the output voltage vector phase, which has good stability.

[0072] After obtaining the reference current, a cost function is also needed to ensure flexible switching of the current. For example, when switching from following the network to building the network, the cost function is initially based on following the network:

[0073]

[0074] becomes:

[0075]

[0076] in, In f sw After changing from 0 to 1, the switching function becomes the following:

[0077]

[0078] Therefore, in this embodiment, a converter control model based on predictive control is established, the switching boundary is judged according to the grid strength, and the grid-connected switching is performed based on the adaptive switching function, so as to realize 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 2 of the present invention is:

[0080] A system 1 for predictive control of flexible switching of a grid-connected converter with a grid-connected network comprises 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, each step of a method for predictive control of flexible switching of a grid-connected converter with a grid-connected network in embodiment 1 is implemented.

[0081] In summary, the present invention provides a method and system for predictive control of flexible switching between following and building a grid of a grid-connected converter. The short-circuit ratio is calculated based on the acquired grid impedance data. If the short-circuit ratio is less than the switching threshold, it indicates that the strength of the grid has decreased. At this time, the predictive control reference current of the switching process is calculated through the adaptive switching function in the form of a first switching function and the current reference value in the following and building modes, and the grid-connected converter is switched from the following mode to the building mode; otherwise, it indicates that the strength of the grid has increased. The predictive control reference current of the switching process is calculated through the adaptive switching function in the form of a second switching function and the current reference value in the following and building modes, and the grid-connected converter is switched from the building mode to the following mode. Therefore, the predictive control method for flexible switching between following and building a grid based on the adaptive switching function ensures that the grid-connected converter module switches quickly and flexibly, takes into account the system stability and the 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.

[0082] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for predictive control of flexible switching of a grid-connected converter, characterized in that: Includes steps: Acquiring detected grid impedance data, and calculating the short circuit ratio according to the grid impedance data; If the short-circuit ratio is less than the switching threshold, a predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-following mode to the grid-forming mode, wherein the adaptive switching function is in the form of a first switching function; Otherwise, the predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes to switch the grid-connected converter from the grid-forming mode to the grid-following mode, and the adaptive switching function is in the form of a second switching function.

2. The method for predictive control of grid-connected converter flexible switching according to claim 1, characterized in that: The predictive control reference current of the switching process is calculated through the adaptive switching function and the current reference values ​​in the grid-following and grid-building modes, including: Set the calculation method of reference current: In the formula, f sw represents the adaptive switching function, and Indicates the current reference value of the α-axis and β-axis in the grid-following mode, and are the current reference values ​​of the α-axis and β-axis in the network mode.

3. The method for predictive control of grid-connected converter flexible switching according to claim 2 is characterized in that: The adaptive switching function is in the form of a first switching function, including: In the first switching function form, the adaptive switching function f sw for: Wherein, t represents the switching time, a represents the adjustable switching speed, and the switching is completed when t=6 / a.

4. The method for predictive control of grid-connected converter flexible switching according to claim 2, characterized in that: The adaptive switching function is in the form of a second switching function, including: In the second switching function form, the adaptive switching function f sw for: Wherein, t represents the switching time, a represents the adjustable switching speed, and the switching is completed when t=6 / a.

5. The method for predictive control of grid-connected converter flexible switching according to claim 1, characterized in that: Calculating the short circuit ratio according to the grid impedance data includes: Calculate the short circuit ratio of SCR: In the formula, S DG Indicates the rated capacity of the distributed generation system connected to the grid, S SC Indicates the short-circuit capacity of the power grid, U g Indicates the rated effective value of the grid voltage, Z g Indicates the grid impedance modulus.

6. A system for predictive control of flexible switching of a grid-connected converter, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the following steps are implemented: Acquiring detected grid impedance data, and calculating the short circuit ratio according to the grid impedance data; If the short-circuit ratio is less than the switching threshold, a predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes, and the grid-connected converter is switched from the grid-following mode to the grid-forming mode, wherein the adaptive switching function is in the form of a first switching function; Otherwise, the predictive control reference current of the switching process is calculated by using an adaptive switching function and current reference values ​​in the grid-following and grid-forming modes to switch the grid-connected converter from the grid-forming mode to the grid-following mode, and the adaptive switching function is in the form of a second switching function.

7. A system for predictive control of grid-connected converter flexible switching according to claim 6, characterized in that: The predictive control reference current of the switching process is calculated through the adaptive switching function and the current reference values ​​in the grid-following and grid-building modes, including: Set the calculation method of reference current: In the formula, f sw represents the adaptive switching function, and Indicates the current reference value of the α-axis and β-axis in the grid-following mode, and are the current reference values ​​of the α-axis and β-axis in the network mode.

8. The system for predictive control of grid-connected converter flexible switching according to claim 7, characterized in that: The adaptive switching function is in the form of a first switching function, including: In the first switching function form, the adaptive switching function f sw for: Wherein, t represents the switching time, a represents the adjustable switching speed, and the switching is completed when t=6 / a.

9. The system for predictive control of grid-connected converter flexible switching according to claim 7, characterized in that: The adaptive switching function is in the form of a second switching function, including: In the second switching function form, the adaptive switching function f sw for: Wherein, t represents the switching time, a represents the adjustable switching speed, and the switching is completed when t=6 / a.

10. The system for predictive control of grid-connected converter flexible switching according to claim 6, characterized in that: Calculating the short circuit ratio according to the grid impedance data includes: Calculate the short circuit ratio of SCR: In the formula, S DG Indicates the rated capacity of the distributed generation system connected to the grid, S SC Indicates the short-circuit capacity of the power grid, U g Indicates the rated effective value of the grid voltage, Z g Indicates the grid impedance modulus.

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